Deciphering signals of late Holocene fluvial and aeolian supply from a shelf sediment depocentre off Senegal (north‐west Africa)

Abstract

High-resolution climatic records of the late Holocene along the north-west African continental margin are scarce. Here we combine sediment grain size, elemental distribution and mineral assemblage data to trace dust and riverine sources at a shallowmarine sediment depocentre in the vicinity of the Senegal River mouth. The aim is to understand how these terrigenous components reflect climate variability during the late Holocene. Major element contents were measured and mineral identification was performed on three sub-fractions of our sediment core: (i) fluvial material < 2 mm, (ii) aeolian material of 18-63 mm and (iii) a sub-fraction of dual-origin material of 2-18 mm. Results show that more than 80% of the total Al and Fe terrigenous bulk content is present in the fluviogenic fraction. In contrast, Ti, K and Si cannot be considered as proxies for one specific source off Senegal. The Al/Ca ratio, recording the continental river runoff, reveals two dry periods from 3010 to 2750 cal a BP and from 1900 to 1000 cal a BP, and two main humid periods from 2750 to 1900 cal a BP and from 1000 to 700 cal a BP. The match between (i) intervals of low river runoff inferred by low Al/Ca values, (ii) reduced river discharge inferred by integrated palynological data from offshore Senegal and (iii) periods of enhanced dune reactivation in Mali confirms this interpretation.

Twenty Years of XRF Core Scanning Marine Sediments: What Do Geochemical Proxies Tell Us?

“…Other aluminosilicate minerals include andalusite, kyanite, sillimanite, beryl, garnet, spinel, and turquoise. Weathering of low Fe and SiO 2 bedrock in tropical climates results in high-alumina clays Use: Al and Fe have been used as fluvial proxies (Nizou et al 2011). In this study, Al was preferred for palaeoclimatic analysis due to potential Fe mobility Limitations: Al is routinely used for normalisation of WD-XRF data, but its closeness to core scanner detection limits, rapid attenuation in air and through any sub-film water layer, low response depth, and dependence on tube type may constrain its use in core scanner data normalisation.…”

“…Use: Al/Ca used to record terrigenous flux off NW Africa (Nizou et al 2010(Nizou et al , 2011 with low Al/Ca values interpreted as low river discharge indicating hinterland aridity and high Al/ Ca values reflecting humid hinterland phases, use supported by palynological records. To avoid dilution effects, Al/Ca was plotted against time (Weltje and Tjallingii 2008).…”

“…To avoid dilution effects, Al/Ca was plotted against time (Weltje and Tjallingii 2008). Water content may influence core scanner measurement for low-atomic-weight elements such as Al (Tjallingii et al 2007) but Nizou et al (2011) found little porosity variation downcore (~10 %), hence water influence not considered significant. This analysis was supported by Fe/Ca (two high-atomic weight elements not influenced by water content) which showed same pattern as Al/Ca.…”

Seasonal provenance changes of present-day Saharan dust collected on- and offshore Mauritania

“…1). Kaolinite and smectite were found in the clay fraction of the surface sediments off Senegal (Nizou et al, 2011) and may have been brought into the ocean by the Senegal River and redistributed by ocean currents (Biscaye, 1964). The season of high Senegal River sediment supply is between July and October/November (Gac and Kane, 1986).…”

“…The PSA 1 is underlain by the eastern Atlas chain and the northern Grand Erg/Ahnet and Ghadames basins. The outcrops in the Atlas uplift are composed of, e.g., limestones, sandstones and evaporites (Piqué, 2001). The thick strata overlying the northern Ahnet and Ghadames basins consist of, e.g., sandstones and mudstones (Selley, 1997b).…”