Thomas Algeo | University of Cincinnati (original) (raw)

Papers by Thomas Algeo

Palaeogeography, Palaeoclimatology, Palaeoecology, Nov 1, 2017

Although oceanic anoxia is regarded as a leading cause of the Permian-Triassic boundary (PTB) mas... more Although oceanic anoxia is regarded as a leading cause of the Permian-Triassic boundary (PTB) mass extinction, its timing, extent, and underlying causes remain unclear. Here, we conducted a high-resolution Fe-S-C-Mo geochemical study of the entire Changhsingian and lowermost Induan succession in a carbonate-ramp setting at Ganxi, western Hubei Province, South China. Six stratigraphic units representing discrete redox intervals were identified based on integrated Fe-S-C-Mo data. Units I-III are of latest Wuchiapingian to early Changhsingian age, with Units I and III characterized by euxinia and Unit II by uncertain redox conditions. Unit IV, of mid-Changhsingian age, records a gradual

Earth and Planetary Science Letters, 2023

Geochimica et Cosmochimica Acta, Oct 1, 2020

Mass-dependent variations in selenium stable isotope ratios have recently been developed as a pal... more Mass-dependent variations in selenium stable isotope ratios have recently been developed as a paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox potential, this system holds promise for probing conditions relevant to the evolution and diversification of eukaryotic and animal life, which required substantial dissolved oxygen levels. Although several studies have identified selenium isotopic variability during oxygenation events in Earth's distant past, we still have only a broad understanding of the mechanisms controlling this isotopic variability. This currently limits the robust interpretation of selenium isotope variability to first-order mechanisms driving large-magnitude changes. Here, we explore selenium isotope variability within and among Paleozoic black shales deposited on the North American craton that have been well-studied using a variety of other paleo-environmental proxies. Using this combined dataset, we attempt to unravel the controls on selenium abundance and isotope ratios in organic-rich ancient marine sedimentary rocks. We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy.

Palaeogeography, Palaeoclimatology, Palaeoecology, Oct 1, 2019

The Late Paleozoic (Early Devonian to Middle Permian) was an interval of profound changes in Eart... more The Late Paleozoic (Early Devonian to Middle Permian) was an interval of profound changes in Earth-surface systems, reflected in dynamic interplay among the biosphere, hydrosphere, atmosphere, and geosphere. Major events transpired, including the colonization of landmasses by vascular plants, the assembly of the supercontinent Pangea, two first-order mass extinctions (the Frasnian-Famennian and Devonian-Carboniferous boundary events), and the most severe icehouse climate mode of the Phanerozoic (the Late Paleozoic Ice Age, LPIA). The

Nature Communications, Apr 4, 2023

During the Marinoan Ice Age (ca. 654-635 Ma), one of the 'Snowball Earth' events in the Cryogenia... more During the Marinoan Ice Age (ca. 654-635 Ma), one of the 'Snowball Earth' events in the Cryogenian Period, continental icesheets reached the tropical oceans. Oceanic refugia must have existed for aerobic marine eukaryotes to survive this event, as evidenced by benthic phototrophic macroalgae of the Songluo Biota preserved in black shales interbedded with glacial diamictites of the late Cryogenian Nantuo Formation in South China. However, the environmental conditions that allowed these organisms to thrive are poorly known. Here, we report carbon-nitrogen-iron geochemical data from the fossiliferous black shales and adjacent diamictites of the Nantuo Formation. Iron-speciation data document dysoxic-anoxic conditions in bottom waters, whereas nitrogen isotopes record aerobic nitrogen cycling perhaps in surface waters. These findings indicate that habitable open-ocean conditions were more extensive than previously thought, extending into mid-latitude coastal oceans and providing refugia for eukaryotic organisms during the waning stage of the Marinoan Ice Age.

Earth and Planetary Science Letters, Jul 1, 2023

Global and Planetary Change, May 1, 2021

Abstract Combustion-derived polycyclic aromatic hydrocarbons (PAHs) are frequently used as molecu... more Abstract Combustion-derived polycyclic aromatic hydrocarbons (PAHs) are frequently used as molecular proxies for wildfire in recent and ancient sediments. Here, we document the abundances of four hydrocarbon compounds, i.e., phenanthrene (Phe), benzo[e]pyrene (BeP), benzo[ghi]perylene (Bpery), and coronene (Cor), across the Permian–Triassic boundary (PTB) in five marine sections of South China. High values of PAHs are present below the end-Permian mass extinction (EPME) horizon, from the Clarkina changxingensis Zone to the Clarkina yini Zone, suggesting strong perturbations to terrestrial ecosystems predating the marine crisis. PAHs peaked in the uppermost C. yini Zone, correlative with the EPME, reflecting simultaneity of the most severe phases of the terrestrial and marine crises. The proxy records document suitable climatic conditions for wildfires (high pO2) throughout the P-Tr transition. The results of the present study are also significant in validating the utility of BeP, Bpery, Cor, and BeP/Phe as proxies for terrestrial wildfires in deep-time marine successions.

Palaeogeography, Palaeoclimatology, Palaeoecology, Apr 1, 2019

The Permian-Triassic boundary (PTB) at ~252 Ma coincided with the largest mass extinction of the ... more The Permian-Triassic boundary (PTB) at ~252 Ma coincided with the largest mass extinction of the Phanerozoic. Previous research on diversity and abundance changes during this event has focused mainly on the terrestrial vertebrate and marine invertebrate records, with little attention to date given to the phytoplankton that form the base of the marine trophic web. Although the fossil record of Permian-Triassic phytoplankton is relatively poor owing to preservational factors, sufficient material is now available to evaluate secular changes in acritarch communities through the mass extinction interval. In this contribution, we evaluate diversity and abundance changes among 8 genera and 25 species of acritarchs, including large-spherical, small-spherical, long-spined, and short-spined forms ranging from the Upper Permian Clarkina yini to the Lower Triassic Isarcicella isarcica zones in eight sections representing different sedimentary facies of the South China Craton. Acritarchs

Palaeogeography, Palaeoclimatology, Palaeoecology, Apr 1, 2016

The end-Devonian was a critical interval in Earth history recording the transition from the Early... more The end-Devonian was a critical interval in Earth history recording the transition from the Early Paleozoic greenhouse climate to the Late Paleozoic icehouse climate. A mass extinction at this time (the Hangenberg extinction) eliminated ~21% of genera and ~16% of families of marine invertebrates, although its causes remain poorly understood. The marine nitrogen cycle is intimately related to the nutrient status of seawater, microbial community composition, and the redox condition of the oceans, and, thus, it may provide insights into the mechanism(s) of the end-Devonian mass extinction and climate change. Here, we analyzed high-stratigraphicresolution bulk-sediment nitrogen isotope variation ( 15 N bulk) in three sections from South China: (1) Long'an, an isolated carbonate platform section, (2) Qilinzhai, a shallow-water carbonate platform section between the Yangtze Oldland and the Youjiang Trough, and (3) Malanbian, a shallow-water carbonate platform section between the Yangtze and Cathaysia oldlands and distant from deep-water areas. Our  15 N bulk records show a major perturbation to the marine nitrogen cycle in the Middle Si. praesulcata Zone, which is marked by a decline in  15 N bullk from +4‰ to 0‰

Palaeogeography, Palaeoclimatology, Palaeoecology

Nature Communications

Decomposition of soil organic matter (SOM) can be stimulated by fresh organic matter input, a phe... more Decomposition of soil organic matter (SOM) can be stimulated by fresh organic matter input, a phenomenon known as the ‘priming effect’. Despite its global importance, the relationship of the priming effect to mineral weathering and nutrient release remains unclear. Here we show close linkages between mineral weathering in the critical zone and primed decomposition of SOM. Intensified mineral weathering and rock-derived nutrient release are generally coupled with primed SOM decomposition resulting from “triggered” microbial activity. Fluxes of organic matter products decomposed via priming are linearly correlated with weathering congruency. Weathering congruency influences the formation of organo-mineral associations, thereby modulating the accessibility of organic matter to microbial decomposers and, thus, the priming effect. Our study links weathering with primed SOM decomposition, which plays a key role in controlling soil C dynamics in space and time. These connections represent ...

Frontiers in Earth Science

GSA Bulletin, 2022

Methane-derived carbonates (MDCs) are common along modern and ancient continental margins, and th... more Methane-derived carbonates (MDCs) are common along modern and ancient continental margins, and the majority of such formations are associated with seafloor cold seeps. Here, we document petrographic, rare earth element + yttrium (REE + Y), carbonate clumped isotope temperature (TΔ47), and carbon-isotopic evidence from a shale succession in southern Tibet spanning a ~28 m.y. interval (ca. 113–85 Ma) that coincided with the mid-Cretaceous greenhouse event. At least nine limestone nodule-bearing horizons exhibit seep-associated sedimentary structures, including carbonate fabrics (e.g., micritic crusts, crystal fans, and botryoidal textures) and 13C-depleted isotopic compositions (δ13Ccarb < –32.3‰), which are indicative of methane-derived carbon sources. Along with sedimentary evidence, the patterns of TΔ47–δ13Ccarb–δ18Ocarb support precipitation of these MDCs over a large temperature range. The REE + Y compositions and europium (Eu) anomalies indicate that the release of methane wa...

Global and Planetary Change, 2021

Geology, 2020

Enhanced regional subduction-related volcanism in the South China craton concurrent with Siberian... more Enhanced regional subduction-related volcanism in the South China craton concurrent with Siberian Traps large igneous province magmatism was a likely contributor to major biotic and environmental stresses associated with the Permian-Triassic boundary (ca. 252 Ma) mass extinction. However, the timing, intensity, and duration of this regional volcanic activity remain uncertain. We analyzed mercury (Hg) concentrations in three widely separated marine sections in the South China craton (Shangsi, Ganxi, and Chaohu) as well as Hg isotopic compositions in one section (Shangsi) from the Upper Permian (Changhsingian) through the lowermost Triassic (Induan) in order to track volcanic inputs. Four mercury enrichment (ME) intervals, dating to the lowermost Changhsingian (ME1), mid–Clarkina changxingensis zone (ME2), upper C. changxingensis to lower C. yini zones (ME3), and latest Permian mass extinction (LPME) interval (ME4), were recognized on the basis of elevated Hg/total organic carbon rati...

Frontiers in Earth Science, 2021

Stable isotope analysis is pivotal for investigating the paleodiet and paleoecology of ancient ma... more Stable isotope analysis is pivotal for investigating the paleodiet and paleoecology of ancient mammals. Recently, a ∼9-Myr-old Hipparion fauna was discovered at an elevation of ∼2,200 m above sea level (a.s.l.) in the Xunhua Basin on the northeastern Tibetan Plateau. These fossils can provide key evidence for the ecosystem structure and regional paleoenvironmental reconstruction. Here we present carbon and oxygen isotopic compositions of 56 bulk and 85 serial tooth enamel samples from this fauna, which includes Hipparion platyodus Selve, Chilotherium sp. and Gazella gaudryi Schlosser. The enamel−δ13°C values display a wide range of variation (−11.4 to −5.0‰), with a mean value of −8.9 ± 1.0‰, indicating that the fauna consumed mainly C3 plants. However, the heaviest δ13C value of Hipparion (−5.0‰) suggests at least some consumption of C4 plants. Combined with pollen records, we infer that the Hipparion fauna in the Xunhua Basin was living in forest-grassland setting at ∼9 Ma. The re...

Earth and Planetary Science Letters, 2021

Abstract Oceanic anoxia is considered as the immediate cause of the Frasnian–Famennian mass extin... more Abstract Oceanic anoxia is considered as the immediate cause of the Frasnian–Famennian mass extinction, yet the frequency of anoxia and associated environmental modulators remains unknown. Here, we demonstrate that astronomical forcing paced oceanic anoxic episodes by mediating land-ocean interactions during the Upper Kellwasser (UKW) Event, using an ultra-high-resolution (one centimeter spacing), multi-proxy geochemical profile of a UKW interval from the Upper Devonian Chattanooga Shale of Tennessee, USA. Organic and inorganic geochemical indices for oceanic anoxia/euxinia, marine primary productivity, terrestrial plant/soil inputs, and clastic sediment inputs show synchronous fluctuations that were paced by the Earth's orbital precession. This study provides the first mechanistic evidence linking the periodicity of Late Devonian oceanic euxinia to astronomical forcing and identifies terrestrial-to-marine inorganic and organic fluxes as the driver of this linkage. These results suggest that astronomical forcing modulated the expression of environmental stressors that led to the Frasnian–Famennian mass extinction.

GSA Bulletin, 2019

The end-Permian mass extinction (ca. 252 Ma) represents the most severe biotic crisis of the Phan... more The end-Permian mass extinction (ca. 252 Ma) represents the most severe biotic crisis of the Phanerozoic, and it was accompanied by profound environmental perturbations, especially to the global carbon cycle, as indicated by sharp negative carbon isotope excursions (CIE) in both carbonates (δ13Ccarb) and organic matter (δ13Corg). To date, carbon isotope records are mostly from marine Permian-Triassic transitional sequences with relatively few high-resolution carbon isotope profiles having been generated for terrestrial facies. Terrestrial Permian-Triassic sequences suitable for high-resolution carbon isotope study are rare globally and are difficult to correlate with better-studied marine sequences. However, carbon isotope records from continental facies are essential to a full understanding of global carbon cycle changes during the Permian-Triassic transition. Here, we present bulk δ13Corg profiles for three terrestrial sections in North China representing Permian-Triassic transiti...