Enhanced hydrological cycling and continental weathering during the Jenkyns Event in a lake system in the Sichuan Basin, China

doi:10.1016/j.gloplacha.2022.103915

Global and Planetary Change

Volume 216, September 2022, 103915

https://doi.org/10.1016/j.gloplacha.2022.103915 Get rights and content

Highlights

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Tempestites and hyperpycnites were recognized mainly in the JE.
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Enhanced hydrological cycling and continental weathering during the JE were registered in the terrestrial sequences.
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Fluctuation of chemical weathering and hydrological cycling during the JE correlated with SST well.

Abstract

Knowledge of past hydrological cycling and continental weathering under greenhouse climate conditions helps understand extreme climatic events that are currently occurring. Previous studies of marine strata have reported enhanced hydrological cycling and continental weathering during the Jenkyns Event (JE, also known as the Toarcian Oceanic Anoxic Event; ca. 183 Ma). We undertook a sedimentological and geochemical investigation of semi-deep to deep lacustrine deposits that correspond to the Jenkyns Event (i.e., the Da'anzhai Member in the well LQ104X) in the Sichuan Basin, China. Statistical analysis of sedimentological data demonstrates that the tempestities (bioclastic limestones, discrete shell beds, and shell laminae, lenses, and aggregates in mudstones) and hyperpycnites (sandstones and siltstones with normally and inversely–normally graded beddings) occurred in the Jenkyns Event, which is indicative of enhanced hydrological cycling. Hyperpycnites associated with flooding events developed in the upper part of the Jenkyns Event. Geochemical data suggests an intensified continental weathering during the Jenkyns Event. However, weaker chemical weathering recorded in the upper part of the Jenkyns Event compared with the lower part. The enhanced hydrological cycling increased the transportation during the upper part of the Jenkyns Event, which weakened the chemical weathering in the provenance area and resulted in low CIA (chemical index of alteration). The correlation with sea surface temperatures implies that elevated temperatures during the Jenkyns Event lead to enhanced hydrological cycling, increased frequency of storms and floods, and intensified continental weathering.

Introduction

Enhanced hydrological cycling and intensified continental weathering due to global warming result in an increased frequency of extreme climatic events, such as storms, floods, and debris flows (Mitchell, 1989; Stott, 2016). Such weather events modify continental and oceanic ecosystems and affect society. Therefore, studies of hydrological cycling and continental weathering in past greenhouse climate intervals are vital to understanding ongoing extreme climate events. There have been numerous warm intervals in Earth's history, including the Late Permian (Montañez et al., 2007), the Early Jurassic (Han et al., 2018; Hu et al., 2020), and the Paleocene–Eocene thermal maximum (Robert and Kennett, 1994). The Jenkyns Event (JE is also known as the Toarcian Oceanic Anoxic Event (T-OAE); ca. 183 Ma) occurred during Early Jurassic hyperthermal intervals (Jenkyns, 1988; Hesselbo et al., 2000, Hesselbo et al., 2007; Dera et al., 2009, Dera et al., 2011; Müller et al., 2017; Suan et al., 2010). Previous studies have shown that sea surface temperatures increased by 7 °C during the JE, and the hyperthermal conditions lasted for ca. 900 kyr (Suan et al., 2008). Increased sea surface temperatures during the JE caused hurricanes and tropical cyclones (Krencker et al., 2015), and tempestites (Leonowicz, 2011) or turbidites (Hesselbo et al., 2007) were widely developed along continental shelf regions. Studies of marine and marginal-marine deposits also suggest a significant increase in continental weathering intensity during the JE (Hesselbo and Pieńkowski, 2011; Hermoso and Pellenard, 2014; Ruebsam et al., 2020). Enhanced hydrological cycling and intensified continental weathering would have theoretically affected marine, marginal-marine and lacustrine settings (Pieńkowski, 2004; Leonowicz, 2011; Brański, 2012). Therefore, lacustrine deposits can provide additional records of continental weathering and hydrological cycling variations in greenhouse climate conditions.

The Lower Jurassic lacustrine sedimentary rocks in the Sichuan Basin, particularly the Da'anzhai Member, consist of bivalve-bearing mudstones and shales (Sichuan Geology and Mineral Resources Bureau, 1991). The lake expanded and was larger than 26 × 10⁴ km² during the deposition of the Da'anzhai Member. The Re single bond Os ages (ca. 183 Ma; Xu et al., 2017) and a ~ 4‰ negative organic carbon isotope excursion (CIE) of mudstones in the Da'anzhai Member suggested that these mudstones corresponded to the Jenkyns Event (Liu et al., 2020). Preliminary studies have identified tempestites that were developed in the Da'anzhai Member (Feng et al., 2015). This paper presents an integrated sedimentological and geochemical study of semi-deep to deep lacustrine facies rocks of the Da'anzhai Member from the well LQ104X in the Sichuan Basin and compares our record of the Jenkyns Event with those from marine sediments.

Section snippets

Geological settings

The Sichuan Basin transitioned from a marine platform to a foreland basin in the middle of the Late Triassic Xujiahe Formation (Xia et al., 1982). The Indosinian Orogeny, from the end-Triassic to the Early Jurassic, involved episodic compressional thrusting eastward and southward of Bayan Har Mountains and Qinling orogenic belts in the west and north of the Sichuan Basin. Subsequently, the region tectonic activity tended to be calm, and the Lower Jurassic sediments were deposited. The

Samples and methods

The well LQ104X is located in Dazhou City, Sichuan Province (Fig. 1a). For the sedimentological investigations, the drill core was logged in detail. Seventy-two samples were collected for thin-section observations under an optical microscope (Fig. 1c), and each thin-section was scanned on an Epson Perfection 3170 flatbed scanner.

Clay minerals were identified by XRD on oriented mounts of non-calcareous clay-sized particles at the Wuxi Research Institute of Petroleum Geology, SINOPEC. The clay

Lithology

The Da'anzhai Member in the well LQ104X contains three major lithofacies: limestone, mudstone, and sandstone. Mudstones account for 95%, and sandstones and limestones account for 5%. A total of seven sub-facies associated with storm and flood events are summarized in Table 2. According to the background deposition of these sub-facies, the sediment transport and depositional processes, as well as the hydrological conditions which are inferred by taphonomic characteristics of shells and

Shell enrichment and mud as indicators of storm and hurricane events

The four shell-rich lithofacies (i.e., bioclastic limestone, shell beds, shell laminae, and shell lenses and aggregates) in the well LQ104X were formed associated with storm or hurricane events. The ubiquitous erosional base (Fig. 3d) and HCS in bioclastic limestones were observed in the well LQ104X and an outcrop section 138 km away. HCS is a typical sedimentary structure indicating the storm action (Dott Jr and Bourgeois, 1982; Duke, 1985; Cheel et al., 1993). High-energy storm flows with

Conclusions

(1)
Bioclastic limestones and shell-rich mudstones in the Da'anzhai Member in the well LQ104X (3517.5–3546.1 m) represent tempestite deposits formed by storms and hurricanes. Siltstones and sandstones with normally and inversely–normally graded beddings are hyperpycnites related to flooding events. Tempestites and hyperpycnites were dominantly deposited during the Jenkyns Event.
(2)
Multiple weathering and paleoclimatic indices and the sedimentary structures suggest that intensified continental

Declaration of Competing Interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Enhanced hydrological cycling and continental weathering during the Jenkyns Event in a lake system in the Sichuan Basin, China”.

Acknowledgments

We thank Tianchen He from University of Leeds for discussions and two anonymous reviewers for help on earlier versions of the manuscript. We are warmly grateful to Fansheng Meng, Bozhi Wang, and Yang Zhou from Southwest Petroleum University for their assistance with sample preparation. This study was funded by the National Natural Science Foundation of China (Grant NO. 41872155).

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The early Toarcian biotic crisis (∼ 183 Ma), characterized in marine environments by abrupt temperature fluctuations, included a hyperthermal event (Jenkyns Event) with sea-level fluctuations, a carbon cycle perturbation, a crisis of carbonate productivity, and oxygen depleted conditions in some basins, resulting in a second order mass extinction. We suggest that the early Toarcian biotic crisis was key in the evolution of primary producers, including chlorophycean Dasycladales. The effect of the Jenkyns Event on Dasycladales has not been studied previously despite the fact that the impact on many groups of organisms have been the subject of many articles. In this work the stratigraphic distribution of Dasycladalean species from Upper Triassic to end of Jurassic is presented, discussed and compared with climatic fluctuations and sea-level changes.
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Citation Excerpt :
Thereby, increased clastic influx during calcareous shale deposition could be ascribed to climate variation. An accelerated hydrological cycle under a warming setting contributes to the supply of clastic material and nutrients to marine and lacustrine basins (Izumi et al., 2018; Chen et al., 2022; Liu et al., 2022). This inference is also supported by the significant relationship between Th/K and Si/Al (R = 0.63) in the studied section.
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Enhanced hydrological cycling and continental weathering during the Jenkyns Event in a lake system in the Sichuan Basin, China

Highlights

Abstract

Introduction

Section snippets

Geological settings

Samples and methods

Lithology

Shell enrichment and mud as indicators of storm and hurricane events

Conclusions

Declaration of Competing Interest

Acknowledgments

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Clay Minerals in North Sea Sandstones

Grain size control of river suspended sediment geochemistry: clues from amazon river depth profiles

Geochem. Geophys. Geosyst.

The mineralogical record of the early Toarcian stepwise climate changes and other environmental variations (Ciechocinek Formation, Polish Basin)

Volumina Jurassica.

Hydrodynamic properties of cephalopod shell ornament

Paleobiology

Clay Formation Through Weathering. Clay Sedimentology