Sabine Wulf

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.

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.

Estimated reservoir ages of the Black Sea since the last glacial

Accelerator mass spectrometry (AMS) radiocarbon dating of ostracod and gastropod shells from the southwestern Black Sea cores combined with tephrochronology provides the basis for studying reservoir age changes in the late-glacial Black Sea. The comparison of our data with records from the northwestern Black Sea shows that an apparent reservoir age of ∼1450 14C yr found in the glacial is characteristic of a homogenized water column. This apparent reservoir age is most likely due to the hardwater effect. Though data indicate that a reservoir age of ∼1450 14C yr may have persisted until the Bolling-Allerod warm period, a comparison with the GISP2 ice-core record suggests a gradual reduction of the reservoir age to ∼1000 14C yr, which might have been caused by dilution effects of inflowing meltwater. During the Bolling-Allerod warm period, soil development and increased vegetation cover in the catchment area of the Black Sea could have hampered erosion of carbonate bedrock, and hence diminished contamination by “old” carbon brought to the Black Sea basin by rivers. A further reduction of the reservoir age most probably occurred contemporary to the precipitation of inorganic carbonates triggered by increased phytoplankton activity, and was confined to the upper water column. Intensified deep water formation subsequently enhanced the mixing/convection and renewal of intermediate water. During the Younger Dryas, the age of the upper water column was close to 0 yr, while the intermediate water was ∼900 14C yr older. The first inflow of saline Mediterranean water, at ∼8300 14C yr BP, shifted the surface water age towards the recent value of ∼400 14C yr.

Marine tephra from the Cape Riva eruption (22 ka) of Santorini in the Sea of Marmara

Abstract A discrete tephra layer has been discovered in three marine sediment cores from the Sea of Marmara, eastern Mediterranean. The rhyodacitic glass chemistry and the stratigraphical position suggest a Santorini provenance and, in particular, a correlation with the marine Y-2 tephra that is known from the southern Aegean Sea and eastern Levantine Basin. This tephra represents the distal facies of the Cape Riva eruption of Santorini, which has been dated by 14C on land at 21 950 cal. yr BP. Hitherto, the Y-2 tephra has been detected only in marine sediment cores recovered south to southeast of its volcanic source. The new occurrence in the Sea of Marmara approximately 530 km NNE of the Santorini eruptive centre suggests a more north-easterly dispersal of fallout products of the Cape Riva eruption than previously supposed.

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.

Varve microfacies and varve preservation record of climate change and human impact for the last 6000 years at Lake Tiefer See (NE Germany)

The Holocene sediment record of Lake Tiefer See exhibits striking alternations between well-varved and non-varved intervals. Here, we present a high-resolution multi-proxy record for the past ~6000 years and discuss possible causes for the observed sediment variability. This approach comprises microfacies, geochemical and microfossil analyses and a multiple dating concept including varve counting, tephrochronology and radiocarbon dating. Four periods of predominantly well-varved sediment were identified at 6000–3950, 3100–2850 and 2100–750 cal. a BP and AD 1924–present. Except of sub-recent varve formation, these periods are considered to reflect reduced lake circulation and consequently, stronger anoxic bottom water conditions. In contrast, intercalated intervals of poor varve preservation or even extensively mixed non-varved sediments indicate strengthened lake circulation. Sub-recent varve formation since AD 1924 is, in addition to natural forcing, influenced by enhanced lake productivity due to modern anthropogenic eutrophication. The general increase in periods of intensified lake circulation in Lake Tiefer See since ~4000 cal. a BP presumably is caused by gradual changes in the northern hemisphere orbital forcing, leading to cooler and windier conditions in Central Europe. Superimposed decadal- to centennial-scale variability of the lake circulation regime is likely the result of additional human-induced changes of the catchment vegetation. The coincidence of major non-varved periods at Lake Tiefer See and intervals of bioturbated sediments in the Baltic Sea implies a broader regional significance of our findings.

Volcanic Lake Sediments as Sensitive Archives of Climate and Environmental Change

In efforts to understand the natural variability of the Earth climate system and the potential for future climate and environmental (e.g., biodiversity) changes, palaeodata play a key role by extending the baseline of environmental and climatic observations. Lake sediments, and particularly sediment archives of volcanic lakes, help to decipher natural climate variability at seasonal to millennial scales, and help identifying causal mechanisms. Their importance includes their potential to provide precise and accurate inter-archive correlations (e.g., based on tephrochronology) and to record cyclicity and high frequency climate signals. We present a few examples of commonly used techniques and proxy-records to investigate past climatic variability and its influence to the history of the lakes and of their biota. This paper is rather a presentation of potentials and limits of palaeolimnological and limnogeological research on crater lakes, than a pervasive review of palaeolimnological studies on crater lakes. We show the importance of seismic stratigraphy for the selection of coring sites, and discuss problems in core chronology. Then we give examples of physical and chemical proxies, including magnetism, micro-facies and oxygen and carbon stable isotopes from crater lake deposits mainly located in central and southern Europe. Finally, we present the use of air-transported (pollens) and lacustrine biological remains. The continuing need to develop new approaches and methods stimulated us to mention, as an example, the potential of the studies of subsurface biosphere, and the effects of microbiological metabolism on mineral diagenesis in sediments.

Site-specific sediment responses to climate change during the last 140 years in three varved lakes in Northern Poland

Accurate dating and unambiguous chronological correlation using cryptotephras provide a powerful tool to compare the varved sediment records of the lakes Głęboczek (JG), Czechowskie (JC) and Jelonek (JEL) (north-central Poland). For the last 140 years, micro-facies analyses and µ-XRF element scanning at seasonal resolution, as well as bulk elemental analyses (organic matter, carbonate) at sub-decadal to decadal resolution, were conducted for all three lakes records. All lakes are located in a region with low population density, and therefore, anthropogenic influences are negligible or only minor. The varve chronologies have been established independently for each record and were synchronized with the Askja AD 1875 cryptotephra. Comparison with monthly temperature data since 1870 and daily temperature data since 1951 revealed different responses of lake deposition to recent climate change. Varves are well preserved over the entire 140 years only at JG, while in the JC record two faintly varved intervals are intercalated and in the JEL record two non-varved intervals occur at the base and top of the profiles. These differences likely are due to variations in lake characteristics and their influence on lake-internal responses. JG is the smallest and best wind-sheltered lake, which favours varve preservation. JC’s attenuated sediment responses can likely be linked to lake productivity changes with respect to climate warming. JEL is lacking a direct sedimentological response to the observed temperature increase, which can be linked to lake size and water depth superimposing regional climate changes. Climate changes at the demise of the ‘Little Ice Age’ around 1900 and the recent warming since the 1980s are expressed in sediment proxies in the lakes with different response times and amplitudes. This detailed comparison study on three nearby lakes demonstrates the influence of local parameters such as lake and catchment size and water depth superimposed on more regional climate-driven changes.