From living communities to fossil assemblages: origin and fate of coccolithophores in the northern Arabian Sea

摘要

The use of coccolithophores as proxies for past environmental conditions in the Arabian Sea is critically evaluated based on a number of recent studies. We investigated coccolithophores in terms of their production in the photic zone, their transformation to settling assemblages, their accumulation on the seatloor, and their final burial in the sediments. The living coccolithophore communities reflect local oceanographic conditions and seem to be more dependent on mixed layer depth and nutrient availability than on temperature and/or salinity changes. We found a strong seasonally in coccolithophore fluxes corresponding to the seasonal monsoonal development. Several species showed specific annual trends, however, the annual cycles of the majority of living species are not preserved and only a smoothed signal with gradual intraannual changes characterize the settling assemblages. In the surface sediments the distribution of species and species groups reflect the environmental parameters of the overlying water masses and may be successfully used to improve paleoclimatic reconstructions. Even though sediments in the Arabian Sea are annually laminated within an expanded oxygen minimum zone, no seasonal signal is preserved due to the event-like deposition of light laminae. However, interannual variability can be studied with a unique resolution. Differences in the species compositions of living coccolithophore communities, settling, and sediment assemblages result from taxonomical and preservational factors. Interpretation of variations of species compositions and abundances in the fossil record are further complicated by evolutionary shifts and events and changing environmental conditions (e.g. fluctuation of sedimentation rate). Despite all limitations and constraints, it is shown that the fossil coccolithophore core records bear great potentials to successfully monitor and reconstruct paleoenvironmental changes.