Publications by authors named "Peter W Crockford"

9 Publications

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Geologic evidence for an icehouse Earth before the Sturtian global glaciation.

Sci Adv 2020 Jun 10;6(24):eaay6647. Epub 2020 Jun 10.

Department of Geosciences, Princeton University, Princeton, NJ, USA.

Snowball Earth episodes, times when the planet was covered in ice, represent the most extreme climate events in Earth's history. Yet, the mechanisms that drive their initiation remain poorly constrained. Current climate models require a cool Earth to enter a Snowball state. However, existing geologic evidence suggests that Earth had a stable, warm, and ice-free climate before the Neoproterozoic Sturtian global glaciation [ca. 717 million years (Ma) ago]. Here, we present eruption ages for three felsic volcanic units interbedded with glaciolacustrine sedimentary rocks from southwest Virginia, USA, that demonstrate that glacially influenced sedimentation occurred at tropical latitudes ca. 751 Ma ago. Our findings are the first geologic evidence of a cool climate teetering on the edge of global glaciation several million years before the Sturtian Snowball Earth.
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http://dx.doi.org/10.1126/sciadv.aay6647DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7286673PMC
June 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

A productivity collapse to end Earth's Great Oxidation.

Proc Natl Acad Sci U S A 2019 08 12;116(35):17207-17212. Epub 2019 Aug 12.

Non-traditional Isotope Research on Various Advanced Novel Applications (NIRVANA) Labs, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

It has been hypothesized that the overall size of-or efficiency of carbon export from-the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma). However, the timing, tempo, and trigger for this decrease remain poorly constrained. Here we test this hypothesis by studying the isotope geochemistry of sulfate minerals from the Belcher Group, in subarctic Canada. Using insights from sulfur and barium isotope measurements, combined with radiometric ages from bracketing strata, we infer that the sulfate minerals studied here record ambient sulfate in the immediate aftermath of the GOE (ca. 2,018 Ma). These sulfate minerals captured negative triple-oxygen isotope anomalies as low as ∼ -0.8‰. Such negative values occurring shortly after the GOE require a rapid reduction in primary productivity of >80%, although even larger reductions are plausible. Given that these data imply a collapse in primary productivity rather than export efficiency, the trigger for this shift in the Earth system must reflect a change in the availability of nutrients, such as phosphorus. Cumulatively, these data highlight that Earth's GOE is a tale of feast and famine: A geologically unprecedented reduction in the size of the biosphere occurred across the end-GOE transition.
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http://dx.doi.org/10.1073/pnas.1900325116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6717284PMC
August 2019

Large sulfur isotope fractionation by bacterial sulfide oxidation.

Sci Adv 2019 07 24;5(7):eaaw1480. Epub 2019 Jul 24.

Center for Geomicrobiology, Department of Bioscience, Aarhus University, Aarhus, Denmark.

A sulfide-oxidizing microorganism, (DA), generates a consistent enrichment of sulfur-34 () in the produced sulfate of +12.5 per mil or greater. This observation challenges the general consensus that the microbial oxidation of sulfide does not result in large enrichments and suggests that sedimentary sulfides and sulfates may be influenced by metabolic activity associated with sulfide oxidation. Since the DA-type sulfide oxidation pathway is ubiquitous in sediments, in the modern environment, and throughout Earth history, the enrichments and depletions in in sediments may be the combined result of three microbial metabolisms: microbial sulfate reduction, the disproportionation of external sulfur intermediates, and microbial sulfide oxidation.
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http://dx.doi.org/10.1126/sciadv.aaw1480DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6656534PMC
July 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

Transient marine euxinia at the end of the terminal Cryogenian glaciation.

Nat Commun 2018 08 1;9(1):3019. Epub 2018 Aug 1.

Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Science, Peking University, Beijing, 100871, China.

Termination of the terminal Cryogenian Marinoan snowball Earth glaciation (~650-635 Ma) is associated with the worldwide deposition of a cap carbonate. Modeling studies suggest that, during and immediately following deglaciation, the ocean may have experienced a rapid rise in pH and physical stratification followed by oceanic overturn. Testing these predictions requires the establishment of a high-resolution sequence of events within sedimentary records. Here we report the conspicuous occurrence of pyrite concretions in the topmost Nantuo Formation (South China) that was deposited in the Marinoan glacial deposits. Sedimentary facies and sulfur isotope data indicate pyrite precipitation in the sediments with HS diffusing from the overlying sulfidic/euxinic seawater and Fe (II) from diamictite sediments. These observations suggest a transient but widespread presence of marine euxinia in an ocean characterized by redox stratification, high bioproductivity, and high-fluxes of sulfate from chemical weathering before the deposition of the cap carbonate.
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http://dx.doi.org/10.1038/s41467-018-05423-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6070556PMC
August 2018

Triple oxygen isotope evidence for limited mid-Proterozoic primary productivity.

Nature 2018 07 18;559(7715):613-616. Epub 2018 Jul 18.

University of Colorado Boulder, Boulder, CO, USA.

The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative ∆O values (down to -0.88‰) observed in sulfates, except for those from the terminal Cryogenian period. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO and O and the photosynthetic O flux at this time. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean and Phanerozoic primary production, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O over geologic time.
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http://dx.doi.org/10.1038/s41586-018-0349-yDOI Listing
July 2018

Publisher Correction: Pelagic barite precipitation at micromolar ambient sulfate.

Nat Commun 2018 01 16;9(1):305. Epub 2018 Jan 16.

Large Lakes Observatory, University of Minnesota Duluth, Duluth, MN, 55812, USA.

The original version of this Article contained an error in the barite saturation state equation in the fourth paragraph of the Introduction and incorrectly read 'Ω=({Ba}⋅{SO})/K)'. The correct version removes the superscript 134 next to 'Ba'. This error has now been corrected in both the PDF and HTML versions of the Article.
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http://dx.doi.org/10.1038/s41467-017-02701-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5768865PMC
January 2018

Pelagic barite precipitation at micromolar ambient sulfate.

Nat Commun 2017 11 7;8(1):1342. Epub 2017 Nov 7.

Large Lakes Observatory, University of Minnesota Duluth, Duluth, MN, 55812, USA.

Geochemical analyses of sedimentary barites (barium sulfates) in the geological record have yielded fundamental insights into the chemistry of the Archean environment and evolutionary origin of microbial metabolisms. However, the question of how barites were able to precipitate from a contemporary ocean that contained only trace amounts of sulfate remains controversial. Here we report dissolved and particulate multi-element and barium-isotopic data from Lake Superior that evidence pelagic barite precipitation at micromolar ambient sulfate. These pelagic barites likely precipitate within particle-associated microenvironments supplied with additional barium and sulfate ions derived from heterotrophic remineralization of organic matter. If active during the Archean, pelagic precipitation and subsequent sedimentation may account for the genesis of enigmatic barite deposits. Indeed, barium-isotopic analyses of barites from the Paleoarchean Dresser Formation are consistent with a pelagic mechanism of precipitation, which altogether offers a new paradigm for interpreting the temporal occurrence of barites in the geological record.
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http://dx.doi.org/10.1038/s41467-017-01229-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5673900PMC
November 2017