Multi-tracer study of continental erosion and sediment transport to the Red Sea and the Gulf of Aden during the last 20 ka

Abstract

Mineralogical compositions and grain-size distributions combined with ⁸⁷Sr/⁸⁶Sr and ε_Nd values of the detrital fraction were studied on cores recovered from the Gulf of Aden (MD92-1002) and the Red Sea (MD92-1008) basins in order to document past changes in Indian monsoon and northwesterly winds during the last glacial-interglacial transition (the last 20 ka), encompassing the African Humid Period (AHP). The ε_Nd vs. ⁸⁷Sr/⁸⁶Sr plot indicates that sediments result from the mixing of two main sedimentary sources corresponding to the Afar volcanic rocks in Ethiopia and to the Arabian-Nubian Shield. Variations of sediment isotopic and mineralogical composition point to a diminution of the volcanic source contribution during the last deglaciation. Changes of mineral-accumulation rates and grain-size distributions denote a decline in the aridity of the source regions during the Holocene, particularly of the Afar volcanic region. In this area, the reduction of detrital supply, from 15 cal ka BP, can be explained by an increase of precipitations during the AHP, which resulted in an expansion of the vegetation cover and lake extensions in East Africa. In the Arabian Peninsula, precipitations were confined to the south, allowing sediments to be transported even during the Holocene. Our data suggest that the southwest monsoon was not the main carrier of aeolian sediments to the Red Sea and Gulf of Aden basins, but the Northwesterlies. In the Red Sea, the isotopic and mineralogical tracers reveal a contribution from Saharan dust between 16 and 12 cal ka BP, transported from the Nile catchment after aridification during Heinrich event 1.

Introduction

The Red Sea and the Gulf of Aden (GOA) basins are located in the midst of the desert belt between the Saharan and Arabian deserts, with precipitations below 100 mm/year (Locke and Thunell, 1988) and where aeolian erosion plays a key role, due to the sparse vegetation cover in the area. In this region, precipitations and low-altitude wind circulation show marked seasonal changes, typical of a monsoon system (Webster, 1981; Laing and Evans, 2011). Southwest winds blow over the Arabian Sea, from the Horn of Africa to the Indian subcontinent, during the wet summer monsoon. In contrast, the Northeast winds prevail during the dry winter monsoon (Clemens and Prell, 1991; Sirocko et al., 1991) (Fig. 1). However, low-altitude atmospheric circulation patterns in the region are more complex than a simple SW-NE seasonal reversal. They reflect the influence of the Inter-Tropical Convergence Zone (ITCZ) shifting position and of local topography (Fleitmann et al., 2007; Lézine et al., 2007; 2010). Above the monsoonal wind system, the mid-tropospheric circulation is dominated by northwesterly winds from the Arabian Peninsula, which appear to play an important role in the transport of dust to the western Arabian Sea (Sirocko et al., 1991, 1993; Sirocko and Lange, 1991; Pourmand et al., 2004), and may also influence the sediment transport to the Red Sea and GOA basins.

On orbital timescale and particularly across the last glacial-interglacial transition, many studies have documented major changes in monsoon climates in response to variations in insolation. Continental and marine records from Asia and North Africa suggest that the summer monsoon intensity in the northern hemisphere was weaker 18–20 ka ago and increased to reach a maximum in early-mid Holocene, coeval with the maximum northern hemisphere summer insolation (Overpeck et al., 1996; Gasse and Van Campo, 1994; Gasse, 2000; Enzel et al., 1999; Sirocko et al., 1991; Fleitmann et al., 2003, 2007; Revel et al., 2014). Similarly, changes in the flux of aeolian dust brought to the Arabian Sea were documented in relation to variations in Indian monsoon intensity. Sedimentary record from core 74 KL, located off the Oman coast, revealed a diminution of windborne dolomite particles since the Last Glacial Maximum (LGM), with a striking minimum corresponding to the early Holocene at the maximum of southwest monsoon intensity (Sirocko et al., 1993).

In the northern half of the African continent, the humid episode that occurred during the first part of the Holocene is known as the African Humid Period (AHP). Increase of precipitations over this time period resulted in the rise of lake levels and the extension of vegetation cover (Enzel et al., 1999; Gasse, 2000; deMenocal and Tierney, 2012; Shanahan et al., 2015; Lézine et al., 2007, 2010; Revel et al., 2015). A detailed and well-dated sedimentary record at ODP Site 658C (off Cap Blanc, Mauritania) suggests very abrupt, large-scale changes in aeolian sediment transport with a well-defined period of low influx between 12.3 and 5.5 cal ka BP associated with the AHP, when the Sahara was nearly completely covered by vegetation (Adkins et al., 2006). Continental data from the African continent and the Arabian Peninsula indicate that the beginning and end of this humid period may have been progressive (Lézine et al., 1998, 2010; 2011), whereas the marine windborne records from the region show abrupt transitions that are out-of-phase with the continental evidence (e.g. abrupt decrease of aeolian proxies as early as ∼15,000 cal yr BP; Lézine et al., 2014). These differences between marine and continental records may be related to the dependence of aeolian sedimentation on changes in wind intensity and/or direction (Lézine et al., 2014).

The study of the amount and composition of aeolian dust in oceanic sediments is of great importance because of the influence they have on climatic, pedogenic and ecological conditions in the Earth. Atmospheric dust can have an effect on the Earth's radioactive balance, since it either absorbs or reflects the solar energy depending on its mineralogy, grain-size and distribution. The erosion of clay minerals and removal from the source areas can also cause the degradation of soils and the supply of nutrients to marine environments, which in turn can have an effect over the removal of carbon dioxide from the atmosphere (Grousset and Biscaye, 2005; Maher et al., 2010; Scheuvens et al., 2013). In order to have a better understanding of the factors affecting dust transport and climatic conditions, methods that can relate aeolian sediment records to its contemporaneous source areas are required. Various types of fingerprint tracers can be applied, including elemental concentrations, mineralogy and grain-size distributions (Grousset and Biscaye, 2005).

In the same way, Strontium and Neodymium isotopic ratios can be used as reliable source tracers. The radiogenic isotopic compositions of Nd and Sr are very different in mantle derived vs. crust derived rocks, allowing the distinction to be made between young volcanic areas and old continental shields (Goldstein and Hemming, 2003; Grousset and Biscaye, 2005). Geological formations surrounding the Red Sea and GOA basins present a large variety of lithologies, with distinct Nd and Sr isotope signatures for crustal terrains of the late Proterozoic Arabian–Nubian Shield (ANS) (Stoeser and Frost, 2006; Stein and Goldstein, 1996), the Saharan Shield (Kuster et al., 2008), Phanerozoic platform sediments and Cenozoic alkali basalts from the Afar region (Betton and Civetta, 1984; Teklay et al., 2009) (Fig. 1a). Previous studies have shown the interest of using mineralogical and geochemical signatures to track changes in the sedimentary material from this region. Sirocko et al. (1991) and Sirocko and Lange (1991) characterized the mineralogy of sediments in the Arabian Sea and determined that the Arabian Peninsula is a major source of Arabian Sea deep-sea sediments, transported by northwesterly winds. Aeolian fluxes in the Arabian Sea were found to be higher during glacial periods due to the discharge from northwesterly winds that was more intense when the southwest monsoon was weaker (Sirocko et al., 2000; Pourmand et al., 2004). Nd and Sr isotopic measurements in sediments throughout this basin allowed the distinction to be made between western Arabian Sea sediments, which arrived from North and Central Arabian Peninsula, and eastern Arabian Sea, that presented riverine contributions from India (Sirocko, 1994).

Stein et al. (2007) measured Sr isotopes in two sediment cores in the Red Sea and the GOA, which covered a time period of 500 ka with a 10 to 30 ka resolution. They identified a hydrothermal Sr component associated with sea-floor spreading in the Red Sea, superimposed with a granitic Arabian source and a loess source. Nd and Sr isotopic compositions from sediment cores in the northern and Central Red Sea revealed sedimentary sources like the Saharan granitoids, the Arabo-Nubian Shield terrains and basalts from the Ethiopian Highlands (Palchan et al., 2013). Nonetheless, these results do not show appreciable variations between the Holocene and the LGM, suggesting that there were not any significant changes in the northern Red Sea during this period. Similarly, Jung et al. (2004) did not find any source variation in the sediments from core 905 in the Arabian Sea off the Somalian coast during the Holocene (stable Nd isotopic composition), and attributed the variations in the ⁸⁷Sr/⁸⁶Sr ratio to weathering changes.

In the present study, we examined the Sr and Nd isotopic composition, clay and bulk mineralogy, and grain size changes in sediments from the GOA and southern Red Sea in order to determine the sedimentary sources and transport processes and to establish their variability since the last glacial period. For this purpose, we studied cores MD92-1002 (GOA) and MD92-1008 (Red Sea), which provide continuous sedimentary records deposited at high sedimentation rates since the LGM (Fig. 1, Bouilloux et al., 2013a, 2013b; Fersi et al., 2016). These cores are positioned near the northern limit of the southwest monsoon winds, whose intensity changed during the last glacial-interglacial transition (Van Campo et al., 1982; Sirocko et al., 1991; Overpeck et al., 1996; Gasse and Van Campo, 1994; Gasse, 2000; Fleitmann et al., 2003, 2007; Revel et al., 2014; Fersi et al., 2016), and are surrounded by very contrasted geological formations making them particularly suitable to the study of sediment-source variations. The region was also subjected to the AHP (Gasse, 2000; deMenocal and Tierney, 2012), which has been documented by changes in the sedimentary record from around 15 ka until 5 ka BP (Juginger and Trauth, 2013; Juginger et al., 2014; Shanahan et al., 2015; Revel et al., 2015). The results make it possible to re-evaluate the role of humidity changes and the relative influence of the northwesterly winds and the southwest monsoon winds on the origin and transport of dust to the GOA and the Red Sea since the LGM, across the deglaciation and the onset of the early Holocene humid period.