Seeking a geochemical identifier for authigenic carbonate.

Ming-Yu Zhao,Yan-Yan Zhao,Yong-Fei Zheng

doi:10.1038/ncomms10885

Ming-Yu Zhao, Yan-Yan Zhao + Show 1 more

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https://doi.org/10.1038/ncomms10885

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Abstract

Authigenic carbonate was recently invoked as a third major global carbon sink in addition to primary marine carbonate and organic carbon. Distinguishing the two carbonate sinks is fundamental to our understanding of Earth's carbon cycle and its role in regulating the evolution of atmospheric oxygen. Here, using microscale geochemical measurements of carbonates in Early Triassic strata, we show that the growth of authigenic carbonate follows a different trajectory from primary marine carbonate in a cross-plot of uranium concentration and carbon isotope composition. Thus, a combination of the two geochemical variables is able to distinguish between the two carbonate sinks. The temporal distribution of authigenic carbonates in the Early Triassic strata suggests that the increase in the extent of carbonate authigenesis acted as a negative feedback to the elevated atmospheric CO2 concentration.

Highlights

Authigenic carbonate was recently invoked as a third major global carbon sink in addition to primary marine carbonate and organic carbon
The more widespread type of authigenic carbonate, probably exists in ordinary sediments without methane seep structures. Identifying this authigenic carbonate sink in deep time presents a unique challenge because it may be morphologically and isotopically indistinguishable from primary marine carbonate, it is possibly mixed with primary marine carbonate at the microscale[4], and the existing modern analogues[13,14] are imperfect
The results show that the uranium concentration combined with the carbon isotope composition can be used to distinguish Earth’s two carbonate sinks

Summary

Introduction

Authigenic carbonate was recently invoked as a third major global carbon sink in addition to primary marine carbonate and organic carbon. Using microscale geochemical measurements of carbonates in Early Triassic strata, we show that the growth of authigenic carbonate follows a different trajectory from primary marine carbonate in a cross-plot of uranium concentration and carbon isotope composition. With carbon derived partially from these processes, authigenic carbonates can have distinct carbon isotopic compositions from those of primary marine carbonates, which makes them important in our understanding of Earth’s coupled carbon and oxygen cycles[4,5]. The more widespread type of authigenic carbonate, probably exists in ordinary sediments without methane seep structures Identifying this authigenic carbonate sink in deep time presents a unique challenge because it may be morphologically and isotopically indistinguishable from primary marine carbonate, it is possibly mixed with primary marine carbonate at the microscale[4], and the existing modern analogues[13,14] are imperfect. The results show that the uranium concentration combined with the carbon isotope composition can be used to distinguish Earth’s two carbonate sinks

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