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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
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... 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). ...
... 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.
... 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 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.
... 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.
... 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.
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.
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.
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.
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.
Greenland Stadial 1 (GS-1) was the last of a long series of severe cooling episodes in the Northern Hemisphere during the last glacial period. Numerous North Atlantic and European records reveal the intense environmental impact of that stadial, whose origin is attributed to an intense weakening of the Atlantic Meridional Overturning Circulation in response to freshening of the North Atlantic. Recent high-resolution studies of European lakes revealed a mid-GS-1 transition in the climatic regimes. The geographical extension of such atmospheric changes and their potential coupling with ocean dynamics still remains unclear. Here we use a subdecadally resolved stalagmite record from the Northern Iberian Peninsula to further investigate the timing and forcing of this transition. A solid interpretation of the environmental changes detected in this new, accurately dated, stalagmite record is based on a parallel cave monitoring exercise. This record reveals a gradual transition from dry to wet conditions starting at 12,500 y before 2000 A.D. in parallel to a progressive warming of the subtropical Atlantic Ocean. The observed atmospheric changes are proposed to be led by a progressive resumption of the North Atlantic convection and highlight the complex regional signature of GS-1, very distinctive from previous stadial events.
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 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.
The Younger Dryas (YD) cold interval is one of the most abrupt climate events of Earth's recent history. The origin of this rapid, severe cooling episode is still widely debated, but it was probably triggered by a large freshwater influx to the North Atlantic resulting in disruption of the Atlantic Meridional Overturning Circulation. The YD termination, despite having been even more abrupt than the onset has, however, received significantly less attention. Here using multi-proxy data from a high-resolution marine sediment record, we present evidence for a gradual decrease of the Labrador Current influence, northward migration of the Gulf Stream oceanic front and a rapid decline of sea-ice cover at the YD termination. Our data indicate a stepwise sequence of events with changes in ocean circulation clearly preceding those in atmospheric conditions, in contrast to the hitherto commonly assumed single-event rapid climatic shift at the YD-Holocene transition.
The present study attempts to carry out a biogeographic and bioclimatic ap-proach using a specialized vegetation type such as that of heathlands (Calluno-Ulice-tea class) in the Iberian Peninsula and Northern Morocco. The territories in which this vegetation occurs have been divided into 63 small units based on the accepted biogeogra-phy of the area. The heathland flora of each unit was surveyed using phytosociological data from bibliography and other sources. Finally, each of the territorial units has a list of species drawn up being representative of the floristic composition of their heathland vegetation. The total amount of plants was limited to 289, as rare plants were discarded. Species from this list were classified into 12 groups according to their phytosociological affinity (Calluno-Ulicetea, Quercetea ilicis, Querco-Fagetea, Festuco-Bro-metea, etc.) in order to identify the floristic influences in each of the territories. The original matrix with 63 columns and 289 rows was set and several ordinations were carried out, particularly PCA and canonical RDA. The floristic composition of the list of total species for each territory, strongly reflects the biogeographic and bioclimatic conditions. This is valid for the genuine Calluno-Ulicetea flora as well as for the rest of the floristic groups. The main climatic parameters which explain the differences between the heathland flora of the territories considered are mostly P (annual precipitation), It (ther-micity index) and Ic (continentality index). Rainy areas, located in the Cantabrian and Atlantic areas, have a large number of Calluno-Ulicetea plants, whilst continental areas, particularly in the Eastern Pyrenees, have low numbers.
THE warming at the end of the last glaciation was characterized by a
series of abrupt returns to glacial climate, the best-known of which is
the Younger Dryas event1. Despite much study of the causes of
this event and the mechanisms by which it ended, many questions remain
unresolved1. Oxygen isotope data from Greenland ice
cores2-4 suggest that the Younger Dryas ended abruptly, over
a period of about 50 years; dust concentrations2,4 in these
cores show an even more rapid transition (<~20 years). This extremely
short timescale places severe constraints on the mechanisms underlying
the transition. But dust concentrations can reflect subtle changes in
atmospheric circulation, which need not be associated with a large
change in climate. Here we present results from a new Greenland ice core
(GISP2) showing that snow accumulation doubled rapidly from the Younger
Dryas event to the subsequent Preboreal interval, possibly in one to
three years. We also find that the accumulation-rate change from the
Oldest Dryas to the Bø11ing/Allerød warm period was large
and abrupt. The extreme rapidity of these changes in a variable that
directly represents regional climate implies that the events at the end
of the last glaciation may have been responses to some kind of threshold
or trigger in the North Atlantic climate system.
We present two new high-resolution sediment records from the southwestern Iceland and Norwegian Seas that were dated by numerous 14C ages up to 54 14C ka BP. Based on various lines of evidence, the local 14C 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 d180 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 14C ages between 20 and 54 14C ka BP. Generally the results confirm the 14C 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 14C shifts after 45 14C 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 14C and 10Be production, giving rise to excess 14C in the atmosphere of up to 1200‰.
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 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 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.
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.
 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
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.
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.
A palaeo sea ice reconstruction for northern Norway for the interval ca
13.8-7.2 cal ka BP has been performed using a combined
organic geochemical proxy-based study of a marine sediment core
(JM99-1200) obtained from Andfjorden (69° 15.95‧ N, 16°
25.09‧ E) and the outcomes compared with a series of other
palaeoclimate studies carried out previously. Within this study,
particular emphasis has been placed on the identification of the sea ice
conditions during the Younger Dryas and the application of different
biomarker-based proxies to both identify and quantify seasonal sea ice
conditions. Firstly, the appearance of the specific sea ice diatom proxy
IP25 at ca 12.9 cal ka BP in the sedimentary record
has provided an unambiguous but qualitative measure of seasonal sea ice
and thus the onset of the Younger Dryas stadial. The near continuous
occurrence of IP25 for the next ca 1400 yr demonstrates
seasonal sea ice during this interval, although variable abundances
suggest that the recurrent conditions in the early-mid Younger Dryas (ca
12.9-11.9 cal ka BP) changed significantly from stable
to highly variable sea ice conditions at ca 11.9 cal ka BP and
this instability in sea ice prevailed for the subsequent ca 400 yr,
during which, an extremely short interval of permanent or near-permanent
sea ice was observed at ca 11.75 cal ka BP. At ca
11.5 cal ka BP, IP25 disappeared from the record
indicating ice-free conditions that signified the beginning of the
Holocene. The IP25 concentrations were also combined with
those of the open water phytoplankton biomarker brassicasterol to
generate PBIP25 data from which more quantitative
measurements of sea ice were determined. PBIP25
data were consistent with, consecutively, seasonal then infrequent sea
ice cover for the early-mid and late Younger Dryas, while further
comparisons of the PBIP25 data with the outcomes
of previous correlations with modelled sea ice concentrations suggested
that these intervals were characterised by ca 60-90% and ca
0-60% sea ice cover, respectively. Both qualitative and
quantitative sea ice records were also consistent with sea surface
temperature and other palaeoclimate estimates derived previously from
various other proxy studies. The contrasting seasonal sea ice conditions
during the Younger Dryas were further verified through a comparison of
the concentrations of IP25 with those of another highly
branched isoprenoid (HBI) alkene that is di-unsaturated and believed to
also be produced by sea ice diatoms. The ratio of the HBI diene to
IP25, termed DIP25, is believed to provide a
useful indicator of stability or variability in sea ice conditions and
complements the outcomes from the IP25 and
PBIP25 index data. The identification of
contrasting seasonal sea ice conditions during the Younger Dryas will
likely contribute to the debate regarding the climatic impacts that took
place during this intriguing interval.