Xiaolin Zhang

Triassic-Jurassic climate in continental high-latitude Asia was dominated by obliquity-paced variations (Junggar Basin, Ürümqi, China).

Significance Geological records of paleoclimate provide the only constraints on Solar System orbital solutions extending beyond the ∼50-Ma limit imposed by chaotic diffusion. Examples of such constraints are coupled high and low latitude, Triassic–Jurassic (∼198–202 Ma) sedimentary cyclicity in coal-bearing outcrops from the ∼60° N-paleolatitude Junggar Basin (Western China), and contemporaneous tropical basins. Analysis reveals climate variability dominated by obliquity-scale cyclicity in the Junggar Basin and precession-scale cyclicity in the tropics. Together, these geological records empirically constrain orbital solutions by providing joint g4 − g3 and s4 − s3 secular frequency estimates of the Earth–Mars orbital resonance. These results demonstrate the opportunity for developing a new class of solutions grounded by geological data extending hundreds of millions of years into the geologic past. Empirical constraints on orbital gravitational solutions for the Solar System can be derived from the Earth’s geological record of past climates. Lithologically based paleoclimate data from the thick, coal-bearing, fluvial-lacustrine sequences of the Junggar Basin of Northwestern China (paleolatitude ∼60°) show that climate variability of the warm and glacier-free high latitudes of the latest Triassic–Early Jurassic (∼198–202 Ma) Pangea was strongly paced by obliquity-dominated (∼40 ky) orbital cyclicity, based on an age model using the 405-ky cycle of eccentricity. In contrast, coeval low-latitude continental climate was much more strongly paced by climatic precession, with virtually no hint of obliquity. Although this previously unknown obliquity dominance at high latitude is not necessarily unexpected in a high CO2 world, these data deviate substantially from published orbital solutions in period and amplitude for eccentricity cycles greater than 405 ky, consistent with chaotic diffusion of the Solar System. In contrast, there are indications that the Earth–Mars orbital resonance was in today’s 2-to-1 ratio of eccentricity to inclination. These empirical data underscore the need for temporally comprehensive, highly reliable data, as well as new gravitational solutions fitting those data.

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.

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

Five-S-isotope evidence of two distinct mass-independent sulfur isotope effects and implications for the modern and Archean atmospheres

Significance Anomalous sulfur isotopic compositions preserved in sedimentary rocks older than ∼2.5 billion years have been widely interpreted as the products of UV photolysis of sulfur dioxide in an anoxic atmosphere and used to track the history of primitive Earth and evolution of early life. In this study, we present strong observational evidence that there is an additional process that produces similar anomalous sulfur isotope signatures. This previously unknown origin not only offers a tool for quantifying the present-day atmospheric sulfur budget and evaluating its influences on climate and public health but also implies that anomalous sulfur isotopic compositions in some of the oldest rocks on Earth might have been produced in a way different from that previously thought. The signature of mass-independent fractionation of quadruple sulfur stable isotopes (S-MIF) in Archean rocks, ice cores, and Martian meteorites provides a unique probe of the oxygen and sulfur cycles in the terrestrial and Martian paleoatmospheres. Its mechanistic origin, however, contains some uncertainties. Even for the modern atmosphere, the primary mechanism responsible for the S-MIF observed in nearly all tropospheric sulfates has not been identified. Here we present high-sensitivity measurements of a fifth sulfur isotope, stratospherically produced radiosulfur, along with all four stable sulfur isotopes in the same sulfate aerosols and a suite of chemical species to define sources and mechanisms on a field observational basis. The five-sulfur-isotope and multiple chemical species analysis approach provides strong evidence that S-MIF signatures in tropospheric sulfates are concomitantly affected by two distinct processes: an altitude-dependent positive 33S anomaly, likely linked to stratospheric SO2 photolysis, and a negative 36S anomaly mainly associated with combustion. Our quadruple stable sulfur isotopic measurements in varying coal samples (formed in the Carboniferous, Permian, and Triassic periods) and in SO2 emitted from combustion display normal 33S and 36S, indicating that the observed negative 36S anomalies originate from a previously unknown S-MIF mechanism during combustion (likely recombination reactions) instead of coal itself. The basic chemical physics of S-MIF in both photolytic and thermal reactions and their interplay, which were not explored together in the past, may be another ingredient for providing deeper understanding of the evolution of Earth’s atmosphere and life’s origin.

Mid-Cretaceous carbon cycle perturbations and Oceanic Anoxic Events recorded in southern Tibet.

The organic carbon isotope (δ13Corg) curve for ~1.7-km-thick mid-Cretaceous strata of the Chaqiela section in Gamba area, southern Tibet is presented in this study. C-isotopic chemostratigraphic correlation combined with biostratigraphic constraints show that the Chaqiela section spans early Aptian through early Campanian period, and that almost all of the carbon cycle perturbations and Oceanic Anoxic Events during the mid-Cretaceous period are well recorded in the continental margin area of the southeastern Tethys Ocean. Significantly, two levels of methane-derived authigenic carbonates were identified at the onset of OAE1b near the Aptian-Albian boundary. We suggest that an increase in methane release from gas hydrates, potentially driven by sea-level fall and bottom water temperature increase, may have contributed to the large negative δ13Corg excursions and global warming during OAE1b.

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.