Decoupled Changes in Western Niger Delta Primary Productivity and Niger River Discharge Across the Last Deglacial

Abstract:

Abrupt droughts in West Africa impart significant socio-economic impacts on the developing countries of this region, and yet a comprehensive understanding of the causes and duration of such droughts remains elusive. Much of the summertime rainfall associated with the West African Monsoon (WAM) falls within the Niger River basin and eventually drains into the eastern Gulf of Guinea, contributing to the low sea-surface salinity of this region. Of the limited number of studies that reconstruct Gulf of Guinea salinity through the deglacial, the most comprehensive of those is located ~ 400 km east of the Niger delta and may not be solely influenced by WAM runoff. Here, we present XRF and foraminiferal trace metal data from two new cores located less than 100 km from the Western Niger Delta. Radiocarbon dating of cores Grand 21 (4.72^oN, 4.48^oE) and Fan 17 (4.81^oN, 4.41^oE) produced near linear sedimentation rates of 20 cm/kyr and 15 cm/kyr respectively. Elemental sediment compositions from XRF core scanning reveal an abrupt 50% increase in SiO₂ between 17-15 ka during Heinrich Event 1. This increase, coeval with increases of CaCO₃ (+12%) content and Ba/Ti ratios suggests a large increase in primary productivity during H1. Values then decrease at the onset of the Bolling-Allerod (~14.6 kyr) until a similar, albeit smaller increase is recorded during the Younger Dryas beginning at 12.7 kyr. In contrast, FeO₂ and TiO₂ are thought to be a proxies of Niger River discharge strength and suggest a more gradual change in riverine discharge across the deglacial that is most likely driven by precession. These proxies suggest Niger River runoff was low from the LGM through Heinrich 1, gradually increasing around 13 ka. FeO₂ and TiO₂ values then peak between 11.5-7.5 kyr, consistent with the African Humid Period, before gradually decreasing through the mid-late Holocene. This deglacial pattern of riverine input is markedly different from previous reconstructions of WAM variability and does not appear to explain the large increases in primary production during H1 or the YD. To further investigate Niger River runoff and water column hydrography change in the Niger Delta across the deglacial, we will also present data from three planktonic foraminifera: Globigerinoides ruber, Neogloboquadrina dutertrei and Globorotalia crassaformis.