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Nearly 13,000 years ago, the warming trend into the Holocene was sharply interrupted by a reversal to near glacial conditions. Climatic causes and ecological consequences of the Younger Dryas (YD) have been extensively studied, however proxy archives from the Mediterranean basin capturing this period are scarce and do not provide annual resolution. Here, we report a hydroclimatic reconstruction from stable isotopes (delta O-18, delta C-13) in subfossil pines from southern France. Growing before and during the transition period into the YD (12 900-12 600 cal BP), the trees provide an annually resolved, continuous sequence of atmospheric change. Isotopic signature of tree sourcewater (delta O-18(sw)) and estimates of relative air humidity were reconstructed as a proxy for variations in air mass origin and precipitation regime. We find a distinct increase in inter-annual variability of sourcewater isotopes (delta O-18(sw)), with three major downturn phases of increasing magnitude beginning at 12 740 cal BP. The observed variation most likely results from an amplified intensity of North Atlantic (low delta O-18(sw)) versus Mediterranean (high delta O-18(sw)) precipitation. This marked pattern of climate variability is not seen in records from higher latitudes and is likely a consequence of atmospheric circulation oscillations at the margin of the southward moving polar front.
While of higher plant origin, a specific source assignment of sedimentary leaf wax n-alkanes remains difficult. In addition, it is unknown how fast a changing catchment vegetation would be reflected in sedimentary leaf wax archives. In particular, for a quantitative interpretation of n-alkane C and H isotope ratios in terms of paleohydrological and paleoecological changes, a better understanding of transfer times and dominant sedimentary sources of leaf wax n-alkanes is required. In this study we tested to what extent compositional changes in leaf wax n-alkanes can be linked to known vegetation changes by comparison with high-resolution palynological data from the same archive. We analyzed leaf wax n-alkane concentrations and distributions in decadal resolution from a sedimentary record from Trzechowskie paleolake (TRZ, northern Poland), covering the Late Glacial to early Holocene (13 360-9940 yr BP). As an additional source indicator of targeted n-alkanes, compound-specific carbon isotopic data have been generated in lower time resolution. The results indicated rapid responses of n-alkane distribution patterns coinciding with major climatic and paleoecological transitions. We found a shift towards higher average chain length (ACL) values at the Allerod-Younger Dryas (YD) transition between 12 680 and 12 600 yr BP, co-evaled with a decreasing contribution of arboreal pollen (mainly Pinus and Betula) and a subsequently higher abundance of pollen derived from herbaceous plants (Poaceae, Cyperaceae, Artemisia), shrubs, and dwarf shrubs (Juniperus and Salix). The termination of the YD was characterized by a successive increase in n-alkane concentrations coinciding with a sharp decrease in ACL values between 11 580 and 11 490 yr BP, reflecting the expansion of woodland vegetation at the YD-Holocene transition. A gradual reversal to longer chain lengths after 11 200 yr BP, together with decreasing n-alkane concentrations, most likely reflects the early Holocene vegetation succession with a decline of Betula. These results show that n-alkane distributions reflect vegetation changes and that a fast (i.e., subdecadal) signal transfer occurred. However, our data also indicate that a standard interpretation of directional changes in biomarker ratios remains difficult. Instead, responses such as changes in ACL need to be discussed in the context of other proxy data. In addition, we find that organic geochemical data integrate different ecological information compared to pollen, since some gymnosperm genera, such as Pinus, produce only a very low amount of n-alkanes and for this reason their contribution may be largely absent from biomarker records. Our results demonstrate that a combination of palynological and n-alkane data can be used to infer the major sedimentary leaf wax sources and constrain leaf wax transport times from the plant source to the sedimentary sink and thus pave the way towards quantitative interpretation of compound-specific hydrogen isotope ratios for paleohydrological reconstructions.
One of the most significant Late Holocene climate shifts occurred around 2800 years ago, when cooler and wetter climate conditions established in western Europe. This shift coincided with an abrupt change in regional atmospheric circulation between 2760 and 2560 cal years BP, which has been linked to a grand solar minimum with the same duration (the Homeric Minimum). We investigated the temporal sequence of hydroclimatic and vegetation changes across this interval of climatic change (Homeric climate oscillation) by using lipid biomarker stable hydrogen isotope ratios (ED values) and pollen assemblages from the annually-laminated sediment record from lake Meerfelder Maar (Germany). Over the investigated interval (3200-2000 varve years BP), terrestrial lipid biomarker ED showed a gradual trend to more negative values, consistent with the western Europe long-term climate trend of the Late Holocene. At ca. 2640 varve years BP we identified a strong increase in aquatic plants and algal remains, indicating a rapid change in the aquatic ecosystem superimposed on this long-term trend. Interestingly, this aquatic ecosystem change was accompanied by large changes in ED values of aquatic lipid biomarkers, such as nC(21) and nC(23) (by between 22 and 30%(0)). As these variations cannot solely be explained by hydroclimate changes, we suggest that these changes in the Wag value were influenced by changes in n-alkane source organisms. Our results illustrate that if ubiquitous aquatic lipid biomarkers are derived from a limited pool of organisms, changes in lake ecology can be a driving factor for variations on sedimentary lipid MN values, which then could be easily misinterpreted in terms of hydro climatic changes. (C) 2017 Elsevier Ltd. All rights reserved.
The sedimentary record of the Dead Sea provides an exceptional high-resolution archive of past climate changes in the drought-sensitive eastern Mediterranean-Levant, a key region for the development of humankind at the boundary of global climate belts. Moreover, it is the only deep hypersaline lake known to have deposited long sequences of finely laminated, annually deposited sediments (i.e. varves) of varied compositions, including aragonite, gypsum, halite and clastic sediments. Vast efforts have been made over the years to decipher the environmental information stored in these evaporitic-clastic sequences spanning from the Pleistocene Lake Amora to the Holocene Dead Sea. A general characterisation of sediment facies has been derived from exposed sediment sections, as well as from shallow- and deep-water sediment cores. During high lake stands and episodes of positive water budget, mostly during glacial times, alternating aragonite and detritus laminae (‘aad’ facies) were accumulated, whereas during low lake stands and droughts, prevailing during interglacials, laminated detritus (‘ld’ facies) and laminated halite (‘lh’ facies) dominate the sequence. In this paper, we (i) review the three types of laminated sediments of the Dead Sea sedimentary record (‘aad’, ‘ld’ and ‘lh’ facies), (ii) discuss their modes of formation, deposition and accumulation, and their interpretation as varves, and (iii) illustrate how Dead Sea varves are utilized for palaeoclimate reconstructions and for establishing floating chronologies.
Nearly 13,000 years ago, the warming trend into the Holocene was sharply interrupted by a reversal to near glacial conditions. Climatic causes and ecological consequences of the Younger Dryas (YD) have been extensively studied, however proxy archives from the Mediterranean basin capturing this period are scarce and do not provide annual resolution. Here, we report a hydroclimatic reconstruction from stable isotopes (delta O-18, delta C-13) in subfossil pines from southern France. Growing before and during the transition period into the YD (12 900-12 600 cal BP), the trees provide an annually resolved, continuous sequence of atmospheric change. Isotopic signature of tree sourcewater (delta O-18(sw)) and estimates of relative air humidity were reconstructed as a proxy for variations in air mass origin and precipitation regime. We find a distinct increase in inter-annual variability of sourcewater isotopes (delta O-18(sw)), with three major downturn phases of increasing magnitude beginning at 12 740 cal BP. The observed variation most likely results from an amplified intensity of North Atlantic (low delta O-18(sw)) versus Mediterranean (high delta O-18(sw)) precipitation. This marked pattern of climate variability is not seen in records from higher latitudes and is likely a consequence of atmospheric circulation oscillations at the margin of the southward moving polar front.
Reading the sediment record in terms of past climates is challenging since linking climate change to the associated responses of sedimentary systems is not always straightforward. Here we analyze the erosional response of landscapes on the Tibetan Plateau to interglacial climate forcing. Using the theory of dynamical systems on Holocene time series of geochemical proxies, we derive a sedimentary response model that accurately simulates observed proxy variation in three lake records. The model suggests that millennial variations in sediment composition reflect a self-organization of landscapes in response to abrupt climate change between 11.6 and 11.9 ka BP. The self-organization is characterized by oscillations in sediment supply emerging from a feedback between physical and chemical erosion processes, with estimated response times between 3,000 to 18,000 years depending on catchment topography. The implications of our findings emphasize the need for landscape response models to decipher the paleoclimatic code in continental sediment records. Plain Language Summary Lake sediments are an important source of information on past climates. Reading the information is not always straightforward. Complex interactions in landscapes can affect the transmission of climatic signals to the sediment record. However, the exact nature of such complex interactions remains unknown. This study compares sediment deposits of three lakes on the Tibetan Plateau. The deposits are continuous records of landscape responses to climate change during the last 12,000 years. We identified a mathematical model that accurately simulates changes in sediment composition at all sites. The model simulations suggest that an abrupt warming at the end of the last glacial period destabilized the landscapes. This caused fluctuations in the transport of sediments, which persisted for several thousand years. Our findings present evidence for a long-lasting impact of abrupt climate change on fundamental Earth surface processes.
Reading the sediment record in terms of past climates is challenging since linking climate change to the associated responses of sedimentary systems is not always straightforward. Here we analyze the erosional response of landscapes on the Tibetan Plateau to interglacial climate forcing. Using the theory of dynamical systems on Holocene time series of geochemical proxies, we derive a sedimentary response model that accurately simulates observed proxy variation in three lake records. The model suggests that millennial variations in sediment composition reflect a self-organization of landscapes in response to abrupt climate change between 11.6 and 11.9 ka BP. The self-organization is characterized by oscillations in sediment supply emerging from a feedback between physical and chemical erosion processes, with estimated response times between 3,000 to 18,000 years depending on catchment topography. The implications of our findings emphasize the need for landscape response models to decipher the paleoclimatic code in continental sediment records. Plain Language Summary Lake sediments are an important source of information on past climates. Reading the information is not always straightforward. Complex interactions in landscapes can affect the transmission of climatic signals to the sediment record. However, the exact nature of such complex interactions remains unknown. This study compares sediment deposits of three lakes on the Tibetan Plateau. The deposits are continuous records of landscape responses to climate change during the last 12,000 years. We identified a mathematical model that accurately simulates changes in sediment composition at all sites. The model simulations suggest that an abrupt warming at the end of the last glacial period destabilized the landscapes. This caused fluctuations in the transport of sediments, which persisted for several thousand years. Our findings present evidence for a long-lasting impact of abrupt climate change on fundamental Earth surface processes.
Millennial-scale Dansgaard Oeschger (DO) variability at northern high latitudes has influenced climatic and environmental conditions in the Mediterranean during the last glacial period. There is evidence that the hemispheric transmission of the DO variability occurred at the end of DO event 25; however, the exact timing and the trigger that activated the environmental response in the Mediterranean remains incompletely understood. Here, we provide evidence that the clear millennial-scale teleconnection between Greenland and the Mediterranean started at similar to 111.4 ka BP and was initiated by a sub-millennial scale cooling in Greenland (GI-25b). High-resolution sediment proxies and the pollen record of Lago Grande di Monticchio (MON), Italy, reflect climatic instability during the last millennium of the last interglacial, which was characterised by a first and short cooling episode (MON 1) at 111.44 +/- 0.69 ka BP, coinciding with the Greenland cold sub-event GI-25b in duration and timing (within dating uncertainties). MON and Greenland (NorthGRIP ice core) also agree in recording a subsequent warm rebound phase that abruptly culminated in the stadial MON 2/GS-25, marking the transition into the last glacial period. Our results show that the GI-25b triggered an early environmental response at MON to centennial-scale climate change in Greenland as a prelude to the millennial-scale teleconnection that was maintained during the glacial period.
Stable carbon isotopes of sediment organic matter (delta C-13(OM)) are widely applied in paleoenvironmental studies. Interpretations of delta C-13(OM), however, remain challenging and factors that influence delta C-13(OM) may not apply across all lakes. Common explanations for stratigraphic shifts in delta C-13(OM) include changes in lake productivity or changes in inputs of allochthonous OM. We investigated the influence of different oxygen conditions (oxic versus anoxic) on the delta C-13(OM) values in the sediments of Lake Tiefer See. We analysed (1) a long sediment core from the deepest part of the lake, (2) two short, sediment-water interface cores from shallower water depths, and (3) OM in the water column, i.e. from sediment traps. Fresh OM throughout the entire water column showed a relatively constant delta C-13(OM) value of approximately -30.5 parts per thousand. Similar values, about -31 parts per thousand, were obtained for well-varved sediments in both the long and short, sediment-water interface cores. In contrast, delta C-13(OM) values from non-varved sediments in all cores were significantly less negative (-29 parts per thousand). The delta C-13(OM) values in the sediment-water interface cores from different water depths differ for sediments of the same age, if oxygen conditions at the time of deposition were different at these sites, as suggested by the state of varve preservation. Sediments deposited from AD 1924 to 1980 at 62m water depth are varved and exhibit delta C-13(OM) values around -31 parts per thousand, whereas sediments of the same age in the core from 35m water depth are not varved and show less negative delta C-13(OM) values of about -29 parts per thousand. The relation between varve occurrence and delta C-13(OM) values suggests that delta C-13(OM) is associated with oxygen conditions because varve preservation depends on hypolimnetic anoxia. A mechanism that likely influences delta C-13(OM) is selective degradation of OM under oxic conditions, such that organic components with more negative delta C-13(OM) are preferably decomposed, leading to less negative delta C-13(OM) values in the remaining, undegraded OM pool. Greater decomposition of OM in non-varved sediments is supported by lower TOC concentrations in these deposits (similar to 5%) compared to well-varved sediments (similar to 15%). Even in lakes that display small variations in productivity and terrestrial OM input through time, large spatial and temporal differences in hypolimnetic oxygen concentrations may be an important factor controlling sediment delta C-13(OM).
The sediment profile from Lake Goscia(z) over dot in central Poland comprises a continuous, seasonally resolved and exceptionally well-preserved archive of the Younger Dryas (YD) climate variation. This provides a unique opportunity for detailed investigation of lake system responses during periods of rapid climate cooling (YD onset) and warming (YD termination). The new varve record of Lake Goscia(z) over dot presented here spans 1662 years from the late Allerod (AL) to the early Preboreal (PB). Microscopic varve counting provides an independent chronology with a YD duration of 1149+14/-22 years, which confirms previous results of 1140 +/- 40 years. We link stable oxygen isotopes and chironomid-based air temperature reconstructions with the response of various geochemical and varve microfacies proxies especially focusing on the onset and termination of the YD. Cooling at the YD onset lasted similar to 180 years, which is about a century longer than the terminal warming that was completed in similar to 70 years. During the AL/YD transition, environmental proxy data lagged the onset of cooling by similar to 90 years and revealed an increase of lake productivity and internal lake re-suspension as well as slightly higher detrital sediment input. In contrast, rapid warming and environmental changes during the YD/PB transition occurred simultaneously. However, initial changes such as declining diatom deposition and detrital input occurred already a few centuries before the rapid warming at the YD/PB transition. These environmental changes likely reflect a gradual increase in summer air temperatures already during the YD. Our data indicate complex and differing environmental responses to the major climate changes related to the YD, which involve different proxy sensitivities and threshold processes.