Publications by authors named "Dan Asael"

4 Publications

  • Page 1 of 1

Subglacial meltwater supported aerobic marine habitats during Snowball Earth.

Proc Natl Acad Sci U S A 2019 12 2;116(51):25478-25483. Epub 2019 Dec 2.

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

The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations.
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http://dx.doi.org/10.1073/pnas.1909165116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6926012PMC
December 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

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

Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere.

Proc Natl Acad Sci U S A 2013 Oct 30;110(42):16736-41. Epub 2013 Sep 30.

Nordic Center for Earth Evolution (NordCEE), and Institute of Biology, University of Southern Denmark, DK-5230 Odense M, Denmark.

The oxygen content of Earth's atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth's oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth's oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest-lived positive ╬┤(13)C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.
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http://dx.doi.org/10.1073/pnas.1315570110DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3801071PMC
October 2013