Ulrich G. Wortmann

Hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single-step microbial sulfate reduction

Coexisting dissolved sulfide and sulfate from hypersulfidic interstitial waters of a 380- m-long sediment core show a large isotopic difference of up to 72‰ caused by in situ microbial sulfate reduction. This is considerably larger than the assumed biological maximum of 46‰ derived from laboratory studies with pure cultures of sulfate-reducing bacteria. Similar high fractionations inferred from sedimentary metal sulfides have been previously explained by a multistage process, involving sulfide reoxidation and disproportionation of sulfur intermediates. Our data show that extreme isotopic differences between sulfate and the reduced sulfur species can also be generated during microbial single-step fractionation. This result indicates that the sulfate-reducing communities and/or their cellular metabolic activities in the deep biosphere may differ from those observed in near-surface sediments or the water column.

Alpine plate kinematics revisited: The Adria Problem

Tectonic evolution of the Alpine Tethys is controlled by the plate movements of Africa, Europe, and the Adriatic microplate. It is, however, unclear to which extent and at what times the motion of Adria was related to Africa. Kinematic models which assume a rigid connection between Africa and Adria have difficulties in explaining the Alpine rock record. Reconstructions based on the Alpine record are, on the other hand, in conflict with the involved kinematics. To resolve these conflicts, they require complicated motions or the introduction of additional microplates. Here we present a solution which is based on a rigid connection between Africa/Adria during Jurassic/Cretaceous times. Our model requires only four plates, involving Africa, Europe, Iberia, and Adria. It describes a self-consistent kinematic evolution of the western Tethyan microplate motions from Jurassic to Miocene times. The initial (Early Jurassic) plate configuration was found by iterative forward modeling. The resulting Jurassic plate configuration is unusual and provides new insights into Alpine geology. The obtained model is, however, in good agreement with the available geological data and suggests that the assumption of independent movements of Adria during Jurassic/Cretaceous times is not a necessity.

Effect of evaporite deposition on Early Cretaceous carbon and sulphur cycling

The global carbon and sulphur cycles are central to our understanding of the Earth’s history, because changes in the partitioning between the reduced and oxidized reservoirs of these elements are the primary control on atmospheric oxygen concentrations. In modern marine sediments, the burial rates of reduced carbon and sulphur are positively coupled, but high-resolution isotope records indicate that these rates were inversely related during the Early Cretaceous period. This inverse relationship is difficult to reconcile with our understanding of the processes that control organic matter remineralization and pyrite burial. Here we show that the inverse correlation can be explained by the deposition of evaporites during the opening of the South Atlantic Ocean basin. Evaporite deposition can alter the chemical composition of sea water, which can in turn affect the ability of sulphate-reducing bacteria to remineralize organic matter and mediate pyrite burial. We use a reaction–transport model to quantify these effects, and the resulting changes in the burial rates of carbon and sulphur, during the Early Cretaceous period. Our results indicate that deposition of the South Atlantic evaporites removed enough sulphate from the ocean temporarily to reduce biologically mediated pyrite burial and organic matter remineralization by up to fifty per cent, thus explaining the inverse relationship between the burial rates of reduced carbon and sulphur during this interval. Furthermore, our findings suggest that the effect of changing seawater sulphate concentrations on the marine subsurface biosphere may be the key to understanding other large-scale perturbations of the global carbon and sulphur cycles.

Altered carbon cycling and coupled changes in Early Cretaceous weathering patterns: Evidence from integrated carbon isotope and sandstone records of the western Tethys

Abstract In this study we investigate if a major perturbation of the Early Cretaceous carbon cycle was accompanied by altered weathering and erosion rates. The large Aptian carbon isotope anomaly records the response of the biosphere to widespread volcanic activity and probably resulting changes in atmospheric pCO 2 levels. Elevated pCO 2 levels should also result in an accelerated hydrological cycle and increased silicate weathering, creating a negative feedback loop removing CO 2 from the atmosphere. We propose to interpret the widespread occurrence of quartz sandstones in the Tethys–Atlantic seaway as a result of altered weathering and erosion rates in the wake of the Aptian carbon cycle excursion. We challenge the traditional notion that these are ‘flysch’ deposits associated with Early Cretaceous orogenic movements in the western Tethys. We propose that these sandstones were most likely part of a large conveyor belt system, acting along the Iberian and European margin of the Tethys seaway. Using chemostratigraphic correlations, we show that the activity of this system was only short-lived and coeval with changes in coastal ecology and the Aptian carbon cycle perturbations. We tentatively relate the existence of this system to a transient climate regime, characterized by fluctuating pCO 2 levels.

Rapid Variability of Seawater Chemistry Over the Past 130 Million Years

More than a Dash of Sea Salt The cycling of major elements, such as sulfur, in the oceans depends on a number of processes, from bacterial respiration of organic matter to venting of gases from hydrothermal vents on the seafloor. Over geologic time, sediment deposited on the seafloor preserves chemical records of major changes in sulfur cycling and seawater chemistry (see the Perspective by Hurtgen). Halevy et al. (p. 331) observed swings in sulfur isotopes in a stratigraphic database covering North America and the Caribbean that, when modeled, corresponded to variable evaporite preservation and high turnover of sedimentary pyrite. Wortmann and Paytan (p. 334) modeled the two most recent major swings in sedimentary sulfur isotopes over the last 130 million years and suggest that short periods of rapid fluxes in sulfur cycling were at least in part caused by the growth and dissolution of evaporite deposits. Modeling the evolution of the marine sulfur cycle suggests that short intervals of rapid change punctuated long periods of stasis. Fluid inclusion data suggest that the composition of major elements in seawater changes slowly over geological time scales. This view contrasts with high-resolution isotope data that imply more rapid fluctuations of seawater chemistry. We used a non–steady-state box model of the global sulfur cycle to show that the global δ34S record can be explained by variable marine sulfate concentrations triggered by basin-scale evaporite precipitation and dissolution. The record is characterized by long phases of stasis, punctuated by short intervals of rapid change. Sulfate concentrations affect several important biological processes, including carbonate mineralogy, microbially mediated organic matter remineralization, sedimentary phosphorous regeneration, nitrogen fixation, and sulfate aerosol formation. These changes are likely to affect ocean productivity, the global carbon cycle, and climate.

Hydrogen sulfide–hydrates and saline fluids in the continental margin of South Australia

During the drilling of the southern Australian continental margin (Leg 182 of the Ocean Drilling Program), fluids with unusually high salinities (to 106‰) were encountered in Miocene to Pleistocene sediments. At three sites (1127, 1129, and 1131), high contents of H2S (to 15%), CH4 (50%), and CO2 (70%) were also encountered. These levels of H2S are the highest yet reported during the history of either the Deep Sea Drilling Project or the Ocean Drilling Program. The high concentrations of H2S and CH4 are associated with anomalous Na+/Cl− ratios in the pore waters. Although hydrates were not recovered, and despite the shallow water depth of these sites (200–400 m) and relative warm bottom water temperatures (11–14 °C), we believe that these sites possess disseminated H2S-dominated hydrates. This contention is supported by calculations using the measured gas concentrations and temperatures of the cores, and depths of recovery. High concentrations of H2S necessary for the formation of hydrates under these conditions were provided by the abundant SO42− caused by the high salinities of the pore fluids, and the high concentrations of organic material. One hypothesis for the origin of these fluids is that they were formed on the adjacent continental shelf during previous lowstands of sea level and were forced into the sediments under the influence of hydrostatic head.

Major‐element analysis of cyclic black shales: Paleoceanographic implications for the Early Cretaceous deep western tethys

Lower Cretaceous sediments are frequently characterized by a well expressed cyclicity. While the processes influencing environments above the carbonate compensation depth (CCD) are reasonably well understood, almost nothing is known about the deep ocean. Cretaceous sub-CCD sediments from the Tethys and Atlantic Oceans typically show rhythmic black/green shale successions. To gain insight into the nature of these black/green shale cycles, we performed detailed geochemical analyses (X-ray fluorescence, Rock-Eval and reactive iron analysis) on a 3 m long section of latest Aptian age. The major-element distribution of the analyzed shale sequence indicates a periodic change from a high-productivity and well-oxygenated green shale mode to a low-productivity oxygen-deficient black shale mode. It is proposed here that the preservation of organic matter was dependent on the strength of salinity-driven deepwater generation. Furthermore, the data show that the Corg content covaries with changes in the detrital composition. Therefore we hypothesize that Tethyan deepwater circulation was sensitive to changes in the monsoonal system. Time series analysis suggests that these changes are periodic in nature, although we are currently unable to prove that the dominant periodicity is related to the precession component of the Milankovitch frequencies.

Microbial sulfate reduction in deep sediments of the Southwest Pacific (ODP Leg 181, Sites 1119-1125): evidence from stable sulfur isotope fractionation and pore water modeling

Abstract Interstitial water samples from seven ODP sites (Leg 181, Sites 1119–1125) of the southwestern Pacific Ocean have been analyzed for the stable sulfur isotopic composition of dissolved sulfate along with major and minor ions. Sulfate from the interstitial fluids (δ 34 S values between +20.7 and +60‰ vs. the SO 2 -based Vienna–Canyon Diablo troilite standard) was enriched in 34 S with respect to modern sea water (δ 34 S≈+20.6‰) indicating that microbial sulfate reduction takes place to different extents at all investigated sites. Microbial sulfate reduction (MSR) was found at all sites, the intensity depending on the availability of organic matter which is controlled by paleo-sedimentation conditions (sedimentation rate, presence of turbidites) and productivity. Microbial net sulfate reduction was additionally confirmed by modeling interstitial water sulfate profiles. Areal net sulfate reduction rates up to 14 mmol m −2 yr −1 have been calculated which were positively related to sedimentation rates. Total reduced inorganic sulfur (TRIS; essentially pyrite) as a product of microbial sulfate reduction was isotopically characterized in squeeze cake samples and gave δ 34 S values between −51 and +9‰ indicating pyrite formation both close to the sediment–water interface and later diagenetic contributions.

Co‐generation of hydrogen sulfide and methane in marine carbonate sediments

Sulfate reduction and methanogenesis are considered to be mutually exclusive microbial reactions in marine sediments. Typically, methane does not appear in significant concentrations in sediment pore waters until almost all dissolved sulfate has been reduced to sulfide. An exception to this commonly accepted pattern occurs in an approximately 500-meter thick sequence of Quaternary carbonates on the continental margin of the Great Australian Bight. An unusual combination of geochemical and sedimentological conditions leads to extensive simultaneous sulfate reduction and methane production throughout the 500-m interval. A probable explanation for the co-production of these reduced gases in this deeper biosphere is the presence of noncompetitive substrates for the two types of microbiota.

Early Aptian carbon and sulphur isotope signatures at ODP Site 765

Current carbon and sulphur isotope ratios (δ13C and δ34S) suggest there were major shifts in partitioning between reduced and oxidised reservoirs of carbon and sulphur during the Early Cretaceous. However, the δ13C and δ34S records are composed from different Ocean Drilling Program sites and are hard to correlate at high resolution. We present high-resolution Aptian δ13Corg and δ34Sbarite values derived from the same set of samples, enabling a higher certainty correlation than previously possible. Two major hypotheses aim to explain the Early Aptian S-isotope excursion: increased volcanic degassing and/or fluctuations in the marine sulphate concentration. Our S-isotope data provide tight constraints on the timing and magnitude of volcanic flux required. We show that the observed S-isotope signature can be explained by a 2 Ma pulse of increased volcanic flux, injecting ∼4.5×1018 mol C into the atmosphere. Further work is needed to evaluate whether these fluxes are compatible with the existing C-isotope record.