Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels: paleoceanographic and paleoecological significance
1Utrecht University Faculty of Geosciences (Department of Earth Sciences – Geochemistry) Budapestlaan, 4, 3508 TA Utrecht, The Netherlands
2Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR) Facultad de Ciencias, Campus Fuentenueva, 18002 Granada, Spain
3Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109-1005, USA
4Institute of Marine Sciences, Earth & Planetary Sciences Department, University of Santa Cruz, Santa Cruz, CA 95064, USA
5Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva, 18002 Granada, Spain
Abstract. Sapropels are organic matter enriched sediments cyclically deposited in the Eastern Mediterranean. We have studied a suite of 35 sapropel sequences from a transect of four ODP sites across the basin to explore for paleoproductivity patterns and their relationship to ecological changes in the region during their deposition. We assessed paleoproductivity variations using TOC and Babio mass accumulation rates and δ15Ntotal and δ13Corg values. Elevated Ba/Al and TOC mass accumulation rates record periods of basin-wide amplified productivity. δ15N values are dramatically lower in the sapropels than in TOC-poor background sediments indicating a major contribution from nitrogen-fixing bacteria to the higher productivity during sapropel deposition. Different degrees of denitrification occurred as a function of water column oxygen depletion. The evolution of sapropel deposition over the past 3 My is characterized by increased productivity together with enhanced preservation of organic matter during the late Pliocene, peaks in primary and export production and sedimentation rates during the middle Pleistocene, and a relatively weak increase in productivity during formation of the Holocene sapropel accompanied by high sedimentation rates. We conclude that sapropels were formed by cyclic increases in primary production of marine organic matter largely sustained by N-fixing bacteria that led to depletion of deepwater dissolved oxygen and consequently improved organic matter preservation. The increase in primary production was helped by nutrient input from continental sources climatically induced.