Euxinic Deep Water Research Articles

Chromium Cycling in Redox-Stratified Basins Challenges δ53Cr Paleoredox Proxy Applications.

Chromium stable isotope composition (δ53Cr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δ53Cr have not been assessed in modern redox-stratified basins. We present new chromium (Cr) concentration and δ53Cr data in dissolved, sinking particulate, and sediment samples from the redox-stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non-quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox-stratified basins show analogous patterns. This challenges assumptions from δ53Cr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non-quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δ53Cr from oxidative continental weathering.

Element enrichment and provenance of the detrital component in Holocene sediments from the western Black Sea

Summary Concentrations of a large set of major and trace elements, and Sr, Nd and Pb isotope ratios were measured in Holocene sediments cored in the western deep Black Sea in order to unravel: (1) the controls of element enrichment, and (2) sources of the detrital component. The transition of the basin from oxic to euxinic resulted in enrichment or depletion in a number of elements in the deep-sea sediments. Authigenic Fe enrichment appears to depend on the amount of Fe mobilized from the sediment through the benthic redox shuttle mechanism and free H2S in the water column (degree of “euxinization”). Manganese enrichment is controlled by diagenetic reactions within the sediment: the dissolution of Mn minerals, Mn2+ diffusion upward and reprecipitation. Barium enrichment is also controlled by diagenetic reactions, sulfate reduction and methanogenesis, that take place above and below the sulfate-methane transition, respectively. The major part of V, Co, Ni, Cu, Zn, Cr, Mo, Cd and Sb is inferred to have co-precipitated with Fe in the euxinic deep waters and to have been incorporated into authigenic Fe-sulfides. Basin reservoir effect additionally influences the Mo enrichment. The U enrichment is interpreted to have a different origin in the two organic-rich stratigraphic units (II and I). It is inferred to be: (i) at the expense of the U inventory of the deepwater pool and a result of inorganic reduction of U at euxinic conditions in the lower Unit II; and (ii) at the expense of the U inventory of the surface water pool and a result of biogenic uptake and transfer to the sediment by the plankton in the upper Unit I. The high field strength elements are closely linked to the detrital component and their depletion in the organic-rich sediments reflects a dilution of the detrital component by the biogenic one. The enrichments of REE, Sn and Th are likely controlled by adsorption on clay minerals. Sr-Nd-Pb isotope compositions of the alumino-silicate component of the studied sediments are relatively uniform. They are most likely controlled by riverine suspended matter supplied mainly in the NW Black Sea (Danube Delta) and transported southward by marine currents, and to a lesser degree by suspended matter from the small rivers draining SE Bulgaria and NW Turkey. Wind-blown dust from the Sahara Desert appears to have a minor contribution to the alumino-silicate component of the sediments. The slight shift in the Pb isotopes in Unit I upper layers is possibly caused by the addition of anthropogenic Pb.

Shelf-to-basin iron shuttle in the Guaymas Basin, Gulf of California

Enrichments of highly reactive iron (Fe) (sum of Fe (oxyhydr)oxide, carbonate and sulfide minerals) in marine sediments and sedimentary rocks are commonly interpreted as an indication of anoxic conditions in the bottom water at the time of deposition. The model system for this proxy rationale is the semi-restricted Black Sea, where sediments underneath the anoxic and sulfidic (i.e., euxinic) deep-water are enriched in reactive Fe, which was mobilized from the surrounding shelf areas. To test whether such a shelf-to-basin Fe shuttle can operate in semi-restricted basins without euxinic deep water, we investigated sedimentary Fe speciation and Fe isotope compositions in sediments of the Guaymas Basin, Gulf of California. Sediments on the slope underneath the eastern equatorial Pacific oxygen minimum zone and sediments within the oxic deep basin are both enriched in reactive Fe, with reactive Fe making up 45 ± 11% of the total Fe pool. The following mechanisms may contribute to these Fe enrichments: (1) Release of dissolved Fe from anoxic shelf and slope sediments followed by lateral transport of dissolved and/or particulate Fe in the water column; (2) preferential transport of fine-grained, terrigenous particles with a high reactive Fe content into the basin; (3) microbially mediated conversion of non-reactive silicate minerals to reactive Fe minerals during transport; (4) hydrothermal venting and lateral Fe transport within the deep water. The first process can explain reactive Fe enrichments in slope sediments, whereas all processes may contribute to sedimentary Fe enrichments in the deeper basin. The δ 56 Fe value of sediments increases from shelf to slope and decreases from the slope into the basin. This lateral pattern of δ 56 Fe, as well as the pattern of Fe enrichment, is similar to that observed in other marine systems with a Fe shuttle. However, the size of the Fe enrichment, and the range in δ 56 Fe (−0.06 to +0.16‰) is smaller. This difference is due to higher terrigenous sedimentation rates in the Guaymas Basin and, therefore, more intense dilution of shuttle-derived reactive Fe. We argue that, depending on the extent of bathymetric restriction and terrigenous background sedimentation, reactive Fe enrichments can form under a broad range of redox conditions and in diverse sedimentary environments. The concepts applied in this study can be used to identify those circumstances in the paleo-record.

Redox condition and organic carbon accumulation mechanism in the Cryogenian Nanhua Basin, South China: Insights from iron chemistry and sulfur, carbon, oxygen isotopes of the Datangpo Formation

Global glaciation, oxidation event and eukaryotic expansion and diversification in the Neoproterozoic period are marked events that characterize the early evolution of the Earth, but how the interactions occurred among these events is not well understood. The organic matters preserved in the black shales of the Datangpo Formation (Cryogenian period) are sensitive to redox conditions, and thus its accumulation and preservation offer beneficial clues to unravel the early evolutional history of the Earth. This study presents new chemostratigraphic data of iron component, TOC content, sulfur isotope of pyrite, carbon and oxygen isotopes of carbonaceous shale of the Datangpo Formation (Cryogenian period) in the Datangpo section, South China. The analyzed results imply abundant nutrients existing in the ocean in the Early Cryogenian. The nutrients, such as phosphorus, resulting from neighbor volcanic eruptions, provided nutrients that enabled microbes to flourish during the Cryogenian interglacial gap. Iron components and sulfur isotopes indicated anoxic, euxinic deep water environments for the black shales in the lower portion of the Datangpo Formation. The anoxic setting was good for the preservation of organic matter, but terrigenous materials inputs, as revealed by the high Al2O3 contents, diluted the organic carbon content (TOC). Cited as : Wang, C., Shi, G. Redox condition and organic carbon accumulation mechanism in the Cryogenian Nanhua Basin, South China: Insights from iron chemistry and sulfur, carbon, oxygen isotopes of the Datangpo Formation. Advances in Geo-Energy Research, 2019, 3(1): 67-75, doi: 10.26804/ager.2019.01.05

Mineralogy and geochemistry of silicate, sulfide, and oxide iron formations in Norway: evidence for fluctuating redox states of early Paleozoic marine basins

Laterally extensive silicate and sulfide iron formation associated with jasper (hematitic chert) beds and volcanogenic massive sulfide (VMS) deposits in Norway provide evidence of early mineral assemblages and redox conditions within coeval early Paleozoic seawater. Calculated detrital-free compositions record mixed hydrothermal (e.g., Fe, Cu) and seawater ± biogenic (e.g., Si, Ni, S, REE, P) components. Rare earth element (REE) patterns are characterized by small to large negative Ce anomalies and insignificant to locally large positive Eu anomalies, reflecting seawater REE carried to the seafloor by Fe–P-rich particles later modified by diagenetic processes. Protoliths of silicate iron formation precipitated in anoxic and intermittently euxinic deep waters by the diagenetic modification of amorphous Si–Fe oxyhydroxides and/or Si–Fe–OOH gels, based on possible modern analogues in the Red Sea. Diagenetic minerals include nontronite, greenalite, stilpnomelane, magnetite, manganosiderite, apatite, and iron sulfides. In sulfide iron formation, a local predominance of pyrrhotite over pyrite records highly reducing conditions caused by organic material. The geochemical data provide evidence for Mn–Fe–P shuttle and redox processes in a stratified basin with oxic or suboxic shallow waters and silica concentrations much higher than those of modern seawater. Hydrothermal plume-derived Fe present within the anoxic layer and near the chemocline formed mixed-valence oxyhydroxides and silicates and, intermittently, sulfides by reaction with aqueous Si and H2S, respectively, the latter derived from bacterial reduction of seawater sulfate at the chemocline. Major sustained fluxes of hydrothermally derived reductants (Fe2+, Mn2+, H2S, H2) produced from large seafloor systems such as Lokken may have changed the redox state of seawater in local, and possibly regional, basins from weakly or moderately oxic to intermittently anoxic or euxinic conditions.

Integrated carbon, sulfur, and nitrogen isotope chemostratigraphy of the Ediacaran Lantian Formation in South China: Spatial gradient, ocean redox oscillation, and fossil distribution.

The Ediacaran Doushantuo Formation in South China is a prime target for geobiological investigation because it offers opportunities to integrate chemostratigraphic and paleobiological data. Previous studies were mostly focused on successions in shallow-water shelf facies, but data from deep-water successions are needed to fully understand basinal redox structures. Here, we report δ13 Ccarb , δ13 Corg , δ34 Spyr , δ34 SCAS , and δ15 Nsed data from a drill core of the fossiliferous Lantian Formation, which is a deep-water equivalent of the Doushantuo Formation. Our data confirm a large (>10‰) spatial gradient in δ13 Ccarb in the lower Doushantuo/Lantian formations, but this gradient is probably due to the greater sensitivity of carbonate-poor deep-water sediments to isotopic mixing with 13 C-depleted carbonate cements. A pronounced negative δ13 Ccarb excursion (EN3) in the upper Doushantuo/Lantian formations, however, is spatially consistent and may be an equivalent of the Shuram excursion. δ34 Spyr is more negative in deeper-water facies than in shallow-water facies, particularly in the lower Doushantuo/Lantian formations, and this spatial pattern is interpreted as evidence for ocean redox stratification: Pyrite precipitated in euxinic deep waters has lower δ34 Spyr than that formed within shallow-water sediments. The Lantian Formation was probably deposited in oscillating oxic and euxinic conditions. Euxinic black shales have higher TOC and TN contents, but lower δ34 Spyr and δ15 Nsed values. In euxinic environments, pyrite was predominantly formed in the water column and organic nitrogen was predominantly derived from nitrogen fixation or NH4+ assimilation because of quantitative denitrification, resulting in lower δ34 Spyr and δ15 Nsed values. Benthic macroalgae and putative animals occur exclusively in euxinic black shales. If preserved in situ, these organisms must have lived in brief oxic episodes punctuating largely euxinic intervals, only to be decimated and preserved when the local environment switched back to euxinia again. Thus, taphonomy and ecology were the primary factors controlling the stratigraphic distribution of macrofossils in the Lantian Formation.

Spatial and temporal trends in Precambrian nitrogen cycling: A Mesoproterozoic offshore nitrate minimum

Fixed nitrogen is an essential nutrient for eukaryotes. As N2 fixation and assimilation of nitrate are catalyzed by metalloenzymes, it has been hypothesized that in Mesoproterozoic oceans nitrate was limited in offshore environments by low trace metal concentrations and high rates of denitrification in anoxic and episodically euxinic deep water masses, restricting eukaryotes to near-shore environments and limiting their evolutionary innovation. To date this hypothesis has only been tested in the Belt Supergroup (∼1.4Ga), with results that support an onshore-offshore nitrate gradient as a potential control on eukaryote ecology. Here we present bulk nitrogen and organic carbon isotopic data from non-isochronous cross-basinal facies across the Bangemall (∼1.5Ga) and the Roper (∼1.4–1.5Ga) basins to better understand the extent and variability of onshore-offshore nitrogen isotope gradients in the Mesoproterozoic. Both basins show an average ∼1–2‰ enrichment in δ15Nbulk from deep to shallow facies, with a maximum range from −1‰ offshore to +7.5‰ onshore. Unlike the Belt basin, the Bangemall and Roper basins show some offshore δ15Nbulk values that are enriched beyond the isotopic range associated with biological N2 fixation alone. This suggests a mixture of aerobic and anaerobic metabolisms offshore. In shallow waters, where δ15Nbulk enrichment peaks, an aerobic nitrogen cycle was evidently operating. Even though isotopic signatures of aerobic nitrogen cycling are seen in all parts of the Bangemall and Roper basins, our data are consistent with a lateral gradient in nitrate availability within the photic zone, with higher concentrations in near-shore environments than offshore. The variability in δ15Nbulk values in each depositional environment and the consistently low δ15Nbulk values from Mesoproterozoic units compared to the Paleoproterozoic and Neoproterozoic suggest that nitrate concentrations in the global ocean were likely low. This trend is now seen in all three Mesoproterozoic basins so far examined, and contrasts with the Paleoproterozoic and Neoproterozoic where nearly all δ15Nbulk data plot above the N2 fixation window. Thus, we propose that the Mesoproterozoic ocean was characterized by a nitrate minimum, with the lowest concentrations in offshore environments. This inference is consistent with a Mesoproterozoic O2 decline following a temporary Paleoproterozoic O2 peak, and it further supports the idea that nitrate limitation offshore may have contributed to the restriction of photosynthetic eukaryotes to near-shore environments, delaying their rise to ecological dominance until the Neoproterozoic Era.

Novel archaeal tetraether lipids with a cyclohexyl ring identified in Fayetteville Green Lake, NY, and other sulfidic lacustrine settings.

The meromictic Fayetteville Green Lake (FGL) is of significant geobiological interest because of microbial cycling of sulfur within and below the permanent chemocline and in the euxinic deep waters. Studies of glycerol dibiphytanyl glycerol tetraethers (GDGTs) may help shed light on understanding the activity of archaeal communities in these habitats. Normal-phase and reversed-phase liquid chromatography/mass spectrometry (LC/MS) analysis on total lipid extracts of environmental samples revealed series of GDGTs with different biphytane structures. Comparison of the mass spectrum of biphytane obtained from separated novel GDGTs with that of a synthetic C40 biphytane confirms our structural assignments. A unique cyclohexyl ring configured in the middle of a C40 biphytane chain was identified in these novel GDGTs. We suggest the trivial name S-GDGTs for these compounds, where 'S' stands for 'sulfidic' and 'six-membered ring'. S-GDGT derivatives composed of biphytanes modified with double bonds and cyclopentane rings were also detected in the samples we analyzed. Intact polar lipid precursors of S-GDGT include compounds with mono- and diglycosyl head groups. The carbon isotopic composition of S-GDGTs and their occurrence in FGL, Messel Shale as well as Salt Pond and salt marshes on Cape Cod suggest that S-GDGTs may be produced by chemoautotrophic archaea that prefer sulfidic conditions. Copyright © 2016 John Wiley & Sons, Ltd.

Re–Os depositional age for black shales from the Kaimur Group, Upper Vindhyan, India

Proterozoic sedimentary deposits from the Vindhyan Supergroup, central India are excellent archives of paleo-environmental conditions. Meaningful interpretation of these geological signatures and their global correlation requires depositional age information for the sediments, data on which are limited for the Upper Vindhyan sediments. In this work, Re–Os isotopic study of the Bijaigarh shale, Kaimur Group from the Upper Vindhyan has been carried out to infer their depositional age and paleo-hydrological conditions. The Re–Os isochron for the Bijaigarh shale, a lagoonal deposit with an open ocean connection, provides a depositional age of 1210±52Ma (Model 3; MSWD=69; n=15; 2σ). This age for the Kaimur Group is younger by ~400Ma than the underlying Semri Group (age ~1600Ma), confirming existence of a long sedimentary hiatus between the two groups. The isochron-derived initial 187Os/188Os (Osi) ratio for these shales (~0.6) is higher than those reported for Mesoproterozoic marine shales (~0.3) from other global margins. Osi ratios of shales could be regulated by regional paleo-hydrological conditions; osmium mass balance calculations for the restricted basin of the Bijaigarh shale show that a minor increase in freshwater influx (as low as ~2.4% i.e. salinity change by ~0.8psu) to the euxinic deep water of the basin is sufficient to elevate Osi ratio from 0.3 (marine value) to ~0.6 (Osi ratio of the lagoonal shale). In contrast to lagoonal shales, the Osi ratios of marine shales seem to mimic the seawater 187Os/188Os during their deposition period. Compilation of marine Osi ratios shows coarse synchrony between temporal changes in Osi during the Proterozoic eon and the atmospheric oxygen level, pointing to the dominant role of oxidative continental weathering on paleo seawater 187Os/188Os ratio.

Heterogeneous oxygenation states in the Atlantic and Tethys oceans during Oceanic Anoxic Event 2

The Cenomanian–Turonian boundary (ca. 93.5 Ma) is marked by an episode of profound environmental change, including a major perturbation of the carbon cycle and an Oceanic Anoxic Event (OAE-2). Here, we present molybdenum (Mo) isotope variations within the OAE-2 interval for four sections from the western Tethys (Furlo and La Contessa) and the North-Atlantic (ODP site 1276 and DSDP site 367). The main target of this study is to investigate the extent of reducing conditions (truly global in extent or restricted to poorly-ventilated restricted deep basins), with particular reference to the relationship between the change in the oxygenation state of the ocean and the link to global perturbations of the carbon cycle recorded in carbon isotopes.All four sections show fluctuations in the redox sensitive trace metal (RSTE) distribution, suggesting rapid variations in local redox conditions, ranging from anoxic to euxinic. The RSTE enrichment factors (EFs) also suggest different depositional conditions and paleoceanographic processes in the western Tethys versus the North Atlantic. Whereas the North Atlantic sites show evidence of weak watermass restriction associated with the action of a particulate shuttle within the water column, the EFs of the Tethyan sections are characteristic of unrestricted marine systems. Mo isotopes show surprisingly negative values through the Tethyan sections. At the onset of OAE-2, an increasing trend in δMo98 is observed, with values ranging from −0.6 to 0.6‰. During the second half of OAE-2, the δMo98 curve shows a progressive shift towards more negative values. In the North Atlantic, δMo98 signatures from ODP site 1276 show a similar behaviour as observed in the western Tethys. At DSDP site 367, Mo isotopes are generally heavier during OAE-2, fluctuating around an average value of 1.1‰. This is consistent with fully euxinic conditions and the black shales deposited may have recorded the seawater signature during OAE-2. The Mo isotope signature is lighter than the modern ocean value (2.3‰) and suggests a higher proportion of bottom water euxinia (∼10%). Although this estimate is greater than the euxinic fraction in the modern ocean, our results challenge the suggestion of a globally euxinic deep waters during OAE-2. The variations in Mo/TOC ratios within the North Atlantic and Tethys oceans are in good agreement with a general drawdown of Mo within the OAE interval, suggesting a general increase in anoxic conditions during OAE-2. Our results highlight the role of local parameters in the development of euxinia, even during period of widespread anoxia such as OAEs.

The manganese and iron shuttle in a modern euxinic basin and implications for molybdenum cycling at euxinic ocean margins

A meaningful application of Mo as a paleo-redox proxy requires an understanding of Mo cycling in modern reducing environments. Stagnant euxinic basins such as the Black Sea are generally regarded as model systems for understanding euxinic systems during early Earth history. However, drawing direct parallels between the Black Sea and open-marine euxinic margins is somewhat complicated by differences in the seawater residence time between these two environments. We report sediment and pore water Mo, U, Mn and Fe data for a euxinic basin with a short seawater residence time; the weakly restricted Gotland Deep in the Baltic Sea. Here, prolonged periods of euxinia alternate with brief inflow events during which well-oxygenated, saline water penetrates into the basin. During these inflow events, dissolved Mn and Fe that has accumulated within the euxinic deep water can be oxidized and precipitated. Co-variations of Mo and U within the sediment suggest that these inflow and oxygenation events may favor Mo accumulation in the sediment through adsorption to freshly oxidized Mn and Fe solid phases. Once Mo is sequestered within the deeper euxinic water and sediments, Mo retention can be further facilitated by conversion to thiomolybdate species and interactions with organic matter and metal sulfides. By comparing our data with those from previous studies where a Mn and Fe “shuttle” for Mo has been demonstrated, we identify two prerequisites for the occurrence of this mechanism. First, there must be a water column oxic–anoxic redox-boundary; this provides a solubility contrast for Mn and Fe. Second, the residence time of seawater in the system has to be short (weeks to a few years). The latter criterion can be met through regular inflow in weakly restricted basins or upwelling in oxygen minimum zones at open-marine continental margins. Based on prior work, we suggest that similar conditions to those currently represented by the Gotland Deep may have prevailed at euxinic ocean margins during the Proterozoic. A boundary between euxinic and oxic water masses overlying the continental shelf may have resulted in accelerated Mo transport through the water column with Mn and Fe (oxyhydr)oxides. We propose that this mechanism, along with Mo isotope fractionation during adsorption, could contribute to the light Mo isotope composition observed in open-marine euxinic sediment facies of the Proterozoic.

Sulfur cycling in the aftermath of a 635-Ma snowball glaciation: Evidence for a syn-glacial sulfidic deep ocean

We have analyzed δ34S (sulfate and pyrite), δ18Ocarbonate and δ13Ccarbonate, and major and trace elemental concentrations, including extractable Fe and Mn phases, in four sections of the Maieberg Formation, the cap-carbonate sequence to the Marinoan glaciation in northern Namibia. δ34Ssulfate profiles and other geochemical characteristics in the basal, transgressive cap dolostone (Keilberg Member) are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations. In the overlying interval that consists of rhythmites deposited during the transgressive high-stand, large geochemical disparities occur between sections that we interpret to have been deposited in open-ocean versus restricted settings. In the former, a large negative shift in δ34Ssulfate of ∼20‰ above the cap dolostone accompanies a change in mineralogy from dolomite to limestone, a sharp decrease in δ18O and δ13C, and a positive spike in Fe and Mn concentrations. In the latter, dolomite persists above the cap dolostone, δ34Ssulfate increases abruptly, δ18O and δ13C are invariant, and a spike in Mn and Fe concentrations is present, but subtle. These contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as cold, euxinic deep water and a cap of warm, oxic, brackish water that flooded the continental shelf during the post-glacial transgression. The geochemical differences between open-ocean and restricted sections diminishes upsection, presumably recording the gradual mixing and homogenization of the waters along the Otavi platform.

Long-Term vs Short-term Variability of Stable Isotopes in Brachiopods of the Silurian of Gotland (Sweden): Palaeoenvironmental Implications

The Silurian of Gotland consists of 440 m carbonate deposits, spanning the late Llandovery to late Ludlow epochs (431 to 411 m.y.). Repeatedly, uniform sequences of micritic limestones and marls are interrupted by complex-structured reefs and adjacent platform sediments. Previously, the alternation of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. Ratios of stable carbon and oxygen isotopes in brachiopod shells (more than 370 specimens, esp. Atrypa reticularis) from the Silurian of Gotland, Sweden, have been analysed. Preservation of biological skeletal ultrastructures, observed in SEM-micrographs, and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs. The Silurian sequence of Gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms.The isotope ratios are influenced by local as well as global factors. The oxygen isotope ratios are interpreted to reflect palaeosalinity changes due to varying freshwater input, rather than changes in palaeotemperature. Consequently, the facies distribution of Gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and fresh water runoff rather than by sea level fluctuations. Periods of arid climate and therefore anti-estuarine downwelling of oxygenated surface water appear as short episodes of reef growing (~< 1.5 m.y.) in an epoche characterized by a tropic humid climate, which causes an estuarine circulation and the upwelling of CO2-rich deep water. Carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13Crich surface water (arid episodes) or upwelling of 13C-depleted deep water (humid episodes) of a Silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales. Although the development of 813C and 81SO values have been shown to be parallel in the secular changes, the underlying long-term trends show antiparallel courses. This fact indicates that the factors controlling the long-term trends in isotopes are different from those assumed to control the secular changes. The long-term trends in the Silurian of Gotland interrupt the general increase of both carbon and oxygen isotope values during the Palaeozoic.

Paleoenvironmental changes in the Silurian indicated by stable isotopes in brachiopod shells from Gotland, Sweden

Ratios of stable carbon and oxygen isotopes in brachiopod shells (more than 370 specimens, esp. Atrypa reticularis) from the Silurian of Gotland, Sweden, have been analysed. Preservation of biological skeletal ultrastructures, observed in SEM-micrographs, and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs. The Silurian of Gotland consists of 440 m carbonate deposits, spanning the late Llandovery to late Ludlow epochs (431-411 m.y.). Repeatedly, uniform sequences of micritic limestones and marks are interrupted by complex-structured reefs and adjacent platform sediments. Previously, the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. The Silurian sequence of Gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms. The isotope ratios are influenced by local as well as global factors. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than changes in paleotemperature. Consequently, the facies distribution of Gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations. Periods of arid climate and, therefore, anti-estuarine downwelling of oxygenated surface water appear as short episodes of reef growing (≤1.5 m.y.) in an epoche characterized by a tropic humid climate, which causes an estuarine circulation and the upwelling of CO 2-rich deep water. Carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13C-rich surface water (arid episodes) or upwelling of 13C-depleted deep water (humid episodes) of a Silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales.

The Silurian of Gotland (Sweden): facies interpretation based on stable isotopes in brachiopod shells

The Silurian of Gotland, Sweden, consists of 440 m of carbonate deposits. Repeatedly, uniform sequences of micritic limestones and marls are interrupted by complex-structured reefs and by adjacent platform sediments. Generally, the alteration of facies is interpreted as the result of sea-level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. The purpose of this study is a facies interpretation based on both field observations and stable isotope measurements of brachiopod shells. Approximately 700 samples from stratigraphically arranged localities in different facies areas have been investigated. The carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic sequence. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than to paleotemperature. Carbon isotope ratios are believed to have been connected to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions. Consequently, the facies succession on Gotland results from global paleoclimatic conditions. Changes in terrigenous input due to different rates of weathering and freshwater runoff, rather than sea-level fluctuations, control the carbonate formation of the Silurian on Gotland.