Publications by authors named "Timothy W Lyons"

55 Publications

Shedding light on manganese cycling in the early oceans.

Proc Natl Acad Sci U S A 2020 10 12;117(42):25960-25962. Epub 2020 Oct 12.

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

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http://dx.doi.org/10.1073/pnas.2016447117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7584896PMC
October 2020

Massive formation of early diagenetic dolomite in the Ediacaran ocean: Constraints on the "dolomite problem".

Proc Natl Acad Sci U S A 2020 06 8;117(25):14005-14014. Epub 2020 Jun 8.

Institute for Geology, Mineralogy, and Geophysics, Ruhr University Bochum, D-44801 Bochum, Germany.

Paleozoic and Precambrian sedimentary successions frequently contain massive dolomicrite [CaMg(CO)] units despite kinetic inhibitions to nucleation and precipitation of dolomite at Earth surface temperatures (<60 °C). This paradoxical observation is known as the "dolomite problem." Accordingly, the genesis of these dolostones is usually attributed to burial-hydrothermal dolomitization of primary limestones (CaCO) at temperatures of >100 °C, thus raising doubt about the validity of these deposits as archives of Earth surface environments. We present a high-resolution, >63-My-long clumped-isotope temperature (T) record of shallow-marine dolomicrites from two drillcores of the Ediacaran (635 to 541 Ma) Doushantuo Formation in South China. Our T record indicates that a majority (87%) of these dolostones formed at temperatures of <100 °C. When considering the regional thermal history, modeling of the influence of solid-state reordering on our T record further suggests that most of the studied dolostones formed at temperatures of <60 °C, providing direct evidence of a low-temperature origin of these dolostones. Furthermore, calculated δO values of diagenetic fluids, rare earth element plus yttrium compositions, and petrographic observations of these dolostones are consistent with an early diagenetic origin in a rock-buffered environment. We thus propose that a precursor precipitate from seawater was subsequently dolomitized during early diagenesis in a near-surface setting to produce the large volume of dolostones in the Doushantuo Formation. Our findings suggest that the preponderance of dolomite in Paleozoic and Precambrian deposits likely reflects oceanic conditions specific to those eras and that dolostones can be faithful recorders of environmental conditions in the early oceans.
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http://dx.doi.org/10.1073/pnas.1916673117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7321997PMC
June 2020

Nitrogen isotope ratios trace high-pH conditions in a terrestrial Mars analog site.

Sci Adv 2020 Feb 26;6(9):eaay3440. Epub 2020 Feb 26.

Virtual Planetary Laboratory, University of Washington, Seattle, WA 98195, USA.

High-pH alkaline lakes are among the most productive ecosystems on Earth and prime targets in the search for life on Mars; however, a robust proxy for such settings does not yet exist. Nitrogen isotope fractionation resulting from NH volatilization at high pH has the potential to fill this gap. To validate this idea, we analyzed samples from the Nördlinger Ries, a Miocene impact crater lake that displayed pH values up to 9.8 as inferred from mineralogy and aqueous modeling. Our data show a peak in δN of +17‰ in the most alkaline facies, followed by a gradual decline to around +5‰, concurrent with the proposed decline in pH, highlighting the utility of nitrogen isotopes as a proxy for high-pH conditions. In combination with independent mineralogical indicators for high alkalinity, nitrogen isotopes can provide much-needed quantitative constraints on ancient atmospheric co (partial pressure of CO) and thus climatic controls on early Earth and Mars.
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http://dx.doi.org/10.1126/sciadv.aay3440DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7043907PMC
February 2020

Correlation between biomarkers of exposure, effect and potential harm in the urine of electronic cigarette users.

BMJ Open Respir Res 2020 02;7(1)

Department of Cell, Molecular, and Developmental Biology, University of California Riverside, Riverside, California, USA

Objectives: To determine if urinary biomarkers of effect and potential harm are elevated in electronic cigarette users compared with non-smokers and if elevation correlates with increased concentrations of metals in urine.

Study Design And Setting: This was a cross-sectional study of biomarkers of exposure, effect and potential harm in urine from non-smokers (n=20), electronic cigarette users (n=20) and cigarette smokers (n=13). Participant's screening and urine collection were performed at the Roswell Park Comprehensive Cancer Center, and biomarker analysis and metal analysis were performed at the University of California, Riverside.

Results: Metallothionein was significantly elevated in the electronic cigarette group (3761±3932 pg/mg) compared with the non-smokers (1129±1294 pg/mg, p=0.05). 8-OHdG (8-hydroxy-2'-deoxyguanosine) was significantly elevated in electronic cigarette users (442.8±300.7 ng/mg) versus non-smokers (221.6±157.8 ng/mg, p=0.01). 8-Isoprostane showed a significant increase in electronic cigarette users (750.8±433 pg/mg) versus non-smokers (411.2±287.4 pg/mg, p=0.03). Linear regression analysis in the electronic cigarette group showed a significant correlation between cotinine and total metal concentration; total metal concentration and metallothionein; cotinine and oxidative DNA damage; and total metal concentration and oxidative DNA damage. Zinc was significantly elevated in the electronic cigarette users (584.5±826.6 µg/g) compared with non-smokers (413.6±233.7 µg/g, p=0.03). Linear regression analysis showed a significant correlation between urinary zinc concentration and 8-OHdG in the electronic cigarette users.

Conclusions: This study is the first to investigate biomarkers of potential harm and effect in electronic cigarette users and to show a linkage to metal exposure. The biomarker levels in electronic cigarette users were similar to (and not lower than) cigarette smokers. In electronic cigarette users, there was a link to elevated total metal exposure and oxidative DNA damage. Specifically, our results demonstrate that zinc concentration was correlated to oxidative DNA damage.
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http://dx.doi.org/10.1136/bmjresp-2019-000452DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7047495PMC
February 2020

Absence of biomarker evidence for early eukaryotic life from the Mesoproterozoic Roper Group: Searching across a marine redox gradient in mid-Proterozoic habitability.

Geobiology 2019 05 10;17(3):247-260. Epub 2019 Jan 10.

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

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 oil window maturity), and previous geochemical data suggest a relatively strong connection to the open ocean compared to other mid-Proterozoic records. Here, we present one of the first integrated investigations of Mesoproterozoic biomarker records performed in parallel with established inorganic redox proxy indicators. Results reveal a temporally variable paleoredox structure through the Velkerri Formation as gauged from iron mineral speciation and trace-metal geochemistry, vacillating between oxic and anoxic. Our combined lipid biomarker and inorganic geochemical records indicate at least episodic euxinic conditions sustained predominantly below the photic zone during the deposition of organic-rich shales found in the middle Velkerri Formation. The most striking result is an absence of eukaryotic steranes (4-desmethylsteranes) and only traces of gammacerane in some samples-despite our search across oxic, as well as anoxic, facies that should favor eukaryotic habitability and in low maturity rocks that allow the preservation of biomarker alkanes. The dearth of Mesoproterozoic eukaryotic sterane biomarkers, even within the more oxic facies, is somewhat surprising but suggests that controls such as the long-term nutrient balance and other environmental factors may have throttled the abundances and diversity of early eukaryotic life relative to bacteria within marine microbial communities. Given that molecular clocks predict that sterol synthesis evolved early in eukaryotic history, and (bacterial) fossil steroids have been found previously in 1.64 Ga rocks, then a very low environmental abundance of eukaryotes relative to bacteria is our preferred explanation for the lack of regular steranes and only traces of gammacerane in a few samples. It is also possible that early eukaryotes adapted to Mesoproterozoic marine environments did not make abundant steroid lipids or tetrahymanol in their cell membranes.
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http://dx.doi.org/10.1111/gbi.12329DOI Listing
May 2019

Heterogeneous and dynamic marine shelf oxygenation and coupled early animal evolution.

Emerg Top Life Sci 2018 Sep;2(2):279-288

Department of Earth Sciences, University of California, Riverside, CA 92521, U.S.A.

It is generally agreed that early diversification of animals and significant rise of atmospheric and oceanic oxygen (O2) levels occurred in the Ediacaran (635-541 million years ago, Ma) and early Cambrian (ca. 541-509 Ma). The strength and nature of their relationship, however, remain unclear and debated. A recent wave of paleoredox research - with a particular focus on the fossiliferous sections in South China - demonstrates high spatial heterogeneity of oceanic O2 (redox) conditions and dynamic marine shelf oxygenation in a dominantly anoxic ocean during the Ediacaran and early Cambrian. This pattern shows a general spatiotemporal coupling to early animal evolution. We attribute dynamic shelf oxygenation to a complex interplay among the evolving atmosphere, continents, oceans, and biosphere during a critical period in Earth history. Our review supports the idea of a complex coevolution between increasing O2 levels and early diversification of animals, although additional work is required to fully delineate the timing and patterns of this coevolution and the mechanistic underpinnings.
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http://dx.doi.org/10.1042/ETLS20170157DOI Listing
September 2018

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

Mid-Proterozoic redox evolution and the possibility of transient oxygenation events.

Emerg Top Life Sci 2018 Sep;2(2):235-245

Department of Earth Sciences and the NASA Astrobiology Institute Alternative Earths Team, University of California, Riverside, CA 92521, U.S.A.

It is often assumed that rising environmental oxygen concentrations played a significant role in the timing of the first appearance of animals and the trajectory of their early proliferation and diversification. The inherent large size and complexity of animals come with large energy requirements - levels of energy that can best, if not only, be acquired through aerobic respiration. There is also abundant geochemical evidence for an increase in ocean-atmosphere O2 concentrations in temporal proximity with the emergence of the group. To adequately test this hypothesis, however, a thorough understanding of the history of environmental oxygenation in the time between the first appearance of eukaryotes and the eventual appearance of animals is necessary. In this review, we summarize the evidence for the prevailing long-term conditions of the Proterozoic Eon prior to the emergence of Metazoa and go on to highlight multiple independent geochemical proxy records that suggest at least two transient oxygenation events - at ca. 1.4 and ca. 1.1 billion years ago (Ga) - during this time. These emerging datasets open the door to an important possibility: while prevailing conditions during much of this time would likely have presented challenges for early animals, there were intervals when oxygenated conditions were more widespread and could have favored yet undetermined advances in eukaryotic innovation, including critical early steps toward animal evolution.
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http://dx.doi.org/10.1042/ETLS20170146DOI Listing
September 2018

Dynamic oxygen and coupled biological and ecological innovation during the second wave of the Ediacara Biota.

Emerg Top Life Sci 2018 Sep;2(2):223-233

Department of Earth Sciences, University of California at Riverside, 900 University Ave, Riverside, CA, U.S.A.

Animal life on Earth is generally accepted to have risen during a period of increasingly well-oxygenated conditions, but direct evidence for that relationship has previously eluded scientists. This gap reflects both the enigmatic nature of the early animal fossil record and the coarse temporal resolution of Precambrian environmental change. Here, we combine paleontological data from the Ediacara Biota, the earliest fossil animals, with geochemical evidence for fluctuating redox conditions. Using morphological and ecological novelties that broadly reflect oxygen demand, we show that the appearance of abundant oxygen-demanding organisms within the Ediacara Biota corresponds with a period of elevated global oxygen concentrations. This correlation suggests that a putative rise in oxygen levels may have provided the necessary environments for the diversification of complex body plans and energetically demanding ecologies. The potential loss of organisms with relatively high oxygen requirements in the latest Ediacaran coupled with an apparent return to low oxygen concentrations further supports the availability of oxygen as a control on early animal evolution. While the advent of animal life was probably the product of a variety of factors, the recognition of a possible connection between changing environmental conditions and the diversification of animal morphologies suggests that the availability of oxygen played a significant role in the evolution of animals on Earth.
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http://dx.doi.org/10.1042/ETLS20170148DOI Listing
September 2018

Early Earth and the rise of complex life.

Emerg Top Life Sci 2018 Sep;2(2):121-124

Department of Earth Science, University of California, Santa Barbara, CA 93106, U.S.A.

The history of life on Earth progressed in parallel with the evolving oxygen state of the atmosphere and oceans, but the details of that relationship remain poorly known and debated. There is, however, general agreement that the first appreciable and persistent accumulation of oxygen in the oceans and atmosphere occurred around 2.3 to 2.4 billion years ago. Following this Great Oxidation Event, biospheric oxygen remained at relatively stable intermediate levels for more than a billion years. Much current research focuses on the transition from the intermediate conditions of this middle chapter in Earth history to the more oxygenated periods that followed - often emphasizing whether increasing and perhaps episodic oxygenation drove fundamental steps in the evolution of complex life and, if so, when. These relationships among early organisms and their environments are the thematic threads that stitch together the papers in this collection. Expert authors bring a mix of methods and opinions to their leading-edge reviews of the earliest proliferation and ecological impacts of eukaryotic life, the subsequent emergence and ecological divergence of animals, and the corresponding causes and consequences of environmental change.
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http://dx.doi.org/10.1042/ETLS20180093DOI 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

Thallium isotopes reveal protracted anoxia during the Toarcian (Early Jurassic) associated with volcanism, carbon burial, and mass extinction.

Proc Natl Acad Sci U S A 2018 06 11;115(26):6596-6601. Epub 2018 Jun 11.

Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306.

For this study, we generated thallium (Tl) isotope records from two anoxic basins to track the earliest changes in global bottom water oxygen contents over the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) of the Early Jurassic. The T-OAE, like other Mesozoic OAEs, has been interpreted as an expansion of marine oxygen depletion based on indirect methods such as organic-rich facies, carbon isotope excursions, and biological turnover. Our Tl isotope data, however, reveal explicit evidence for earlier global marine deoxygenation of ocean water, some 600 ka before the classically defined T-OAE. This antecedent deoxygenation occurs at the Pliensbachian/Toarcian boundary and is coeval with the onset of initial large igneous province (LIP) volcanism and the initiation of a marine mass extinction. Thallium isotopes are also perturbed during the T-OAE interval, as defined by carbon isotopes, reflecting a second deoxygenation event that coincides with the acme of elevated marine mass extinctions and the main phase of LIP volcanism. This suggests that the duration of widespread anoxic bottom waters was at least 1 million years in duration and spanned early to middle Toarcian time. Thus, the Tl data reveal a more nuanced record of marine oxygen depletion and its links to biological change during a period of climatic warming in Earth's past and highlight the role of oxygen depletion on past biological evolution.
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http://dx.doi.org/10.1073/pnas.1803478115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6042096PMC
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

Late inception of a resiliently oxygenated upper ocean.

Science 2018 07 31;361(6398):174-177. Epub 2018 May 31.

Department of Earth Sciences, Syracuse University, Syracuse, NY, USA.

Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine-to-calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in the partial pressure of oxygen in the atmosphere at ~400 million years (Ma) ago and reveals a step change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma ago. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma ago.
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http://dx.doi.org/10.1126/science.aar5372DOI 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

Reactive Oxygen Species: Radical Factors in the Evolution of Animal Life: A molecular timescale from Earth's earliest history to the rise of complex life.

Bioessays 2018 03 7;40(3). Epub 2018 Feb 7.

Department of Earth Sciences University of California, University of California, Riverside, 900 University Ave. Riverside, 92521 California, California, USA.

Introduction of O to Earth's early biosphere stimulated remarkable evolutionary adaptations, and a wide range of electron acceptors allowed diverse, energy-yielding metabolic pathways. Enzymatic reduction of O yielded a several-fold increase in energy production, enabling evolution of multi-cellular animal life. However, utilization of O also presented major challenges as O and many of its derived reactive oxygen species (ROS) are highly toxic, possibly impeding multicellular evolution after the Great Oxidation Event. Remarkably, ROS, and especially hydrogen peroxide, seem to play a major part in early diversification and further development of cellular respiration and other oxygenic pathways, thus becoming an intricate part of evolution of complex life. Hence, although harnessing of chemical and thermo-dynamic properties of O for aerobic metabolism is generally considered to be an evolutionary milestone, the ability to use ROS for cell signaling and regulation may have been the first true breakthrough in development of complex life.
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http://dx.doi.org/10.1002/bies.201700158DOI Listing
March 2018

The oceanic budgets of nickel and zinc isotopes: the importance of sulfidic environments as illustrated by the Black Sea.

Philos Trans A Math Phys Eng Sci 2016 11;374(2081)

Department of Earth Sciences, University of California, Riverside, CA 92521, USA.

Isotopic data collected to date as part of the GEOTRACES and other programmes show that the oceanic dissolved pool is isotopically heavy relative to the inputs for zinc (Zn) and nickel (Ni). All Zn sinks measured until recently, and the only output yet measured for Ni, are isotopically heavier than the dissolved pool. This would require either a non-steady-state ocean or other unidentified sinks. Recently, isotopically light Zn has been measured in organic carbon-rich sediments from productive upwelling margins, providing a potential resolution of this issue, at least for Zn. However, the origin of the isotopically light sedimentary Zn signal is uncertain. Cellular uptake of isotopically light Zn followed by transfer to sediment does not appear to be a quantitatively important process. Here, we present Zn and Ni isotope data for the water column and sediments of the Black Sea. These data demonstrate that isotopically light Zn and Ni are extracted from the water column, probably through an equilibrium fractionation between different dissolved species followed by sequestration of light Zn and Ni in sulfide species to particulates and the sediment. We suggest that a similar, non-quantitative, process, operating in porewaters, explains the Zn data from organic carbon-rich sediments.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.
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http://dx.doi.org/10.1098/rsta.2015.0294DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5069529PMC
November 2016

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

Carbon and Sulfur Cycling below the Chemocline in a Meromictic Lake and the Identification of a Novel Taxonomic Lineage in the FCB Superphylum, Candidatus Aegiribacteria.

Front Microbiol 2016 27;7:598. Epub 2016 Apr 27.

Penn State Astrobiology Research Center, Department of Geosciences, Pennsylvania State University University Park, TX, USA.

Mahoney Lake in British Columbia is an extreme meromictic system with unusually high levels of sulfate and sulfide present in the water column. As is common in strongly stratified lakes, Mahoney Lake hosts a dense, sulfide-oxidizing phototrophic microbial community where light reaches the chemocline. Below this "plate," the euxinic hypolimnion is anoxic, eutrophic, saline, and rich in sulfide, polysulfides, elemental sulfur, and other sulfur intermediates. While much is known regarding microbial communities in sunlit portions of euxinic systems, the composition and genetic potential of organisms living at aphotic depths have rarely been studied. Metagenomic sequencing of samples from the hypolimnion and the underlying sediments of Mahoney Lake indicate that multiple taxa contribute to sulfate reduction below the chemocline and that the hypolimnion and sediments each support distinct populations of sulfate reducing bacteria (SRB) that differ from the SRB populations observed in the chemocline. After assembling and binning the metagenomic datasets, we recovered near-complete genomes of dominant populations including two Deltaproteobacteria. One of the deltaproteobacterial genomes encoded a 16S rRNA sequence that was most closely related to the sulfur-disproportionating genus Dissulfuribacter and the other encoded a 16S rRNA sequence that was most closely related to the fatty acid- and aromatic acid-degrading genus Syntrophus. We also recovered two near-complete genomes of Firmicutes species. Analysis of concatenated ribosomal protein trees suggests these genomes are most closely related to extremely alkaliphilic genera Alkaliphilus and Dethiobacter. Our metagenomic data indicate that these Firmicutes contribute to carbon cycling below the chemocline. Lastly, we recovered a nearly complete genome from the sediment metagenome which represents a new genus within the FCB (Fibrobacteres, Chlorobi, Bacteroidetes) superphylum. Consistent with the geochemical data, we found little or no evidence for organisms capable of sulfide oxidation in the aphotic zone below the chemocline. Instead, comparison of functional genes below the chemocline are consistent with recovery of multiple populations capable of reducing oxidized sulfur. Our data support previous observations that at least some of the sulfide necessary to support the dense population of phototrophs in the chemocline is supplied from sulfate reduction in the hypolimnion and sediments. These studies provide key insights regarding the taxonomic and functional diversity within a euxinic environment and highlight the complexity of biogeochemical carbon and sulfur cycling necessary to maintain euxinia.
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http://dx.doi.org/10.3389/fmicb.2016.00598DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4846661PMC
May 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

The rise of oxygen in Earth's early ocean and atmosphere.

Nature 2014 Feb;506(7488):307-15

1] Department of Earth Sciences, University of California, Riverside, California 92521, USA [2] Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA.

The rapid increase of carbon dioxide concentration in Earth's modern atmosphere is a matter of major concern. But for the atmosphere of roughly two-and-half billion years ago, interest centres on a different gas: free oxygen (O2) spawned by early biological production. The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth's history.
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http://dx.doi.org/10.1038/nature13068DOI Listing
February 2014

Sulfur isotopes track the global extent and dynamics of euxinia during Cretaceous Oceanic Anoxic Event 2.

Proc Natl Acad Sci U S A 2013 Nov 29;110(46):18407-12. Epub 2013 Oct 29.

Department of Earth Sciences, University of California, Riverside, CA 92521.

The Mesozoic Era is characterized by numerous oceanic anoxic events (OAEs) that are diagnostically expressed by widespread marine organic-carbon burial and coeval carbon-isotope excursions. Here we present coupled high-resolution carbon- and sulfur-isotope data from four European OAE 2 sections spanning the Cenomanian-Turonian boundary that show roughly parallel positive excursions. Significantly, however, the interval of peak magnitude for carbon isotopes precedes that of sulfur isotopes with an estimated offset of a few hundred thousand years. Based on geochemical box modeling of organic-carbon and pyrite burial, the sulfur-isotope excursion can be generated by transiently increasing the marine burial rate of pyrite precipitated under euxinic (i.e., anoxic and sulfidic) water-column conditions. To replicate the observed isotopic offset, the model requires that enhanced levels of organic-carbon and pyrite burial continued a few hundred thousand years after peak organic-carbon burial, but that their isotope records responded differently due to dramatically different residence times for dissolved inorganic carbon and sulfate in seawater. The significant inference is that euxinia persisted post-OAE, but with its global extent dwindling over this time period. The model further suggests that only ~5% of the global seafloor area was overlain by euxinic bottom waters during OAE 2. Although this figure is ~30× greater than the small euxinic fraction present today (~0.15%), the result challenges previous suggestions that one of the best-documented OAEs was defined by globally pervasive euxinic deep waters. Our results place important controls instead on local conditions and point to the difficulty in sustaining whole-ocean euxinia.
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http://dx.doi.org/10.1073/pnas.1305304110DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3831968PMC
November 2013