Yanan Shen

Calibrating the End-Permian Mass Extinction

High-precision geochronologic dating constrains probable causes of Earths largest mass extinction. The end-Permian mass extinction was the most severe biodiversity crisis in Earth history. To better constrain the timing, and ultimately the causes of this event, we collected a suite of geochronologic, isotopic, and biostratigraphic data on several well-preserved sedimentary sections in South China. High-precision U-Pb dating reveals that the extinction peak occurred just before 252.28 ± 0.08 million years ago, after a decline of 2 per mil (‰) in δ13C over 90,000 years, and coincided with a δ13C excursion of −5‰ that is estimated to have lasted ≤20,000 years. The extinction interval was less than 200,000 years and synchronous in marine and terrestrial realms; associated charcoal-rich and soot-bearing layers indicate widespread wildfires on land. A massive release of thermogenic carbon dioxide and/or methane may have caused the catastrophic extinction.

Evidence for sulfidic deep water during the Late Permian in the East Greenland Basin

A detailed study of the size distribution of framboidal pyrites in the black shales of the Upper Permian Ravnefjeld Formation was performed to evaluate the redox state of the Late Permian ocean. In contrast to framboidal pyrites in bioturbated sediments, the smaller and less variable size distribution of pyrite framboids in the laminated shales of the Ravnefjeld Formation provides persuasive evidence for sulfidic (H 2 S-rich) bottom-water conditions in the East Greenland Basin. However, the S isotope compositions of both pyrite populations show a similar distribution. The widespread δ 3 4 S values of pyrites (-41.2‰ to -28.2‰) in the black shales of the Ravnefjeld Formation indicate a large fractionation (up to 52.7‰) relative to seawater sulfate, and may record different pathways of sulfur cycling in sulfidic water columns as well as within sediments. The new data from the East Greenland Basin indicate that environmental stress such as widespread sulfidic conditions could have caused the biotic crisis in the Late Permian.

Large perturbations of the carbon and sulfur cycle associated with the Late Ordovician mass extinction in South China

High-resolution δ 13 C data of organic carbon from a continuous section of the Late Ordovician–Early Silurian reveal two positive δ 13 C excursions that are associated with the mass extinction in South China. The fi rst stratigraphic δ 34 S measurements on pyrite tied to well-established biostratigraphy indicate a large perturbation of the sulfur cycle, consistent with major sea-level changes related to the glaciation. The elevated δ 34 S values of pyrites and a large, short-lived negative δ 34 S excursion of ~20‰ associated with the decay of the glaciation suggest deep-water anoxia during the Hirnantian Stage, in contrast to the conventional view that the global oceans were oxygenated. We suggest that deep-water anoxia may have contributed to the Late Ordovician mass extinction in South China and possibly elsewhere.

Redox chemistry changes in the Panthalassic Ocean linked to the end-Permian mass extinction and delayed Early Triassic biotic recovery

Significance To understand how most life on Earth went extinct 250 million years ago, we used multiple sulfur isotopes to investigate redox chemistry changes in the Panthalassic Ocean, comprising ∼85–90% of the contemporaneous global ocean. The S-isotopic anomalies from Canada and Japan provide evidence for the timing of the onset of euxinia and mixing of sulfidic and oxic waters. Our data suggest that shoaling of H2S-rich waters may have driven the mass extinction and delayed the recovery of the marine ecosystem. This study illustrates how environmental changes could have had a devastating effect on Earth’s early biosphere, and may have present-day relevance because global warming and eutrophication are causing development of sulfidic zones on modern continental shelves, threatening indigenous marine life. The end-Permian mass extinction represents the most severe biotic crisis for the last 540 million years, and the marine ecosystem recovery from this extinction was protracted, spanning the entirety of the Early Triassic and possibly longer. Numerous studies from the low-latitude Paleotethys and high-latitude Boreal oceans have examined the possible link between ocean chemistry changes and the end-Permian mass extinction. However, redox chemistry changes in the Panthalassic Ocean, comprising ∼85–90% of the global ocean area, remain under debate. Here, we report multiple S-isotopic data of pyrite from Upper Permian–Lower Triassic deep-sea sediments of the Panthalassic Ocean, now present in outcrops of western Canada and Japan. We find a sulfur isotope signal of negative Δ33S with either positive δ34S or negative δ34S that implies mixing of sulfide sulfur with different δ34S before, during, and after the end-Permian mass extinction. The precise coincidence of the negative Δ33S anomaly with the extinction horizon in western Canada suggests that shoaling of H2S-rich waters may have driven the end-Permian mass extinction. Our data also imply episodic euxinia and oscillations between sulfidic and oxic conditions during the earliest Triassic, providing evidence of a causal link between incursion of sulfidic waters and the delayed recovery of the marine ecosystem.

Rapid enhancement of chemical weathering recorded by extremely light seawater lithium isotopes at the Permian–Triassic boundary

Significance Estimates of seawater Li isotopic composition at the Permian–Triassic boundary (PTB) reveal extremely light seawater Li isotopic signatures accompanying the most severe mass extinction in the history of animal life. Theoretical modeling indicates a rapid enhancement of continental weathering during this time, which was likely triggered by the eruption of the Siberian Traps, rapid global warming, and acid rains. Our results provide independent geochemical evidence for an enhanced continental chemical weathering at the PTB, illustrating that continental weathering may provide a key link between terrestrial and marine ecological crises. Lithium (Li) isotope analyses of sedimentary rocks from the Meishan section in South China reveal extremely light seawater Li isotopic signatures at the Permian–Triassic boundary (PTB), which coincide with the most severe mass extinction in the history of animal life. Using a dynamic seawater lithium box model, we show that the light seawater Li isotopic signatures can be best explained by a significant influx of riverine [Li] with light δ7Li to the ocean realm. The seawater Li isotope excursion started ≥300 Ky before and persisted up to the main extinction event, which is consistent with the eruption time of the Siberian Traps. The eruption of the Siberian Traps exposed an enormous amount of fresh basalt and triggered CO2 release, rapid global warming, and acid rains, which in turn led to a rapid enhancement of continental weathering. The enhanced continental weathering delivered excessive nutrients to the oceans that could lead to marine eutrophication, anoxia, acidification, and ecological perturbation, ultimately resulting in the end-Permian mass extinction.

Late inception of a resiliently oxygenated upper ocean

The rise of oxygen To understand the evolution of the biosphere, we need to know how much oxygen was present in Earths atmosphere during most of the past 2.5 billion years. However, there are few proxies sensitive enough to quantify O2 at the low levels present until slightly less than 1 billion years ago. Lu et al. measured iodine/calcium ratios in marine carbonates, which are a proxy for dissolved oxygen concentrations in the upper ocean. They found that a major, but temporary, rise in atmospheric O2 occurred at around 400 million years ago and that O2 levels underwent a step change to near-modern values around 200 million years ago. Science, this issue p. 174 The I/Ca ratio in marine carbonates tracks atmospheric oxygen levels for the past 2.5 billion years. Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine-to-calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in the partial pressure of oxygen in the atmosphere at ~400 million years (Ma) ago and reveals a step change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma ago. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma ago.

High-resolution C-isotope chemostratigraphy of the uppermost Cambrian stage (Stage 10) in South China: implications for defining the base of Stage 10 and palaeoenvironmental change

The Wa9ergang section in South China has been proposed as a potential Global Stratotype Section and Point (GSSP) for the base of Stage 10, the uppermost stage of the Cambrian System. In this study, high-resolution C-isotopic compositions are reported and we identified three large negative δ 13 C excursions, namely N1, N2 and N3, at Wa9ergang. The N1 is located just above the First Appearance Datum (FAD) of Lotagnostus americanus , corresponding to the possible base of the Proconodontus posterocostatus conodont Zone. The N2 was identified within the Micragnostus chuishuensis trilobite Zone and the Proconodontus muelleri conodont Zone. The N3 is located in the lowermost part of the Leiagnostus cf. bexelli – Archaeuloma taoyuanense trilobite Zone or Eoconodontus conodont Zone. The N1 and N2 can be correlated with the negative δ 13 C excursions preceding the Top of Cambrian Carbon Isotope Excursion (TOCE) observed globally. The N3 can be correlated with the TOCE or the HEllnmaria–Red Tops Boundary (HERB) Event. The inter-basinal correlation of N1 and L. americanus strongly supports that the base of Stage 10 may be best defined by the FAD of L. americanus . We also used a box model to quantitatively explore the genesis of the negative δ 13 C excursions from South China. Our numerical simulations suggest that weathering of the organic-rich sediments on the platform, probably driven by intermittent sea level fall and/or the oxygenation of the Dissolved Organic Carbon (DOC) reservoir in seawater, may have contributed to the generation of the negative δ 13 C excursions observed in the Stage 10 at Wa9ergang in South China.

87 Sr/ 86 Sr evidence from the epeiric Martin Ridge Basin for enhanced carbonate weathering during the Hirnantian

A pronounced positive δ13C excursion in the Hirnantian Age has been documented globally, reflecting large perturbations of carbon cycling in the Late Ordovician oceans. Increased organic-carbon burial or enhanced carbonate weathering during glacioeustatic sea-level regression has been proposed to account for this anomalous C-isotope excursion. To test the two competing hypotheses, we measured 87Sr/86Sr and δ13C of carbonates from the Copenhagen Canyon section in Nevada, USA. Our data reveal two rapid negative 87Sr/86Sr shifts that coincide with two prominent positive δ13C excursions and glacial advances. Numerical model simulations suggest that enhanced weathering of carbonates driven by glacio-eustatically controlled sea-level fall is required to produce the observed drops of 87Sr/86Sr and the coeval large positive δ13C excursions, possibly with or without increased organic carbon burial.