Research Report on the Pulse of Intense Oxidative Weathering During the Latest Paleoproterozoic

Academic Background

The level of atmospheric oxygen on Earth has a profound impact on the ecology and biogeochemical functioning of marine ecosystems. The Great Oxidation Event (GOE), which occurred approximately 2.5 to 2.2 billion years ago, marked a significant rise in atmospheric oxygen levels from trace amounts to over 10^-5 of the present atmospheric level. However, over the next ~1 billion years, atmospheric oxygen levels declined to a lower level, although the exact level remains uncertain. Recent studies suggest that the redox environment during this period was more complex than previously thought, with evidence of multiple oxygenation pulses. Nevertheless, the causes and extent of these oxygenation pulses remain unclear.

To explore these issues, researchers analyzed 1.64-billion-year-old black shales and dolostones from the North China Craton (NCC). These rocks represent sediments deposited during the breakup of the Columbia supercontinent. By studying the elemental concentrations, rhenium-osmium (Re-Os), and copper (Cu) isotopic compositions of these rocks, the researchers aimed to evaluate terrestrial weathering processes and the evolution of atmospheric oxygen levels during the latest Paleoproterozoic.

Source of the Paper

This paper was co-authored by Xiuqing Yang, Guowei Yang, Chao Li, Kurt O. Konhauser, Changzhi Wu, Fang Huang, and Jingwen Mao. The authors are affiliated with the School of Earth Science and Resources at Chang’an University, the National Research Center for Geoanalysis at the Chinese Academy of Geological Sciences, the Department of Earth & Atmospheric Sciences at the University of Alberta, the School of Earth and Space Sciences at the University of Science and Technology of China, and the Institute of Mineral Resources at the Chinese Academy of Geological Sciences. The paper was published online on September 25, 2024, in the journal Geology, with the DOI 10.1130/g52373.1.

Research Process

1. Sample Collection and Processing

The researchers collected black shale and dolostone samples from the Chuanlinggou Formation in the ZK83-6 drill core located in Chicheng City, Hebei Province. After detailed petrographic examination, the samples were analyzed for total organic carbon (TOC), total sulfur (TS), major and trace elements, as well as Re-Os and Cu isotopes.

2. Experimental Methods

• Total Organic Carbon and Total Sulfur Analysis: Standard methods were used to determine TOC and TS content.
• Major and Trace Element Analysis: Determined using X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma mass spectrometry (ICP-MS).
• Re-Os Isotope Analysis: Samples were dissolved using the inverse aqua regia method to reduce blank levels. Re and Os isotopic compositions were measured using thermal ionization mass spectrometry (TIMS).
• Cu Isotope Analysis: Cu isotopic compositions were determined using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS).

3. Data Analysis

• Re-Os Isotope Data: Initial 187Os/188Os ratios were calculated to reflect the Os isotopic composition of seawater at the time of deposition.
• Cu Isotope Data: δ65Cu values were calculated to assess oxidative weathering processes.
• Th/U Ratios and Chemical Index of Alteration (CIA-K): Used to evaluate the intensity of terrestrial weathering.

Key Results

1. Elemental Composition of Black Shale and Dolostone

The TOC and TS contents of black shale samples ranged from 0.13-4.68 wt% and 0.03-2.13 wt%, respectively, while dolostone samples had TOC and TS contents of 0.05-0.64 wt% and 0.02-0.62 wt%, respectively. The Th/U ratios of black shale ranged from 0.94-7.21, with an average of 4.53, indicating intense oxidative weathering.

2. Re-Os Isotopic Composition

The initial 187Os/188Os ratios of black shale samples ranged from 0.84-1.34, with an average of 1.06±0.14, similar to modern seawater values (~1.06). The initial 187Os/188Os ratios of dolostone samples ranged from 0.58-0.93, with an average of 0.72±0.13.

3. Cu Isotopic Composition

The δ65Cu values of black shale samples ranged from -0.19‰ to 0.43‰, with an average of +0.12‰±0.17‰, similar to those of GOE and post-GOE shales.

4. Chemical Index of Alteration

The CIA-K values for the entire section ranged from 84-99, indicating intense chemical weathering.

Discussion and Conclusions

1. Evidence of Oxidative Weathering

High initial 187Os/188Os ratios, positive δ65Cu values, high Th/U ratios, and CIA-K values suggest that terrestrial weathering processes at 1.64 billion years ago were characterized by intense oxidative weathering. These findings are similar to post-GOE weathering processes, indicating a possible oxygenation pulse during this period.

2. Origin and Causes of Intense Terrestrial Weathering

The researchers propose that the breakup of the Columbia supercontinent led to enhanced terrestrial weathering. During the supercontinent breakup, the weathering of large amounts of fresh mafic rocks increased the supply of phosphorus to the oceans, stimulating cyanobacterial photosynthesis and subsequently increasing oxygen production rates. Additionally, the breakup of the supercontinent expanded coastlines, providing more shallow-water habitats for cyanobacterial mats.

3. Scientific and Practical Value

This study provides new evidence for understanding the evolution of atmospheric oxygen levels during the Paleoproterozoic and reveals a possible link between supercontinent breakup and oxygenation pulses. The findings are significant for understanding the complexity of early Earth’s redox environment and the driving mechanisms of oxygen production.

Research Highlights

• Key Discovery: Terrestrial weathering processes at 1.64 billion years ago were characterized by intense oxidative weathering, possibly associated with an oxygenation pulse.
• Methodological Innovation: The use of the inverse aqua regia method for Re-Os isotope analysis reduced blank levels and improved data accuracy.
• Unique Research Object: The study samples are from the Chuanlinggou Formation in the North China Craton, which contains some of the best-preserved black shales from 1.8-0.8 billion years ago in the region.

Additional Valuable Information

The study also found that the large igneous province (LIP) activity during the breakup of the Columbia supercontinent may have further promoted terrestrial weathering and oxygen production. These findings provide new perspectives for understanding the evolution of early Earth’s redox environment.