THE SEDIMENTARY RECORD OF PELAGIC BIOGEOGRAPHY

Sedimentary remains of fossilizable marine organisms, chiefly microplanktonic ones (coccolithophorids, silicoflagellates, diatoms, dinoflagellates, radiolarians, foraminifers) have long been used for biogeographic surveys, usually with the aim of unravelling paleoenvironmental settings. Utilization of bottom deposits (rather than plankton samples) has some advantages for this purpose: thanatocoenoses require smaller sample sizes, materials are more readily available, and they eliminate the small time-scale variability which obscures long-term trends. On the other hand, the sedimentary imprint of planktonic distributional patterns is often severely distorted by fragmentation and dissolution of the biogenic remains, reworking of sediments, winnowing and lateral advection of shells, especially toward lower latitudes, seasonally and interannually variable output rates, and the integration of vertically heterogeneous assemblages. The biases derived from these processes rue reviewed, and the need for more information on selective dissolution, vertical distribution patterns and planktonic reproduction and output rates is stressed. Given the assumptions used in paleoenvironmental studies, distributional information drawn from sediment samples is less prone to yielding biased information when applied to these surveys, than to bio- or paleobiogeographic investigations; albeit the former can also be significantly affected as a result of changes in the traits that govern the formation of biogenic sediments,