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Authigenic clay formation during early diagenesis of marine sediments, termed “reverse weathering,” is an important process for regulating ocean pH, seawater chemistry, and atmospheric CO₂over geologic time scales. Although the importance of reverse weathering has been increasingly recognized, the rates and mechanisms remain poorly constrained. This study investigated the mechanisms, kinetics, and mineral products derived from diatom biogenic silica. We show the formation of Fe(II)-bearing smectite and mica in 40 days, the most rapid process and first specific mineral phases reported to date. Unraveling the kinetics and mechanisms of authigenic clay formation suggests that reverse weathering is far more dynamic and responsive to changes in ocean chemistry than previously envisioned, with a potential to impact marine alkalinity cycling on a shorter timescale. 

Abstract The evolution and expansion of land plants brought about one of the most dramatic shifts in the history of the Earth system — the birth of modern soils — and likely stimulated massive changes in marine biogeochemistry and climate. Multiple marine extinctions characterized by widespread anoxia, including the Late Devonian mass extinction around 372 million years ago, may have been linked to terrestrial release of the nutrient phosphorus driven by newly-rooted landscapes. Here we use recently published Devonian lake records as variable inputs in an Earth system model of the coupled carbon-nitrogen-phosphorus-oxygen-sulfur biogeochemical cycles to evaluate whether recorded changes to phosphorus fluxes could sustain Devonian marine anoxia sufficient to drive mass extinction. Results show that globally scaled increases in riverine phosphorus export during the Late Devonian mass extinction could have generated widespread marine anoxia, as modeled perturbations in carbon isotope, temperature, oxygen, and carbon dioxide data are generally consistent with the geologic record. Similar results for large scale volcanism suggest the Late Devonian mass extinction was likely multifaceted with both land plants and volcanism as contributing factors. 

Abstract Atmospheric oxygen is thought to have played a vital role in the evolution of large, complex multicellular organisms. Challenging the prevailing theory, we show that the transition from an anaerobic to an aerobic world can strongly suppress the evolution of macroscopic multicellularity. Here we select for increased size in multicellular ‘snowflake’ yeast across a range of metabolically-available O 2 levels. While yeast under anaerobic and high-O 2 conditions evolved to be considerably larger, intermediate O 2 constrained the evolution of large size. Through sequencing and synthetic strain construction, we confirm that this is due to O 2 -mediated divergent selection acting on organism size. We show via mathematical modeling that our results stem from nearly universal evolutionary and biophysical trade-offs, and thus should apply broadly. These results highlight the fact that oxygen is a double-edged sword: while it provides significant metabolic advantages, selection for efficient use of this resource may paradoxically suppress the evolution of macroscopic multicellular organisms. 

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure ( p O 2 ) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating p O 2 and productivity during the Proterozoic. O-MIF, reported as Δ′ 17 O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS 2 ) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air–sea gas exchange. Previous analyses of these data concluded that p O 2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on p O 2 is essentially unconstrained by these data. Indeed, p O 2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that p CO 2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O 2 ) between the atmosphere and surface ocean is controlled more by air–sea gas transfer rates than by biological productivity. Improved estimates of p CO 2 and/or improved proxies for Δ′ 17 O of atmospheric O 2 would allow tighter constraints to be placed on mid-Proterozoic p O 2 . 

For most of Earth’s history, the ocean’s interior was pervasively anoxic and showed occasional shifts in ocean redox chemistry between iron-buffered and sulfide-buffered states. These redox transitions are most often explained by large changes in external inputs, such as a strongly altered delivery of iron and sulfate to the ocean, or major shifts in marine productivity. Here, we propose that redox shifts can also arise from small perturbations that are amplified by nonlinear positive feedbacks within the internal iron and sulfur cycling of the ocean. Combining observational evidence with biogeochemical modeling, we show that both sedimentary and aquatic systems display intrinsic iron–sulfur bistability, which is tightly linked to the formation of reduced iron–sulfide minerals. The possibility of tipping points in the redox state of sediments and oceans, which allow large and nonreversible geochemical shifts to arise from relatively small changes in organic carbon input, has important implications for the interpretation of the geological rock record and the causes and consequences of major evolutionary transitions in the history of Earth’s biosphere.