Gert J. de Lange

Preservation of organic-walled dinoflagellate cysts in different oxygen regimes: a 10,000 year natural experiment

Abstract The occurrence of organic-walled dinoflagellate cysts in (fossil) sediments depends on several factors, including as the ecological preferences of the cyst-forming dinoflagellates, cyst production, transport and preservation. Although laboratory experiments have shown that several cyst species are sensitive to chemical treatment, no information about the selective preservation of dinoflagellate cyst species in natural environments has previously been presented. Here, we present data on the effects of oxygen availability in bottom sediments on a cyst assemblage from the ungraded Madeira Abyssal Plain f-turbidite of which only the upper layer has been oxidized. Based on differences in species composition between the oxidized and underlying, unoxidized layers of this turbidite, the influence of oxygen availability on the preservation of individual species has been estimated. Cyst species have been classified in ascending order of resistance to oxygen availability in sediments as: (1) highly sensitive (cysts formed by Protoperidinium species), (2) moderately sensitive (e.g. Spiniferites species), (3) moderately resistant (e.g. Impagidinium paradoxum and Nematosphaeropsis labyrinthus) and (4) resistant (e.g. Impagidinium aculeatum).

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.

Manganese solubility control in marine pore waters

Abstract A theoretical study of the system CaCO3-MnCO3-H2O provides evidence that solid solution is limited to three mole fraction ranges corresponding to Mn-calcite, kutnohorite and calcic-rhodochrosite. All these authigenic phases may control the concentration of manganese in suboxic marine pore waters. As a consequence the solubility of manganese cannot be adequately described by a single thermodynamic equilibrium constant.

Biogeochemistry and Community Composition of Iron- and Sulfur-Precipitating Microbial Mats at the Chefren Mud Volcano (Nile Deep Sea Fan, Eastern Mediterranean)

ABSTRACT In this study we determined the composition and biogeochemistry of novel, brightly colored, white and orange microbial mats at the surface of a brine seep at the outer rim of the Chefren mud volcano. These mats were interspersed with one another, but their underlying sediment biogeochemistries differed considerably. Microscopy revealed that the white mats were granules composed of elemental S filaments, similar to those produced by the sulfide-oxidizing epsilonproteobacterium “Candidatus Arcobacter sulfidicus.” Fluorescence in situ hybridization indicated that microorganisms targeted by a “Ca. Arcobacter sulfidicus”-specific oligonucleotide probe constituted up to 24% of the total the cells within these mats. Several 16S rRNA gene sequences from organisms closely related to “Ca. Arcobacter sulfidicus” were identified. In contrast, the orange mat consisted mostly of bright orange flakes composed of empty Fe(III) (hydr)oxide-coated microbial sheaths, similar to those produced by the neutrophilic Fe(II)-oxidizing betaproteobacterium Leptothrix ochracea. None of the 16S rRNA gene sequences obtained from these samples were closely related to sequences of known neutrophilic aerobic Fe(II)-oxidizing bacteria. The sediments below both types of mats showed relatively high sulfate reduction rates (300 nmol·cm−3·day−1) partially fueled by the anaerobic oxidation of methane (10 to 20 nmol·cm−3·day−1). Free sulfide produced below the white mat was depleted by sulfide oxidation within the mat itself. Below the orange mat free Fe(II) reached the surface layer and was depleted in part by microbial Fe(II) oxidation. Both mats and the sediments underneath them hosted very diverse microbial communities and contained mineral precipitates, most likely due to differences in fluid flow patterns.

Possible diagenetic mobilization of barium in sapropelic sediment from the eastern Mediterranean

Abstract In the last few years it has frequently been suggested that Ba is a useful indicator of paleoproductivity. The formation of some sapropels in the Eastern Mediterranean is considered to be related to, or to coincide with, periods of enhanced productivity. A high-resolution sampling study has been undertaken in order to investigate whether the Ba distribution in sapropels reflects a primary input signal or whether it has been altered by diagenetic processes. On the basis of our results we suggest that three diagenetic stages determine the distribution of Ba. During deposition of the sapropel (stage 1) Ba is mobilized as anoxic conditions prograde. After deposition of the sapropel (stage 2), a progressive oxidation front develops. This front induces the formation of Mn and Fe enrichments and barite precipitation at the oxic/anoxic boundary. Barite precipitation is believed to be caused mainly by a rise in the porewater sulphate concentration after sulphides have been oxidized by the front. Upon burial (stage 3), suboxic conditions develop as the oxygen becomes exhausted again. In contrast to Fe- and Mn-oxyhydroxides which dissolve and reprecipitate at higher levels, barite is preserved because dissolved sulphate is not depleted. The interpretation of the Ba distribution in organic-rich sediment is not straightforward. Diagenetic reallocation of a primary Ba signal will possibly disturb the relationship between Ba and organic production. Consequently, one must be very cautious when invoking Ba as a paleoproductivity indicator.

Geochemistry of eastern Mediterranean sediments: Primary sediment composition and diagenetic alterations

Abstract Two cores recovered in the eastern Mediterranean were analysed for major, minor and trace elements. The primary chemical composition of the sediment is different at each location, probably because the lithological sources and the relative biogenic contributions differ. Carbonates are important for the concentration of Ca, Mg and Sr, whereas aluminosilicates determine the concentration of Si, Al, K, Li, Y and Be, and to a lesser extent that of Fe, Cr, Ti, Mg, Zn and Zr. In sapropels, organic carbon and sulphur seem to be closely related. Bromine, Mo, P, Fe, V, Cu, Zn, Co, Ni and Cr are closely associated with organic and sulphidic compounds. The concentration versus depth profile for organic carbon in two sapropels points to a rapid establishment of conditions that gave rise to sapropel formation, followed by a gradual transition back to “normal” conditions. The primary composition is overprinted by diagenetic processes. Sulphate-reducing conditions occurred during and just after sapropel deposition. A progressive oxidation front mechanism, which became active after sapropel deposition, is responsible for additional major geochemical changes. This diagenetic phenomenon has strong implications for the chemistry of Fe, Mn, Ni, Co, Zn, Cu, Cr, V, U, As and Sb.

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.

Oxygenation and organic-matter preservation in marine sediments: Direct experimental evidence from ancient organic carbon–rich deposits

Clarification of the factors involved in the formation of unusual ancient organic carbon- rich deposits (like eastern Mediterranean sapropels) is central in understanding oceanic carbon cycling. The role of oxygenation remains a subject of controversy primarily due to two major uncertainties: (1) it is unknown if ancient organic-rich deposits reflect an accumulation of refractory organic matter (OM) or oxygenation-related aberrant sediment OM recycling, and (2) although marine OM degradation may be slower under anoxic conditions, its ultimate impact on organic carbon (Corg) preservation over geological time remains unclear. Here we provide direct experimental evidence that the Corg in eastern Mediterranean S1 sapropels (deposited .5 ka) is still highly reactive and that a shutdown in labile organic matter degradation under anoxic conditions played a key role in the formation of these deposits.

Sulphur speciation in anoxic hypersaline sediments from the eastern Mediterranean Sea

Abstract The anoxic hypersaline Tyro and Bannock Basins are among the most sulphidic bodies of water in the marine environment (H2S > 2 mM). We report the distribution of elemental sulphur, Acid Volatile Sulphur (AVS), organic polysulphides, humic sulphur (0.5 M NaOH extractable), and pyritic sulphur in the sediments from these basins. Pyritic sulphur appears to be the main phase of inorganic reduced sulphur (50–80% of the total sulphur pool) and is at the same level (about 250 μmoles per gram dry weight) in cores recovered from the two basins. Remarkably, humic sulphur has been found to account for 17–28% of the total sulphur pool in the Tyro Basin, and for 10–43% in the Bannock Basin. Sulphur isotope data show negative δ34S values for both pyritic sulphur (δ34S: −19%c to −39%) and humic sulphur (δ34S: −15% to −30%). This indicates that both pyritic and humic sulphur originate from microbially produced HZS. Neither basin shows a significant correlation between pyritic sulphur and organic carbon, suggesting syngenetic pyrite formation. Additionally, the degree of pyritization in these sediments (DOP is 0.62) indicates that pyrite formation is limited by the reactivity of Fe. Humic sulphur correlates with the pyritic sulphur distribution and seems to be related with gelatinous pellicles (bacterial mats at the interface between oxic seawater and brine). Both pyritic and humic sulphur can be formed in the water column at the interface of oxic seawater and brine by a syngenetic pathway; about 5% of the pyrite is formed in the sediments by diagenetic processes. The presence of a bacterial mat and high HZS content in the brines favour such conditions. However, most of the humic sulphur inventory can be supported by diagenetic processes. Although these basins differ in their major element chemistry, their reduced sulphur species chemistry appears to be similar.