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Abstract: Iron oxides are important terminal electron acceptors for organic carbon (OC) remineralization in subterranean estuaries, particularly where oxygen and nitrate concentrations are low. In Indian River Lagoon, Florida, USA, terrestrial Fe-oxides dissolve at the seaward edge of the seepage face and flow upward into overlying marine sediments where they precipitate as Fe-sulfides. The dissolved Fe concentrations vary by over three orders of magnitude, but Fe-oxide dissolution rates are similar across the 25-m wide seepage face, averaging around 0.21mg/cm2/yr. The constant dissolution rate, but differing concentrations, indicate Fe dissolution is controlled by a combination of increasing lability of dissolved organic carbon (DOC) and slower porewater flow velocities with distance offshore. In contrast, the average rate constants of Fe-sulfide precipitation decrease from 21.9×10−8 s−1 to 0.64×10−8 s−1 from the shoreline to the seaward edge of the seepage face as more oxygenated surface water circulates through the sediment. The amount of OC remineralized by Fe-oxides varies little across the seepage face, averaging 5.34×10−2 mg/cm2/yr. These rates suggest about 3.4kg of marine DOC was remineralized in a 1-m wide, shore-perpendicular strip of the seepage face as the terrestrial sediments were transgressed over the past 280years. During this time, about 10 times more marine solid organic carbon (SOC) accumulated in marine sediments than were removed from the underlying terrestrial sediments. Indian River Lagoon thus appears to be a net sink for marine OC. [Copyright &y& Elsevier]

Abstract It is widely assumed that changes in the rate of biological O 2 supply to the atmosphere played little, if any, role in driving the Great Oxidation Event (GOE) because extensive biological O 2 production via oxygenic photosynthesis significantly predates atmospheric oxygenation on Earth. Moreover, the C isotope record seemingly precludes a dramatic increase in O 2 -liberating burial of organic C in the late Archean. However, organic C burial is not the only way in which the biosphere may oxidize the Earth's atmosphere. Biologically mediated burial of reduced mineral phases may also yield net oxidation. For example, reducing power can be transferred from organic matter to H 2 S via anaerobic respiration and ultimately stored in sedimentary pyrite (FeS 2). Changes in pyrite burial may thus impact Earth's oxidation trajectory. Here, we revisit the earliest recognized episode of sulfide-rich (euxinic), pyrite-precipitating conditions in Earth's ocean, ∼200–300 million years prior to the GOE, as preserved in the ∼2.7–2.6 billion-year-old (Ga) Roy Hill Shale of the Jeerinah Formation. Applying Fe speciation and trace metal proxies to drillcore samples from two sites, we characterize both spatial and temporal heterogeneity in the accumulation of H 2 S in the otherwise Fe-rich Hamersley Basin. We argue that extensive pyrite burial in these locally euxinic environments was the consequence of enhanced volcanic emanations of SO 2 to the atmosphere rather than oxidative weathering of crustal sulfides. Because SO 2 reduction and burial as pyrite is chemically analogous to CO 2 reduction and burial as organic matter, we further posit that this alternative oxidation pathway contributed to Earth's protracted oxygenation during the GOE. Our results thus highlight the need for continued study of the interplay between solid-Earth, volcanic, and biological processes and their joint modulation of oceanic and atmospheric composition. Highlights • The ∼2.7 to 2.6 Ga Roy Hill Shale (Hamersley Basin) is highly pyritic. • Extensive pyrite burial is the result of pulsed S delivery to the Hamersley Basin. • Anomalous S fluxes are attributed to episodes of enhanced subaerial volcanism. • Microbial reduction and burial of S as pyrite oxidizes the surface environment. • Subaerial volcanism and pyrite burial contributed to whiffs of oxygen and the GOE. [ABSTRACT FROM AUTHOR]

Abstract: Pore water radon (222Rn) distributions from Indian River Lagoon, Florida, are characterized by three zones: a lower zone where pore water 222Rn and sediment-bound radium (226Ra) are in equilibrium and concentration gradients are vertical; a middle zone where 222Rn is in excess of sediment-bound 226Ra and concentration gradients are concave-downward; and an upper zone where 222Rn concentration gradients are nearly vertical. These 222Rn data are simulated in a one-dimensional numerical model including advection, diffusion, and non-local exchange to estimate magnitudes of submarine groundwater discharge components (fresh or marine). The numerical model estimates three parameters, fresh groundwater seepage velocity, irrigation intensity, and irrigation attenuation, using two Monte Carlo (MC) simulations that (1) ensure the minimization algorithm converges on a global minimum of the merit function and the parameter estimates are consistent within this global minimum, and (2) provide 90% confidence intervals on the parameter estimates using the measured 222Rn activity variance. Model estimates of seepage velocities and discharge agree with previous estimates obtained from numerical groundwater flow models and seepage meter measurements and show the fresh water component decreases offshore and varies seasonally by a factor of nine or less. Comparison between the discharge estimates and precipitation patterns suggests a mean residence time in unsaturated and saturated zones on the order of 5 to 7 months. Irrigation rates generally decrease offshore for all sampling periods. The mean irrigation rate is approximately three times greater than the mean seepage velocity although the ranges of irrigation rates and seepage velocities are the same. Possible mechanisms for irrigation include density-driven convection, wave pumping, and bio-irrigation. Simulation of both advection and irrigation allows the separation of submarine groundwater discharge into fresh groundwater and (re)circulated lagoon water. [Copyright &y& Elsevier]