Coccolithophore size rules - Reconstructing ancient cell geometry and cellular calcite quota from fossil coccoliths

Algal cell size exerts a fundamental influence on the stable isotopic fractionation during photosynthesis and biocalcification, as well as the incorporation of trace elements in coccolith calcite by calcifying haptophytes (the coccolithophores). Therefore, it is important to evaluate the influence of ancient coccolithophore cell size on coccolithophorid-derived geochemical proxies that are applied in paleoceanographic studies. However, in contrast to planktonic foraminifera, for which cell size may be directly estimated from their fossilized calcite shells, the reconstruction of ancient coccolithophore cell dimensions appears hampered because the sediment record is dominated by single coccoliths, rather than intact coccospheres - the armor of multiple interlocking calcite platelets that surround live cells. Solving this apparent problem, this paper presents quantitative constraints on coccolith size, coccosphere and cell diameters, for three main Cenozoic genera of coccolithophore (Reticulofenestra, Cyclicargolithus and Coccolithus) through detailed biometry of rarely fossilized coccospheres of middle Eocene to early Miocene age. Together, these taxa are most dominant in Cenozoic deep-sea sediments, providing an exquisitely detailed record of their evolution and the bulk of biogenic carbonate burial through time. Accurate estimates of coccolithophore cell size can be made by relatively simple size measurements of individual coccoliths in fossil assemblages, since coccolith size is strongly linearly correlated to both coccosphere- and cell diameter. Additional general coccolith 'size rules' are confirmed by the morphometric data: (1) coccolith thickness increases with size, (2) elliptical placoliths are more circular at larger size, and (3) the number of coccoliths per coccosphere is rather invariable within the investigated Cenozoic genera. The latter observation offers better control on (minimum) estimates of cellular calcite quota in the ancestors of today's prolific bloom-forming species Emiliania huxleyi, Gephyrocapsa spp. and Coccolithus pelagicus. (C) 2008 Elsevier B.V. All rights reserved.