Bottom-trawling signals lost in sediment: A combined biogeochemical and modeling approach to early diagenesis in a perturbed coastal area of the southern Baltic Sea

Trawl-fishing is broadly considered to be one of the most destructive anthropogenic activities toward benthic ecosystems. In this study, we examine the effects of bottom-contact fishing by otter trawls on the geochemistry and macrofauna in sandy silt sediment in an area of the Baltic Sea where clear spatial patterns in trawling activity were previously identified by acoustic mapping. We calibrated an early diagenetic model to biogeochemical data from various coring locations. Fitting measured mercury profiles allowed for the determination of the sediment mixing and burial velocity. For all sites, independent of the trawl mark density, good fits were obtained by applying the model with the same organic matter loading and parameter values, while iron fluxes scaled linearly with the burial velocity. A sensitivity analysis revealed that the fitted sulfate reduction rate, solid sulfur contents, ammonium concentration, and both the isotopic composition and concentration of dissolved inorganic carbon provided reliable constraints for the total mineralization rate, which exhibited a narrow range of variability (around ±20 % from the mean) across the sites. Also, the trawling intensity did not significantly correlate with total organic carbon contents in surficial sediment, indicating limited loss of organic matter due to trawling. The fits to the reactive iron, acid volatile sulfur, chromium(II) reducible sulfur contents, and porewater composition demonstrate that sediment burial and mixing primarily determine the redox stratification. The mixing depth did not correlate with trawling intensity and is more likely the result of bioturbation, as the analyzed macrofaunal taxonomy and density showed a high potential for sediment reworking. The extraordinarily long-lived Arctica islandica bivalve dominated the infaunal biomass, despite the expectation that trawling leads to the succession from longer-lived to shorter-lived and bigger to smaller macrofauna. Our results further suggest that a clear geochemical footprint of bottom-trawling may not develop in sediments actively reworked by tenacious macrofauna.

Clumped isotope analysis of (bio)apatite using a Kiel IV device in long-integration dual-inlet mode

RATIONALE

Clumped isotope (Δ47 ) analysis of bioapatite-derived CO2 is a powerful tool to determine body temperatures of extinct vertebrates. The common acid bath technique in combination with dual-inlet-based mass spectrometric measurements has been the preferred method of choice for this purpose, but the large amount of material necessary and the presence of secondary calcite represent obstacles.

METHODS

We analyzed the Δ47 composition of carbonate-bearing (bio)apatites using a Kiel IV device, which - in general - allows a reduction of sample replicate size by a factor of ~40 over dual-inlet-based techniques. The Kiel IV device was tested in two different modes: without and with additional water sinks for improved water removal. Furthermore, we tested a pretreatment technique based on 1 M acetic acid (pH = 5) to selectively remove secondary calcite from the carbonate-bearing (bio)apatite phase.

RESULTS

Significantly lower Δ47 values were obtained for a given bioapatite after the installation of the two water sinks. With this setup, Δ47 of (bio)apatites followed a temperature relationship that is indistinguishable from the unified one for pure carbonates, provided a dentine sample, rich in organic matter, was excluded. The original bioapatite Δ47 value was restored from a bioapatite/calcite mixture if the mixed material was treated for 1 h with 1 M acetic acid (pH = 5).

CONCLUSIONS

(Bio)apatites having low organic matter content such as enamel(oid) can be analyzed accurately for Δ47 using a Kiel IV equipped with water sinks that ensure effective removal of water. Secondary calcite can be effectively removed from carbonate-bearing apatite by pretreatment with 1 M acetic acid (pH = 5) for 1 h.

Are Large Sulfur Isotope Variations Biosignatures in an Ancient, Impact-Induced Hydrothermal Mars Analog?

Discrepancies have emerged concerning the application of sulfur stable isotope ratios as a biosignature in impact crater paleolakes. The first in situ δ34S data from Mars at Gale crater display a ∼75‰ range that has been attributed to an abiotic mechanism. Yet biogeochemical studies of ancient environments on Earth generally interpret δ34S fractionations >21‰ as indicative of a biological origin, and studies of δ34S at analog impact crater lakes on Earth have followed the same approach. We performed analyses (including δ34S, total organic carbon wt%, and scanning electron microscope imaging) on multiple lithologies from the Nördlinger Ries impact crater, focusing on hydrothermally altered impact breccias and associated sedimentary lake-fill sequences to determine whether the δ34S properties define a biosignature. The differences in δ34S between the host lithologies may have resulted from thermochemical sulfate reduction, microbial sulfate reduction, hydrothermal equilibrium fractionation, or any combination thereof. Despite abundant samples and instrumental precision currently exclusive to Earth-bound analyses, assertions of biogenicity from δ34S variations >21‰ at the Miocene Ries impact crater are tenuous. This discourages the use of δ34S as a biosignature in similar environments without independent checks that include the full geologic, biogeochemical, and textural context, as well as a comprehensive acknowledgment of alternative hypotheses.

Kinetic oxygen isotope fractionation upon acid liberation of CO2 from artificial and natural norsethite (BaMg(CO3 )2 ), a mineral analogue of dolomite

RATIONALE

The analytical method to determine the stable oxygen isotope (¹⁸ O/¹⁶ O) composition of carbonates via phosphoric acid digestion leads to temperature- and solid-dependent kinetic isotope fractionation. Values for the double carbonate norsethite (BaMg(CO₃ )₂ ) have been unknown so far.

METHODS

The temperature dependence of kinetic oxygen isotope fractionation during the reaction of synthetic and natural BaMg(CO₃ )₂ with orthophosphoric acid (H₃ PO₄ ) according to the overall reaction BaMg(CO₃ )₂ + 2H₃ PO₄ = Ba²⁺ + Mg²⁺ + 2HPO₄ ^2- + 2CO₂ + 2H₂ O has been examined for the first time using separate carbonate decomposition via fluorination or phosphoric acid digestion, with the resulting gases analyzed by isotope ratio monitoring mass spectrometry.

RESULTS

In the temperature range between 25 and 70°C the kinetic fractionation factor between acid-generated CO₂ and artificial and natural norsethite is described by (T in K): [Formula: see text] with A = 4.15 and B = 6.47 for natural norsethite, and A = 4.77 and B = 5.94 for synthetic norsethite. The fractionation factor measured for a poorly crystallized synthetic carbonate agrees with those for the other samples at 25°C, but is slightly lower at 50 and 70°C. No carbon isotope fractionation was found during the unidirectional acid dissolution.

CONCLUSIONS

The kinetic oxygen isotope fractionation during phosphoric acid liberation of CO₂ from BaMg(CO₃ )₂ is quantified. Based on published results for endmember carbonates, the results at 25°C for other double carbonates are estimated.

The impact of temperature on the water isotope (2H/1H, 17O/16O, 18O/16O) fractionation upon transport through a low-density polyethylene membrane

In the present study we investigated the isotope effects associated with water loss from closed low-density polyethylene (LDPE) bottles via diffusion at temperatures between 4 and 60 °C. While at low temperatures (4 and 10 °C) no substantial diffusional loss of water was observed within storage time, a pronounced loss was found for the experiments at room temperature and 60 °C. The latter was associated with a substantial increase in δ ¹⁸O, δ ¹⁷O, and δ ²Η values, and a decrease in the deuterium excess. The magnitude of the isotope effects essentially depended on the extent of water evaporation from the closed bottles through the LDPE membrane.

Linking sedimentary sulfur and iron biogeochemistry to growth patterns of a cold-water coral mound in the Porcupine Basin, S.W. Ireland (IODP Expedition 307)

Challenger Mound, a 150-m-high cold-water coral mound on the eastern flank of the Porcupine Seabight off SW Ireland, was drilled during Expedition 307 of the Integrated Ocean Drilling Program (IODP). Retrieved cores offer unique insight into an archive of Quaternary paleo-environmental change, long-term coral mound development, and the diagenetic alteration of these carbonate fabrics over time. To characterize biogeochemical carbon-iron-sulfur transformations in the mound sediments, the contents of dithionite- and HCl-extractable iron phases, iron monosulfide and pyrite, and acid-extractable calcium, magnesium, manganese, and strontium were determined. Additionally, the stable isotopic compositions of pore-water sulfate and solid-phase reduced sulfur compounds were analyzed. Sulfate penetrated through the mound sequence and into the underlying Miocene sediments, where a sulfate-methane transition zone was identified. Small sulfate concentration decreases (<7 mM) within the top 40 m of the mound suggested slow net rates of present-day organoclastic sulfate reduction. Increasing δ(34)S-sulfate values due to microbial sulfate reduction mirrored the decrease in sulfate concentrations. This process was accompanied by oxygen isotope exchange with water that was indicated by increasing δ(18)O-sulfate values, reaching equilibrium with pore-water at depth. Below 50 mbsf, sediment intervals with strong (34)S-enriched imprints on chromium-reducible sulfur (pyrite S), high degree-of-pyritization values, and semi-lithified diagenetic carbonate-rich layers characterized by poor coral preservation, were observed. These layers provided evidence for the occurrence of enhanced microbial sulfate-reducing activity in the mound in the past during periods of rapid mound aggradation and subsequent intervals of non-deposition or erosion when geochemical fronts remained stationary. During these periods, especially during the Early Pleistocene, elevated sulfate reduction rates facilitated the consumption of reducible iron oxide phases, coral dissolution, and the subsequent formation of carbonate cements.

Stable sulfur isotope fractionation during the reduction of thiosulfate by Dethiosulfovibrio russensis

Stable sulfur isotope fractionation was investigated during reduction of thiosulfate by growing batch cultures of Dethiosulfovibrio russensis at a cell-specific reduction rate of 2.4 +/- 0.72 fmol cell(-1) d(-1) (28 degrees C). Citrate was used as carbon and energy source. The hydrogen sulfide produced by this sulfur- and thiosulfate-reducing bacterium was depleted in 34S by 11% compared to total thiosulfate sulfur, in agreement with previous results observed for sulfate-reducing bacteria. This indicates the operation of a similar pathway for thiosulfate reduction in these phylogenetically different bacteria.

The Stable Isotopic Geochemistry of the Sulfur and Carbon Cycles in a Modern Karst Environment

The stable isotopic geochemistry of the sulfur and carbon cycles in a modern karst environment at the southwest border of the Harz Mountains (Germany) has been studied during a 7-year period. Beside major, minor and trace elements, river and sulfate-carbonate ground water were analyzed for the ¹³C/¹²C ratios of the dissolved carbonate species, the ¹⁸O/¹⁶O ratio of water, and the ³⁴S/³²S and ¹⁸O/¹⁶O ratios of dissolved sulfate. Additionally, the carbon, sulfur and oxygen isotopic composition of the major karst aquifers (carbonate and gypsum anhydrite rocks) of the Permian Zechstein formation was measured. Nowadays, the stable isotope biogeochemistry of the river and karst water in the studied area results from the complex interactions between dissolution of biogenic CO₂ in the water-unsaturated zone, (minor) subterrestrial microbial decomposition reactions of organic matter (recent DOC or fossil organic matter), interactions with carbonate and sulfate aquifer minerals, and input of acids from atmospheric pollution (e.g., sulfuric acid). Subsurface precipitation of secondary calcite due to the incongruent dissolution of gypsum and/or dolomite is deduced from the hydrogeochemical composition of selected ground waters and the isotopical composition of calcite found in gypsum aquifer material. Spring and river waters were additionally influenced by the liberation of carbon dioxide into the Earth's atmosphere, a process which is accompanied by the preferrential desorbtion of ¹²CO₂. The sulfur isotopic composition of dissolved sulfate from ground and spring waters indicates a mixture of sulfate derived from surface waters (mainly originating from atmospheric pollution) and geogenic sulfate from the subsurface dissolution of gypsum/anhydrite of the Zechstein formation. The oxygen isotopic composition of dissolved sulfate is generally far away from the exchange equilibrium with water, but consistent with the two-component mixing model derived from the sulfur isotope ratios.

Methane-derived carbonates in a native sulfur deposit: stable isotope and trace element discriminations related to the transformation of aragonite to calcite

Abstract Stable isotope ((13)C, (18)O, (34)S) and trace element (Sr(2+), Mg(2+), Mn(2+), Ba(2+), Na(+)) investigations of elemental sulfur, primary calcites and mixtures of aragonite with secondary, post-aragonitic calcite from sulfur-bearing limestones have provided new insights into the geochemistry of the mineral forming environment of the native sulfur deposit at Machów (SE-Poland). The carbon isotopic composition of carbonates (δ(13)C = -41 to -47‰ vs. PDB) associated with native sulfur (δ(34)S = + 10 to + 15‰ vs. V-CDT) relates their formation to the microbiological anaerobic oxidation of methane and the reduction of sulfate derived from Miocene gypsum. From a comparison with experimentally derived fractionation factors the element ratios of the aqueous fluids responsible for carbonate formation are estimated. In agreement with field and laboratory observations, ratios near seawater composition are obtained for primary aragonite, whereas the fluids were relatively enriched in dissolved calcium during the formation of primary and secondary calcites. Based on the oxygen isotope composition of the carbonates (δ(18)O = -3.9 to -5.9‰ vs. PDB) and a secondary SrSO(4) (δ(18)O = + 20‰ vs. SMOW; δ(34)S = + 59‰ vs. V-CDT), maximum formation temperatures of 35°C (carbonates) and 47°C (celestite) are obtained, in agreement with estimates for West Ukraine sulfur ores. The sulfur isotopic composition of elemental sulfur associated with carbonates points to intense microbial reduction of sulfate derived from Miocene gypsum (δ(34)S ≈ + 23‰) prior to the re-oxidation of dissolved reduced sulfur species.

(18)O/(16)O and (13)c/(12)c fractionation during the reaction of carbonates with phosphoric Acid: effects of cationic substitution and reaction temperature

Abstract The factors for (18)O/(16)O fractionation between carbonates and CO(2) gas produced by the dissolution of the carbonates in phosphoric acid (sealed vessel method) have been investigated as a function of reaction temperature (20-90°C) and cationic substitution in the solid. Synthetic CaCO(3), Ca(0.75) Mn(0.25) CO(3), MnCO(3), BaCO(3) and SrCO(3) powders, and a natural kutnahorite sample were used as solids. The δ(18)O values of the gaseous CO(2) liberated by the reaction with phosphoric acid decrease with increasing temperature and seem to be a linear function of T(°K)(-2). The slopes are specific for different carbonates. No temperature-depended (13)C/(12)C fractionation seems to exist.