The Devonian-Carboniferous transition marks a fundamental shift in the surface environment primarily related to changes in ocean-atmosphere oxidation states(1,2), resulting from the continued proliferation of vascular land plants that stimulated the hydrological cycle and continental weathering(3,4), glacioeustasy(5,6), eutrophication and anoxic expansion in epicontinental seas(3,4), and mass extinction events(2,7,8). Here we present a comprehensive spatial and temporal compilation of geochemical data from 90 cores across the entire Bakken Shale (Williston Basin, North America). Our dataset allows for the detailed documentation of stepwise transgressions of toxic euxinic waters into the shallow oceans that drove a series of Late Devonian extinction events. Other Phanerozoic extinctions have also been related to the expansion of shallow-water euxinia, indicating that hydrogen sulfide toxicity was a key driver of Phanerozoic biodiversity.

Biogeochemical cycling of sulfur is relatively understudied in terrestrial environments compared to marine environments. However, the comparative ease of access, observation, and sampling of terrestrial settings can expand our understanding of organisms and processes important in the modern sulfur cycle. Furthermore, these sites may allow for the discovery of useful process analogs for ancient sulfur-metabolizing microbial communities at times in Earth's past when atmospheric O2 concentrations were lower and sulfide was more prevalent in Earth surface environments. We identified a new site at Santa Paula Creek (SPC) in Ventura County, CA-a remarkable freshwater, gravel-bedded mountain stream charged with a range of oxidized and reduced sulfur species and heavy hydrocarbons from the emergence of subsurface fluids within the underlying sulfur- and organic-rich Miocene-age Monterey Formation. SPC hosts a suite of morphologically distinct microbial biofacies that form in association with the naturally occurring hydrocarbon seeps and sulfur springs. We characterized the geology, stream geochemistry, and microbial facies and diversity of the Santa Paula Creek ecosystem. Using geochemical analyses and 16S rRNA gene sequencing, we found that SPC supports a dynamic sulfur cycle that is largely driven by sulfide-oxidizing microbial taxa, with contributions from smaller populations of sulfate-reducing and sulfur-disproportionating taxa. This preliminary characterization of SPC revealed an intriguing site in which to study geological and geochemical controls on microbial community composition and to expand our understanding of sulfur cycling in terrestrial environments.

Whiting events – the episodic precipitation of fine-grained suspended calcium cabonates in the water column – have been documented across a variety of marine and lacustrine environments. Whitings likely are a major source of carbonate muds, a constituent of limestones, and important archives for geochemical proxies of Earth history. While several biological and physical mechanisms have been proposed to explain the onset of these precipitation events, no consensus has been reached thus far. Fayetteville Green Lake (New York, USA), is a meromictic lake that experiences annual whitings. Materials suspended in the water column collected through the whiting season were characterized using scanning electron microscopy and scanning transmission X-ray microscopy. Whitings in Fayetteville Green Lake are initiated in the spring within the top few meters of the water column, by precipitation of fine amorphous calcium carbonate (ACC) phases nucleating on Synechococcus cells (cyanobacteria), as well as on extracellular polymeric substances (EPS), including abundant β-chitin fibrils exuded by centric diatoms. Whiting particles found in the summer consist of 5-7 µm calcite grains forming aggregates with diatoms and their EPS. Simple calculations demonstrate that calcite particles continuously grow over several days, then sink quickly through the water column. In the late summer, partial calcium carbonate dissolution is observed deeper in the water column. Settling whiting particles however reach the bottom of the lake, where they form a major constituent of the sediment, along with diatom frustules. The importance of diatoms and their EPS in whitings at Fayetteville Green Lake is described for the first time here, a largely overlooked mechanism for other whiting events in modern and ancient environments.

D. Jablonski [Proc. Natl. Acad. Sci. U.S.A. 99, 8139-8144 (2002)] coined the term "dead clades walking" (DCWs) to describe marine fossil orders that experience significant drops in genus richness during mass extinction events and never rediversify to previous levels. This phenomenon is generally interpreted as further evidence that the macroevolutionary consequences of mass extinctions can continue well past the formal boundary. It is unclear, however, exactly how long DCWs are expected to persist after extinction events and to what degree they impact broader trends in Phanerozoic biodiversity. Here we analyze the fossil occurrences of 134 skeletonized marine invertebrate orders in the Paleobiology Database (paleobiodb.org) using a Bayesian method to identify significant change points in genus richness. Our analysis identifies 70 orders that experience major diversity losses without recovery. Most of these taxa, however, do not fit the popular conception of DCWs as clades that narrowly survive a mass extinction event and linger for only a few stages before succumbing to extinction. The median postdrop duration of these DCW orders is long ( >30 Myr), suggesting that previous studies may have underestimated the long-term taxonomic impact of mass extinction events. More importantly, many drops in diversity without recovery are not associated with mass extinction events and occur during background extinction stages. The prevalence of DCW orders throughout both mass and background extinction intervals and across phyla ( >50% of all marine invertebrate orders) suggests that the DCW pattern is a major component of macroevolutionary turnover.

Numerous Paleozoic biotic crises correlate with sedimentological and geochemical evidence for widespread low-O 2 and high-H 2 S, shallow-marine conditions (anoxia and euxinia). Hypothesized factors that promoted the recurrent anoxic events range from external (volcanic outgassing, continental configuration) to internal (biogeochemical cycling) [1]. Recent work has suggested that seafloor-hydrothermal activity, recorded by large volcanogenic massive sulfide (VMS) deposits, may have acted as an additional external driver of basinal-to-regional shifts in shallow-marine redox states [2]. Forming during mid-ocean ridge, arc, and back-arc volcanism, these hydrothermal systems potentially sustained significant fluxes of reductants (Fe, Mn, H 2 S, H 2 ) over ~10 5 years, acting as a sink for oxidants and a mechanism for limiting nutrients. Here we provide the first test of ocean redox sensitivity to seafloor-hydrothermal activity using an intermediate-complexity, 3D Earth system model (cGENIE). Across a range of Paleozoic pO 2 , climate, and nutrient scenarios, our results demonstrate that benthic reductant fluxes associated with VMS mineralization are capable of driving dysoxia and impacting biogeochemical cycling at a basinal to regional scale. These simulations suggest that VMS

Carbonate sediments and rocks are valuable archives of Earth's past whose geochemical compositions inform our understanding of Earth's surface evolution. Yet carbonates are also reactive minerals and often undergo compositional alteration between the time of deposition and sampling and analysis. These changes may be mineralogical, structural, and/or chemical, and they are broadly referred to as diagenesis. Building on work over the past 40 years, we present an overview of key carbonate diagenesis terminology and a process-based framework for evaluating the geochemical impacts of carbonate diagenesis; we also highlight recent experimental and field observations that suggest metal isotopes as valuable diagenetic indicators. Our primary objectives are to demonstrate the value of coupling quantitative and analytical approaches, specifically with regard to metal isotopes and Mg/Ca, and to focus attention on key avenues for future work, including the role of authigenesis in impacting global geochemical cycles and the isotopic composition of the rock record. ▪ Quantitative frameworks utilizing well-understood diagenetic indicators and basic geochemical parameters allow us to assess the extent of diagenetic alteration in carbonate sediments. ▪ The reactivity, duration of reaction, and degree of isotopic or elemental/chemical disequilibrium determine the extent to which carbonates may be altered. ▪ Metal isotopic ratios (δ44Ca, δ26Mg, 87Sr/86Sr) can be used to constrain the extent and rate of carbonate recrystallization. ▪ Diagenetic signals may be globally synchronous, while diagenetic fluxes may impact global geochemical cycles.

Late Devonian (Famennian) marine successions globally are typified by organic‐rich black shales deposited in anoxic and euxinic waters and the cessation of shelf carbonate sedimentation. This global ‘carbonate crisis’, known as the Hangenberg Event, coincides with a major extinction of reef‐building metazoans and perturbations to the global carbon cycle, evidenced by positive carbon‐isotope excursions of up to 4‰. It has been suggested that authigenic carbonate, formed as cements in sedimentary pore spaces during early burial diagenesis, is a significant mass fraction of the total global carbon burial flux, particularly during periods of low oxygen concentration. Because some authigenic carbonate could have originated from remineralization of organic carbon in sediments, it is possible for this reservoir to be isotopically depleted and thereby drive changes in the carbon isotopic composition of seawater. This study presents bulk isotopic and elemental analyses from fine‐grained siliciclastics of the Late Devonian–Early Mississippian Bakken Formation (Williston Basin, USA) to assess the volume and isotopic composition of carbonates in these sediments. Carbonate in the Bakken black shales occurs primarily as microscopic disseminated dolomite rhombs and calcite cements that, together, comprise a significant mass‐fraction (ca 9%). The elemental composition of the shales is indicative of a dynamic anoxic to sulphidic palaeoenvironment, likely supported by a fluctuating chemocline. Despite forming in an environment favourable to remineralization of organic matter and the precipitation of isotopically depleted authigenic carbonates, the majority of carbon isotope measurements of disseminated carbonate fall between −3‰ and +3‰, with systematically more depleted carbonates in the deeper‐water portions of the basin. Thus, although there is evidence for a significant total mass‐fraction of carbonate with contribution from remineralized organic matter, Bakken authigenic carbonates suggest that Famennian black shales are unlikely to be sufficiently 13C‐depleted relative to water column dissolved inorganic carbon to serve as a major lever on seawater isotopic composition.