To understand carbon and oxygen dynamics in sediments with deep oxygen penetration, we investigated eight locations (160–318‐m depth) throughout Lake Superior. Despite the 2–4 weight percent organic carbon content, oxygen penetrated into the sediment by 3.5 to > 12 cm at all locations. Such deep penetration is explained by low sedimentation rates (0.01–0.04 cm yr⁻¹), high solubility of oxygen in freshwater, and a shallow (~ 2 cm) bioturbation zone. In response mainly to oxygen variations in the bottom waters, the sediment oxygen penetration varied seasonally by as much as several centimeters, suggesting that temporal variability in deeply oxygenated sediments may be greater than previously acknowledged. The oxygen uptake rates (4.4–7.7 mmol m⁻² d⁻¹, average 6.1 mmol m⁻² d⁻¹) and carbon mineralization efficiency (~ 90% of deposited carbon) were similar to those in marine hemipelagic and pelagic sediments of comparable sedimentation rates. The reactivity of organic carbon was found to decrease with age similarly to the power‐law documented in marine environments. The burial flux of carbon into the deep sediment (0.7 mmol m⁻² d⁻¹) was 2.5% of the previously estimated primary production. Maximum volume‐specific carbon degradation rates were 0.3–1.5 µmol cm⁻³ d⁻¹; bioturbation coefficient near the sediment surface was 3–8 cm² yr⁻¹. These results indicate that carbon cycling in large freshwater systems conforms to many of the same trends as in marine systems.

Significance: The ecosystems of the Laurentian Great Lakes have been dramatically reengineered by invasive bottom-dwelling dreissenid mussels. A key question is whether this biological change has altered whole-ecosystem biogeochemistry, in particular the cycling of phosphorus, a key nutrient that limits biological productivity in freshwater ecosystems. We show that phosphorus cycling in the invaded Great Lakes is now regulated by the population dynamics of a single benthic species, the quagga mussel. This qualitatively changes the responses of the affected lakes to phosphorus inputs from watersheds, complicates predictions, and necessitates a new paradigm for managing these large aquatic ecosystems. Similar changes likely play out in many other dreissenid-invaded lakes across Europe and North America.

The chemistry of the Early Earth is widely inferred from the elemental and isotopic compositions of sulfidic sedimentary rocks, which are presumed to have formed globally through the reduction of seawater sulfate or locally from hydrothermally supplied sulfide. Here we argue that, in the anoxic Archean oceans, pyrite could form in the absence of ambient sulfate from organic sulfur contained within living cells. Sulfides could be produced through mineralization of reduced sulfur compounds or reduction of organic-sourced sulfite. Reactive transport modeling suggests that, for sulfate concentrations up to tens of micromolar, organic sulfur would have supported 20 to 100% of sedimentary pyrite precipitation and up to 75% of microbial sulfur reduction. The results offer an alternative explanation for the low range of δ³⁴S in Archean sulfides, and raise a possibility that sulfate scarcity delayed the evolution of dissimilatory sulfate reduction until the initial ocean oxygenation around 2.7 Ga.

Abrupt disappearance of mass-independent fractionation of sulfur isotopes (MIF-S) from the geologic record and an apparent ingrowth in seawater sulfate around 2.45 billion years ago (Ga) signal the first large-scale oxygenation of the atmosphere [the Great Oxygenation Event (GOE)]. Pre-GOE O₂ production is evident from multiple other terrestrial and marine proxies, but oceanic O₂ concentrations remain poorly constrained. Furthermore, current interpretations of S isotope records do not explain a concurrent expansion in the range of both MIF-S—diagnostic for low atmospheric O₂—and δ³⁴S beginning at 2.7 Ga. To address these unknowns, we developed a reaction-transport model to analyze the preservation patterns of sulfur isotopes in Archean sedimentary pyrites, one of the most robust and widely distributed proxies for early Earth biogeochemistry. Our modeling, paradoxically, reveals that micromolar levels of O₂ in seawater enhance the preservation of large MIF-S signals, whereas concomitant ingrowth of sulfate expands the ranges in pyrite δ³⁴S. The 2.7- to 2.45-Ga expansion in both Δ³³S and δ³⁴S ranges thus argues for a widespread and protracted oxygenation of seawater, at least in shallow marine environments. At the micromolar levels predicted, the surface oceans would support a strong flux of O₂ to the atmosphere, where O₂ sinks balanced these fluxes until the GOE. This microoxic seawater would have provided habitat for early aerobic microorganisms and supported a diversity of new O₂-driven biogeochemical cycles in the Neoarchean.