Publications by authors named "Terry T Isson"

4 Publications

  • Page 1 of 1

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 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

Reverse weathering as a long-term stabilizer of marine pH and planetary climate.

Nature 2018 08 8;560(7719):471-475. Epub 2018 Aug 8.

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

For the first four billion years of Earth's history, climate was marked by apparent stability and warmth despite the Sun having lower luminosity. Proposed mechanisms for maintaining an elevated partial pressure of carbon dioxide in the atmosphere ([Formula: see text]) centre on a reduction in the weatherability of Earth's crust and therefore in the efficiency of carbon dioxide removal from the atmosphere. However, the effectiveness of these mechanisms remains debated. Here we use a global carbon cycle model to explore the evolution of processes that govern marine pH, a factor that regulates the partitioning of carbon between the ocean and the atmosphere. We find that elevated rates of 'reverse weathering'-that is, the consumption of alkalinity and generation of acidity during marine authigenic clay formation-enhanced the retention of carbon within the ocean-atmosphere system, leading to an elevated [Formula: see text] baseline. Although this process is dampened by sluggish kinetics today, we propose that more prolific rates of reverse weathering would have persisted under the pervasively silica-rich conditions that dominated Earth's early oceans. This distinct ocean and coupled carbon-silicon cycle state would have successfully maintained the equable and ice-free environment that characterized most of the Precambrian period. Further, we propose that during this time, the establishment of a strong negative feedback between marine pH and authigenic clay formation would have also enhanced climate stability by mitigating large swings in [Formula: see text]-a critical component of Earth's natural thermostat that would have been dominant for most of Earth's history. We speculate that the late ecological rise of siliceous organisms and a resulting decline in silica-rich conditions dampened the reverse weathering buffer, destabilizing Earth's climate system and lowering baseline [Formula: see text].
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http://dx.doi.org/10.1038/s41586-018-0408-4DOI Listing
August 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