Gwyneth Gordon

Arizona State University, School of Earth and Space Exploration, Research Scientist & Lab Manager

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Papers by Gwyneth Gordon

Constraining the Molybdenum Isotope Budget of 2.63 – 2.50 Ga Oceans

GSA Annual Meeting in Seattle, Washington, USA - 2017, 2017

Extreme change in sulfide concentrations near the Bosporus Inlet (Black Sea) during the Little Ice Age tracked using molybdenum isotopes

AGUFM, Dec 1, 2011

ABSTRACT The Black Sea is the largest and most studied anoxic basin in the modern world. Much of ... more ABSTRACT The Black Sea is the largest and most studied anoxic basin in the modern world. Much of this research has focused on the redox structure of the water column, specifically the driving forces behind variations in the position and stability of the oxic-anoxic interface (chemocline). However, none of these studies has been able to quantify the historical sulfide concentrations associated with the changes in chemocline depth. Using the isotopic composition of molybdenum in sediments as a proxy, we show for the first time that varying concentrations of dissolved sulfide can be fingerprinted in historical systems. Our molybdenum isotope data indicate that in the region of the Bosporus inlet the chemocline rose more than 65 m, reaching concentrations over 100 μM sulfide in the bottom water approximately 300 ybp. This historical shoaling of the chemocline and extreme change in bottom water sulfide concentration exceeds the modern changes that have been observed directly and attributed to anthropogenic influences on the Black Sea chemistry/hydrology. Our preliminary findings demonstrate the potential of the Mo isotope proxy to reconstruct historical, basin-wide changes in water-column redox structure and sulfide concentrations in the Black Sea and other marine basins by analogy.

An expanded shale δ98Mo record permits recurrent shallow marine oxygenation during the Neoarchean

Chemical Geology, 2020

, et al., An expanded shale δ98Mo record permits recurrent shallow marine oxygenation during the ... more

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Extreme change in sulfide concentrations in the Black Sea during the Little Ice Age reconstructed using molybdenum isotopes

Geology, Jul 1, 2012

The Black Sea is the largest and most studied anoxic basin in the modern world. Much of this rese... more The Black Sea is the largest and most studied anoxic basin in the modern world. Much of this research has focused on the redox structure of the water column, specifi cally on the driving forces behind variations in the position, stability, and structure of the oxic-anoxic interface (chemocline). However, none of these studies has been able to quantify the historical sulfi de concentrations associated with the changes in chemocline depth. Using the isotopic composition of molybdenum in sediments as a proxy, we show for the fi rst time that varying concentrations of dissolved sulfi de can be fi ngerprinted in historical systems. Our molybdenum isotope data indicate that in the region of the Bosporus inlet, the chemocline rose more than 65 m, reaching concentrations over 100 μM sulfi de in the bottom water ca. 300 yr B.P. This historical shoaling of the chemocline and extreme change in bottom-water sulfi de concentration exceeds the modern changes that have been observed directly and attributed to anthropogenic infl uences on the Black Sea chemistry/hydrology. The fi rst cold interval of the Little Ice Age, when temperature and circulation changes occurred in the Black Sea basins, may have provided the natural trigger for this extreme rise in bottom-water sulfi de concentrations.

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Fully oxygenated water columns over continental shelves before the Great Oxidation Event

Nature Geoscience, Feb 25, 2019

Late Archaean sedimentary rocks contain compelling geochemical evidence for episodic accumulation... more Late Archaean sedimentary rocks contain compelling geochemical evidence for episodic accumulation of dissolved oxygen in the oceans along continental margins before the Great Oxidation Event. However, the extent of this oxygenation remains poorly constrained. Here we present thallium and molybdenum isotope compositions for anoxic organic-rich shales of the 2.5 billion-year-old Mount McRae Shale from Western Australia, which previously yielded geochemical evidence of a transient oxygenation event. During this event, we observe an anticorrelation between thalium and molybdenum isotope data, including two shifts to higher molybdenum and lower thalium isotope compositions. Our data indicate pronounced burial of manganese oxides in sediments elsewhere in the ocean at these times, which requires that water columns above portions of the ocean floor were fully oxygenated: all the way from the air-sea interface to well below the sediment-water interface. Well-oxygenated continental shelves were likely the most important sites of manganese oxide burial and mass-balance modeling results suggest that fully oxygenated water columns were at least a regional-scale feature of early-Earth's oceans 2.5 billion years ago.

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Technical comment on “Reexamination of 2.5-Ga ‘whiff’ of oxygen interval points to anoxic ocean before GOE”

Science Advances, Apr 7, 2023

Many lines of inorganic geochemical evidence suggest transient “whiffs” of environmental oxygenat... more Many lines of inorganic geochemical evidence suggest transient “whiffs” of environmental oxygenation before the Great Oxidation Event (GOE). Slotznick et al. assert that analyses of paleoredox proxies in the Mount McRae Shale, Western Australia, were misinterpreted and hence that environmental O 2 levels were persistently negligible before the GOE. We find these arguments logically flawed and factually incomplete.

Significance of 56Fe depletions in late-Archean shales and pyrite

Geochimica et Cosmochimica Acta, 2022

Shale Heavy Metal Isotope Records of Low Environmental O2 Between Two Archean Oxidation Events

Frontiers in Earth Science, Apr 26, 2022

Evidence of molecular oxygen (O 2) accumulation at Earth's surface during the Archean (4.0-2.5 bi... more Evidence of molecular oxygen (O 2) accumulation at Earth's surface during the Archean (4.0-2.5 billion years ago, or Ga) seems to increase in its abundance and compelling nature toward the end of the eon, during the runup to the Great Oxidation Event. Yet, many details of this late-Archean O 2 story remain under-constrained, such as the extent, tempo, and location of O 2 accumulation. Here, we present a detailed Fe, Tl, and U isotope study of shales from a continuous sedimentary sequence deposited between~2.6 and~2.5 Ga and recovered from the Pilbara Craton of Western Australia (the Wittenoom and Mt. Sylvia formations preserved in drill core ABDP9). We find a progressive decrease in bulk-shale Fe isotope compositions moving up core (as low as δ 56 Fe =-0.78 ± 0.08‰; 2SD) accompanied by invariant authigenic Tl isotope compositions (average ε 205 Tl A =-2.0 ± 0.6; 2SD) and bulk-shale U isotope compositions (average δ 238 U =-0.30 ± 0.05‰; 2SD) that are both not appreciably different from crustal rocks or bulk silicate Earth. While there are multiple possible interpretations of the decreasing δ 56 Fe values, many, to include the most compelling, invoke strictly anaerobic processes. The invariant and near-crustal ε 205 Tl A and δ 238 U values point even more strongly to this interpretation, requiring reducing to only mildly oxidizing conditions over ten-million-year timescales in the late-Archean. For the atmosphere, our results permit either homogenous and low O 2 partial pressures (between 10 −6.3 and 10 −6 present atmospheric level) or heterogeneous and spatially restricted O 2 accumulation nearest the sites of O 2 production. For the ocean, our results permit minimal penetration of O 2 in marine sediments over large areas of the seafloor, at most sufficient for the burial of Fe oxide minerals but insufficient for the burial of Mn oxide minerals. The persistently low background O 2 levels implied by our dataset between~2.6 and 2.5 Ga contrast with the timeframes immediately before and after, where strong evidence is presented for transient Archean Oxidation Events. Viewed in this broader context, our data support the emerging narrative that Earth's initial oxygenation was a dynamic process that unfolded in fits-and-starts over many hundreds-of-millions of years.

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Using natural stable calcium isotopes to rapidly assess changes in bone mineral balance using a bed rest model to induce bone loss

The FASEB Journal

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Developing a Clinically Useful Calcium Isotope Biomarker

Naturally-occurring Ca is mixture of six isotopes Ca-40, Ca-42, Ca-43, Ca-44, Ca-46, Ca-48). Biol... more Naturally-occurring Ca is mixture of six isotopes Ca-40, Ca-42, Ca-43, Ca-44, Ca-46, Ca-48). Biological reaction rates and equilibrium constants depend slightly, but measurably, on atomic mass, causing the relative abundances of Ca isotopes to vary between different tissues. During bone formation, light isotopes of Ca are preferentially incorporated into bone, leaving soft tissue depleted in light isotopes. In contrast, bone resorption exhibits no isotopic preference, and thus transfers the light isotope signature of bone back to soft tissue. This balance makes the Ca isotope composition of soft tissue (e.g. serum, urine) a highly sensitive, quantitative tracer for whole-body bone mineral balance (BMB).

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Re-assessing the Molybdenum Isotope Composition of Pre-GOE Seawater: Evidence for Dynamic Ocean Redox

Redox dynamics of later Cambrian oceans

Palaeogeography, Palaeoclimatology, Palaeoecology, 2021

Abstract A growing body of evidence suggests that the deep oceans during the early Paleozoic Era ... more Abstract A growing body of evidence suggests that the deep oceans during the early Paleozoic Era were widely oxygen deficient, despite evidence for increased marine oxygenation during the Neoproterozoic. However, the temporal and geographic extents and dynamics of reducing marine conditions within these oceans are not well understood. Here, we investigate marine redox history during the Drumian through the earliest Jiangshanian International Stages of the Cambrian Period, using concentrations of redox-sensitive metals (vanadium, uranium, and molybdenum), iron speciation, and Mo isotope stratigraphy of the Alum Shale Formation of Scandinavia. These data suggest a major perturbation occurred in trace metal cycling during the later Cambrian Period that was linked to a transient change in marine redox conditions coincident with the well-known Steptoean Positive Isotope Excursion or SPICE. The δ98Mo measurements of the Alum shale show systematic variations during the interval that contains the SPICE which are broadly consistent with a transient expansion of sulfidic, reducing marine environments — indicating a significant exacerbation of an already-common condition during the Cambrian Period. Additionally, iron speciation data record a local transition from predominantly anoxic, ferruginous (Fe+2 containing) to anoxic, euxinic (sulfide containing) water column conditions near the initiation of the SPICE. Trace metal abundances, however, appear to decline well before the start of the SPICE, suggesting an earlier initiation of the global expansion of reducing environments. More broadly, our data and modeling support the notion that significant portions of the oceans remained oxygen deficient throughout the later portion of the Cambrian, and that these oceans were also prone to transient intervals of more reducing conditions similar to the Oceanic Anoxic Events of the Mesozoic.

Multiple negative molybdenum isotope excursions in the Doushantuo Formation (South China) fingerprint complex redox-related processes in the Ediacaran Nanhua Basin

Geochimica et Cosmochimica Acta, 2019

The Ediacaran Doushantuo Formation offers one of the most complete and extensively studied record... more The Ediacaran Doushantuo Formation offers one of the most complete and extensively studied records of end-Neoproterozoic biotic and environmental change. Here, we report multiple coeval negative molybdenum (Mo) isotope excursions (to as low as d 98 Mo NIST+0.25 = À2.24 ± 0.10‰; 2SD) in shales from four separate sites in South China (Rongxi, Taoying, Wuhe, and Yuanjia) that preserve the Doushantuo Formation. The negative d 98 Mo excursions appear coincident with previously discovered and seemingly peculiar redox-sensitive element (RSE) patterns in the same sedimentary rocks. We propose that these geochemical trends can be explained by some combination of (a) enhanced local marine oxygenation in the sedimentary basin where the Doushantuo Formation was originally deposited (the Nanhua Basin) and (b) changes in the degree of connectivity between this paleo basin and the open ocean. Enhanced local marine oxygenation, by exposing more sediments in the Nanhua basin to H 2 S-poor conditions, could have hindered quantitative tetrathiomolybdate formation within these sediments. Local marine oxygenation could have also stimulated the operation of a Mn oxide shuttle. Today, both of these processes are shown to promote the retention of lighter-mass Mo isotopes in sediments and also govern RSE enrichment patterns. Alternatively, or in addition, the Nanhua Basin may not have maintained an uninterrupted connection with the open ocean during the entirety of the Ediacaran Period. The negative d 98 Mo excursions occur coincident with sea level highstands that could have also exposed more sediments in the basin to H 2 S poor conditions and/or catalyzed the operation of a local Mn oxide shuttle. When trying to infer temporal changes in ancient global ocean redox, it is important to consider the influence of sea level changes and associated variations in local depositional conditions on stratigraphic trends in RSE enrichments and isotope compositions.

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From water to edible fish. Transfer of metals and metalloids in the San Roque Reservoir (Córdoba, Argentina). Implications associated with fish consumption

Ecological Indicators, 2016

and Hg was analysed in water, sediments, and aquatic organisms from the San Roque Reservoir (Córd... more and Hg was analysed in water, sediments, and aquatic organisms from the San Roque Reservoir (Córdoba-Argentina), sampled during the wet and dry season, to evaluate their transfer through the food web. Stable nitrogen (␦ 15 N) isotopes were used to investigate trophic interactions. According to this, samples were divided into three trophic groups: plankton, shrimp (Palaemonetes argentinus) and fish (Silverside, Odontesthes bonariensis). Liver and gills are the main heavy metal storage tissues in fish. Hg and As concentrations in the muscle of O. bonariensis exceed the Oral Reference doses for metals established by USEPA (2009). Trophic magnification factors (TMFs) for each element were determined from the slope of the regression between trace element concentrations and ␦ 15 N. Calculated TMFs showed fundamental differences in the trophodynamics of the studied elements during the wet and dry season in the San Roque Reservoir.

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Predicting multiple myeloma disease activity by analyzing natural calcium isotopic composition

Leukemia, 2014

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The global anoxic 'SPICE' event ∼500 Myrs ago. Trace metal depletion observed from Molybdenum stable isotope compositions

IOP Conference Series: Earth and Environmental Science, 2009

This article was submitted without an abstract, please refer to the full-text PDF file.

Uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late Ediacaran Period

Geochimica et Cosmochimica Acta, 2015

To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we meas... more To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average  238 U of most samples is 0.24 ± 0.16 ‰ (2SD; relative to standard CRM145), which is slightly higher than the average  238 U of 0.02 ± 0.12 ‰ for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled  238 U value of 0.2 ‰ for open ocean euxinic sediments in the modern well-oxygenated oceans. Because 238 U is preferentially removed to euxinic sediments compared to 235 U, expanded ocean anoxia will deplete seawater of 238 U relative to 235 U, ultimately leading to deposition of ORM with low  238 U. Hence, the high  238 U of Member IV ORM points to a common occurrence of extensive ocean oxygenation ca. 560 to 551 Myr ago. The Mo isotope composition of sediments deposited from strongly euxinic bottom waters ([H2S]aq > 11 µM) either directly records the global seawater Mo isotope composition (if Mo removal from deep waters is quantitative) or represents a minimum value for seawater (if Mo removal is not quantitative). Near the top of Member IV,  98 Mo approaches the modern seawater value of 2.34 ± 0.10‰. High  98 Mo points to widespread ocean oxygenation because the preferential removal of isotopically light Mo to sediments occurs to a greater extent in O2-rich compared to O2-deficient marine environments. However, the  98 Mo value for most Member IV ORM is near 0‰ (relative to standard NIST SRM 3134 = 0.25‰), suggesting extensive anoxia. The low  98 Mo is at odds with the high Mo concentrations of Member IV ORM, which suggest a large seawater Mo inventory in well-oxygenated oceans, and the high  238 U. Hence, we propose that the low  98 Mo of most Member IV ORM was fractionated from contemporaneous seawater.

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Rapidly assessing changes in bone mineral balance using natural stable calcium isotopes

Proceedings of the National Academy of Sciences of the United States of America, Jan 19, 2012

The ability to rapidly detect changes in bone mineral balance (BMB) would be of great value in th... more The ability to rapidly detect changes in bone mineral balance (BMB) would be of great value in the early diagnosis and evaluation of therapies for metabolic bone diseases such as osteoporosis and some cancers. However, measurements of BMB are hampered by difficulties with using biochemical markers to quantify the relative rates of bone resorption and formation and the need to wait months to years for altered BMB to produce changes in bone mineral density large enough to resolve by X-ray densitometry. We show here that, in humans, the natural abundances of Ca isotopes in urine change rapidly in response to changes in BMB. In a bed rest experiment, use of high-precision isotope ratio MS allowed the onset of bone loss to be detected in Ca isotope data after about 1 wk, long before bone mineral density has changed enough to be detectable with densitometry. The physiological basis of the relationship between Ca isotopes and BMB is sufficiently understood to allow quantitative translation...

Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish

Proceedings of the National Academy of Sciences, 2010

The evolution of Earth’s biota is intimately linked to the oxygenation of the oceans and atmosphe... more The evolution of Earth’s biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550–560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, anima...

Reply to Butterfield: The Devonian radiation of large predatory fish coincided with elevated atmospheric oxygen levels

Proceedings of the National Academy of Sciences, 2011

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