Achim Brauer

Rapid environmental changes in southern Europe during the last glacial period

Oxygen-isotope records from Greenland ice cores, indicate numerous rapid climate fluctuations during the last glacial period. North Atlantic marine sediment cores show comparable variability in sea surface temperature and the deposition of ice-rafted debris. In contrast, very few continental records of this time period provide the temporal resolution and environmental sensitivity necessary to reveal the extent and effects of these environmental fluctuations on the continents. Here we present high-resolution geochemical, physical and pollen data from lake sediments in Italy and from a Mediterranean sediment core, linked by a common tephrochronology. Our lacustrine sequence extends to the past 102,000 years. Many of its features correlate well with the Greenland ice-core records, demonstrating that the closely coupled ocean–atmosphere system of the Northern Hemisphere during the last glacial extended its influence at least as far as the central Mediterranean region. Numerous vegetation changes were rapid, frequently occurring in less than 200 years, showing that the terrestrial biosphere participated fully in last-glacial climate variability. Earlier than 65,000 years ago, our record shows more climate fluctuations than are apparent in the Greenland ice cores. Together, the multi-proxy data from the continental and marine records reveal differences in the seasonal character of climate during successive interstadials, and provide a step towards determining the underlying mechanisms of the centennial–millennial-scale variability.

Correlation and synchronisation of Lateglacial continental sequences in northern central Europe based on annually laminated lacustrine sediments

Abstract The present study focusses on correlation and synchronisation of Weichselian Lateglacial varved lake sediments from western Germany (Meerfelder Maar, Eifel region), northern Germany (Hamelsee, Lower Saxony), central Poland (Lake Gości a z) and eastern Poland (Lake Perespilno) by using varve chronology, tephrochronology, palynostratigraphy and stable isotopes. Comparison of the several independent time scales shows that biotic and abiotic parameters respond abruptly and quasi-synchronously, within the errors of the different chronologies, during the Younger Dryas/Preboreal transition. Moreover, there is a consensus about the length of the Younger Dryas cold stage of 1100–1150 varve years. In the Allerod the prominent Laacher See tephra (12,880 varve years BP) can be used to fix floating varve chronologies. The relative duration of this biozone has been determined in Meerfelder Maar and Hamelsee at between 625 and 670 varve years. In the Meerfelder Maar a combination of continuous varve counting and biostratigraphy has been possible for the almost entire Lateglacial. The comparison between continental limnic sequences and Greenland ice-core records should be made on the basis of independent chronologies in both archives. It is more practicable to develop regional stratotypes on the continental regions instead of simply using ice cores as stratotypes for the Lateglacial for terrestrial European records. In this respect, annually laminated lacustrine sequences have a great potential.

High resolution sediment and vegetation responses to Younger Dryas climate change in varved lake sediments from Meerfelder Maar, Germany

Abstract This work focuses on a 1900-year section of varved sediments from Lake Meerfelder Maar (MFM) extending from the Late Allerod 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.

Lateglacial calendar year chronology based on annually laminated sediments from Lake Meerfelder Maar, Germany

Abstract Annually laminated sediments from Lake Meerfelder Maar (MFM) in the Eifel, Germany, provide an independent calendar year timescale for the last glacial/interglacial transition. Changes in varve thickness and varve facies reflect significant environment variations during this period. The general micro facies pattern confirms that diatomaceous organic varves are favoured during warm climatic phases while cold periods generate higher allochthonous sediment fluxes. An exception is the significant increase in diatom blooms at the beginning of the Younger Dryas. Six Lateglacial biozones have been defined by Litt and Stebich (Quaternary International, 1999). The sediment record comprises two tephra layers as chronomarkers. The Laacher See Tephra (12,880 varve years BP) is a continent-wide correlation horizon while the Ulmener Maar Tephra provides regional chronological links. The beginning of the lateglacial warming is dated at 14,450 varve years (vyrs) BP which is regarded as the minimum age. Three abrupt cold spells interrupted the lateglacial interstadial at 13,800 vyrs BP (Oldest Dryas), 13,540 vyrs BP (Older Dryas) and 12,680 vyrs BP (Younger Dryas). The Oldest Dryas (130 years) and the Older Dryas (190 years) were rather short while the Younger Dryas lasted much longer (1090 years). The MFM chronology was compared with ice core timescales from GISP2 and GRIP. There is a good agreement between these timescales, with the GRIP model age (ss09) providing the best match to the MFM chronology.

Evidence for last interglacial chronology and environmental change from Southern Europe

Establishing phase relationships between earth-system components during periods of rapid global change is vital to understanding the underlying processes. It requires records of each component with independent and accurate chronologies. Until now, no continental record extending from the present to the penultimate glacial had such a chronology to our knowledge. Here, we present such a record from the annually laminated sediments of Lago Grande di Monticchio, southern Italy. Using this record we determine the duration (17.70 ± 0.20 ka) and age of onset (127.20 ± 1.60 ka B.P.) of the last interglacial, as reflected by terrestrial ecosystems. This record also reveals that the transitions at the beginning and end of the interglacial spanned only ≈100 and 150 years, respectively. Comparison with records of other earth-system components reveals complex leads and lags. During the penultimate deglaciation phase relationships are similar to those during the most recent deglaciation, peaks in Antarctic warming and atmospheric methane both leading Northern Hemisphere terrestrial warming. It is notable, however, that there is no evidence at Monticchio of a Younger Dryas-like oscillation during the penultimate deglaciation. Warming into the first major interstadial event after the last interglacial is characterized by markedly different phase relationships to those of the deglaciations, warming at Monticchio coinciding with Antarctic warming and leading the atmospheric methane increase. Diachroneity is seen at the end of the interglacial; several global proxies indicate progressive cooling after ≈115 ka B.P., whereas the main terrestrial response in the Mediterranean region is abrupt and occurs at 109.50 ± 1.40 ka B.P.

A complete terrestrial radiocarbon record for 11.2 to 52.8 kyr B.P

Dating Carbon Radiocarbon dating is the best way to determine the age of samples that contain carbon and that are younger than ∼50,000 years, the limit of precision for the method. There are several factors that complicate such age determinations, however, some of the most important of which include variability of the 14C production in the atmosphere (which affects organic samples whose radiocarbon inventories are derived from atmospheric CO2), surface ocean reservoir effects (which affect marine samples that acquire their radiocarbon signatures from seawater), and variable dead carbon fraction effects (which affect speleothems that derive their carbon from groundwaters). Bronk Ramsey et al. (p. 370; see the Perspective by Reimer) avoid the need to make such assumptions, reporting the 14C results of sediments from Lake Suigetsu, Japan. Analysis of terrestrial plant macrofossils in annually layered datable sediments yielded a direct record of atmospheric radiocarbon for the entire measurable interval up to 52.8 thousand years ago. Radiocarbon measurements of samples from Lake Suigetsu, Japan, extend the 14C time scale back to more than 50,000 years ago. Radiocarbon (14C) provides a way to date material that contains carbon with an age up to ~50,000 years and is also an important tracer of the global carbon cycle. However, the lack of a comprehensive record reflecting atmospheric 14C prior to 12.5 thousand years before the present (kyr B.P.) has limited the application of radiocarbon dating of samples from the Last Glacial period. Here, we report 14C results from Lake Suigetsu, Japan (35°35′N, 135°53′E), which provide a comprehensive record of terrestrial radiocarbon to the present limit of the 14C method. The time scale we present in this work allows direct comparison of Lake Suigetsu paleoclimatic data with other terrestrial climatic records and gives information on the connection between global atmospheric and regional marine radiocarbon levels.

Volcanic ash reveals time-transgressive abrupt climate change during the Younger Dryas

Knowledge of regional variations in response to abrupt climatic transitions is essential to understanding the climate system and anticipating future changes. Global climate models typically assume that major climatic changes occur synchronously over continental to hemispheric distances. The last major reorganization of the ocean-atmosphere system in the North Atlantic realm took place during the Younger Dryas (YD), an ∼1100 yr cold period at the end of the last glaciation. Within this region, several terrestrial records of the YD show at least two phases, an initial cold phase followed by a second phase of climatic amelioration related to a resumption of North Atlantic overturning. We show that the onset of climatic amelioration during the YD cold period was locally abrupt, but time-transgressive across Europe. Atmospheric proxy signals record the resumption of thermohaline circulation midway through the Younger Dryas, occurring 100 yr before deposition of ash from the Icelandic Vedde eruption in a German varve lake record, and 20 yr after the same isochron in western Norway, 1350 km farther north. Synchronization of two high-resolution continental records, using the Vedde Ash layer (12,140 ± 40 varve yr B.P.), allows us to trace the shifting of the polar front as a major control of regional climate amelioration during the YD in the North Atlantic realm. It is critical that future climate models are able to resolve such small spatial and chronological differences in order to properly encapsulate complex regional responses to global climate change.

Reduced Interannual Rainfall Variability in East Africa During the Last Ice Age

Extreme rainfall was weaker and less frequent in East Africa during the last ice age. Interannual rainfall variations in equatorial East Africa are tightly linked to the El Niño Southern Oscillation (ENSO), with more rain and flooding during El Niño and droughts in La Niña years, both having severe impacts on human habitation and food security. Here we report evidence from an annually laminated lake sediment record from southeastern Kenya for interannual to centennial-scale changes in ENSO-related rainfall variability during the last three millennia and for reductions in both the mean rate and the variability of rainfall in East Africa during the Last Glacial period. Climate model simulations support forward extrapolation from these lake sediment data that future warming will intensify the interannual variability of East Africa’s rainfall.

Annually dated late Weichselian continental paleoclimate record from the Eifel, Germany

A record extending back to 23 220 calendar yr B.P. was obtained from varved sediments from Lake Holzmaar, Germany, by applying multiple dating methods and interlake comparison with Meerfelder Maar. Variations in the Weichselian inorganic carbon content resulting from intensified eolian activities coincide with proxy parameters of dust accumulation in Greenland. The timing for the end of the last glacial maximum at Holzmaar agrees with the Greenland Ice Sheet Project (GISP2) data but suggests dating discrepancies with the Greenland Ice Core Project data (GRIP). Assuming that periods of maximum dust deposition in the Eifel and Greenland coincide with ice advances in northeastern Germany, the Holzmaar record provides a means of dating three late Weichselian terminal moraines.

Abrupt environmental oscillations during the Early Weichselian recorded at Lago Grande di Monticchio, southern Italy

Abstract This study presents multi-proxy data from a new core from Lago Grande di Monticchio (Italy) extending back to about 100,000 calendar years BP. The time scale of this record is independent from deep-sea and ice-core chronologies and has been established from a combination of varve counts and high-resolution sedimentation rate calculations based on microscopic sediment investigation. This paper focuses on environmental variability between 100,000 and 65,000 calendar years BP. Several abrupt cold periods have been recognised and correlated to other terrestrial as well as to ice-core and deep-sea records. The longest of the Monticchio cold phases corresponds to Marine Isotopic Stage 5b and lasted from ca 87,000 to 84,000 calendar years BP. Two shorter events (about 1000 years long) occurred during the transition from interstadial to full glacial conditions between 79,000 and 75,000 calendar years BP. The increased frequency of cold events during this time interval probably indicates a higher degree of climatic instability in periods of major changes. Dating discrepancies between Monticchio, GRIP, and deep-sea chronologies range between a few hundred to a few thousand years. However, these differences in dating are not constant but vary significantly for different sections of the records. Nevertheless, the good match of environmental fluctuations at Monticchio with atmospheric variability at the Greenland Summit as well as variations in the oceanic circulation pattern suggests a close link of the Mediterranean to the Atlantic climate regime.