Modifications of the C₃₇ alkenone and alkenoate composition in the water column and sediment: Possible implications for sea surface temperature estimates in paleoceanography

The literature pertaining to C 37 alkenone and C 37 and C 38 alkenoate production and diagenesis has been reviewed and evaluated for issues that might jeopardize their usefulness in paleotemperature estimation. We also examined the use of the C 37 alkenones as paleoproductivity indicators, the stability of their δ 13 C isotopic compositions, and their incorporation into the nonsolvent extractable organic matter fraction. Biological transformation of organic matter by bacteria and zooplankton does not appear to cause significant changes to the ratio of C 37:2 and C 37:3 alkenones, but there are major alterations in the relative composition of alkenoates. Studies of water column processes and postdepositional sedimentary changes indicate overall stability in the C 37:2 /(C 37:2 + C 37:3 ) ratio, leading to effective preservation of the paleotemperature signal. This is not the case for alkenoates, and there also appear to be some doubts about the dependence between sea surface temperature and the sedimentary abundance of the C 37:4 alkenone. C 37 alkenones may provide useful qualitative estimates of marine paleoproductivity. Although these compounds are apparently only derived from Haptophycea, changes in their quantitative composition might reflect changes in nutrient availability that on sedimentary timescales should also be generally significant of other algal species. One caveat is that in highly productive upwelling systems, productivity may be expressed mostly in diatom growth and thus not measurable by the C 37 alkenones. No changes in δ 13 C C 37 alkenone composition are observed upon zooplankton ingestion. Although this is expected from a theoretical perspective, further studies are encouraged in order to fully assess the stability of the isotopic signal during C 37 alkenone diagenesis. Finally, alkenones bound in the nonextractable fraction of sedimentary organics have not been adequately studied and further work is required to clarify any possible correspondence with a temperature signal.