Marine methane hydrates are a huge and dynamic carbon reservoir found mainly at the continental margins, and their stability might be affected by climate-associated pressure and temperature changes on the seafloor. Reconstructing the growth history of cold-seep carbonates, which formed during seafloor methane leakage, could help to constrain methane hydrate stability in the geological past. However, U-Th isotope systematics of these complex carbonate cements have not yet been systematically investigated on the micro-scale, leaving uncertainties in the U-series geochronology which is commonly applied to date the seep carbonates. In this study, we have developed multiple in-situ analytical methods, including U-Th isotope analysis by laser ablation MC-ICPMS, elemental concentration mapping by laser ablation ICPMS, as well as organic distribution mapping by Raman Spectroscopy, to provide insights into the U-series geochemistry and geochronology for the different types of cold-seep carbonates. Our result demonstrates that 238U and 232Th of these carbonates are dominantly derived from seawater and detrital particles, respectively. As the [230Th/232Th] (activity ratio) of micro-domains with high [232Th/238U] is negligibly affected by U decay, we have directly determined the initial [230Th/232Th] of the cold-seep carbonates which is 0.7 ± 0.1 (2 SD, n = 12). In general, U-series isotopes show closed-system behavior within our studied analytical precision in the interior of the seep cements, and the initial [230Th/232Th] derived from the isochron approach is consistent with the in-situ direct determination. A notable exception is a calcitic pipe sample with highly enriched U in its rim which has also experienced post-depositional mobilization. Our method is then applied to a large set of seep carbonate samples from the upper continental slope of the northern South China Sea (SCS). The obtained U-series ages provide evidence that cold seep was likely continuously active since at least ∼72 ka in the northern SCS, but the timing and duration of methane leakage vary between different sites within this region. While the upper continental slope methane hydrates are highly susceptible to destabilization under changing bottom water conditions, our extensive dating on seep carbonates from the SCS further indicates that bottom water P-T conditions could exert contrasting impacts on methane hydrate stability at different depths within the same geological setting. Overall, the in-situ U-series geochronological method developed in our study has the potential to date cold-seep carbonates efficiently and reliably, offering new opportunities to probe into the history of methane leakage in the late Quaternary.
|Number of pages||17|
|Journal||Geochimica et Cosmochimica Acta|
|State||Published - 1 Mar 2022|
Bibliographical noteFunding Information:
We are grateful to the Editors and Dr. Germain Bayon as well as another anonymous reviewer for the constructive comments that helped improving the quality of manuscript. This study is supported by National Natural Science Foundation of China ( 41822603 , 41991325 , 42021001 ), the Guangdong Basic and Applied Basic Research Foundation ( 2019B030302004 ), the gas hydrate program initiated by the China Geological Survey ( DD20190218 ) and CAS Interdisciplinary Innovation Team (JCTD-2018-12). The authors are grateful to the crews of the Haiyong-4, Haiyang-6, Xiangyanghong-9, Tan Kah Kee as well as Jiaolong, Haima, ROPOS team for sampling during multiple research cruises. Harry H. Roberts is thanked for providing the Gulf of Mexico cold-seep carbonate. The authors thank assistance from Menglong Zhang for SEM analysis, Haiyan Shen for C and O isotopes analysis, Yibo Lin for solution trace elements analysis, Huan Hu and Ennong Tian for LA-ICP-MS analysis and Xiaoyu Zhang for figure preparations.
© 2021 Elsevier Ltd
- Cold-seep carbonate
- Element mapping
- Methane leakage duration
- U-Th dating
ASJC Scopus subject areas
- Geochemistry and Petrology