Enhanced regeneration of phosphorus during formation of the most recent eastern Mediterranean sapropel (S1)

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Abstract

Phosphorus regeneration and burial fluxes during and after formation of the most recent sapropel S1 were determined for two deep-basin, low-sedimentation sites in the eastern Mediterranean Sea. Organic C/P ratios and burial fluxes indicate enhanced regeneration of P relative to C during deposition of sapropel S1. This is largely due to the enhanced release of P from organic matter during sulfate reduction. Release of P from Fe-bound P also increased, but this was only a relatively minor source of dissolved P. Pore-water HPO42− concentrations remained too low for carbonate fluorapatite formation. An increased burial of biogenic Ca-P (i.e., fish debris) was observed for one site. Estimated benthic fluxes of P during sapropel formation were elevated relative to the present day (∼900 to 2800 vs. ∼70 to 120 μmol m−2 yr−1). The present-day sedimentary P cycle in the deep-basin sediments is characterized by two major zones of reaction: (1) the zone near the sediment-water interface where substantial release of HPO42− from organic matter takes place, and (2) the oxidation front at the top of the S1 where upward-diffusing HPO42− from below the sapropel is sorbed to Fe-oxides. The efficiency of aerobic organisms in retaining P is reflected in the low organic C/P ratios in the oxidized part of the sapropel. Burial efficiencies for reactive P were significantly lower during S1 times compared with the present day (∼7 to 15% vs. 64 to 77%). Budget calculations for the eastern Mediterranean Sea demonstrate that the weakening of the antiestuarine circulation and the enhanced regeneration of P both contributed to a significant increase in deep-water HPO42− concentrations during sapropel S1 times. Provided that sufficient vertical mixing occurred, enhanced regeneration of P at the seafloor may have played a key role in maintaining increased productivity during sapropel S1 formation.

Introduction

Eastern Mediterranean Sea sediments consist of an alternation of organic-poor hemipelagic layers and organic-rich sapropel units. The formation of these sapropels is most likely due to the combined effect of increased surface water productivity and improved preservation of organic matter under anoxic bottom-water conditions Calvert et al 1992, Rohling 1994, Passier et al 1999, Thomson et al 1999. The episodes of increased productivity were presumably initiated by a climate-induced increase in the continental runoff of freshwater carrying nutrients Rossignol-Strick et al 1982, Hilgen 1991. This enhanced input of low-salinity water led to a weakening of the antiestuarine water circulation (Rohling, 1994) or to a circulation reversal Sarmiento et al 1988, Thunell and Williams 1989 in the basin and, as a consequence, to a decreased ventilation of the deep water with oxygen.

At present, phosphorus (P) is the limiting nutrient in the eastern Mediterranean Sea Berland et al 1980, Krom et al 1991. This means that changes in the biogeochemical cycle of P linked to changes in deep-water oxygenation may have played a key role during times of sapropel formation. Enhanced regeneration of P relative to C can be the result of redox-dependent release of P from organic matter Gächter et al 1988, Ingall et al 1993, Van Cappellen and Ingall 1994 and Fe-oxides Einsele 1936, Mortimer 1941. In sediments accumulating under O2-depleted bottom water, this is reflected in low ratios of the organic C decomposition rate to dissolved P flux and a reduced burial of reactive P relative to organic C in the sediment, as has been demonstrated for a number of modern and ancient lacustrine and marine environments (e.g., Ingall and Jahnke 1994, Ingall and Jahnke 1997, Van Cappellen and Ingall 1994, Arthur and Dean 1998, Schenau and de Lange 2001). The mechanism for enhanced mineralization of P from organic matter under anoxia is still incompletely understood and has been questioned by some (e.g., Colman et al 1997, McManus et al 1997, Colman and Holland 2000). Furthermore, enhanced release of P from organic matter and Fe oxides in anoxic environments may partly be counteracted by an increased burial of other reactive P forms, such as authigenic calcium phosphate minerals Ingall et al 1993, Van Cappellen and Ingall 1994, Van Cappellen and Ingall 1996, Schenau et al 2000 and phosphatic fish debris, here referred to as biogenic Ca-P DeVries and Pearcy 1982, Schenau and de Lange 2000.

Despite the potential importance of redox-related changes in P regeneration efficiency, this process is not included in current models of sapropel formation. For example, in recent model calculations of nutrient cycling and thermohaline circulation in sapropel times, Stratford et al. (2000) assumed that remineralization of both C and P is slower under anoxia when compared with oxic conditions. This situation may at least be partly due to the limited and contradictory reports on P in sapropels and its diagenetic alteration. Pruysers et al. (1991) reported total P enrichments in sapropel S1, S5, and S6 for a site near Crete and attributed this entirely to organic matter assuming a Redfield organic C/P molar ratio of 106. Eijsink et al. (1997), in contrast, observed an organic C/P ratio in sapropel S1 of ∼400. This latter value is in line with the enhanced regeneration hypothesis. P enrichments above sapropel S1 have been suggested to be associated with Fe-oxides formed by diagenetic processes after sapropel formation (Thomson et al., 1995).

In this article, we use high-resolution pore-water and solid-phase data for two low-sedimentation rate, deep-basin sites in the eastern Mediterranean Sea to determine whether there is indeed evidence for a reduced burial (i.e., enhanced regeneration) of reactive P relative to organic C during the formation of the most recent sapropel S1 (also termed Si2, Lourens et al., 1996) between 6.0 and 9.5 kyr BP (radiocarbon convention years; Mercone et al., 2000). In addition, we determine whether increased burial of authigenic calcium P, biogenic Ca-P, or both occurs in sapropel S1 and elucidate the diagenetic reactions affecting P during and after sapropel formation. Finally, the potential role of benthic P regeneration in sapropel S1 formation in the eastern Mediterranean is discussed.

Section snippets

Sampling

Three box cores from two locations in the central part of the eastern Mediterranean Sea (Fig. 1) were selected for this study. Box core BC19 was recovered during the Marflux-I MD69 cruise of the RV Marion Dufresnein 1991. Box cores SL35 (BC19 site) and SL114 were obtained during the Medineth cruise of the RV Logachevin 1999. The BC19 site was revisited in 1999 to obtain pore-water data to complement the solid-phase data collected for the original BC19 core. Subsamples were taken from the box

General sediment geochemistry

In both cores BC19 and SL114, the dark-colored visual sapropel is characterized by higher Corg (1 to 4%) and S (∼0.5 to 5%) concentrations than the surrounding sediment (Fig. 2). Depth profiles of Ba show a gaussian-shaped increase starting at the base of the sapropel and returning to baseline values at ∼5.5 cm above the top of the visual sapropel in both cores. Porosity profiles follow the general trend observed for Ba. Enrichments of Mn and Fe are found above the visual sapropel. Both cores

Original sapropel S1 and the oxidation front

Detailed studies of the sulfur geochemistry of sapropel S1 Passier et al 1996, Passier et al 1997, Passier et al 1999 indicate that sulfate reduction was the main respiratory pathway in the surface sediment during its formation. This resulted in fast formation of framboidal pyrite in the sapropel. Directly below the sapropel, in the formerly oxic sediment, Fe (hydr-)oxides acted as the main electron acceptor for organic matter decomposition. In this part of the sediment, slow formation of

Conclusions

The results of solid-phase P speciation for sediments from the deep basin of the eastern Mediterranean Sea indicate enhanced benthic regeneration of P relative to C during formation of the most recent eastern Mediterranean sapropel S1. This is largely due to the enhanced release of P from organic matter in the absence of oxic conditions in the bottom waters. Release of P from Fe-oxides was a relatively minor source of dissolved P. Increased burial of biogenic Ca-P (i.e., phosphatic fish debris)

Acknowledgements

M. S. Principato is acknowledged for providing the sieve fractions for the fish debris counts. T. Van Wijk, H. de Waard, and A. Gebhardt are thanked for their contribution to the laboratory analyses. J. van Ooijen and G. Nobbe performed the onboard nutrient analyses. We are grateful to G. J. Reichart for his assistance in the microscopic identification of the fish debris. We thank P. Van Cappellen, G. Filipelli, K. C. Ruttenberg, and E. Ingall for valuable comments on the article in manuscript.

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