Hans-Joachim Pachur

Simulation of an abrupt change in Saharan vegetation in the Mid‐Holocene

Climate variability during the present inter- glacial, the Holocene, has been rather smooth in compar- ison with the last glacial. Nevertheless, there were some rather abrupt climate changes. One of these changes, the desertication of the Saharan and Arabian region some 4 - 6 thousand years ago, was presumably quite important for human society. It could have been the stimulus leading to the foundation of civilizations along the Nile, Euphrat and Tigris rivers. Here we argue that Saharan and Arabian de- sertication was triggered by subtle variations in the Earths orbit which were strongly amplied by atmosphere- vegeta- tion feedbacks in the subtropics. The timing of this tran- sition, however, was mainly governed by a global interplay between atmosphere, ocean, sea ice, and vegetation.

Mid-Holocene land-surface conditions in northern Africa and the Arabian peninsula : A data set for the analysis of biogeophysical feedbacks in the climate system

Large changes in the extent of northern subtropical arid regions during the Holocene are attributed to orbitally forced variations in monsoon strength and have been implicated in the regulation of atmospheric trace gas concentrations on millenial timescales. Models that omit biogeophysical feedback, however, are unable to account for the full magnitude of African monsoon amplification and extension during the early to middle Holocene (˜9500–5000 years B.P.). A data set describing land-surface conditions 6000 years B.P. on a 1° × 1° grid across northern Africa and the Arabian Peninsula has been prepared from published maps and other sources of palaeoenvironmental data, with the primary aim of providing a realistic lower boundary condition for atmospheric general circulation model experiments similar to those performed in the Palaeoclimate Modelling Intercomparison Project. The data set includes information on the percentage of each grid cell occupied by specific vegetation types (steppe, savanna, xerophytic woods/scrub, tropical deciduous forest, and tropical montane evergreen forest), open water (lakes), and wetlands, plus information on the flow direction of major drainage channels for use in large-scale palaeohydrological modeling. The data set is available in digital form by anonymous ftp.

Paleoclimatic implications of late quaternary lacustrine sediments in Western Nubia, Sudan

Abstract Owing to the hypercontinental location of Western Nubia, secular fluctuations of climate have been filtered and wet phases can be considered as representative of conditions throughout the southeastern Sahara. The study area is crossed by the 20-mm isohyet; between 9300 and about 4000 yr B.P., however, there were widespread lake and swamp environments with freshwater molluscs, ostracods, and diatoms, and a species-rich savanna mammal fauna. The center of the West Nubian Basin (approx. 18°N), an area of about 20,000 km 2 , was occupied by a semiaquatic landscape which was situated at the same latitude as Paleolake Chad. From extensive lake carbonates up to about 4 m thick, a long-term rise of the groudwater table is inferred. Environments developed that now exist at about latitude 13°N. Radiocarbon dates from lake sediment sequences cluster between 30,000 and 21,000 yr B.P., indicating a Pleistocene wet phase. A gap in radiocarbon dates between 21,000 and 11,000 yr B.P. signals a phase of hyperaridity, similar to the present hyperarid phase, with eolian deflation and deposits of sand being the dominant forms of erosion and accumulation.

Late quaternary palaeoecology and palaeoclimates of the eastern Sahara

Abstract Latest field research and palaeoenvironmental reconstructions have revealed that within less than 6000 years the eastern Sahara experienced a dramatic climatic change similar to that in the western Sahara, passing from hyperaridity to semi-aridity (dry savanna) to its present hyperarid state. Groundwater levels started to rise about 9300 years before present ( 14 C years BP), leading to the formation of a mosaic of freshwater lakes and swamps. Within a few decades, the aquifers were loaded and the palaeopiezometric surface was as much as 25 m higher than it is today. The uplands generated up to 800 km long fluvial systems, which put an end to the endorheic drainage of the region and functioned as migration paths for large savanna mammals. These wetter conditions persisted in Western Nubia during the Holocene until ca 5000 years BP The climatic deterioration began around 5700 years BP as shown by evaporitic sediments. Reversal events prior to aridification during the Late Holocene were not recorded systematically in the sediments of the eastern Sahara because of the stability of the ecosystems. Changes in land-surface conditions such as palaeolakes, swamps and vegetation created water vapour sources that generated local rainfall and buffered short dry spells. Radiocarbon-dated charcoal indicates that Neolithic human occupation culminated during this Early Holocene wet phase and ended ca 2000 years after the fading of the wet phase at about 3000 years BP, when the shallow aquifers were exhausted.

22. Interglacial and glacial fingerprints from lake deposits in the Gobi Desert, NW China

Abstract A 230-m long sediment core from the centre of the Gaxun Nur Basin, Gobi Desert, NW China provides evidence for climate induced changes in water balance during the last glacial cycle. Millennial scale and short-term variations of geochemical precipitates and grain size show that freshwater fluxes from the Tibetan Plateau by surface run-off were the main controlling factors for lake evolution in the Tibetan dry forelands for about the last 250 kyr. Periods of positive water balance with strong lake extension and reverse developments generally coincide with changes in the global ice volume and with oxygen-18 records from Tibet and Greenland as well, documenting the close relationship between environmental conditions in remote desert regions of NW China and orbitally forced Northern Hemisphere high mountain mid-latitude and high-latitude climates on a regional and global scale. Our data imply that both the East Asian summer monsoon and the extra-tropical westerlies are the major feedback mechanisms for effective moisture supply over NW China. During the 10-kyr long interglacial warmmoist substage 5.5, summer monsoon moisture dominated owing to its strong northward shift beyond the modern limit. At that time, a large and slightly saline lake filled the entire Gaxun Nur basin as a result of strong river inflow from the Tibetan catchment by melt water supply and by enhanced summer monsoon precipitation. Aeolian transport was weak. The Eemian interglacial in the Gaxun Nur region started at about 129 kyr, with warm and moist environmental conditions between 128 and 121 kyr BP and terminated around 119 kyr, documented by a strong climate shift towards dry conditions and enhanced mobilization of aeolian sand. During interstadial climates, contemporaneous with D/O events in Greenland ice cores, both wind systems most likely supplemented each other, while in transitional phases towards cold conditions, moisture supply by the westerlies seems to have dominated. Cold-dry stages, recorded in the Gaxun Nur core, are synchronous with the global climate. They induced strong lake-level declines and promoted aeolian transport of exposed lake sediments southwards due to the enhanced winter monsoon. Loess records from the Chinese Loess Plateau confirm that the temporal distribution of loess mobilization recorded in the Gaxun Nur sediments was synchronous with depositional phases on the Loess Plateau.

Palaeolake and palaeoenvironment between 42 and 18 kaBP in Tengger Desert, NW China

Comprehensive field investigations and laboratory analyses show that palaeolakes, including freshmesohaline water Megalake Tengger and other semi-connected, isolated water bodies, during late Pleistocene covered an area of more than 20000 km2, which is more than half of the Tengger Desert in NW China. Stratigraphic correlation and chronological evidence indicate that before ca. 42000 aBP the area was more arid. The palaeolakes started to develop around 40000 uncal.14C aBP but until 3700014C aBP their scope was limited. High water levels established from 3500014C aBP lasted until 2200014C aBP. Lake levels regressed between 22000 and 2000014C aBP but transgressed from 20000 to 1860014C aBP. Subsequently, water level declined further and the Megalake Tengger finally desiccated at around 1800014C aBP. Megalake Tengger possessed a fresh-mesohaline water property, implying that the regional precipitation increased significantly. During the period of Megalake Tengger, the climate was warmer-humid than present. The annual rainfall was 250 to 350 mm more than that of today and the temperature was 1.5 to 3.0°C higher.

Palaeolake evolution and abrupt climate changes during Last Glacial Period in NW China

Field investigations and laboratory analyses show that there existed long-lasting and extensive palaolakes between 18,000 and 42,000 yr B.P. in the Tengger Desert, NW China. This palaolake phase developed in a mild humid-warm climate that was very different from that of present. Geochemical data, palynological evidence and grain size variations demonstrate that the lake levels had fluctuated greatly with the climate changes and these fluctuations can be correlated with the Dansgaard-Oeschger and Heinrich events.

The Holocene Palaeoclimatic change in southern vicinity of Tengger Desert

Through stratigraphy and chronology studies on Hongshui River section located at the southern vicinity of Tengger Desert, and based on the analysis results of TOC, TIC, elements, stable oxygen isotope that possess a high resolution of 40–50 a, the paleaoclimatic change history during last 8 000–3 000 a has been reconstructed.