Understanding the taphonomic bias on the benthic foraminiferal bottom current proxy

Benthic foraminifera provide key information for the reconstruction of ocean bottom currents. In the Iberian Margin Contourite Drift System, abundances of a distinct group of benthic foraminifera are controlled by the strength and nutrient load of bottom currents. This so-called elevated epifauna (EEF) comprises highly specialized suspension feeding benthic foraminifera attached to elevated substrates as an adaptation to strong bottom currents. Schönfeld (2002) demonstrated a direct relation between EEF abundances and Mediterranean Outflow Water strength at the Iberian Margin in modern surface samples and established EEF abundances as a bottom current proxy. Faunal and geochemical data from Pliocene-Pleistocene contourites drilled during IODP Exp. 339 in the Gulf of Cádiz, however, showed a poor correlation of EEF abundances with other bottom current proxy data, indicating a bias by depositional or taphonomic processes.

The tectonically active Iberian Margin is characterized by a complex depositional environment shaped by alongslope and downslope transport. Taphonomic processes linked to transport and loss of foraminiferal tests during early fossilization potentially bias fossil EEF abundances in marine slope sediments and may result in overestimation of bottom current speeds. Our understanding of these taphonomic processes is still rather poor. Stable oxygen and carbon isotope ratios from foraminiferal tests in Pleistocene contourites and turbidites can be used to identify allochthonous tests and provide insights into the extent of downslope transport at the Iberian Margin. The aim of this study is to quantify the effects of taphonomic processes on fossil foraminiferal assemblages and to establish an improved EEF proxy method. The improved method will provide researchers with a reliable, well-calibrated and easily applicable tool for bottom current reconstruction.

 

Reference: Schönfeld, J. (2002). A new benthic foraminiferal proxy for near-bottom current velocities in the Gulf of Cadiz, northeastern Atlantic Ocean. Deep Sea Research Part I: Oceanographic Research Papers, 49(10), 1853-1875.