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Coupled ocean and atmospheric changes during Greenland stadial 1 in southwestern Europe
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Abstract
Paleoclimate reconstructions suggest that the complex variability within the Greenland stadial 1 (GS-1)
over western Europe was governed by coupled ocean and atmospheric changes. However, few works
from the North Atlantic mid-latitudes document both the GS-1 onset and its termination, which are often
considered as single abrupt transition events. Here, we present a direct comparison between marine
(alkenone-based sea surface temperatures) and terrestrial (pollen) data, at very high resolution (28 years
mean), from the southwestern Iberian shelf record D13882. Our results reveal a rather complex climatic
period with internally changing conditions. The GS-1 onset (GS-1a: 12890-12720 yr BP) is marked by a
progressive cooling and drying; GS-1b (12720-12390 yr BP) is the coldest and driest phase; GS-1c
(12390-12030 yr BP) is marked by a progressive warming and increase in moisture conditions; GS-1
termination (GS-1d: 12030-11770 yr BP) is marked by rapid switches between cool wet, cold dry and
cool wet conditions. Although hydroclimate response was very unsteady throughout the GS-1 and in
particular during its termination phase, the persistence of an open temperate and Mediterranean forest
in southwestern Iberia during the entire episode suggests that at least some moisture was delivered via
the Westerlies. We propose coupled ocean and atmospheric mechanisms to reproduce these scenaria.
Changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) as well as variations
in the North Atlantic sea-ice growth have favoured the displacement of the polar jet stream's latitudinal
position and contributed to a complex spatial pattern and strength of the Westerlies across western
Europe.
... Strengthening of storminess throughout the YD was confirmed by higher aeolian activity (formation and movement of transgressive dunefields) in Europe and on the west coast of Portugal, generally coinciding with global cooling events, and associated with enhanced westerly storm activity (e.g., Costas et al., 2012). According to pollenbased interpretations from lakes and wetlands of Serra da Estrela (van der Knaap and Van Leeuwen, 1997) and nearby marine records (Naughton et al., 2019), the YD was a complex event, with a first phase characterized by the driest and coldest atmospheric conditions (12.85-12.44 ka cal BP; van der Knaap and Van Leeuwen, 1997). ...
... ka cal BP; van der Knaap and Van Leeuwen, 1997). This driest and coldest phase is referred to as the GS-1b, i.e., the onset of the YD (Naughton et al., 2019) and, more recently, as the second phase of the YDb , ranging between 12.72 ka cal BP and 12.39 ka cal BP. The Lake Peixão records of Br/TOC and Cl/TOC spanning the initial phase of the YD (12.83-12.73 ...
... point to a sustained oceanic influx that already occurred during the B/A, and was probably triggered by the repositioning and strengthening of the mid-latitude westerlies or storminess, as supported by other data from the region (e.g. Costas et al., 2012;Naughton et al., 2019). The climatic pattern given by these two proxies is consistent with the shift of the westerlies to the SW-NE route and the gradual increase of moisture (12.89 to 12.73 ka cal BP) proposed by Obreht et al. (2020). ...
Iberian lacustrine sediments are a valuable archive to document environmental changes since the last glacial termination, seen as key for anticipating future climate/environmental changes and their far-reaching implications for generations to come. Herein, multi-proxy-based indicators of a mountain lake record from Serra da Estrela were used to reconstruct atmospheric (in)fluxes and associated climatic/environmental changes over the last ~13.5 ka. Depositions of long-range transported dust (likely from Sahara) and halogens (primarily derived from seawater) were higher for the pre-Holocene, particularly in the late Bølling-Allerød-Younger Dryas period, compared to the Holocene. This synchronous increase could be related to a recognized dust-laden atmosphere, along with the combined effect of (i) an earlier proposed effective transport of Sahara dust for higher latitudes during cold periods and (ii) the progressive Polar Front expansion southwards, with the amplification of halogen activation reactions in lower latitudes due to greater closeness to snow/sea ice (halide-laden) surfaces. Additionally, the orographic blocking of Serra da Estrela may have played a critical role in increasing precipitation of Atlantic origin at higher altitudes, with the presence of snow prompting physical and chemical processes involving halogen species. In the late Holocene, the dust proxy records highlighted two periods of enhanced input to Lake Peixão, the first (~3.5-2.7 ka BP) after the end of the last African Humid Period and the second, from the 19th century onwards, agreeing with the advent of commercial agriculture, and human contribution to land degradation and dust emission in the Sahara/Sahel region. The oceanic imprints throughout the Holocene matched well with North Atlantic rapid climatic changes that, in turn, coincided with ice-rafted debris or Bond events and other records of increased storminess for the European coasts. Positive parallel peaks in halogens were found in recent times, probably connected to fire extinction by halogenated alkanes and roadway de-icing.
... In the last decade several very-high-resolution studies have been performed in the North Atlantic and Europe showing that the YD is characterised by several phases and, therefore, much more complex than previously thought (e.g. Magny & Bégeot, 2004;Brauer et al., 2008;Bakke et al., 2009;Lane et al., 2013;Rach et al., 2014;Renssen et al., 2018;Naughton et al., 2019). In particular, they show a complex distribution in space and time of hydroclimate across Europe that cannot be explained by the AMOC slowdown alone but would necessitate also changes in atmospheric dynamics. ...
... Specifically, the combination of changes in the AMOC strength and North Atlantic sea ice extent produced latitudinal shifts of the polar jet stream and, therefore, changes in the position and strength of the Westerlies across western Europe that are responsible for seaÀland moisture transfer (e.g. Magny & Bégeot, 2004;Brauer et al., 2008;Bakke et al., 2009;Lane et al., 2013;Rach et al., 2014;Naughton et al., 2019;Rea et al., 2020). ...
... A gradual cooling in the mid-and high latitudes of the western North Atlantic and Greenland is revealed by a decrease in the regional sea surface temperatures (SST) and of δ 18 O of water in Greenland ice cores (Figs 7.1 and 7.2) (e.g. Alley et al., 1993;Rasmussen et al., 2006;Bakke et al., 2009;Thornalley et al., 2011;Muschitiello et al., 2015;Naughton et al., 2019). The AMOC started to slow down slightly, as supported by the 231 Pa/230Th record from the western North Atlantic mid-latitudes and the compiled North Atlantic dataset (Figs 7.1 and 7.2) (McManus et al., 2004;Ng et al., 2018), and the sea ice started to expand somewhat further south (Cabedo-Sanz et al., 2013;Müller & Stein, 2014). ...
The Younger Dryas (YD) stadial is the last extreme cold event detected in the northern hemisphere during the last deglaciation. Many theories have been proposed to explain the causes of this event. Currently, the most accepted one is that the increased meltwater discharges into the North Atlantic or Arctic Sea produced a series of changes in ocean circulation and sea ice cover that triggered substantial shifts in the atmospheric circulation patterns. The impact of these changes is marked by a series of climate shifts within the YD stadial with a complex spatial distribution of hydroclimate in Europe in each phase.
... The so-called Mystery Interval (Denton et al., 2005;2006;Williams et al., 2012) was initially identified in other Iberian ranges ~16.56-14.7 ka cal BP (Jalut et al., 2010;Domínguez-Villar et al., 2013;Palacios et al., 2016). This period has also been recorded in marine pollen sequences around the Iberian Peninsula as a cold and dry interval (Naughton et al., 2007(Naughton et al., , 2019Fletcher et al., 2010. The ND shows the lowest sedimentation rate (0.23 mm/yr) in the Oldest Dryas (Fig. 2). ...
... The Bølling sub-interstadial was cooler than the Allerød interstadial, as indicated by the weaker development of organic matter (LOI, Fig. 6). The latter suggests a warming trend from the Bølling to the Allerød, which is archived in both the marine and the terrestrial records in the southwestern Mediterranean region and southern Europe (Watts et al., 1996;van der Knaap and van Leeuwen, 1997;Renssen et al., 2010;Yu and Eicher, 2001;Naughton et al., 2007Naughton et al., , 2019Morellón et al., 2009;Carrión et al., 2010;González-Sampériz et al., 2006, 2010Jalut et al., 2010;Rodrigo-Gámiz et al., 2011;Moreno et al., 2014;de Beaulieu et al., 2017;Sadori, 2018). However, a second cold and dry period were detected later in the Bølling/Allerød interstadial at ~13.1-12.9 ...
... 6), has also been documented in the Iberian records. It was a cold and wet period in the northern Peninsula (González-Sampériz et al., 2006; Morellón et al., 2009;Iriarte-Chiapusso et al., 2016;Pontevedra-Pombal et al., 2019) and a dry period in south Iberia Carrión, 2002;Naughton et al., 2007Naughton et al., , 2019Carrión et al., 2010;Mesa-Fernández et al., 2018) suggesting a latitudinal-like boundary of palaeoclimatic and palaeohydrological conditions around 40 • N. ...
The Iberian Central System (ICS) is a clue region to reveal Mediterranean/Atlantic inferences over Iberia. We present a multidisciplinary study from western Spain conducted in the Navamuño depression (ND), covering the last 16.8 ka (cal BP). A reconstruction of the palaeotemperature from the resulting geochemical data highlights four cold and dry intervals, namely, the Oldest Dryas, Older Dryas, Intra-Allerød Cold Period (IACP), and the Younger Dryas, along with warmer intervals: the Bølling (14.7-14 ka) and the Allerød (12.9-12.6 ka); however, the Greenland Interstadial GI-1c (13.4-13.1 ka) is barely distinguishable in the ND. Despite the shortage of biomass to sustain fire, the earliest charcoals are from ~14.4-13.8 ka. Evidence of ash/dust events overprinting the geochemical background starts at ~13.8-12.8 ka. Significant fire activity in the Early Holocene at ~11.7-10.6 ka affected the ND, matching the westernmost ICS data. This period includes short oceanic spells inferred from Cl peaks at ~10.9-10.2 ka and three cold intervals at 11.4, 9.3, and 8.2 ka disrupted the progressive temperature increase. The Mid-Holocene showed a continuously increasing trend towards an arid climate, peaking at 4.2 ka under a pervasive dust influx from North Africa, which has prevailed since almost ~7.9 ka. A prominent volcanic event at ~6.8-5.8 ka is in Navamuño and Roñanzas (Asturias, N Spain; Gallego et al., 2013) identified from heavy metal-rich layer, synchronous with the last known eruption of the Calatrava volcanic field (South-Central Spain; Poblete-Piedrabuena et al., 2019). This volcanic eruption could affect many other regions half north of Iberia. The pervasive presence of oceanic aerosols in the last three millennia (2.8 ka ~) allowed the formation of a Cl-rich peat layer during the Ibero-Roman humid period ~2.1 ka, before a changing around ~0.4 ka toward colder and drier conditions at the Little Ice Age (LIA) period.
... The transition to the YD is marked by a subtle decrease of TMF, mainly MF ( Fig. 4c and e), reflecting a cooling episode with moderately drier conditions. However, the modest abundances of TMF (Table 1, Figs. 3 and 4c) suggest that some moisture was still delivered to the region during the YD/GS-1 interval (Naughton et al., 2019). A rapid increase in TMF, both TF and MF, marks the onset of the Holocene (11,700 cal kyr BP) (Fig. 4c, d and e), suggesting a response to climate warming and increase in moisture availability without lags, due to the proximity to glacial refugia for temperate and Mediterranean taxa (Figueiral and Terral, 2002;García-Amorena et al., 2007;López de Heredia et al., 2007). ...
... In particular, the maximum expansion of deciduous oak (TF, Figs. 3a and 4c) requires both high temperatures and winter precipitation (Fletcher et al., 2010;Naughton et al., 2019) while evergreen oak, Olea and Pistacia (MF, Fig. 3a) reflects not only high temperatures and winter precipitation but also, intense summer drought (Costa et al., 2012;Espírito-Santo et al., 2017). Similar forest composition is detected in both continental (LSA, Santos and Sánchez Goñi, 2003) records from the same region (Fig. 1). ...
... The Early Holocene spatial precipitation gradient is quite similar to that documented for IP under present-day conditions (e.g. Serrano et al., 1999;Gimeno et al., 2010;Trigo et al., 2013) or even in the past during episodes with very different boundary conditions (such as during Heinrich Stadials and the Younger Dryas: Kageyama et al., 2005;Naughton et al., 2019). The delayed response of TMF, associated with a deficit in precipitation in the south, especially in the southeast, could likely also result from other particular baseline climate conditions that affected the Early Holocene, such as the persistence of Northern Hemisphere ice sheets as previously proposed by Desprat et al. (2013). ...
... Undoubtedly, it is the most widely studied deglacial period. Although climate varied extraordinarily during this period (Naughton et al., 2019), its effects in the Northern Hemisphere are clearthe AMOC weakened (Meissner, 2007;Muschitiello et al., 2019), sea ice expanded, and winter and spring temperatures dropped drastically (Steffensen et al., 2008;Mangerud et al., 2016); summers remained relatively warm (Schenk et al., 2018). Glaciers in Europe advanced (Ivy-Ochs, 2015;Mangerud et al., 2016), and the Asian monsoon weakened (Wang et al., 2008). ...
... The causes of the abrupt YD anomaly continue to be a topic of debate. Changes in deep-water circulation in the Nordic seas, weakening of the AMOC (Muschitiello et al., 2019), moderate negative radiative forcing and altered atmospheric circulation (Renssen et al., 2015;Naughton et al., 2019) likely played a role. Draining of Glacial Lake Agassiz after intense melting of the Laurentide Ice Sheet during the B-A would have weakened the AMOC and is supported by geomorphic evidence of this lake draining into the Gulf of St. Lawrence and the North Atlantic at the end of the B-A (Leydet et al., 2018). ...
... Fogwill et al. (2017) argue that, once deglaciation starts, it is driven by global oceanic and atmospheric teleconnections. New data support the idea that meltwater cooling of the Northern Hemisphere reduced the AMOC strength (Deaney et al., 2017;Muschitiello et al., 2019) and pushed the northern westerlies southward in Asia (Chen et al., 2019), Europe (Naughton et al., 2019), and North America (Hudson et al., 2019). The Asian summer monsoon weakened during these cold periods in the Northern Hemisphere Chen et al., 2019), and the Indian summer monsoon transferred Southern Hemisphere heat northward, promoting subsequent Northern Hemisphere deglaciation (Nilsson-Kerr et al., 2019). ...
This paper reviews current understanding of deglaciation in North, Central and South America from the Last Glacial Maximum to the beginning of the Holocene. Together with paleoclimatic and paleoceanographic data, we compare and contrast the pace of deglaciation and the response of glaciers to major climate events. During the Global Last Glacial Maximum (GLGM, 26.5-19 ka), average temperatures decreased 4° to 8°C in the Americas, but precipitation varied strongly throughout this large region. Many glaciers in North and Central America achieved their maximum extent during the GLGM, whereas others advanced even farther during the subsequent Heinrich Stadial 1 (HS-1). Glaciers in the Andes also expanded during the GLGM, but that advance was not the largest, except on Tierra del Fuego. HS-1 (17.5-14.6 ka) was a time of general glacier thickening and advance throughout most of North and Central America, and in the tropical Andes; however, glaciers in the temperate and subpolar Andes thinned and retreated during this period. During the Bølling-Allerød interstadial (B-A, 14.6-12.9 ka), glaciers retreated throughout North and Central America and, in some cases, completely disappeared. Many glaciers advanced during the Antarctic Cold Reversal (ACR, 14.6-12.9 ka) in the tropical Andes and Patagonia. There were small advances of glaciers in North America, Central America and in northern South America (Venezuela) during the Younger Dryas (12.9-11.7 ka), but glaciers in central and southern South America retreated during this period, except on the Altiplano where advances were driven by an increase in precipitation. Taken together, we suggest that there was a climate compensation effect, or ‘seesaw’, between the hemispheres, which affected not only marine currents and atmospheric circulation, but also the behavior of glaciers. This seesaw is consistent with the opposing behavior of many glaciers in the Northern and Southern Hemispheres.
... The Last Glacial Maximum (LGM), a period of more moderate stadial conditions compared to HS2 preceding and HS1 proceeding it, is characterized by higher PP attributed to intensified trade winds and subsequent upwelling (Abrantes, 1991;Palumbo et al., 2013;Voelker et al., 2009). Few PP and Pexp reconstructions are available for the Younger Dryas (YD), although existing Iberian Margin records demonstrate contrasting estimations (Incarbona et al., 2010;Palumbo et al., 2013;Salgueiro et al., 2014), which are typically not sufficiently resolved to confidently address productivity variations during its several phases (Naughton et al., 2019). Despite the majority of paleoproductivity reconstructions for the last deglaciation focus on the stadials, high PP is observed for the interstadial Bølling/Allerød (B/A) (Incarbona et al., 2010;Pailler and Bard, 2002;Palumbo et al., 2013). ...
... Each of these abrupt climate changes has been associated with shifts in the strength of the Atlantic Meridional Overturning Circulation (AMOC) in the North Atlantic (McManus et al., 2004;Ng et al., 2018), including the Iberian margin (Gherardi et al., 2005). Proxy evidence and model simulations suggest the weakening and/ or shutdown of AMOC via freshwater input in the North Atlantic during HS2, HS1 and the YD led to Northern Hemispheric cooling, decreased precipitation and reorganization of the trade wind circulation (Jackson et al., 2015;Menviel et al., 2008;Naughton et al., 2016Naughton et al., , 2019. In the ocean, AMOC reduction has been linked to latitudinal migrations of North Atlantic oceanic gyres (Reibig et al., 2019;Repschl€ ager et al., 2015) and a large reduction in export production, the latter ascribed to shoaling of the mixed layers and the subsequent decreased in nutrient input (Menviel et al., 2008;Schmittner, 2005). ...
... The opposite is true for YDb, suggesting warmer conditions during the second phase. Recently, the YD has been described as a four-phase interval from alkenone-derived SST and pollen records at decadal resolution from nearby core D13882 (Naughton et al., 2019). Briefly, a cool and dry onset (12890-12720 yr BP) followed by an exacerbation of these conditions (12720-12390 yr BP) constitute the first two phases, which likely correspond to YDa in this work. ...
This study combines high-resolution records of nannofossil abundances, oxygen and carbon stable isotopes, core scanning X-ray fluorescence (XRF), and ice rafted debris (IRD) to assess the paleoceanographic changes that occurred during the last deglaciation on the SW Iberian Margin. Our results reveal parallel centennial-scale oscillations in coccolithophore productivity, nutricline depth and upwelling phenomena not previously observed, explained by means of arrival of iceberg-melting waters, iceberg-induced turbulent conditions, SST changes and riverine discharges. On millennial time-scales, higher primary productivity (PP), shallower nutricline, and upwelling occurrence/invigoration are observed for the Last Glacial Maximum (LGM) and Bølling-Allerød (B/A). The opposite scenario (i.e., lower productivity, deeper nutricline and upwelling weakening/absence) is linked to cold spells such as Heinrich Stadials 2 and 1 (HS2 and HS1) and the Younger Dryas (YD). Such paleoproductivity variations are attributed to latitudinal migrations of the thermal fronts associated with oceanic gyres in the North Atlantic, in parallel to oscillations in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Moderate-to-high PP during the Holocene is ascribed to the development of the modern seasonal surface hydrography, with a more persistent Iberian Poleward Current (IPC) and seasonal wind-induced upwelling.
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... The IP have experienced a wide spectrum of climatic and microclimate conditions, a result of the effects such as latitude (e.g., Moreno, et al., 2014a;Morellón et al., 2018;Naughton et al., 2019) and the orographic complexity and proximity to the ocean (continentality). These regional diversities and their interactions with the above-mentioned climate drivers could explain the spatiotemporal climate variability observed in the Iberia throughout the Holocene (e.g., Carrión et al., 2010;Tarroso et al., 2014;Sánchez-López et al. 2016;Abrantes, et al., 2017;Morellón et al., 2018;Baldini et al., 2019;Thatcher et al., 2020a;Naughton et al., 2019;Gomes et al., 2020). ...
... The IP have experienced a wide spectrum of climatic and microclimate conditions, a result of the effects such as latitude (e.g., Moreno, et al., 2014a;Morellón et al., 2018;Naughton et al., 2019) and the orographic complexity and proximity to the ocean (continentality). These regional diversities and their interactions with the above-mentioned climate drivers could explain the spatiotemporal climate variability observed in the Iberia throughout the Holocene (e.g., Carrión et al., 2010;Tarroso et al., 2014;Sánchez-López et al. 2016;Abrantes, et al., 2017;Morellón et al., 2018;Baldini et al., 2019;Thatcher et al., 2020a;Naughton et al., 2019;Gomes et al., 2020). Consequently, although challenging due to the diversity of competing for climatic factors, the paleoclimate studies of this region can provide key insights into how the North Atlantic zone, particularly the Eastern North Atlantic, Southwestern European, and Western Mediterranean areas, respond to global climate change. ...
... More recently, evidence for a time-transgressive climate shift midway through the YD has been reported from lacustrine sediments (e.g. Bakke et al., 2009;Brauer et al., 2008;Schlolaut et al., 2017), speleothems (Baldini et al., 2015;Bartolomé et al., 2015;Rossi et al., 2018) and marine sediments in Europe (Naughton et al., 2019). This climate shift has been attributed to a gradual northward movement of the polar front driven by the resumption of the AMOC and concomitant sea-ice retreat in the North Atlantic. ...
... It took about 300 years for this shift to propagate to central and finally to northern Europe, where it is documented as a rapid change in Ti content and varve thickness (Bakke et al., 2009;Lane et al., 2013). While many records from SW (Baldini et al., 2015;Bartolomé et al., 2015;Naughton et al., 2019;Rossi et al., 2018) and N Europe indicate that the first half of the YD was colder and drier than the second one, biomarker data from lacustrine sediments of the Gemündener Maar in W Germany suggest an opposite trend (Hepp et al., 2019). ...
In the European Alps, the Younger Dryas (YD) was
characterised by the last major glacier advance, with equilibrium line
altitudes being ∼ 220 to 290 m lower than during the Little
Ice Age, and also by the development of rock glaciers. Dating of these
geomorphic features, however, is associated with substantial uncertainties,
leading to considerable ambiguities regarding the internal structure of this
stadial, which is the most intensively studied one of the last glacial period. Here,
we provide robust physical evidence based on 230Th-dated cryogenic cave
carbonates (CCCs) from a cave located at 2274 m a.s.l. in the Dolomites of
northern Italy coupled with thermal modelling, indicating that early YD
winters were only moderately cold in this part of the Alps. More precisely,
we find that the mean annual air temperature dropped ≤ 3 ∘C
at the Allerød–YD transition. Our data suggest that autumns and early
winters in the early part of the YD were relatively snow-rich, resulting in
stable winter snow cover. The latter insulated the shallow subsurface in
winter and allowed the cave interior to remain close to the freezing point
(0 ∘C) year-round, promoting CCC formation. The main phase of CCC
precipitation at ∼ 12.2 ka coincided with the mid-YD
transition recorded in other archives across Europe. Based on thermal
modelling we propose that CCC formation at ∼ 12.2 ka was
most likely associated with a slight warming of approximately
+1 ∘C in conjunction with drier autumns and early winters in the
second half of the YD. These changes triggered CCC formation in this Alpine
cave as well as ice glacier retreat and rock glacier expansion across the
Alps.
... Collectively, the regional comparisons discussed above support that during the abrupt climatic events of the LGIT, winter sea-ice expansion in the North Atlantic initiated a southerly migration of the NAPF (e.g. Denton et al., 2005;Renssen et al., 2018;Muschitiello et al., 2019;Naughton et al., 2019), the focussing of stronger and more zonal westerly winds devoid of moisture across Northern Europe (Brauer et al., 2008), and the depression of moisture bearing subpolar cyclonic activity into Central and Southern Europe (e.g. Marchal et al., 2016;Pauly et al., 2018). ...
... Changes in hydroclimatic signals through GS-1 (including the Younger Dryas in Europe) are more complex than those in GI-1d and GI-1b, and have also been attributed to the position of the NAPF across the European continent (e.g. Brauer et al., 2008;Bakke et al., 2009;Lane et al., 2013;Rach et al., 2014;Naughton et al., 2019). The findings presented here broadly supports these interpretations (i.e. ...
European paleoenvironmental records through the Last Glacial-Interglacial Transition (LGIT; ca 16-8 cal ka BP) record a series of climatic events occurring over decadal to multi-centennial timescales. Changes in components of the climatic system other than temperature (e.g. hydrology) through the LGIT are relatively poorly understood however, and further records of hydroclimatic changes are required in order to develop a more complete understanding on the phasing of environmental and anthropogenic responses in Europe to abrupt climate change. Here, we present a multiproxy palaeoenvironmental record (macroscale and microscale sedimentology, macrofossils, and carbonate stable isotopes) from a palaeolake sequence in the Vale of Pickering (VoP), NE England, which enables the reconstruction of hydro-climatic changes constrained by a radiocarbon-based chronology. Relative lake-level changes in the VoP occurred in close association (although not necessarily in phase) to threshold shifts across abrupt climate change transitions, most notably lowering during cooling intervals of the LGIT (~GI-1d,~GI-1b, and~GS-1). This reflects more arid hydroclimates associated with these cooling episodes in the British Isles. Comparisons to hydrological records elsewhere in Europe show a latitudinal bifurcation, with Northern Europe (50e60 N) becoming more arid (humid), and Southern Europe (40e50 N) becoming more humid (arid) in response to these cooling (warming) intervals. We attribute these bifurcating signals to the relative positions of the Atlantic storm tracks, sea-ice margin, and North Atlantic Polar Front (NAPF) during the climatic events of the LGIT.
... This climatic improvement, that allowed rapid forest recolonization by a general development of high-mountain pine forests, has been documented in all European mountain areas (Watts et al., 1996;Ammann et al., 2007;Beaulieu et al., 2017;Giesecke et al., 2017). Therefore, it suggests a warming trend from the Bølling to the Allerød period that has also been evidenced in both marine and terrestrial records in the southern Mediterranean region (Watts et al., 1996;Naughton et al., 2007Naughton et al., , 2019Lowe et al., 2008;Gonz alez-Samp eriz et al., 2010;Beaulieu et al., 2017;Sadori, 2018). In summary, in the Navamuño depression, it can be assumed that pioneer open pine woodlands were developing in the vicinity of the palaeolake during the Allerød oscillation. ...
... Nevertheless, sequences from other Iberian Mediterranean continental and southern Italian mountain areas generally show little changes in vegetation composition and the persistence of conifers, deciduous oaks and open landscapes (Watts et al., 1996;Aranbarri et al., 2014;Beaulieu et al., 2017). The Younger Dryas is clearly recorded in Greenland ice cores (Fig. 6) by low and oscillating d 18 O values (Alley, 2000;Rasmussen et al., 2008), as well as in many other palaeoenvironmental studies across the western Mediterranean (Morell on et al., 2009;Fletcher et al., 2010;Tomasso et al., 2018;Naughton et al., 2019). ...
A new record from a long sediment core (S3) in Navamu~ no (1505 m asl, western Iberian Central System) provides the reconstruction of the vegetation history and environmental changes in the region between 15.6 and 10.6 ka cal BP, namely during the Late Glacial and the early Holocene, using a multiproxy analysis (pollen-based vegetation and climate reconstruction, sedimentary macrocharcoals, loss-on-ignition, magnetic susceptibility and X-ray fluorescence (XRF) measurements). The results are then compared with other sequences from the Iberian Central System and the whole Iberian Peninsula in order to better understand the past dynamics of the main forest constituents. The pollen record shows a shift from open pine forests ~15.6e14.7 ka cal BP (Oldest Dryas) to mixed open pine-birch woodlands ~14.7e14.0 ka cal BP (Bølling). Woodlands were succeeded by a steppe-like landscape until ~13.4 ka cal BP (Older Dryas), which was replaced again by high-mountain pine forests and riparian woodlands ~13.4 e12.6 ka cal BP (Allerød). A great development of cold steppe grasslands linked to the decline of birch woodlands is documented ~12.6e11.7 ka cal BP (Younger Dryas). The early Holocene (11.7e10.6 ka cal BP) was characterized by a progressive reforestation of the study area by pine and birch forests in the highlands and oak woods in the lowlands. Temperate tree taxa (Carpinus betulus, Castanea sativa, Corylus avellana, Fraxinus, Juglans, Tilia, and Ulmus) were also common but likely at lower elevations. Pollen of Fagus sylvatica was already recorded during the Late Glacial and the early Holocene. The marked increasing local fire activity during the warmer and wetter Allerød oscillation could be related to a rise in tree cover, supporting the climatically driven character of these fires. Nevertheless, the strong increase in fire activity during the Younger Dryas would probably be related to growing tree and shrub mortality, as well as to the wet/dry biphasic structure of this stadial. The standard "Modern Analogue Technique" has been also applied to the Navamu~ no sequence to provide quantitative climate estimations for the Late Glacial and the early Holocene periods. This record is one of the few continental archives that show the climatic trend between the Late Glacial and the early Holocene in central Iberia, agreeing with many other regional records from the Western Mediterranean.
... Therefore, the IPCC AR6 has changed its assessment of AMOC's abrupt change before 2100 to medium confidence (Fox-Kemper et al., 2021) and categorized AMOC collapse as a low-probability, high-impact event, known as a black swan event. Such a collapse can cause sudden changes in regional weather patterns and water cycles (Chiang and Bitz, 2005;Chiang and Friedman, 2012;Jackson et al., 2015;Renssen et al., 2018;Sandeep et al., 2020), for example, shifting the tropical ITCZ southward (Peterson et al., 2000;McGee et al., 2014;Schneider et al., 2014;Mohtadi et al., 2016;Reimi and Marcantonio, 2016), weakening the Asian-African monsoon (Tierney and deMenocal, 2013;Otto-Bliesner et al., 2014;Cheng et al., 2016;Grandey et al., 2016;Wurtzel et al., 2018), strengthening the monsoon in the SH (Cruz Jr et al., 2005;Ayliffe et al., 2013;Stríkis et al., 2015;Stríkis et al., 2018), cooling in North America , increasing precipitation in the mid-latitudes of North America (Polyak et al., 2004;Grimm et al., 2006;Wagner et al., 2010;Voelker et al., 2015), and drying in Europe (Genty et al., 2006;Haarsma et al., 2015;Jackson et al., 2015;Rach et al., 2017;Naughton et al., 2019). ...
Based on modern observations, historical proxy data, and climate model simulations, this paper provides a comprehensive overview of the past, present and future evolution characteristics of the Atlantic Meridional Overturning Circulation (AMOC), as well as its impact on the surface air temperature (SAT) at regional and hemispherical scales. The reconstruction results based on the proxy data indicate that the AMOC has weakened since the late 19th century and experienced overall weakening throughout the 20th century with low confidence. Direct observations show that the AMOC weakened during 2004–2016, but it is not possible to distinguish between its decadal variability and long-term trend. Climate models predict that if greenhouse gas emissions continue to increase, AMOC will weaken in the future, but there will not be a sudden collapse before 2100. For the thermodynamic effects of AMOC, the increased surface heat flux release and meridional heat transport (MHT) over the North Atlantic associated with the strong AMOC cause an increase in the hemispherical SAT. At the millennial scale, climate cooling (warming) periods correspond to a weakened (strengthened) AMOC. The enhanced MHT of a strong AMOC can affect Arctic warming and thus influence regional SAT anomalies and SAT extremes through mutual feedback between Arctic sea ice and AMOC. In terms of dynamic effects, a strong AMOC modulates the Rossby wave trains originating from the North Atlantic and spreading across mid-to-high latitudes in the Northern Hemisphere and causes an increase in the variabilities in the circulation anomalies over the Ural and Siberian regions. Ultimately, a strong AMOC significantly affects the frequencies of extreme cold and warm events in the mid-to-high latitude regions over Eurasia. In addition, AMOC can also influence regional and global SAT anomalies through its dynamic adjustment of planetary-scale circulation. Decadal variation in AMOC is closely related to the Atlantic Multidecadal Oscillation (AMO). During positive phases of AMO and AMOC, enhanced surface heat fluxes over the North Atlantic lead to abnormal warming in the Northern Hemisphere, while during negative phases, the reverse case occurs. Under high emission scenarios in the future, the possibility of AMOC collapse increases due to freshwater forcing. However, most advanced climate models underestimate the strength of the AMOC and its impact on the AMO and relevant climate change, which presents a major challenge for future understanding and prediction of the AMOC and its climate effects.
... Therefore, the IPCC AR6 has changed its assessment of AMOC's abrupt change before 2100 to medium confidence (Fox-Kemper et al., 2021) and categorized AMOC collapse as a low-probability, high-impact event, known as a black swan event. Such a collapse can cause sudden changes in regional weather patterns and water cycles (Chiang and Bitz, 2005;Chiang and Friedman, 2012;Jackson et al., 2015;Renssen et al., 2018;Sandeep et al., 2020), for example, shifting the tropical ITCZ southward (Peterson et al., 2000;McGee et al., 2014;Schneider et al., 2014;Mohtadi et al., 2016;Reimi and Marcantonio, 2016), weakening the Asian-African monsoon (Tierney and deMenocal, 2013;Otto-Bliesner et al., 2014;Cheng et al., 2016;Grandey et al., 2016;Wurtzel et al., 2018), strengthening the monsoon in the SH (Cruz et al., 2005;Ayliffe et al., 2013;Stríkis et al., 2015;Stríkis et al., 2018), cooling in North America , increasing precipitation in the mid-latitudes of North America (Polyak et al., 2004;Grimm et al., 2006;Wagner et al., 2010;Voelker et al., 2015), and drying in Europe (Genty et al., 2006;Haarsma et al., 2015;Jackson et al., 2015;Rach et al., 2017;Naughton et al., 2019). ...
... On the contrary, the MEN d 18 O composite shows three different phases, in good correspondence with a simplified version of the three stages recorded by marine sediments (Naughton et al., 2019): (i) a cool/dry climate at the YD onset, (ii) a slightly warmer (and wetter) phase at about 12.3 kyr BP, and (iii) a cool and dry (but quite unstable phase) just before the onset of the Holocene. The age model (Fig. 3B) precludes establishing the precise timing of these stages. ...
Recent hydroclimate studies on the Iberian Peninsula have shown a complex regional pattern in timing and intensity of climate change spanning the Younger Dryas and the Holocene. These changes are due to multifaceted interactions between climate variability that characterizes the Atlantic Ocean region and hydroclimatic processes associated with the Mediterranean climate, thus making it difficult to reconstruct centennial-and millennial-scale variability in rainfall. In this study we present a composite and continuous isotopic record (d 13 C and d 18 O) consisting of four stalagmites from Mendukilo cave (MEN composite) in the western Pyrenees covering the Younger Dryas and the entire Holocene. This record reveals millennial-scale shifts in carbon isotopes in response to changes in the hydroclimate in the northern part of Iberia. The MEN oxygen isotopes show little variation on millenial time scales but reveal centennial changes that correlate with North Atlantic events (e.g., the 8.2 kyr BP cooling event). We observe a delay in the onset of humid conditions in the early Holocene and a subsequent trend towards drier and colder conditions between 6.0 and 2.5 kyr BP. This new, high-resolution and replicated spe-leothem record denotes the complex connection that exists between the North Atlantic and Western Europe during last millennia and the strong regional heterogeneity of the hydroclimate of Iberia during this time.
... to assess interhemispheric phasing of climate change (e.g., bipolar seesaw), which has been independently verified by methane synchronization of ice cores for the last glacial period (Blunier and Brook, 2001;WAIS Divide Project Members, 2015). Moreover, the narrow continental shelf off Portugal results in the rapid delivery of terrestrial material to the deep-sea environment, thereby permitting correlation of marine, ice core, and European terrestrial records (Margari et al., 2010(Margari et al., , 2014Naughton et al., 2019;Oliveira et al., 2016Oliveira et al., , 2017Oliveira et al., , 2018Oliveira et al., , 2020Sánchez Goñi et al., 1999;Shackleton et al., 2003;Tzedakis et al., 2009Tzedakis et al., , 2004. ...
... to assess interhemispheric phasing of climate change (e.g., bipolar seesaw), which has been independently verified by methane synchronization of ice cores for the last glacial period (Blunier and Brook, 2001;WAIS Divide Project Members, 2015). Moreover, the narrow continental shelf off Portugal results in the rapid delivery of terrestrial material to the deep-sea environment, thereby permitting correlation of marine, ice core, and European terrestrial records (Margari et al., 2010(Margari et al., , 2014Naughton et al., 2019;Oliveira et al., 2016Oliveira et al., , 2017Oliveira et al., , 2018Oliveira et al., , 2020Sánchez Goñi et al., 1999;Shackleton et al., 2003;Tzedakis et al., 2009Tzedakis et al., , 2004. ...
From 11 October to 11 December 2022, International Ocean Discovery Program Expedition 397 took place off the coast of Portugal southwest of Lisbon. The main objective was to recover the
exceptional sedimentary archive preserved beneath the seafloor on the Iberian margin to study past climate change at high temporal resolution. During the expedition, which carried 26 international
scientists, four sites were drilled, recovering 6.2 km of marine sediments that accumulated rapidly, thereby providing a high-fidelity record of past climate change on timescales of hundreds to thousands of years and extending back millions of years ago. Climate signals from these marine sediment cores will be correlated precisely to polar ice cores from both hemispheres and with European pollen records, providing a rare opportunity to link oceanic, atmospheric, and terrestrial climate and environmental changes. The four drill sites are located at different water depths (1339, 2590, 3479 and 4691 m below sea level), permitting scientists to study how deep-ocean
circulation and chemistry changed in the past, including its role in deep-sea carbon storage and atmospheric CO2 changes. The sediment cores recovered during Expedition 397 will provide
benchmark records of North Atlantic climate change at high temporal resolution from the late Miocene (about 8 million years ago) to present. This period includes the last 3 million years when changes in the Earth's orbit resulted in the growth and decay of large ice sheets in the Northern Hemisphere and a warmer world before this time when atmospheric CO2 was similar to today. All cores recovered show strong changes in physical properties (such as color) that represent a response to known cyclic changes in Earth's orbit, which will aid in accurately dating the sediment. Many years of research will be needed to extract the detailed climatic signals from the kilometers of core recovered during Expedition 397, but the records to be produced will be vital for testing numerical climate models and understanding how the climate system works and how it might change in the future.
... ka BP (ca. 0.8 ka) coincides with the coldest part of the YD cold event (U4; Naughton et al., 2019Naughton et al., , 2022. These cold conditions led to a longer duration of ice and snow cover in the lake and its catchment, leading to a decrease in high-energy runoff episodes. ...
Understanding the environmental response to the last glacial termination in regions located in transitional climate zones such as the Atlantic Iberian mountains is crucial to estimate potential changes in regions affected by current glacial melting. We present an 8.5‐m‐long, solid last deglaciation and Holocene chronostratigraphic record including detailed sediment analysis from Lake Peixão, a pro‐glacial lake in the Serra da Estrela (Central Portugal). The age‐depth model relies on a Bayesian approach that includes 16 AMS 14C dates and 210Pb‐137CS measurements, robustly dating the lake formation at 14.7±0.32 cal. ka BP. This chronological reconstruction shows an average sedimentation rate of ca. 0.07 cm·yr‐1 (15 yrs·cm‐1), enabling proxy analyses at decadal timescales. The sediment sequence is composed of 5 lithological units: U1) coarse and unsorted fluvioglacial lacustrine deposits; U2) massive fluvioglacial lacustrine deposits (863‐790 cm below surface (bsf); 14.7±0.32‐13.8±0.12 cal. ka BP); U3) water current fluvioglacial lacustrine deposits (790‐766 cm bsf; 13.8±0.12‐12.9±0.29 cal. ka BP); U4) laminated/banded lacustrine deposits characterised by terrigenous deposits from ice‐covered lake periods and episodic events of ice and snow melting (766‐752 cm bsf; 12.9±0.29‐11.7±0.15 cal. ka BP); and U5) massive muddy lacustrine deposits (752‐0 cm bsf; 11.7±0.15 cal. ka BP‐present). The occurrence of U2 to U4 deposits defines the transition from glacial cold (U1) to net warm postglacial conditions (U5). These climate transitions are marked by changes in sediments and the presence of very low sedimentation rate periods possibly related to the Intra‐Allerød Cold Period and the coldest phase of the Younger Dryas. Our results support the previously proposed timing of the retreat of the Serra da Estrela glaciers ca. 13.8±0.12 cal. ka BP. The robust chronology of Lake Peixão highlights the potential of Iberian pro‐glacial lakes for dating deglaciation processes and will lead to unprecedented decadal‐to‐centennial timescale palaeoclimate reconstructions in this region since the last glacial‐interglacial transition.
... These oceanic and atmospheric reorganisations contributed to the complex spatial and temporal distribution of the European hydroclimate reflected by both marine and terrestrial climate indicators (Fig. 4.1) (e.g. Brauer et al., 2008;Bakke et al., 2009;Naughton et al., 2019;Rach et al., 2014;). In contrast, the B-A warming episode of the Last Deglaciation, 14.7 and 12.9 ka, was marked by warm North Atlantic SST and the gradual expansion of temperate trees, reflecting a steady increase of moisture from the Bølling to the Allerød in Western Europe (Fig. 4.1) (Litt & Stebich, 1999;Bard et al., 2000;Zaragosi et al., 2001;Hoek, 2009;Fletcher et al., 2010;Naughton et al., 2016). ...
This chapter briefly describes the long-term climate evolution, as well as, the superimposed abrupt climate shifts that have punctuated the Last Deglaciation, that is, the pre-Heinrich Stadial 1 and the Heinrich Stadial 1, the Bølling–Allerød Interstadial and the Younger Dryas Stadial. A short description of the impact of these abrupt changes in the North Atlantic, Greenland and Europe is also provided as a prelude to the following chapters.
... These oceanic and atmospheric reorganisations contributed to the complex spatial and temporal distribution of the European hydroclimate reflected by both marine and terrestrial climate indicators (Fig. 4.1) (e.g. Brauer et al., 2008;Bakke et al., 2009;Naughton et al., 2019;Rach et al., 2014;). ...
The geography of the Earth at the end of the Tertiary, with the new arrangement of continents, oceans and the distribution of mountain ranges, especially since the opening of the Drake Strait and the closing of the Isthmus of Panama, favoured a global cooling trend that culminated in the Quaternary. In addition to the ice sheets of Antarctica and Greenland, glaciers during the Quaternary tended to expand, especially on the Northern Hemisphere continents and in the mountains. The expansion ended abruptly for short periods, of about 10 ka, during which these glaciers largely disappeared. These periods are called terminations and mark the end of different glacial cycles. In the first half of the Quaternary, terminations occurred every 41 ka, but in the last 800 ka, terminations have been delayed, whilst glaciers could extend over larger areas, occurring every 100 ka. The onset of the terminations and their dynamics remains a mystery, but it coincides with a series of processes, where it is difficult to know what the cause is and what is the effect. It appears that when the glaciers in the Northern Hemisphere reach their maximum extent, an increase in insolation in the mid-latitudes of the Northern Hemisphere causes the onset of global termination. Once termination has begun, a series of temperature changes take place, intense in the Northern Hemisphere and milder in the Southern Hemisphere, but with inverse trends, and in direct relation to changes in the intensity of the Atlantic Meridional Overturning Circulation (AMOC) and to latitudinal changes in atmospheric circulation. Despite these changes in temperature, CO2 in the atmosphere increases throughout the termination, albeit with varying intensity. Once the balance between the AMOC and the proportion of CO2 in the atmosphere is in equilibrium, the temperature stabilises and the termination ends, leading to the onset of an interglacial optimum. This occurs when the northern continental ice sheets have disappeared or have reduced their extension.
... Among a series of millennial-scale climate events during the last glacialedeglaciation periods (Rasmussen et al., 2014), the Younger Dryas (YD) (nominally~12,900 to 11,600 y BP (before present, where present ¼ 1950 CE) was the most recent one and has received widespread attention in the scientific community (Alley, 2000). During this period, a number of climate records have shown that the climate was highly unstable (e.g., Bakke et al., 2009;Ma et al., 2012;Liu et al., 2013;Rach et al., 2014;Naughton et al., 2019;Yao et al., 2020;Cheng et al., 2020;Zhang et al., 2021b). Particularly for the warm peaks in the North Atlantic domain, the coherency between NGRIP (North Greenland Ice Core Project) ice core and SE09-6 speleothem (Seso Cave, North Spain) d 18 O records is evident, both suggesting four centennial-scale warm events occurred within the YD (Rasmussen et al., 2014;Bartolom e et al., 2015;Cheng et al., 2020). ...
High-resolution and precisely dated hydroclimate records in the southeastern Tibetan Plateau (TP) remain sparse beyond the Holocene, which hampers our ability to understand the hydroclimate variability in this important Indian summer monsoon (ISM) fringe area and its global teleconnection. Here we present 3-y resolution δ¹⁸O and δ¹³C records from a laminated stalagmite (RG-3) from Rige Cave in the southeastern TP, spanning the Younger Dryas (YD). The records allow us to precisely characterize the timing, structure, and particularly centennial-scale events within the YD, and probe the control factors of precipitation δ¹⁸O (δ¹⁸Op) in the ISM fringe area. On centennial–millennial timescales, the Rige δ¹⁸O record shows coherent pattern with the East Asian summer monsoon (EASM) counterpart, combined with modeled δ¹⁸Op results and spatial analysis, suggesting that δ¹⁸Op in this part of the TP is largely controlled by the large-scale atmosphere circulation (or the ISM strength), and the altitude increase in the TP may not potentially result in an opposite δ¹⁸Op pattern at least in the monsoonal TP regime. We also found significant δ¹³C-δ¹⁸O covariation on the centennial timescale, suggesting a coincided ISM rainfall and biomass change during the YD. In the Rige records, one weak centennial-scale ISM event (namely the intra-Allerød cold period, IACP) and three strong centennial-scale ISM events within the YD (namely A1′-A2′-A3′) were prominent and occurred between ∼12,470 and 12,310 ± 14, ∼12,210–12,090 ± 12 and ∼12,010–11,920 ± 12 y BP (before present, where present = 1950 CE), respectively. Spectral analyses of Rige records also revealed a significant ∼200-y periodicity, which is nearly in-phase with observed centennial-scale variations of the North Atlantic temperature and mid-latitude westerly-jet during that time. These observations support the hypothesis that the solar de Vries cycle (207-y) triggered the centennial-scale climate variations in high northern latitude, leading to the ISM variations via fast atmospheric processes.
... BP (Zazo et al., 2005), as well as the macro-remains from the same stratigraphical unit contain thermophilous elements such as Alnus, Betula and Quercus (Morales- Molino et al., 2011). Finally, marine cores such as SU81-13 (Parra, 1994), MD95-2043 (Fletcher andSánchez-Goñi, 2008;Fletcher et al., 2010), MD95-2042 (Pailler andBard, 2002;Chabaud et al., 2014), ODP site 976 (Combourieu Nebout et al., 2009;Dormoy et al., 2009), SU81-18 (Bard et al., 2000;Turon et al., 2003), U1385 (Oliveira et al., 2018), and D13882 (Naughton et al., 2019) depict the changes in the coastal and mountainous vegetation near the coasts of the Iberian Peninsula and Morocco throughout the last 20,000 years. They show the persistence of pine woodlands mixed with holm and cork oak. ...
The Doñana area in southern Iberia is one of the most renowned protected areas of Europe, mostly due to the diversity and value of its wetland ecosystems. The large biogeographical significance of this territory and the outstanding availability of sedimentary archives have made this region a hotspot of paleobotanical research in the Iberian Peninsula. Specifically, the organic deposits on El Asperillo Cliff have been studied during the past few decades from the geomorphological and paleobotanical (pollen, macrofossils) points of view. However, large uncertainties remain concerning the chronology of certain sections of the exposed profile and the paleobotanical potential of this site has not been fully exploited yet. In this study, we revisited El Asperillo with the aims of completing the paleobotanical record and refining the chronology of this site. The age of the studied deposits ranges from ca. 22,000 to 30,900 cal. yr BP according to the radiocarbon dates obtained, thus embracing the particularly cold and dry Heinrich Event 2 and the Last Glacial Maximum. Our palynological results allow inferring the presence of a coastal marshland system. Additionally, the new pollen records highlight the relevance and diversity of pines (Pinus nigra-sylvestris type, P. pinaster, P. halepensis-pinea type) in the Late Pleistocene landscape of Doñana, reinforcing the native status of pines. Last but not least, the results stress the persistence of a highly diverse woody flora in Doñana during the harshest periods of the last glacial cycle, highlighting the importance of this enclave in postglacial vegetation recolonization of the Iberian Peninsula.
... Moreover, the wetter conditions during GS-1 compared to GS-2.1a are independently confirmed by the speleothem geochemistry of Seso cave (Central Pyrenees, Spain;Bartolom e et al., 2015) and pollen-based climate reconstructions from the Albor an Sea Rodrigo-G amiz et al., 2011). Finally, multi-proxy analysis of lacustrine (Fletcher et al., 2010;Rius et al., 2014;Moreno et al., 2012Moreno et al., , 2014Gonz alez-Samp eriz et al., 2017;Oliva-Urcia et al., 2017) and marine Dormoy et al., 2009;Melki et al., 2009;Naughton et al., 2019) sequences from within the wider region of SW Europe indicate similar chronologies, all of these in step with global records (Lisiecki and Raymo, 2005;Rasmussen et al., 2014;Lambeck et al., 2014). ...
The chronology of the most extensive Late Pleistocene glaciation in the eastern Pyrenees is now well established, but climatic variations during the subsequent Last Glacial-to-Interglacial Transition (LGIT) are much less well documented. The aim of this research is to fill that gap. We present a new series of 22 10 Be exposure ages from a sequence of moraines in the upper Ari ege catchment, and update 64 previously published exposure ages from seven other valleys. Based on (i) geomorphological maps of successive generations of ice-marginal deposits and (ii) their age distributions, followed by (iii) glaciological modeling to infer corresponding mass-balance gradients and equilibrium line altitudes (ELAs), we reconstruct glacier behaviour in the eastern Pyrenees during the LGIT and retrieve paleoclimatic parameters for each glacial stillstand. Results document rapid deglaciation involving four relative still-stands, with an ELA rise of~410 m and increases in temperature and precipitation of~4 C and~40%, respectively, between~18 ka (Greenland Stadial GS-2.1b) and~14.5 ka (Greenland Interstadial GI-1). A cold and dry environment prevailed from~18 ka onward, and deglaciation was almost complete by GI-1. Only small glaciers persisted as late as~12.3 ka in cirques where local conditions allowed it. These patterns are compatible with a transient Polar Front Jet Stream repositioning to latitudes of 40e42 N between 18 and 16 ka, with Westerlies-dominated circulation not yet feeding precipitation to the mountain belt abundantly from the northwest, and outbursts of polar air meeting with cool sea waters in the western Mediterranean mostly suppressing precipitation from Mediterranean sources.
... Temporal match of various 14 C plateaus with deglacial periods of major atmospheric CO 2 rise and ocean warmings (AA stands for Antarctic, and GIS stands for Greenland Interstadial).Deglacial events of pCO 2 rise vs. age of planktic 14 C plateaus (in cal ka)Age control based on aNaughton et al. (2019). Age control based on b ...
Changes in the geometry of ocean meridional overturning circulation (MOC)
are crucial in controlling past changes of climate and the carbon inventory
of the atmosphere. However, the accurate timing and global correlation of
short-term glacial-to-deglacial changes of MOC in different ocean basins
still present a major challenge. The fine structure of jumps and plateaus in
atmospheric and planktic radiocarbon (14C) concentration reflects
changes in atmospheric 14C production, ocean–atmosphere 14C
exchange, and ocean mixing. Plateau boundaries in the atmospheric 14C
record of Lake Suigetsu, now tied to Hulu Cave U∕Th model ages instead of optical
varve counts, provide a stratigraphic “rung ladder” of up to 30 age tie
points from 29 to 10 cal ka for accurate dating of planktic oceanic 14C records. The age differences between contemporary planktic and atmospheric
14C plateaus record the global distribution of 14C reservoir ages
for surface waters of the Last Glacial Maximum (LGM) and deglacial Heinrich
Stadial 1 (HS-1), as documented in 19 and 20 planktic 14C records, respectively. Elevated
and variable reservoir ages mark both upwelling regions and high-latitude
sites covered by sea ice and/or meltwater. 14C ventilation ages of LGM
deep waters reveal opposed geometries of Atlantic and Pacific MOC. Like
today, Atlantic deep-water formation went along with an estuarine inflow of
old abyssal waters from the Southern Ocean up to the northern North Pacific
and an outflow of upper deep waters. During early HS-1, 14C ventilation
ages suggest a reversed MOC and ∼1500-year flushing of
the deep North Pacific up to the South China Sea, when estuarine circulation
geometry marked the North Atlantic, gradually starting near 19 ka. High
14C ventilation ages of LGM deep waters reflect a major drawdown of
carbon from the atmosphere. The subsequent major deglacial age drop reflects
changes in MOC accompanied by massive carbon releases to the atmosphere as
recorded in Antarctic ice cores. These new features of MOC and the carbon
cycle provide detailed evidence in space and time to test and refine ocean
models that, in part because of insufficient spatial model resolution and
reference data, still poorly reproduce our data sets.
Mediterranean climates are characterized by warm, dry summers and mild, rainy winters. Previous studies suggest that over the last 1.36 Myr, Mediterranean winter rainfalls were in phase with the African monsoon. Here we present a high-resolution terrestrial and marine dataset for the Marine Isotope Stage 17 interglacial (Middle Pleistocene) from Southern Italy, showing that precipitation rates and regimes in the central Mediterranean varied independently of the monsoon system. Specifically, events of extreme summer precipitation were promoted by increased regional insolation rates and/or extratropical cyclones, and their magnitude was further enhanced by the advection of cool and humid North Atlantic air during stadials. Our findings provide new information on the short- to mid-term natural hydroclimatic variability of the Mediterranean basin, and offer new critical insights on land–ocean interactions at the regional scale by complementing previous analyses on the displacement of storm tracks toward southern Europe.
The Younger Dryas (GS-1: ~12.9–11.7 ka) constituted a climate reversal, abruptly interrupting the rapid deglaciation prevailing in Termination I, and the previous consistent trend to warming that characterised the Bølling/Allerød interstadial. There is a general agreement on the causes of the abrupt cooling of the Younger Dryas: the arrival of large volumes of cold meltwater from Lake Agassiz and the Fennoscandian ice sheet to the North Atlantic, resulting in a weakening of the Atlantic Overturning Meridional Circulation (AMOC), although climate models suggest that other factors also need to be considered. As a consequence, the transfer of heat from subtropical regions of the Atlantic Ocean to the coastal areas of Europe was interrupted, causing an estimated temperature drop of 8ºC–9ºC in the vicinity of the Arctic Circle, attenuated towards southern latitudes. Both the Fennoscandian ice sheet and the glaciers of the European mountains underwent a halt in their general retreat, followed by a limited expansion leading to the formation of voluminous moraines.
The Bølling–Allerød (B-A), the terrestrial counterpart of Dansgaard–Oeschger 1, is the first abrupt northern hemisphere climate warming episode of the last deglaciation. Several hypotheses have been proposed to explain this event: all involve the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) prior or during the Heinrich Stadial 1/B-A transition and the consequent warming of the northern hemisphere. Terrestrial proxies (mainly pollen and speleothems) reflect a progressive increase in moisture availability from the Bølling to the Allerød in Europe contrasting with Greenland isotope records which show maxima in temperature and precipitation at the onset of the Bølling and a gradual decrease to the end of the Allerød. Changes in moisture availability in Europe are triggered by coupled interactions between the AMOC and changes in atmospheric dynamics led by the westerlies.
In the European Alps, the Younger Dryas (YD) was characterized by the last major glacier advance with equilibrium line altitudes being ~ 220 to 290 m lower than during the Little Ice Age and also by the development of rock glaciers. Dating of these geomorphic features, however, is associated with substantial uncertainties leading to considerable ambiguities on the internal structure of this stadial, the most intensively studied one of the last glacial period. Here we provide robust physical evidence based on precise 230Th-dated cryogenic cave carbonates (CCC) coupled with thermal modelling indicating that early YD winters were only moderately cold in the Southern Alps, challenging the commonly held view of extreme YD seasonality. Our data argue for a negative temperature anomaly of ≤ 3 °C in mean annual air temperature at the Allerød-YD transition in a mountain cave (Cioccherloch, 2274 m a.s.l.) in the Dolomites of northern Italy. Our data suggest that autumns and early winters in the early part of the YD were relatively snow-rich, resulting in a stable winter snow cover. The latter insulated the shallow subsurface in winter and allowed the cave interior to remain close to the freezing point (0 °C) year-round, promoting CCC formation. The main phase of CCCs precipitation at ~ 12.2 ka BP coincides with the mid-YD transition recorded in other archives across Europe. Based on thermal modelling we propose that CCC formation at ~ 12.2 ka BP was most likely associated with a slight warming of approximately +1 °C in conjunction with drier autumns and early winters in the second half of the YD. These changes triggered CCC formation in this alpine cave as well as ice glacier retreat and rock glacier expansion in the Alps.
The regional patterns and timing of the Younger Dryas cooling in the North Atlantic realm were complex and are mechanistically incompletely understood. To enhance understanding of regional climate patterns, we present molecular biomarker records at subannual to annual resolution by mass spectrometry imaging (MSI) of sediments from the Lake Meerfelder Maar covering the Allerød-Younger Dryas transition. These analyses are supported by conventional extraction-based molecular-isotopic analyses, which both validate the imaging results and constrain the sources of the target compounds. The targeted fatty acid biomarkers serve as a gauge of the response of the local aquatic and terrestrial ecosystem to climate change. Based on the comparison of our data with existing data from Meerfelder Maar, we analyse the short-term environmental evolution in Western Europe during the studied time interval and confirm the previously reported delayed hydrological response to Greenland cooling. However, despite a detected delay of Western European environmental change of ~135 years, our biomarker data show statistically significant correlation with deuterium excess in Greenland ice core at ~ annual resolution during this time-transgressive cooling. This suggests a coherent atmospheric forcing across the North Atlantic realm during this transition. We propose that Western European cooling was postponed due to major reorganization of the westerlies that were intermittently forcing warmer and wetter air masses from lower latitudes to Western Europe and thus resulted in delayed cooling relative to Greenland.
We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.
Abrupt climate changes in the past have been attributed to variations in Atlantic Meridional Overturning Circulation (AMOC) strength. However, the exact timing and magnitude of past AMOC shifts remain elusive, which continues to limit our understanding of the driving mechanisms of such climate variability. Here we show a consistent signal of the ²³¹Pa/²³⁰Th proxy that reveals a spatially coherent picture of western Atlantic circulation changes over the last deglaciation, during abrupt millennial scale climate transitions. At the onset of deglaciation, we observe an early slowdown of circulation in the western Atlantic from around 19 to 16.5 thousand years ago (ka), consistent with the timing of accelerated Eurasian ice melting. The subsequent weakened AMOC state persists for over a millennium (~16.5–15 ka), during which time there is substantial ice rafting from the Laurentide ice sheet. This timing indicates a role for melting ice in driving a two-step AMOC slowdown, with a positive feedback sustaining continued iceberg calving and climate change during Heinrich Stadial 1.
The Atlantic meridional overturning circulation (AMOC)-a system of ocean currents in the North Atlantic-has a major impact on climate, yet its evolution during the industrial era is poorly known owing to a lack of direct current measurements. Here we provide evidence for a weakening of the AMOC by about 3 ± 1 sverdrups (around 15 per cent) since the mid-twentieth century. This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature 'fingerprint'-consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region-and is calibrated through an ensemble of model simulations from the CMIP5 project. We find this fingerprint both in a high-resolution climate model in response to increasing atmospheric carbon dioxide concentrations, and in the temperature trends observed since the late nineteenth century. The pattern can be explained by a slowdown in the AMOC and reduced northward heat transport, as well as an associated northward shift of the Gulf Stream. Comparisons with recent direct measurements from the RAPID project and several other studies provide a consistent depiction of record-low AMOC values in recent years.
The suitability of MIS 11c and MIS 19c as analogues of our present interglacial and its natural evolution is still debated. Here we examine the regional expression of the Holocene and its orbital analogues over SW Iberia using a model–data comparison approach. Regional tree fraction and climate based on snapshot and transient experiments using the LOVECLIM model are evaluated against the terrestrial–marine profiles from Site U1385 documenting the regional vegetation and climatic changes. The pollen-based reconstructions show a larger forest optimum during the Holocene compared to MIS 11c and MIS 19c, putting into question their analogy in SW Europe. Pollen-based and model results indicate reduced MIS 11c forest cover compared to the Holocene primarily driven by lower winter precipitation, which is critical for Mediterranean forest development. Decreased precipitation was possibly induced by the amplified MIS 11c latitudinal insolation and temperature gradient that shifted the westerlies northwards. In contrast, the reconstructed lower forest optimum at MIS 19c is not reproduced by the simulations probably due to the lack of Eurasian ice sheets and its related feedbacks in the model. Transient experiments with time-varying insolation and CO2 reveal that the SW Iberian forest dynamics over the interglacials are mostly coupled to changes in winter precipitation mainly controlled by precession, CO2 playing a negligible role. Model simulations reproduce the observed persistent vegetation changes at millennial time scales in SW Iberia and the strong forest reductions marking the end of the interglacial “optimum”.
The Younger Dryas event, which began approximately 12,900 years ago, was a period of rapid cooling in the Northern Hemisphere, driven by large-scale reorganizations of patterns of atmospheric and oceanic circulation1, 2, 3. Environmental changes during this period have been documented by both proxy-based reconstructions3 and model simulations4, but there is currently no consensus on the exact mechanisms of onset, stabilization or termination of the Younger Dryas5, 6, 7, 8. Here we present high-resolution records from two sediment cores obtained from Lake Kråkenes in western Norway and the Nordic seas. Multiple proxies from Lake Kråkenes are indicative of rapid alternations between glacial growth and melting during the later Younger Dryas. Meanwhile, reconstructed sea surface temperature and salinity from the Nordic seas show an alternation between sea-ice cover and the influx of warm, salty North Atlantic waters. We suggest that the influx of warm water enabled the westerly wind systems to drift northward, closer to their present-day positions. The winds thus brought relatively warm maritime air to Northern Europe, resulting in rising temperatures and the melting of glaciers. Subsequent input of this fresh meltwater into the ocean spurred the formation of sea ice, which forced the westerly winds back to the south, cooling Northern Europe. We conclude that rapid alternations between these two states immediately preceded the termination of the Younger Dryas and the permanent transition to an interglacial state.
Abrupt changes in Western Mediterranean climate during the last deglaciation (20 to 6 cal ka BP) are detected in marine core MD95-2043 (Alboran Sea) through the investigation of high-resolution pollen data and pollen-based climate reconstructions by the modern analogue technique (MAT) for annual precipitation ( P <sub>ann</sub>) and mean temperatures of the coldest and warmest months (MTCO and MTWA). Changes in temperate Mediterranean forest development and composition and MAT reconstructions indicate major climatic shifts with parallel temperature and precipitation changes at the onsets of Heinrich stadial 1 (equivalent to the Oldest Dryas), the Bölling-Allerød (BA), and the Younger Dryas (YD). Multi-centennial-scale oscillations in forest development occurred throughout the BA, YD, and early Holocene. Shifts in vegetation composition and ( P <sub>ann</sub> reconstructions indicate that forest declines occurred during dry, and generally cool, episodes centred at 14.0, 13.3, 12.9, 11.8, 10.7, 10.1, 9.2, 8.3 and 7.4 cal ka BP. The forest record also suggests multiple, low-amplitude Preboreal (PB) climate oscillations, and a marked increase in moisture availability for forest development at the end of the PB at 10.6 cal ka BP. Dry atmospheric conditions in the Western Mediterranean occurred in phase with Lateglacial events of high-latitude cooling including GI-1d (Older Dryas), GI-1b (Intra-Allerød Cold Period) and GS-1 (YD), and during Holocene events associated with high-latitude cooling, meltwater pulses and N. Atlantic ice-rafting. A possible climatic mechanism for the recurrence of dry intervals and an opposed regional precipitation pattern with respect to Western-central Europe relates to the dynamics of the westerlies and the prevalence of atmospheric blocking highs. Comparison of radiocarbon and ice-core ages for well-defined climatic transitions in the forest record suggests possible enhancement of marine reservoir ages in the Alboran Sea by 200 years (surface water age 600 years) during the Lateglacial.
We present two new high-resolution sediment records from the southwestern Iceland and Norwegian Seas that were dated by numerous 14 C ages up to 54 14 C ka bp. Based on various lines of evidence, the local 14 C reservoir effect was restricted to 400–1600 yr. The planktic stable isotope records reveal several meltwater spikes that were sampled with an average time resolution of 50 yr in PS2644 and 130 yr in core 23071 during isotope stage 3. Most of the δ 18 O spikes correlate peak-by-peak to the stadials and cold rebounds of the Dansgaard-Oeschger cycles in the annual-layer counted GISP2 ice core, with the major spikes reflecting the Heinrich events 1–6. This correlation indicates large fluctuations in the calibration of 14 C ages between 20 and 54 14 C ka bp. Generally the results confirm the 14 C age shifts as predicted by the geomagnetic model of Laj, Mazaud and Duplessy (1996). However, the amplitude and speed of the abrupt decrease and subsequent major increase of our 14 C shifts after 45 14 C ka bp clearly exceed the geomagnetic prediction near 40–43 and 32–34 calendar (cal) ka bp. At these times, the geomagnetic field intensity minima linked to the Laschamp and the Mono Lake excursions and confirmed by a local geomagnetic record, probably led to a sudden increase in cosmogenic 14 C and 10 Be production, giving rise to excess 14 C in the atmosphere of up to 1200%.
The Iberian Peninsula (IP) was hit in 2011-12 by one of the most severe droughts ever recorded in this increasingly dry region of the world. The IP winter precipitation is influenced not only by the preferred path of synoptic disturbances (storm tracks), and associated atmospheric instability that forces air masses to rise, but also by the supply of moisture from the major moisture source regions (MSR) in the North Atlantic. It is quantified how much less moisture from these MSRs was received in the IP during the winter of 2011-12 compared with a long-term climatology. For that purpose, a Lagrangian analysis was used, which in summary, consists of identifying all trajectories originating from MSRs and coming to the IP. By quantifying the net rate of change of water vapor along each trajectory and adding this for all the trajectories over the 10 days of transport, it is possible to quantify moisture received by the IP from each MSR in terms of E-P.
The Younger Dryas is the last major abrupt climate change event of the last deglaciation occurring ~12 900–11 700 years ago. Large portions of the Northern Hemisphere cooled and much of the Southern Hemisphere warmed during the event in a bipolar seesaw pattern. While changes in net precipitation were more variable at higher latitudes, Northern Hemisphere subtropics and tropics were generally drier and the Southern Hemisphere subtropics wetter from southward migration of the Intertropical Convergence Zone. Many of the climate changes related to the Younger Dryas were likely a response to increased freshwater discharge to the North Atlantic and the attendant reduction in Atlantic meridional overturning strength. Although multiple freshwater forcing hypotheses have been proposed, the existing terrestrial and marine records indicate that the northward retreat of the southern margin of the Laurentide Ice Sheet from the Great Lakes caused a routing of freshwater from the western Canadian Plains from the Mississippi River to the St. Lawrence River, with the increased freshwater discharge to the North Atlantic slowing ocean circulation and ultimately causing the Younger Dryas.
Sources and timing of freshwater forcing relative to hydroclimate shifts recorded in Greenland ice cores at the onset of Younger Dryas, ~12,800 years ago, remain speculative. Here we show that progressive Fennoscandian Ice Sheet (FIS) melting 13,100–12,880 years ago generates a hydroclimate dipole with drier–colder conditions in Northern Europe and wetter–warmer conditions in Greenland. FIS melting culminates 12,880 years ago synchro- nously with the start of Greenland Stadial 1 and a large-scale hydroclimate transition lasting B180 years. Transient climate model simulations forced with FIS freshwater reproduce the initial hydroclimate dipole through sea-ice feedbacks in the Nordic Seas. The transition is attributed to the export of excess sea ice to the subpolar North Atlantic and a subsequent southward shift of the westerly winds. We suggest that North Atlantic hydroclimate sensi- tivity to FIS freshwater can explain the pace and sign of shifts recorded in Greenland at the climate transition into the Younger Dryas.
Significance
This study presents robust evidence of two hydrological phases within the Greenland Stadial 1 (GS-1) cold event (12.8−11.7 ka B.P.) in Southern Europe. We present a well-dated high-resolution speleothem record (Seso Cave, Central Pyrenees) where temperature and hydrological signals are independently reconstructed. Detailed interpretation of stable isotopes and trace elements allow characterizing a first dry period followed, after 12,500 y before 2000 A.D., by more humid conditions. Our findings point to the resumption of the Atlantic overturning circulation as the main mechanism behind the hydrological response in Europe during this mid–GS-1 transition. The second phase, cold in Greenland but humid in Western Europe, represents a new paradigm in the well-established model of dry, cold stadials during the last glacial period.
High-temporal resolution analysis of different climatic tracers (pollen, foraminiferal-based winter sea surface temperature (SST), benthic foraminiferal delta O-18) from marine core MD95-2042, retrieved off SW Iberia, allows us to directly compare, without any chronological ambiguity, Mediterranean vegetation and eastern North Atlantic winter SST changes for the last 14.2 kyr. We identify on land and in the ocean several climatic phases such as the end of the warm and humid Bolling-Allerod, the cold and dry Younger Dryas, and the warm and humid Holocene with the Mediterranean forest (MF) optimum between 9.6 and 8.1 kyr. This record shows that, at multi-centennial timescale (similar to 800 years), declines in forest cover generally related to dry and cool periods in southern Iberia are synchronous with cold SST in the eastern part of the North Atlantic subtropical gyre. At multi-centennial timescale, changes in thermohaline circulation, via freshwater content fluctuations, appear to be responsible for the coupling between dryness in Iberia and SST cooling in eastern North Atlantic subtropical gyre. In contrast, some Holocene events include centennial-scale oscillations (similar to 100 years) marked by MF declines in southern Iberia concomitant with SST warming in the eastern North Atlantic subtropical gyre. This climatic pattern is similar to that observed at decadal timescale under the influence of the positive mode of the North Atlantic Oscillation (NAO). We suggest, therefore, that synchronous SW Iberian dryness and SST warming at centennial timescale could be explained by atmospheric fluctuations related to NAO changes.
Due to their outstanding resolution and well-constrained chronologies, Greenland ice-core records provide a master record of past climatic changes throughout the Last Interglacial–Glacial cycle in the North Atlantic region. As part of the INTIMATE (INTegration of Ice-core, MArine and TErrestrial records) project, protocols have been proposed to ensure consistent and robust correlation between different records of past climate. A key element of these protocols has been the formal definition and ordinal numbering of the sequence of Greenland Stadials (GS) and Greenland Interstadials (GI) within the most recent glacial period. The GS and GI periods are the Greenland expressions of the characteristic Dansgaard–Oeschger events that represent cold and warm phases of the North Atlantic region, respectively. We present here a more detailed and extended GS/GI template for the whole of the Last Glacial period. It is based on a synchronization of the NGRIP, GRIP, and GISP2 ice-core records that allows the parallel analysis of all three records on a common time scale. The boundaries of the GS and GI periods are defined based on a combination of stable-oxygen isotope ratios of the ice (δ18O, reflecting mainly local temperature) and calcium ion concentrations (reflecting mainly atmospheric dust loading) measured in the ice. The data not only resolve the well-known sequence of Dansgaard–Oeschger events that were first defined and numbered in the ice-core records more than two decades ago, but also better resolve a number of short-lived climatic oscillations, some defined here for the first time. Using this revised scheme, we propose a consistent approach for discriminating and naming all the significant abrupt climatic events of the Last Glacial period that are represented in the Greenland ice records. The final product constitutes an extended and better resolved Greenland stratotype sequence, against which other proxy records can be compared and correlated. It also provides a more secure basis for investigating the dynamics and fundamental causes of these climatic perturbations.
The general warming trend of the last deglaciation was interrupted by the Younger Dryas, a period of abrupt cooling and widespread environmental change. Ice core records suggest the abrupt cooling began 12,846 years ago in Greenland, about 170 years before the significant environmental and vegetation change in western Europe classically defined as the Younger Dryas. However, this difference in timing falls within age model uncertainties. Here we use the hydrogen isotope composition of lipid biomarkers from precisely dated varved sediments from Lake Meerfelder Maar to reconstruct hydroclimate over western Europe. We observe a decrease in the hydrogen isotope values of both aquatic and terrestrial lipids 12,850 years ago, indicating cooling climate in this region synchronous with the abrupt cooling in Greenland. A second drop occurs 170 years later, mainly in the hydrogen isotope record of aquatic lipids but to a lesser extent in the terrestrial lipids, which we attribute to aridification, as well as a change in moisture source and pathway. We thus confirm that there was indeed a lag between cooling and substantial hydrologic and environmental change in western Europe. We suggest the delay is related to the expansion of sea ice in the North Atlantic Ocean and the subsequent southward migration of the westerly wind system. We further suggest that these hydrological changes amplified environmental change in western Europe at the onset of the Younger Dryas.
Pollen-based climate reconstructions were performed on two
high-resolution pollen marines cores from the Alboran and Aegean Seas in
order to unravel the climatic variability in the coastal settings of the
Mediterranean region between 15 000 and 4000 years BP (the Lateglacial,
and early to mid-Holocene). The quantitative climate reconstructions for
the Alboran and Aegean Sea records focus mainly on the reconstruction of
the seasonality changes (temperatures and precipitation), a crucial
parameter in the Mediterranean region. This study is based on a
multi-method approach comprising 3 methods: the Modern Analogues
Technique (MAT), the recent Non-Metric Multidimensional
Scaling/Generalized Additive Model method (NMDS/GAM) and Partial Least
Squares regression (PLS). The climate signal inferred from this
comparative approach confirms that cold and dry conditions prevailed in
the Mediterranean region during the Oldest and Younger Dryas periods,
while temperate conditions prevailed during the
Bølling/Allerød and the Holocene. Our records suggest a
West/East gradient of decreasing precipitation across the Mediterranean
region during the cooler Late-glacial and early Holocene periods,
similar to present-day conditions. Winter precipitation was highest
during warm intervals and lowest during cooling phases. Several
short-lived cool intervals (i.e. Older Dryas, another oscillation after
this one (GI-1c2), Gerzensee/Preboreal Oscillations, 8.2 ka event, Bond
events) connected to the North Atlantic climate system are documented in
the Alboran and Aegean Sea records indicating that the climate
oscillations associated with the successive steps of the deglaciation in
the North Atlantic area occurred in both the western and eastern
Mediterranean regions. This observation confirms the presence of strong
climatic linkages between the North Atlantic and Mediterranean regions.
The IntCal09 and Marine09 radiocarbon calibration curves have been revised utilizing newly available and updated data sets from 14C measurements on tree rings, plant macrofossils, speleothems, corals, and foraminifera. The calibration curves were derived from the data using the random walk model (RWM) used to generate IntCal09 and Marine09, which has been revised to account for additional uncertainties and error structures. The new curves were ratified at the 21st International Radiocarbon conference in July 2012 and are available as Supplemental Material at www.radiocarbon.org. The database can be accessed at http://intcal.qub.ac.uk/intcal13/. © 2013 by the Arizona Board of Regents on behalf of the University of Arizona.
The Younger Dryas cooling 12,700 years ago is one of the most abrupt climate changes observed in Northern Hemisphere palaeoclimate records. Annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes, as the seasonal deposition responds immediately to climate, and the varve counts provide an accurate estimate of the timing of the change. Here, we present sub-annual records of varve microfacies and geochemistry from Lake Meerfelder Maar in western Germany, providing one of the best dated records of this climate transition. Our data indicate an abrupt increase in storminess during the autumn to spring seasons, occurring from one year to the next at 12,679yr BP, broadly coincident with other changes in this region. We suggest that this shift in wind strength represents an abrupt change in the North Atlantic westerlies towards a stronger and more zonal jet. Changes in meridional overturning circulation alone cannot fully explain the changes in European climate; we suggest the observed wind shift provides the mechanism for the strong temporal link between North Atlantic Ocean overturning circulation and European climate during deglaciation.
We present a new common stratigraphic timescale for the North Greenland Ice Core Project (NGRIP) and GRIP ice cores. The timescale covers the period 7.9–14.8 kyr before present and includes the Bølling, Allerød, Younger Dryas, and early Holocene periods. We use a combination of new and previously published data, the most prominent being new high-resolution Continuous Flow Analysis (CFA) impurity records from the NGRIP ice core. Several investigators have identified and counted annual layers using a multiparameter approach, and the maximum counting error is estimated to be up to 2% in the Holocene part and about 3% for the older parts. These counting error estimates reflect the number of annual layers that were hard to interpret, but not a possible bias in the set of rules used for annual layer identification. As the GRIP and NGRIP ice cores are not optimal for annual layer counting in the middle and late Holocene, the timescale is tied to a prominent volcanic event inside the 8.2 kyr cold event, recently dated in the DYE-3 ice core to 8236 years before A. D. 2000 (b2k) with a maximum counting error of 47 years. The new timescale dates the Younger Dryas-Preboreal transition to 11,703 b2k, which is 100–150 years older than according to the present GRIP and NGRIP timescales. The age of the transition matches the GISP2 timescale within a few years, but viewed over the entire 7.9–14.8 kyr section, there are significant differences between the new timescale and the GISP2 timescale. The transition from the glacial into the Bølling interstadial is dated to 14,692 b2k. The presented timescale is a part of a new Greenland ice core chronology common to the DYE-3, GRIP, and NGRIP ice cores, named the Greenland Ice Core Chronology 2005 (GICC05). The annual layer thicknesses are observed to be log-normally distributed with good approximation, and compared to the early Holocene, the mean accumulation rates in the Younger Dryas and Bølling periods are found to be 47 ± 2% and 88 ± 2%, respectively.
This study investigated the main sources of moisture in the atmosphere over the Iberian Peninsula (IP) at annual and seasonal scales using FLEXPART, a powerful new 3D Lagrangian diagnosis method that identifies the humidity contributions to the moisture budget of a region. This method can identify moisture sources at lower cost and with greater accuracy than standard isotopic content methods. The results are based on back-tracking analysis of all air masses residing over the IP in the 5-yr period from 2000 to 2004. The results show that the two most important moisture source regions affecting the IP are in a tropical-subtropical North Atlantic corridor that extends from the Gulf of Mexico to the IP, and the IP itself and the surrounding Mediterranean. The importance of these two source areas varies throughout the year, and also with respect to different climatic regions inside the IP. The former source region is the dominant moisture source for the entire IP during winter and in western regions throughout the year, whereas the latter source region dominates the moisture supply to the IP in summer and in the eastern Mediterranean region of the IP throughout the year. The results also demonstrate that winter precipitation in the IP is influenced by both atmospheric instability that forces air masses to rise, and the supply of moisture from the tropical-subtropical North Atlantic corridor on a daily scale and a seasonal basis. Thus, a combination of high (low) moisture supply from the North Atlantic corridor and high (low) atmospheric instability appears to be responsible for the most recent wet (dry) winter in the IP.
High-resolution proxy data analyzed on two high-sedimentation shallow water sedimentary sequences (PO287-26B and PO287-28B) recovered off Lisbon (Portugal) provide the means for comparison to long-term instrumental time series of marine and atmospheric parameters (sea surface temperature (SST), precipitation, total river flow, and upwelling intensity computed from sea level pressure) and the possibility to do the necessary calibration for the quantification of past climate conditions. XRF Fe is used as proxy for river flow, and the upwelling-related diatom genus Chaetoceros is our upwelling proxy. SST is estimated from the coccolithophore-synthesized alkenones and U37 k' index. Comparison of the Fe record to the instrumental data reveals its similarity to a mean average run of the instrumentally measured winter (JFMA) river flow on both sites. The upwelling diatom record concurs with the upwelling indices at both sites; however, high opal dissolution, below 20-25 cm, prevents its use for quantitative reconstructions. Alkenone-derived SST at site 28B does not show interannual variation; it has a mean value around 16°C and compares quite well with the instrumental winter/spring temperature. At site 26B the mean SST is the same, but a high degree of interannual variability (up to 4°C) appears to be determined by summer upwelling conditions. Stepwise regression analyses of the instrumental and proxy data sets provided regressions that explain from 65 to 94% of the variability contained in the original data, and reflect spring and summer river flow, as well as summer and winter upwelling indices, substantiating the relevance of seasons to the interpretation of the different proxy signals. The lack of analogs and the small data set available do not allow quantitative reconstructions at this time, but this might be a powerful tool for reconstructing past North Atlantic Oscillation conditions, should we be able to find continuous high-resolution records and overcome the analog problem.
An attempt is made to find the main monthly modes of variation of precipitation over the Iberian Peninsula. The modes of variation of precipitation were derived from principal component analysis. The dataset used consists of records of monthly precipitation from 40 meteorological stations over 74 yr (1919-92). The stations are spatially representative of most of the Iberian Peninsula. To take into account the seasonality of precipitation over the Iberian Peninsula, one analysis was performed separately for each calendar month. The modes of variation resulting from the different analyses were compared and clustered in groups according to their loading patterns. Seven main patterns were found to be of importance during various months of the year. These seven patterns explained more than 75% of the variance of the precipitation field from December to April. However, less than 20% of the total variance is explained for July and August. It is concluded that, depending on the month or season of interest, different modes of variation should be considered in order to achieve a better description of the monthly precipitation field.
Abstract Comparison of selected, well-dated, lacustrine, speleothem and terrestrial pollen records spanning the Holocene onset and the Early Holocene (ca. 11.7–8 cal kyrs BP) in the Iberian Peninsula shows large hydrological fluctuations and landscape changes with a complex regional pattern in timing and intensity. Marine pollen records from Alboran, the Mediterranean and off shore Atlantic sites show a step-wise increase in moisture and forest during this transition. However, available continental records point to two main patterns of spatial and temporal hydrological variability: i) Atlantic-influenced sites located at the northwestern areas (Enol, Sanabria, Lucenza, PRD-4), characterized by a gradual increase in humidity from the end of the Younger Dryas to the Mid Holocene, similarly to most North Atlantic records; and ii) continental and Mediterranean-influenced sites (Laguna Grande, Villarquemado, Fuentillejo, Padul, Estanya, Banyoles, Salines), with prolonged arid conditions of variable temporal extension after the Younger Dryas, followed by an abrupt increase in moisture at 10-9 cal kyrs BP. Different local climate conditions influenced by topography or the variable sensitivity (gradual versus threshold values) of the proxies analyzed in each case are evaluated. Vegetation composition (conifers versus mesothermophilous taxa) and resilience would explain a subdued response of vegetation in central continental areas while in Mediterranean sites, insufficient summer moisture availability could not maintain high lake levels and promote mesophyte forest, in contrast to Atlantic-influenced areas. Comparison with available climate models, Greenland ice cores, North Atlantic marine sequences and continental records from Central and Northern Europe and the whole Mediterranean region underlines the distinctive character of the hydrological changes occurred in inner Iberia throughout the Early Holocene. The persistent arid conditions might be explained by the intensification of the summer drought due to the high seasonality contrast at these latitudes caused by the orbital-induced summer insolation maximum. New records, particularly from western and southernmost Iberia, and palaeoclimate models with higher spatial resolution would help to constrain these hypotheses.
This was the third cruise using the new SOC giant piston corer. The objectives were to complete the testing of the giant piston corer which was begun on cruise D219 in November/December 1995 and cruise D225 in February/March 1997, and to collect giant piston cores from the Gulf of Cadiz on sites selected from the TOBI survey carried out on cruise D244 in December 1999. As on previous cruises we had problems with both the ships equipment (outboard sheave on coring gantry), and the giant piston corer. The sheave problem necessitated 2.5 days in port plus transit time. This combined with some weather downtime severely limited the amount of work which could be carried out. Nevertheless, we resolved some coring issues, especially proving that elastic rebound of the kevlar is not a major problem. We also managed to take a consistent series of good cores with a 15 metre barrel, and although all the 27 metre barrel cores bent, the bending took place above the sediment surface leaving a good quality core below. The longest core recovered was 18.1 metres and 21 cores were obtained in all. The cruise also involved testing the scatterometer. This proved to be a failure due to multiple electronic problems and no data was recorded in any of its two deployments.
The Younger Dryas cooling event disrupted the overall warming trend in the North Atlantic region during the last
deglaciation. Climate change during the Younger Dryas was abrupt, and thus provides insights into the sensitivity of the climate system to perturbations. The sudden Younger Dryas cooling has traditionally been attributed to a shutdown of the Atlantic Meridional Overturning Circulation by meltwater discharges. However, alternative explanations such as strong negative radiative forcing and a shift in atmospheric circulation have also been
off�ered. Here we investigate the importance of these diff�erent forcings in coupled climate model experiments constrained by data assimilation.We find that the Younger Dryas climate signal as registered in proxy evidence is best simulated using a combination of processes: a weakened Atlantic Meridional Overturning Circulation, moderate negative radiative forcing and an altered atmospheric circulation. We conclude that none of the individual mechanisms alone provide a plausible explanation for the Younger Dryas cold period.We suggest that the triggers for abrupt climate changes such as the Younger Dryas are more complex than suggested so far, and that studies on the response of the climate system to perturbations should account for this complexity.
The direct comparison between marine and terrestrial data from the NW Iberian margin, core MD03-2697, allows us to accurately evaluate, without chronological ambiguity, the vegetation response to North Atlantic climate events across the last deglaciation. Comparison of MD03-2697 data with other marine and terrestrial records from a vast area stretching from the Azores to western (W) France, Iberia and its margin, the W Mediterranean and NW Africa reveals the importance of enhanced winter North Atlantic westerlies episodes in driving a heterogeneous regional climatic signal during particular events of the last deglaciation. Heinrich Stadial 1 (HS1)/Oldest Dryas is a complex event marked by three synchronous main phases (a: extremely cold/relatively wet; b: cool/dry; c: relatively warmer/increasing moisture availability) in regions directly influenced by the North Atlantic while it is characterized by a single phase (cold and dry) in most inland and high altitude areas. Changes in the strength and position of North Atlantic westerlies could explain the variability in moisture during HS1 from W Pyrenees to W Mediterranean. The Bølling-Allerød (B-A) event is marked by a synchronous progressive increase of ocean and atmospheric temperatures and precipitation from the Bølling to the Allerød in W Iberia and W Pyrenees contrasting with the Greenland temperature pattern. Mid-to high latitudes thermal contrast and the gradual strengthening of the Atlantic Meridional Overturning Circulation (AMOC) triggered the continuing enhancement of westerlies, and moisture, along this period.
The North Atlantic subtropical gyre (STG) circulates warm waters between 10 and 40°N and is a potential area of heat storage during periods of reduced North Atlantic Meridional Overturning Circulation (AMOC), when warm salt-rich waters are retained in the subtropics. In this study, we investigated multi-centennial to millennial scale changes in subtropical North Atlantic hydrography in response to AMOC changes during the last deglaciation and early Holocene, using sediment cores MD08-3180 and GEOFAR KF16. The coring site (38°N), is situated near the boundary between transitional eastern North Atlantic waters and STG waters that is formed by the Azores Front. Hydrographic changes are reconstructed using new stable isotope data of benthic and subsurface dwelling planktonic foraminifera, Mg/Ca measurements on planktonic foraminifera, and planktonic foraminifera abundances that are supplemented with published Sea Surface Temperature and stable isotope data. These multiproxy data indicate a close coupling between the latitudinal position of the northern STG boundary and deglacial AMOC modes. During weak AMOC phases (Heinrich event 1, Younger Dryas (YD), 8.2 ka event), Northern Hemisphere subpolar water reached down to the northern STG boundary, displacing the boundary southward. During the Bølling-Allerød warm period, a strong warming trend of the subtropical region to 19°C is observed. A cooling of the SST by 6°C during the YD is accompanied by ongoing northward transport of warm subsurface water that might have contributed to the restart of AMOC.
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Precisely-dated, high-resolution stable isotope and trace element data from a stalagmite from La Garma Cave, northern Spain, reveal several stages of distinct climatic variability along the northern Iberian Atlantic margin, and provide new constraints on the latitude of North Atlantic westerlies during the Younger Dryas Event (YD). Westerly wind position (reconstructed using sub-annually resolved Mg data as a proxy for sea spray contributions associated with wind strength at this coastal cave site) during the early YD (12.85–12.15 kyr) oscillated meridionally, resembling the decadal-scale component of the modern North Atlantic Oscillation (NAO). Northward repositioning of westerly storm tracks over northern Iberia began at ∼12.15 kyr, consistent with other high-resolution wind proxy reconstructions from central and northern Europe, but occurred more gradually nearer the Atlantic margin. From ∼12.15 kyr to the YD termination, atmospheric circulation resembled a persistently positive NAO, with westerlies reaching their maximum northward extent at 11.8 kyr (reflected by a Mg concentration minimum at this time). Air temperature (reflected by our δ 18 O data) and Iberian wind strength were predominantly coupled throughout the YD suggesting that temperature modulated sea-ice extent, and consequently controlled westerly wind latitude. However our data suggest that abrupt warming at 12.1 kyr was followed by much more gradual northward shifts in westerly position, and that a lag existed between the warming and sea-ice retreat. This gradual return of the westerlies to the north beginning at 12.1 kyr is consistent with inferred changes in wind strength at other European sites. Additionally, atmospheric circulation inferred from our northern Iberian wind strength proxy record generally tracked low-frequency meridional shifts in Intertropical Convergence Zone position, corroborating past research that suggested closely coupled low-and high-latitude atmospheric teleconnections over this period.
The transition from last glacial to deglacial and subsequently to modern interglacial climate conditions was accompanied by abrupt shifts in the palaeoceanographic setting in the subpolar North Atlantic. Knowledge about the role that sea ice coverage played during these rapid climate reversals is limited since most marine sediment cores from the higher latitudes provide only a coarse temporal resolution and often poorly preserved microfossils. Here we present a highly resolved reconstruction of sea ice conditions that characterised the eastern Fram Strait - a key area for water mass exchange between the Arctic Ocean and the North Atlantic - for the past 30 ka BP. This reconstruction is based on the distribution of the sea ice biomarker IP25 and phytoplankton derived biomarkers in a sediment core from the continental slope of western Svalbard. During the late glacial (30 ka to 19 ka BP), recurrent advances and retreats of sea ice characterised the study area and point to a hitherto less considered oceanic (and/or atmospheric) variability. A long-lasting perennial sea ice coverage in eastern Fram Strait persisted only at the very end of the Last Glacial Maximum (i.e. from 19.2 to 17.6 ka BP) and was abruptly reduced at the onset of Heinrich Event 1 - coincident with or possibly even inducing the collapse of the Atlantic Meridional Overturning Circulation (AMOC). Further maximum sea ice conditions prevailed during the Younger Dryas cooling event and support the assumption of an AMOC reduction due to increased formation and export of Arctic sea ice through Fram Strait. A significant retreat of sea ice and sea surface warming are observed for the Early Holocene.
We have analysed alkenones in 149 surface sediments from the eastern South Atlantic in order to establish a sediment-based calibration of the U37K′ paleotemperature index. Our study covers the major tropical to subpolar production systems and sea-surface temperatures (SST’s) between 0° and 27°C. In order to define the most suitable calibration for this region, the U37K′ values were correlated to seasonal, annual, and production-weighted annual mean atlas temperatures and compared to previously published culture and core-top calibrations. The best linear correlation between U37K′ and SST was obtained using annual mean SST from 0 to 10 m water depth (U37K′ = 0.033 T + 0.069, r2 = 0.981). Data scattering increased significantly using temperatures of waters deeper than 20 m, suggesting that U37K′ reflects mixed-layer SST and that alkenone production at thermocline depths was not high enough to significantly bias the mixed-layer signal. Regressions based on both production-weighted and on actual annual mean atlas SST were virtually identical, indicating that regional variations in the seasonality of primary production have no discernible effect on the U37K′ vs. SST relationship. Comparison with published core-top calibrations from other oceanic regions revealed a high degree of accordance. We, therefore, established a global core-top calibration using U37K′ data from 370 sites between 60°S and 60°N in the Atlantic, Indian, and Pacific Oceans and annual mean atlas SST (0–29°C) from 0 m water depth. The resulting relationship (U37K′ = 0.033 T + 0.044, r2 = 958) is identical within error limits to the widely used E. huxleyi calibrations of Prahl and Wakeham (1987) and Prahl et al. (1988) attesting their general applicability. The observation that core-top calibrations extending over various biogeographical coccolithophorid zones are strongly linear and in better accordance than culture calibrations suggests that U37K′ is less species-dependent than is indicated by culture experiments. The results also suggest that variations in growth rate of algae and nutrient availability do not significantly affect the sedimentary record of U37K′ in open ocean environments.
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High-resolution multiproxy analyses were performed on a 128 cm
section of organic sediments accumulated in a small mountain lake in NW
Iberia (Laguna de la Roya, 1608 m asl). The pollen
stratigraphy together with radiocarbon dating provided the basis for a
chronology ranging from 15,600 to 10,500 cal yr BP.
Chironomid-inferred July air temperatures suggest a temperature range
from 7 to 13 °C, also evidencing two well-established cold
periods which may be equivalent to the INTIMATE stages GS-2a and GS-1.
Furthermore, a number of short cold events (with summer temperatures
dropping about 0.5-1 °C) appear intercalated within the
Lateglacial Interstadial (possibly equivalent to the INTIMATE cold
events GI-1d, GI-1c2 and GI-1b) and the early Holocene (possibly
equivalent to the 11.2 k event). The temperature variations
predicted by our reconstruction allow explaining the changes in local
conditions and productivity of the lake inferred from the biological
record of the same sediment core. Furthermore, they also agree with the
local and regional vegetation dynamics, and the main oscillations
deduced for the vegetation belts. Based on its chronology our multiproxy
record indicates a similar temperature development in NW Iberia as
inferred by the Greenland δ18O record, the marine
deep-sea records off the Atlantic Iberian Margin, and other
chironomid-based Lateglacial temperature reconstructions from Europe.
Nevertheless, the impact of most of the less intense Lateglacial/early
Holocene cold events in NW Iberia was most probably limited to very
sensitive sites that were very close to ecotonal situations.
Particularly, our new pollen record indicates that they were represented
as three minor environmental crises occurring during the Lateglacial
Interestadial in this area. The Older Dryas event (in our usage
corresponding to the Aegelsee Oscillation in Central Europe and event
GI-1d in central Greenland) has previously been described in this
region, but its age and duration
(ca 14,250-14050 cal yr BP) is now better
constrained. The two subsequent stages, La Roya I
(ca 13,600-13,400 cal yr BP) and La Roya II
(ca 13,300-12,900 cal yr BP) have been
described for first time in NW Iberia.
[1] Constraints on the timing of Atlantic meridional overturning circulation (AMOC) changes during the last deglaciation are fundamental to understanding the climate's rapid response to insolation forcing. However, uncertainty about high-latitude North Atlantic (HLNA) radiocarbon reservoir ages has previously precluded robust age model development for this critical region. HLNA reservoir ages also serve as a proxy for AMOC strength. We present regionally averaged HLNA reservoir ages for 0 to 41 thousand years before the present (kyr BP) based on over 500 radiocarbon dates from 33 North Atlantic cores. An early deglacial increase to >1000 14C yr reservoir ages between 18.5 and 16.5 kyr BP suggests reduced AMOC before peak Heinrich Stadial 1 (HS1) ice-rafted debris (IRD). A rapid decrease in reservoir ages coincident with the IRD maximum at 16 kyr BP indicates strong stratification of the upper water column caused by massive freshwater release.
The relationship between circulation weather types (WTs) and the spatial variability of precipitation across
the Iberian Peninsula were studied using a high density, quality controlled, homogenized monthly precipitation database with approximately 3000 stations and interpolated to a 10 km grid. The circulation WTs were computed using an objective version of the Lamb classification centered on the Iberian Peninsula. A total of 26 WTs were selected for the period 1948–2003. Daily WTs were grouped to obtain their monthly frequencies, and used as potential independent variables in a linear least-square non-negative regression model with a forward stepwise selection. Results show the impact of each WT on precipitation in the Iberian Peninsula with a spatial resolution which had never been achieved before and additionally were obtained on a monthly scale highlighting the large seasonal cycle observed in each class, and including significantly different patterns in winter and summer. Nevertheless, results confirm that most of the precipitation in the Iberian Peninsula is produced by just a few WTs, with W, SW and C being the most influential. The association between
WTs and precipitation is more robust in winter months and for the western IP areas, while it is lower during summer months and for the eastern IP areas. Spatial analysis revealed that precipitation on the Mediterranean coastland is mostly related with easterly flows (NE, E, SE and their hybrid counterparts), while on the Cantabrian coastland. N and NW flows are the most influential WTs. In general, cyclone-related types are the least frequent ones and also the most efficient in generating precipitation; while anticyclone-related types have the highest frequencies, but also the lowest contribution to total monthly rainfall in the Iberian Peninsula.
The Atlantic meridional overturning circulation affects the latitudinal
distribution of heat, and is a key component of the climate system.
Proxy reconstructions, based on sedimentary
231Pa/230Th ratios and the difference between
surface- and deep-water radiocarbon ages, indicate that during the last
glacial period, the overturning circulation was reduced during
millennial-scale periods of cooling. However, much debate exists over
the robustness of these proxies. Here we combine proxy reconstructions
of sea surface and air temperatures and a global climate model to
quantitatively estimate changes in the strength of the Atlantic
meridional overturning circulation during the last glacial period. We
find that, relative to the Last Glacial Maximum, the overturning
circulation was reduced by approximately 14Sv during the cold Heinrich
event 1. During the Younger Dryas cold event, the overturning
circulation was reduced by approximately 12Sv, relative to the preceding
warm interval. These changes are consistent with qualitative estimates
of the overturning circulation from sedimentary
231Pa/230Th ratios. In addition, we find that the
strength of the overturning circulation during the Last Glacial Maximum
and the Holocene epoch are indistinguishable within the uncertainty of
the reconstruction.
Paired Mg/Ca–δ18O measurements on multiple species of planktic foraminifera are combined with published benthic isotope records from south of Iceland in order to assess the role North Atlantic freshwater input played in determining the evolution of hydrography and climate during the last deglaciation. We demonstrate that Globigerina bulloides and Globorotalia inflata are restricted to intervals when warm Atlantic waters reached the area south of Iceland, and therefore Mg/Ca–δ18O data from these species monitor changes in the temperature and seawater δ18O signature of the northward inflow of Atlantic water to the area. In contrast, Neogloboquadrina pachyderma (sinistral) calcifies within local subpolar/polar waters and new Mg/Ca–δ18O analyses on this species document changes in this water mass. We observe two major surface ocean events during Heinrich Stadial 1 (∼ 17–14.7 ka): an early freshening of the Atlantic Inflow (∼ 17–16 ka), and a later interval (16–14.7 ka) of local surface freshening, sea-ice formation and brine rejection that was associated with a further reduction in deep ocean ventilation. Centennial-scale cold intervals during the Bølling–Allerød (BA, 14.7–12.9 ka) were likely triggered by the rerouting of North American continental run-off during ice-sheet retreat. However, the relative effects of these freshwater events on deep ventilation and climate south of Iceland appear to have been modulated by the background climate deterioration. Two freshwater events occurred during the Younger Dryas cold interval (YD, 12.9–11.7 ka), both accompanied by a reduction in deep ventilation south of Iceland: an early YD freshening of the Atlantic Inflow and local subpolar/polar waters, and a late YD ice-rafted detritus event that was possibly related to brine formation south of Iceland. Based on our reconstructions, the strengthening of the Atlantic Meridional Overturning Circulation at the onset of BA and Holocene may have been promoted by the subsurface warming of subpolar/polar water, brine formation that drew warm saline Atlantic water northwards, and the high background salinity of the Atlantic Inflow.
We present a high resolution speleothem record of the last deglaciation from NW Spain, which provides as an important link between the millennial climate variability well characterized in the North Atlantic and Greenland, and the correlative abrupt climate changes observed in high accumulation rate marine cores in the western Mediterranean. The nearly continuous record from stalagmite CANDELA, from 25.5 to 11.6 ky BP documents with high resolution and precise chronology the climate change in NW Iberia from the Late Glacial period through the end of the Younger Dryas. By combining trace element indicators of aridity with oxygen and carbon isotopic tracers sensitive to temperature and moisture-source, this record provides an integrated perspective on the climate changes experienced by the region. Carbon isotopic variations reflect temperature and humidity regulation of vegetation and soil respiration and dripwater degassing. Oxygen isotopic variations reflect a more complex array of processes including temperature-driven changes in isotopic fractionation during calcite precipitation, changes in sources of moisture in the hydrological cycle, and changes in seasonality of precipitation. Mg/Ca and Ba/Ca respond to the hydrological balance (P-E) through soil contact times and extent of prior calcite precipitation. This location in NW Spain is particularly sensitive to climate disruptions caused by changes in the North Atlantic Meridional Overturning Circulation (MOC). Stalagmite growth ceases only during the 2 ky shutdown of the MOC known as the Mystery Interval, but not during the preceding glacial maximum or GS-3 stages which are colder in Greenland and are periods in which speleothem growth is absent farther north on the Atlantic or Mediterranean coasts of France. Thus, in NW Iberia this Mystery Interval is potentially the coldest and driest interval of the presented record. Cold interludes in the North Atlantic region, such as Heinrich event 2, were characterized by more arid and cold conditions in NW Iberian Peninsula. In contrast, warm Greenland interstadial 2 (GI-2) was characterized by more humid conditions. The major glacial-interglacial transition is not synchronous among all climate indicators in the stalagmite. In oxygen isotopes, the main transition occurs during the hiatus between 18.2 and 15.4 ky BP; values after the hiatus are ~ 1 0/00 lighter than before and may reflect changes in precipitation source regions with the northward retreat of the polar front. In contrast, the other indicators (Mg/Ca, Ba/Ca and carbon isotopes) suggest that the major shift in humidity between dry glacial conditions and more humid interglacial conditions occurred between 15.4 to 13.4 ky BP. The increase in humidity is gradual and reaches its peak at 13.4 ky BP. This gradual change is consistent with that of speleothems from the Atlantic coast of France and lakes in the Pre-Pyrenees, but contrasts with the more abrupt change in temperature in Greenland and in the hydrological cycle in the Mediterranean which occurred at the onset of the Bølling about 14.7 ky BP. Carbon isotopes and Ba/Ca ratios indicate that the Younger Dryas represented a return to more arid conditions analogous to glacial times. Although both this site in NW Spain and the Mediterranean show a generally similar response of greater aridity during cold periods in Greenland, the different rates of response during deglaciation are suggestive of a different climate threshold for Mediterranean vs. Atlantic margin precipitation.
Prior reconstructions of the last glacial maximum and the Holocene did not include detailed paleoclimate data from the Greenland, Iceland and Norwegian (GIN) seas, which cover sensitive areas for oceanographic boundaries, sea ice distribution and deep water formation. In order to fill this gap, we studied nine well dated sediment cores from this area and reconstructed the paleoceanographic conditions of the surface GIN seas at 2000-year intervals since 13,400 BP based on the diatom record. Results show that a N.-S. extending sea-ice-free corridor had opened along Norway already at 13,400 BP, indicating a northward flow of a branch of the North Atlantic Drift, possibly caused by a change in the jet stream flow due to the decreased height of the North American ice sheet. A major change of climatic conditions occurred over the GIN seas as the insolation anomaly reached its maximum around 9000 BP, when the sea ice cover and the oceanic fronts retreated to a northwesterly position along Greenland, and the ocean temperatures rose. The first half of the Holocene is recorded as the warmest period during the last 13,400 BP in the GIN seas. The duration of this climatic optimum decreased both toward the north and toward the margins of the area. The second half of the Holocene is characterized by a cooling trend in step with the decreasing insolation. These results support the proposal of Imbrie et al. (1992) that the GIN seas region acts as an initial responder to insolation changes.
We present evidence for freshwater forcing from the Greenland Ice Sheet during the last deglaciation, explore its potential involvement in the Younger Dryas cooling, and investigate the role of Atlantic Intermediate Water in the retreat of the Greenland Ice Sheet. Paleoceanographic conditions and ice-sheet retreat history are interpreted from IRD, stable isotopes and foraminiferal assemblages in sediment cores retrieved along the length of the Kangerlussuaq Trough, a cross-shelf trough on the southeast Greenland shelf. The Vedde Ash (11,980±cal. yr BP), which is a marker bed within the Younger Dryas, and the Saksunarvatn tephra (10,180±cal. yr BP) an early Holocene marker horizon, have been located in these cores. The tephra markers and calibrated 14C dates on foraminifers and molluscs provide the basis for the age models in the cores. The stable isotope records indicate a 0.5 0/00 depletion of the 18O between 13.4 and 12.8 cal ka BP. A pronounced light isotope event, exceeding 1.5 0/00 on the inner shelf and 0.8 0/00 on the outer shelf, occurs at the beginning of the Younger Dryas Cold period (12.8 cal. ka BP) and ends by 12 cal ka BP, immediately before deposition of the Vedde Ash. The light isotope spikes are interpreted as glacial meltwater spikes associated with retreat of the adjacent Greenland Ice Sheet. Associated with the light isotope events are episodic high percentages of the Atlantic Intermediate Water benthic foraminiferal species, Cassidulina neoteretis, which continue from the Bølling/Allerød into the Younger Dryas. High percentages of this species suggest that Atlantic Intermediate Water entered the Kangerlussuaq Trough during the Allerød and the Younger Dryas via the Irminger Current, as it does today. We conclude that advection of Atlantic Intermediate Water into the Kangerlussuaq Trough, south of the Denmark Strait, promoted melting and retreat of the marine portions of the Greenland Ice Sheet, explaining the observed glacial meltwater spikes.
This work focuses on a 1900-year section of varved sediments from Lake Meerfelder Maar (MFM) extending from the Late Allerød to the Preboreal. Varve counting provides the chronological framework and determines the length of Younger Dryas to 1025–1090 years. A strong relation between climate change, environment response and depositional processes has been found. In consequence, varve microfacies variations are a sensitive proxy for environment changes. These are reflected, for example, in erosion processes within the lake’s catchment (minerogenic input) and lake productivity (diatom blooms). The observed varve changes have been quantified by multiproxy analyses of physical and chemical sediment parameters with a resolution of between 8 and 40 years depending on sedimentation rate. In addition, high resolution palynological investigations provide the biostratigraphical subdivision based on changes in the vegetation occurring during the same time interval. Varve observations reveal that environment changes at the beginning and the end of the Younger Dryas occurred within 20–50 years. Furthermore, sediment and vegetation changes were synchronous. Within the actual precision of the MFM and GRIP chronologies (divergence of only a few decades) terrestrial responses in Western Europe occurred quasi-synchronous to temperature changes in Greenland.
The IMAGES core MD99-2343, recovered from a sediment drift north of the island of Minorca, in the north-western Mediterranean Sea, holds a high-resolution sequence that is perfectly suited to study the oscillations of the overturning system of the Western Mediterranean Deep Water (WMDW). Detailed analysis of grain-size and bulk geochemical composition reveals the sensitivity of this region to climate changes at both orbital and centennial–millennial temporal scales during the last 50 kyr. The dominant orbital pattern in the K/Al record indicates that sediment supply to the basin was controlled by the insolation evolution at 40°N, which forced changes in the fluvial regime, with more efficient sediment transport during insolation maxima. This orbital control also modulated the long-term pattern of the WMDW intensity as illustrated by the silt/clay ratio.
High resolution reconstructions of sea surface temperature (Uk′37-SST), coccolithophore associations and continental input (total organic carbon, higher plant n-alkanes, n-alkan-1-ols) in core D13882 from the shallow Tagus mud patch are compared to SST records from deep-sea core MD03-2699 and other western Iberian Margin cores. Results reveal millennial-scale climate variability over the last deglaciation, in particular during the LGIT. In the Iberian margin, Heinrich event 1 (H1) and the Younger Dryas (YD) represent two extreme episodes of cold sea surface condition separated by a marine warm phase that coincides with the Bølling–Allerød interval (B–A) on the neighboring continent. Following the YD event, an abrupt sea surface warming marks the beginning of the Holocene in this region. SSTs recorded in core D13882 changed, however, faster than those at deep-sea site MD03-2699 and at the other available palaeoclimate sequences from the region. While the SST values from most deep-sea cores reflect the latitudinal gradient detected in the Iberian Peninsula atmospheric temperature proxies during H1 and the B-A, the Tagus mud patch (core D13882) experienced colder SSTs during both events. This is most certainly related to a supplementary input of cold freshwater from the continent to the Tagus mud patch, a hypothesis supported by the high contents of terrigenous biomarkers and total organic carbon as well as by the dominance of tetra-unsaturated alkenone (C37:4) observed at this site.