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Volume 8, issue 2
Biogeosciences, 8, 415–431, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 8, 415–431, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

  16 Feb 2011

16 Feb 2011

Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels: paleoceanographic and paleoecological significance

D. Gallego-Torres1,5, F. Martinez-Ruiz2, P. A. Meyers3, A. Paytan4, F. J. Jimenez-Espejo2, and M. Ortega-Huertas5 D. Gallego-Torres et al.
  • 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, Univ. 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. We have studied a suite of 35 sapropel sequences from a transect of four ODP sites across the Eastern Mediterranean to explore for paleoproductivity patterns and provide new insights on ecological changes during their deposition. Paleoproductivity variations were identified 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. Our data further support that sapropels were formed by cyclic increases in primary production of marine organic matter largely sustained by N-fixing bacteria. This productivity increase was triggered by climate factors leading to increased fluvial discharge and amplified nutrient input that also favored the establishment of N-fixing bacteria. Enhanced productivity led to depletion of deepwater dissolved oxygen and consequently improved organic matter preservation. Primary production was more intense during the middle to Late Pleistocene compared to Pliocene equivalents, coinciding with increasing total sedimentation rates. 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. Additionally, different degrees of denitrification occurred as a consequence of water column oxygenation which in turns evolved from stagnant anoxic bottom waters during Pliocene sapropels to oxygen depleted and sluggish circulation in late Quaternary layers. These differences between sapropel layers provide new evidences for the general evolution of the Eastern Mediterranean basin during the last 3 Mys in terms of paleoceanographic conditions and the intensity of climate variability leading to sapropel deposition.

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