Sr-Nd isotopes and mineralogy as tracers of continental erosion and sediment transport to the Red Sea and the Gulf of Aden during the last 20,000 years

Abstract:

During the transition from the last glacial maximum to the Holocene, production and transport of matter into the southern part of Red Sea and Gulf of Aden were sensitive to changes in monsoon winds, rain intensity and ITCZ position.

We studied two marine cores on each side of the Bab-el-Mandeb strait in order to compare the open ocean and a more isolated basin at the Eastern limit of the Arabic peninsula. Erosion and matter transfer from the continent have been documented by different approaches, which include radiogenic isotopes and mineralogy of bulk samples and clay fraction. The Sr-Nd fingerprint of detrital sediments is an effective tool for discriminating between sediment sources. Mineralogical composition also depends on lithology of the source area and can provide insight on the environmental conditions at the time of deposition.

ε_Nd and ⁸⁷Sr/⁸⁶Sr from the detrital fraction in both cores show large changes linked to the Glacial-Interglacial transition, following the foraminifera δ¹⁸O data. The isotopic values evolved towards a lower volcanic contribution from the last glaciation to the Holocene with a more pronounced change in the Gulf of Aden and more volcanic values for the Red Sea.

The sediment mineralogy includes quartz, feldspar, muscovite, biotite, amphibole and pyroxene. Clay mineralogy is dominated by smectite and lesser amounts of illite and palygorskite. Correlation of percentage of smectite and Nd-Sr isotope composition agrees with a volcanic origin of smectite and shows a 2-pole mixture of sediment sources. The first end member is characterized by a large amount of smectite, positive values of ε_Nd and a low ⁸⁷Sr/⁸⁶Sr ratio and was identified as Afar volcanic rocks. The second one with more negative values of ε_Nd and a higher ⁸⁷Sr/⁸⁶Sr ratio is enriched in illite and palygorskite and was defined as originating from the Central Arabian region. Changes in sediment composition during the Glacial-Interglacial transition denote a variation in the relative proportions of both sources. They may be linked to a larger contribution of the Northwesterly winds during the Holocene than during the last glacial period, which affected the Gulf of Aden basin more than the Red Sea.