Hilde F. Passier

Sulphidic Mediterranean surface waters during Pliocene sapropel formation

Sapropels—organic-matter rich layers—are common in Neogene sediments of the eastern Mediterranean Sea. The formation of these layers has been attributed to climate-related increases in organic-matter production and increased organic-matter preservation due to oxygen depletion in more stagnant bottom waters,. Here we report that eastern Mediterranean Pliocene sapropels contain molecular fossils of a compound (isorenieratene) known to be synthesized by photosynthetic green sulphur bacteria, suggesting that sulphidic (euxinic)—and therefore anoxic—conditions prevailed in the photic zone of the water column. These sapropels also have a high trace-metal content, which is probably due to the efficient scavenging of these metals by precipitating sulphides in a euxinic water column. The abundance and sulphur-isotope composition of pyrite are consistent with iron sulphide formation in the water column. We conclude that basin-wide water-column euxinia occurred over substantial periods during Pliocene sapropel formation in the eastern Mediterranean Sea, and that the ultimate degradation of the increased organic-matter production was strongly influential in generating and sustaining the euxinic conditions.

DIAGENETIC PYRITISATION UNDER EASTERN MEDITERRANEAN SAPROPELS CAUSED BY DOWNWARD SULPHIDE DIFFUSION

Recurrent organic-rich layers (sapropels) in eastern Mediterranean sediments are enriched in Corg, Fe, and S. Sulphur and Fe are enriched in a zone immediately below the sapropels, whereas Corg is not. δ34S values of bulk sediments and simple mass-balance calculations indicate that SO42− reduction has taken place in an open system, with all HS− formed at, or close to, the sediment surface. Formation of pyrite in the sapropel was Fe-limited and consequently, excess HS− was able to migrate downwards (downward sulphidisation). This resulted in the formation of pyrite below the sapropel by reaction of this HS− with solid-phase ferric iron and Fe2+ diffusing upwards from underlying sediments. The Fe2+ source probably includes Fe (hydr) oxide layers formed at former oxidation fronts above previously deposited and buried sapropels. This downward sulphidisation mechanism allows accumulation of twice as much S in alternating organic-rich-anoxic/organic-poor-suboxic sediments compared to what is preserved in organic-rich anoxic sediments.

Modes of sapropel formation in the eastern Mediterranean: some constraints based on pyrite properties

Abstract Pyrite formation within and directly below sapropels in the eastern Mediterranean was governed by the relative rates of sulphide production and Fe liberation and supply to the organic-rich layers. At times of relatively high SO2−4 reduction, sulphide could diffuse downward from the sapropel and formed pyrite in underlying sediments. The sources of Fe for pyrite formation comprised detrital Fe and diagenetically liberated Fe(II) from sapropel-underlying sediments. In organic-rich sapropels, input of Fe from the water column via Fe sulphide formation in the water may have been important as well. Rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels, whereas below the sapropels slow euhedral pyrite formation at low saturation levels occurred. δ34S values of pyrite are −33‰ to −50‰. Below the sapropels δ34S is lower than within the sapropels, as a result of increased sulphide re-oxidation at times of relatively high sulphide production and concentration when sulphide could escape from the sediment. The percentage of initially formed sulphide that was re-oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment. It ranges from 34% to 80%, varying significantly between sapropels. Increased palaeoproductivity as well as enhanced preservation contributed to magnified accumulation of organic matter in sapropels.

Pyrite contents, microtextures, and sulfur isotopes in relation to formation of the youngest eastern Mediterranean sapropel

Pyrite within and below sapropels in the eastern Mediterranean is a result of microbial SO42− reduction within the sapropel, and the subsequent reaction of sulfide (HS−) with detrital Fe and Fe2+ diffusing upward from underlying sediments. Below the youngest Mediterranean sapropel, S1, pyrite (as much as 281 µmol pyritic S/g) is mostly present as euhedral crystals, whereas within the sapropel only framboidal pyrite (as much as 360 µmol pyritic S/g) has been detected. Framboidal microtextures indicate pyrite formation at the site of HS− production within the sapropel. Euhedral pyrite, below the sapropel, forms when sulfate reduction in the sapropel outcompetes iron liberation and supply, and HS− diffuses out of the sapropel. Sulfur isotope values of pyrite are extremely light in the sapropel (−37.3‰ to −38.2‰) as well as below the sapropel (−45.6‰ and −49.6‰), indicating that HS− has formed in a system with abundant SO42− and in the presence of oxidants.

Early diagenesis and sulphur speciation in sediments of the Oman Margin, northwestern Arabian Sea

The pore-waters from two boxcores from the Oman Margin, from sites NIOP484 and NIOP487 (respectively in and below the oxygen minimum zone in the water column), were analysed for concentrations of iodide, sulphide, thiosulphate, phosphate, silica, ammonia, nitrate, nitrite, dissolved iron, dissolved manganese and dissolved sulphur. In addition, the solid phase of samples from the same cores was analysed for organic carbon, pyritic sulphur, acid volatile sulphide, elemental sulphur, organic polysulphides, humic sulphur, reactive iron phases and reactive manganese phases. Pore-water profiles show that post-oxic diagenesis dominates in these Holocene sediments. Only minor amounts of reduced sulphur species were found in pore-waters, despite the low oxygen concentration of the bottom-water and the relatively high organic-matter content of the sediments. Significant amounts of solid-phase reduced sulphur were only found in the lower half of the boxcore at station NIOP484. Consequently, sulphate reduction is not as significant in these sediments relative to sediments from other upwelling areas, and thus more sedimentary organic matter is preserved.

Sediment chemistry and magnetic properties in an anomalously reducing core from the eastern Mediterranean Sea

Abstract In Core KC19C (19.6 m long), recovered in the abyssal plain between Crete and Cyprus in the eastern Mediterranean, a large number of organic-rich layers (sapropels) occur, which correlate to maxima in the insolation curve. In contrast to other sites in the eastern Mediterranean, porewaters contain sulfide below a few meters below seafloor (mbsf). Geochemical analyses were performed on the porewaters (ammonium, alkalinity, sulfate, manganese, iron, bisulfide, chloride, Eh, pH) and sediments (organic carbon, barium, total sulfur, total iron, manganese, pyrite, dithionite-extractable iron). In addition, a series of magnetic parameters and ratios (NRM, ARM, IRM, χ in , ARM/IRM, S-ratio, ARM 20mT /ARM in ) were determined. In the top of the sediments, Fe and Mn (hydr)oxides occur at the oxic–suboxic boundary just above the youngest sapropel Si2; these (hydr)oxides appear to be more easily demagnetized by means of alternating fields than deeper (hydr)oxides which presumably are of detrital origin. The transition from suboxic to anoxic sediments is located at ∼2 mbsf. Sulfide is produced, possibly during sulfate reduction through anoxic methane oxidation, at ∼17.5 mbsf. From 17.5 mbsf sulfide migrates upward, titrating reactive Fe, resulting in pyrite formation in the entire sediment column up to ∼2 mbsf. At this depth, the upward sulfide flux has been totally consumed by reaction with solid phase Fe and dissolved Fe(II) diffusing downward from the suboxic zone above. From ∼2 mbsf downward magnetic intensities are significantly reduced, indicating reductive dissolution of Fe-oxide minerals and pyrite formation. As a consequence, no reliable NRM data can be obtained in the lower half of the core.

Sulphur Enrichment in Organic Matter of Eastern Mediterranean Sapropels: A Study of Sulphur Isotope Partitioning

Sulphur isotope compositions and S/C ratios of organic matter were analysed in detail by combustion-isotope ratio monitoring mass spectrometry (C-irmMS) in eastern Mediterranean sediments containing three sapropels of different ages and with different organic carbon contents (sapropel S1 in core UM26, formed from 5–9 ka ago with a maximum organic carbon content of 2.3 wt%; sapropel 967 from ODP Site 160-967C, with an age of 1.8 Ma and a maximum organic carbon content of 7.4 wt%; and sapropel 969 from ODP Site 160-969E, with an age of 2.9 Ma and a maximum organic carbon content of 23.5 wt%). Sulphur isotopic compositions (δ34S) of the organic matter ranged from -29.5 to +15.8‰ and the atomic S/C ratio was 0.005 to 0.038. The organic sulphur in the sediments is a mixture of sulphur derived from (1) incorporation of 34S-depleted inorganic reduced sulphur produced by dissimilatory microbial sulphate reduction; and (2) biosynthetic sulphur with an isotopic signature close to seawater sulphate. The calculated biosynthetic fraction of organic sulphur in non-sapropelic sediments ranges from 68–87%. The biosynthetic fraction of the organic sulphur of the sapropels (60–22%) decreases with increasing organic carbon content of the sapropels. We propose that uptake of reduced sulphur into organic matter predominantly took place within sapropels where pyrite formation was iron-limited and thus an excess of dissolved sulphide was present for certain periods of time. Simultaneously, sulphide escaped into the bottom water and into sediments below the sapropels where pyrite formation occurred.

Changes in Magnetic Parameters After Sequential Iron Phase Extraction of Eastern Mediterranean Sapropel S1 Sediments

Iron is distributed over different minerals (i.e. silicates, pyrite, detrital oxides) that are present in a sediment sequence that formed under anoxic conditions. After post-depositional re-oxidation of the sediments pyrite is no longer present and diagenetic iron phases constitute an important portion of the iron in the oxidised part of the sapropel. They are very fine-grained making them amenable to analysis by means of sequential extraction and mineral-magnetic methods. The sequential extraction shows that besides iron in silicates, iron mainly occurs in ‘amorphous’ oxides in the oxidised part of the S1 sapropel. Pyrite constitutes an important fraction in the still reduced part of the S1 sapropel. Some silicon is dissolved during the extraction for the ‘amorphous’ oxides, suggesting that ‘amorphous’ iron also occurs as ferro-silicate coatings. Mineral-magnetic analysis involved component analysis of the isothermal remanent magnetisation (IRM) and hysteresis loop measurements. Three coercivity phases could be identified in the IRM component analysis; these were interpreted as ‘detrital’ magnetite, hematite, and biogenic magnetite. The diagenetically formed iron phases influence the parameters of the IRM components. Hysteresis measurements together with the IRM component analysis, indicate the importance of bacterial magnetite in the oxidised sapropel, particularly in the lower part of the active oxidation zone.