Evaluation of dissolution indices for planktic foraminifera in various environmental settings

Fossil planktic foraminifera is a useful indicator of the paleoenvironment of the surface ocean. Because of its susceptibility to dissolution, various indices for preservation state of planktic foraminifera have been used to determine better the degree of bias introduced by the species-dependent resistance to dissolution. This study aims to evaluate six dissolution indices of foraminifera from four Pacific sites, each representing different surface and deep ocean condition. Compared to the fragmentation rate (FLS), demanding and time-consuming but one of the most reliable dissolution indices regardless of the ambient environment, each index has different sensitivity on preservation state owing to the environmental factors. Carbonate fraction (CaCO3) and coarse sized fraction (CSF) are fairly well correlated with FLS in the foraminifera-dominated tropical ocean where the influx of other sedimentary components is insignificant. The assemblage dissolution index (FDX) and percentage of resistant species (Res) based on the relative enrichment of dissolution-resistant species are best applicable at the central equatorial Pacific site without any significant changes in the primary faunal assemblage. The benthic/planktic ratio (B) varies with FLS similarly at a productive Northwestern Pacific site, but should be carefully applied in many cases owing to the strong dependence of benthic foraminifera on food availability. The results of this study suggest the importance of rigorous application criteria of the foraminifera dissolution index in various depositional environments.