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
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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, N2 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.
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