Biological productivity during sapropel S5 formation in the Eastern Mediterranean Sea: evidence from stable isotopes of nitrogen and carbon

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Abstract

We determined 15N/14N ratios in modern surface and sapropel S5 sediments of the Mediterranean Sea to clarify differences in the nutrient regime associated with sapropel formation. In the modern situation, high δ15N of unused nitrate (15–20 ‰) remaining in the surface waters during the winter phytoplankton bloom evidences P-limitation of biological production in winter. δ15N of surface sediments decrease towards the east of the basin (5 to >2.5‰). This is a consequence of either eastward increasing nitrogen fixation during the summer months, or of particulate matter being supplied predominantly by the P-limited winter bloom. Very low (−1–1‰) δ15N values in sapropel S-5 from four locations require a very light source of nutrient-N assimilated at a minimum of ten times the modern export flux. Because the isochronous records show no spatial gradient in δ15N, we exclude both Ekman-type upwelling and direct riverine discharge as likely sources of nutrients. Our data are consistent with an anti-estuarine thermohaline circulation in the upper 500m during S5 time, allowing for the trapping of nutrients in the eastern basin. The most likely scenario for S5 is that phosphorus release from a relatively shallow redox boundary resulted in an imbalanced supply of N:P (<16:1) to the photic zone. The result was a slow assimilation of carbon during summer stratification and extensive N2-fixation providing the majority of the export flux from a N-limited system.

Introduction

The modern Mediterranean Sea is a highly oligotrophic (nutrient poor) marine environment and the flux of organic carbon from the sea surface is very low and is controlled by the addition of new nutrients from the continent (Béthoux, 1989). Gross primary productivity in the Eastern Mediterranean is limited by phosphorus (Krom et al., 1991), and is based mainly on regenerated nutrients Krom et al 1992, Yacobi et al 1995, Zohary and Robarts 1998. The unusual P limitation of the Eastern Mediterranean may be caused by unequal supply of N and P from external sources (Herut et al., 1999), by adsorption of phosphate by inorganic particles (Krom et al., 1991), or by N2-fixation similar to that observed in the Central Pacific (Karl et al., 1997; see also Wu et al., 2000).

The sediments in the Eastern Mediterranean Sea, however, intermittently recorded dramatic increases in the burial of organic carbon since 5.3 Million years ago (and possibly earlier). Sapropels Kidd et al 1978, Kullenberg 1952, Olausson 1961 containing organic carbon concentrations of up to 30% were deposited during these periods, and each sapropel event lasted several thousand years (Vergnaud-Grazzini et al., 1977). The accumulation rate of organic matter increased abruptly, which has been attributed to higher biologic production (Calvert et al., 1992), better preservation of organic carbon (Cita and Grignani, 1982), or both (Emeis et al., 2000a). Sedimentological Emeis et al 1996, Kidd et al 1978, geochemical (Nijenhuis et al., 1998) and micropaleontological (Schmiedl et al., 1998) facies are consistent with anoxic conditions in deep water bodies and H2S may have even been present in the euphotic zone during some episodes of sapropel deposition (Passier et al., 1999).

The reasons behind increased organic carbon burial are at the heart of the dispute over the significance of sapropels and their environment of formation. If the productive surface layer was replete with nutrients, then where did they come from? If it was not, then how could organic matter sedimentation and burial increase so enormously? Our aim was to reconstruct the nutrient regime during formation of one sapropel (S5, deposited during marine isotope stage 5e in the interval from 128–123 kyr) and our tools are isotope ratios of 13C/12C and 15N/14N. Nitrogen sources associated with the increased export flux during sapropel formation might be identified based on distinctive isotopic ratios. In accordance with the Raleigh model of isotope fractionation (e.g., Mariotti et al., 1981), biotic utilization of a nutrient pool such as nitrate in which the utilization is selective for the lighter isotope of nitrogen, 14N, will result in N products (biomass) which are depleted in 15N relative to the source nitrate. In a closed system, as such N utilization precedes the remaining nitrate will become progressively enriched in 15N, in turn enriching the biomass subsequently produced. Nearing complete N utilization, the 15N/14N of the accumulated products must, via mass balance considerations, approach that of the original source nitrate. Under circumstances where biotic N utilization is relatively non-isotopically selective and the N substrate is abundant (e.g., N fixation) the 15N/14N of the biomass produced will consistently differ little from that of the N substrate (e.g., N fixer δ15N ≈ −2–0‰).

A second isotopic signature, δ13C of organic matter from marine phytoplankton, is theoretically dependent on such factors as CO2 concentration and its δ13C signature, growth rate, cell size and morphology, and species composition Laws et al 1997, Popp et al 1998, Rau et al 1996. Low δ13C of the dissolved inorganic carbon pool (e.g., riverine DIC or DIC from anoxic deep waters) can therefore lead to the production of isotopically light organic matter. Low values are for example found in chemo-autotrophic and photo-autotrophic bacterial production in the deep chlorophyll maximum of the Black Sea, which assimilates isotopically depleted DIC from anoxic water (Fry et al., 1991). An increase in the biotic uptake rate of CO2 (relative to CO2 supply) can be expected to increase the 13C/12C of the biomass produced either through Raleigh fractionation considerations (above) or, more effectively, through elevation of the CO213C/12C within photosynthesizing cells (e.g., Rau et al., 1996). Exceptions to such biomass 13C/12C behavior, however, have been observed both experimentally (e.g., Burkhardt et al 1999, Popp et al 1998) and in the field Pancost et al 1999, Rau et al 2001.

Together, the stable isotope ratios of organic carbon and nitrogen are potentially powerful tools, because they may characterize sources of nitrogen and growth conditions. They also trace changes in the composition of that nutrient pool away from that source (Altabet and Francois, 1994), permitting an identification of the point source of nutrients, e.g., a river or an upwelling area. A possible limit on their use as paleo-proxies is diagenesis. Differential degradation of biologic products under oxic and anoxic conditions has been postulated from studies on δ15N of bulk nitrogen and compound-specific δ15N (Sachs and Repeta, 1999). Such a possible diagenetic overprint would imply that the sedimentary signal is not the original signal of exported production. Another potential isotopic overprint in bulk marine organic material is the presence of terrestrial detritus (e.g., Jasper and Hayes, 1993).

To test for possible diagenetic effects required that we determine δ15N in dissolved nitrate, total suspended matter, and surface sediments of the modern Mediterranean Sea to confirm the robustness of the signal with regard to diagenesis. These data also provide valuable new insight into the nutrient status of the modern eastern Mediterranean. We then present high-resolution data sets from an array of 4 cores on an E-W transect in the eastern Mediterranean Sea that constrain sources of nutrient-N and conditions of biologic productivity in the surface mixed layer during S5 time, using published and new data on the isotopic ratios of carbon and nitrogen. Ancillary information available for the time-slice reconstruction are sea surface temperature (SST) variations from alkenone unsaturation ratios and oxygen and carbon isotope ratios of planktonic foraminiferal calcite.

Section snippets

Material and methods

Sapropel S5 samples of Ocean Drilling Project Sites 967 from Erathostenes Seamount south of Cyprus (Fig. 1, Table 1) and gravity corer core KC01B from the Ionian Sea were analyzed for organic carbon and nitrogen contents and their stable isotope compositions. High resolution data sets across S5 of δ18O and δ13C of G. ruber at Sites 967 and 969 and the low-resolution data set of Site 969 have in part been published in Emeis et al. (1998). They also reported alkenone temperature estimates for

Modern δ15N distribution patterns

Nitrogen isotope composition of dissolved nitrate (3 stations) and suspended matter (7 stations) in the upper water column (10–400 m water depth) and of 38 surface sediment samples (Fig. 1; Table 2, Table 3) illustrate the oligotrophic nutrient regime in the modern Mediterranean Sea.

Nitrate concentrations were low (around 1.0 μMol) in the surface waters (Fig. 2) in January 1998 and were matched by elevated Chl a, POC and PN concentrations in the uppermost 100m. Previous measurements carried

The modern situation

The isotope ratios found in particulate nitrogen collected in January 1998 reflect those that are produced from new nutrients after convective overturn during winter cooling; in the Eastern Mediterranean Sea, this is the time when biologic productivity in the surface is highest (Krom et al., 1992). The presence of residual nitrate in the photic zone after phosphate has been entirely consumed has been interpreted as direct evidence for P limitation of primary productivity (Krom et al., 1991). A

Conclusions

Nitrogen isotope ratios of nitrate remaining in the present surface waters during the winter phytoplankton bloom provide evidence for P-limitation in winter. The nitrogen isotope ratios of the surface sediments of the modern Mediterranean are increasingly lighter towards the east of the basin. In one view, this could be caused by either an increasing contribution of nitrogen fixed from the atmosphere. In this scenario there would be a 30% or even higher contribution of export flux from N2

Acknowledgements

This work was funded under contracts Em 37/4 and Em 37/8 by the German Research Foundation and ENV4-CT97 to 0564 (TEMPUS) by the European Community. K.E. also acknowledges a grant by the Japanese Society for the Advancement of Science and the hospitality of T. Sakamoto, I. Koizumi and N. Suzuki during an extended visit to Sapporo. We appreciate the careful reviews of Dan Sigman, Martine Rossignol-Strick and one anonymous reviewer that greatly helped to improve the manuscript.

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    Present address: Dep. of Paleontology and Historical Geology, Munich University, Richard Wagner Straße 10, D-80333 Munich, Germany

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