Linda C. Kah

Low marine sulphate and protracted oxygenation of the Proterozoic biosphere

Progressive oxygenation of the Earths early biosphere is thought to have resulted in increased sulphide oxidation during continental weathering, leading to a corresponding increase in marine sulphate concentration. Accurate reconstruction of marine sulphate reservoir size is therefore important for interpreting the oxygenation history of early Earth environments. Few data, however, specifically constrain how sulphate concentrations may have changed during the Proterozoic era (2.5–0.54 Gyr ago). Prior to 2.2 Gyr ago, when oxygen began to accumulate in the Earths atmosphere, sulphate concentrations are inferred to have been <1 mM and possibly <200 µM, on the basis of limited isotopic variability preserved in sedimentary sulphides and experimental data showing suppressed isotopic fractionation at extremely low sulphate concentrations. By 0.8 Gyr ago, oxygen and thus sulphate levels may have risen significantly. Here we report large stratigraphic variations in the sulphur isotope composition of marine carbonate-associated sulphate, and use a rate-dependent model for sulphur isotope change that allows us to track changes in marine sulphate concentrations throughout the Proterozoic. Our calculations indicate sulphate levels between 1.5 and 4.5 mM, or 5–15 per cent of modern values, for more than 1 Gyr after initial oxygenation of the Earths biosphere. Persistence of low oceanic sulphate demonstrates the protracted nature of Earths oxygenation. It links biospheric evolution to temporal patterns in the depositional behaviour of marine iron- and sulphur-bearing minerals, biological cycling of redox-sensitive elements and availability of trace metals essential to eukaryotic development.

Active Microbial Sulfur Disproportionation in the Mesoproterozoic

The environmental expression of sulfur compound disproportionation has been placed between 640 and 1050 million years ago (Ma) and linked to increases in atmospheric oxygen. These arguments have their basis in temporal changes in the magnitude of 34S/32S fractionations between sulfate and sulfide. Here, we present a Proterozoic seawater sulfate isotope record that includes the less abundant sulfur isotope 33S. These measurements imply that sulfur compound disproportionation was an active part of the sulfur cycle by 1300 Ma and that progressive Earth surface oxygenation may have characterized the Mesoproterozoic.

Geochemistry of a 1.2 Ga carbonate-evaporite succession, northern Baffin and Bylot Islands: implications for Mesoproterozoic marine evolution

A 4‰ positive shift in the carbon isotopic composition of the oceans, recorded globally in marine carbonate rocks at ∼1.3 Ga, suggests a significant change in Mesoproterozoic carbon cycling. Enhanced burial fluxes of organic carbon, relative to inorganic carbon, implied by this isotopic shift may have resulted in increased oxygenation of the Earths biosphere, as has been suggested for similar Paleoproterozoic and Neoproterozoic carbon isotope events. This hypothesized Mesoproterozoic oxygenation event may be recorded in the geologic record by the appearance of the oldest preserved, laterally extensive, bedded marine CaSO4 evaporites in the ∼1.2 Ga Grenville and Bylot supergroups. Speculation that the appearance of extensively preserved marine gypsum and/or anhydrite reflects increased biospheric oxygenation has been challenged, however, by the hypothesis that CaSO4 precipitation prior to the Mesoproterozoic may have been inhibited by significantly higher marine carbonate saturation, which would have facilitated carbonate precipitation and effectively limited Ca2+ availability during seawater evaporation (Grotzinger, J.P., 1989. Controls on Carbonate Platform and Basin Development, vol. 44, SEPM, Tulsa, OK, pp. 79–106), regardless of O2 levels. The 1.2 Ga Society Cliffs Formation (Bylot Supergroup, northern Baffin Island) consists of ∼720 m of peritidal carbonates, evaporites, and minor siliciclastic rocks. Evaporites occur predominantly in the lowermost 300 m of the Society Cliffs Formation, where gypsum beds (1–250 cm thick) constitute up to 15% of the exposed strata. Stratigraphic and sedimentologic constraints, as well as isotopic (C, O, Sr) and elemental (Ca, Sr, Na, K, Ba) compositions of evaporites and associated carbonates, indicate a marine origin for Society Cliffs gypsum. An upsection increase in δ34S of Society Cliffs gypsum (from +22‰ to +32‰ VCDT) is therefore interpreted to reflect primary variation in Mesoproterozoic marine sulfate compositions, although the inferred rapidity of isotopic change requires a marine sulfate reservoir significantly smaller than that of the modern ocean. Examination of the maximum fractionation between coeval sulfide and sulfate reservoirs, however, indicates that Mesoproterozoic oceans were not sulfate-limited with respect to bacterial sulfate reduction either before or after the hypothesized 1.3 Ga oxygenation event. Although increased ocean-atmosphere oxygenation may have increased marine sulfate concentrations at this time, the exact role of a Mesoproterozoic oxygenation event cannot be ascertained. Furthermore, high Mg/Ca ratios measured in Society Cliffs gypsum suggest that elevated Mg2+ concentrations in Proterozoic marine systems may have helped sustain carbonate hypersaturation, and that Ca2+-limitation may have played a significant role in the Proterozoic record of evaporite deposition.

Changes in organic matter production and accumulation as a mechanism for isotopic evolution in the Mesoproterozoic ocean

Mesoproterozoic marine successions worldwide record a shift in average delta(13)C values from 0 to +3.5parts per thousand, with the latter value evident in successions younger than 1250 Ma. New carbon isotope data from the similar to 1300 to 1270 Ma Dismal Lakes Group, Arctic Canada, provide further insight into this fundamental transition. Data reveal that the shift to higher VC values was gradual and marked by occasional excursions to values less than 0 parts per thousand. When compared to records from older and younger marine successions, it is evident that the difference between isotopic minima and maxima increased with time, indicating that the marine system evolved to become isotopically more variable. We interpret these patterns to record an increase in the crustal inventory of organic carbon, reflecting eukaryotic diversification and a change in the locus of organic carbon burial to include anoxic deep marine sites where preservation potential was high. We speculate that the release of O-2 to Earths surface environments associated with increased organic carbon storage induced irreversible changes in the Mesoproterozoic biosphere, presaging the more extreme environmental and evolutionary developments of the Neoproterozoic.

Covariance of microfossil assemblages and microbialite textures across an upper Mesoproterozoic carbonate platform

ABSTRACT Early diagenetic chert nodules and beds in the upper Mesoproterozoic Angmaat (formerly Society Cliffs) Formation, Baffin and Bylot islands, preserve microfossils and primary petrofabrics that record microbial mat deposition and lithification across a range of peritidal carbonate environments. Five distinct microfossil assemblages document the distribution of mat-building and mat-dwelling populations across a gradient from restricted, frequently exposed flats to more persistently subaqueous environments. Mats built primarily by thin filamentous or coccoidal cyanobacteria give way to a series of more robust forms that show increasing assemblage diversity with decreasing evidence of subaerial exposure. Distinct fabric elements are associated with each microbial assemblage, and aspects of these petrofabrics are recognizably preserved within unsilicified carbonate in the same beds. These include some features that are distinctly geologic in nature (e.g., seafloor cements) and others that reflect microbial growth and decomposition (e.g., tufted microbialites). A particularly distinctive, micronodular fabric is here interpreted as carbonate infilling of primary voids within microbial mat structures. Such structures mark the co-occurrence of cyanobacterial photosynthesis that produced oxygen gas, filamentous mat builders that imparted the coherence necessary to trap gas bubbles, elevated carbonate saturation required to preserve void fabrics via penecontemporaneous cementation, and a relative paucity of detrital sediment that would have inhibited mat growth. Petrofabrics preserved in Angmaat samples are widespread in upper Paleoproterozoic and Mesoproterozoic carbonate successions but are rare thereafter, perhaps recording, at least in part, the declining carbonate saturation state of seawater. Covariation of microfossil assemblages with petrofabrics in both silicified and unsilicified portions of carbonate beds supports hypotheses that link stromatolite microstructure to the composition and diversity of mat communities.

Deep-water microbialites of the Mesoproterozoic Dismal Lakes Group: microbial growth, lithification, and implications for coniform stromatolites

Offshore facies of the Mesoproterozoic Sulky Formation, Dismal Lakes Group, arctic Canada, preserve microbialites with unusual morphology. These microbialites grew in water depths greater than several tens of meters and correlate with high-relief conical stromatolites of the more proximal September Lake reef complex. The gross morphology of these microbial facies consists of ridge-like vertical supports draped by concave-upward, subhorizontal elements, resulting in tent-shaped cuspate microbialites with substantial primary void space. Morphological and petrographic analyses suggest a model wherein penecontemporaneous upward growth of ridge elements and development of subhorizontal draping elements initially resulted in a buoyantly supported, unlithified microbial form. Lithification began via precipitation within organic elements during microbialite growth. Mineralization either stabilized or facilitated collapse of initially neutrally buoyant microbialite forms. Microbial structures and breccias were then further stabilized by precipitation of marine herringbone cement. During late-stage diagenesis, remaining void space was occluded by ferroan dolomite cement. Cuspate microbialites are most similar to those found in offshore facies of Neoarchean carbonate platforms and to unlithified, buoyantly supported microbial mats in modern ice-covered Antarctic lakes. We suggest that such unusual microbialite morphologies are a product of the interaction between motile and non-motile communities under nutrient-limiting conditions, followed by early lithification, which served to preserve the resultant microbial form. The presence of marine herringbone cement, commonly associated with high dissolved inorganic carbon (DIC), low O2 conditions, also suggests growth in association with reducing environments at or near the seafloor or in conjunction with a geochemical interface. Predominance of coniform stromatolite forms in the Proterozoic--across a variety of depositional environments--may thus reflect a combination of heterogeneous nutrient distribution, potentially driven by variable redox conditions, and an elevated carbonate saturation state, which permits preservation of these unusual microbialite forms.

Microbialites in a high-altitude Andean lake: multiple controls on carbonate precipitation and lamina accretion

ABSTRACT Microbialites comprise the mineralized record of early life on Earth and preserve a spectrum of fabrics that reflect complex physical, chemical, and biological interactions. The relatively rarity of microbialites in modern environments, however, challenges our interpretation of ancient structures. Here we report the occurrence of microbial mats, mineral precipitates, and oncoids in the Laguna Negra, a high-altitude hypersaline Andean lake in Catamarca Province, Argentina. Laguna Negra is a Ca-Na-Cl brine where abundant carbonate precipitation takes place. Extreme environmental conditions, including high UV radiation, elevated salinity, and temperature extremes, restrict multicellular life so that mineralization reflects a combination of local hydrologic conditions, lake geochemistry, and microbial activity. The resulting carbonates consist of micritic laminae, botryoidal cement fans, and isopachous cement laminae that are strikingly similar to those observed in Proterozoic stromatolites, providing insight into mechanisms of mineralization. Here, increased saturation with respect to carbonate minerals reflects mixing of spring-fed inlets and lake waters, favoring microbialite formation and preservation. This highlights the importance of hydrological mixing zones in microbialite formation and as taphonomic windows to record microbial activity. Recent discoveries of minerals related to evaporating playa-lake systems on Mars further highlights the potential of Laguna Negra to provide critical insight into biosignature preservation in both terrestrial and extraterrestrial settings.

Proterozoic microbial mats and their constraints on environments of silicification

The occurrence of microfossiliferous, early diagenetic chert in Proterozoic successions is broadly restricted to peritidal marine environments. Such coastal environments are amongst the most environmentally variable of marine environments, experiencing both enhanced evaporation and potential influx of terrestrial freshwaters. To better understand potential conditions under which silicification occurs, we focus on microfossiliferous early diagenetic chert from the Mesoproterozoic Bylot Supergroup, northern Baffin Island. Spectacular preservation of silicified microbial mats, their associated mineral phases, and the petrographic fabrics of the chert itself require that silicification occurred at the sediment-water interface, penecontemporaneously with mat growth. In some cases, silica is the primary precipitated mineral phase and is not associated with replacement of precursor mineral phases. In other cases, silica deposition includes the mimetic replacement of carbonate, gypsum, and halite mineral phases. These petrographic constraints suggest that silicification potentially occurred under a range of fluid chemistries associated with environmental variability in nearshore peritidal environments. Here we provide the first direct thermodynamic modeling of hypothetical Proterozoic seawater solutions, seawater-derived brines, and mixed seawater-freshwater solutions, and demonstrate that peritidal environments are capable of providing a wide range of fluid chemistries under which early diagenetic silica can both precipitate and replace primary mineralogical phases. Despite the thermodynamic potential for silica deposition under a wide range of fluid compositions, chert is not ubiquitous in Proterozoic nearshore environments, suggesting that the kinetics of silica polymerization exert a primary control over deposition.

Mercury isotope signatures record photic zone euxinia in the Mesoproterozoic ocean

Significance The occurrence of toxic H2S-rich water in the photic zone of the ocean, a phenomenon called photic zone euxinia (PZE), had profound negative impacts on ancient biological evolution and modern marine ecosystems. We explore the possibility of using mercury (Hg) stable isotopes as a new proxy for PZE. We found that Hg-isotope compositions in sedimentary rocks deposited under oxic versus H2S-rich conditions are distinguishable. The difference in Hg isotopes is most likely caused by contrasting Hg chemical behavior in response to changes in surface ocean redox states. Thus, our data demonstrate that Hg isotopes in marine sediments are a promising proxy of PZE, useful in future studies to refine our understanding on PZE and its impact on life. Photic zone euxinia (PZE) is a condition where anoxic, H2S-rich waters occur in the photic zone (PZ). PZE has been invoked as an impediment to the evolution of complex life on early Earth and as a kill mechanism for Phanerozoic mass extinctions. Here, we investigate the potential application of mercury (Hg) stable isotopes in marine sedimentary rocks as a proxy for PZE by measuring Hg isotope compositions in late Mesoproterozoic (∼1.1 Ga) shales that have independent evidence of PZE during discrete intervals. Strikingly, a significantly negative shift of Hg mass-independent isotope fractionation (MIF) was observed during euxinic intervals, suggesting changes in Hg sources or transformations in oceans coincident with the development of PZE. We propose that the negative shift of Hg MIF was most likely caused by (i) photoreduction of Hg(II) complexed by reduced sulfur ligands in a sulfide-rich PZ, and (ii) enhanced sequestration of atmospheric Hg(0) to the sediments by thiols and sulfide that were enriched in the surface ocean as a result of PZE. This study thus demonstrates that Hg isotope compositions in ancient marine sedimentary rocks can be a promising proxy for PZE and therefore may provide valuable insights into changes in ocean chemistry and its impact on the evolution of life.