TY - JOUR

T1 - Paired U and Mo isotope evidence for pervasive anoxia in the Cryogenian early interglacial ocean

AU - Pan, Wen

AU - Cao, Mengchun

AU - Du, Yuansheng

AU - Cheng, Meng

AU - Zhou, Yao Qi

AU - Algeo, Thomas J.

AU - Zhao, Ming Yu

AU - Thibault, Nicolas

AU - Li, Chao

AU - Wei, Guang Yi

AU - Dahl, Tais W.

N1 - Funding Information: M.C. acknowledges financial support from the National Natural Science Foundation of China (No. 41772190) and Fundamental Research Funds for the Central Universities (No. 18CX06026A). P. W. acknowledges financial support from the National Natural Science Foundation of China (No. 42072131). TWD acknowledges financial support from the Danish Council for Independent Research (no. 7014-00295B). We thank Drs. Gwyneth W. Gordon and Chadlin M. Ostrander for laboratory assistant at Arizona State University for Mo and U isotope analysis. We also thank Dr. Frances Westall for editorial handling and the two anonymous reviewers whose comments helped to improve this manuscript. This paper is part of Mengchun Cao's Ph.D. dissertation Funding Information: M.C. acknowledges financial support from the National Natural Science Foundation of China (No. 41772190 ) and Fundamental Research Funds for the Central Universities (No. 18CX06026A ). P. W. acknowledges financial support from the National Natural Science Foundation of China (No. 42072131 ). TWD acknowledges financial support from the Danish Council for Independent Research (no. 7014-00295B). We thank Drs. Gwyneth W. Gordon and Chadlin M. Ostrander for laboratory assistant at Arizona State University for Mo and U isotope analysis. We also thank Dr. Frances Westall for editorial handling and the two anonymous reviewers whose comments helped to improve this manuscript. This paper is part of Mengchun Cao's Ph.D. dissertation Publisher Copyright: © 2021 Elsevier B.V.

PY - 2021

Y1 - 2021

N2 - The delayed diversification of early animals until the middle Ediacaran Period, despite their likely origin in the Cryogenian Period or earlier, has been attributed to low levels of dissolved O2 in global seawater. To provide more robust constraints on the extent of global-ocean oxygenation during the mid-Cryogenian Period, we analyzed paired U (δ238U) and Mo (δ98Mo) isotope compositions of organic-rich shale in the basal Datangpo Formation from two separate drillcores in South China. This unit yielded an average δ238U of −0.39 ± 0.23‰ (2SD; relative to standard CRM145), which is significantly lower than the average δ238U (+0.02 ± 0.12‰) of euxinic sediments of the modern restricted Black Sea and well below a modeled δ238U of +0.2‰ for modern unrestricted euxinic facies (e.g., Saanich Inlet). The U/Al ratios of our samples indicate that ~ 40% of U was derived from detrital materials (δ238Udetrital = ~−0.3‰), suggesting that the δ238U of authigenic U is even lower than the measured bulk-sample values. Taking into consideration the 0.4–0.8‰ enrichment of 238U in euxinic sediments relative to seawater, the δ238U values of the Datangpo shales correspond to contemporaneous seawater δ238U of −1.20‰ to −0.73‰. Owing to mass balance considerations, these low seawater δ238U values are indicative of widespread development of anoxia in the Cryogenian interglacial ocean (~658–654 Ma). These shales yield mostly low δ98Mo values (+0.56 ± 0.46‰), although the youngest samples have values as high as +1.21 ± 0.23‰ (relative to NIST SRM 3134 = +0.25‰), all of which are significantly lower than modern seawater δ98Mo (+2.34 ± 0.10‰). The Datangpo shales either directly record the global seawater δ98Mo signature or provide a minimum estimate thereof (in the event of local influences). The consistently low δ98Mo values of the Datangpo shales thus confirm widespread anoxia in the Cryogenian interglacial ocean. These findings based on paired δ238U-δ98Mo analyses thus provide key information regarding global-ocean redox conditions during the interval immediately preceding the rise of macroscopic and morphologically complex animals

AB - The delayed diversification of early animals until the middle Ediacaran Period, despite their likely origin in the Cryogenian Period or earlier, has been attributed to low levels of dissolved O2 in global seawater. To provide more robust constraints on the extent of global-ocean oxygenation during the mid-Cryogenian Period, we analyzed paired U (δ238U) and Mo (δ98Mo) isotope compositions of organic-rich shale in the basal Datangpo Formation from two separate drillcores in South China. This unit yielded an average δ238U of −0.39 ± 0.23‰ (2SD; relative to standard CRM145), which is significantly lower than the average δ238U (+0.02 ± 0.12‰) of euxinic sediments of the modern restricted Black Sea and well below a modeled δ238U of +0.2‰ for modern unrestricted euxinic facies (e.g., Saanich Inlet). The U/Al ratios of our samples indicate that ~ 40% of U was derived from detrital materials (δ238Udetrital = ~−0.3‰), suggesting that the δ238U of authigenic U is even lower than the measured bulk-sample values. Taking into consideration the 0.4–0.8‰ enrichment of 238U in euxinic sediments relative to seawater, the δ238U values of the Datangpo shales correspond to contemporaneous seawater δ238U of −1.20‰ to −0.73‰. Owing to mass balance considerations, these low seawater δ238U values are indicative of widespread development of anoxia in the Cryogenian interglacial ocean (~658–654 Ma). These shales yield mostly low δ98Mo values (+0.56 ± 0.46‰), although the youngest samples have values as high as +1.21 ± 0.23‰ (relative to NIST SRM 3134 = +0.25‰), all of which are significantly lower than modern seawater δ98Mo (+2.34 ± 0.10‰). The Datangpo shales either directly record the global seawater δ98Mo signature or provide a minimum estimate thereof (in the event of local influences). The consistently low δ98Mo values of the Datangpo shales thus confirm widespread anoxia in the Cryogenian interglacial ocean. These findings based on paired δ238U-δ98Mo analyses thus provide key information regarding global-ocean redox conditions during the interval immediately preceding the rise of macroscopic and morphologically complex animals

KW - Cryogenian

KW - Datangpo Formation

KW - Early metazoans

KW - Iron speciation

KW - Paleoredox

KW - South China

U2 - 10.1016/j.precamres.2021.106244

DO - 10.1016/j.precamres.2021.106244

M3 - Journal article

AN - SCOPUS:85105731692

VL - 361

JO - Precambrian Research

JF - Precambrian Research

SN - 0301-9268

M1 - 106244

ER -