Interpreting carbonate and organic carbon isotope covariance in the sedimentary record

Implications of dietary carbon incorporation in fish carbonates for the global carbon cycle

Marine bony fish are important participants in Earth's carbon cycle through their contributions to the biological pump and the marine inorganic carbon cycle. However, uncertainties in the composition and magnitude of fish contributions preclude their integration into fully coupled carbon-climate models. Here, we consider recent upwards revisions to global fish biomass estimates (2.7-9.5×) and provide new stable carbon isotope measurements that show marine fish are prodigious producers of carbonate with unique composition. Assuming the median increase (4.17×) in fish biomass estimates is linearly reflected in fish carbonate (ichthyocarbonate) production rate, marine fish are estimated to produce between 1.43 and 3.99 Pg CaCO3 yr-1, but potentially as much as 9.03 Pg CaCO3 yr-1. Thus, marine fish carbonate production is equivalent to or potentially higher than contributions by coccolithophores or pelagic foraminifera. New stable carbon isotope analyses indicate that a significant proportion of ichthyocarbonate is derived from dietary carbon, rather than seawater dissolved inorganic carbon. Using a statistical mixing model to derive source contributions, we estimate ichthyocarbonate contains up to 81 % dietary carbon, with average compositions of 28-56 %, standing in contrast to contents <10 % in other biogenic carbonate minerals. Results also indicate ichthyocarbonate contains 5.5-40.4 % total organic carbon. When scaled to the median revised global production of ichthyocarbonate, an additional 0.08 to 1.61 Pg C yr-1 can potentially be added to estimates of fish contributions to the biological pump, significantly increasing marine fish contributions to total surface carbon export. Our integration of geochemical and physiological analyses identifies an overlooked link between carbonate production and the biological pump. Since ichthyocarbonate production is anticipated to increase with climate change scenarios, due to ocean warming and acidification, these results emphasize the importance of quantitative understanding of the multifaceted role of marine fish in the global carbon cycle.

Revisiting marine redox conditions during the Ediacaran Shuram carbon isotope excursion

The Neoproterozoic carbonate record contains multiple carbon isotope anomalies, which are the subject of intense debate. The largest of these anomalies, the Shuram excursion (SE), occurred in the mid-Ediacaran (~574-567 Ma). Accurately reconstructing marine redox landscape is a clear path toward making sense of the mechanism that drives this δ¹³ C anomaly. Here, we report new uranium isotopic data from the shallow-marine carbonates of the Wonoka Formation, Flinders Ranges, South Australia, where the SE is well preserved. Our data indicate that the δ²³⁸ U trend during the SE is highly reproducible across globally disparate sections from different depositional settings. Previously, it was proposed that the positive shift of δ²³⁸ U values during the SE suggests an extensive, near-modern level of marine oxygenation. However, recent publications suggest that the fractionation of uranium isotopes in ferruginous and anoxic conditions is comparable, opening up the possibility of non-unique interpretations of the carbonate uranium isotopic record. Here, we build on this idea by investigating the SE in conjunction with additional geochemical proxies. Using a revised uranium isotope mass balance model and an inverse stochastic carbon cycle model, we reevaluate models for δ¹³ C and δ²³⁸ U trends during the SE. We suggest that global seawater δ²³⁸ U values during the SE could be explained by an expansion of ferruginous conditions and do not require a near-modern level of oxygenation during the mid-Ediacaran.

Mercury evidence from southern Pangea terrestrial sections for end-Permian global volcanic effects

The latest Permian mass extinction (LPME) was triggered by magmatism of the Siberian Traps Large Igneous Province (STLIP), which left an extensive record of sedimentary Hg anomalies at Northern Hemisphere and tropical sites. Here, we present Hg records from terrestrial sites in southern Pangea, nearly antipodal to contemporaneous STLIP activity, providing insights into the global distribution of volcanogenic Hg during this event and its environmental processing. These profiles (two from Karoo Basin, South Africa; two from Sydney Basin, Australia) exhibit significant Hg enrichments within the uppermost Permian extinction interval as well as positive Δ¹⁹⁹Hg excursions (to ~0.3‰), providing evidence of long-distance atmospheric transfer of volcanogenic Hg. These results demonstrate the far-reaching effects of the Siberian Traps as well as refine stratigraphic placement of the LPME interval in the Karoo Basin at a temporal resolution of ~10⁵ years based on global isochronism of volcanogenic Hg anomalies.

Direct measurement of the deposition of submicron soot particles on leaves of Platanus acerifolia tree

Submicron soot particles (<1.0 μm in aerodynamic diameter) are responsible for global warming and health burdens worldwide. However, studies on bio-monitoring of submicron soot particles and their associated sources by using tree leaves are not comprehensively illustrated. Here, we determined the seasonal trends of submicron soot particles on the leaves of the Platanus acerifolia collected from two cities (Lu'an, Anhui Province, and Nanjing, Jiangsu Province) in the Yangtze River Delta region, China. The source apportionment of submicron soot particles was performed using stable carbon isotopic analyses. Significant seasonal trends of submicron soot particles were observed in two cities with averaged levels of 0.41-1.36 mg m^-2 in cold seasons and averaged levels of 0.13-0.24 mg m^-2 in warm seasons. The levels of δ¹³C for submicron soot at the suburban site of Lu'an city were observed to be in the range of -25.6‰ to -18.2‰ with fossil fuels dominated (∼58%) in summer and -23.0‰ to -15.6‰ with biomass burning dominated in winter (∼67%). In comparison, the ranges in the levels of δ¹³C in submicron soot were found to be from -26.5‰ to -20.4‰ in winter, and -24.2‰ to -17.9‰ in summer at the urban site of Nanjing. Fossil fuels accounted for a large fraction of submicron soot with average contributions of 53% in winter and 73% in summer, respectively. These findings demonstrate that Platanus acerifolia trees could be used as an effective and low-cost bio-monitoring tool for monitoring the pollution status of submicron soot and associated source contribution.

The origin of carbonate mud and implications for global climate

Carbonate mud represents one of the most important geochemical archives for reconstructing ancient climatic, environmental, and evolutionary change from the rock record. Mud also represents a major sink in the global carbon cycle. Yet, there remains no consensus about how and where carbonate mud is formed. Here, we present stable isotope and trace-element data from carbonate constituents in the Bahamas, including ooids, corals, foraminifera, and algae. We use geochemical fingerprinting to demonstrate that carbonate mud cannot be sourced from the abrasion and mixture of any combination of these macroscopic grains. Instead, an inverse Bayesian mixing model requires the presence of an additional aragonite source. We posit that this source represents a direct seawater precipitate. We use geological and geochemical data to show that "whitings" are unlikely to be the dominant source of this precipitate and, instead, present a model for mud precipitation on the bank margins that can explain the geographical distribution, clumped-isotope thermometry, and stable isotope signature of carbonate mud. Next, we address the enigma of why mud and ooids are so abundant in the Bahamas, yet so rare in the rest of the world: Mediterranean outflow feeds the Bahamas with the most alkaline waters in the modern ocean (>99.7th-percentile). Such high alkalinity appears to be a prerequisite for the nonskeletal carbonate factory because, when Mediterranean outflow was reduced in the Miocene, Bahamian carbonate export ceased for 3-million-years. Finally, we show how shutting off and turning on the shallow carbonate factory can send ripples through the global climate system.

Biogeochemical properties and potential risk of shallow arsenic-rich sediment layers to groundwater quality in Western Bangladesh

The arsenic-contaminated groundwater has attracted attention in much south and southeast Asian deltas, however, mainly on the deep aquifers. Here, arsenic (As) concentration and its fractionation of the sediment cores in a shallow aquifer in Bangladesh were investigated using ICP-MS, FE-EPMA, XRD and ¹⁴C-AMS chronology techniques. The results of the present study indicated that the peak concentrations of As (54.7-79.1 µg/g) were in peat layers (at a depth of 7.5-8.0 m). Several types of iron (oxyhidr)oxides and framboidal pyrite, which contain As also, were found in the peat samples. The high concentrations of As were in an exchangeable form, As-bearing iron crystalline and As-bearing organic materials. We revealed that the As-rich peat layers were formed from 3170 to 3901 cal yrs before, due to the sea level decrease in this area. The 16S rRNA gene-based phylogenetic analysis revealed that the bacterial strains in the As-rich peats were mainly affiliated with genera Acinetobacter, Enterobacter, Escherichia, Bacillus, Clostridiaceae and Acinetobacter. The geo-accumulation index (I_geo) and ecological risk index assessment were calculated for the sediments, which shows that As-rich sediment layers were in range of moderately to heavily contaminated and considerable classes, respectively. Under the permanent saturated condition, the As-rich peat layers should be considered as an important potential driver of the groundwater As in this area.

Mercury evidence for combustion of organic-rich sediments during the end-Triassic crisis

The sources of isotopically light carbon released during the end-Triassic mass extinction remain in debate. Here, we use mercury (Hg) concentrations and isotopes from a pelagic Triassic–Jurassic boundary section (Katsuyama, Japan) to track changes in Hg cycling. Because of its location in the central Panthalassa, far from terrigenous runoff, Hg enrichments at Katsuyama record atmospheric Hg deposition. These enrichments are characterized by negative mass independent fractionation (MIF) of odd Hg isotopes, providing evidence of their derivation from terrestrial organic-rich sediments (Δ¹⁹⁹Hg < 0‰) rather than from deep-Earth volcanic gases (Δ¹⁹⁹Hg ~ 0‰). Our data thus provide evidence that combustion of sedimentary organic matter by igneous intrusions and/or wildfires played a significant role in the environmental perturbations accompanying the event. This process has a modern analog in anthropogenic combustion of fossil fuels from crustal reservoirs.

Mercury (Hg) concentrations and isotopes from a deep-ocean Triassic–Jurassic (~201 Ma) boundary section provide evidence of large inputs from terrestrial organic-rich sources through combustion by magmatic sills and wildfires.

Intensified continental chemical weathering and carbon-cycle perturbations linked to volcanism during the Triassic-Jurassic transition

Direct evidence of intense chemical weathering induced by volcanism is rare in sedimentary successions. Here, we undertake a multiproxy analysis (including organic carbon isotopes, mercury (Hg) concentrations and isotopes, chemical index of alteration (CIA), and clay minerals) of two well-dated Triassic-Jurassic (T-J) boundary sections representing high- and low/middle-paleolatitude sites. Both sections show increasing CIA in association with Hg peaks near the T-J boundary. We interpret these results as reflecting volcanism-induced intensification of continental chemical weathering, which is also supported by negative mass-independent fractionation (MIF) of odd Hg isotopes. The interval of enhanced chemical weathering persisted for ~2 million years, which is consistent with carbon-cycle model results of the time needed to drawdown excess atmospheric CO₂ following a carbon release event. Lastly, these data also demonstrate that high-latitude continental settings are more sensitive than low/middle-latitude sites to shifts in weathering intensity during climatic warming events.

Formation environments and mechanisms of multistage paleokarst of Ordovician carbonates in Southern North China Basin

With the reduction of oil and gas reserves and the increase of mining difficulty in Northern China, the carbonate rocks in Southern North China Basin are becoming a significant exploration target for carbonate reservoirs. However, the development characteristics, formation stages, formation environments and mechanisms of the carbonate reservoirs in Southern North China Basin are still unclear, which caused the failures of many oil and gas exploration wells. This study focused on addressing this unsolved issue from the Ordovician carbonate paleokarst in the Huai-Fu Basin, which is located in the southeast of Southern North China Basin and one of the key areas for oil and gas exploration. Based on petrology, mineralogy and geochemical data, pore types, distribution characteristics, and formation stages of the Ordovician paleokarst were analyzed. Then, in attempt to define the origins of porosity development, the formation environments and mechanisms were illustrated. The results of this study showed that pore types of the Ordovician carbonates in the Huai-Fu Basin are mainly composed of intragranular pores, intercrystalline (intergranular) pores, dissolution pores (vugs), fractures, channels, and caves, which are usually in fault and fold zones and paleoweathering crust. Furthermore, five stages and five formation environments of the Ordovician paleokarst were identified. Syngenetic karst, eogenetic karst, and paleoweathering crust karst were all developed in a relatively open near-surface environment, and their formations are mainly related to meteoric water dissolution. Mesogenetic karst was developed in a closed buried environment, and its formation is mainly related to the diagenesis of organic matters and thermochemical sulfate reduction in the Permian-Carboniferous strata. Hydrothermal (water) karst was developed in a deep-buried and high-temperature environment, where hydrothermal fluids (waters) migrated upward through structures such as faults and fractures to dissolve carbonate rocks and simultaneously deposited hydrothermal minerals and calcites. Lastly, a paleokarst evolution model, combined with the related porosity evolution processes, nicely revealed the Ordovician carbonate reservoir development. This study provides insights and guidance for further oil and gas exploration in the Southern North China Basin, and also advances our understanding of the genesis of carbonate paleokarst around the world.

Carbonate facies-specific stable isotope data record climate, hydrology, and microbial communities in Great Salt Lake, UT

Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ¹⁸ O_water , δ¹³ C_DIC ), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ¹⁸ O_carb , δ¹³ C_carb , ∆₄₇ ), as well as carbon isotopic compositions of bulk organic matter (δ¹³ C_org ) and dissolved inorganic carbon (DIC; δ¹³ C_DIC ) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time-averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ∆₄₇ isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(∆₄₇ )), δ¹⁸ O_carb , and calculated δ¹⁸ O_water in isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation-precipitation balance, as well as identify microbially mediated carbonate formation.

Calibrating the coevolution of Ediacaran life and environment

The rise of animals occurred during an interval of Earth history that witnessed dynamic marine redox conditions, potentially rapid plate motions, and uniquely large perturbations to global biogeochemical cycles. The largest of these perturbations, the Shuram carbon isotope excursion, has been invoked as a driving mechanism for Ediacaran environmental change, possibly linked with evolutionary innovation or extinction. However, there are a number of controversies surrounding the Shuram, including its timing, duration, and role in the concomitant biological and biogeochemical upheavals. Here we present radioisotopic dates bracketing the Shuram on two separate paleocontinents; our results are consistent with a global and synchronous event between 574.0 ± 4.7 and 567.3 ± 3.0 Ma. These dates support the interpretation that the Shuram is a primary and synchronous event postdating the Gaskiers glaciation. In addition, our Re-Os ages suggest that the appearance of Ediacaran macrofossils in northwestern Canada is identical, within uncertainty, to similar macrofossils from the Conception Group of Newfoundland, highlighting the coeval appearance of macroscopic metazoans across two paleocontinents. Our temporal framework for the terminal Proterozoic is a critical step for testing hypotheses related to extreme carbon isotope excursions and their role in the evolution of complex life.

Isotopically anomalous organic carbon in the aftermath of the Marinoan snowball Earth

Throughout most of the sedimentary record, the marine carbon cycle is interpreted as being in isotopic steady state. This is most commonly inferred via isotopic reconstructions, where two export fluxes (organic carbon and carbonate) are offset by a constant isotopic fractionation of ~25 (termed εorg-carb ). Sedimentary deposits immediately overlying the Marinoan snowball Earth diamictites, however, stray from this prediction. In stratigraphic sections from the Ol Formation (Mongolia) and Sheepbed Formation (Canada), we observe a temporary excursion where the organic matter has anomalously heavy δ13 C and is grossly decoupled from the carbonate δ13 C. This signal may reflect the unique biogeochemical conditions that persisted in the aftermath of snowball Earth. For example, physical oceanographic modeling suggests that a strong density gradient caused the ocean to remain stratified for about 50,000 years after termination of the Marinoan snowball event, during which time the surface ocean and continental weathering consumed the large atmospheric CO₂ reservoir. Further, we now better understand how δ13 C records of carbonate can be post-depostionally altered and thus be misleading. In an attempt to explain the observed carbon isotope record, we developed a model that tracks the fluxes and isotopic values of carbon between the surface ocean, deep ocean, and atmosphere. By comparing the model output to the sedimentary data, stratification alone cannot generate the anomalous observed isotopic signal. Reproducing the heavy δ13 C in organic matter requires the progressively diminishing contribution of an additional anomalous source of organic matter. The exact source of this organic matter is unclear.

Weathering, alteration and reconstructing Earth's oxygenation

Deciphering the role-if any-that free oxygen levels played in controlling the timing and tempo of the radiation of complex life is one of the most fundamental questions in Earth and life sciences. Accurately reconstructing Earth's redox history is an essential part of tackling this question. Over the past few decades, there has been a proliferation of research employing geochemical redox proxies in an effort to tell the story of Earth's oxygenation. However, many of these studies, even those considering the same geochemical proxy systems, have led to conflicting interpretations of the timing and intensity of oxygenation events. There are two potential explanations for conflicting redox reconstructions: (i) that free oxygen levels were incredibly dynamic in both time and space or (ii) that collectively, as a community-including the authors of this article-we have frequently studied rocks affected by secondary weathering and alteration (particularly secondary oxidation) while neglecting to address the impact of this alteration on the generated data. There are now multiple case studies that have documented previously overlooked secondary alteration, resolving some of the conflicting constrains regarding redox evolution. Here, an analysis of a large shale geochemistry database reveals significant differences in cerium (Ce) anomalies, a common palaeoredox proxy, between outcrop and drill core samples. This inconsistency provides support for the idea that geochemical data from altered samples are frequently published in the peer-reviewed literature. As individuals and a geochemical community, most of us have been slow to appreciate how pervasive the problem is but there are examples of other communities that have faced and met the challenges raised by such quality control crises. Further evidence of the high potential for alteration of deep-time geochemical samples, and recognition of the manner in which this may lead to spurious results and palaeoenvironmental interpretations, indicate that sample archiving, in publicly accessible collections needs to become a prerequisite for publication of new palaeoredox data. Finally, the geochemical community need to think about ways to implement additional quality control measures to increase the fidelity of palaeoredox proxy work.

On the co-evolution of surface oxygen levels and animals

Few topics in geobiology have been as extensively debated as the role of Earth's oxygenation in controlling when and why animals emerged and diversified. All currently described animals require oxygen for at least a portion of their life cycle. Therefore, the transition to an oxygenated planet was a prerequisite for the emergence of animals. Yet, our understanding of Earth's oxygenation and the environmental requirements of animal habitability and ecological success is currently limited; estimates for the timing of the appearance of environments sufficiently oxygenated to support ecologically stable populations of animals span a wide range, from billions of years to only a few million years before animals appear in the fossil record. In this light, the extent to which oxygen played an important role in controlling when animals appeared remains a topic of debate. When animals originated and when they diversified are separate questions, meaning either one or both of these phenomena could have been decoupled from oxygenation. Here, we present views from across this interpretive spectrum-in a point-counterpoint format-regarding crucial aspects of the potential links between animals and surface oxygen levels. We highlight areas where the standard discourse on this topic requires a change of course and note that several traditional arguments in this "life versus environment" debate are poorly founded. We also identify a clear need for basic research across a range of fields to disentangle the relationships between oxygen availability and emergence and diversification of animal life.

Microbially influenced lacustrine carbonates: A comparison of Late Quaternary Lahontan tufa and modern thrombolite from Fayetteville Green Lake, NY

Carbonate microbialites in lakes can serve as valuable indicators of past environments, so long as the biogenicity and depositional setting of the microbialite can be accurately determined. Late Pleistocene to Early Holocene frondose draping tufa deposits from Winnemucca Dry Lake (Nevada, USA), a subbasin of pluvial Lake Lahontan, were examined in outcrop, petrographically, and geochemically to determine whether microbially induced precipitation is a dominant control on deposition. These observations were compared to modern, actively accumulating microbialites from Fayetteville Green Lake (New York, USA) using similar methods. In addition, preserved microbial DNA was extracted from the Lahontan tufa and sequenced to provide a more complete picture of the microbial communities. Tufas are texturally and geochemically similar to modern thrombolitic microbialites from Fayetteville Green Lake, and the stable isotopic composition of organic C, N, inorganic C, and O supports deposition associated with a lacustrine microbial mat environment dominated by photosynthetic processes. DNA extraction and sequencing indicate that photosynthetic microbial builders were present during tufa deposition, primarily Chloroflexi and Proteobacteria with minor abundances of Cyanobacteria and Acidobacteria. Based on the sequencing results, the depositional environment of the tufas can be constrained to the photic zone of the lake, contrasting with some previous interpretations that put tufa formation in deeper waters. Additionally, the presence of a number of mesothermophilic phyla, including Deinococcus-Thermus, indicates that thermal groundwater may have played a role in tufa deposition at sites not previously associated with groundwater influx. The interpretation of frondose tufas as microbially influenced deposits provides new context to interpretations of lake level and past environments in the Lahontan lake basins.

Enhanced weathering as a trigger for the rise of atmospheric O2 level from the late Ediacaran to the early Cambrian

A shift toward a higher oxygen level in both ocean and atmosphere systems during the late Ediacaran to the early Cambrian has been suggested from multiple indirect proxies. However, the mechanism and magnitude of this oxidation remain unclear. To solve this issue, we measured carbon isotopes in both carbonate and organic matter as well as their trace element compositions for an Ediacaran-Cambrian sequence in the Lower Yangtze basin, South China. The δ13Corg and δ13Ccarb excursions of this sequence are coupled and can be compared with contemporaneous global carbon isotope curves. A 2‰ rise in Δ13Ccarb-org occurred from the late Ediacaran to the early Cambrian, suggesting a substantial increase in atmospheric oxygen level from 16% to 30% of the present atmospheric level (PAL). Furthermore, the distribution pattern of rare earth elements and the concentrations of water-insoluble elements in the carbonates indicate a sudden enhancement in chemical weathering of the continental crust during the early Cambrian, which may be a trigger for the rise of atmospheric O2 level. Both the supply of a large amount of nutrients due to the enhanced continental weathering and the contemporary increase of atmospheric oxygen concentrations may have promoted the appearance of large metazoans in the early Cambrian.

Sedimentology and chemostratigraphy of the terminal Ediacaran Dengying Formation at the Gaojiashan section, South China

The late Ediacaran Dengying Formation (ca. 551.1-538.8 Ma) in South China is one of two successions where Ediacara-type macrofossils are preserved in carbonate facies along with skeletal fossils and bilaterian animal traces. Given the remarkable thickness of carbonate-bearing strata deposited in less than 12.3 million years, the Dengying Formation holds the potential for a relatively continuous chemostratigraphic profile for the terminal Ediacaran stage. In this study, a detailed sedimentological and chemostratigraphic (δ¹³C_carb, δ¹⁸O_carb, δ¹³C_org, δ³⁴S_pyrite, and ⁸⁷Sr/⁸⁶Sr) investigation was conducted on the Dengying Formation at the Gaojiashan section, Ningqiang County of the southern Shaanxi Province, South China. Sedimentological results reveal an overall shallow marine depositional environment. Carbonate breccia, void-filling botryoidal precipitates, and aragonite crystal fans are common in the Algal Dolomite Member of the Dengying Formation, suggesting that peritidal facies were repeatedly karstified. The timing of karstification was likely early, probably soon after the deposition of the dolomite sediments. The presence of authigenic aragonite cements suggests high alkalinity in the terminal Ediacaran ocean. Geochemical analysis of micro-drilled samples shows that distinct compositions are registered in different carbonate phases, which should be considered when constructing chemostratigraphic profiles representative of true temporal variations in seawater chemistry. Integrated chemostratigraphic data suggest enhanced burial of organic carbon and pyrite, and the occurrence of extensive marine anoxia (at least in the Gaojiashan Member). Rapid basinal subsidence and carbonate accumulation during a time of elevated seawater alkalinity and increased rates of pyrite burial may have facilitated the evolutionary innovation of early biomineralizing metazoans.

Detecting Environment-Dependent Diversification From Phylogenies: A Simulation Study and Some Empirical Illustrations

Understanding the relative influence of various abiotic and biotic variables on diversification dynamics is a major goal of macroevolutionary studies. Recently, phylogenetic approaches have been developed that make it possible to estimate the role of various environmental variables on diversification using time-calibrated species trees, paleoenvironmental data, and maximum-likelihood techniques. These approaches have been effectively employed to estimate how speciation and extinction rates vary with key abiotic variables, such as temperature and sea level, and we can anticipate that they will be increasingly used in the future. Here we compile a series of biotic and abiotic paleodatasets that can be used as explanatory variables in these models and use simulations to assess the statistical properties of the approach when applied to these paleodatasets. We demonstrate that environment-dependent models perform well in recovering environment-dependent speciation and extinction parameters, as well as in correctly identifying the simulated environmental model when speciation is environment-dependent. We explore how the strength of the environment-dependency, tree size, missing taxa, and characteristics of the paleoenvironmental curves influence the performance of the models. Finally, using these models, we infer environment-dependent diversification in two empirical phylogenies: temperature-dependence in Cetacea and $\delta^{13}C$-dependence in Ruminantia. We illustrate how to evaluate the relative importance of abiotic and biotic variables in these two clades and interpret these results in light of macroevolutionary hypotheses. Given the important role paleoenvironments are presumed to have played in species evolution, our statistical assessment of how environment-dependent models behave is crucial for their utility in macroevolutionary analysis.

Bioturbation and directionality in Earth's carbon isotope record across the Neoproterozoic-Cambrian transition

Mixing of sediments by moving animals becomes apparent in the trace fossil record from about 550 million years ago (Ma), loosely overlapping with the tail end of the extreme carbonate carbon isotope δ¹³ C_carbonate fluctuations that qualitatively distinguish the Proterozoic geochemical record from that of the Phanerozoic. These Precambrian-scale fluctuations in δ¹³ C_carbonate (PSF-δ¹³ C_carbonate ) remain enigmatic, due to their high amplitude and inclusion of global-scale negative δ¹³ C_carbonate values, below anything attributable to mantle input. Here, we note that different biogeochemical-model scenarios plausibly explaining globally synchronous PSF-δ¹³ C_carbonate converge: via mechanistic requirements for extensive anoxia in marine sediments to support sedimentary build-up of ¹³ C-depleted carbon. We hypothesize that bioturbation qualitatively reduced marine sediment anoxia by exposing sediments to oxygenated overlying waters, which ultimately contributed to decreasing the carbon cycle's subsequent susceptibility to PSF- δ¹³ C_carbonate . Bioturbation may also have reduced the quantity of (isotopically light) organic-derived carbon available to contribute to PSF- δ¹³ C_carbonate via ocean crust carbonatization at depth. We conduct a comparative modelling exercise in which we introduce bioturbation to existing model scenarios for PSF- δ¹³ C_carbonate : expressing both the anoxic proportion of marine sediments, and the global organic carbon burial efficiency, as a decreasing function of bioturbation. We find that bioturbation's oxygenating impact on sediments has the capacity to prevent PSF- δ¹³ C_carbonate caused by authigenic carbonate precipitation or methanogenesis. Bioturbation's impact on the f-ratio via remineralization is partially offset by liberation of organic phosphate, some of which feeds back into new production. We emphasize that this study is semiquantitative, exploratory and intended merely to provide a qualitative theoretical framework within which bioturbation's impact on long-term, first-order δ¹³ C_carbonate can be assessed (and it is hoped quantified in more detail by future work). With this proviso, we conclude that it is entirely plausible that bioturbation made a decisive contribution to the enigmatic directionality in the δ¹³ C_carbonate record, from the Neoproterozoic-Cambrian boundary onwards.

Local paleoenvironmental controls on the carbon-isotope record defining the Bitter Springs Anomaly

Large magnitude (>10‰) carbon-isotope (δ¹³ C) excursions recorded in carbonate-bearing sediments are increasingly used to monitor environmental change and constrain the chronology of the critical interval in the Neoproterozoic stratigraphic record that is timed with the first appearance and radiation of metazoan life. The ~10‰ Bitter Springs Anomaly preserved in Tonian-aged (1000-720 Ma) carbonate rocks in the Amadeus Basin of central Australia has been offered as one of the best preserved examples of a primary marine δ¹³ C excursion because it is regionally reproducible and δ¹³ C values covary in organic and carbonate carbon arguing against diagenetic exchange. However, here we show that δ¹³ C values defining the excursion coincide with abrupt lithofacies changes between regularly cyclic grainstone and microbial carbonates, and desiccated red bed mudstones with interbedded evaporite and dolomite deposits, recording local environmental shifts from restricted marine conditions to alkaline lacustrine and playa settings that preserve negative (-4‰) and positive (+6‰) δ¹³ C values, respectively. The stratigraphic δ¹³ C pattern in both organic and carbonate carbon recurs within the basin in a similar way to associated sedimentary facies, reflecting the linkage of local paleoenvironmental conditions and δ¹³ C values. These local excursions may be time transgressive or record a relative sea-level influence manifest through exposure of sub-basins isolated by sea-level fall below shallow sills, but are independent of secular seawater variation. As the shallow intracratonic setting of the Bitter Springs Formation is typical of other Neoproterozoic carbonate successions used to construct the present δ¹³ C seawater record, it identifies the potential for local influences on δ¹³ C excursions that are neither diagenetic nor representative of the global exogenic cycle.

Complex patterns in fossilized stromatolites revealed by hyperspectral imaging (400-2496 nm)

Hyperspectral imaging (400-2496 nm) was used to quantitatively map surface textures and compositional variations in stromatolites to determine whether complexity of textures could be used as evidence to support biogenicity in the absence of preserved biomarkers. Four samples of 2.72-2.4 Ga stromatolites from a variety of settings, encompassing marine and lacustrine environments, were selected for hyperspectral imaging. Images of the sawn surfaces of samples were processed to identify reflectance and mineral absorption features and quantify their intensity (as an index of mineral abundance) using automated feature extraction. Amounts of ferrous iron were quantified using a ratio of reflectance at 1650 and 1299 nm. Visible near infrared imagery (400-970 nm) did not reveal additional textural patterns to those obtained from visual inspection. Shortwave infrared imagery (1000-2496 nm), however, revealed complex laminar and convoluted patterns, including a distinctive texture of sharp peaks and broad, low troughs in one sample, similar to living tufted microbial mats. Spectral analysis revealed another sample to be composed of dolomite. Two other samples were dominated by calcite or chlorite ± illite. Large variations in amounts of ferrous iron were found, but ferric iron was exclusively located in the oxidation crust. Hyperspectral imaging revealed large differences between parts of a sample of biogenic and non-biogenic origin. The former was characterized by calcite with varying amounts of ferrous iron, distributed in lenticular, convoluted patterns; the latter by Mg-Fe chlorite with large amounts of aluminium silicate, distributed as fine laminar layers. All minerals identified by hyperspectral imaging were confirmed by thin section petrography and XRD analyses. Spatial statistics generated from quantitative minerals maps showed different patterns between these different parts of the sample. Thus, hyperspectral imaging was shown to be a powerful tool for detecting structures in stromatolites that could be used, together with other lines of evidence, to support biogenicity.

A diagenetic control on the Early Triassic Smithian-Spathian carbon isotopic excursions recorded in the marine settings of the Thaynes Group (Utah, USA)

In the aftermath of the end-Permian mass extinction, Early Triassic sediments record some of the largest Phanerozoic carbon isotopic excursions. Among them, a global Smithian-negative carbonate carbon isotope excursion has been identified, followed by an abrupt increase across the Smithian-Spathian boundary (SSB; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the Late Smithian. It is commonly assumed that Smithian and Spathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the Thaynes Group (Utah, USA) to evaluate the extent to which the Early Triassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ(13) Ccarb variations obtained here reproduce the known Smithian δ(13) Ccarb -negative excursion. However, the δ(13) C signal of the bulk organic matter is invariant across the SSB and variations in the δ(34) S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that Middle to Late Smithian δ(13) Ccarb signal in the shallow marine environments of the Thaynes Group does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment-water interface leading to authigenic carbonate formation during early diagenetic processes.

Ocean acidification and the Permo-Triassic mass extinction

Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.