A quantitative reconstruction of organic matter and nutrient diagenesis in Mediterranean Sea sediments over the Holocene

Abstract A multicomponent diagenetic model was developed and applied to reconstruct the conditions under which the most recent sapropel, S1, was deposited in the eastern Mediterranean Sea. Simulations demonstrate that bottom waters must have been anoxic and sulphidic during the formation of S1 and that organic matter deposition was approximately three times higher than at present. Nevertheless, most present day sediment and pore water profiles — with the exception of pyrite, iron oxyhydroxides, iron-bound phosphorus and phosphate — can be reproduced under a wide range of redox conditions during formation of S1 by varying the depositional flux of organic carbon. As a result, paleoredox indicators (e.g., C org :S ratio, C org :P org ratio, trace metals) are needed when assessing the contribution of oxygen-depletion and enhanced primary production to the formation of organic-rich layers in the geological record. Furthermore, simulations show that the organic carbon concentration in sediments is a direct proxy for export production under anoxic bottom waters. The model is also used to examine the post-depositional alteration of the organic-rich layer focussing on nitrogen, phosphorus, and organic carbon dynamics. After sapropel formation, remineralisation is dominated by aerobic respiration at a rate that is inversely proportional to the time since bottom waters became oxic once again. A sensitivity analysis was undertaken to identify the most pertinent parameters in regulating the oxidation of sapropels, demonstrating that variations in sedimentation rate, depositional flux of organic carbon during sapropel formation, bottom water oxygen concentration, and porosity have the largest impact. Simulations reveal that sedimentary nutrient cycling was markedly different during the formation of S1, as well as after reoxygenation of bottom waters. Accumulation of organic nitrogen in sediments doubled during sapropel deposition, representing a significant nitrogen sink. Following reventilation of deep waters, N 2 production by denitrification was almost 12 times greater than present day values. Phosphorus cycling also exhibits a strong redox sensitivity. The benthic efflux of phosphate was up to 3.5 times higher during the formation of S1 than at present due to elevated depositional fluxes of organic matter coupled with enhanced remineralisation of organic phosphorus. Reoxygenation of bottom waters leads to a large phosphate pulse to the water column that declines rapidly with time due to rapid oxidation of organic material. The oxidation of pyrite at the redox front forms iron oxyhydroxides that bind phosphorus and, thus, attenuate the benthic phosphate efflux. These results underscore the contrasting effects of oxygen-depletion on sedimentary nitrogen and phosphorus cycling. The simulations also confirm that the current conceptual paradigm of sapropel formation and oxidation is valid and quantitatively coherent.

[1]  G. Lange,et al.  Source and development of large manganese enrichments above eastern Mediterranean sapropel S1 , 2006 .

[2]  E. Rohling,et al.  The eastern Mediterranean climate at times of sapropel formation: a review , 2007 .

[3]  J. Leventhal.  Carbon-sulfur plots to show diagenetic and epigenetic sulfidation in sediments , 1995 .

[4]  Karline Soetaert,et al.  Solving Differential Equations in R: Package deSolve , 2010 .

[5]  Karline Soetaert,et al.  Inverse Modelling, Sensitivity and Monte Carlo Analysis in R Using Package FME , 2010 .

[6]  B. Boudreau.  Is burial velocity a master parameter for bioturbation , 1994 .

[7]  P. Hall,et al.  Dissolved organic matter in abyssal sediments: Core recovery artifacts , 2007 .

[8]  C. Slomp,et al.  Enhanced regeneration of phosphorus during formation of the most recent eastern Mediterranean sapropel (S1) , 2002 .

[9]  Karline Soetaert,et al.  Oxygenation and organic-matter preservation in marine sediments: Direct experimental evidence from ancient organic carbon–rich deposits , 2005 .

[10]  Robert M. Key,et al.  A global model for the early diagenesis of organic carbon and organic phosphorus in marine sediments , 1995 .

[11]  Peter Berg,et al.  Dynamic Modeling of Early Diagenesis and Nutrient Cycling. A Case Study in an Artic Marine Sediment , 2003 .

[12]  D. Hammond,et al.  Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis , 1979 .

[13]  Bernard P. Boudreau,et al.  A method-of-lines code for carbon and nutrient diagenesis in aquatic sediments , 1996 .

[14]  C. Corselli,et al.  Indications of low macrobenthic activity in the deep sediments of the Eastern Mediterranean Sea , 2004 .

[15]  R. Berner,et al.  Atmospheric oxygen over Phanerozoic time. , 1999, Proceedings of the National Academy of Sciences of the United States of America.

[16]  C. Slomp,et al.  Oxidation and Origin of Organic Matter in Surficial Eastern Mediterranean Hemipelagic Sediments , 2002 .

[17]  Erwin Suess,et al.  Particulate organic carbon flux in the oceans—surface productivity and oxygen utilization , 1980, Nature.

[18]  R. Bustin,et al.  Influence of water column anoxia on the burial and preservation of carbon and phosphorus in marine shales , 1993 .

[19]  C. Slomp,et al.  Sedimentary organic carbon to phosphorus ratios as a redox proxy in Quaternary records from the Mediterranean , 2010 .

[20]  M. Cita,et al.  STRATIGRAPHY OF EASTERN MEDITERRANEAN SAPROPEL SEQUENCES RECOVERED DURING DSDP LEG 42A AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE , 1978 .

[21]  M. Kuypers,et al.  Phosphorus cycling from the margin to abyssal depths in the proto-Atlantic during oceanic anoxic event 2 , 2010 .

[22]  A. Mangini,et al.  WHY SOME MEDITERRANEAN SAPROPELS SURVIVED BURN-DOWN (AND OTHERS DID NOT) , 1997 .

[23]  M. Krom,et al.  Nutrient budget for the Eastern Mediterranean: Implications for phosphorus limitation , 2004 .

[24]  A. Eisenhauer,et al.  A spike of CO2 in the atmosphere at glacial‐interglacial boundaries induced by rapid deposition of manganese in the oceans , 1991 .

[25]  G. Lange,et al.  Magnetic properties and geochemistry of the active oxidation front and the youngest sapropel in the eastern Mediterranean Sea , 2001 .

[26]  Karline Soetaert,et al.  A Practical Guide to Ecological Modelling: Using R as a Simulation Platform , 2008 .

[27]  F. Wilkerson,et al.  Nutrient sources and primary production in the Eastern Mediterranean , 1988 .

[28]  D. Hydes,et al.  The diagenetic formation of metal-rich layers in sapropel-containing sediments in the eastern Mediterranean , 1993 .

[29]  C. Slomp,et al.  Controls on phosphorus regeneration and burial during formation of eastern Mediterranean sapropels , 2004 .

[30]  Bernard P. Boudreau,et al.  Diagenetic Models and Their Implementation: Modelling Transport and Reactions in Aquatic Sediments , 1996 .

[31]  F. Prahl,et al.  Possible early diagenetic alteration of palaeo proxies , 1994 .

[32]  H. Brumsack.  The trace metal content of recent organic carbon-rich sediments; implications for Cretaceous black shale formation , 2006 .

[33]  Philippe Van Cappellen,et al.  A multicomponent reactive transport model of early diagenesis: Application to redox cycling in coastal marine sediments , 1996 .

[34]  D. Burdige.  Geochemistry of Marine Sediments , 2006 .

[35]  Robert A. Berner,et al.  Early Diagenesis: A Theoretical Approach , 1980 .

[36]  David R. Schink,et al.  Quantitative estimates of biological mixing rates in abyssal sediments , 1975 .

[37]  M. Gaffey,et al.  The Chemical Evolution of the Atmosphere and Oceans , 1984 .

[38]  J. Middelburg,et al.  Modes of sapropel formation in the eastern Mediterranean: some constraints based on pyrite properties , 1999 .

[39]  M. Krom,et al.  Active post-depositional oxidation of the most recent sapropel (S1) in sediments of the eastern Medi , 1996 .

[40]  Wang Yifeng,et al.  Metal cycling in surface sediments; modeling the interplay of transport and reaction , 1995 .

[41]  M. Krom,et al.  Phosphorus limitation of primary productivity in the eastern Mediterranean Sea , 1991 .

[42]  R. Wayne.  Chemical Evolution of the Atmosphere , 2007 .

[43]  R. Jahnke,et al.  Evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters , 1994 .

[44]  R. Zahn,et al.  Carbon cycling in the glacial ocean : constraints on the ocean's role in global change : quantitative approaches in paleoceanography , 1994 .

[45]  E. Tadmor,et al.  New High-Resolution Central Schemes for Nonlinear Conservation Laws and Convection—Diffusion Equations , 2000 .

[46]  G. Lange,et al.  REVIEW OF RECENT ADVANCES IN THE INTERPRETATION OF EASTERN MEDITERRANEAN SAPROPEL S1 FROM GEOCHEMICAL EVIDENCE , 1999 .

[47]  C. Slomp,et al.  A key role for iron-bound phosphorus in authigenic apatite formation in North Atlantic continental platform sediments , 1996 .

[48]  George W. Luther,et al.  Kinetics of pyrite formation by the H2S oxidation of iron (II) monosulfide in aqueous solutions between 25 and 125°C: The rate equation , 1997 .

[49]  C. Slomp,et al.  Synchronous basin-wide formation and redox-controlled preservation of a Mediterranean sapropel , 2008 .

[50]  S. Calvert,et al.  Geochemistry of Recent oxic and anoxic marine sediments: Implications for the geological record , 1993 .

[51]  S. Poulet,et al.  Etude des spectres de taille des particules en suspension dans l'estuaire et le golfe du Saint-Laurent. I. Variations spatiales , 1986 .

[52]  J. Middelburg,et al.  Diagenetic pyritisation under eastern Mediterranean sapropels caused by downward sulphide diffusion , 1996 .

[53]  B. Boudreau,et al.  Mathematics of tracer mixing in sediments; I, Spatially-dependent, diffusive mixing , 1986 .

[54]  Edward D. Goldberg,et al.  Geochronological studies of deep sea sediments by the ionium/thorium method , 1962 .

[55]  R. Rosenberg,et al.  Spreading Dead Zones and Consequences for Marine Ecosystems , 2008, Science.