Co-variations of climate and silicate weathering in the Nile Basin during the Late Pleistocene

Abstract

We have investigated provenance and weathering proxies of the clay-size sediment exported from the Nile River basin over the last 110,000 years. Using neodymium isotope composition of sediments from both the Nile Deep Sea-Fan and Lake Tana, we show that the Nile River branches draining the Ethiopian Highlands have remained the main contributors of clays to the Nile delta during the Late Quaternary. We demonstrate that fluctuations of clay-size particle contribution to the Nile Delta are mainly driven by orbital precession cycle, which controls summer insolation and consequently the African monsoon intensity changes. Our results indicate that -over the last 110,000 yearsthe proportion of clays coming from Ethiopian Traps fluctuates accordingly to the intensity of the last 5 precession cycles (MIS 5 to MIS 1). However, there is a threshold effect in the transport efficiency during the lowest insolation minima (arid periods), in particular during the MIS3. Several arid events corresponding to the Heinrich Stadial periods are associated with small or negligible clay source changes while chemical weathering proxies, such as δ 7 Li, Mg/Ti and K/Ti, vary significantly. This suggests a straightforward control of weathering by hydro-climate changes over centennial to millennial timescales. Our data also suggests a significant but more progressive influence of the temperature decrease between 110kyr and 20kyr. Taken altogether, the observed tight coupling between past climate variations and silicate weathering proxies leads us to conclude that precipitation changes in northeast Africa can impact soil development over a few hundred years only, while the influence of temperature appears more gradual.

Climate, vegetation and fire history during the past 18,000 years, recorded in high altitude lacustrine sediments on the Sanetti Plateau, Bale Mountains (Ethiopia)

“…The allochthonous elements decreased abruptly at about 15.7 cal kyr BP, likely correlating with the onset of the AHP, and evoked by rapid northward migration of the rain belt and the ITCZ in relation to precession-driven insolation changes (Bastian et al 2021). We assume that this resulted in a relatively rapid establishment of a denser vegetation cover and a drastic reduction of dust deflation from northern source areas (Figs.…”

“…The abrupt decrease of most allochthonous elements and ratios (Fig. 5) at about 15.7 cal kyr BP indicates a drastic atmospheric reorganization with an abrupt decrease of dust inputs related to the beginning of the AHP, attributed to the northward migration of the rain belt and the ITCZ (Bastian et al 2021).…”

“…In addition, the southward shift of the Intertropical convergence zone (ITCZ) during ice rafting episodes throughout Heinrich event 1 (H1) are considered as a major cause for the dry event in northern and southeastern Africa around 16-17 cal kyr BP (Tierney et al 2008;Marshall et al 2007;Stager et al 2011;Mohtadi et al 2014). During the African humid period (AHP) maximum northern Hemisphere summer insolation shifted the rain belt associated with the ITCZ to the north (Bastian et al 2021), generating increased rainfall across Northern Africa which turned the Saharan desert into green savanna (Gasse 2000). Palaeoclimatic studies suggest the southward extension of the AHP to eastern Africa (Tierney et al 2008;Tierney and DeMenocal 2013;Costa et al 2014).…”

“…Besides, lacustrine sediments (Tiercelin et al 2008;Bittner et al 2020; and many others), tropical glaciers (Gasse 2000;Thompson et al 2002), peat bogs (Bonnefille and Mohammed 1994; Brown et al 2007) and marine sediments (Camuera et al 2021;Tierney and deMenocal 2013) have been serving as potential archives for the reconstruction of paleoenvironmental fluctuations in eastern Africa. However, the regional, topographic, and climate complexities, together with dating inaccuracy and lack of unambiguous proxies, lead to differences in the reconstructed timing of the onset and termination of major climatic events such as H1 and AHP across sites (Lamb et al 2007;Costa et al 2014;Bastian et al 2021). Moreover, the impact of these drastic climatic events on the migration of humans and on vegetation dynamics is yet not fully understood.…”

Lithium Isotope Geochemistry in the Barton Peninsula, King George Island, Antarctica

“…The δ 7 Li values of the suspended sediment samples ranging from -0.4 to +13.4‰ (mean +8.3‰, n = 20, Table 2) are higher than those of bedrock (mean +5.2‰) and soil (mean +3.8‰) samples, but much lower than meltwater samples (mean +26.4‰). Due to the high solubility of Na during chemical weathering compared to immobile Al, the Na/Al ratio can be regarded as "a weathering index" (Millot et al, 2010) or an index of the leaching intensity, such as the K/Mg (Bastian et al, 2017;Bastian et al, 2021). A positive correlation between Na/Al and Li/Al ratios suggests that Al-rich phases, such as clays, mainly controls Li concentrations in the suspended sediment (Figure 6A), which conforms observations from the dissolved δ 7 Li (see previous section).…”