HELP INDEX:HOEGLUNDINA ELEGANSPRESERVATION INDEX FOR MARINE SEDIMENTS IN THE WESTERN SOUTH ATLANTIC

Abstract

The purpose of this study is to evaluate the abundance and preservation state of the tests of Hoeglundina elegans , with the goal to propose a new preservation index for deep-sea sediments. In seawater, aragonite is more soluble than calcite; the benthic foraminiferal species H. elegans produces aragonitic tests, thereby providing potential proxies for subtle variations in carbonate preservation. Using two proxies based on H. elegans , as well as three other carbonate-dissolution proxies, we interpret the main drivers of carbonate dissolution in the western South Atlantic Ocean from a core, GL-854 (water depth 2220 m) from the western South Atlantic Ocean, Santos Basin. The H. elegans (>150 μm) abundance and the HelP Index ( Hoeglundina elegans Preservation Index), based on five test-preservation degrees, were assessed to determine the state of aragonite preservation. Images were taken using SEM to demonstrate each degree of preservation and to observe the ultrastructural breakdown of the tests’ walls. The other parameters determined were planktonic foraminiferal Fragmentation Index (FI), %CaCO3 and sand fraction (>63 μm). The H. elegans abundance, HelP Index, and sand fraction correlated positively, and those proxies correlated inversely with the FI. The weakest correlations were between %CaCO3, and the other proxies. Subtle differences in proxy responses were consistent with specific stages of carbonate dissolution, ranging from the aragonite lysocline to the Calcite Compensation Depth. The results revealed alternating modes of high and low carbonate preservation throughout the core and the primary driver of carbonate dissolution appeared to be variation in the aragonite saturation state influenced by circulation-induced changes in water masses. The carbonate dissolution observed in specific intervals, probably due to shoaling of the Aragonite Compensation Depth, suggests a mixing zone with more corrosive southern-sourced water and less corrosive northern-sourced water at the depth where the core was located.