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1. Patterns of local and global redox variability during the Cenomanian–Turonian Boundary Event (Oceanic Anoxic Event 2) recorded in carbonates and shales from central Italy

   https://doi.org/10.1111/sed.12352  

   Owens, Jeremy D. ; Lyons, Timothy W. ; Hardisty, Dalton S. ; Lowery, Chris M. ; Lu, Zunli ; Lee, Bridget ; Jenkyns, Hugh C. ; Heimhofer, ed., Ulrich ( January 2017 , Sedimentology)

   Careful evaluation of the local geochemical conditions in past marine settings can provide a window to the average redox state of the global ocean during episodes of extensive organic carbon deposition. These comparisons aid in identifying the interplay between climate and biotic feedbacks contributing to and resulting from these events. Well‐documented examples are known from the Mesozoic Era, which is characterized by episodes of widespread organic carbon deposition known as Oceanic Anoxic Events. This organic carbon burial typically leads to coeval positive carbon‐isotope excursions. Geochemical data are presented here for several palaeoredox proxies (Cr/Ti, V, Mo, Zn, Mn, Fe speciation, I/Ca and sulphur isotopes) from a section exposed at Furlo in the Marche–Umbrian Apennines of Italy that spans the Cenomanian–Turonian boundary. Here, Oceanic Anoxic Event 2 is represented by aca1 m thick radiolarian‐rich millimetre‐laminated organic‐rich shale known locally as the Bonarelli Level. Iron speciation data for thin organic‐rich intervals observed below the Bonarelli Level imply a local redox shift going into the Oceanic Anoxic Event, with ferruginous conditions (i.e. anoxic with dissolved ferrous iron) transiently developed prior to the event and euxinia (i.e. anoxic and sulphidic bottom waters) throughout the event itself. Pre‐Oceanic Anoxic Event enrichments of elements sensitive to anoxic water columns were due to initial development of locally ferruginous bottom waters as a precursor to the event. However, the greater global expanse of dysoxic to euxinic conditions during the Oceanic Anoxic Event greatly reduced redox‐sensitive trace‐metal concentrations in seawater. Pyrite sulphur isotopes document a positive excursion during the Oceanic Anoxic Event. Carbonate I/Ca ratios were generally low, suggesting locally reduced bottom‐water oxygen conditions preceding the event and relatively increased oxygen concentrations post‐event. Combined, the Furlo geochemical data suggest a redox‐stratified water column with oxic surface waters and a shallow chemocline overlying locally ferruginous bottom waters preceding the event, globally widespread euxinic bottom waters during the Oceanic Anoxic Event, followed by chemocline shallowing but sustained local redox stratification following the event.

    

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2. Associations between redox‐sensitive trace metals and microbial communities in a Proterozoic ocean analogue

   https://doi.org/10.1111/gbi.12388  

   Rico, Kathryn I. ; Sheldon, Nathan D. ; Kinsman‐Costello, Lauren E. ( March 2020 , Geobiology)

   Constraints on Precambrian ocean chemistry are dependent upon sediment geochemistry. However, diagenesis and metamorphism can destroy primary biosignatures, making it difficult to consider biology when interpreting geochemical data. Modern analogues for ancient ecosystems can be useful tools for identifying how sediment geochemistry records an active biosphere. The Middle Island Sinkhole (MIS) in Lake Huron is an analogue for shallow Proterozoic waters due to its low oxygen water chemistry and microbial communities that exhibit diverse metabolic functions at the sediment–water interface. This study uses sediment trace metal contents and microbial abundances in MIS sediments and an oxygenated Lake Huron control site (LH) to infer mechanisms for trace metal burial. The adsorption of trace metals to Mn‐oxyhydroxides is a critical burial pathway for metals in oxic LH sediments, but not for the MIS mat and sediments, consistent with conventional understanding of Mn cycling. Micronutrient trace metals (e.g., Zn) are associated with organic matter regardless of oxygen and sulfide availability. Although U and V are conventionally considered to be organically complexed in suboxic and anoxic conditions, U and organic covary in oxic LH sediments, and Mn‐oxyhydroxide cycling dominates V deposition in the anoxic MIS sediments. Significant correlations between Mo and organic matter across all redox regimes have major implications for our interpretations of Mo isotope systematics in the geologic record. Finally, while microbial groups vary between the sampling locales (e.g., the cyanobacteria in the MIS microbial mat are not present in LH sediments), LH and MIS ultimately have similar relationships between microbial assemblages and metal burial, making it difficult to link trace metal burial to microbial metabolisms. Together, these results indicate that bulk sediment trace metal composition does not capture microbiological processes; more robust trace metal geochemistry such as isotopes and speciation may be critical for understanding the intersections between microbiology and sediment geochemistry.

    

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3. Holocene Spatiotemporal Redox Variations in the Southern Baltic Sea

   https://doi.org/10.3389/feart.2021.671401  

   Hardisty, Dalton S. ; Riedinger, Natascha ; Planavsky, Noah J. ; Asael, Dan ; Bates, Steven M. ; Lyons, Timothy W. ( May 2021 , Frontiers in Earth Science)

   null (Ed.)

   Low oxygen conditions in the modern Baltic Sea are exacerbated by human activities; however, anoxic conditions also prevailed naturally over the Holocene. Few studies have characterized the specific paleoredox conditions (manganous, ferruginous, euxinic) and their frequency in southern Baltic sub-basins during these ancient events. Here, we apply a suite of isotope systems (Fe, Mo, S) and associated elemental proxies (e.g., Fe speciation, Mn) to specifically define water column redox regimes through the Baltic Holocene in a sill-proximal to sill-distal transect (Lille Belt, Bornholm Basin, Landsort Deep) using samples collected during the Integrated Ocean Drilling Program Expedition 347. At the sill-proximal Lille Belt, there is evidence for anoxic manganous/ferruginous conditions for most of the cored interval following the transition from the Ancylus Lake to Littorina Sea but with no clear excursion to more reducing or euxinic conditions associated with the Holocene Thermal Maximum (HTM) or Medieval Climate Anomaly (MCA) events. At the sill-distal southern sub-basin, Bornholm Basin, a combination of Fe speciation, pore water Fe, and solid phase Mo concentration and isotope data point to manganous/ferruginous conditions during the Ancylus Lake-to-Littorina Sea transition and HTM but with only brief excursions to intermittently or weakly euxinic conditions during this interval. At the western Baltic Proper sub-basin, Landsort Deep, new Fe and S isotope data bolster previous Mo isotope records and Fe speciation evidence for two distinct anoxic periods but also suggest that sulfide accumulation beyond transient levels was largely restricted to the sediment-water interface. Ultimately, the combined data from all three locations indicate that Fe enrichments typically indicative of euxinia may be best explained by Fe deposition as oxides following events likely analogous to the periodic incursions of oxygenated North Sea waters observed today, with subsequent pyrite formation in sulfidic pore waters. Additionally, the Mo isotope data from multiple Baltic Sea southern basins argue against restricted and widespread euxinic conditions, as has been demonstrated in the Baltic Proper and Bothnian Sea during the HTM or MCA. Instead, similar to today, each past Baltic anoxic event is characterized by redox conditions that become progressively more reducing with increasing distance from the sill. 

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4. Microbial Diversity and Sulfur Cycling in an Early Earth Analogue: From Ancient Novelty to Modern Commonality

   https://doi.org/10.1128/mbio.00016-22  

   Hahn, C. Ryan ; Farag, Ibrahim F. ; Murphy, Chelsea L. ; Podar, Mircea ; Elshahed, Mostafa S. ; Youssef, Noha H. ( April 2022 , mBio)

   Sousa, Filipa L. ; Schleper, Christa M. (Ed.)

   ABSTRACT Life emerged and diversified in the absence of molecular oxygen. The prevailing anoxia and unique sulfur chemistry in the Paleo-, Meso-, and Neoarchean and early Proterozoic eras may have supported microbial communities that differ from those currently thriving on the earth’s surface. Zodletone spring in southwestern Oklahoma represents a unique habitat where spatial sampling could substitute for geological eras namely, from the anoxic, surficial light-exposed sediments simulating a preoxygenated earth to overlaid water column where air exposure simulates oxygen intrusion during the Neoproterozoic era. We document a remarkably diverse microbial community in the anoxic spring sediments, with 340/516 (65.89%) of genomes recovered in a metagenomic survey belonging to 200 bacterial and archaeal families that were either previously undescribed or that exhibit an extremely rare distribution on the current earth. Such diversity is underpinned by the widespread occurrence of sulfite, thiosulfate, tetrathionate, and sulfur reduction and the paucity of sulfate reduction machineries in these taxa. Hence, these processes greatly expand lineages mediating reductive sulfur-cycling processes in the tree of life. An analysis of the overlaying oxygenated water community demonstrated the development of a significantly less diverse community dominated by well-characterized lineages and a prevalence of oxidative sulfur-cycling processes. Such a transition from ancient novelty to modern commonality underscores the profound impact of the great oxygenation event on the earth’s surficial anoxic community. It also suggests that novel and rare lineages encountered in current anaerobic habitats could represent taxa that once thrived in an anoxic earth but have failed to adapt to earth’s progressive oxygenation. IMPORTANCE Life on earth evolved in an anoxic setting; however, the identity and fate of microorganisms that thrived in a preoxygenated earth are poorly understood. In Zodletone spring, the prevailing geochemical conditions are remarkably similar to conditions prevailing in surficial earth prior to oxygen buildup in the atmosphere. We identify hundreds of previously unknown microbial lineages in the spring and demonstrate that these lineages possess the metabolic machinery to mediate a wide range of reductive sulfur processes, with the capacity to respire sulfite, thiosulfate, sulfur, and tetrathionate, rather than sulfate, which is a reflection of the differences in sulfur-cycling chemistry in ancient versus modern times. Collectively, such patterns strongly suggest that microbial diversity and sulfur-cycling processes in a preoxygenated earth were drastically different from the currently observed patterns and that the Great Oxygenation Event has precipitated the near extinction of a wide range of oxygen-sensitive lineages and significantly altered the microbial reductive sulfur-cycling community on earth. 

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5. A geochemical study of the Ediacaran discoidal fossil Aspidella preserved in limestones: Implications for its taphonomy and paleoecology

   https://doi.org/10.1111/gbi.12240  

   Bykova, N. ; Gill, B. C. ; Grazhdankin, D. ; Rogov, V. ; Xiao, S. ( April 2017 , Geobiology)

   The Ediacara biota features the rise of macroscopic complex life immediately before the Cambrian explosion. One of the most abundant and widely distributed elements of the Ediacara biota is the discoidal fossilAspidella, which is interpreted as a subsurface holdfast possibly anchoring a frondose epibenthic organism. It is a morphologically simple fossil preserved mainly in siliciclastic rocks, which are unsuitable for comprehensive stable isotope geochemical analyses to decipher its taphonomy and paleoecology. In this regard, three‐dimensionally preservedAspidellafossils from upper Ediacaran limestones of the Khatyspyt Formation in the Olenek Uplift of northern Siberia offer a rare opportunity to leverage geochemistry for insights into their taphonomy and paleoecology. To take advantage of this opportunity, we analyzed δ¹³C_carb, δ¹⁸O_carb, δ¹³C_org, δ³⁴S_pyr, and iron speciation of the KhatyspytAspidellafossils and surrounding sediment matrix in order to investigate whether they hosted microbial symbionts, how they were fossilized, and the redox conditions of their ecological environments.Aspidellaholdfasts and surrounding sediment matrix show indistinguishable δ¹³C_orgvalues, suggesting they did not host and derive significant amount of nutrients from microbial symbionts such as methanogens, methylotrophs, or sulfide‐oxidizing bacteria. δ¹³C_carb, δ¹⁸O_carb, and δ³⁴S_pyrdata, along with petrographic observations, suggest that microbial sulfate reduction facilitated the preservation ofAspidellaby promoting early authigenic calcite cementation in the holdfasts before matrix cementation and sediment compaction. Iron speciation data are equivocal, largely because of the low total iron concentrations. However, consideration of published sulfur isotope and biomarker data suggests thatAspidellalikely lived in non‐euxinic waters. It is possible thatAspidellawas an opportunistic organism, colonizing the seafloor in large numbers when paleoenvironments were favorable. This study demonstrates that geochemical data of Ediacaran fossils preserved in limestones can offer important insights into the taphonomy and paleoecology of these enigmatic organisms living on the eve of the Cambrian explosion.

    

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