Norbert R Nowaczyk

Influence of the intertropical convergence zone on the East Asian monsoon

The Asian–Australian monsoon is an important component of the Earth’s climate system that influences the societal and economic activity of roughly half the world’s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling–Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period ad 700 to 900 (refs 4–6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.

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.

2.8 Million Years of Arctic Climate Change from Lake El’gygytgyn, NE Russia

Crater Core The high-northern latitudes of the Arctic have an important influence on climate and constitute a region with a unique array of complex feedbacks that make it difficult to understand the workings of its climate. Melles et al. (p. 315, published online 21 June) developed a 2.8-million-year record of Arctic climate, using a sediment core from a lake in northeastern Russia that was formed more than 3.5 million years ago by a meteorite impact. Pronounced glacial episodes began 2.6 million years ago but did not achieve orbital pacing for another 700,000 years. A sediment core from a Russian lake provides a high-latitude climate record where prior terrestrial records have been sparse. The reliability of Arctic climate predictions is currently hampered by insufficient knowledge of natural climate variability in the past. A sediment core from Lake El’gygytgyn in northeastern (NE) Russia provides a continuous, high-resolution record from the Arctic, spanning the past 2.8 million years. This core reveals numerous “super interglacials” during the Quaternary; for marine benthic isotope stages (MIS) 11c and 31, maximum summer temperatures and annual precipitation values are ~4° to 5°C and ~300 millimeters higher than those of MIS 1 and 5e. Climate simulations show that these extreme warm conditions are difficult to explain with greenhouse gas and astronomical forcing alone, implying the importance of amplifying feedbacks and far field influences. The timing of Arctic warming relative to West Antarctic Ice Sheet retreats implies strong interhemispheric climate connectivity.

Arctic Ocean evidence for late Quaternary initiation of northern Eurasian ice sheets

A high-resolution multiparameter stratigraphy allows the identification of late Quaternary glacial and interglacial cycles in a central Arctic Ocean sediment core. Distinct sandy layers in the upper part of the otherwise fine-grained sediment core from the Lomonosov Ridge (lat 87.5°N) correlate to four major glacials since ca. 0.7 Ma. The composition of these ice-rafted terrigenous sediments points to a glaciated northern Siberia as the main source. In contrast, lithic carbonates derived from North America are also present in older sediments and indicate a northern North American glaciation since at least 2.8 Ma. We conclude that large-scale northern Siberian glaciation began much later than other Northern Hemisphere ice sheets.

A multiproxy approach to reconstruct the environmental changes along the Eurasian continental margin over the last 150 000 years

Abstract Sediment cores located along the Eurasian continental margin (Arctic Ocean) have been studied to reconstruct the environmental changes in terms of waxing and waning of the Barents/Kara Sea ice-sheets, Atlantic water inflow, and sea-ice distribution over the last 150 kyr. The stratigraphy of the cores is based on stable oxygen isotopes, AMS 14 C , and paleomagnetic data. We studied variations in marine and terrigenous input by a multiproxy approach, involving direct comparison of sedimentological and organo-geochemical data. Extensive episodes of northern Barents Sea ice-sheet growth during marine isotope stages (MIS) 6 and 2 have been supported by, at least, subsurface Atlantic water inflow, moisture-bearing storms, low summer insolation, and minimal calving of ice. Ice advance during MIS 4 was probably restricted to the shallow shelf. Between MIS 4 and MIS 2, large ice-sheet fluctuations correspond to contemporary Laurentide surging events and indicate short-term climatic changes in the Arctic Ocean as has been recorded in lower latitudes. In contrast, in low precipitation areas in eastern Eurasia, glacial activity was rather limited. Only distinct ice-rafted debris (IRD) input during Termination II and early MIS 3 reflects severe glaciations on the northern Severnaya Semlya margin during MIS 6 and MIS 4. We conclude that (1) oscillations of ice-sheets are less frequent along the eastern Eurasian margin than in areas with continuous moisture supply like the western Eurasian margins and that (2) major fluctuations of the Kara Sea ice-sheet during the last 150 kyr apparently followed the major interglacial/glacial MIS 5/4 and MIS 7/6 transitions rather than the precession (23 kyr) and the tilt (41 kyr) cyclicity of the Earths orbit as observed for the Scandinavian (SIS) and the Svalbard ice-sheets, respectively [Mangerud, J., Jansen, E., Landvik, J.Y., 1996. Late Cenozoic history of the Scandinavian and Barents Sea ice-sheets. In: Solheim, A., Riis, F., Elverhoi, A., Faleide, J.J., Jensen, L.N., Cloetingh, S. (Eds.), Impact of Glaciations on Basin Evolution: Data and Models from the Norwegian Margins and Adjacent Basins. Global and Planetary Chance, Special Issue 12, pp. 11-26.]. Surface and/or subsurface Atlantic water masses coupled with seasonally ice-free conditions penetrated continuously to at least the Franz Victoria Trough during the last 150 kyr. However, sustained periods of open water were largely restricted to substages 5.5, 5.1, and the Holocene as indicated by distinct carbonate dissolution and higher accumulation of marine organic matter (MOM). Signals of periodic open-water conditions along the northern margin of Severnaya Semlya are of less importance. Higher production of foraminifera, probably due to Atlantic water inflow occurred between 38 and 12 14 C kyr and corresponds to periodic Atlantic water advection penetrating into the Arctic Ocean. However, marine organic proxies indicate a continuous decrease of surface-water productivity from the western to the eastern Eurasian continental margin due to a more extensive sea-ice cover over the last 150 kyr.

Marine ice-rafted debris records constrain maximum extent of Saalian and Weichselian ice-sheets along the northern Eurasian margin

Ž. Ž . Ž . Ice-rafted debris IRD ) 2 mm , input in eight sediment cores along the Eurasian continental margin Arctic Ocean , have been studied over the last two glacialrinterglacial cycles. Together with the revised chronologies and new Ž. Ž . micropaleontological data of two cores from the northern Barents Sea PS2138 and northeastern Kara Sea PS2741 Ž. spanning Marine Isotope Stages MIS 6 to 1, the IRD data give new insights into the glacial history of northern Eurasian ice-sheets over the last 150 ka. The chronologies of the cores are based on stable isotope records, AMS 14 C datings, paleomagnetic and biostratigraphic data. Extensive episodes of northern Barents Sea ice-sheet growth, probably to the shelf edge, occurred during the late Ž. Ž . Weichselian MIS 2 and the Saalian MIS 6 . Major IRD discharge at the MIS 4r3-transition hints to another severe glaciation, probably onto the outer shelf, during MIS 4. IRD-based instabilities of the marine-based ice margin along the northern Barents Sea between MIS 4 and 2 are similar in timing with North Atlantic Heinrich events and Nordic Seas IRD events, suggesting similar atmospheric cooling over a broad region or linkage of ice-sheet fluctuations through small sea-level events. In the relatively low-precipitation areas of eastern Eurasia, IRD peak values during Termination II and MIS 4r3-transi- tion suggest a Kara Sea ice-sheet advance onto the outer shelf, probably to the shelf edge, during glacial MIS 6 and 4. This suggests that during the initial cooling following the interglacials MIS 5, and possibly MIS 7, the combined effect of sustained inflow of Atlantic water into the Arctic Ocean and penetration of moisture-bearing cyclones into easterly direction Ž. Ž . supported major ice build-up during Saalian MIS 6 and Mid-Weichselian MIS 4 glaciation. IRD peak values in MIS 5 indicate at least two advances of the Severnaya Semlya ice-sheet to the coast line during the Early Weichselian. In contrast, Ž.

Near East Desertification: Evidence from the Dead Sea

Christoph Heim, Norbert R. Nowaczyk, Jo¨rg F.W. NegendankGeoForschungsZentrum, Bereich 3.3 Sedimente und Beckenbildung,Telegrafenberg A26, D-14473 Potsdam, GermanySuzanne A.G. LeroyCentre for Palaeoecology, School of Geosciences,Queen’s University of Belfast, Northern Ireland, UKZvi Ben-AvrahamDead Sea Research Center, Tel Aviv University, Ramat Aviv, 69978, Israel

Combined high-resolution magnetostratigraphy and nannofossil biostratigraphy for late Quaternary Arctic Ocean sediments

A detailed paleomagnetic and calcareous nannofossil study has been carried out on long sediment cores taken during the 1987 ARK IV/3 expedition of R.V. Polarstern in the eastern Arctic Ocean. The Brunhes-Matuyama boundary was not recovered in any of the cores. The magnetostratigraphies reflect the complex behaviour of the geomagnetic field during the Brunhes Chron of predominent normal polarity. Several short excursions and polarity events of the earths magnetic field are documented in the cores. Their regional correlation was possible on the basis of the characteristic downcore magnetic susceptibility patterns and other physical properties of the sediments. Using the calcareous nannofossil chronostratigraphic framework, seven geomagnetic events of short duration were identified for the last 200 ky. Sedimentation rates derived from the stratigraphic data are highly variable for the Nansen-Gakkel Ridge area, ranging from a few mm ky-1 to several cm ky-1 . More uniform rates of 2-3 cm ky-1 were obtained for the central Fram Strait, the eastern Yermak Plateau, and the southern Nansen Basin and up to 9 cm ky-1 at the Svalbard continental slope and the northern edge of the Yermak Plateau. The chronostratigraphic scheme of geomagnetic events and coccolith abundance patterns developed for the Fram Strait area was applied to the sites of the ARK IV/3 expedition further north. It was then possible to relate the occurrence of coccoliths in the sediments to time. Towards the north, coccoliths occur later in the interglacial cycle, reflecting the influx of southerly currents carrying coccolith stocks to the Arctic Ocean.

Late Quaternary dinoflagellate cyst stratigraphy at the Eurasian continental margin, Arctic Ocean: indications for Atlantic water inflow in the past 150,000 years

Four sediment cores located at the Eurasian continental margin underlying the Atlantic layer have been studied for their dinoflagellate cyst content. Concentrations of distinct dinoflagellate cyst taxa display fluctuations in the late Quaternary, which are linked to changes in the inflow of relatively warm Atlantic surface and near-surface waters, resulting in increased local production of cysts in certain time intervals. Based on the assumption that marked changes in strength of inflow occurred synchronously at the Eurasian continental margin, concentration maxima can be used to correlate sediment cores. A dinoflagellate cyst record from the northern Barents Sea continental margin has been related to the stable oxygen isotope and paleomagnetic records to provide direct chronological information. The combination of these methods permits definition of stratigraphic sections equivalent to oxygen isotope stages in carbonate-poor sequences from the Eurasian continental margin. Previous age models of sediment cores are revised, based on dinoflagellate cyst abundance peaks and species distribution, but a firm chronostratigraphy of sedimentary sequences at the eastern Laptev Sea continental margin cannot be established because of the weak signal at the sites furthest from Fram Strait. Ž. In the past 150,000 years, the influence of Atlantic sub- surface waters generally decreased from west to east along the Eurasian continental margin, in particular during the glacials. Pronounced concentration maxima of cosmopolitan and temperate–subpolar dinoflagellate cysts indicate the inflow of Atlantic waters and seasonally increased production of cysts in the Holocene and Eemian. The Holocene is well-marked at the entire Eurasian continental margin but it is more difficult Ž.

Sedimentation rates in the Makarov Basin, central Arctic Ocean: A paleomagnetic and rock magnetic approach

Three long sediment cores from the Makarov Basin have been subjected to detailed paleomagnetic and rock magnetic analyses. Investigated sediments are dominated by normal polarity including short reversal excursions, indicating that most of the sediments are of Brunhes age. In general, the recovered sediments show only low to moderate variability in concentration and grain size of the remanence-carrying minerals. Estimations of relative paleointensity variations yielded a well-documented succession of pronounced lows and highs that could be correlated to published reference curves. However, together with five accelerator mass spectrometry C-14 ages and an incomplete Be-10 record, still two different interpretations of the paleomagnetic data are possible, with long-term sedimentation rates of either 1.3 or 4 cm kyr(-1) However, both models implicate highly variable sedimentation rates of up to 10 cm kyr(-1), and abrupt changes in rock magnetic parameters might even indicate several hiatuses.