October 2023 LIP of the Month

Roller-coaster atmospheric-terrestrial-oceanic-climatic system during Ordovician-Silurian transition: Consequences of large igneous provinces

Licai Song ^{a, b, c} , Qing Chen ^d , Huijun Li ^b , Changzhou Deng ^{c, *}

^a School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China;

^b Key Laboratory of Paleomagnetism and Tectonic Reconstruction, Institute of Geomechanics, Chinese Academy of Geological Sciences, China Geological Survey, Ministry of Natural Resources, Beijing 100081, China;

^c State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;

^d State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing 210008, China

* Corresponding author: dengchangzhou@mail.gyig.ac.cn (C. Deng)

E-mail address: ww3songlicai@163.com (L. Song)

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What we have done and why it matters?

1. We present the first high-resolution multi-proxy record of Hg (concentrations and isotopic compositions), organic carbon isotopes (δ¹³C_org), and whole-rock geochemical data (including trace elements and P) from the continuous strata of Wufeng-Lungmachi black shales that are calibrated by graptolite biostratigraphy from the Yangtze Platform, South China.
2. We integrate our data with that previously published for various continents to investigate the possible coupling relationships between the pulses of LIP magmatism and the fluctuations in oceanic redox, productivity, P recycling, and C cycling during the Ordovician-Silurian transition (OST).
3. Besides the temporal correlation of pulsed LIP magmatism and the ATOCS, we further analyze the feedback loops among the atmospheric-terrestrial-oceanic-climatic system (ATOCS) to test the causal connections.
4. Our study provides an excellent framework for better illuminating the essential role of LIPs in driving the behavior of the ATOCS and thus biotic crisis (the first of the “Big Five” mass extinctions) during the pivotal period of the OST.
5. Our study provides feedback loops or mechanisms for LIPs to trigger and sustain the Hirnantian Glaciation, Oceanic Anoxia Events, Late Ordovician Mass Extinction, and global-scale deposition of black shales based on multiple geochemical data from different continents.
6. Our study proposes that the Wufeng-Lungmachi black shales are just part of the coeval global-scale black shale record (indicating Oceanic Anoxia Events) owing to the Earth System changes caused by LIPs.
7. Our study proposes that the pulsed Late Ordovician Mass Extinction is largely due to the alternating extreme environments triggered by LIPs, and subsequent recovery (though slow) during each stage of extreme environments.

Graphical abstract

Abstract

The Ordovician-Silurian transition (OST) hosted profound and frequent changes in the atmospheric-terrestrial-oceanic-climatic system (ATOCS). Previous studies have found contrasting stages for such changes, primarily based on hiatus-interrupted sections. However, the dominant driving factors and mechanisms reconciling such frequent changes remain controversial. Mercury isotopes, which undergo both mass-dependent and mass-independent fractionation, can provide critical insights into the deep-time ATOCSs, especially for those impacted by large igneous province(LIP) events. Here, we build a high-resolution multi-proxy record of Hg (concentrations and isotopic compositions) combined with organic carbon isotopes (δ¹³C_org) and whole-rock geochemical data (including trace elements and phosphorus) from continuous cores in the Yangtze Platform, South China. Our data, combined with reported ones, indicate the occurrence of LIP eruptions against localized volcanism, and four successive, yet contrasting stages of ATOCSs during the OST. Moreover, we identified the coupling between two-pulse LIP magmatism and extreme ATOCSs, each with special pCO₂, weathering rate, primary productivity, redox condition, climatic mode, and biotic evolution. For stage I, the first pulse of LIP magmatism triggered global warming, enhanced terrestrial weathering, oceanic acidification, eutrophication, anoxia, P recycling, and thereby widespread deposition of black shales. During stage II, the Hirnantian glaciation and oxygenation arose from the intense chemical weathering and black shale deposition of stage I; dramatically decreased terrestrial weathering and oceanic oxygenation facilitating CO₂ accumulation. In stage III, another pulse of LIP magmatism triggered the de-glaciation, and the ATOCS was largely similar to that of stage I. This led to another round of oxygenation and positive δ¹³C_org excursion in stage IV. Compared with the environmental pressure by the peculiar ATOCS of each stage, their transitions might have been more devastating in triggering the prolonged Late Ordovician Mass Extinction (LOME). Moreover, limited biotic recovery was possible in the later portion of stages I and III. The multi-proxy study of continuous strata of the OST provides an excellent framework for better illuminating the essential role of LIPs in driving the “roller-coaster” behavior of the ATOCS and thus biotic crisis during the pivotal period of the OST.

Keywords: Earth system, Large igneous province, Mercury isotope, Late Ordovician mass extinction, Phosphorus recycling

Introduction

Frequent and profound changes occurred in the atmospheric-terrestrial-oceanic-climatic system (ATOCS) during the Ordovician-Silurian transition (from 449.13 to 439.43 Ma) (Gradstein et al., 2020) (Fig. 1). Such changes include fluctuations of atmospheric CO₂ levels (Lenton et al., 2018), low but rising atmospheric O₂ levels (Brand et al., 2021), origination and expansion of the earliest land plants (Lenton et al., 2012), sea-level fluctuations (Li et al., 2021), oceanic anoxic events (OAEs) (Melchin et al., 2013; Stockey et al., 2020), the Hirnantian glaciation (Finnegan et al., 2011), and, most noticeably, Late Ordovician mass extinction (LOME) (Sheehan, 2001) (Fig. 1).

As the first of the “Big Five” mass extinctions, the LOME has long been the focus of research on the ATOCS during the Ordovician-Silurian transition (Sheehan, 2001; Harper et al., 2014; Fan et al., 2020). Various hypotheses of the ATOCS have been proposed based on the swift, two- or single-pulse models of extinction, centering on the Hirnantian glaciation and OAEs (Wang et al., 2019; Bond and Grasby, 2020; Gradstein et al., 2020). However, recent high-resolution analyses of paleontological data have challenged such swift models of extinction (Rasmussen et al., 2019; Fan et al., 2020). Thus, the causal mechanism in the ATOCS for the LOME remains enigmatic.

Geochemical studies on strata of the Ordovician-Silurian transition used to be relatively rare (Melchin et al., 2013), especially on continuously deposited outcrops or cores (Calner et al., 2021; Li et al., 2021). China’s shale gas exploitation has inspired intense investigations on the continuous strata of the Ordovician-Silurian transition in the Yangtze Platform, South China (Ma et al., 2018) (Figs. 1 and 2). Previous studies have proposed that localized factors prevailed in the deposition of the black organic-rich graptolitic shales (“black shales” for short), including semi-enclosed paleogeography (Chen et al., 2004) and localized volcanism (Zhao et al., 2020; Du et al., 2021). However, these black shales belong to the earliest and second-largest of the six Phanerozoic global-scale petroleum source rock black shales (Klemme and Ulmishek, 1991; Arthur and Sageman, 1994), indicating ubiquitous OAEs along the northern margin of Gondwana at that time (Melchin et al., 2013). The Ordovician-Silurian OAEs are remarkably more protracted than the Mesozoic OAEs (Bartlett et al., 2018; Stockey et al., 2020; Dahl et al., 2021). Moreover, the positive feedback between the OAEs and the recycling of phosphorus (P) (Schobben et al., 2020) during the Ordovician-Silurian transition has been barely touched.

Global changes in the ATOCS can be recorded by isotopic carbon excursions (ICEs), which indicate major perturbations in the global carbon cycle (Jenkyns, 2010; Black and Gibson, 2019). Remarkably positive ICEs have been documented accompanying the roller-coaster ATOCS (Bergström and Goldman, 2019). Over geological time, volcanism-related carbon is the principal input for CO₂ in the atmospheric-terrestrial-oceanic system (McKenzie and Jiang, 2019), and large igneous provinces (LIPs) can release voluminous CO₂ in geologically short duration (often <2 Myr) or multiple discrete shorter pulses through a longer period (Kasbohm et al., 2021). Flood basalts and (ultra)mafic intrusions are among the most remarkable records of LIPs, upon which intensified chemical weathering and enhanced oceanic productivity effectively enhance CO₂ sequestration as negative feedback on the sharp CO₂ inputs (Kasbohm et al., 2021; Torsvik et al., 2021). Therefore, LIPs have long been linked with geologically rapid ATOCS changes (Kasbohm, 2022). However, such volcanic inputs cannot be detected or identified solely by the carbon isotope records (Black and Gibson, 2019; Schobben et al., 2019). Moreover, the poor radiogenic isotope geochronology of the late Katian-Hirnantian sequences poses great difficulties in matching LIPs with the profound ATOCS and biological changes (Torsvik et al., 2021) and further challenges the LIPs as the trigger for such changes.

Therefore, albeit the close temporal coincidence and possible interconnections among the changes in the ATOCS, the dominant driving factors and mechanisms reconciling such changes are still controversial. Sedimentary mercury enrichment and isotopic characteristics are “smoking guns” for LIPs, and allow for direct correlation with concomitant geological records (Grasby et al., 2019; Yager et al., 2021; Nauter-Alves, 2022), thereby providing a unique opportunity for exploring the ATOCS during the Ordovician-Silurian transition.

Recent studies have observed high Hg anomalies near the Ordovician-Silurian boundary, and proposed the immediate environmental deterioration by LIP eruption as the primary trigger for the swift, two- or single-pulse models of LOME (Gong et al., 2017; Smolarek-Lach et al., 2019; Hu et al., 2021) (Fig. 2). The contributions of Hg enrichment by pyrites were also explored (Shen et al., 2019; Shen et al., 2022a). Furthermore, although precise isotopic dating ages are lacking, previous studies have proposed possible relics of LIPs at the Ordovician-Silurian transition (Kasbohm et al., 2021; Torsviket al., 2021), for instance, the mafic igneous rocks in Korea of Sino-Korean Craton (445.0 ± 3.7 Ma and 452.5 ± 3.2 Ma) (Cho et al., 2014), Suordakh of Eastern Siberia Craton (458 ± 13 Ma) (Khudoley et al., 2020), and the Alborz LIP in northern Iran (443.7 ± 2.1 Ma) (Derakhshi et al., 2022) (Fig. 2a, sites of a-c). They were located nearly vertically above the margins of the large low shear-wave velocity provinces (namely, ‘‘TUZO” and ‘‘JASON”), which have been suggested to conform with the distribution of the past LIPs if their positions and shapes are largely unchanged (Torsviket al., 2021) (Fig. 2a). Their dates might be highly valuable for tectonics, but higher-resolution dates are desired for scrutinizing the potential causal relationships between LIP magmatism and the ATOCS during the Ordovician-Silurian transition. Therefore, to illuminate the cause-and-effect relationships between LIPs and successive stages of the ATOCSs and their transitions in the Ordovician-Silurian transition, as well as the interactions among the ATOCS, an integration of data of multiple proxies that are well-constrained by graptolite biostratigraphy remains lacking.

Successive strata spanning the Ordovician-Silurian transition are well-developed and preserved in the Yangtze Platform, South China, and serve as commercial shale gas reservoirs (Ma et al., 2018). Generalized graptolite zonation of these strata enables a remarkably higher-resolution division than U-Pb geochronology can do (Chen et al., 2015; Gradstein et al., 2020) (Fig. 1). Thanks to the plausibility of the global correlation of both the graptolite zonation (Chen et al., 2015; Gradstein et al., 2020) and the strata themselves (Arthur and Sageman, 1994; Chen et al., 2015), a unique opportunity exists for exploring the changes in the ATOCS and their driving mechanisms from a global perspective.

Here, we present the first high-resolution multi-proxy record of Hg (concentrations and isotopic compositions), organic carbon isotopes (δ¹³C_org), and whole-rock geochemical data (including trace elements and P) from a continuous shale gas well core (Y#) that is calibrated by graptolite biostratigraphy from the Yangtze Platform (Fig. 2). We integrate our data with previously published ones to investigate the possible coupling relationships between the pulses of LIP magmatism and the fluctuations in oceanic redox, productivity, P recycling, and C cycling. Besides the temporal correlation of pulsed LIP magmatism and the ATOCS, we further analyze the feedback loops among them to test the causal connections. Our study explores the driving role of LIP magmatism on the roller-coaster ATOCS during the Ordovician-Silurian transition from a global view and attempts to raise attention to similar variations in other geological periods.

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