publications
Peer-reviewed publications in reversed chronological order.
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2023
- Why the Early Paleozoic was intrinsically prone to marine extinctionAlexandre Pohl, Richard G. Stockey, Xu Dai, and 6 more authorsScience Advances 2023
The geological record of marine animal biodiversity reflects the interplay between changing rates of speciation versus extinction. Compared to mass extinctions, background extinctions have received little attention. To disentangle the different contributions of global climate state, continental configuration, and atmospheric oxygen concentration (pO2) to variations in background extinction rates, we drive an animal physiological model with the environmental outputs from an Earth system model across intervals spanning the past 541 million years. We find that climate and continental configuration combined to make extinction susceptibility an order of magnitude higher during the Early Paleozoic than during the rest of the Phanerozoic, consistent with extinction rates derived from paleontological databases. The high extinction susceptibility arises in the model from the limited geographical range of marine organisms. It stands even when assuming present-day pO2, suggesting that increasing oxygenation through the Paleozoic is not necessary to explain why extinction rates apparently declined with time. Climate and continental configuration combined to make Early Paleozoic animals susceptible to extinction.
2022
- Oceanic anoxia and extinction in the latest OrdovicianMu Liu, Daizhao Chen, Lei Jiang, and 7 more authorsEarth and Planetary Science Letters 2022
The Late Ordovician (Hirnantian) mass extinction (LOME) was marked by two discrete pulses of high species turnover rates attributed to glacial cooling (LOME-1) and subsequent expansion of anoxic marine conditions (LOME-2). However, the mechanisms and extent of global marine anoxia remain controversial. In this study, we present uranium isotope ( U) data from a new Ordovician-Silurian (O-S) boundary carbonate section in the Southwest China to explore the extent/duration of the global marine anoxia, and links to the LOME. This section was found to continuously record the characteristic Hirnantian paired carbon isotope excursion (HICE) and complete benthic faunal turnover across the O-S boundary based on detailed stratigraphic constraint. The poor correlations between Ucarb data, local redox proxy (Ce anomalies), and diagenetic indicators (e.g., Mn and Sr contents, Mn/Sr ratio, O), as well as microscopic petrographic examinations of O-S carbonates suggest that most of the U data are not produced by either local redox conditions or post-depositional diagenetic processes. By coupling our new uranium isotope data with a stochastic U isotope mass balance model, we suggest there were two episodes of widespread marine anoxia over this time interval revealed by systematic changes of U with two negative shifts punctuated by one positive one in between. The former took place in the late Katian, predating the LOME-1 episode and the pronounced climatic cooling; while the latter occurred synchronously with the LOME-2 in the mid-late Hirnantian, separated by a potential oceanic oxygenation event. This presents a more complicated picture of anoxia—as inferred from U isotope records—as a driver of the end-Ordovician mass extinction. The insignificant biotic response to late Katian anoxia is consistent with the idea that the effects of anoxia on marine ecosystem can be highly variable. On the other hand, the presence of anoxia in mid-late Hirnantian strengthen the connection between the oceanic deoxygenation and fauna turnover in LOME-2.
- Uranium isotope evidence for extensive shallow water anoxia in the early Tonian oceansFeifei Zhang, Richard G Stockey, Shuhai Xiao, and 8 more authorsEarth and Planetary Science Letters 2022
The Earth’s redox evolution has been commonly assumed to have played a key role in shaping the evolutionary history of the biosphere. However, whether and how shifts in marine redox conditions are linked to key biotic events – foremost the rise of animals and the ecological expansion of eukaryotic algae in the late Proterozoic oceans – remains heavily debated. Our current picture of global marine redox evolution during this critical interval is incomplete. This is particularly the case for the Tonian Period (∼1.0 to ∼0.717 Ga), when animals may have diverged and when eukaryotic algae began their rise in ecological importance. Here, we present new uranium isotope ( U) measurements from Tonian carbonates to fill this outstanding gap. These Tonian carbonates (∼1000–800 Ma) record variable U values, indicating temporal variation in global marine redox through this under-investigated time interval. Arguably the most interesting feature of this new U dataset is an interval of anomalously negative U values (<−1‰) that represent among the most negative stratigraphically continuous values reported to date. These low U values are best explained by prevalent shallow-water anoxia, potentially driven by increases in productivity in a low-O2 Tonian Earth system. We thus provide compelling evidence for extensive shallow marine anoxia just prior to or coincident with Neoproterozoic ecological shifts.
- Continental configuration controls ocean oxygenation during the PhanerozoicNature 2022
The early evolutionary and much of the extinction history of marine animals is thought to be driven by changes in dissolved oxygen concentrations ([O2]) in the ocean1,2,3. In turn, [O2] is widely assumed to be dominated by the geological history of atmospheric oxygen (pO2)4,5. Here, by contrast, we show by means of a series of Earth system model experiments how continental rearrangement during the Phanerozoic Eon drives profound variations in ocean oxygenation and induces a fundamental decoupling in time between upper-ocean and benthic [O2]. We further identify the presence of state transitions in the global ocean circulation, which lead to extensive deep-ocean anoxia developing in the early Phanerozoic even under modern pO2. Our finding that ocean oxygenation oscillates over stable thousand-year (kyr) periods also provides a causal mechanism that might explain elevated rates of metazoan radiation and extinction during the early Palaeozoic Era6. The absence, in our modelling, of any simple correlation between global climate and ocean ventilation, and the occurrence of profound variations in ocean oxygenation independent of atmospheric pO2, presents a challenge to the interpretation of marine redox proxies, but also points to a hitherto unrecognized role for continental configuration in the evolution of the biosphere.
- Breathless through Time: Oxygen and Animals across Earth’s HistoryThe Biological Bulletin 2022
Oxygen levels in the atmosphere and ocean have changed dramatically over Earth history, with major impacts on marine life. Because the early part of Earth’s history lacked both atmospheric oxygen and animals, a persistent co-evolutionary narrative has developed linking oxygen change with changes in animal diversity. Although it was long believed that oxygen rose to essentially modern levels around the Cambrian period, a more muted increase is now believed likely. Thus, if oxygen increase facilitated the Cambrian explosion, it did so by crossing critical ecological thresholds at low O2. Atmospheric oxygen likely remained at low or moderate levels through the early Paleozoic era, and this likely contributed to high metazoan extinction rates until oxygen finally rose to modern levels in the later Paleozoic. After this point, ocean deoxygenation (and marine mass extinctions) is increasingly linked to large igneous province eruptions—massive volcanic carbon inputs to the Earth system that caused global warming, ocean acidification, and oxygen loss. Although the timescales of these ancient events limit their utility as exact analogs for modern anthropogenic global change, the clear message from the geologic record is that large and rapid CO2 injections into the Earth system consistently cause the same deadly trio of stressors that are observed today. The next frontier in understanding the impact of oxygen changes (or, more broadly, temperature-dependent hypoxia) in deep time requires approaches from ecophysiology that will help conservation biologists better calibrate the response of the biosphere at large taxonomic, spatial, and temporal scales.
2021
- Global marine redox evolution from the late Neoproterozoic to the early Paleozoic constrained by the integration of Mo and U isotope recordsGuang-Yi Wei, Noah J Planavsky, Tianchen He, and 5 more authorsEarth-Science Reviews 2021
The emergence and diversification of early animals is commonly thought to have coincided with atmosphere and ocean oxygenation across the terminal Neoproterozoic and early Paleozoic, during which oxygen levels on Earth’s surface were sufficient to support the metabolism of early multicellular metazoans in the ocean. Although surface oxygen levels are likely to have broadly risen through the Paleozoic, ocean oxygenation levels during this period are still disputed and poorly constrained. While the community is actively developing high time-resolution records of redox proxies in marine sediments, uncertainties remain about how these records can be used to reconstruct the global marine redox landscape. In this review, we compile newly published molybdenum and uranium isotope data from the late Neoproterozoic to the early Paleozoic (ca. 680–480 Ma) to provide an updated look at the secular changes in global ocean redox state and the potential drivers of these shifts. Integrations of Mo and U isotope records suggest a gradual transition from a pervasive anoxic condition to a highly dynamic condition for global marine redox state from the late Neoproterozoic to the Cambrian. We further concentrate on the marine redox landscape of early Cambrian, by comparing the carbon, sulfur, nitrogen and uranium isotope records and reproducing the variations in carbon-sulfur-uranium isotope records with biogeochemical box models. Changes in marine primary productivity under a relatively low atmospheric oxygen level, are proposed to play a first-order control on the early Cambrian ocean redox dynamics.
- A long-term record of early to mid-Paleozoic marine redox changeErik A Sperling, Michael J Melchin, Tiffani Fraser, and 8 more authorsScience Advances 2021
The extent to which Paleozoic oceans differed from Neoproterozoic oceans and the causal relationship between biological evolution and changing environmental conditions are heavily debated. Here, we report a nearly continuous record of seafloor redox change from the deep-water upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. Bottom waters were largely anoxic in the Richardson trough during the entirety of Road River Group deposition, while independent evidence from iron speciation and Mo/U ratios show that the biogeochemical nature of anoxia changed through time. Both in Yukon and globally, Ordovician through Early Devonian anoxic waters were broadly ferruginous (nonsulfidic), with a transition toward more euxinic (sulfidic) conditions in the mid–Early Devonian (Pragian), coincident with the early diversification of vascular plants and disappearance of graptolites. This 80-million-year interval of the Paleozoic characterized by widespread ferruginous bottom waters represents a persistence of Neoproterozoic-like marine redox conditions well into the Phanerozoic.
- Decreasing Phanerozoic extinction intensity as a consequence of Earth surface oxygenation and metazoan ecophysiologyProceedings of the National Academy of Sciences 2021
The decline in background extinction rates of marine animals through geologic time is an established but unexplained feature of the Phanerozoic fossil record. There is also growing consensus that the ocean and atmosphere did not become oxygenated to near-modern levels until the mid-Paleozoic, coinciding with the onset of generally lower extinction rates. Physiological theory provides us with a possible causal link between these two observations—predicting that the synergistic impacts of oxygen and temperature on aerobic respiration would have made marine animals more vulnerable to ocean warming events during periods of limited surface oxygenation. Here, we evaluate the hypothesis that changes in surface oxygenation exerted a first-order control on extinction rates through the Phanerozoic using a combined Earth system and ecophysiological modeling approach. We find that although continental configuration, the efficiency of the biological carbon pump in the ocean, and initial climate state all impact the magnitude of modeled biodiversity loss across simulated warming events, atmospheric oxygen is the dominant predictor of extinction vulnerability, with metabolic habitat viability and global ecophysiotype extinction exhibiting inflection points around 40% of present atmospheric oxygen. Given this is the broad upper limit for estimates of early Paleozoic oxygen levels, our results are consistent with the relative frequency of high-magnitude extinction events (particularly those not included in the canonical big five mass extinctions) early in the Phanerozoic being a direct consequence of limited early Paleozoic oxygenation and temperature-dependent hypoxia responses.
- The sedimentary geochemistry and paleoenvironments projectÚna C Farrell, Rifaat Samawi, Savitha Anjanappa, and 8 more authorsGeobiology 2021
Geobiology explores how Earth’s system has changed over the course of geologic history and how living organisms on this planet are impacted by or are indeed causing these changes. For decades, geologists, paleontologists, and geochemists have generated data to investigate these topics. Foundational efforts in sedimentary geochemistry utilized spreadsheets for data storage and analysis, suitable for several thousand samples, but not practical or scalable for larger, more complex datasets. As results have accumulated, researchers have increasingly gravitated toward larger compilations and statistical tools. New data frameworks have become necessary to handle larger sample sets and encourage more sophisticated or even standardized statistical analyses. In this paper, we describe the Sedimentary Geochemistry and Paleoenvironments Project (SGP; Figure 1), which is an open, community-oriented, database-driven research consortium. The goals of SGP are to (1) create a relational database tailored to the needs of the deep-time (millions to billions of years) sedimentary geochemical research community, including assembling and curating published and associated unpublished data; (2) create a website where data can be retrieved in a flexible way; and (3) build a collaborative consortium where researchers are incentivized to contribute data by giving them priority access and the opportunity to work on exciting questions in group papers. Finally, and more idealistically, the goal was to establish a culture of modern data management and data analysis in sedimentary geochemistry.10.1111/gbi.12462
- Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulationAlexandre Pohl, Zunli Lu, Wanyi Lu, and 8 more authorsNature Geoscience 2021
Geochemical redox proxies indicate that seafloor anoxia occurred during the latest Ordovician glacial maximum, coincident with the second pulse of the Late Ordovician mass extinction. However, expanded anoxia in a glacial climate strikingly contrasts with the warming-associated Mesozoic anoxic events and raises questions as to both the causal mechanism of ocean deoxygenation and its relationship with extinction. Here we firstly report iodine-to-calcium ratio (I/Ca) data that document increased upper-ocean oxygenation despite the concurrent expansion of seafloor anoxia. We then resolve these apparently conflicting observations as well as their relationship to global climate by means of a series of Earth system model simulations. Applying available Late Ordovician (Hirnantian) sea-surface temperature estimates from oxygen isotope studies as constraints, alongside our I/Ca data, leads us to identify a scenario in which Hirnantian glacial conditions permit both the spread of seafloor anoxia and increased upper-ocean oxygenation. Our simulated mechanism of a reorganization of global ocean circulation, with reduced importance of northern-sourced waters and a poorer ventilated and deoxygenated deep ocean has parallels with Pleistocene state transitions in Atlantic meridional overturning (despite a very different continental configuration) and suggests that no simple and predictable relationship between past climate state and oxygenation may exist.
- Metabolic tradeoffs control biodiversity gradients through geological timeCurrent Biology 2021
The latitudinal gradient of increasing marine biodiversity from the poles to the tropics is one of the most conspicuous biological patterns in modern oceans.1, 2, 3 Low-latitude regions of the global ocean are often hotspots of animal biodiversity, yet they are set to be most critically affected by anthropogenic climate change.4 As ocean temperatures rise and deoxygenation proceeds in the coming centuries, the volume of aerobically viable habitat is predicted to decrease in these zones.5,6 In contrast to the slightly asymmetrical modern latitudinal biodiversity gradient,7 compilations of fossil occurrences indicate peaks in biodiversity may have existed much further away from the equator in the past, with transitions between climate states hypothesized to explain this trend.8, 9, 10, 11, 12, 13 We combine a new compilation of fossil mollusc occurrences, paleotemperature proxies, and biogeographic data to reveal a non-monotonic relationship between temperature and diversity in the paleontological record over the last 145 million years. We derive a metabolic model that integrates the kinetic effects of temperature on biodiversity14 with the recently described Metabolic Index that calculates aerobic habitat availability based on the effect of temperature on hypoxia sensitivity.5,15,16 Although factors such as coastal habitat area and homeothermy are important,17,18 we find strong congruence between our metabolic model and our fossil and paleotemperature meta-analysis. We therefore suggest that the effects of ocean temperature on the aerobic scope of marine ectotherms is a primary driver of migrating biodiversity peaks through geologic time and will likely play a role in the restructuring of biodiversity under projected future climate scenarios.
2020
- Persistent global marine euxinia in the early SilurianRichard G Stockey, Devon B Cole, Noah J Planavsky, and 3 more authorsNature communications 2020
The second pulse of the Late Ordovician mass extinction occurred around the Hirnantian-Rhuddanian boundary ( 444 Ma) and has been correlated with expanded marine anoxia lasting into the earliest Silurian. Characterization of the Hirnantian ocean anoxic event has focused on the onset of anoxia, with global reconstructions based on carbonate δ238U modeling. However, there have been limited attempts to quantify uncertainty in metal isotope mass balance approaches. Here, we probabilistically evaluate coupled metal isotopes and sedimentary archives to increase constraint. We present iron speciation, metal concentration, δ98Mo and δ238U measurements of Rhuddanian black shales from the Murzuq Basin, Libya. We evaluate these data (and published carbonate δ238U data) with a coupled stochastic mass balance model. Combined statistical analysis of metal isotopes and sedimentary sinks provides uncertainty-bounded constraints on the intensity of Hirnantian-Rhuddanian euxinia. This work extends the duration of anoxia to >3 Myrs – notably longer than well-studied Mesozoic ocean anoxic events.
2018
- The temporal and environmental context of early animal evolution: Considering all the ingredients of an “explosion”Integrative and Comparative Biology 2018
Animals originated and evolved during a unique time in Earth history—the Neoproterozoic Era. This paper aims to discuss (1) when landmark events in early animal evolution occurred, and (2) the environmental context of these evolutionary milestones, and how such factors may have affected ecosystems and body plans. With respect to timing, molecular clock studies—utilizing a diversity of methodologies—agree that animal multicellularity had arisen by ∼800 million years ago (Ma) (Tonian period), the bilaterian body plan by ∼650 Ma (Cryogenian), and divergences between sister phyla occurred ∼560–540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. This view of animal origins contrasts with data from the fossil record, and the taphonomic question of why animals were not preserved (if present) remains unresolved. Neoproterozoic environments demanding small, thin, body plans, and lower abundance/rarity in populations may have played a role. Considering environmental conditions, geochemical data suggest that animals evolved in a relatively low-oxygen ocean. Here, we present new analyses of sedimentary total organic carbon contents in shales suggesting that the Neoproterozoic ocean may also have had lower primary productivity—or at least lower quantities of organic carbon reaching the seafloor—compared with the Phanerozoic. Indeed, recent modeling efforts suggest that low primary productivity is an expected corollary of a low-O2 world. Combined with an inability to inhabit productive regions in a low-O2 ocean, earliest animal communities would likely have been more food limited than generally appreciated, impacting both ecosystem structure and organismal behavior. In light of this, we propose the “fire triangle” metaphor for environmental influences on early animal evolution. Moving toward consideration of all environmental aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event.
- Oxygen, temperature and the deep-marine stenothermal cradle of Ediacaran evolutionProceedings of the Royal Society B 2018
Ediacaran fossils document the early evolution of complex megascopic life, contemporaneous with geochemical evidence for widespread marine anoxia. These data suggest early animals experienced frequent hypoxia. Research has thus focused on the concentration of molecular oxygen (O2) required by early animals, while also considering the impacts of climate. One model, the Cold Cradle hypothesis, proposed the Ediacaran biota originated in cold, shallow-water environments owing to increased O2 solubility. First, we demonstrate using principles of gas exchange that temperature does have a critical role in governing the bioavailability of O2—but in cooler water the supply of O2 is actually lower. Second, the fossil record suggests the Ediacara biota initially occur approximately 571 Ma in deep-water facies, before appearing in shelf environments approximately 555 Ma. We propose an ecophysiological underpinning for this pattern. By combining oceanographic data with new respirometry experiments we show that in the shallow mixed layer where seasonal temperatures fluctuate widely, thermal and partial pressure (pO2) effects are highly synergistic. The result is that temperature change away from species-specific optima impairs tolerance to low pO2. We hypothesize that deep and particularly stenothermal (narrow temperature range) environments in the Ediacaran ocean were a physiological refuge from the synergistic effects of temperature and low pO2.