Speleothem calcite farmed in situ: Modern calibration of δO and δC paleoclimate proxies in a continuously-monitored natural cave system

Understanding the relationships between speleothem stable isotopes (δ¹³C δ¹⁸O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ¹⁸O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ¹⁸O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave.Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2 m s⁻¹ in winter and 0.4 m s⁻¹ in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO₂. A clear relationship is found between calcite δ¹³C and cave air ventilation rates estimated by proxies pCO₂ and ²²²Rn. Calcite δ¹³C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ¹³C_CaCO3 = −7‰. A whole-cave “Hendy test” at multiple contemporaneous farming sites reveals that ventilation induces a +1.9 ± 0.96‰ δ¹³C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the “Hendy test” has implications for interpreting δ¹³C records in ancient speleothems. Calcite δ¹³C_CaCO3 may be a proxy not only for atmospheric CO₂ or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating and plugging ventilation windows.Farmed calcite δ¹⁸O was found to exhibit a +0.82 ± 0.24‰ offset from values predicted by both theoretical calculations and laboratory-grown inorganic calcite. Unlike δ¹³C_CaCO3, oxygen isotopes showed no ventilation effects, i.e. Δδ¹⁸O_CaCO3 appears to be a function of growth temperature only although we cannot rule out a small effect of (unmeasured) gradients in relative humidity (evaporation) accompanying ventilation. Our results support the findings of other cave investigators that water-calcite fractionation factors observed in speleothem calcite are higher that those measured in laboratory experiments. Cave and laboratory calcite precipitates may differ mainly in the complex effects of kinetic isotope fractionation. Combining our data with other recent speleothem studies, we find a new empirical relationship for cave-specific water-calcite oxygen isotope fractionation across a range of temperatures and cave environments:

1000lnα=16.1(10³T^-1)-24.6     

Copper isotope fractionation during surface adsorption and intracellular incorporation by bacteria

Copper isotopes may prove to be a useful tool for investigating bacteria-metal interactions recorded in natural waters, soils, and rocks. However, experimental data which attempt to constrain Cu isotope fractionation in biologic systems are limited and unclear. In this study, we utilized Cu isotopes (δ⁶⁵Cu) to investigate Cu-bacteria interactions, including surface adsorption and intracellular incorporation. Experiments were conducted with individual representative species of Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria, as well as with wild-type consortia of microorganisms from several natural environments. Ph-dependent adsorption experiments were conducted with live and dead cells over the pH range 2.5-6. Surface adsorption experiments of Cu onto live bacterial cells resulted in apparent separation factors (Δ⁶⁵Cu_{solution-solid} = δ⁶⁵Cu_solution − δ⁶⁵Cu_solid) ranging from +0.3‰ to +1.4‰ for B. subtilis and +0.2‰ to +2.6‰ for E. coli. However, because heat-killed bacterial cells did not exhibit this behavior, the preference of the lighter Cu isotope by the cells is probably not related to reversible surface adsorption, but instead is a metabolically-driven phenomenon. Adsorption experiments with heat-killed cells yielded apparent separation factors ranging from +0.3‰ to −0.69‰ which likely reflects fractionation from complexation with organic acid surface functional group sites. For intracellular incorporation experiments the lab strains and natural consortia preferentially incorporated the lighter Cu isotope with an apparent Δ⁶⁵Cu_{solution-solid} ranging from ∼+1.0‰ to +4.4‰. Our results indicate that live bacterial cells preferentially sequester the lighter Cu isotope regardless of the experimental conditions. The fractionation mechanisms involved are likely related to active cellular transport and regulation, including the reduction of Cu(II) to Cu(I). Because similar intracellular Cu machinery is shared by fungi, plants, and higher organisms, the influence of biological processes on the δ⁶⁵Cu of natural waters and soils is probably considerable.     

Comparison of fluid geochemistry and microbiology of multiple organic-rich reservoirs in the Illinois Basin, USA: Evidence for controls on methanogenesis and microbial transport

Microbial methane in sedimentary basins comprises approximately 20% of global natural gas resources, yet little is known about the environmental requirements and metabolic rates of these subsurface microbial communities. The Illinois Basin, located in the midcontinent of the United States, is an ideal location to investigate hydrogeochemical factors controlling methanogenesis as microbial methane accumulations occur: (1) in three organic-rich reservoirs of different geologic ages and organic matter types - Upper Devonian New Albany Shale (up to 900 m depth), Pennsylvanian coals (up to 600 m depth), and Quaternary glacial sediments (shallow aquifers); (2) across steep salinity gradients; and (3) with variable concentrations of . For all three organic-rich reservoirs aqueous geochemical conditions are favorable for microbial methanogenesis, with near neutral pH, concentrations <2 mM, and Cl⁻ concentrations <3 M. Also, carbon isotopic fractionation of CH₄, CO₂, and DIC is consistent with microbial methanogenesis, and increased carbon isotopic fractionation with average reservoir depth corresponds to a decrease of groundwater flushing rates with average depth of reservoir. Plots of stable isotopes of water and Cl⁻ show mixing between a brine endmember and freshwater, suggesting that meteoric groundwater recharge has affected all microbial methanogenic systems. Additionally, similar methanogenic communities are present in all three reservoirs with comparable cell counts (8.69E3-2.58E6 cells/mL). TRFLP results show low numbers of archaea species with only two dominant groups of base pairs in coals, shale, and limestone aquifers. These results compare favorably with other methanogen-containing deep subsurface environments. Individual hydrogeochemical parameters that have a Spearman correlation coefficient greater than 0.3 to variations in methanogenic species include stable isotopes of water (δ¹⁸O and δD), type of substrate (i.e. coals versus shale), pH, and Cl⁻ concentration. The matching of variations between methanogenic TRFLP data and conservative tracers suggests that deep circulation of meteoric waters influenced archaeal communities in the Illinois Basin. In addition, coalification and burial estimates suggest that in the study area, coals and shale reservoirs were previously sterilized (>80 °C in nutrient poor environments), necessitating the re-introduction of microbes into the subsurface via groundwater transport.     

The significance of isotope specific diffusion coefficients for reaction-transport models of sulfate reduction in marine sediments

Modeling isotopic signatures in systems affected by diffusion, advection, and a reaction which modifies the isotopic abundance of a given species, is a discipline in its infancy. Traditionally, much emphasis has been placed on kinetic isotope effects during biochemical reactions, while isotope effects caused by isotope specific diffusion coefficients have been neglected. A recent study by Donahue et al. (2008) suggested that transport related isotope effects may be of similar magnitude as microbially mediated isotope effects. Although it was later shown that the assumed differences in the isotope specific diffusion coefficients were probably overstated by one or two orders of magnitude (Bourg, 2008), this study raises several important issues: (1) Is it possible to directly calculate isotopic enrichment factors from measured concentration data without modeling the respective system? (2) Do changes in porosity and advection velocity modulate the influence of isotope specific diffusion coefficients on the fractionation factor α? (3) If one has no a priori knowledge whether diffusion coefficients are isotope specific or not, what is the nature and magnitude of the error introduced by either assumption? Here we argue (A) That the direct substitution of measured data into a differential equation is problematic and cannot be used as a replacement for a reaction-transport model; (B) That the transport related fractionation scales linearly with the difference between the respective diffusion coefficients of a given isotope system, but depends in a complex non-linear way on the interplay between advection velocity, and downcore changes of temperature and porosity. Last but not least, we argue that the influence of isotope specific diffusion coefficients on microbially mediated sulfate reduction in typical marine sediments is considerably smaller than the error associated with the determination of the fractionation factor.     

Hydrography and circulation in the northwestern Bay of Bengal during the retreat of southwest monsoon

The distribution of temperature and salinity in the upper 500 m of the northwestern Bay of Bengal, adjoining the east coast of India, during the retreat of southwest monsoon (September) of 1983 is presented. This study reveals coastal upwelling (limited to the upper 40 m) induced by the local winds. Waters of higher surface salinity near the coast characterize the upwelling. The freshwater influx near the head of the Bay diluted the surface salinity to as low as 26.0 × 10⁻³. The surface circulation was weak and led to a net transport of 2.0 × 10⁶m³.s⁻¹ directed towards northeast.     

Microfossils in the ‘Complexe de base’ of the Prealpine Helminthoid Flysch (Dranse nappe,Switzerland). - Preliminary results

The Swiss Prealpine Dranse nappe consists of the badly known shaley Complexe de base series and the overlying Campanian to Maastrichtian Helminthoid Flysch. Our preliminary data from the shaley series indicate an Albian to Early Campanian age. It represents mid-slope to abyssal plain starved basin sediments. The occurrence of similar and coeval series in the Carpathians and Ligurian Alps suggest common tectonic and oceanographic conditions.     

Nitrate reductase activity: A solution to nitrate problems tested in free and Immobilized algal cells in presence of heavy metals

Every organism has different potential to accumulate NO₃ ^?from the environment. Nitrate reduction processes are perhaps most significant in maintaining water quality by alteration of nitrate to nitrite. A comparative study between the nitrate reductase NR activity of green and blue green algae in presence of heavy metals is being conducted to present a situation where nitrate reductase process may be affected in presence of heavy metals. Metals interacted negatively with the nitrate reductase activity of a blue green alga, Anacystis nidulans and green algae, Chlorella vulgaris in both free and immobilized state. The activity was more repressed in C. vulgaris in presence of Ni compared to Zn and Cd. However, Cd was more toxic to NR activity in A. nidulans (free state). Metal dependent variation between free and immobilized cells were found to be significant (P< 0.01) however, the concentration dependent pattern in the activity between free and immobilized state was non significant in both the test organisms. C.vulgaris is more efficient in conversion of nitrate to nitrite compared to A.nidulans in presence of heavy metals.     

Spawning densities,egg densities,size structure,and movement patterns of spawning horseshoe crabs,<Emphasis Type="Italic">Limulus polyphemus</Emphasis>, within four coastal embayments on Cape Cod,Massachusetts

Spawning densities, spawning indices, egg densities, size distributions, and movement patterns of horseshoe crabs (Limulus polyphemus) were quantified for four coastal embayments (Monomoy National Wildlife Refuge, Pleasant Bay, Nauset Estuary, and Cape Cod Bay) on Cape Cod, Massachusetts from 2000 to 2002. Spawning activity was highest from mid May through mid June, but densities varied throughout the Cape Cod region. Average spawning densities (male and female crabs combined), measured using 25-m² quadrats, were lower than 1 crab 25 m^?2, although certain locations had consistently higher densities averaging 2 to 3 crabs 25 m^?2 with individual survey densities recorded as high as 17 crabs 25 m^?2. Spawning densities during night surveys were either similar or slightly higher than day surveys, except at a few sites within Pleasant Bay. Spawning indices were considerably lower ranging from 0 to 1.3 females 25 m^?2 throughout the Cape Cod region. Spawning sex ratios varied from 1∶1.6 to 1∶3.1 (females:males) throughout the region, except within Pleasant Bay where highly male skewed ratios were observed (e.g., 1∶5.8, 3-yr average). Egg densities were low overall (<1 egg cm^?2) throughout Cape Cod and egg densities tended to be higher in deeper sediments (5–20 cm deep) compared to shallow sediments (0–5 cm deep) at most locations. Over 7,800 horseshoe crabs were tagged on Cape Cod from 2000 to 2002. Average size and size frequency distributions of tagged crabs varied among regions. Larger individuals were observed at Monomoy National Wildlife Refuge while the smallest individuals were from Cape Cod Bay. We documented an overall recapture rate of 6.7% and our tag-recapture data indicated that 62% of crabs were recaptured at the original tagging location and 70% of recaptures traveled less than 2 km from the original tagging location, providing evidence for localized populations on Cape Cod. We have observed that horseshoe crabs differ among embayments within a regional area, suggesting the potential need for management plans specific to embayments or subregions depending on the characteristics of a population.     

Denitrification rates measured along a salinity gradient in the eutrophic Neuse River estuary, North Carolina, USA

Denitrification rates along a salinity gradient in the eutrophic Neuse River Estuary, North Carolina, were quantified using membrane inlet mass spectrometry (MIMS) within short-term batch incubations. Denitrification rates within the system were highly variable, ranging from 0 to 275 μmol N m⁻² h⁻¹. Intrasite variability increased with salinity, but no significant differences were observed across the salinity gradient. Denitrification rates were positively correlated with sediment oxygen demand at the upstream sampling site where sediment organic carbon levels were lowest. This relationship was not observed in the more saline sampling sites. Denitrification rates were highest during winter. On an annual basis, denitrification accounted for 26% of the dissolved inorganic nitrogen and 12% of the total nitrogen supplied to the system.