A Drop in Sahara Dust Fluxes Records the Northern Limits of the African
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ARTICLE https://doi.org/10.1038/s41467-019-11701-z OPEN A drop in Sahara dust fluxes records the northern limits of the African Humid Period Daniel Palchan 1,2 & Adi Torfstein 1,2 Northern and eastern Africa were exposed to significantly wetter conditions relative to present during the early Holocene period known as the African Humid Period (AHP), although the latitudinal extent of the northward expansion of the tropical rain belt remains 230 1234567890():,; poorly constrained. New records of Thxs-normalized accumulation rates in marine sedi- ment cores from the Red Sea and Gulf of Aden are combined with existing records of western Africa dust and terrestrial records across the Sahara Desert, revealing that fluxes of dust transported east from the Sahara decreased by at least 50% during the AHP, due to the development of wetter conditions as far north as ~22°N. These results provide the first quantitative record of sediment and dust accumulation rates in the Red Sea and the Gulf of Aden over the past 20 kyrs and challenge the paradigm of vast vegetative cover across the north and northeastern Sahara Desert during the AHP. 1 The Fredy & Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, The Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel. 2 Interuniversity Institute for Marine Sciences, Eilat 88103, Israel. Correspondence and requests for materials should be addressed to D.P. (email: [email protected]) NATURE COMMUNICATIONS | (2019) 10:3803 | https://doi.org/10.1038/s41467-019-11701-z | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11701-z he primary source of dust to Earth’s atmosphere is the Methods) over the last 20 kyrs between the Gulf of Aden, central Sahara Desert in northern Africa with an estimated annual Red Sea, and northern Red Sea (Fig. 1; cores KL15; KL11; and Tfl – 1 ux of up to 1600 1700 Tg . Atmospheric dust is strongly KL23, respectively; for exact locations see Table 1). We combine linked with global climate and environmental conditions through the new dust accumulation rates with additional dust and various mechanisms, including the scattering and absorption of hydrology records from sites in the Red Sea, the Atlantic Ocean solar and terrestrial radiation, modification of cloud properties, and the Sahara Desert, to elucidate the latitudinal extent of the and fertilization of oligotrophic oceans with a subsequent AHP impact in northern Africa and the eastern dust emissions enhancement of marine photosynthesis, which in turn modulates during the last deglacial and Holocene. The results indicate a 2,3 marine carbon uptake and atmospheric CO2 concentrations .It relatively limited northern extent of the AHP in eastern Africa is well established that during the early-mid Holocene, wetter (~ 22°N), and hence, challenge the paradigm of a vast vegetative conditions than today prevailed in northern Africa because of the cover across the north and northeastern Sahara Desert during intensification and northward expansion of the African summer the AHP. monsoon rains4,5 triggered by both external changes in insolation fl 6–9 ux and internal feedbacks from albedo variations over land . Results Yet, the extent of the latitudinal expansion of the African summer Dust sources and eastward transport. Considering that the monsoon precipitation is still debated10. Some studies support – drainage basins that surround the Red Sea are relatively small, more-conservative estimations11 13, whereas others suggest a and given the regional hyperarid conditions that limit direct green Sahara with vast vegetation cover, large waterbodies, rivers, fluvial contributions to be negligible relative to the desert dust and tropical rainfall that persisted in the currently hyperarid – plumes30, the terrigenous fraction in the Red Sea bottom sedi- region14 18. Indeed, pollen records suggest that the Sahara Desert ments is considered to be overwhelmingly of eolian origin. The (north of 25°N) accommodated some tropical plants, but only sources of the dust were identified using backward trajectory along perennial rivers and waterbodies13,19. Accordingly, these do analyses (Supplementary Note 1): dust reaching the northern Red not necessarily reflect regional climate conditions but rather, the Sea typically originates from northern Libya and Egypt, dust migration of some plants through designated waterways. Recent reaching the central Red Sea originates from both Sudan and the compilations of Holocene radiocarbon ages from terrestrial Afar region, and dust reaching the Gulf of Aden is delivered from sites in the Sahara Desert, including lacustrine and palustrine the Horn of Africa. Only a small fraction of the air parcels that environments5,8,9 along with vegetation reconstructions11, sug- are transported toward the Red Sea and the Gulf of Aden origi- gest these records are insufficiently resolved or discontinuous due nate from the Arabian Peninsula (Supplementary Fig. 1). The to erosion and other hiatuses13,20. The influence of the intensi- provenance of the terrigenous fraction of downcore records in the fication and migration of the monsoon rains was also recorded Red Sea, based on their radiogenic isotope composition, confirms along the western margins of Africa in well dated, continuous that the latter dust sources were similarly active during the late marine sediment cores7,21 as well as a lacustrine sediment core20. Quaternary and the Holocene30. Multiple studies of riverine and windblown sediments from the Through the last deglaciation, dust accumulation rates in the marine environment at the Atlantic Ocean suggest that more- Gulf of Aden and the central Red Sea dropped from maximum humid conditions prevailed through the Sahara and the Sahel − − − − values of 0.72 g cm 2 ka 1 and 0.89 g cm 2 ka 1 during the between ~ 12 and 5 ka4,7,20 with an abrupt termination and − − − deglacial to minimum values of 0.40 g cm 2 ka 1 and 0.35 g cm 2 Saharan aridification at ~ 5.5 ka4,8,20,22. Dust emissions westward − ka 1 at ca. 7 ka, respectively (Figs. 1, 2a & Table 2). This ~twofold from the Sahara, co-varied with these environmental changes, drop in dust accumulation rates is too big to be attributed to where lower fluxes characterize the humid period4,21,23,24. Med- possible effects of grain size sorting on 230Th -normalized iterranean sediment cores recorded the AHP environmental xs fluxes31, and is not the result of dilution of the dust component in impact on dust as a shift in the kaolinite/chlorite clay proportions, the core sediments (i.e., by enhanced deposition of marine which was interpreted to reflect extensive lake cover across microfossils) as the 230Th -normalized dust fluxes are relatively northern Sahara16. Furthermore, a 3 Ma long record of eastern xs insensitive to this32. It is much more reasonable to attribute this Mediterranean sediment (core 967) displays lower hematite change, which correlates with the monsoon index that is a concentrations in sapropel layers, including during the Holocene measure of the intensity of the African Monsoons33 (Fig. 2b), to AHP interval. This is interpreted to reflect decreased dust flux changes in environmental conditions in the region that owing to the greening of the Sahara18. Marine records from the suppressed dust uptake from the source regions (Supplementary Gulf of Aden25 and from the western Arabian Sea26 present an Note 1). In addition, all three cores display stable and close to 1 overall similar shift to wetter and dryer conditions at the early focusing factors (Supplementary Fig. 2), suggesting the calculated and mid Holocene, respectively. The latter observations, per- dust fluxes reflect vertical fluxes with minor effects of sediment taining to conditions in eastern Africa are corroborated by a focusing or winnowing (see Methods). record of Nile Delta sediments, which reflects the climate patterns in the eastern Africa headwaters of the Nile River27. The north- eastern flanks of Africa remain poorly studied in the context of Additional environmental proxies. Records from the Nile the environmental impact of the AHP apart of some controversial Delta and from the Gulf of Aden reflect wetter conditions at evidence for carbonate deposition in terrestrial sites28, which will eastern Africa during the AHP. Enhanced Nile River outflow is be discussed below. It is worth mentioning, however, the recent evident from the increased presence of basaltic detritus and study of speleothems from northeastern Africa29, where episodes depleted Mediterranean foraminifera δ18O values (Fig. 2c) in of growth occurred during previous interglacials but not during Nile Delta core MS2727 (Fig. 3), and a leaf wax δD record from the Holocene. These carbonate deposits are further characterized core RC09 from the Gulf of Aden25 (Fig. 2i). These records are by distinctly light oxygen isotopes, which were interpreted to well correlated and point to a coeval precipitation increase in reflect a northward expansion of summer African monsoon rains the Horn of Africa and its vicinity. The increased precipitation that brought Atlantic Ocean moisture to the Desert29 rather than resulted in the enhanced growth of the vegetation cover34 that a proximal source in the Mediterranean Sea. inhibited dust uptake, a connection that is well expressed by the Here, we focus on the northeastern part of Africa and present positive correlations between leaf wax δD and dust accumula- 230 2 = new records of Thxs-normalized dust accumulation rates (see tion rates in the Gulf of Aden (R 0.3) and the Central Red 2 NATURE COMMUNICATIONS | (2019) 10:3803 | https://doi.org/10.1038/s41467-019-11701-z | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11701-z ARTICLE ab20°W 15°W 10°W 5°W 35°E 40°E 45°E 50°E AOD 1 30°N GC27 0.85 KL23 30°N 0.71 GC37 0.57 20°N KL11 GC49 0.43 20°N GC66 0.29 GC68 KL15 0.14 10°N 0 cdDetrital flux (g cm–2 ka–1) Detrital flux (g cm–2 ka–1) 2 0.9 30°N 25°N 0.8 28°N 1.5 0.7 26°N 20°N AHP AHP 1 HS-1 0.6 Latitude 24°N Latitude HS-1 0.5 22°N 0.5 15°N 0.4 20°N Ocean Data View/DIVA Ocean Data 0 View Ocean Data 0.3 051015 20 051015 20 Age (ka) Age (ka) Fig.