Repercussions of differential settling on sediment assemblages and multi-proxy palaeo-reconstructions

Abstract. Microfossils preserved in marine sediments are at the centre of numerous proxies for palaeoenvironmental reconstructions. Their precision is based on the assumption that they accurately represent the overlying watercolumn properties and faunas. In this paper, we assess the possibility of a pre-depositional bias in sediment assemblages caused by horizontal drift, due to differential settling velocities of sedimenting particles based on their shape, size and density. We calculate the lateral transport undergone by planktic foraminifera and a range of other proxy carriers in several regions with high current velocities. Lateral transport of different planktic foraminiferal species is minimal due to high settling velocities; no significant shape- or size-dependent sorting occurs before reaching the sediment, making planktic foraminiferal ideal proxy carriers. Diatoms, radiolaria and faecal pellets can be transported up to 500 km in some areas. This transport bias suggests that sediment assemblages could contain different proportions of local and imported particles, decreasing the precision of proxies based on these groups and the accuracy of the temperature reconstruction. For example in the Agulhas current, transport can lead to differences of up to 2 °C in temperature reconstructions between different proxies. For future palaeoenvironmental reconstructions, further sediment-trapping studies and multi-proxy analyses should attempt to quantify the margin of error associated with particle transport.