Reconstruction of changes in export productivity during Pliocene sapropel deposition: a biomarker approach

摘要

Biomarkers (alkenones, loliolide/isololiolide, dinosterol and C_(30), 1,15-diol/keto-ol) representative of four major classes of marine primary producers (haptophytes, diatoms, dinoflagellates, eustigmatophytes, respectively) from three sites of an east-west transfer spanning the entire eastern Mediterranean basin were studied for changes in phytoplankton export production before, during and after deposition of a Pliocene, total organic carbon-rich (up to 27%) sapropel (2.943 Ma). Biomarker contents showed substantial differences between non-sapropelic and sapropelic sediments. In non-sapropelic sediments, small contents are observed reflecting extensive oxic degradation in oxic pore waters during deposition. In the sapropels, biomarker accumulation rates showed a substantial increase from the base to the centre, likely resulting from increased biological productivity since the bottom waters were continuously anoxic and preservation conditions are similar. The diatom biomarker, loliolide/isololiolide, became distinctly more abundant in the sapropels indicating that the eastern Mediterranean basin was enriched in nutrients. This caused enhanced export production by the major phytoplankton classes leading to periods of sapropel deposition. Alkenone-based sea surface temperature reconstructions indicated that this changes in phytoplankton composition was not influenced by changes in sea surface temperature, but was mainly nutrient-controlled. Compound-specific δ~(13)C measurements did not provide additional evidence for substantially increased primary production rates: the δ~(13)C values of C_(37:2) alkenones and loliolide/isololiolide showed a slightly negative shift instead of the anticipated positive shift. This was probably caused by the enrichment of dissolved inorganic carbon in the upper water column, caused by enhanced recycling of respired CO_2 due to the shallow chemocline.