Eolian‐transported freshwater diatoms and phytoliths across the equatorial Atlantic record: Temporal changes in Saharan dust transport patterns

Abstract

The deposition of freshwater diatoms (FD) and phytotliths (P) was determined from sediment traps for a wide region (20°N–7°S) in the tropical and equatorial Atlantic, along a N‐S transect in the eastern equatorial Atlantic around 10°W and in the western equatorial Atlantic around 25°W. These siliceous organisms are derived from the Sahara and Sahel regions of Africa, and eolian transport with direct settling over the open ocean is assumed to be the transport agent. Depositional rates of FD and P revealed strong coupling with seasonal changes in Saharan dust transport that are associated with seasonal precipitation patterns, major wind systems, and the geographical extension of the dust plume across the Atlantic. Mean daily fluxes were highest south of Cape Verde (FD = 9 × 104 valves m−2 d−1; P = 2 × 104 bodies m−2 d−1), moderately high off Cape Blanc and in the Guinea Basin north of the equator (of the order of 3 × 104 valves m−2 d−1 for FD, and 0.7 × 104 bodies m−2 d−1 for P), and consistently low south of the equator and in the western equatorial Atlantic. In traps north of the equator, seasonal changes were marked. Aulacoseira granulata and A. islandica were the most abundant FD in the traps, regardless of trap location and season. However, the number of FD species was higher in the Cape Blanc and Cape Verde areas. The morphological diversity (shape and size) of the P assemblage decreased with increasing distance from the African continent. Patterns of FD and P accumulation rates in surface sediments coincided with those in the traps. Robust freshwater diatom and phytolith records associated with seasonal eolian transport from Saharan and Sahelian regions into the Atlantic furnish clues that can help in our present understanding of the processes linking transport between land, atmosphere, and ocean.