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Climate changes during the Lateglacial in South Europe: new insights based on pollen and brGDGTs of Lake Matese in Italy

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Abstract

The Lateglacial (14,700–11,700 cal BP) is a key climate period marked by rapid but contrasted changes in the Northern Hemisphere. Indeed, regional climate differences have been evidenced during the Lateglacial in Europe and the Northern Mediterranean areas. However, past climate patterns are still debated since temperature and precipitation changes are poorly investigated towards the lower European latitudes. Lake Matese in Southern Italy is a key site in the Central Mediterranean to investigate climate patterns during the Lateglacial. This study uses a multi-proxy approach including magnetic susceptibility, geochemistry (XRF core scanning), pollen data and molecular biomarkers like branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) to reconstruct climate changes and their impacts at Matese. Palaeotemperatures and -precipitation patterns are quantitatively inferred from pollen assemblages (multi-method approach: Modern Analogue Technique, Weighted Averaging Partial Least Squares regression, Random Forest, and Boosted Regression Trees) and brGDGTs calibrations. The results are compared to a latitudinal selection of regional climate reconstructions in Italy to better understand climate processes in Europe and in the circum-Mediterranean region. A warm Bølling–Allerød and a marked cold Younger Dryas are revealed in all climate reconstructions inferred from various proxies (chironomids, ostracods, speleothems, pollen, brGDGTs), showing no latitudinal differences in terms of temperatures across Italy. During the Bølling–Allerød, no significant changes in terms of precipitation are recorded, however, a contrasted pattern is visible during the Younger Dryas. Slightly wet conditions are recorded south of latitude 42° N whereas dry conditions are recorded north of latitude 42° N. During the Younger Dryas, cold conditions can be attributed to the southward position of North Atlantic sea-ice and of the Polar Frontal JetStream whereas the increase of precipitation is Southern Italy seems to be linked to relocation of Atlantic storm tracks into the Mediterranean, induced by the Fennoscandian ice sheet and the North European Plain. On the contrary, during the Bølling–Allerød warm conditions can be linked to the northward position of North Atlantic sea-ice and of the Polar Frontal JetStream.

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Soil-derived branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine river fan sediments have a potential use for determining changes in the mean annual temperature (MAT) and pH of the river watershed soils. Prior to their incorporation in marine sediments, the compounds are transported to the marine system by rivers. However, emerging evidence suggests that the brGDGTs in freshwater systems can be derived from both soil run-off and in situ production. The production of brGDGTs in the river system can complicate the interpretation of the brGDGT signal delivered to the marine system. Therefore, we studied the distribution of brGDGT lipids in suspended particulate matter (SPM) of the Yenisei River. Chromatographic improvements allowed quantification of the recently described hexamethylated brGDGT isomer, characterized by having two methyl groups at the 6/6′ instead of the 5/5′ positions, in an environmental dataset for the first time. This novel compound was the most abundant brGDGT in SPM from the Yenisei. Its fractional abundance correlated well with that of the 6-methyl isomer of the hexamethylated brGDGT that contains one cyclopentane moiety. The Yenisei River watershed is characterized by large differences in MAT (>11 °C) as it spans a large latitudinal range (46–73°N), which would be expected to be reflected in brGDGT distributions of its soils. However, the brGDGT distributions in its SPM show little variation. Furthermore, the reconstructed pH values are high compared to the watershed soil pH. We, therefore, hypothesize that the brGDGTs in the Yenisei River SPM are predominantly produced in situ and not primarily derived from erosion of soil. This accounts for the absence of a change in the temperature signal, as the river water temperature is more stable. Using a lake calibration, the reconstructed temperature values agree with the mean summer temperatures (MST) recorded. The brGDGTs delivered to the sea by the Yenisei River during this season are thus not soil-derived, possibly complicating the use of brGDGTs in marine sediments for palaeoclimate reconstructions.
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Lake-level records from Italy suggest that patterns of precipitation in the central Mediterranean during the Holocene were divided between the north and south, but a scarcity of reliable palaeoclimatic records in the north and central-southern Mediterranean means new evidence is needed to validate this hypothesis. We provide robust quantitative estimates of Holocene climate in the Mediterranean region using four high-resolution pollen records from northern (Lakes Ledro and Accesa) and southern (Lakes Trifoglietti and Pergusa) Italy. Multiple methods are used to provide an improved assessment of the palaeoclimatic reconstruction uncertainty. The multi-method approach uses the pollen-based weighted averaging, weighted-average partial least-squares regression, modern analogue technique, and the non-metric multidimensional scaling/generalized additive model methods. We use independent lake-level data to validate the precipitation reconstructions. Our results support a climatic partition between northern and southern Italy during the Holocene, confirming the hypothesis of opposing mid-Holocene summer precipitation regimes in the Mediterranean. The northern sites (Ledro, Accesa) are characterized by minima for summer precipitation and lake levels during the early to mid-Holocene, while the southern sites (Trifoglietti, Pergusa) are marked by maxima for precipitation and lake levels at the same time. Both pollen-inferred precipitation and lake levels indicate the opposite pattern during the late Holocene, a maximum in northern Italy and a minimum in southern Italy/Sicily. Summer temperatures show the same partitioning, with warm conditions in northern Italy and cool conditions in Sicily during the early/mid-Holocene, and a reversal during the late Holocene. Comparison with marine cores from the Aegean Sea suggests that climate trends and gradients observed in Italy show strong similarities with those recognized from the Aegean Sea, and more generally speaking in the eastern Mediterranean.
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A high-resolution pollen record from Lago Trifoglietti in Calabria (southern Italy) provides new insights into the paleoenvironmental and palaeoclimatic changes which characterise the Holocene period in the southern Italy. The chronology is based on 11 AMS radiocarbon dates from terrestrial organic material. The Holocene history of the vegetation cover shows the persistence of an important and relatively stable Fagus forest present over that entire period, offering a rare example of a beech woodstand able to withstand climate changes for more than 11 000 yr. Probably in relation with early Holocene dry climate conditions which affected southern Italy, the Trifoglietti pollen record supports a southward delay in thermophyllous forest expansion dated to ca. 13 500 cal BP at Monticchio, ca. 11 000 cal BP at Trifoglietti, and finally ca. 9800 cal BP in Sicily. Regarding the human impact history, the Trifoglietti pollen record shows only poor imprints of agricultural activities and anthopogenic indicators, apart from those indicating pastoralism activities beneath forest cover. The selective exploitation of Abies appears to have been the strongest human impact on the Trifoglietti surroundings. On the basis of (1) a specific ratio between hygrophilous and terrestrial taxa, and (2) the Modern Analogue Technique, the pollen data collected at Lago Trifoglietti led to the establishment of two palaeoclimatic records tracing changes in (1) lake depth and (2) annual precipitation. On a millennial scale, these records give evidence of increasing moisture from ca. 11 000 to ca. 9400 cal BP and maximum humidity from ca. 9400 to ca. 6200 cal BP, prior to a general trend towards the drier climate conditions that have prevailed up to the present. In addition, several successive centennial-scale oscillations appear to have punctuated the entire Holocene. The identification of a cold dry event around 11 300 cal BP, responsible for a marked decline in timberline altitude and possibly equivalent to the PBO, remains to be confirmed by further investigations verifying both chronology and magnitude. Two cold and possibly drier Boreal oscillations developed at ca. 9800 and 9200 cal BP. At Trifoglietti, the 8.2 kyr event corresponds to the onset of cooler and drier climatic conditions which persisted until ca. 7500 cal BP. Finally, the second half of the Holocene was characterised by dry phases at ca. 6100-5200, 4400-3500, and 2500-1800 cal BP, alternating with more humid phases at ca. 5200-4400 and ca. 3500-2500 cal BP. Considered as a whole, these millennial-scale trends and centennial-scale climatic oscillations support contrasting patterns of palaeohydrological changes recognised between the north- and south-central Mediterranean.
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This study presents pollen-based climate reconstructions of Holocene temperature and precipitation seasonality for two high-resolution pollen sequences from the central (Lake Accesa, central Italy) and eastern Mediterranean (Tenaghi Philippon, Greece) regions. The quantitative climate reconstruction uses multiple methods to provide an improved assessment of the uncertainties involved in palaeoclimate reconstructions. The multimethod approach comprises Partial Least Squares regression, Weighted Average Partial Least Squares regression, the Modern Analogues Technique, and the Non-Metric-Multidimensional Scaling/Generalized Additive Model method. We find two distinct climate intervals during the Holocene. The first is a moist period from 9500 to 7800 cal. BP characterised by wet winters and dry summers, resulting in a strongly seasonal hydrological contrast (stronger than today) that is interrupted by a short-lived event around 8200 cal. BP. This event is characterised by wet winters and summers at Accesa whereas at Tenaghi Philippon the signal is stronger, reversing the established seasonal pattern, with dry winters and wet summers. The second interval represents a later aridification phase, with a reduced seasonal contrast and lower overall precipitation, lasting from 7800 to 5000 cal. BP. Present-day Mediterranean conditions were established between 2500 and 2000 cal. BP. Many studies show the Holocene to have a complex pattern of climatic change across the Mediterranean regions. Our results confirm the traditional understanding of an evolution from wetter (early Holocene) to drier climatic conditions (late Holocene), but highlight the role of changing seasonality during this time. Our data yield new insights into the aspect of seasonality changes, and explain the apparent discrepancies between the previously available climate information based on pollen, lake-levels and isotopes by invoking changes in precipitation seasonality.
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To reconstruct changes in vegetation, temperature, and sediment geochemistry through the last 6.5 cal ka BP, in the Subcarpathian belt of the Eastern Carpathians (Romania), pollen, branched glycerol dialkyl glycerol tetraethers (brGDGTs) and X-ray fluorescence analyses have been integrated. Pollen and brGDGTs (a bacterial lipid biomarker proxy) are used as paleothermometers for reconstructing the mean annual air temperature (MAAT) and mean temperature above freezing (MAF), respectively. Both proxies show roughly consistent records. The highest MAAT and MAF occurs during the oldest part of the record (from 6.5 to 4.2 cal ka BP), and the Middle to the Late Holocene shift is marked by a prominent decrease in temperature between 5.4 and 4.2 cal ka BP, coinciding with Bond event 4 and 3. This transition is coeval with a decrease in summer insolation, shift from consistent NAO- conditions to a predominance of NAO+ phase and coincides with the beginning of the Neoglacial cooling in northern latitudes. The warm bias in the MAF reconstruction during the Late Holocene is explained as a change in the lipid provenance or in the composition of the brGDGT producers after 4.2 cal ka BP.
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The Lateglacial period is marked by strong changes in environment and climate in Europe, associated to the different steps of the deglaciation. This paper aims to reconstruct the vegetation and climate changes from the deglaciation to the early Holocene (15.2–10.8 ka cal BP) in Western Europe. Pollen investigations are performed at a centennial resolution on a sedimentary core from the Lapsou peat bog (Cantal, France). The chronology is well constrained from three radiocarbon dates and a double tephra layer. The pollen record and quantitative climate reconstructions evidence strong changes associated with the Lateglacial period, in agreement with the regional vegetation and climate dynamics. At the onset of Lateglacial Interstadial (14.5–14.1 ka cal BP), a Juniperus - Betula succession is attributed to the Bølling. Associated climate reconstructions indicate a winter warming of 10 °C and precipitations above 1000 mm. A development of Pinus forests is then evidenced for the Allerød, for which rather constant warm conditions and lower precipitations (drop of 200 mm) are reconstructed. A two-phase pattern characterizes the Younger Dryas (12.6–11.5 ka cal BP): after a first phase with rather constant vegetation and climate conditions compared to the Allerød, a more steppic vegetation developed from 12.2 ka cal BP, indicating a marked cooling and a pronounced aridity (precipitations around 400 mm). Finally, the reconstruction of a cool deciduous forest dominated by Betula and temperate climate conditions defines the early Holocene. A comparison with chironomid-inferred reconstructions of July temperature from the close La Roustière site allows to support pollen-inferred climate quantifications, also showing the interest of applying multi-proxy approach in palaeo-environmental studies.
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Relationships between steppe vegetation, human practices and climate changes in the past are crucial to disentangle human development in Eurasia. In this frame, our study investigates (1) modern pollen-vegetation relationships and (2) changes in vegetation, human activity and climate in the Holocene record of Vanevan peat (south-eastern shore of Lake Sevan, Armenia), using a multiproxy approach including sediment geochemistry (XRF), pollen, Non-Pollen Palynomorphs (NPPs), and branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs). Climate reconstructions are provided by (1) water-level changes, (2) brGDGTs, and (3) pollen transfer functions (multi-method approach: Modern Analogue Technique, Weighted Averaging Partial Least Squares regression, Random Forest, and Boosted Regression Trees). Modern pollen assemblages are selected along an altitudinal transect in Armenia. They show a dominance of Chenopodiaceae in semi-desert/steppe regions while meadows steppes, subalpine, and alpine meadows are dominated by Poaceae. Past vegetation is characterized by steppes dominated by Poaceae surrounded during the Mid-Holocene (8200–4200 a cal BP) by scarce open woodlands. Humans have influenced the local vegetation, mainly through their agricultural practices present since 5200 a cal BP with several intensification steps. Our reconstruction indicates a climate shift from a cold and arid Early Holocene toward a warmer and more humid Mid-Late Holocene. An aridification trend marks the last 5000 years causing a drop in water level, which allowed humans to live and cultivate on Lake Sevan shores. Arid events are recorded at 6.2 ka, 5.2 ka, 4.2 ka and 2.8 ka a cal BP, which are commonly related to multi-centennial-scale variations of Westerlies activity (North Atlantic Oscillation). Through our temperature reconstruction, we can assign (1) the 5.2 and 2.8 ka events as being cold and probably related to a strong Siberian High, and (2) the 4.2 ka event as being warm associated with high Arabian subtropical pressures in the South Caucasus and the Near East. Our study suggests a significant impact of these arid events on the Lake Sevan shore populations and they are consistent with cultural phases in the South Caucasus, thus showing the impact of climatic variations on cultural, land use and occupation mode development in this crossroad region between Europe, Africa and Asia.
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A coupled pollen-brGDGT paleoclimate reconstruction approach has been tested to provide independent and robust estimates of Holocene climate and environment changes in the extremely arid environment of the mountainous areas ranging from northern Arid Central Asia (ACA) to the Mongolian Plateau. The two proxies were calibrated for both global and local modern data sets (NMSDB). This multi-proxy approach was then applied to a sediment core collected from Lake Ayrag, Arkhangai, covering the Late Holocene. In addition to brGDGTs and pollen, we also performed magnetic susceptibility, micro-XRF, elemental and isotopic bulk chemistry, and Non-Pollen Palynomorph (NPP) analyses on the Lake Ayrag sediments in order to better understand the lake system and human impact dynamics. While the globally calibrated record (both for pollen and brGDGTs) displayed a slight millennial-scale cooling, the locally calibrated results exhibit centennial-scale climate oscillations such as the 4.2 and 3.5 kyr events, the Roman Warm Period (RWP), Dark Ages Cold Period (DACP), Medieval Warm Period (MWP) and Little Ice Age (LIA). These climate oscillations and vegetation changes are discussed with regard to the main Mongolian human historical occupation events documented by pastoralism proxies, especially the Xiongnu, Mongol Empire, Mandchou and Soviet periods. The climate systems currently dominating the Mongolian Plateau are difficult to resolve because inter-annual climate variability is pronounced. However, precipitation mainly occurs in summer (easterly monsoon driven) when the winter Westerlies lead the air mass movement. In the past, both pollen and biomarkers exhibited anti-correlated trends with annual precipitation and temperature: over the last 4000 kcal yr BP, the warm periods (MWP, RWP) were dry and the cold periods (LIA, DACP, 3.5 kyrs) were humid. Thus, the East Asian Summer Monsoon (i.e., warm and wet conditions dominant during summer) seems not to have influenced central Mongolian climate during the Late Holocene, which could have remained dominated by the Westerlies/Siberian High cells conflict. A comparison between the Ayrag record and other paleoclimate records from the Baikal area (Dulikha), Mongolian Plateau (D3L6, D1L1, NRX, ATM), and continental China (Kesang, Baluk and Tonnel caves, XRD section) to the Loess Plateau (Huangye and Xianglong caves) suggests that the monsoon front has oscillated since the Early Holocene. A climate synthesis following strictly the same approach (locally calibrated brGDGTs vs. pollen-inferred climate) for all the ACA records available for the Late Holocene helps us to resolve the climate systems paced by centennial to millennial-scale oscillations and their consequences for human societies.
Article
Despite widespread use of branched glycerol dialkyl glycerol tetraethers (brGDGTs) for paleo-temperature reconstruction, no global calibration for their application in lakes has been generated since improved analytical methods have allowed for the separation of the structural isomers. This is a substantial obstacle for the application of this tool as soil calibrations underestimate temperature values when applied to lake sediments. Here, we present a comprehensive dataset (N=272) of lacustrine brGDGT distributions, consisting of both new and previously reported samples, spanning a wide range of geographical locations, air temperatures, and lakewater pH values. Empirical Orthogonal Function (EOF) analysis on the fractional abundance of brGDGTs indicates that temperature exerts a strong control (explaining 58% of the variance) on brGDGT distribution. The influence of water chemistry is weaker, with pH and conductivity explaining 24% of the variance. We use our dataset to generate a new Bayesian calibration to the mean temperature of Months Above Freezing (MAF), which has an R2=0.82 and RMSE = 2.9°C. Application of the new temperature calibration to a previously published lake core record spanning the last 900 years demonstrates that it generates values comparable to instrumental observations of MAF. Our new calibration facilitates the use of lacustrine brGDGTs to reconstruct continental temperatures, a vital piece of information for understanding past climates.
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The top 12 m of a lacustrine sediment sequence from Lago di Mezzano (42°36′N, 11°46′E, 452 a.s.l., Latium, central Italy) have been palynologically investigated. The chronology was established on the basis of radiocarbon dates, measures of annual laminations and volcanic ashes. The continuous sequence provides new fundamental information on the Lateglacial and Holocene, periods often fragmented in Italian pollen records, identifying vegetation dynamics and climate changes occurring in the last 15,300 years. The record starts at the end of the last glacial, closely following the climate changes found at a global scale, with clear stadial and interstadial oscillations, and evidence of cold/dry and wet/warm climatic changes. The Holocene is mostly characterized by forests, evolving rapidly to mature mixed deciduous oak (starting from 11,000 years BP) and, passing through beech forests (starting from ca. 9000 years BP) and alder carrs (starting from ca. 7000 years) to the deeply human-modified present-day landscape. Evaluating the importance of the local and regional human impact is not straightforward. A peculiarity of Lago di Mezzano, as of other Mediterranean lacustrine sites, is that it behaves at times as an “on-site” locale, when human populations were immediately by the lake, and at times as an “off-site” when settlements were found further away. This pattern changed according to cultural and climatic variations, both correlated to the water level of the lake.
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This study presents, for the first time, a detailed quantitative reconstruction of winter (January) and summer (July) palaeotemperatures from the Late Pleistocene to Holocene transition in central Italy based on ostracod assemblages in an 8.59-m-long sediment core retrieved in Lake Trasimeno. Of 19 ostracod species recovered, 13 were calibrated according to their living temperature ranges, enabling us to reconstruct mean January and July temperature ranges using the mutual ostracod temperature range (MOTR) method. The occurrences of Cytheromorpha fuscata and Limnocythere suessenbornensis from 44 000 to 25 500 cal. a BP showed mean January temperatures at least 7°C colder and mean July temperature at least 1°C cooler in some intervals compared to present-day temperatures. Comparison of the MOTR-derived January minima curve with a Greenland oxygen isotope record (NGRIP) shows a remarkable correlation of warmer Greenland Interstadial and the colder Greenland Stadial events with clear peaks and troughs in the MOTR signal. These correlationswere tested successfully by tuning the MOTR curve to the NGRIP record, resulting in an improved age-depth model combining radiocarbon ages with MOTR tie points. The results demonstrate that a record of rapid climate change in the North Atlantic region is archived in lacustrine ostracod assemblages in central Italy.
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The evolution of temperatures during the Holocene is controversial, especially for the early Holocene. The occurrence of the Holocene Thermal Maximum (HTM) during the early Holocene has recently been reconsidered and seasonal biases have been suggested in the paleoclimatic proxies. High regional variability and a low number of reliable and continuous quantitative reconstructions compared with the oceanic realm further complicate study of the Holocene climate in the continental realm. We analyzed branched glycerol dialkyl glycerol tetraethers (brGDGTs), an organic paleothermometer, and palyno-logical signals as part of a multiproxy analysis of the sedimentary record from Lake St Front, in the Massif Central (France). Identification of a shift in brGDGT sources through the Holocene required removing terrigenous influences from the temperature signal. BrGDGT-and pollen-inferred paleotemperature reconstructions (based on the Modern Analog Technique and the Weighted Averaging Partial Least Squares method) were compared. Both showed a thermal maximum during the early Holocene followed by a decrease of temperatures. We evaluated biases which could potentially influence the reconstructed signal. There was no evidence for a summer temperature bias either for brGDGT-derived temperatures or for pollen-derived temperatures. The Lake St Front data, in agreement with other regional records, confirm the occurrence of the HTM as a general warm period during the early Holocene followed by mid-Holocene cooling in Western Europe and suggest that seasonal biases are not the main explanation of the Holocene conundrum d the disagreement between model simulations and proxy-based temperature reconstructions for the northern hemisphere.
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Rugged peaks, large intermontane basins and frequent seismicity all characterize the active extensional tectonic setting of the southern Apennines. The Matese ridge typifies the active tectonic setting of the southern Apennines with steep carbonate mountain fronts and large depositional centres. Moderate to high magnitude earthquakes have affected the northern, western and eastern sectors of the Matese ridge in historical times. However, the seismogenic potential of the extensional fault system bounding the southern Matese mountain front has not been fully assessed to date. To unravel the active tectonic setting of the southern Matese mountain front, we have carried out a comprehensive geomorphological and tectonic-geomorphology investigation of the mountain front and its piedmont and have constrained results through chronological (i.e., tephrostratigraphical and 40Ar/39Ar) and structural data. Our study highlights that in the last ~600 ka, activity along E-Wtrending normal faults has identified a locus of higher slip rate tectonic activity in the central part of the analysed mountain front. These active E-W-striking normal faults are inherited, reactivated structures, which have interacted with newly formed NW-SE-striking normal faults during NE-SWextension active on the regional scale, causing fault bending and local extension to be oriented N-S. Consequently, lower slip rates have been recorded along the NW-SE-striking normal faults at the north-western and south-eastern tips of the southern Matese front. The long-term displacement rate of the fault system at the boundary of the central part of the southern Matese front is consistent with mean values of displacement of faults that, in the southern Apennines, show evidence of activity during the late Quaternary. Despite strong historical seismicity clustering primarily around the study area, our data highlight that it cannot be ruled out that moderate to high magnitude seismicity could affect the southern Matese mountain front. Our case study represents an example of the possible modes of formation and evolution of mountain front-basin systems in extensional setting, and shows how the combination of different data sets allows unravelling the interaction between tectonic, erosional and sedimentary processes, which lead to landscape evolution of active mountain belts.
Article
Among the new proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs), the BIT index (Branched and Isoprenoid Tetraether index) is one of the most difficult to determine accurately, as shown by two round-robin GDGT studies. Sensitivity to mass spectrometer settings and tuning, and a diversity of mass spectrometry techniques may explain the relatively large observed interlaboratory scatter. However, the mass defect difference between crenarchaeol and branched GDGTs (brGDGTs) has never been specifically scrutinized. In this study, we analyzed five sediment samples with contrasting BIT values using about 60 m/z values to assess the shape of GDGT peaks using selected ion monitoring. We then assessed the biases in relative GDGT signals and mass spectrometry-derived BIT values under two scenarios which ignore the systematic mass defect difference between crenarchaeol and brGDGTs. Our results show that approximate mass selection for GDGT analysis using selected ion monitoring generates losses of relative GDGT signals of up to 36%. The observed effects on BIT values are maximal for intermediate BIT values, with shifts of BIT values of ±0.1 unit. The shifts of BIT values due to approximate mass selection are thus not negligible compared to the interlaboratory scatter evidenced by the latest round-robin GDGT study.
Article
Our understanding of deglacial climate history in the southern Great Lakes region of the United States is primarily based upon fossil pollen data, with few independent and multi-proxy climate reconstructions. Here we introduce a new, well-dated fossil pollen record from Stotzel-Leis, OH, and a new deglacial temperature record based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) at Silver Lake, OH. We compare these new data to previously published records and to a regional stack of pollen-based temperature reconstructions from Stotzel-Leis, Silver Lake, and three other well-dated sites. The new and previously published pollen records at Stotzel-Leis are similar, but our new age model brings vegetation events into closer alignment with known climatic events such as the Younger Dryas (YD). brGDGT-inferred temperatures correlate strongly with pollen-based regional temperature reconstructions, with the strongest correlation obtained for a global soil-based brGDGT calibration (r² = 0.88), lending confidence to the deglacial reconstructions and the use of brGDGT and regional pollen stacks as paleotemperature proxies in eastern North America. However, individual pollen records show large differences in timing, rates, and amplitudes of inferred temperature change, indicating caution with paleoclimatic inferences based on single-site pollen records. From 16.0 to 10.0ka, both proxies indicate that regional temperatures rose by ∼10 °C, roughly double the ∼5 °C estimates for the Northern Hemisphere reported in prior syntheses. Change-point analysis of the pollen stack shows accelerated warming at 14.0 ± 1.2ka, cooling at 12.6 ± 0.4ka, and warming from 11.6 ± 0.5ka into the Holocene. The timing of Bølling-Allerød (B-A) warming and YD onset in our records lag by ∼300–500 years those reported in syntheses of temperature records from the northern mid-latitudes. This discrepancy is too large to be attributed to uncertainties in radiocarbon dating, and correlation between pollen and brGDGT temperature reconstructions rules out vegetation lags as a cause. However, the YD termination appears synchronous among the brGDGT record, regional pollen stack, and Northern Hemisphere stack. The cause of the larger and lagged temperature changes in the southern Great Lakes relative to Northern Hemisphere averages remains unclear, but may be due to the effects of continentality and ice sheet extent on regional climate evolution.
Article
Understanding past climate trends is key for reliable projections of global warming and associated risks and hazards. Uncomfortably large discrepancies between vegetation-based summer temperature reconstructions (mainly based on pollen) and climate model results have been reported for the current interglacial, the Holocene. For the Mediterranean region these reconstructions indicate cooler-than-present mid-Holocene summers, in contrast with expectations based on climate models and long-term changes in summer insolation. We present new quantitative and replicated Holocene summer temperature reconstructions based on fossil chironomid midges from the northern central Mediterranean region. The Holocene thermal maximum is reconstructed 9,000–5,000 years ago and estimated to have been 1–2 °C warmer in mean July temperature than the recent pre-industrial period, consistent with glacier and marine records, and with transient climate model runs. This combined evidence implies that widely used pollen-based summer temperature reconstructions in the Mediterranean area are significantly biased by precipitation or other forcings such as early land use. Our interpretation can resolve the previous discrepancy between climate models and quantitative palaeotemperature records for millennial-scale Holocene summer temperature trends in the Mediterranean region. It also suggests that pollen-based evidence for cool mid-Holocene summers in other semi-arid to arid regions of the Northern Hemisphere may have to be reconsidered, with potential implications for global-scale reconstructions.
Article
An integrated investigation including geological, geomorphological, geophysical and structural survey, tephra analyses, ¹⁴C and ⁴⁰Ar/³⁹Ar dating, as well as paleoseismic trenching along the N-Matese fault system is presented. The study allowed the characterization of the tectonic mobility of this structure as well as the associated Bojano basin sedimentary-tectonic evolution since the early Middle Pleistocene, providing also new clues concerning the fault historical activity and the associated Mw > 6.5 earthquakes. We have found lines of evidence for > 1 mm/yr slip rate along the presently buried Bojano fault during the mid Middle Pleistocene, and similar rates for the main fault segments paralleling the Matese flanks. The buried Bojano fault significantly slowed down during the last 300 kyr, ceasing its activity before the Holocene. In turn, the segments outcropping along the Matese flanks reactivated at the onset of Late Pleistocene, after a long period of quiescence (480–110 ka), with robust slip rates that would seem even accelerating in post LGM times. Paleoseismic data suggest the occurrence of four Mw > 6.6 earthquakes in the past 3 ka, three of which match the little known 280 BCE event, and the devastating 1456 and 1805 earthquakes.
Chapter
The importance of downy oak as an integral component of the “submediterranean” woods has been underscored by many studies. Nevertheless, terms like “submediterranean” and “downy oak” are some of the most poorly understood concepts in European phytogeographic and taxonomic research. Downy oak is well known to be a problematic taxon. The name “Quercus pubescens” (= Q. humilis) combines populations characterized by increasing phenotypic and genomic polymorphisms along north-south gradients, which is explained as the result of a “founder effect” produced by a relatively fast post-glacial re-colonization of the northern areas through rare long-distance dispersal events.
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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.
Article
Since multi-site reconstructions are less affected by site-specific climatic effects and artefacts, regional palaeotemperature reconstructions based on a number of sites can provide more robust estimates of centennial- to millennial-scale temperature trends than individual, site-specific records. Furthermore, reconstructions based on multiple records are necessary for developing continuous climate records over time scales longer than covered by individual sequences. Here, we present a procedure for developing such reconstructions based on relatively short (centuries to millennia), discontinuously sampled records as are typically developed when using biotic proxies in lake sediments for temperature reconstruction. The approach includes an altitudinal correction of temperatures, an interpolation of individual records to equal time intervals, a stacking procedure for sections of the interval of interest that have the same records available, as well as a splicing procedure to link the individual stacked records into a continuous reconstruction. Variations in the final, stacked and spliced reconstruction are driven by variations in the individual records, whereas the absolute temperature values are determined by the stacked segment based on the largest number of records. With numerical simulations based on the NGRIP δ18O record, we demonstrate that the interpolation and stacking procedure provides an approximation of a smoothed palaeoclimate record if based on a sufficient number of discontinuously sampled records. Finally, we provide an example of a stacked and spliced palaeotemperature reconstruction 15000–90 calibrated 14C yr BP based on six chironomid records from the northern and central Swiss Alps and eastern France to discuss the potential and limitations of this approach.
Article
Comparisons of climate model hindcasts with independent proxy data are essential for assessing model performance in non-analogue situations. However, standardized palaeoclimate data sets for assessing the spatial pattern of past climatic change across continents are lacking for some of the most dynamic episodes of Earth's recent past. Here we present a new chironomid-based palaeotemperature dataset designed to assess climate model hindcasts of regional summer temperature change in Europe during the late-glacial and early Holocene. Latitudinal and longitudinal patterns of inferred temperature change are in excellent agreement with simulations by the ECHAM-4 model, implying that atmospheric general circulation models like ECHAM-4 can successfully predict regionally diverging temperature trends in Europe, even when conditions differ significantly from present. However, ECHAM-4 infers larger amplitudes of change and higher temperatures during warm phases than our palaeotemperature estimates, suggesting that this and similar models may overestimate past and potentially also future summer temperature changes in Europe. http://www.nature.com/ncomms/2014/140911/ncomms5914/full/ncomms5914.html
Article
Tephrochronology commonly relies upon grain-discrete analysis of glass shards to reveal the subtle geochemical differences between tephras from past explosive volcanic eruptions. The use of electron probe microanalysis for this purpose is widely accepted by tephrochronologists. It is widely recognised that both precision and accuracy must be maximised, and that rigorous standardisation procedures must be followed. In this paper the performances of seven northern European electron microprobe centres are compared, using a geochemically homogeneous obsidian secondary standard. The results reveal that a number of tephra-oriented probe centres could benefit from exercises similar to the programme outlined within. In particular, it is evident that a number of these centres undertaking tephrochronological investigations have generated geochemical data that may be inadequate in terms of distinguishing between tephras. Inaccuracy in the determination of sodium is seen to be the main difficulty.In recent years the application of distal tephrochronology to stratigraphical problems in the northern European Quaternary record has become increasingly common. Geochemical standardisation is essential at an early stage in the development of a regional tephrochronological framework and, we hope, exercises such as presented here will encourage the production of reliable tephra geochemical data.
Article
Until recently, pollen-stratigraphic research in the Northern Apennines (Northern Italy) provided only generalized pollen diagrams that lacked reliable chronologies, and few records provided complete and detailed postglacial sequences equipped with radiocarbon dates. We present a new Late-Glacial and Holocene pollen sequence from Pavullo nel Frignano in the Emilian Apennines (Modena, Italy). The chronology relies on AMS-radiocarbon-dated samples of terrestrial plant origin. Our pollen record suggests that open Late-Glacial Pinus and Juniperus dominated woodlands were established at the site before 14,000cal. BP. Thermophilous trees such as Quercus, Ulmus, and Tilia as well as Abies alba expanded already during the Late-Glacial (ca. 14,000–13,000cal. BP), but did not form closed forests. After climate cooling of the Younger Dryas A. alba re-expanded at the onset of the Holocene at ca. 11,500cal. BP and remained the dominant species until at ca. 6000–5500cal. BP. The decline of A. alba was associated with a marked opening of forests, and the expansion of deciduous trees such as Fagus and Quercus. Vegetational composition did not change substantially during the past 5000years, and cultivated tree taxa such as Juglans and Castanea played only a transient or marginal role. Although the vegetation history of Pavullo is consistent with previous investigations in the study area, comparison is hampered by the absence of other records from the same vegetational (colline) belt. Our pollen-inferred human-impact history is in agreement with archaeological evidence. In addition, our results suggest a rather close link between vegetational change in the Northern Apennines and the Southern Alps. Common features between these two climatically-similar regions are the initial expansion of thermophilous trees and Abies alba at ca. 13,000cal. BP, the mid-Holocene collapse of A. alba (probably as a consequence of human disturbance) as well as the subsequent expansions of Quercus and Fagus.
Article
We compared palaeoclimate proxy records of two deep-sea sediment cores from the Tyrrhenian Sea to provide new insights into the climate variability in the Mediterranean basin during Termination I and the Holocene. High-resolution sequences of Globigerina bulloides and Globigerinoides ruber oxygen stable isotope analyses and planktonic foraminifera abundances for cores BS7937 and BS7938 are presented here. The sedimentation rates (on average 18 cm/ky for BS7937 and 25 cm/ky for BS7938) calculated for these cores are among the highest recorded in the Tyrrhenian Sea and the sampling resolution of core BS7938 provides the first centennial-scale study available for the Holocene in this area. At least seven cold episodes, with an average duration of 650 years (from 400 to 1000 years), have been clearly recognised in the record of the last 12 ky in the planktonic foraminiferal and δ18O profiles of both cores. The timing and intensity of such coolings is in good agreement with others previously identified in the Mediterranean basin and this testifies to their wide distribution in an area previously thought to be characterised by Holocene climatic stability. This environmental variability has also been further highlighted by the revised biozonation applied to the relative abundances of the main planktonic foraminifera species recognised in the south Tyrrhenian Sea.
Article
Chironomid head capsules were extracted and identified in 52 samples along a sediment core retrieved from the Ech palaeololake, (710 m a.s.l. in the west-central Pyrenees). The pollen stratigraphy together with radiocarbon dates provided the basis for the chronology of the record. Since temperature changes were identified as one of the major determining factors for shifts in the Ech chironomid assemblages during the Lateglacial, July air temperatures were tentatively inferred from the chironomid record using a transfer function based on a combined Swiss and Norwegian calibration dataset. The model reconstructed July air temperatures between 10 and 13 °C during the Oldest Dryas, a two step increase in temperatures to 16 and 17.5 °C during the first part of the Interstadial followed by a decrease to 16.5 °C at the end of the Interstadial. The Ech record features three centennial-scale 1.5 °C coolings during the Interstadial. Inferred temperatures decreased to 15–15.5 °C during the Younger Dryas, and reached ca. 17 °C at the beginning of the Holocene. The applicability of the combined Swiss–Norwegian transfer function to the Pyrenean fossil record was substantiated by the good statistics of the inference and the similar distribution of taxa with respect to temperature in the Alps and the Pyrenees. The reliability of the Ech record, including its chronology, was supported by its good concordance with other previously published chironomid-based lateglacial temperature reconstructions from Europe and the NGRIP δ18O record.