Publications by authors named "Linda C Kah"

9 Publications

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Structural and chemical heterogeneity of Proterozoic organic microfossils of the ca. 1 Ga old Angmaat Formation, Baffin Island, Canada.

Geobiology 2021 Jul 23. Epub 2021 Jul 23.

Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France.

Organic microfossils in Meso- and Neoproterozoic rocks are of key importance to track the emergence and evolution of eukaryotic life. An increasing number of studies combine Raman spectroscopy with synchrotron-based methods to characterize these microfossils. A recurring observation is that Raman spectra of organic microfossils show negligible variation on a sample scale and that variation between different samples can be explained by differences in thermal maturation or in the biologic origin of organic precursor material. There is a paucity of work, however, that explores the extent to which the petrographic framework and diagenetic processes might influence the chemical structure of organic materials. We present a detailed Raman spectroscopy-based study of a complex organic microfossil assemblage in the ca. 1 Ga old Angmaat Formation, Baffin Island, Canada. This formation contains abundant early diagenetic chert that preserves silicified microbial mats with numerous, readily identifiable organic microfossils. Individual chert beds show petrographic differences with discrete episodes of cementation and recrystallization. Raman spectroscopy reveals measurable variation of organic maturity between samples and between neighboring organic microfossils of the same taxonomy and taphonomic state. Scanning transmission X-ray microscopy performed on taphonomically similar coccoidal microfossils from the same thin section shows distinct chemical compositions, with varying ratios of aromatic compounds to ketones and phenols. Such observations imply that geochemical variation of organic matter is not necessarily coupled to thermal alteration or organic precursor material. Variation of the Raman signal across single samples is most likely linked to the diagenetic state of analyzed materials and implies an association between organic preservation and access to diagenetic fluids. Variation in the maturity of individual microfossils may be a natural outcome of local diagenetic processes and potentially exceeds differences derived from precursor organic material. These observations stress the importance of detailed in situ characterization by Raman spectroscopy to identify target specimens for further chemical analysis.
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http://dx.doi.org/10.1111/gbi.12463DOI Listing
July 2021

Photogeologic Map of the Perseverance Rover Field Site in Jezero Crater Constructed by the Mars 2020 Science Team.

Space Sci Rev 2020 Dec 3;216(8). Epub 2020 Nov 3.

Plancius Research, Severna Park, MD, USA.

The Mars 2020 Perseverance rover landing site is located within Jezero crater, a ∼ 50 km diameter impact crater interpreted to be a Noachian-aged lake basin inside the western edge of the Isidis impact structure. Jezero hosts remnants of a fluvial delta, inlet and outlet valleys, and infill deposits containing diverse carbonate, mafic, and hydrated minerals. Prior to the launch of the Mars 2020 mission, members of the Science Team collaborated to produce a photogeologic map of the Perseverance landing site in Jezero crater. Mapping was performed at a 1:5000 digital map scale using a 25 cm/pixel High Resolution Imaging Science Experiment (HiRISE) orthoimage mosaic base map and a 1 m/pixel HiRISE stereo digital terrain model. Mapped bedrock and surficial units were distinguished by differences in relative brightness, tone, topography, surface texture, and apparent roughness. Mapped bedrock units are generally consistent with those identified in previously published mapping efforts, but this study's map includes the distribution of surficial deposits and sub-units of the Jezero delta at a higher level of detail than previous studies. This study considers four possible unit correlations to explain the relative age relationships of major units within the map area. Unit correlations include previously published interpretations as well as those that consider more complex interfingering relationships and alternative relative age relationships. The photogeologic map presented here is the foundation for scientific hypothesis development and strategic planning for Perseverance's exploration of Jezero crater.
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http://dx.doi.org/10.1007/s11214-020-00739-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7116714PMC
December 2020

A persistently low level of atmospheric oxygen in Earth's middle age.

Nat Commun 2021 01 13;12(1):351. Epub 2021 Jan 13.

Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, 20015, USA.

Resolving how Earth surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time. The redox sensitivity of cerium relative to other rare earth elements and its uptake in carbonate minerals make the Ce anomaly (Ce/Ce*) a particularly useful proxy for capturing redox conditions in the local marine environment. Here, we report Ce/Ce* data in marine carbonate rocks through 3.5 billion years of Earth's history, focusing in particular on the mid-Proterozoic Eon (i.e., 1.8 - 0.8 Ga). To better understand the role of atmospheric oxygenation, we use Ce/Ce* data to estimate the partial pressure of atmospheric oxygen (pO) through this time. Our thermodynamics-based modeling supports a major rise in atmospheric oxygen level in the aftermath of the Great Oxidation Event (~ 2.4 Ga), followed by invariant pO of about 1% of present atmospheric level through most of the Proterozoic Eon (2.4 to 0.65 Ga).
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http://dx.doi.org/10.1038/s41467-020-20484-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7806885PMC
January 2021

Is a Linear or a Walkabout Protocol More Efficient When Using a Rover to Choose Biologically Relevant Samples in a Small Region of Interest?

Astrobiology 2020 03 5;20(3):327-348. Epub 2020 Feb 5.

Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado.

We conducted a field test at a potential Mars analog site to provide insight into planning for future robotic missions such as Mars 2020, where science operations must facilitate efficient choice of biologically relevant sampling locations. We compared two data acquisition and decision-making protocols currently used by Mars Science Laboratory: (1) a linear approach, where sites are examined as they are encountered and (2) a walkabout approach, in which the field site is first examined with remote rover instruments to gain an understanding of regional context followed by deployment of time- and power-intensive contact and sampling instruments on a smaller subset of locations. The walkabout method was advantageous in terms of both the time required to execute and a greater confidence in results and interpretations, leading to enhanced ability to tailor follow-on observations to better address key science and sampling goals. This advantage is directly linked to the walkabout method's ability to provide broad geological context earlier in the science analysis process. For Mars 2020, and specifically for small regions to be explored (., <1 km), we recommend that the walkabout approach be considered where possible, to provide early context and time for the science team to develop a coherent suite of hypotheses and robust ways to test them.
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http://dx.doi.org/10.1089/ast.2019.2090DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7071088PMC
March 2020

Mineralized microbialites as archives of environmental evolution, Laguna Negra, Catamarca Province, Argentina.

Geobiology 2019 03 13;17(2):199-222. Epub 2018 Dec 13.

Department of Earth & Planetary Sciences, The University of Tennessee, Knoxville, Tennessee.

Environmental fluctuations are recorded in a variety of sedimentary archives of lacustrine depositional systems. Geochemical signals recovered from bottom sediments in closed-basin lakes are among the most sensitive paleoenvironmental indicators and are commonly used in reconstructing lake evolution. Microbialites (i.e., organosedimentary deposits accreted through microbial trapping and binding of detrital sediment or in situ mineral precipitation on organics [Palaios, 2, 1987, 241]), however, have been largely overlooked as paleoenvironmental repositories. Here, we investigate concentrically laminated mineralized microbialites from Laguna Negra, a high-altitude (4,100 m above sea level) hypersaline, closed-basin lake in northwestern Argentina, and explore the potential for recovery of environmental signals from these unique sedimentary archives. Spatial heterogeneity in hydrological regime helps define zones inside Laguna Negra, each with their own morphologically distinct microbialite type. Most notably, platey microbialites (in Zone 3A) are precipitated by evaporative concentration processes, while discoidal oncolites (in Zone 3C) are interpreted result from fluid mixing and biologically mediated nucleation. This spatial heterogeneity is reflected in petrographically distinct carbonate fabrics: micritic, botryoidal, and isopachous. Fabric type is interpreted to reflect a combination of physical and biological influences during mineralization, and paired C-isotope measurement of carbonate and organic matter supports ecological differences as a dominant control on C-isotopic evolution between zones. Laminae of Laguna Negra microbialites preserve a range of δ C from +5.75‰ to +18.25‰ and δ O from -2.04‰ to +9.28‰. Temporal trends of lower carbon and oxygen isotopic compositions suggest that the influence of CO degassing associated with evaporation has decreased over time. Combined, these results indicate that microbialite archives can provide data that aid in interpretation of both lake paleohydrology and paleoenvironmental change.
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http://dx.doi.org/10.1111/gbi.12327DOI Listing
March 2019

Mercury isotope signatures record photic zone euxinia in the Mesoproterozoic ocean.

Proc Natl Acad Sci U S A 2018 10 1;115(42):10594-10599. Epub 2018 Oct 1.

School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287.

Photic zone euxinia (PZE) is a condition where anoxic, HS-rich waters occur in the photic zone (PZ). PZE has been invoked as an impediment to the evolution of complex life on early Earth and as a kill mechanism for Phanerozoic mass extinctions. Here, we investigate the potential application of mercury (Hg) stable isotopes in marine sedimentary rocks as a proxy for PZE by measuring Hg isotope compositions in late Mesoproterozoic (∼1.1 Ga) shales that have independent evidence of PZE during discrete intervals. Strikingly, a significantly negative shift of Hg mass-independent isotope fractionation (MIF) was observed during euxinic intervals, suggesting changes in Hg sources or transformations in oceans coincident with the development of PZE. We propose that the negative shift of Hg MIF was most likely caused by () photoreduction of Hg(II) complexed by reduced sulfur ligands in a sulfide-rich PZ, and () enhanced sequestration of atmospheric Hg(0) to the sediments by thiols and sulfide that were enriched in the surface ocean as a result of PZE. This study thus demonstrates that Hg isotope compositions in ancient marine sedimentary rocks can be a promising proxy for PZE and therefore may provide valuable insights into changes in ocean chemistry and its impact on the evolution of life.
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http://dx.doi.org/10.1073/pnas.1721733115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6196510PMC
October 2018

The Mars Science Laboratory (MSL) Mast cameras and Descent imager: Investigation and instrument descriptions.

Earth Space Sci 2017 08 19;4(8):506-539. Epub 2017 Aug 19.

Malin Space Science Systems, Inc San Diego California USA.

The Mars Science Laboratory Mast camera and Descent Imager investigations were designed, built, and operated by Malin Space Science Systems of San Diego, CA. They share common electronics and focal plane designs but have different optics. There are two Mastcams of dissimilar focal length. The Mastcam-34 has an f/8, 34 mm focal length lens, and the M-100 an f/10, 100 mm focal length lens. The M-34 field of view is about 20° × 15° with an instantaneous field of view (IFOV) of 218 μrad; the M-100 field of view (FOV) is 6.8° × 5.1° with an IFOV of 74 μrad. The M-34 can focus from 0.5 m to infinity, and the M-100 from ~1.6 m to infinity. All three cameras can acquire color images through a Bayer color filter array, and the Mastcams can also acquire images through seven science filters. Images are ≤1600 pixels wide by 1200 pixels tall. The Mastcams, mounted on the ~2 m tall Remote Sensing Mast, have a 360° azimuth and ~180° elevation field of regard. Mars Descent Imager is fixed-mounted to the bottom left front side of the rover at ~66 cm above the surface. Its fixed focus lens is in focus from ~2 m to infinity, but out of focus at 66 cm. The f/3 lens has a FOV of ~70° by 52° across and along the direction of motion, with an IFOV of 0.76 mrad. All cameras can acquire video at 4 frames/second for full frames or 720p HD at 6 fps. Images can be processed using lossy Joint Photographic Experts Group and predictive lossless compression.
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http://dx.doi.org/10.1002/2016EA000252DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5652233PMC
August 2017

Active microbial sulfur disproportionation in the Mesoproterozoic.

Science 2005 Dec;310(5753):1477-9

Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, USA.

The environmental expression of sulfur compound disproportionation has been placed between 640 and 1050 million years ago (Ma) and linked to increases in atmospheric oxygen. These arguments have their basis in temporal changes in the magnitude of 34S/32S fractionations between sulfate and sulfide. Here, we present a Proterozoic seawater sulfate isotope record that includes the less abundant sulfur isotope 33S. These measurements imply that sulfur compound disproportionation was an active part of the sulfur cycle by 1300 Ma and that progressive Earth surface oxygenation may have characterized the Mesoproterozoic.
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http://dx.doi.org/10.1126/science.1117824DOI Listing
December 2005

Low marine sulphate and protracted oxygenation of the Proterozoic biosphere.

Nature 2004 Oct;431(7010):834-8

Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA.

Progressive oxygenation of the Earth's early biosphere is thought to have resulted in increased sulphide oxidation during continental weathering, leading to a corresponding increase in marine sulphate concentration. Accurate reconstruction of marine sulphate reservoir size is therefore important for interpreting the oxygenation history of early Earth environments. Few data, however, specifically constrain how sulphate concentrations may have changed during the Proterozoic era (2.5-0.54 Gyr ago). Prior to 2.2 Gyr ago, when oxygen began to accumulate in the Earth's atmosphere, sulphate concentrations are inferred to have been <1 mM and possibly <200 microM, on the basis of limited isotopic variability preserved in sedimentary sulphides and experimental data showing suppressed isotopic fractionation at extremely low sulphate concentrations. By 0.8 Gyr ago, oxygen and thus sulphate levels may have risen significantly. Here we report large stratigraphic variations in the sulphur isotope composition of marine carbonate-associated sulphate, and use a rate-dependent model for sulphur isotope change that allows us to track changes in marine sulphate concentrations throughout the Proterozoic. Our calculations indicate sulphate levels between 1.5 and 4.5 mM, or 5-15 per cent of modern values, for more than 1 Gyr after initial oxygenation of the Earth's biosphere. Persistence of low oceanic sulphate demonstrates the protracted nature of Earth's oxygenation. It links biospheric evolution to temporal patterns in the depositional behaviour of marine iron- and sulphur-bearing minerals, biological cycling of redox-sensitive elements and availability of trace metals essential to eukaryotic development.
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http://dx.doi.org/10.1038/nature02974DOI Listing
October 2004
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