Abstract

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 (Δ[CO32−]bw), the observed decrease in test weights by ~0.3 μg per μmol kg−1 was accompanied by decreasing Mg/Ca ratios (~0.12 mmol mol−1 per μg) and increasing stable oxygen isotope values δ18O; ~−0.13‰ per μg). 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 Δ[CO32−]bw>21 μmol kg−1 (no dissolution), three dissolution stages can be distinguished: 1) Gentle dissolution (Δ[CO32−]bw ~10–21 μmol kg−1, little alteration of planktonic foraminiferal Mg/Ca and δ18O) 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 (Δ[CO32−]bw ~0–10 μmol kg−1, clear alteration of Mg/Ca and δ18O) 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 (Δ[CO32−]bw <0 μmol kg−1, significant alteration of planktonic foraminiferal Mg/Ca and δ18O) indicated by only white tests showing several calcite layers, holes and broken final chambers.

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