Title: Spatiotemporal variation of dissolved oxygen in the Ediacaran surface ocean and its implication for oceanic carbon cycling
Author: Zihu ZHANG, Meng CHENG*，Haiyang WANG，Xinyang CHEN，Wei GUO，Chao LI*
Journal: Science China- Earth Sciences
Year of Publication: 2023
Volume: 66(8)
Link: https://doi.org/10.1007/s11430-022-1116-3
Abstract：The Ediacaran Period（635–539 Ma） witnessed the largest negative excursion in inorganic carbon isotope(δ13Ccarb)over the Earth’s geological history, also known as the Shuram Excursion（SE） event. The occurrence of the SE has been widely attributed to an increase in atmospheric-oceanic oxygen levels and the subsequent oxidation of organic matters in Earth’s surface system. However, the oxygen levels in the Ediacaran ocean during the SE remain poorly constrained, limiting our ability to better understand the cause and mechanisms behind the SE. Recently, the ratio of I/（Ca+Mg） in carbonate has emerged as an effective proxy for quantifying dissolved oxygen（[O2]） in the local surface seawaters. In this study, we analyzed I/（Ca+Mg） ratios in the Shuiquan Formation at the Mochia-Khutuk（MK） section, which records the SE event in the Tarim continent. The I/（Ca+Mg）ratio shows synchronous variation with δ13Ccarbin the MK section, with the average value decreasing from 2.2 μmol/mol at the bottom of the section to 0.8 μmol/mol in the middle and then increasing to 3.4 μmol/mol at the very top along with the decline and recovery of δ13Ccarb. According to the relationship between I/（Ca+Mg） and oxygen content in minimum oxygen zones of the modern ocean, we infer that [O2] of surface water in the MK section decreased from ＞20–70 μmol/L to ＜20–70 μmol/L during the SE, which may reflect the upwelling of the deep seawater enriched dissolved organic carbon（DOC） and reduced substance(such as Fe2+) together with its subsequent consumption of [O2] in the surface ocean. The I/（Ca+Mg） pattern in the MK section is significantly different from those of other contemporaneous SE records on other continents, indicating the surface [O2] in the Ediacaran ocean could have been temporally and spatially heterogeneous. Local factors, such as latitude, temperature, productivity, and input of anoxic water masses could play important roles in regulating the surface ocean redox conditions. This observation further suggests that the atmospheric oxygen level during the Ediacaran was relatively low and insufficient to dominate the regulation of [O2] in the surface ocean. The results of our study imply that the oxidation of the ocean and in turn the DOC reservoir therein during the SE could be spatially restricted to the continental shelf, rather than the whole ocean.
Key Words: I/(Ca+Mg), Carbon isotope, Dissolved oxygen, Shuram Excursion, Oceanic oxygenation
