Benthic foraminiferal signal in Southwetern South Atlantic: Changes in vertical water mass structure or changes in organic matter availability?

Abstract

Benthic foraminifera are constituents of marine fauna widely used for deep-sea environmental reconstructions. Assemblage structure is conditioned by several environmental factors such as depth, grain size, calcium carbonate saturation, temperature, salinity, oxygenation and quantity of organic matter (OM). Accordingly, specific combinations of species have been used as proxies of past water masses vertical distribution as well as oxygenation and quantity of OM of bottom waters. The Southwestern South Atlantic (SWSA) presents a highly dynamic hydrographic structure defined by the presence of contrasting deep water masses. In turn, this determines a similarly complex primary productivity pattern in the area and widely differing organic matter sources. Since, the present-day type water masses distribution and surface primary production would have changed between glacial e interglacial periods, which controlled the benthic foraminiferal assemblages in the SWSA: the water masses vertical distribution or the oxygenation of bottom waters related with quantity/quality of OM? In order to answer this question, a Q-mode Factor Analysis was performed on benthic foraminiferal assemblages of sediment core SP1251 (northern Argentine Continental Margin - 38°29.7?S / 53°40.7?W ~3500 m depth), which records the Late Pleistocene. Results reveal two distinct benthic foraminiferal assemblages, which account for 85% of the total variance. Factor 1, dominated by Uvigerina peregrina, Cassidulina inflata, Alabaminella weddellensis and pistominella exigua; and Factor 2, dominated by Globocassidulina subglobosa and other uvigerinids, explain 75% of data variability and were considered related to high levels of OM. The remaining 10%, explained by Factor 3, was accounted for mostly by low oxygen related species such as Globobulimina ovula. These results allow to infer that during Late Pleistocene benthic foraminiferal assemblages were structured by quantity of OM and oxygenation of bottom waters rather than water masses physic-chemical properties, with changes in the productivity patterns and the origin of the OM being identified.