Shuang Zhang

Bio: Shuang Zhang is an academic researcher from Yale University. The author has contributed to research in topics: Weathering & Carbon cycle. The author has an hindex of 9, co-authored 17 publications receiving 361 citations. Previous affiliations of Shuang Zhang include Carnegie Institution for Science.

Stepwise oxygenation of the Paleozoic atmosphere.

Abstract: Oxygen is essential for animal life, and while geochemical proxies have been instrumental in determining the broad evolutionary history of oxygen on Earth, much of our insight into Phanerozoic oxygen comes from biogeochemical modelling. The GEOCARBSULF model utilizes carbon and sulphur isotope records to produce the most detailed history of Phanerozoic atmospheric O2 currently available. However, its predictions for the Paleozoic disagree with geochemical proxies, and with non-isotope modelling. Here we show that GEOCARBSULF oversimplifies the geochemistry of sulphur isotope fractionation, returning unrealistic values for the O2 sourced from pyrite burial when oxygen is low. We rebuild the model from first principles, utilizing an improved numerical scheme, the latest carbon isotope data, and we replace the sulphur cycle equations in line with forwards modelling approaches. Our new model, GEOCARBSULFOR, produces a revised, highly-detailed prediction for Phanerozoic O2 that is consistent with available proxy data, and independently supports a Paleozoic Oxygenation Event, which likely contributed to the observed radiation of complex, diverse fauna at this time.

Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact.

Abstract: Mass extinction at the Cretaceous-Paleogene (K-Pg) boundary coincides with the Chicxulub bolide impact and also falls within the broader time frame of Deccan trap emplacement. Critically, though, empirical evidence as to how either of these factors could have driven observed extinction patterns and carbon cycle perturbations is still lacking. Here, using boron isotopes in foraminifera, we document a geologically rapid surface-ocean pH drop following the Chicxulub impact, supporting impact-induced ocean acidification as a mechanism for ecological collapse in the marine realm. Subsequently, surface water pH rebounded sharply with the extinction of marine calcifiers and the associated imbalance in the global carbon cycle. Our reconstructed water-column pH gradients, combined with Earth system modeling, indicate that a partial ∼50% reduction in global marine primary productivity is sufficient to explain observed marine carbon isotope patterns at the K-Pg, due to the underlying action of the solubility pump. While primary productivity recovered within a few tens of thousands of years, inefficiency in carbon export to the deep sea lasted much longer. This phased recovery scenario reconciles competing hypotheses previously put forward to explain the K-Pg carbon isotope records, and explains both spatially variable patterns of change in marine productivity across the event and a lack of extinction at the deep sea floor. In sum, we provide insights into the drivers of the last mass extinction, the recovery of marine carbon cycling in a postextinction world, and the way in which marine life imprints its isotopic signal onto the geological record.

Evolution of the Global Carbon Cycle and Climate Regulation on Earth

Abstract: 14 The existence of stabilizing feedbacks on Earth is generally thought to be necessary for the persistence of 15 liquid water and life. Earth’s atmospheric composition appears to have adjusted to the gradual increase in 16 solar luminosity over time, resulting in persistently habitable surface temperatures. With limited exceptions, 17 the Earth system recovered rapidly from climatic perturbations. Carbon dioxide (CO2) regulation via negative 18 feedbacks within the coupled global carbon-silica cycles are classically viewed as the main processes giving 19 rise to climate stability on Earth. Here we review the long-term global carbon cycle budget and how the 20 processes modulating Earth’s climate system have evolved over time. Specifically, we focus on the relative 21 roles that shifts in carbon sources and sinks have played in driving long-term changes in atmospheric pCO2. 22 We make a case that marine processes are an important component of the canonical silicate weathering 23 feedback, and have played a much more important role in pCO2 regulation than traditionally imagined. The 24 weathering of marine sediments and off-axis basalt alteration are major carbon sinks. However, this sink 25 was potentially dampened during Earth’s early history when oceans had higher levels of dissolved silicon 26 (Si), iron (Fe) and magnesium (Mg), and instead likely fostered more extensive reverse weathering—which in 27 turn fostered higher ocean-atmosphere CO2. 28 29

No evidence for high atmospheric oxygen levels 1,400 million years ago.

Abstract: Zhang et al. (1) recently proposed atmospheric oxygen levels of ∼4% present atmospheric levels (PAL) based on modeling a paleoenvironment reconstructed from trace metal and biomarker data from the 1,400 Ma Xiamaling Formation in China. Intriguingly, this pO2 level is above the threshold oxygen requirements of basal animals and clashes with evidence for atmospheric oxygen levels <<1% PAL in the mid-Proterozoic (2). However, there are fundamental problems with the inorganic and organic geochemical work presented by Zhang et al. (1).

The aerobic diagenesis of Mesoproterozoic organic matter

Abstract: The Xiamaling Formation in the North China Block contains a well-preserved 1400 Ma sedimentary sequence with a low degree of thermal maturity. Previous studies have confirmed the dynamic and complex nature of this evolving marine setting, including the existence of an oxygen-minimum zone, using multi-proxy approaches, including iron speciation, trace metal dynamics, and organic geochemistry. Here, we investigate the prevailing redox conditions during diagenesis via the biomarkers of rearranged hopanes from the finely laminated sediments of the organic-rich black shales in Units 2 and 3 of the Xiamaling Formation. We find that rearranged hopanes are prominent in the biomarker composition of the oxygen-minimum zone sediment, which is completely different from that of the sediment in the overlying anoxic strata. Since the transition process from hopanes to rearranged hopanes requires oxygen via oxidation at the C-l6 alkyl position of 17α(H)-hopanes, we infer that dissolved oxygen led to the transformation of hopane precursors into rearranged hopanes during the early stages of diagenesis. The use of hopanoid hydrocarbons as biomarkers of marine redox conditions has rarely been previously reported, and the hydrocarbon signatures point towards oxic bottom waters during the deposition of Unit 3 of the Xiamaling Formation, which is consistent with the earlier oxygen-minimum zone environmental interpretation of this Unit.

Co2 In Seawater Equilibrium Kinetics Isotopes

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A shale-hosted Cr isotope record of low atmospheric oxygen during the Proterozoic

Abstract: The emergence and expansion of animal life on Earth represents a dramatic shift in the structure and complexity of the biosphere. A lack of firm constraints on surface oxygen levels during the mid-Proterozoic has resulted in heated debate as to whether the rise and earliest diversification of animals was directly linked to a change in environmental oxygen levels or, instead, simply reflects the timing of innovations in gene expression and developmental regulation and was independent of a direct environmental trigger. Here, we present chromium (Cr) isotope data from marine black shales that provide evidence for minimal Cr oxidation throughout the mid-Proterozoic leading up to the diversification of eukaryotes and the rise of animals during the late Neoproterozoic. This observation requires very low background oxygen levels (<1% of present atmospheric levels). Accepting previously proposed estimates of p O2 levels needed to induce Cr isotope fractionation, our data provide support for the persistence of an Earth system in which baseline atmospheric p O2 would have been low enough to inhibit the diversification of animals until ca. 800 Ma. More generally, evidence for a delayed rise of atmospheric oxygen strongly suggests that environmental factors have played a fundamental role in controlling the emergence and expansion of complex life on Earth.

Past climates inform our future

Abstract: As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation—a practice that we argue should be widely adopted.