Euxinic Bottom Waters Research Articles

Coniacian–Santonian deep ocean anoxia/euxinia inferred from molecular and inorganic markers: Results from the Demerara Rise (ODP Leg 207)

We investigated Coniacian to early Santonian sediments from ODP site 1261, Demerara Rise, using organic, inorganic and molecular methods to reconstruct the development of ocean water anoxia/euxinia. High lycopane/ n-C 31 values of up to 4.5 suggest oxygen-depleted conditions in the water column for most of the time. Lower values (∼ 1) across the early to mid-Coniacian transition probably mark a period of stronger oxygenation. In two higher resolution intervals we observe strong enrichment in Zn, concurrent with an increase in the lycopane/ n-C 31 ratio, marking the onset of euxinic bottom waters, whereas decreases in lycopane/ n-C 31 values and decreases in Ni enrichment characterize the termination of sulfidic conditions. Photic zone euxinia appears to be restricted to the early Coniacian interval, as isorenieratene derivatives were exclusively detected in this part of the section. The combination of biomarker and inorganic geochemical proxies suggests a general decoupling of surface and mid-water oxygenation from deep water oxygenation from the mid-Coniacian onwards. This process might be related to the successive opening of the Equatorial Atlantic Gateway, allowing a migration of surface and intermediate water masses before the establishment of a deep water connection.

Tracing the stepwise oxygenation of the Proterozoic ocean

Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth's atmosphere. Geochemical data suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon (2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox-sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen. Subsequent expansion of sulphidic conditions after about 1,800 Myr ago (refs 8, 9) maintained a mid-Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic (sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms. By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column.

Biomarker and 16S rDNA evidence for anaerobic oxidation of methane and related carbonate precipitation in deep-sea mud volcanoes of the Sorokin Trough, Black Sea

Many mud volcanoes were recently discovered in the euxinic bottom waters of the Sorokin Trough (NE Black Sea). Three of them, i.e., NIOZ, Odessa, and Kazakov, were selected for a detailed biogeochemical investigation. Four methane-related carbonate crusts covered with microbial mats, and sediments (‘mud breccia’) from these mud volcanoes were collected during the 11th Training-Through-Research cruise (TTR-11) in 2001, the first finding of methanotrophic microbial mats associated with authigenic carbonates in the deep Black Sea. We measured the concentrations and δ 13C values of methane and specific archaeal and bacterial lipids, and determined archaeal and bacterial 16S rRNA gene sequences. The δ 13C of the microbial lipids reflected the carbon isotopic values of the methane, indicating that methane was the main carbon source for microorganisms inducing carbonate formation. Anaerobic oxidation of methane (AOM) in these settings was performed by archaea affiliated with the so-called ANME-1 group. None of the identified archaeal sequences were closely related to known methanogens. The combined 16S rRNA gene sequence and biomarker data revealed a distinct difference in archaeal assemblage between the carbonate crusts and mud breccias. Besides gene sequences of sulfate-reducing bacteria, DNA analysis of bacterial communities revealed a diversity of bacteria with apparent contrasting metabolic properties. The methane utilization via AOM processes was detected in the uppermost sediments where it subsequently induces authigenic carbonate precipitation most probably below seafloor. The results of integrated biomarker and 16S rRNA gene study reveal a crucial role of AOM processes in formation of authigenic carbonates in methane seep environments.

Reactive iron enrichment in sediments deposited beneath euxinic bottom waters: constraints on supply by shelf recycling

Abstract Modern and ancient euxinic sediments are often enriched in iron that is highly reactive towards dissolved sulphide, compared to continental margin and deep-sea sediments. It is proposed that iron enrichment results from the mobilization of dissolved iron from anoxic porewaters into overlying seawater, followed by transport into deep-basin environments, precipitation as iron sulphides, and deposition into sediments. A diagenetic model shows that diffusive iron fluxes are controlled mainly by porewater dissolved iron concentrations, the thickness of the surface oxygenated layer of sediment and to a lesser extent by pH and temperature. Under typical diagenetic conditions (pH < 8, porewater Fe 2+ = 10 −6 g cm −3 ) iron can diffuse from the porewaters in continental margin sediments to the oxygenated overlying seawater at fluxes of 100–1000 μg cm −2 a −1 . The addition of reactive iron to deep-basin sediments is determined by the magnitude of this diffusive flux, the export efficiency (ɛ) of recycled iron from the shelf, the ratio of source area ( S ) to basin sink area ( B ) and the trapping of reactive iron in the deep basin. Values of ɛ are poorly constrained but modern enclosed or semi-enclosed sedimentary basins show a wide variation in S/B ratios (0.25–13) where the shelf source area is defined as sediments at less than 200 m water depth. Diffusive fluxes in the range 100–1000 μg cm −2 a −1 are able to produce the observed reactive iron enrichments in the Black Sea, the Cariaco Basin and the Gotland Deep for values of ɛ × S/B from 0.1–5. Transported reactive iron can be trapped physically and/or chemically in deep basins. Physical trapping is controlled by basin geometry and chemical capture by the presence of euxinic bottom water. The S/B ratios in modern basins may not be representative of those in ancient euxinic/semi-euxinic sediments but preliminary data suggest that ɛ × S/B in ancient euxinic sediments has a similar range as in modern euxinic sediments.

Late Ordovician (Turinian–Chatfieldian) carbon isotope excursions and their stratigraphic and paleoceanographic significance

Five positive carbon isotope excursions are reported from Platteville–Decorah strata in the Upper Mississippi Valley. All occur in subtidal carbonate strata, and are recognized in the Mifflin, Grand Detour, Quimbys Mill, Spechts Ferry, and Guttenberg intervals. The positive carbon isotope excursions are developed in a Platteville–Decorah succession in which background δ 13C values increase upward from about −2‰ at the base to about 0‰ Vienna Pee Dee belemnite (VPDB) at the top. A regional north–south δ 13C gradient, with lighter values to the north and heavier values to the south is also noted. Peak excursion δ 13C values of up to +2.75 are reported from the Quimbys Mill excursion, and up to +2.6 from the Guttenberg excursion, although there are considerable local changes in the magnitudes of these events. The Quimbys Mill, Spechts Ferry, and Guttenberg carbon isotope excursions occur in units that are bounded by submarine disconformities, and completely starve out in deeper, more offshore areas. Closely spaced chemostratigraphic profiles of these sculpted, pyrite-impregnated hardground surfaces show that they are associated with very abrupt centimeter-scale negative δ 13C shifts of up to several per mil, possibly resulting from the local diagenetic effects of incursions of euxinic bottom waters during marine flooding events.

Pyritization: a palaeoenvironmental and redox proxy reevaluated

Pyrite as a proxy for the redox states of bottom waters was evaluated in the context of modern salt marsh sediments. Two sites, 40 m apart, with very different physicochemical properties were sampled seasonally. The sites differed from each other in (1) the amount of tidal flushing, (2) vegetation, (3) redox condition below the sediment–water interface, (4) carbon content and (5) solid phase iron oxyhydroxide and iron sulfide content of the sediment. The calculated degree of pyritization (DOP) varies between 0.44 and 1.0, depending on the site and the season. Different DOP values were obtained for the same samples when different techniques were applied for extraction of reactive iron. Framboidal pyrite was dominant; however, other textural forms of pyrite were also observed. The size distribution of the individual pyrite crystals and the framboids was fairly narrow; with the majority of them having diameters <10 μm. Despite deposition taking place in aerobic waters (fully oxygenated waters) at the sampled sites, both the DOP and pyrite textures indicated dysoxic (waters with low oxygen concentrations) to euxinic (waters with no oxygen present, and H 2S may be continually or intermittently present) bottom waters. These results indicate that caution must be applied when using DOP and pyrite morphologies as proxies for interpreting the redox conditions of different depositional environments.

Timing and genesis of base-metal mineralisation in black shales of the Upper Permian Ravnefjeld Formation, Wegener Halvø, East Greenland

Bituminous mud shales of the Upper Permian Ravnefjeld Formation (Zechstein 1 equivalent) are mineralised with zinc, lead and copper within a ca. 50 km2 area on Wegener Halvo in central East Greenland. The occurrence of base-metal sulphides in shale nodules cemented prior to compaction indicates an early commencement of base-metal mineralisation. In other cases, post-compactional sulphide textures are observed. Homogeneous lead isotope signatures of galena and sphalerite from the shales (206Pb/204Pb: 18.440–18.466; 207Pb/204Pb: 16.554–16.586; 208Pb/204Pb: 38.240–38.326) suggest that all base metals were introduced during a single hydrothermal event. Therefore, post-compactional textures are believed to result from recrystallisation of early diagenetic sulphides during deep burial in the Upper Cretaceous to Tertiary. Lead isotope signatures of galena hosted in Upper Permian carbonate build-ups are relatively heterogeneous compared to those of the shale-hosted sulphides. The observed relations indicate a shared lead source for the two types of mineralisation, but different degrees of homogenisation during mineralisation. This suggests that lead was introduced to the carbonate rocks and black shales during two separate events. δ34S of base-metal sulphides in the Ravnefjeld Formation lie between –12 and –4‰, whereas synsedimentary and early diagenetic pyrite in unmineralised shales in general have δ34S between –47 and –16.5‰. Early diagenetic pyrite in the Wegener Halvo area in general has δ34S 15 to 20‰ higher than the same pyrite morphotype in Triaselv in the western part of the basin. This relatively high δ34S can be explained by extensive microbial sulphate reduction within persistent euxinic (super-anoxic) bottom waters under which supply of isotopically light seawater sulphate (and disproportionation of intermediate sulphur compounds) was restricted. The sulphur in the base-metal sulphides is believed to represent sulphide-dominated pore water, enriched in 34S due to preferential removal of 32S by sulphate-reducing bacteria and precipitation of diagenetic pyrite in the near-seafloor environment. We suggest that the sulphide-dominated pore water was trapped in the shale formation prior to introduction of base-metal-bearing fluids through fractures in the underlying carbonates, and that sulphide precipitation took place when the two fluids met. δ34S values of carbonate-hosted base-metal sulphides fall within the same range as the shale-hosted ones. The relationship between barite and sulphides and evidence for pre-mineralisation entrapment of liquid hydrocarbons in the carbonates suggest that the sulphide in this case is derived by in-situ thermochemical sulphate reduction (TSR). Measured fractionation between sulphide and sulphate ranges from 18.5 to 24.4‰, suggesting temperatures of TSR around 70 to 100 °C. Vitrinite reflectance measurements in mineralised shale samples are all between 1.7 and 2.0%, except for samples taken close to a Tertiary dyke giving ca. 3.0%. Vitrinite reflectance data are comparable to previously published data from unmineralised shale samples in the area and could not be proven to correlate with the degree of mineralisation. This indicates that any early hydrothermal effect has been overprinted later, probably during deep burial in the Late Cretaceous to Early Tertiary as previously proposed.

Varve calibrated records of carbonate and organic carbon accumulation over the last 2000 years in the Black Sea

Sedimentologic and geochemical studies of box and gravity cores recovered from the Black Sea during the first leg of a multileg international Black Sea expedition in 1988 allow reconstruction of the basinwide Holocene environmental history of the Black Sea. In the deeper parts of the basin, box cores typically recovered a flocculent surface layer (“fluff”), laminated coccolith marls of Unit I (25–45 cm thick), and the upper 5–10 cm of finely laminated, dark‐colored sapropels of Unit II. Fine‐grained, homogeneous mud turbidites are interbedded with Units I and II over much of the basin, but the stratigraphie position of these turbidites differs, from site to site. The deposition of individual turbidites up to 15 cm thick does not appear to have significantly disturbed underlying laminae. Sediment trap deployments in the Black Sea suggest that light and dark laminae couplets represent annual increments of sedimentation (i.e., varves); we have therefore constructed a varve chronology for the sequence in order to correlate and date distinctive sedimentation and paleoenvironmental events. Distinctive groups of laminae in Unit I can be correlated across the entire deeper basin (a distance of more than 1000 km). This implies a remarkable homogeneity in production, accumulation, and preservation of biogenic material over much of the Black Sea during deposition of Unit I. The change from deposition of finely laminated, organic carbon‐rich sapropels (Unit II) to laminated, more calcareous, coccolith‐rich marls (Unit I) is thought to represent the crossing of a salinity threshold for Emiliania huxleyi. The varve chronology sets this change at about 1.63 ka (1633±100 yr B.P.), but the record of magnetic secular variation measured in several cores produces an age estimate of about 2.0 ka for the base of Unit I, or about 1.2 times the varve age. The average of six calibrated accelerator mass spectrometry radiocarbon ages for the base of Unit I is 2.7 ka, or about 1.7 times the varve age. Following the initial change to coccolith‐dominated sedimentation, deposition of sapropel resumed for at least one significant period, 1.56–1.25 ka. Since 1.25 ka, cycles of carbonate deposition with quasi‐decadal periodicities have produced characteristic darker and lighter assemblages of laminae. These cycles may have been climatically driven. Geochemical analyses coupled with the varve ages adopted herein indicate that accumulation rates of carbonate are nearly an order of magnitude higher in Unit I (averaging 35–45 g m−2 yr−1) than in sapropelic Unit II. which contains primarily detrital carbonate. The accumulation of lithogenic components in parts of Unit I is only 1.5 times the rate in Unit II. Deepwater organic carbon accumulation rates are somewhat higher in Unit I (3.5–4.5 g m−2 yr−1) than in the upper part of Unit II. Organic carbon accumulation rates in Unit I are somewhat antithetic to those of carbonate, and on the basis of this and additional constraints placed by pyrolysis and carbon isotopic analyses of organic material, it appears that terrestrial organic matter is an important component (perhaps &gt;25%) of total organic carbon burial in the basin. Unit I in the western part of the Black Sea has a higher terrestrial organic component and higher accumulation rates of terrigenous clastic material than Unit I in the eastern part. This difference between eastern and western Black Sea is to be expected because of the major rivers that empty into the western Black Sea from eastern Europe, Ukraine, and Russia. Shallow slope sites, but still within euxinic bottom waters, have lower organic carbon accumulation rates and lower pyrolysis hydrogen indices than deepwater basinal sites, suggesting selective resuspension and oxidation of organic matter at basin margins and focusing of organic matter deposition toward the basin center. A comparison of the Black Sea data with those from several open ocean sites with similar water depths showed no significant difference between organic carbon accumulation rates under oxic and anoxic conditions. For a given bulk accumulation rate the organic carbon accumulation rates, normalized to primary productivity, are about the same in both settings.

Pyrite formation in anoxic environments of the Baltic

Reactive iron enrichment in sediments deposited beneath euxinic bottom waters: constraints on supply by shelf recycling

Stable isotopic fractionation, climate change and episodic stagnation in the eastern Mediterranean during the late Quaternary

Fluctuations in the δ 18O composition of planktonic foraminifers in eastern Mediterranean piston cores indicate cycles with amplitudes much greater than those which can be attributed alone to global ice-volume changes. Isotopic values become markedly negative within lithostratigraphic levels characterized by the apparition of organicrich sapropels These owe their origin to the development of euxinic bottom water during episodes of basin-wide stagnation. The depletion of δ 18O in many of the sapropels is accompanied by the occurrence of poorly diversified planktonic faunas, and both phenomena are attributed to a strong dilution of the local eastern Mediterranean surface water mass by a combination of glacial run off from large continental ice sheets and by an important increase of regional precipitation synchronous with the transition from pleniglacial to kataglacial climate. Although sapropel accumulation occurs generally during intervals of warming of the Mediterranean region as determined from a quantitative evaluation of planktonic foraminiferal assemblages, the pre-125,000 years, local warming of eastern Mediterranean surface water lagged the acme of glacial melting by up to 15,000 years. Climatic and isotopic cycles are correlative within the eastern Mediterranean for lateral distances in excess of 1000 km and, except for amplitude and phase, they are in most aspects remarkably similar to those recorded in the equatorial Pacific and Caribbean.