Dirk C Leuschner

The low-latitude monsoon climate during Dansgaard–Oeschger cycles and Heinrich Events

During the last 100,000 years Dansgaard–Oeschger cycles (D/O cycles) and Heinrich Events have been the dominant signal of past climate variability over Greenland and the North Atlantic. The succession of stadials (cold) and interstadials (warm) associated with these cycles has been documented in records from the entire northern hemisphere, South America, New Zealand, Antarctica, the South Atlantic and the Southern Ocean. Evidently, climate forcing in the D/O band affects both hemispheres. The origin and cause of these teleconnected patterns is still unknown, even if a large proportion of the cooling in Europe and northern Asia during Heinrich Events is a meteorological response to cold surface water in the North Atlantic resulting from the surge of the Laurentian and Scandinavian ice sheets. But, this does not imply that the D/O cycles are a priori caused by the collapse of these large icesheets. A severe challenge to a primary origin from the northern ice sheet and North Atlantic comes from the observation that warming in Antarctica and the circumpolar current leads the North Atlantic changes by some 1500 years (Charles et al., 1996; Blunier et al., 1998). In the second part of the paper we investigate new deep sea records from the low-latitude monsoon system, mainly corroborating the result of Schulz et al. (1998) that the Indian monsoon showed D/O variability. The abundance of eolian dust (indicating continental humidity/aridity) in core 70KL from the Arabian Sea shows humid intervals which seem to correlate with temperature maxima in the Antarctic Vostok ice core. Apparently, the low-frequency, sub-Milankovitch variability of the monsoon is associated with the southern hemisphere. The D/O-scale component in the monsoonal climate, on the other hand, shows a succession of short humid intervals. The sequence is most closely comparable to the Greenland temperature record and to the stadial/interstadial succession in the Pacific Santa Barbara Basin ODP 893. In order to understand the global forcing of climate change in the D/O band, it appears necessary to quantify the phase relationships not only between D/O cycles in the polar ice cores (Bender et al., 1994; Blunier et al., 1998; Steig et al., 1998), but also between the low latitude Pacific, Indian Ocean and the high-latitude records.

Orbital insolation forcing of the Indian Monsoon – a motor for global climate changes?

Abstract Both modern and ancient Indian summer monsoons are driven by transequatorial pressure differences, directly coupled with the insolation difference between the Northern and Southern subtropical Hemispheres. A high-resolution record of upwelling and dust flux from the western Arabian Sea resembles an insolation-based Indian Summer Monsoon Index. This index and the observed monsoonal climate variations share major elements on the orbital obliquity and precessional band with the Specmap marine oxygen isotope record, representing global ice volume. The long-term evolution of the index mirrors almost exactly the insolation changes at 65°N, showing that the forcing of low latitude climate variability has a structure similar to that of the insolation forcing in the high northern latitudes. Moreover, insolation forcing in the low latitudes directly controls atmospheric processes in the African, Indian and Asian Monsoon, being responsible for a huge amount of transequatorial water vapour and therefore latent heat transport. Millennial-scale variability of the monsoonal climate is concentrated at periodicities near 1100, 1450, 1750 and 2300 years. These cycles are not strictly periodic, but occur in bands, with specific activity phases and amplitude increases during warm stages and interstadials for the 1100- and 1750-yr cycles, whereas the 1450-yr cycle dominates the cold intervals.

Palaeoenvironmental changes in the arid and sub arid belt (Sahara-Sahel-Arabian Peninsula) from 150 kyr to present

The PEP III Arid to Subarid Belt includes the largest hot desert in the world, the Sahara- Arabian desert and the Sahel zone. The region of interest extends south of the Atlas Mountains and south and east of the Mediterranean Sea to approximately 10 °N and shows a broadly zonal pattern with a varying seasonal distribution of precipitation. In the north (ca. 20–23 °N), rainfall results from the southward displacement of the midlatitude westerlies during winter whereas the south is governed by seasonal northward migration of the Intertropical Convergence Zone (ITCZ). Contraction and expansion phases of these presently semi-arid to hyper-arid desert areas result from significant changes in local precipitation. Palaeoenvironmental records from Northern Africa (north of 10 °N) and the surrounding seas document long-term changes in the magnitude and extent of the African monsoon in response to orbitally-forced changes in insolation. However, marine records as well as terrestrial palaeohydrological indicators (e.g., lakes, speleothems, rivers, pollen and charcoal) show that there have been changes in the hydrological cycle superimposed on the long-term waxing and waning of the monsoon which cannot be explained exclusively by changes in insolation. These fluctuations in space, time and magnitude were on a regional to continental scale. Here, we review available data on near-surface palaeohydrological indicators and vegetational changes in arid North Africa and the Arabian Peninsula as well as changes in the intensity of the South Asian Monsoon identified from marine sediments of the Arabian Sea. A comparison of regional environmental changes can clarify relations between the environment and changes in the Earth’s climate system. Each data-set is initially presented independently because they represent heteregeneous records from different regions and time periods and thereby emphasise their potential to provide evidence of continental chronostratigraphic palaeoenvironmental changes. Data-sets of lake status and vegetational change are complementary as they strongly reflect hydrological variation. Deep-sea sediments from the Arabian Sea were used to generate continuous records of oceanic upwelling, continental humidity, and dust and river discharge, that are closely related to palaeoenvironmental changes on the surrounding continents.After presenting the individual data-sets we compare the palaeoclimatic reconstructions derived from the different types of evidence.

Possible influence of Zoophycos bioturbation on radiocarbon dating and environmental interpretation

Abstract In paleoenvironmental studies of marine sediments bioturbation is often neglected and/or only treated as a diffusion-like process affecting only the uppermost sediment with decreasing intensity with depth. Deep dwelling animals, like the Zoophycos producing animal, however, affect the sediment composition by transporting material over vertical distances of up to 1 m below the seafloor. In Arabian Sea sediment cores 70KL, 64KL and 57KL a significant downward transport of particles by Zoophycos can be observed. Within the Zoophycos burrows the faunal composition of both planktonic and benthic foraminiferal assemblages as well as the isotopic signature of foraminiferal carbonate differ significantly from the background sediment. Radiocarbon ages obtained from foraminiferal tests derived from the Zoophycos burrows reveal ages of up to 10 000 yr younger than those from the background sediment, indicating the time when the burrow was active. Faunal and isotopic compositions within the Zoophycos burrows resemble the surface sediment conditions at the time the Zoophycos producing animal lived. These features may result in misleading environmental interpretations regarding for example transfer functions of paleotemperature and -salinity or ventilation reconstructions. Moreover, the results show that Zoophycos bioturbation may significantly bias the radiocarbon age assessment for paleoenvironmental events if sampling is not carried out as careful to avoid possible incorporation of Zoophycos bioturbation material.

Geochemical implications for changing dust supply by the Indian Monsoon system to the Arabian Sea during the last glacial cycle

Element concentrations of 43 elements as well as inorganic and organic carbon content of sediment core 70KL from the western Arabian Sea were measured with high (1 cm) sample resolution. Principal components of the sediment’s chemical composition were determined with the help of statistical principle component analysis. These components are representing the major environmental factors at the site. The most important processes controlling the observed variations are the changing lithogenic influx derived from the major wind systems of the region (i.e., the Arabian northwesterly winds, the northeast winter monsoon and the southwest summer monsoon), summer monsoon associated upwelling and biogenic productivity as well as the redox conditions at the sediment-water interface.