Dynamics of North African Humid Periods of the last 130 kyr
Final Report Abstract
North Africa experienced dramatic changes in hydrology and vegetation during the late Quaternary driven by insolation-induced shifts of the tropical rain belt and vegetation-climate feedbacks. Our project aimed to reconstruct the dynamics of arid and humid North African phases from an Eastern Mediterranean Sea (EMS) perspective. We concentrated on sediment cores GeoTü SL71 (SE of Crete), SL99 (off Libya) and SL110 (off Israel) in high resolution. Additionally, we integrated own data from other sediment core from the EMS. We investigated mainly the clay mineral composition, grain size of the terrigenous sediment fraction and benthic foraminifer assemblages. The African Humid Periods (AHPs) are documented both in the aeolian influx and the river discharge to the EMS. Minimum aeolian transport of clay-sized particles as documented by kaolinite/chlorite ratios occurred during the AHPs, when deflation was hampered by humidity and vegetation. Instead, kaolinite derived from weathering of kaolinite-bearing rocks was stored in lake basins, river beds and soils. During the subsequent dry phases, fine-grained dust was mobilised from the desiccated lakes, rivers and soils resulting in dust pulses in EMS sediments. Kaolinite transport decreased again when these sediment sources exhausted. The intensity of the dust pulses correlates with the monsoon intensity and maximum insolation of the preceding humid period. The strongest AHP occurred during the Eemian and was followed by two weaker phases centred at ca. 100 ka and ca. 80 ka. Because of the glacial boundary conditions, the dust pulse at ca. 160 ka is lower than could be expected. In contrast to the influx of clay-sized dust to the EMS, the influx of coarser, silt-sized dust is not controlled by availability but by aridity and wind activity. Its influx stayed on a relatively high level between the AHPs. Differences in the dust flux were observed according to the proximal versus distal position of the sediment cores. The benthic foraminiferal faunas of the studied sites do not respond to the expected dustinduced fertilisation of surface waters. Instead, the post-sapropel diversity patterns and successions of different deep-sea taxa are likely controlled by the seasonality of food fluxes, repopulation dynamics and inter-specific competition. Depending on the strength and duration of the anoxic sapropel phase, the subsequent recovery of deep-sea benthic ecosystems lasted between approximately 2500 and 6000 years. The phases of enhanced river discharge as expressed by the smectite concentrations lasted longer than the sapropel formation in the EMS. Data from core 64PE349-5 (Gulf of Sirte) document that the palaeo-river systems Irharhar, Kufrah and Sahabi were active during all AHPs. The time lag between the patterns of fluvial and aeolian influx can be explained by successive migration of the rain belt, possibly combined with a vegetation feedback. Vegetation feedbacks are also responsible for the much less than expected fluvial sediment delivery to 64PE349-5 and SL99 during AHP5. The orbital changes in fluvial and aeolian influx of fine-grained particles to the EMS are superimposed by millennial-scale changes on the Dansgaard-Oeschger and Heinrich bands during both glacial and interglacial intervals. These short-term variations are best documented in the semi-distal site and illustrate close links between the African monsoon and North Atlantic climate regimes.
Publications
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(2020) Reconstructing Saharan dust transport to the Eastern Mediterranean Sea during the last 180 ka using endmember modelling of grain size data. Quaternary Research 94 156-173
Beuscher, Sarah; Ehrmann, Werner; Krüger, Stefan; Schmiedl, Gerhard
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2016. A distal 140 kyr sediment record of Nile discharge and East African monsoon variability. Climate of the Past, 12: 713-727
Ehrmann, W., Schmiedl, G., Seidel, M., Krüger, S., Schulz, H.
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2017. End-member modelling as a tool for climate reconstruction – An Eastern Mediterranean case study. PLoS ONE 12(9): e0185136
Beuscher, S, Krüger, S., Ehrmann, W., Schmiedl, G., Milker, Y., Arz, H., Schulz, H.
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2017. Intensity of African Humid Periods estimated from Saharan dust fluxes. PLoS ONE 12 (1): e0170989
Ehrmann, W., Schmiedl, G., Beuscher, S., Krüger, S.
