Despite a surge of recent work, the evolution of mid‐Proterozoic oceanic–atmospheric redox remains heavily debated. Constraining the dynamics of Proterozoic redox evolution is essential to determine the role, if any, that anoxia played in protracting the development of eukaryotic diversity. We present a multiproxy suite of high‐resolution geochemical measurements from a drill core capturing the ~1.4 Ga Xiamaling Formation, North China Craton. Specifically, we analyzed major and trace element concentrations, sulfur and molybdenum isotopes, and iron speciation not only to better understand the local redox conditions but also to establish how relevant our data are to understanding the contemporaneous global ocean. Our results suggest that throughout deposition of the Xiamaling Formation, the basin experienced varying degrees of isolation from the global ocean. During deposition of the lower organic‐rich shales (130–85 m depth), the basin was extremely restricted, and the reservoirs of sulfate and trace metals were drawn down almost completely. Above a depth of 85 m, shales were deposited in dominantly euxinic waters that more closely resembled a marine system and thus potentially bear signatures of coeval seawater. In the most highly enriched sample from this upper interval, the concentration of molybdenum is 51 ppm with a δ98Mo value of +1.7‰. Concentrations of Mo and other redox‐sensitive elements in our samples are consistent with a deep ocean that was largely anoxic on a global scale. Our maximum δ98Mo value, in contrast, is high compared to published mid‐Proterozoic data. This high value raises the possibility that the Earth's surface environments were transiently more oxygenated at ~1.4 Ga compared to preceding or postdating times. More broadly, this study demonstrates the importance of integrating all available data when attempting to reconstruct surface O2dynamics based on rocks of any age.
By about 2.0 billion years ago (Ga), there is evidence for a period best known for its extended, apparent geochemical stability expressed famously in the carbonate–carbon isotope data. Despite the first appearance and early innovation among eukaryotic organisms, this period is also known for a rarity of eukaryotic fossils and an absence of organic biomarker fingerprints for those organisms, suggesting low diversity and relatively small populations compared to the Neoproterozoic era. Nevertheless, the search for diagnostic biomarkers has not been performed with guidance from paleoenvironmental redox constrains from inorganic geochemistry that should reveal the facies that were most likely hospitable to these organisms. Siltstones and shales obtained from drill core of the ca. 1.3–1.4 Ga Roper Group from the McArthur Basin of northern Australia provide one of our best windows into the mid‐Proterozoic redox landscape. The group is well dated and minimally metamorphosed (of
- NSF-PAR ID:
- 10375273
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Geobiology
- Volume:
- 17
- Issue:
- 3
- ISSN:
- 1472-4677
- Page Range / eLocation ID:
- p. 247-260
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
Terrestrial environments have been suggested as an oxic haven for eukaryotic life and diversification during portions of the Proterozoic Eon when the ocean was dominantly anoxic. However, iron speciation and Fe/Al data from the ca. 1.1-billion-year-old Nonesuch Formation, deposited in a large lake and bearing a diverse assemblage of early eukaryotes, are interpreted to indicate persistently anoxic conditions. To shed light on these distinct hypotheses, we analyzed two drill cores spanning the transgression into the lake and its subsequent shallowing. While the proportion of highly reactive to total iron (Fe HR /Fe T ) is consistent through the sediments and typically in the range taken to be equivocal between anoxic and oxic conditions, magnetic experiments and petrographic data reveal that iron exists in three distinct mineral assemblages resulting from an oxycline. In the deepest waters, reductive dissolution of iron oxides records an anoxic environment. However, the remainder of the sedimentary succession has iron oxide assemblages indicative of an oxygenated environment. At intermediate water depths, a mixed-phase facies with hematite and magnetite indicates low oxygen conditions. In the shallowest waters of the lake, nearly every iron oxide has been oxidized to its most oxidized form, hematite. Combining magnetics and textural analyses results in a more nuanced understanding of ambiguous geochemical signals and indicates that for much of its temporal duration, and throughout much of its water column, there was oxygen in the waters of Paleolake Nonesuch.more » « less
-
more » « less
Abstract 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.
-
more » « less
Abstract The potent greenhouse gas nitrous oxide (N2O) may have been an important constituent of Earth's atmosphere during Proterozoic (~2.5–0.5 Ga). Here, we tested the hypothesis that chemodenitrification, the rapid reduction of nitric oxide by ferrous iron, would have enhanced the flux of N2O from ferruginous Proterozoic seas. We empirically derived a rate law,
, and measured an isotopic site preference of +16‰ for the reaction. Using this empirical rate law, and integrating across an oceanwide oxycline, we found that low nM NO and μM‐lowmM Fe2+concentrations could have sustained a sea‐air flux of 100–200 Tg N2O–N year−1, if N2fixation rates were near‐modern and all fixed N2was emitted as N2O. A 1D photochemical model was used to obtain steady‐state atmospheric N2O concentrations as a function of sea‐air N2O flux across the wide range of possiblep OPAL ). At 100–200 Tg N2O–N year−1and >0.1PAL O2, this model yielded low‐ppmv N2O, which would produce several degrees of greenhouse warming at 1.6 ppmvCH 4and 320 ppmvCO 2. These results suggest that enhanced N2O production in ferruginous seawater via a previously unconsidered chemodenitrification pathway may have helped to fill a Proterozoic “greenhouse gap,” reconciling an ice‐free Mesoproterozoic Earth with a less luminous early Sun. A particularly notable result was that high N2O fluxes at intermediate O2concentrations (0.01–0.1PAL ) would have enhanced ozone screening of solarUV radiation. Due to rapid photolysis in the absence of an ozone shield, N2O is unlikely to have been an important greenhouse gas if Mesoproterozoic O2was 0.001PAL . At low O2, N2O might have played a more important role as life's primary terminal electron acceptor during the transition from an anoxic to oxic surface Earth, and correspondingly, from anaerobic to aerobic metabolisms. -
more » « less
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.
