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Featured researches published by Philipp Hoelzmann.


Geophysical Research Letters | 1999

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

Martin Claussen; Claudia Kubatzki; Victor Brovkin; Andrey Ganopolski; Philipp Hoelzmann; Hans-Joachim Pachur

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.


Global Biogeochemical Cycles | 1998

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

Philipp Hoelzmann; D. Jolly; Sandy P. Harrison; F. Laarif; Raymonde Bonnefille; Hans-Joachim Pachur

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.


Archive | 2004

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

Philipp Hoelzmann; Françoise Gasse; Lydie M Dupont; Ulrich Salzmann; Michael Staubwasser; Dirk C Leuschner; Franko Sirocko

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.


Quaternary Research | 1991

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

Hans-Joachim Pachur; Philipp Hoelzmann

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.


Global and Planetary Change | 2000

Precipitation estimates for the eastern Saharan palaeomonsoon based on a water balance model of the West Nubian Palaeolake Basin

Philipp Hoelzmann; Hans-Joachim Kruse; Frank Rottinger

During the Holocene, the eastern Sahara underwent a drastic climatic change: in less than 6000 years, the climate changed from hyperaridity to semiaridity and back to its current hyperarid state. The West Nubian Palaeolake Basin in NW Sudan (ca 18.5°N/25.5°E) contains widespread lake carbonates, which indicate development of a freshwater lake between approximately 9500 and 4000 years BP. The size of the lake has been estimated as between 1100 and 7000 km2. Water balance calculations and a dense drainage net surrounding the terminal lake indicate predominance of superficial and minor importance of groundwater inflow. A lake with an area of 1100 km2 would require an annual precipitation of approximately 500 mm to balance the respective aerial evaporation. Similar precipitation values are currently observed at a latitude of 12°N, some 600 km farther south, while the West Nubian Palaeolake Basin receives an annual rainfall of less than 15 mm/year. A lake size of 7000 km2 would have required 900 mm of annual rainfall, an amount characteristic of sub-tropical conditions. This value could indicate inhomogenities in the wet phase climate, which could strongly influence the water level of a terminal lake located in a flat terrain. The highly depleted oxygen isotope values of the lake carbonates indicate that the Holocene wet phase was characterized by intense tropical summer (monsoonal) rainfall with heavy thunderstorms. Thus, an intensified southwest palaeomonsoon apparently transported large quantities of moisture to the eastern Sahara, producing the necessary conditions for the persisting existence of freshwater in the West Nubian Palaeolake Basin. The reconstructed palaeoclimate implies high groundwater levels and corresponds to precipitation rates from other sites in North Africa.


Journal of African Earth Sciences | 2000

Late quaternary palaeoecology and palaeoclimates of the eastern Sahara

Hans-Joachim Pachur; Philipp Hoelzmann

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.


Global and Planetary Change | 2000

Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs

Paul I. Abell; Philipp Hoelzmann

Abstract Gastropod shells and bulk sedimentary carbonate deposits found in palaeolake sediments in the presently hyperarid regions of NW Sudan provide proxy materials for the evaluation of the vastly different and wetter climatic conditions that prevailed for several thousand years in that region at the beginning of the Holocene. Oxygen and carbon stable isotope ratio measurements on these shells and carbonates suggest that the African Summer Monsoon provided extensive rainfall up to 800 km further north than at present, creating substantial lakes and refilling the deep aquifers of the region with isotopically depleted water up to 21°N. Variations in stable isotope ratios, as one proceeds upwards through the sediments, indicate that the wettest phase occured about 9000 years B.P., and that a considerably drier period began after about 5600 years B.P., after which the record is obliterated by decreasing rainfall and subsequent deflation of the sediments. During the wet phase, large quantities of isotopically depleted (light) moisture — corroborating the convective origin of the rainfall — were brought to the Eastern Sahara by intensified monsoonal rains. Variations in the oxygen isotope ratios during the growth of individual shells demonstrate that considerable seasonality existed in yearly rainfall.


Science | 2016

Repeated catastrophic valley infill following medieval earthquakes in the Nepal Himalaya

Wolfgang Schwanghart; Anne Bernhardt; Amelie Stolle; Philipp Hoelzmann; Basanta Raj Adhikari; Christoff Andermann; Stefanie Tofelde; Silke Merchel; Georg Rugel; Monique Fort; Oliver Korup

Nepals quake-driven landslide hazards Large earthquakes can trigger dangerous landslides across a wide geographic region. The 2015 Mw 7.8 Gorhka earthquake near Kathmandu, Nepal, was no exception. Kargal et al. used remote observations to compile a massive catalog of triggered debris flows. The satellite-based observations came from a rapid response team assisting the disaster relief effort. Schwanghart et al. show that Kathmandu escaped the historically catastrophic landslides associated with earthquakes in 1100, 1255, and 1344 C.E. near Nepals second largest city, Pokhara. These two studies underscore the importance of determining slope stability in mountainous, earthquake-prone regions. Science, this issue p. 10.1126/science.aac8353; see also p. 147 Sediment records are used to identify catastrophic debris flows from paleoquakes near Pokhara, Nepal. Geomorphic footprints of past large Himalayan earthquakes are elusive, although they are urgently needed for gauging and predicting recovery times of seismically perturbed mountain landscapes. We present evidence of catastrophic valley infill following at least three medieval earthquakes in the Nepal Himalaya. Radiocarbon dates from peat beds, plant macrofossils, and humic silts in fine-grained tributary sediments near Pokhara, Nepal’s second-largest city, match the timing of nearby M > 8 earthquakes in ~1100, 1255, and 1344 C.E. The upstream dip of tributary valley fills and x-ray fluorescence spectrometry of their provenance rule out local sources. Instead, geomorphic and sedimentary evidence is consistent with catastrophic fluvial aggradation and debris flows that had plugged several tributaries with tens of meters of calcareous sediment from a Higher Himalayan source >60 kilometers away.


The Holocene | 2010

Hydrological response of an east-Saharan palaeolake (NW Sudan) to early-Holocene climate

Philipp Hoelzmann; Antje Schwalb; Neil Roberts; P. Cooper; A. Burgess

We present a palaeolimnological record in western Nubia (NW Sudan, eastern Sahara) isolated from large-scale artesian systems and thus reflecting hydrological changes resulting from local rainfall. Past changes in lake water chemistry and water depth are interpreted by combining results from biological indicators (diatoms, ostracodes and charophytes) with geochemical and stable oxygen and carbon isotope data. Palaeolake Gureinat existed from ~10.9 cal. ka BP until at least 7.1 cal. ka BP and is devided into five lake phases. For the first two millennia the lake was shallow and fluctuating salinity conditions prevailed (500 to 3000 μS/cm). After ~8.7 cal. ka BP, the lake level rose, waters became fresh to oligosaline, and the lake had a minimum depth of at least 8 m. Both transgressive and stable lake phases were punctuated by at least three drops in lake level with complete desiccation at around 10.9 and 8.9 cal. ka BP. The third of these regressions may be coeval with the 8.2 ka climatic event. Sediments younger than 7.1 cal. ka BP representing the mid-Holocene fading of the wet phase were removed by deflation. This record shows that lakes in the eastern Sahara registered short-term climatic events, as long as they were isolated from large-scale artesian groundwater systems. Consequently, even in this continental region, early-Holocene climatic fluctuations affected water levels and salinities of hydrologically isolated lake systems.


Marine Pollution Bulletin | 2017

Ecological risk assessment of a coastal zone in Southern Vietnam: Spatial distribution and content of heavy metals in water and surface sediments of the Thi Vai Estuary and Can Gio Mangrove Forest

Sandra Costa-Böddeker; Philipp Hoelzmann; Lê Xuân Thuyên; Hoang Duc Huy; Hoang Anh Nguyen; Otto Richter; Antje Schwalb

Enrichment of heavy metals was assessed in the Thi Vai Estuary and in the Can Gio Mangrove Forest (SE, Vietnam). Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn contents in water and in sediments were measured. Total organic carbon, nitrogen, phosphorus and C/N ratios were determined. Cu and Cr values were higher than threshold effect level of toxicity, while Ni exceeded probable effect level, indicating the risk of probable toxicity effects. Enrichment factors (EF), contamination factor (CF) and Geo-accumulation index (I-geo) were determined. CF reveals moderate to considerable pollution with Cr and Ni. EF suggests anthropogenic sources of Cr, Cu and Ni. I-geo indicates low contamination with Co, Cu and Zn and moderate contamination with Cr and Ni. Overall metal contents were lower than expected for this highly industrialized region, probably due to dilution, suggesting that erosion rates and hydrodynamics may also play a role in metal contents distribution.

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Antje Schwalb

Braunschweig University of Technology

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Daniel Knitter

Free University of Berlin

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Anne Bernhardt

Free University of Berlin

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Michael Meyer

Free University of Berlin

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