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This study evaluates rates and pathways of methane (CH4) oxidation and uptake using 14C-based tracer experiments throughout the oxic and anoxic waters of ferruginous Lake Matano. Methane oxidation rates in Lake Matano are low compared to other lakes, but are sufficiently high to preclude strong CH4 fluxes to the atmosphere. In addition to aerobic CH4 oxidation, which takes place in Lake Matano's oxic mixolimnion, we also detected CH4 oxidation in Lake Matano's anoxic ferruginous waters. Here, CH4 oxidation proceeds in the apparent absence of oxygen (O2) and instead appears to be coupled to nitrate (NO3-), nitrite (NO2-), iron (Fe), or manganese (Mn) reduction. Throughout the lake, the fraction of CH4 carbon that is assimilated vs. oxidized to carbon dioxide (CO2) is high, indicating extensive CH4 conversion to biomass and underscoring the importance of CH4 as a carbon and energy source in Lake Matano and potentially other ferruginous or low productivity environments. [ABSTRACT FROM AUTHOR]

Long-term secular variation in seawater sulfate concentrations ([SO42-¯ ]SW) is of interest owing to its relationship to the oxygenation history of Earth's surface environment. In this study, we develop two complementary approaches for quantification of sulfate concentrations in ancient seawater and test their application to late Neoproterozoic (635 Ma) to Recent marine units. The "rate method" is based on two measurable parameters of paleomarine systems: (1) the S-isotope fractionation associated with microbial sulfate reduction (MSR), as proxied by Δ34S CAS-PY, and (2) the maximum rate of change in seawater sulfate, as proxied by ∂ δ34SCAS = ∂t(max). The "MSR-trend method" is based on the empirical relationship of Δ34SCAS-PY to aqueous sulfate concentrations in 81 modern depositional systems. For a given paleomarine system, the rate method yields an estimate of maximum possible [SO42-¯ ]SW (although results are dependent on assumptions regarding the pyrite burial flux, FPY), and the MSR-trend method yields an estimate of mean [SO42-¯ ]SW. An analysis of seawater sulfate concentrations since 635Ma suggests that [SO42-¯]SW was low during the late Neoproterozoic (<5 mM), rose sharply across the Ediacaran--Cambrian boundary (~ 5-10 mM), and rose again during the Permian (~ 10-30 mM) to levels that have varied only slightly since 250 Ma. However, Phanerozoic seawater sulfate concentrations may have been drawn down to much lower levels (~ 1-4 mM) during short (< ~ 2 Myr) intervals of the Cambrian, Early Triassic, Early Jurassic, and Cretaceous as a consequence of widespread ocean anoxia, intense MSR, and pyrite burial. The procedures developed in this study offer potential for future high-resolution quantitative analyses of paleo-seawater sulfate concentrations. [ABSTRACT FROM AUTHOR]

Incubations of intact sediment cores and sediment slurries reveal that anammox is an important sink for fixed nitrogen in sediments from the Lower St. Lawrence Estuary (LSLE), where it occurs at a rate of 5.5± 1.7 µmol N m-2 h-1. Canonical denitrification occurs at a rate of 11.3±1.1 µmol N m-2 h-1, and anammox is thus responsible for up to 33% of the total N2 production. Both anammox and denitrification are mostly (>95%) fueled by nitrate and nitrite produced in situ through benthic nitrification. Nitrification accounts for >15% of the benthic oxygen demand and may, therefore, contribute significantly to the development and maintenance of hypoxic conditions in the LSLE. The rate of dissimilatory nitrate reduction to ammonium is three orders of magnitude lower than denitrification and anammox, and it is insignificant to N-cycling. NH4+ oxidation by sedimentary Fe(III) and Mn(III/IV) in slurry incubations with N isotope labels did not occur at measurable rates; moreover, we found no evidence for NH4+ oxidation by added Mn(III)-pyrophosphate. [ABSTRACT FROM AUTHOR]

This study explores Mn biogeochemistry in a stratified, ferruginous lake, a modern analogue to ferruginous oceans. Intense Mn cycling occurs in the chemocline where Mn is recycled at least 15 times before sedimentation. The product of biologically catalyzed Mn oxidation in Lake Matano is birnessite. Although there is evidence for abiotic Mn reduction with Fe(II), Mn reduction likely occurs through a variety of pathways. The flux of Fe(II) is insufficient to balance the reduction of Mn at 125 m depth in the water column, and Mn reduction could be a significant contributor to CH4 oxidation. By combining results from synchrotron-based X-ray fluorescence and X-ray spectroscopy, extractions of sinking particles, and reaction transport modeling, we find the kinetics of Mn reduction in the lake's reducing waters are sufficiently rapid to preclude the deposition of Mn oxides from the water column to the sediments underlying ferruginous water. This has strong implications for the interpretation of the sedimentary Mn record. [ABSTRACT FROM AUTHOR]

Incubations of intact sediment cores and sediment slurries reveal that anammox is an important sink for fixed nitrogen in the Lower St. Lawrence Estuary (LSLE), where it occurs at a rate of 5.5 ± 1.7 µmol N m-2 h-1 in the sediment. Anammox is responsible for up to 33% of the total N2 production, and both anammox and denitrification are mostly (>95%) fueled by nitrate and nitrite produced in situ through benthic nitrification. Nitrification accounts for >15% of the benthic oxygen demand and contributes significantly to the development and maintenance of hypoxic conditions in the LSLE. The rate of dissimilatory nitrate reduction to ammonium is three orders of magnitude lower than denitrification and anammox and is therefore insignificant to N-cycling. Tests for Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed oxidation by sedimentary Fe(III) and Mn(III/IV), using slurry incubations with N isotope labels, revealed that it does not occur at measurable rates, and we found no evidence for Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed oxidation by added Mn(III)-pyrophosphate. [ABSTRACT FROM AUTHOR]

The concentration of dissolved organic carbon (DOC) in groundwater emanating as spring discharge at several locations in Florida, USA, and the net rate of DOC increase in the downstream receiving waters were measured as part of a larger investigation of carbon dynamics in flowing waters. Springs with high discharge (>2.8 m³ s-1) were found to be the most organic-poor natural waters yet reported (13 ± 1.6 μmol C L-1), while springs with lesser discharge exhibited somewhat higher DOC concentrations (values ranging from 30 to 77 μmol C L-1). DOC concentrations increased rapidly downstream from the point of spring discharge, with the calculated net areal input rate of DOC ranging from 0.04 to 1.64 mol C m-2 d-1 across springs. Rates of DOC increase were generally greater in those springs with high discharge rates. These input rates compare favorably with values reported for gross primary production in these macrophyte-dominated spring systems, assuming that 17% of macrophyte primary production is lost, on average, as DOC. The measures reported here are possible only because of the remarkably low DOC levels in the up-surging groundwaters and the short residency times of the water in the spring-runs themselves. [ABSTRACT FROM AUTHOR]