THE SEDIMENTARY RECORD OF PELAGIC BIOGEOGRAPHY

Sedimentary remains of fossilizable marine organisms, chiefly micropla nktonic ones (coccolithophorids, silicoflagellates, diatoms, dinoflage llates, radiolarians, foraminifers) have long been used for biogeograp hic surveys, usually with the aim of unravelling paleoenvironmental se ttings. 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 p atterns is often severely distorted by fragmentation and dissolution o f 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 verticall y heterogeneous assemblages. The biases derived from these processes r ue reviewed, and the need for more information on selective dissolutio n, vertical distribution patterns and planktonic reproduction and outp ut rates is stressed. Given the assumptions used in paleoenvironmental studies, distributional information drawn from sediment samples is le ss 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 trait s that govern the formation of biogenic sediments,