Publications by authors named "Nathan D Sheldon"

6 Publications

  • Page 1 of 1

C plant carbon isotope discrimination does not respond to CO concentration on decadal to centennial timescales.

New Phytol 2021 Mar 22;229(5):2576-2585. Epub 2020 Nov 22.

Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, 1100 N University Avenue, Ann Arbor, MI, 48109, USA.

Plant carbon isotope discrimination is complex, and could be driven by climate, evolution and/or edaphic factors. We tested the climate drivers of carbon isotope discrimination in modern and historical plant chemistry, and focus in particular on the relationship between rising [CO ] over Industrialization and carbon isotope discrimination. We generated temporal records of plant carbon isotopes from museum specimens collected over a climo-sequence to test plant responses to climate and atmospheric change over the past 200 yr (including Pinus strobus, Platycladus orientalis, Populus tremuloides, Thuja koraiensis, Thuja occidentalis, Thuja plicata, Thuja standishii and Thuja sutchuenensis). We aggregated our results with a meta-analysis of a wide range of C plants to make a comprehensive study of the distribution of carbon isotope discrimination and values among different plant types. We show that climate variables (e.g. mean annual precipitation, temperature and, key to this study, CO in the atmosphere) do not drive carbon isotope discrimination. Plant isotope discrimination is intrinsic to each taxon, and could link phylogenetic relationships and adaptation to climate quantitatively and over ecological to geological time scales.
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http://dx.doi.org/10.1111/nph.17030DOI Listing
March 2021

Associations between redox-sensitive trace metals and microbial communities in a Proterozoic ocean analogue.

Geobiology 2020 07 17;18(4):462-475. Epub 2020 Mar 17.

Department of Biological Sciences, Kent State University, Kent, OH, USA.

Constraints on Precambrian ocean chemistry are dependent upon sediment geochemistry. However, diagenesis and metamorphism can destroy primary biosignatures, making it difficult to consider biology when interpreting geochemical data. Modern analogues for ancient ecosystems can be useful tools for identifying how sediment geochemistry records an active biosphere. The Middle Island Sinkhole (MIS) in Lake Huron is an analogue for shallow Proterozoic waters due to its low oxygen water chemistry and microbial communities that exhibit diverse metabolic functions at the sediment-water interface. This study uses sediment trace metal contents and microbial abundances in MIS sediments and an oxygenated Lake Huron control site (LH) to infer mechanisms for trace metal burial. The adsorption of trace metals to Mn-oxyhydroxides is a critical burial pathway for metals in oxic LH sediments, but not for the MIS mat and sediments, consistent with conventional understanding of Mn cycling. Micronutrient trace metals (e.g., Zn) are associated with organic matter regardless of oxygen and sulfide availability. Although U and V are conventionally considered to be organically complexed in suboxic and anoxic conditions, U and organic covary in oxic LH sediments, and Mn-oxyhydroxide cycling dominates V deposition in the anoxic MIS sediments. Significant correlations between Mo and organic matter across all redox regimes have major implications for our interpretations of Mo isotope systematics in the geologic record. Finally, while microbial groups vary between the sampling locales (e.g., the cyanobacteria in the MIS microbial mat are not present in LH sediments), LH and MIS ultimately have similar relationships between microbial assemblages and metal burial, making it difficult to link trace metal burial to microbial metabolisms. Together, these results indicate that bulk sediment trace metal composition does not capture microbiological processes; more robust trace metal geochemistry such as isotopes and speciation may be critical for understanding the intersections between microbiology and sediment geochemistry.
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http://dx.doi.org/10.1111/gbi.12388DOI Listing
July 2020

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

Rapid response to anthropogenic climate change by : implications for past climate reconstructions and future climate predictions.

PeerJ 2019 26;7:e7378. Epub 2019 Jul 26.

Department of Earth and Environmental Sciences, University of Michigan-Ann Arbor, Ann Arbor, MI, USA.

Carbon isotope values of leaves (δC) from meta-analyses and growth chamber studies of C plants have been used to propose generalized relationships between δC and climate variables such as mean annual precipitation (MAP), atmospheric concentration of carbon dioxide ([CO]), and other climate variables. These generalized relationships are frequently applied to the fossil record to create paleoclimate reconstructions. Although plant evolution influences biochemistry and response to environmental stress, few studies have assessed species-specific carbon assimilation as it relates to climate outside of a laboratory. We measured δC values and C:N ratios of a wide-ranging evergreen conifer with a long fossil record, (Cupressaceae) collected 1804-2017, in order to maximize potential paleo-applications of our focal species. This high-resolution record represents a natural experiment from pre-Industrial to Industrial times, which spans a range of geologically meaningful [CO] and δC values. Δ values (carbon isotope discrimination between δC and δC) remain constant across climate conditions, indicating limited response to environmental stress. Only δC and δC values showed a strong relationship (linear), thus, δC is an excellent record of carbon isotopic changes in the atmosphere during Industrialization. In contrast with previous free-air concentration enrichment experiments, no relationship was found between C:N ratios and increasing [CO]. Simultaneously static C:N ratios and Δ in light of increasing CO highlights plants' inability to match rapid climate change with increased carbon assimilation as previously expected; Δ values are not reliable tools to reconstruct MAP and [CO], and δC values only decrease with [CO] in line with atmospheric carbon isotope changes.
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http://dx.doi.org/10.7717/peerj.7378DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6662565PMC
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

Terrestrial cooling in Northern Europe during the eocene-oligocene transition.

Proc Natl Acad Sci U S A 2013 May 22;110(19):7562-7. Epub 2013 Apr 22.

Department of Chemistry and Center for Integrative Geoscience, University of Connecticut, Storrs, CT 06269, USA.

Geochemical and modeling studies suggest that the transition from the "greenhouse" state of the Late Eocene to the "icehouse" conditions of the Oligocene 34-33.5 Ma was triggered by a reduction of atmospheric pCO2 that enabled the rapid buildup of a permanent ice sheet on the Antarctic continent. Marine records show that the drop in pCO2 during this interval was accompanied by a significant decline in high-latitude sea surface and deep ocean temperature and enhanced seasonality in middle and high latitudes. However, terrestrial records of this climate transition show heterogeneous responses to changing pCO2 and ocean temperatures, with some records showing a significant time lag in the temperature response to declining pCO2. We measured the Δ47 of aragonite shells of the freshwater gastropod Viviparus lentus from the Solent Group, Hampshire Basin, United Kingdom, to reconstruct terrestrial temperature and hydrologic change in the North Atlantic region during the Eocene-Oligocene transition. Our data show a decrease in growing-season surface water temperatures (~10 °C) during the Eocene-Oligocene transition, corresponding to an average decrease in mean annual air temperature of ~4-6 °C from the Late Eocene to Early Oligocene. The magnitude of cooling is similar to observed decreases in North Atlantic sea surface temperature over this interval and occurs during major glacial expansion. This suggests a close linkage between atmospheric carbon dioxide concentrations, Northern Hemisphere temperature, and expansion of the Antarctic ice sheets.
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http://dx.doi.org/10.1073/pnas.1210930110DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3651463PMC
May 2013