Dynamics of North African Humid Periods of the last 180.000 years
Prof. Dr. Werner Ehrmann
Dr. Stefan Krüger
MSc Sarah Beuscher
Dipl.-Geol. Martin Seidel
Prof. Dr. Gerhard Schmiedl
Geological-Paleontological Institute, University of Hamburg
Dr. Hartmut Schulz
Department of Geosciences, University of Tübingen
Prof. Dr. Helge Arz
Leibniz Institute for Baltic Sea Research, Warnemünde (IOW)
Deutsche Forschungsgemeinschaft, Bonn
Geological data and model simulations document repeated African Humid Periods during the late Quaternary. Each of them spans ca. 10,000 years (10 ka) and corresponds to a much shorter interval of sapropel formation in the deep Eastern Mediterranean Sea. Because only few detailed geological records are available and numerical circulation models do not draw a uniform picture, there is still a debate regarding the dynamics of the humid periods, especially the nature of the landscape evolution, the asymmetry between West and East African records, the dynamics during onset and termination of humid periods, and possible biophysical feedbacks between climate and vegetation.
In our study we plan to approach these open questions from a distal perspective by reconstructing the intensity and dynamics of dust fluxes from North Africa into the EMS. We will achieve this by producing high-resolution data sets from three sediment cores recovered from key positions in the EMS. The cores contain sediments of the last 180 ka including sapropel layers S1 to S6 and the corresponding humid periods. The core from the Nile River discharge plume in the SE Levantine Sea, close to the coast of Israel, should document changes in the monsoon-driven riverine sediment input. The core from the SW Levantine Sea, close to the North African coast off Libya should be primarily influenced by dust fluxes from the central and eastern Sahara, and should record the hydrological history of that region. The third core comes from the Ionian Sea, SE of Crete. Our preliminary study shows that in this core we can distinguish between climatic signals from the north and the south.
We want to investigate especially the interplay of solar insolation, monsoon activity, precipitation, vegetation cover, river runoff, wind erosion and aeolian transport. We are most interested in the timing of the African Humid Periods, on their gradual versus abrupt onsets and terminations and the relation between aridification/dust transport and precipitation/river discharge. To this end, we intend to apply different sedimentological and geochemical proxies for investigating processes involved in the transfer of climatic signals from the central and eastern part of North Africa to the Eastern Mediterranean Sea. Suitable proxies are clay minerals, grain sizes and major and trace elements. Furthermore we want to study the species composition and microhabitat structure of the benthic foraminiferal fauna in order to assess the fertilization potential of Saharan dust on the marine ecosystem of the Eastern Mediterranean Sea.
(1) The ratio between the clay minerals kaolinite and chlorite has been investigated in high resolution in the late Quaternary sediment core GeoTü SL143 from the central Aegean Sea. The record spans the last ca. 105 ka. The kaolinite/chlorite ratio was used to reconstruct the fine-grained aeolian dust influx from the North African deserts, mainly derived from desiccated lake depressions. It therewith can be used as a proxy for wind activity, aridity and vegetation cover in the source area. The data document three major humid phases in North Africa bracketing the formation of sapropel layers S4, S3 and S1. They occur at >105–95 ka, 83.5–72 ka and 14–2 ka. The first two phases are characterised by relatively abrupt lower and upper boundaries suggesting a non-linear response of vegetation to precipitation, with critical hydrological thresholds. In contrast, the onset and termination of the last humid period were more gradual. Highest kaolinite/chlorite ratios indicating strongest aeolian transport and aridity occur during Marine Isotope Stage 5b, at ca. 95–84 ka. The long-term decrease in kaolinite/chlorite ratios during the last glacial period indicates a gradual decline of deflatable lake sediments in the source areas.
(2) Clay mineral assemblages in a sediment core from the distal Nile discharge plume off Israel have been used to reconstruct the late Quaternary Nile sediment discharge into the Eastern Mediterranean Sea (EMS). The record spans the last ca. 140 ka. Smectite abundances indicate the influence of the Blue Nile and Atbara that have their headwaters in the volcanic rocks of the Ethiopian highlands. Kaolinite abundances indicate the influence of wadis, which contribute periodically to the suspension load of the Nile. Due to the geographical position, the climate and the sedimentary framework of the EMS is controlled by two climate systems. The long-term climate regime was governed by the African monsoon that caused major humid periods with enhanced sediment discharge at 132 to <126 ka (AHP5), 116 to 99 ka (AHP4), and 89 to 77 ka (AHP3). They lasted much longer than the formation of the related sapropel layers S5 (>2 ka), S4 (3.5 ka) and S3 (5 ka). During the last glacial period (MIS 4–2) the long-term changes of the monsoonal system were superimposed by millennial-scale changes of an intensified mid-latitude glacial system. This climate regime caused short but pronounced drought periods in the Nile catchment, which are linked to Heinrich Events and alternate with more humid interstadials. The clay mineral record further implies that feedback mechanisms between vegetation cover and sediment discharge of the Nile are detectable but of minor importance for the sedimentary record in the southeastern Mediterranean Sea during the investigated African Humid Periods.
(3) We established a high-resolution record of fine-grained eastern Saharan dust from the Eastern Mediterranean Sea spanning the last 180 ka. It is based on the clay mineral composition of the marine sediment core GeoTü SL71 (50 km SE of Crete), especially the kaolinite/chlorite ratio. Minimum aeolian kaolinite transport occurred during the African Humid Periods because kaolinite deflation was hampered by increased humidity and vegetation cover. Instead, kaolinite weathering from kaolinite-bearing Cenozoic rocks was stored in lake basins, river beds and soils during these periods. During the subsequent dry phases, fine-grained dust was mobilised from the desiccated lakes, rivers and soils resulting in maximum aeolian uptake and transport of kaolinite. The kaolinite transport decreased again while these sediment sources exhausted. We conclude that the amount of clay-sized dust blown out of the Sahara into the Eastern Mediterranean Sea is proportional to the intensity of the kaolinite weathering and accumulation in soils and lake sediments, and thus to the strength of the preceding humid period. The strongest humid period occurred during the Eemian supporting earlier evidence for the migration of modern humans out of Africa during this time.
Ehrmann, W., Schmiedl, G., Beuscher, S. & Krüger, S. (submitted): Intensity of African Humid Periods quantified by Saharan dust fluxes. - PLoS ONE
Ehrmann, W., Schmiedl, G., Seidel, M., Krüger, S. & Schulz, H. (2016): A distal 140 kyr sediment record of Nile discharge and East African monsoon variability. - Climate of the Past, 12: 713-727. doi: 10.5194/cp-12-713-2016
Ehrmann; W., Seidel, M. & Schmiedl, G. (2013): Dynamics of Late Quaternary North African humid periods documented in the clay mineral record of central Aegean Sea sediments. - Global and Planetary Change, 107: 186-195. doi:10.1016/j.gloplacha.2013.05.010
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Figure 1: Reconstruction of humid and arid phases in North Africa during the last 140 ka. (a) June insolation at 65°N; (b) Influx of Saharan dust to the central Aegean Sea (GeoTü SL143; Ehrmann et al. 2013); (c) Nile sediment discharge (core MS27PT from the western Nile delta; Revel et al., 2010; Caley et al., 2011); (d, e) Sediment discharge by the Nile River, reconstructed from core GeoTü SL110 in SE Levantine Sea (Ehrmann et al. 2016), with contribution from Egyptian wadis (d) and contribution by the Blue Nile and the Atbara (e). The African Humid Periods AHP1, AHP3, AHP4 and AHP5 are well documented by both the strongly reduced aeolian influx and the enhanced Nile sediment discharge. S sapropels, MIS = Marine Isotope Stages, H = Heinrich Events.
Figure 2: Top: Saharan dust record from core GeoTü SL71 in the Ionian Sea documented by the kaolinite/chlorite ratio, with dust pulses D1 to D6 (Ehrmann et al., submitted). Grey bars indicate sapropel layers (S). Younger Dryas (YD), Heinrich Events (H) and other cold events (o–t) are labelled. MIS = Marine Isotope Stage. Bottom: Conceptual model of dust dynamics (Ehrmann et al., submitted). (A) Changes in the quantity of fine-grained dust influx to the EMS with time. 1: Background level; 2: Pluvial phase with sapropel formation; 3: Dust phase, 4: Glacial drought. The dotted area was used to calculate a dust index. (B) Sketches for the individual phases labelled in panel (A). 1: Arid periods with restricted deflation of kaolinite from kaolinite-bearing sedimentary rocks. 2: Cessation of aeolian transport of kaolinite during humid periods due to vegetation and reduced wind activity. Clay-rich erosion products accumulate in lake basins. 3: With the return of arid conditions, the kaolinite-rich lake sediments are blown out from the desiccated basins leading to maximum kaolinite influx to the Eastern Mediterranean Sea. 4: Intensified aeolian activity during glacial droughts (Heinrich Events) causes minor kaolinite maxima.