Study on Continental Weathering and Reductive Surface Runoff During the Triassic-Jurassic Transition

Academic Background

The Triassic-Jurassic Transition (TJT) marks one of the most significant mass extinction events in Earth’s history, known as the End-Triassic Extinction (ETE). This event led to a widespread decline in biodiversity across both marine and terrestrial ecosystems, primarily triggered by the volcanic activity of the Central Atlantic Magmatic Province (CAMP). The eruptions of CAMP not only caused global climate changes but also significantly enhanced continental weathering intensity and altered terrestrial redox conditions. However, the specific changes in continental weathering intensity and surface runoff redox conditions during this period remain poorly understood.

To address this research gap, the authors of this study analyzed stratigraphic sections from high-latitude and mid-to-low-latitude regions, combining iron (Fe) and zinc (Zn) isotope data to investigate the enhanced continental weathering and reductive surface runoff conditions during the ETE-TJT interval. This research not only contributes to understanding major environmental changes in Earth’s history but also provides important references for modern climate change and ecosystem responses.

Source of the Paper

This paper was co-authored by Ruoqi Wan, Chengshuai Yuan, Sheng-Ao Liu, Linhao Fang, Jun Shen, and Xiaomei Wang. The authors are affiliated with the State Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences (Beijing), PetroChina Research Institute of Petroleum Exploration and Development, the State Key Laboratory of Petroleum Resources and Engineering at China University of Petroleum (Beijing), and the State Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences (Wuhan). The paper was published online on October 17, 2024, in the journal Geology, with the DOI 10.1130/G52551.1.

Research Process and Results

Research Process

1. Sample Collection and Stratigraphic Analysis
   The study selected two stratigraphic sections: the high-latitude Haojiagou Section (HJG) and the mid-to-low-latitude Qilixia Section (QLX). These sections are located in the Junggar Basin in northwestern China and the Sichuan Basin in southwestern China, respectively. Through detailed lithological descriptions and chronological analyses of these sections, the researchers identified the stratigraphic positions corresponding to the ETE-TJT transition.

2. Isotope and Chemical Weathering Index Analysis
   The researchers conducted iron (δ56Fe) and zinc (δ66Zn) isotope analyses on the collected sedimentary rock samples, combined with the Chemical Index of Alteration (CIA) for comprehensive study. CIA is an indicator of chemical weathering intensity, with higher values indicating stronger weathering.

3. Polycyclic Aromatic Hydrocarbons (PAHs) Analysis
   To further explore the redox conditions of surface runoff, the researchers also analyzed the content of Polycyclic Aromatic Hydrocarbons (PAHs) in the samples. PAHs are products of wildfire combustion, and their variations can reflect the intensity of wildfire activity.

Key Findings

1. Variations in Iron and Zinc Isotopes
   The results showed significant positive excursions in δ56Fe values in both sections during the ETE-TJT interval, indicating enhanced iron isotope fractionation. Meanwhile, δ66Zn values decreased in the HJG section but exhibited large fluctuations in the QLX section. These isotopic changes were synchronous with increases in CIA values, suggesting a significant enhancement in continental weathering intensity during the ETE-TJT interval.

2. Changes in Chemical Weathering Index
   CIA values increased significantly during the ETE-TJT interval, particularly in beds 43–49 of the HJG section and beds 15–18 of the QLX section. The increase in CIA values correlated positively with the positive excursions in δ56Fe, indicating changes in the redox conditions of surface runoff under intensified weathering.

3. Variations in Polycyclic Aromatic Hydrocarbons
   The study also found that the positive excursions in δ56Fe correlated with increases in PAHs content. Notably, PAHs content peaked during the TJT interval, indicating heightened wildfire activity. Biochar produced by wildfires may have further enhanced the reductive conditions of surface runoff through microbial-mediated iron reduction.

Conclusions and Significance

1. Enhanced Continental Weathering
   The results demonstrate that continental weathering intensity significantly increased during the ETE-TJT interval, closely linked to global warming triggered by CAMP volcanic activity. Enhanced weathering not only altered the chemical composition of surface runoff but also impacted the global carbon cycle.

2. Reductive Conditions of Surface Runoff
   Through iron isotope and PAHs analyses, the researchers identified significant changes in the redox conditions of surface runoff during the ETE-TJT interval. Biochar from wildfires likely enhanced the reductive nature of surface runoff through microbial-mediated iron reduction.

3. Implications for Global Environmental Changes
   This study provides new evidence for understanding major environmental changes in Earth’s history, particularly the impact of CAMP volcanic activity on global climate and ecosystems. Additionally, the findings offer important insights for modern climate change and ecosystem responses.

Research Highlights

1. Multi-Isotope Combined Analysis
   This study is the first to combine iron and zinc isotopes with the Chemical Index of Alteration, providing a comprehensive understanding of changes in continental weathering and surface runoff redox conditions during the ETE-TJT interval.

2. Impact of Wildfire Activity
   Through PAHs analysis, the researchers linked wildfire activity to the reductive conditions of surface runoff, revealing the profound impact of CAMP volcanic activity on terrestrial ecosystems.

3. Global Comparative Study
   By comparing high-latitude and mid-to-low-latitude sections, the study provides critical data support for understanding global environmental changes.

Additional Valuable Information

1. Microbial-Mediated Iron Reduction
   The study also explored the role of microbes in iron reduction, highlighting the importance of biochar as an electron donor in this process.

2. Feedback Mechanisms in the Carbon Cycle
   By analyzing variations in PAHs and CIA, the researchers further investigated the feedback mechanisms of wildfire activity on the global carbon cycle, offering new perspectives on carbon cycle changes in Earth’s history.

This study, through interdisciplinary approaches, comprehensively reveals changes in continental weathering and surface runoff redox conditions during the ETE-TJT interval, providing new evidence and perspectives for understanding major environmental changes in Earth’s history.