Sediment Provenance in the Equatorial Atlantic and its Implications on Paleocirculation

Ocean circulation in the Equatorial Atlantic is composed of a diverse range of currents from the surface level equatorial currents and countercurrents to the North Atlantic Deep Water (NADW). Sediments deposited in this region are derived from Africa and South America by a combination of aeolian and fluvial pathways and then transported by ocean currents. Fifteen cores from the Equatorial Atlantic (10°N to 10°S, 0° to 45°W) were used to determine sediment provenance during the Holocene and Last Glacial Maximum (LGM), and specifically to test whether sediment was transported by surface or deep water currents. The radiogenic isotopic composition of any rock depends on its age, initial composition and its geologic history. The strontium isotopic composition of the terrigenous portion of marine sediments can often be used to identify the source rock(s) from which they weathered. By incorporating this new sediment core data from both Holocene and LGM depths with previous research designating the 87Sr/86Sr ratios for source areas in Africa and South America we were able to connect possible source areas with core locations. In general, the 87Sr/86Sr ratios from the Holocene are higher than those from the LGM, indicating a greater contribution from geologically older sediment sources during the Holocene. Primary sediment transport from African sources is best explained by wind transport, while sediment derived from South American sources is most easily explained by river input. The geographic pattern of 87Sr/86Sr ratios appears to be correlated to deep ocean circulation, specifically the NADW which flows southward along the western side of the Atlantic near South America and then cuts east along the Romanche Fracture Zone (RFZ, ~0°) before continuing south once again in the eastern basin near Africa. Based on our results, there is no observable difference in transport mechanisms between the Holocene and LGM; however, the difference in ratios between the two time periods is strikingly more prominent in the cores near South America and extending east along the RFZ. The results indicate that while the sediment transported from Africa (primarily by the Saharan Air Layer and North East Trades) does not differ significantly between the Holocene and LGM, sediment flux from the Amazon does differ between these two time periods, contributing more terrigenous sediment during the LGM than during the Holocene. The Amazon derives the majority of its sediment from Andean rock source, while smaller rivers such as the Orinoco and Sao Francisco weather the more radiogenic lowlands and Brazilian Shield. Therefore, it is probable that the Amazon dominates the South American terrigenous input during the LGM, while smaller rivers had a relatively greater contribution during the Holocene.