Publications by authors named "Christopher T Reinhard"

40 Publications

Bistability in the redox chemistry of sediments and oceans.

Proc Natl Acad Sci U S A 2020 12 14;117(52):33043-33050. Epub 2020 Dec 14.

Department of Biology, University of Antwerp, 2160 Wilrijk, Belgium;

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.
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http://dx.doi.org/10.1073/pnas.2008235117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7776822PMC
December 2020

Novel insights into the taxonomic diversity and molecular mechanisms of bacterial Mn(III) reduction.

Environ Microbiol Rep 2020 10 16;12(5):583-593. Epub 2020 Aug 16.

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA.

Soluble ligand-bound Mn(III) can support anaerobic microbial respiration in diverse aquatic environments. Thus far, Mn(III) reduction has only been associated with certain Gammaproteobacteria. Here, we characterized microbial communities enriched from Mn-replete sediments of Lake Matano, Indonesia. Our results provide the first evidence for the biological reduction of soluble Mn(III) outside the Gammaproteobacteria. Metagenome assembly and binning revealed a novel betaproteobacterium, which we designate 'Candidatus Dechloromonas occultata.' This organism dominated the enrichment and expressed a porin-cytochrome c complex typically associated with iron-oxidizing Betaproteobacteria and a novel cytochrome c-rich protein cluster (Occ), including an undecaheme putatively involved in extracellular electron transfer. This occ gene cluster was also detected in diverse aquatic bacteria, including uncultivated Betaproteobacteria from the deep subsurface. These observations provide new insight into the taxonomic and functional diversity of microbially driven Mn(III) reduction in natural environments.
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http://dx.doi.org/10.1111/1758-2229.12867DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7775658PMC
October 2020

Publisher Correction: The role of calcium in regulating marine phosphorus burial and atmospheric oxygenation.

Nat Commun 2020 06 11;11(1):3046. Epub 2020 Jun 11.

Department of Geology and Geophysics, Yale University, 210 Whitney Ave, New Haven, CT, 06511, USA.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.
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http://dx.doi.org/10.1038/s41467-020-16793-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7289855PMC
June 2020

The role of calcium in regulating marine phosphorus burial and atmospheric oxygenation.

Nat Commun 2020 05 6;11(1):2232. Epub 2020 May 6.

Department of Geology and Geophysics, Yale University, 210 Whitney Ave, New Haven, CT, 06511, USA.

The marine phosphorus cycle plays a critical role in controlling the extent of global primary productivity and thus atmospheric pO on geologic time scales. However, previous attempts to model carbon-phosphorus-oxygen feedbacks have neglected key parameters that could shape the global P cycle. Here we present new diagenetic models to fully parameterize marine P burial. We have also coupled this diagenetic framework to a global carbon cycle model. We find that seawater calcium concentration, by strongly influencing carbonate fluorapatite (CFA) formation, is a key factor controlling global phosphorus cycling, and therefore plays a critical role in shaping the global oxygen cycle. A compilation of Cenozoic deep-sea sedimentary phosphorus speciation data provides empirical support for the idea that CFA formation is strongly influenced by marine Ca concentrations. Therefore, we propose a previously overlooked coupling between Phanerozoic tectonic cycles, the major-element composition of seawater, the marine phosphorus cycle, and atmospheric pO.
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http://dx.doi.org/10.1038/s41467-020-15673-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7203231PMC
May 2020

Large Mass-Independent Oxygen Isotope Fractionations in Mid-Proterozoic Sediments: Evidence for a Low-Oxygen Atmosphere?

Astrobiology 2020 05 31;20(5):628-636. Epub 2020 Mar 31.

Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, Israel.

Earth's ocean-atmosphere system has undergone a dramatic but protracted increase in oxygen (O) abundance. This environmental transition ultimately paved the way for the rise of multicellular life and provides a blueprint for how a biosphere can transform a planetary surface. However, estimates of atmospheric oxygen levels for large intervals of Earth's history still vary by orders of magnitude-foremost for Earth's middle history. Historically, estimates of mid-Proterozoic (1.9-0.8 Ga) atmospheric oxygen levels are inferred based on the kinetics of reactions occurring in soils or in the oceans, rather than being directly tracked by atmospheric signatures. Rare oxygen isotope systematics-based on quantifying the rare oxygen isotope O in addition to the conventionally determined O and O-provide a means to track atmospheric isotopic signatures and thus potentially provide more direct estimates of atmospheric oxygen levels through time. Oxygen isotope signatures that deviate strongly from the expected mass-dependent relationship between O, O, and O develop during ozone formation, and these "mass-independent" signals can be transferred to the rock record during oxidation reactions in surface environments that involve atmospheric O. The magnitude of these signals is dependent upon O, CO, and the overall extent of biospheric productivity. Here, we use a stochastic approach to invert the mid-Proterozoic ΔO record for a new estimate of atmospheric O, leveraging explicit coupling of O and biospheric productivity in a biogeochemical Earth system model to refine the range of atmospheric O values that is consistent with a given observed ΔO. Using this approach, we find new evidence that atmospheric oxygen levels were less than ∼1% of the present atmospheric level (PAL) for at least some intervals of the Proterozoic Eon.
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http://dx.doi.org/10.1089/ast.2019.2060DOI Listing
May 2020

On the co-evolution of surface oxygen levels and animals.

Geobiology 2020 05 16;18(3):260-281. Epub 2020 Mar 16.

Department of Geology and Geophysics, Yale University, New Haven, Connecticut.

Few topics in geobiology have been as extensively debated as the role of Earth's oxygenation in controlling when and why animals emerged and diversified. All currently described animals require oxygen for at least a portion of their life cycle. Therefore, the transition to an oxygenated planet was a prerequisite for the emergence of animals. Yet, our understanding of Earth's oxygenation and the environmental requirements of animal habitability and ecological success is currently limited; estimates for the timing of the appearance of environments sufficiently oxygenated to support ecologically stable populations of animals span a wide range, from billions of years to only a few million years before animals appear in the fossil record. In this light, the extent to which oxygen played an important role in controlling when animals appeared remains a topic of debate. When animals originated and when they diversified are separate questions, meaning either one or both of these phenomena could have been decoupled from oxygenation. Here, we present views from across this interpretive spectrum-in a point-counterpoint format-regarding crucial aspects of the potential links between animals and surface oxygen levels. We highlight areas where the standard discourse on this topic requires a change of course and note that several traditional arguments in this "life versus environment" debate are poorly founded. We also identify a clear need for basic research across a range of fields to disentangle the relationships between oxygen availability and emergence and diversification of animal life.
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http://dx.doi.org/10.1111/gbi.12382DOI Listing
May 2020

The impact of marine nutrient abundance on early eukaryotic ecosystems.

Geobiology 2020 03 17;18(2):139-151. Epub 2020 Feb 17.

NASA Astrobiology Institute, Alternative Earths Team, Riverside, California.

The rise of eukaryotes to ecological prominence represents one of the most dramatic shifts in the history of Earth's biosphere. However, there is an enigmatic temporal lag between the emergence of eukaryotic organisms in the fossil record and their much later ecological expansion. In parallel, there is evidence for a secular increase in the availability of the key macronutrient phosphorus (P) in Earth's oceans. Here, we use an Earth system model equipped with a size-structured marine ecosystem to explore relationships between plankton size, trophic complexity, and the availability of marine nutrients. We find a strong dependence of planktonic ecosystem structure on ocean nutrient abundance, with a larger ocean nutrient inventory leading to greater overall biomass, broader size spectra, and increasing abundance of large Zooplankton. If existing estimates of Proterozoic marine nutrient levels are correct, our results suggest that increases in the ecological impact of eukaryotic algae and trophic complexity in eukaryotic ecosystems were directly linked to restructuring of the global P cycle associated with the protracted rise of surface oxygen levels. Our results thus suggest an indirect but potentially important mechanism by which ocean oxygenation may have acted to shape marine ecological function during late Proterozoic time.
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http://dx.doi.org/10.1111/gbi.12384DOI Listing
March 2020

Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans.

Sci Adv 2019 11 27;5(11):eaav2869. Epub 2019 Nov 27.

Departments of Microbiology and Immunology and Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, Canada.

Banded iron formation (BIF) deposition was the likely result of oxidation of ferrous iron in seawater by either oxygenic photosynthesis or iron-dependent anoxygenic photosynthesis-photoferrotrophy. BIF deposition, however, remains enigmatic because the photosynthetic biomass produced during iron oxidation is conspicuously absent from BIFs. We have addressed this enigma through experiments with photosynthetic bacteria and modeling of biogeochemical cycling in the Archean oceans. Our experiments reveal that, in the presence of silica, photoferrotroph cell surfaces repel iron (oxyhydr)oxides. In silica-rich Precambrian seawater, this repulsion would separate biomass from ferric iron and would lead to large-scale deposition of BIFs lean in organic matter. Excess biomass not deposited with BIF would have deposited in coastal sediments, formed organic-rich shales, and fueled microbial methanogenesis. As a result, the deposition of BIFs by photoferrotrophs would have contributed fluxes of methane to the atmosphere and thus helped to stabilize Earth's climate under a dim early Sun.
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http://dx.doi.org/10.1126/sciadv.aav2869DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6881150PMC
November 2019

Ecosystem-bedrock interaction changes nutrient compartmentalization during early oxidative weathering.

Sci Rep 2019 10 18;9(1):15006. Epub 2019 Oct 18.

Biosphere 2, The University of Arizona, Tucson, AZ, USA.

Ecosystem-bedrock interactions power the biogeochemical cycles of Earth's shallow crust, supporting life, stimulating substrate transformation, and spurring evolutionary innovation. While oxidative processes have dominated half of terrestrial history, the relative contribution of the biosphere and its chemical fingerprints on Earth's developing regolith are still poorly constrained. Here, we report results from a two-year incipient weathering experiment. We found that the mass release and compartmentalization of major elements during weathering of granite, rhyolite, schist and basalt was rock-specific and regulated by ecosystem components. A tight interplay between physiological needs of different biota, mineral dissolution rates, and substrate nutrient availability resulted in intricate elemental distribution patterns. Biota accelerated CO mineralization over abiotic controls as ecosystem complexity increased, and significantly modified the stoichiometry of mobilized elements. Microbial and fungal components inhibited element leaching (23.4% and 7%), while plants increased leaching and biomass retention by 63.4%. All biota left comparable biosignatures in the dissolved weathering products. Nevertheless, the magnitude and allocation of weathered fractions under abiotic and biotic treatments provide quantitative evidence for the role of major biosphere components in the evolution of upper continental crust, presenting critical information for large-scale biogeochemical models and for the search for stable in situ biosignatures beyond Earth.
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http://dx.doi.org/10.1038/s41598-019-51274-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6800431PMC
October 2019

A paleosol record of the evolution of Cr redox cycling and evidence for an increase in atmospheric oxygen during the Neoproterozoic.

Geobiology 2019 11 22;17(6):579-593. Epub 2019 Aug 22.

Department of Geology and Geophysics, Yale University, New Haven, CT, USA.

Atmospheric oxygen levels control the oxidative side of key biogeochemical cycles and place limits on the development of high-energy metabolisms. Understanding Earth's oxygenation is thus critical to developing a clearer picture of Earth's long-term evolution. However, there is currently vigorous debate about even basic aspects of the timing and pattern of the rise of oxygen. Chemical weathering in the terrestrial environment occurs in contact with the atmosphere, making paleosols potentially ideal archives to track the history of atmospheric O levels. Here we present stable chromium isotope data from multiple paleosols that offer snapshots of Earth surface conditions over the last three billion years. The results indicate a secular shift in the oxidative capacity of Earth's surface in the Neoproterozoic and suggest low atmospheric oxygen levels (<1% PAL pO ) through the majority of Earth's history. The paleosol record also shows that localized Cr oxidation may have begun as early as the Archean, but efficient, modern-like transport of hexavalent Cr under an O -rich atmosphere did not become common until the Neoproterozoic.
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http://dx.doi.org/10.1111/gbi.12360DOI Listing
November 2019

Anoxygenic photosynthesis and the delayed oxygenation of Earth's atmosphere.

Nat Commun 2019 07 9;10(1):3026. Epub 2019 Jul 9.

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332, USA.

The emergence of oxygenic photosynthesis created a new niche with dramatic potential to transform energy flow through Earth's biosphere. However, more primitive forms of photosynthesis that fix CO into biomass using electrons from reduced species like Fe(II) and H instead of water would have competed with Earth's early oxygenic biosphere for essential nutrients. Here, we combine experimental microbiology, genomic analyses, and Earth system modeling to demonstrate that competition for light and nutrients in the surface ocean between oxygenic phototrophs and Fe(II)-oxidizing, anoxygenic photosynthesizers (photoferrotrophs) translates into diminished global photosynthetic O release when the ocean interior is Fe(II)-rich. These results provide a simple ecophysiological mechanism for inhibiting atmospheric oxygenation during Earth's early history. We also find a novel positive feedback within the coupled C-P-O-Fe cycles that can lead to runaway planetary oxygenation as rising atmospheric pO sweeps the deep ocean of the ferrous iron substrate for photoferrotrophy.
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http://dx.doi.org/10.1038/s41467-019-10872-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6616575PMC
July 2019

A sluggish mid-Proterozoic biosphere and its effect on Earth's redox balance.

Geobiology 2019 01 3;17(1):3-11. Epub 2018 Oct 3.

Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Japan.

The possibility of low but nontrivial atmospheric oxygen (O ) levels during the mid-Proterozoic (between 1.8 and 0.8 billion years ago, Ga) has important ramifications for understanding Earth's O cycle, the evolution of complex life and evolving climate stability. However, the regulatory mechanisms and redox fluxes required to stabilize these O levels in the face of continued biological oxygen production remain uncertain. Here, we develop a biogeochemical model of the C-N-P-O -S cycles and use it to constrain global redox balance in the mid-Proterozoic ocean-atmosphere system. By employing a Monte Carlo approach bounded by observations from the geologic record, we infer that the rate of net biospheric O production was 3 . 5 - 1.1 + 1.4 Tmol year (1σ), or ~25% of today's value, owing largely to phosphorus scarcity in the ocean interior. Pyrite burial in marine sediments would have represented a comparable or more significant O source than organic carbon burial, implying a potentially important role for Earth's sulphur cycle in balancing the oxygen cycle and regulating atmospheric O levels. Our statistical approach provides a uniquely comprehensive view of Earth system biogeochemistry and global O cycling during mid-Proterozoic time and implicates severe P biolimitation as the backdrop for Precambrian geochemical and biological evolution.
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http://dx.doi.org/10.1111/gbi.12317DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6585969PMC
January 2019

A case for low atmospheric oxygen levels during Earth's middle history.

Emerg Top Life Sci 2018 Sep;2(2):149-159

NASA Astrobiology Institute Alternative Earths Team, Riverside, CA, U.S.A.

The oxygenation of the atmosphere - one of the most fundamental transformations in Earth's history - dramatically altered the chemical composition of the oceans and provides a compelling example of how life can reshape planetary surface environments. Furthermore, it is commonly proposed that surface oxygen levels played a key role in controlling the timing and tempo of the origin and early diversification of animals. Although oxygen levels were likely more dynamic than previously imagined, we make a case here that emerging records provide evidence for low atmospheric oxygen levels for the majority of Earth's history. Specifically, we review records and present a conceptual framework that suggest that background oxygen levels were below 1% of the present atmospheric level during the billon years leading up to the diversification of early animals. Evidence for low background oxygen levels through much of the Proterozoic bolsters the case that environmental conditions were a critical factor in controlling the structure of ecosystems through Earth's history.
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http://dx.doi.org/10.1042/ETLS20170161DOI Listing
September 2018

Nitrous oxide from chemodenitrification: A possible missing link in the Proterozoic greenhouse and the evolution of aerobic respiration.

Geobiology 2018 11 22;16(6):597-609. Epub 2018 Aug 22.

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia.

The potent greenhouse gas nitrous oxide (N O) 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 N O from ferruginous Proterozoic seas. We empirically derived a rate law, d N 2 O d t = 7.2 × 10 - 5 [ Fe 2 + ] 0.3 [ NO ] 1 , 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-low mM Fe concentrations could have sustained a sea-air flux of 100-200 Tg N O-N year , if N fixation rates were near-modern and all fixed N was emitted as N O. A 1D photochemical model was used to obtain steady-state atmospheric N O concentrations as a function of sea-air N O flux across the wide range of possible pO values (0.001-1 PAL). At 100-200 Tg N O-N year and >0.1 PAL O , this model yielded low-ppmv N O, which would produce several degrees of greenhouse warming at 1.6 ppmv CH and 320 ppmv CO . These results suggest that enhanced N O 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 N O fluxes at intermediate O concentrations (0.01-0.1 PAL) would have enhanced ozone screening of solar UV radiation. Due to rapid photolysis in the absence of an ozone shield, N O is unlikely to have been an important greenhouse gas if Mesoproterozoic O was 0.001 PAL. At low O , N O 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.
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http://dx.doi.org/10.1111/gbi.12311DOI Listing
November 2018

Constraints on Paleoproterozoic atmospheric oxygen levels.

Proc Natl Acad Sci U S A 2018 08 23;115(32):8104-8109. Epub 2018 Jul 23.

Department of Geology and Geophysics, Yale University, New Haven, CT 06511.

The oxygenation of Earth's surface environment dramatically altered key biological and geochemical cycles and ultimately ushered in the rise of an ecologically diverse biosphere. However, atmospheric oxygen partial pressures (O) estimates for large swaths of the Precambrian remain intensely debated. Here we evaluate and explore the use of carbonate cerium (Ce) anomalies (Ce/Ce*) as a quantitative atmospheric O proxy and provide estimates of Proterozoic O using marine carbonates from a unique Precambrian carbonate succession-the Paleoproterozoic Pethei Group. In contrast to most previous work, we measure Ce/Ce* on marine carbonate precipitates that formed in situ across a depth gradient, building on previous detailed sedimentology and stratigraphy to constrain the paleo-depth of each sample. Measuring Ce/Ce* across a full platform to basin depth gradient, we found only minor depleted Ce anomalies restricted to the platform and upper slope facies. We combine these results with a Ce oxidation model to provide a quantitative constraint on atmospheric O 1.87 billion years ago (Ga). Our results suggest Paleoproterozoic atmospheric oxygen concentrations were low, near 0.1% of the present atmospheric level. This work provides another crucial line of empirical evidence that atmospheric oxygen levels returned to low concentrations following the Lomagundi Event, and remained low enough for large portions of the Proterozoic to have impacted the ecology of the earliest complex organisms.
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http://dx.doi.org/10.1073/pnas.1806216115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6094116PMC
August 2018

Exoplanet Biosignatures: Future Directions.

Astrobiology 2018 06;18(6):779-824

1 School of Earth and Space Exploration, Arizona State University , Tempe, Arizona.

We introduce a Bayesian method for guiding future directions for detection of life on exoplanets. We describe empirical and theoretical work necessary to place constraints on the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate and geophysics, geochemical cycling, the universalities of physics and chemistry, the contingencies of evolutionary history, the properties of life as an emergent complex system, and the mechanisms driving the emergence of life. We provide examples for how the Bayesian formalism could guide future search strategies, including determining observations to prioritize or deciding between targeted searches or larger lower resolution surveys to generate ensemble statistics and address how a Bayesian methodology could constrain the prior probability of life with or without a positive detection. Key Words: Exoplanets-Biosignatures-Life detection-Bayesian analysis. Astrobiology 18, 779-824.
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http://dx.doi.org/10.1089/ast.2017.1738DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6016573PMC
June 2018

Tracking the rise of eukaryotes to ecological dominance with zinc isotopes.

Geobiology 2018 07 5;16(4):341-352. Epub 2018 Jun 5.

Geology and Geophysics, Yale University, New Haven, Connecticut.

The biogeochemical cycling of zinc (Zn) is intimately coupled with organic carbon in the ocean. Based on an extensive new sedimentary Zn isotope record across Earth's history, we provide evidence for a fundamental shift in the marine Zn cycle ~800 million years ago. We discuss a wide range of potential drivers for this transition and propose that, within available constraints, a restructuring of marine ecosystems is the most parsimonious explanation for this shift. Using a global isotope mass balance approach, we show that a change in the organic Zn/C ratio is required to account for observed Zn isotope trends through time. Given the higher affinity of eukaryotes for Zn relative to prokaryotes, we suggest that a shift toward a more eukaryote-rich ecosystem could have provided a means of more efficiently sequestering organic-derived Zn. Despite the much earlier appearance of eukaryotes in the microfossil record (~1700 to 1600 million years ago), our data suggest a delayed rise to ecological prominence during the Neoproterozoic, consistent with the currently accepted organic biomarker records.
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http://dx.doi.org/10.1111/gbi.12289DOI Listing
July 2018

Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment.

Astrobiology 2018 06 10;18(6):630-662. Epub 2018 May 10.

12 German Aerospace Center, Institute of Planetary Research , Extrasolar Planets and Atmospheres, Berlin, Germany .

We describe how environmental context can help determine whether oxygen (O) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, interdisciplinary example of O biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O without life being present. Consequently, our ability to interpret both the presence and absence of O in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment to identify how the interplay of sources and sinks may have suppressed O release into the atmosphere for several billion years, producing a false negative for biologically generated O. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O, planetary processes that may generate abundant atmospheric O without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures-Oxygenic photosynthesis-Exoplanets-Planetary atmospheres. Astrobiology 18, 630-662.
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http://dx.doi.org/10.1089/ast.2017.1727DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6014580PMC
June 2018

Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life.

Astrobiology 2018 06 4;18(6):663-708. Epub 2018 May 4.

1 Department of Earth Sciences, University of California , Riverside, California.

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets-Biosignatures-Habitability markers-Photosynthesis-Planetary surfaces-Atmospheres-Spectroscopy-Cryptic biospheres-False positives. Astrobiology 18, 663-708.
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http://dx.doi.org/10.1089/ast.2017.1729DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6016574PMC
June 2018

Redox-independent chromium isotope fractionation induced by ligand-promoted dissolution.

Nat Commun 2017 11 17;8(1):1590. Epub 2017 Nov 17.

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332, USA.

The chromium (Cr) isotope system has emerged as a potential proxy for tracing the Earth's atmospheric evolution based on a redox-dependent framework for Cr mobilization and isotope fractionation. Although studies have demonstrated that redox-independent pathways can also mobilize Cr, no quantitative constraints exist on the associated isotope fractionations. Here we survey the effects of common environmental ligands on the dissolution of Cr(III)-(oxy)hydroxide solids and associated Cr isotope fractionation. For a variety of organic acids and siderophores, δCr values of dissolved Cr(III) are -0.27 to 1.23‰, within the range of previously observed Cr isotope signatures in rock records linked to Cr redox cycling. Thus, ligand-promoted dissolution of Cr-containing solids, a redox-independent process, must be taken into account when using sedimentary Cr isotope signatures to diagnose atmospheric oxygen levels. This work provides a step towards establishing a more robust framework for using Cr isotopes to track the evolution of the Earth's atmosphere.
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http://dx.doi.org/10.1038/s41467-017-01694-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5693864PMC
November 2017

False Negatives for Remote Life Detection on Ocean-Bearing Planets: Lessons from the Early Earth.

Astrobiology 2017 04;17(4):287-297

1 NASA Astrobiology Institute .

Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary changes throughout its long history, with potentially significant ramifications for the emergence and long-term stability of atmospheric biosignatures. Though a great deal of work has centered on refining our understanding of false positives for remote life detection, much less attention has been paid to the possibility of false negatives, that is, cryptic biospheres that are widespread and active on a planet's surface but are ultimately undetectable or difficult to detect in the composition of a planet's atmosphere. Here, we summarize recent developments from geochemical proxy records and Earth system models that provide insight into the long-term evolution of the most readily detectable potential biosignature gases on Earth-oxygen (O), ozone (O), and methane (CH). We suggest that the canonical O-CH disequilibrium biosignature would perhaps have been challenging to detect remotely during Earth's ∼4.5-billion-year history and that in general atmospheric O/O levels have been a poor proxy for the presence of Earth's biosphere for all but the last ∼500 million years. We further suggest that detecting atmospheric CH would have been problematic for most of the last ∼2.5 billion years of Earth's history. More broadly, we stress that internal oceanic recycling of biosignature gases will often render surface biospheres on ocean-bearing silicate worlds cryptic, with the implication that the planets most conducive to the development and maintenance of a pervasive biosphere will often be challenging to characterize via conventional atmospheric biosignatures. Key Words: Biosignatures-Oxygen-Methane-Ozone-Exoplanets-Planetary habitability. Astrobiology 17, 287-297.
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http://dx.doi.org/10.1089/ast.2016.1598DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5399744PMC
April 2017

Evolution of the global phosphorus cycle.

Nature 2017 01 21;541(7637):386-389. Epub 2016 Dec 21.

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada.

The macronutrient phosphorus is thought to limit primary productivity in the oceans on geological timescales. Although there has been a sustained effort to reconstruct the dynamics of the phosphorus cycle over the past 3.5 billion years, it remains uncertain whether phosphorus limitation persisted throughout Earth's history and therefore whether the phosphorus cycle has consistently modulated biospheric productivity and ocean-atmosphere oxygen levels over time. Here we present a compilation of phosphorus abundances in marine sedimentary rocks spanning the past 3.5 billion years. We find evidence for relatively low authigenic phosphorus burial in shallow marine environments until about 800 to 700 million years ago. Our interpretation of the database leads us to propose that limited marginal phosphorus burial before that time was linked to phosphorus biolimitation, resulting in elemental stoichiometries in primary producers that diverged strongly from the Redfield ratio (the atomic ratio of carbon, nitrogen and phosphorus found in phytoplankton). We place our phosphorus record in a quantitative biogeochemical model framework and find that a combination of enhanced phosphorus scavenging in anoxic, iron-rich oceans and a nutrient-based bistability in atmospheric oxygen levels could have resulted in a stable low-oxygen world. The combination of these factors may explain the protracted oxygenation of Earth's surface over the last 3.5 billion years of Earth history. However, our analysis also suggests that a fundamental shift in the phosphorus cycle may have occurred during the late Proterozoic eon (between 800 and 635 million years ago), coincident with a previously inferred shift in marine redox states, severe perturbations to Earth's climate system, and the emergence of animals.
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http://dx.doi.org/10.1038/nature20772DOI Listing
January 2017

Cyanobacterial Diazotrophy and Earth's Delayed Oxygenation.

Front Microbiol 2016 23;7:1526. Epub 2016 Sep 23.

Department of Earth Sciences, University of California, Riverside, Riverside, CA USA.

The redox landscape of Earth's ocean-atmosphere system has changed dramatically throughout Earth history. Although Earth's protracted oxygenation is undoubtedly the consequence of cyanobacterial oxygenic photosynthesis, the relationship between biological O production and Earth's redox evolution remains poorly understood. Existing models for Earth's oxygenation cannot adequately explain the nearly 2.5 billion years delay between the origin of oxygenic photosynthesis and the oxygenation of the deep ocean, in large part owing to major deficiencies in our understanding of the coevolution of O and Earth's key biogeochemical cycles (e.g., the N cycle). For example, although possible links between O and N scarcity have been previously explored, the consequences of N limitation for net biological O production have not been examined thoroughly. Here, we revisit the prevailing view that N fixation has always been able to keep pace with P supply and discuss the possibility that bioavailable N, rather than P, limited export production for extended periods of Earth's history. Based on the observation that diazotrophy occurs at the expense of oxygenesis in the modern ocean, we suggest that an N-limited biosphere may be inherently less oxygenic than a P-limited biosphere-and that cyanobacterial diazotrophy was a primary control on the timing and tempo of Earth's oxygenation by modulating net biogenic O fluxes. We further hypothesize that negative feedbacks inhibit the transition between N and P limitation, with the implication that the pervasive accumulation of O in Earth's ocean-atmosphere system may not have been an inevitable consequence of oxygenic photosynthesis by marine cyanobacteria.
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http://dx.doi.org/10.3389/fmicb.2016.01526DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5033965PMC
September 2016

Limited role for methane in the mid-Proterozoic greenhouse.

Proc Natl Acad Sci U S A 2016 10 26;113(41):11447-11452. Epub 2016 Sep 26.

NASA Astrobiology Institute, University of California, Riverside, CA 92521; Department of Earth Science, University of California, Riverside, CA 92521.

Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH to the atmosphere, enhancing the climatic significance of CH early in Earth's history. Indeed, the assumption of elevated pCH during the Proterozoic underlies most models for both anomalous climatic stasis during the mid-Proterozoic and extreme climate perturbation during the Neoproterozoic; however, the geologic record cannot directly constrain atmospheric CH levels and attendant radiative forcing. Here, we revisit the role of CH in Earth's climate system during Proterozoic time. We use an Earth system model to quantify CH fluxes from the marine biosphere and to examine the capacity of biogenic CH to compensate for the faint young Sun during the "boring billion" years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH coupled to SO reduction is a highly effective obstacle to CH accumulation in the atmosphere, possibly limiting atmospheric pCH to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO If recent pO constraints from Cr isotopes are correct, we predict that reduced UV shielding by O should further limit pCH to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH for the majority of Earth history-possibly reviving the faint young Sun paradox during Proterozoic time and challenging existing models for the initiation of low-latitude glaciation that depend on the oxidative collapse of a steady-state CH greenhouse.
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http://dx.doi.org/10.1073/pnas.1608549113DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5068276PMC
October 2016

Earth's oxygen cycle and the evolution of animal life.

Proc Natl Acad Sci U S A 2016 08 25;113(32):8933-8. Epub 2016 Jul 25.

Department of Paleobiology, National Museum of Natural History, Washington, DC 20560.

The emergence and expansion of complex eukaryotic life on Earth is linked at a basic level to the secular evolution of surface oxygen levels. However, the role that planetary redox evolution has played in controlling the timing of metazoan (animal) emergence and diversification, if any, has been intensely debated. Discussion has gravitated toward threshold levels of environmental free oxygen (O2) necessary for early evolving animals to survive under controlled conditions. However, defining such thresholds in practice is not straightforward, and environmental O2 levels can potentially constrain animal life in ways distinct from threshold O2 tolerance. Herein, we quantitatively explore one aspect of the evolutionary coupling between animal life and Earth's oxygen cycle-the influence of spatial and temporal variability in surface ocean O2 levels on the ecology of early metazoan organisms. Through the application of a series of quantitative biogeochemical models, we find that large spatiotemporal variations in surface ocean O2 levels and pervasive benthic anoxia are expected in a world with much lower atmospheric pO2 than at present, resulting in severe ecological constraints and a challenging evolutionary landscape for early metazoan life. We argue that these effects, when considered in the light of synergistic interactions with other environmental parameters and variable O2 demand throughout an organism's life history, would have resulted in long-term evolutionary and ecological inhibition of animal life on Earth for much of Middle Proterozoic time (∼1.8-0.8 billion years ago).
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http://dx.doi.org/10.1073/pnas.1521544113DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4987840PMC
August 2016

No evidence for high atmospheric oxygen levels 1,400 million years ago.

Proc Natl Acad Sci U S A 2016 May 20;113(19):E2550-1. Epub 2016 Apr 20.

Department of Earth Science, University of California, Riverside, CA 92521;

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http://dx.doi.org/10.1073/pnas.1601925113DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4868423PMC
May 2016

Transient episodes of mild environmental oxygenation and oxidative continental weathering during the late Archean.

Sci Adv 2015 Nov 20;1(10):e1500777. Epub 2015 Nov 20.

School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA. ; Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, USA.

It is not known whether environmental O2 levels increased in a linear fashion or fluctuated dynamically between the evolution of oxygenic photosynthesis and the later Great Oxidation Event. New rhenium-osmium isotope data from the late Archean Mount McRae Shale, Western Australia, reveal a transient episode of oxidative continental weathering more than 50 million years before the onset of the Great Oxidation Event. A depositional age of 2495 ± 14 million years and an initial (187)Os/(188)Os of 0.34 ± 0.19 were obtained for rhenium- and molybdenum-rich black shales. The initial (187)Os/(188)Os is higher than the mantle/extraterrestrial value of 0.11, pointing to mild environmental oxygenation and oxidative mobilization of rhenium, molybdenum, and radiogenic osmium from the upper continental crust and to contemporaneous transport of these metals to seawater. By contrast, stratigraphically overlying black shales are rhenium- and molybdenum-poor and have a mantle-like initial (187)Os/(188)Os of 0.06 ± 0.09, indicating a reduced continental flux of rhenium, molybdenum, and osmium to seawater because of a drop in environmental O2 levels. Transient oxygenation events, like the one captured by the Mount McRae Shale, probably separated intervals of less oxygenated conditions during the late Archean.
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http://dx.doi.org/10.1126/sciadv.1500777DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4681338PMC
November 2015

Earth history. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals.

Science 2014 Oct;346(6209):635-8

Department of Earth Sciences, University of California, Riverside, CA, USA.

The oxygenation of Earth's surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth's surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.
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http://dx.doi.org/10.1126/science.1258410DOI Listing
October 2014

Evolution: a fixed-nitrogen fix in the early ocean?

Curr Biol 2014 Mar;24(7):R276-8

Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA.

A new study asserts that a late evolutionary leap in cyanobacterial nitrogen fixation terminated a long history of nitrogen-limited primary production in the ocean--and contributed to a dramatic increase in biospheric oxygen coincident with the rise of animals.
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http://dx.doi.org/10.1016/j.cub.2014.02.034DOI Listing
March 2014