TEST APPEARANCE OF THE PLANKTONIC FORAMINIFER PULLENIATINA OBLIQUILOCULATA AS AN INDICATOR OF CALCITE DISSOLUTION IN DEEP-SEA SEDIMENTS

Deep-sea calcareous sediments are subject to dissolution that can alter the appearance and geochemistry of planktonic foraminiferal tests and faunal assemblages. Assessment of the impact of dissolution is often based on assumptions [e.g., position of a sediment sample with respect to the seawater calcite-saturation horizon (CSH)] and elaborate techniques (e.g., scanning-electron microscopy). To assess dissolution effects on calcareous sediments, we weighed and photographed tests of Pulleniatina obliquiloculata from South China Sea (SCS) sediment-surface samples spanning deposition above the CSH down to the calcite-compensation depth. Controlled by the calcite-saturation state of bottom waters (Delta[CO32-](bw)), the observed decrease in test weights by similar to 0.3 mu g per mu mol kg(-1) was accompanied by decreasing Mg/Ca ratios (similar to 0.12 mmol mol(-1) per mu g) and increasing stable oxygen isotope values delta O-18; similar to-0.13 parts per thousand per mu g). Concomitant changes in test appearance demonstrated the potential of P. obliquiloculata to indicate distinct preservation stages. We present an economical, fast and non-destructive guideline to evaluate sediment preservation based on P. obliquiloculata appearance under a light microscope. In addition to the initial P. obilquiloculata test preservation, characterized by a smooth and shiny surface of slightly brownish-pinkish color in samples deposited in bottom waters with a Delta[CO32-](bw) > 21 mu mol kg(-1) (no dissolution), three dissolution stages can be distinguished: 1) Gentle dissolution (Delta[CO32-](bw) similar to 10-21 mu mol kg(-1), little alteration of planktonic foraminiferal Mg/Ca and delta O-18) indicated by slightly brownish-pinkish tests with smooth and shiny surfaces only slightly damaged by fissures on top of the pores along the test's periphery; 2) Moderate dissolution (Delta[CO32-](bw) similar to 0-10 mu mol kg(-1), clear alteration of Mg/Ca and delta O-18) with extensive disintegration and coexistence of both slightly brownish-pinkish tests and white tests with initially smooth surfaces already decayed to small prismatic units; 3) Severe dissolution (Delta[CO32-](bw) <0 mu mol kg(-1), significant alteration of planktonic foraminiferal Mg/Ca and delta O-18) indicated by only white tests showing several calcite layers, holes and broken final chambers.