Research Interests: Geology and Geochemistry
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Research Interests: Geology, Geochemistry, Geobiology, Coevolution, Biology, and 7 moreCyanobacteria, Ecology, Algae, Medicine, Nitrogen, Biological evolution, and Metals
Research Interests: Geology and Geochemistry
Critical thinking and scientific reasoning are central to higher education in the United States, but many courses (in-person and online) teach students information about science much more than they teach the actual process of science and... more
Critical thinking and scientific reasoning are central to higher education in the United States, but many courses (in-person and online) teach students information about science much more than they teach the actual process of science and its associated knowledge and skills. In the online arena specifically, the tools available for course construction exacerbate this problem by making it difficult to build the types of active learning activities that research shows to be the most effective. Here, we present a report on Habitable Worlds, offered by Arizona State University for 12 semesters over the past 6 years. This is a unique online course that uses an array of novel technologies to deliver an active, inquiry-driven learning experience. Learning outcomes and quantitative data from more than 3000 students demonstrate the success of our approach but also identify several remaining challenges. The design and development of this course offers valuable lessons for instructional designer...
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Recently debate has arisen over the redox state of the mid-Proterozoic oceans. The occurrence of banded iron formations (BIF) in the Archean and Paleoproterozoic strongly suggests oceans with little dissolved O2 or H2S until ca. 1.8 Ga.... more
Recently debate has arisen over the redox state of the mid-Proterozoic oceans. The occurrence of banded iron formations (BIF) in the Archean and Paleoproterozoic strongly suggests oceans with little dissolved O2 or H2S until ca. 1.8 Ga. The disappearance of BIF after this time is commonly taken to indicate ocean oxygenation. Alternatively, the extent of sulfidic conditions in the oceans
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It is not known whether environmental O2 levels increased in a linear fashion or fluctuated dynamically between the evolution of oxygenic photosynthesis and the later Great Oxidation Event. New rhenium-osmium isotope data from the late... more
It is not known whether environmental O2 levels increased in a linear fashion or fluctuated dynamically between the evolution of oxygenic photosynthesis and the later Great Oxidation Event. New rhenium-osmium isotope data from the late Archean Mount McRae Shale, Western Australia, reveal a transient episode of oxidative continental weathering more than 50 million years before the onset of the Great Oxidation Event. A depositional age of 2495 ± 14 million years and an initial (187)Os/(188)Os of 0.34 ± 0.19 were obtained for rhenium- and molybdenum-rich black shales. The initial (187)Os/(188)Os is higher than the mantle/extraterrestrial value of 0.11, pointing to mild environmental oxygenation and oxidative mobilization of rhenium, molybdenum, and radiogenic osmium from the upper continental crust and to contemporaneous transport of these metals to seawater. By contrast, stratigraphically overlying black shales are rhenium- and molybdenum-poor and have a mantle-like initial (187)Os/(1...
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We present the first observations of natural mass-dependent fractionation of the isotopic composition of molybdenum (Mo), using multi-collector inductively coupled plasma mass spectrometry. Variations in the isotopic composition of Mo are... more
We present the first observations of natural mass-dependent fractionation of the isotopic composition of molybdenum (Mo), using multi-collector inductively coupled plasma mass spectrometry. Variations in the isotopic composition of Mo are reported as δ97/95Mo (=((97Mo/95Mo)sample/(97Mo/95Mo)standard−1)×1000‰). External analytical precision of δ97/95Mo is 1‰ between sediments deposited under anoxic conditions (δ97/95Mo=+1.02 to +1.52‰ relative to our in-house standard) and ferromanganese nodules (δ97/95Mo=−0.63
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ABSTRACT We have measured iridium and platinum in ancient metasediments to obtain the first direct constraints on the extraterrestrial mass flux at the Earth's surface before 3.8 Gyr (the Hadean era). The craters of the Moon... more
ABSTRACT We have measured iridium and platinum in ancient metasediments to obtain the first direct constraints on the extraterrestrial mass flux at the Earth's surface before 3.8 Gyr (the Hadean era). The craters of the Moon record an intense "late heavy bombardment" (LHB) beginning as early as 4.15 Gyr, and terminating ca.3.85 +/- 0.05 Gyr. Recent geochemical studies of metasediments and other supracrustal rocks from Akilia Island, in southern West Greenland, appear to extend both the record of marine sedimentation and the record of metabolically-sophisticated life to > 3.8 Gyr. Additional information is contained in the original extended abstract.
A continuous sequence through the Late Archean Mt. McRae Shale has been recovered as part of the NASA Astrobiology Drilling Program. The provision of fresh unweathered drill core enables iron paleoredox proxies to provide a high... more
A continuous sequence through the Late Archean Mt. McRae Shale has been recovered as part of the NASA Astrobiology Drilling Program. The provision of fresh unweathered drill core enables iron paleoredox proxies to provide a high resolution record of environmental change arising from a transient or secular increase in ocean/atmospheric oxygenation and the episodic enrichment of iron from hydrothermal sources.
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Research Interests: Speciation, Earth, Science, Multidisciplinary, Weathering, and 5 moreArchean, Precambrian, Shale, Oxidation, and Sedimentary Rocks
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Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thus the bulk of living matter, it is theoretically possible... more
Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thus the bulk of living matter, it is theoretically possible that some other elements in the periodic table could serve the same functions. Here, we describe a bacterium, strain GFAJ-1 of the Halomonadaceae, isolated from Mono Lake, California, that is able to substitute arsenic for phosphorus to sustain its growth. Our data show evidence for arsenate in macromolecules that normally contain phosphate, most notably nucleic acids and proteins. Exchange of one of the major bio-elements may have profound evolutionary and geochemical importance.