Abstract

This study is a comprehensive, stable isotope survey of the marine carbonate-dominated, upper Paleo- to lower Neoproterozoic stratigraphy of Jixian County, China. Carbonate-associated sulfate (CAS) was extracted and measured for δ 34S CAS using the same samples analyzed for δ 13C carbonate. This integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of Proterozoic seawater. We specifically sampled marine carbonate intervals from the lower section of the Chuanlinggou Formation, Changcheng Group (ca. 1700 Ma) to the top of the Jingeryu Formation, Qingbaikou Group (ca. 800 Ma). δ 13C carbonate values are mostly negative in the upper Paleoproterozoic Changcheng Group, with an ascending trend from −3‰ to 0‰. We observed variation of approximately 0 ± 1‰ in the Mesoproterozoic Jixian Group, and positive values of +2 ± 2‰ characterize the lower Neoproterozoic Qingbaikou Group. Stratigraphic variations in δ 34S CAS are more remarkable in their ranges and magnitudes, including conspicuously high values exceeding +30‰ in the three intervals at ca. 1700 Ma, 1300–1100 Ma, and 1000–900 Ma. In the Changcheng Group, δ 34S CAS values are typically higher than +25‰, with only a few values of less than +15‰. In contrast, most of the data spanning from the Mesoproterozoic Tieling Formation of the Jixian Group to the lower Neoproterozoic Jingeryu Formation of the Qingbaikou Group are highly variable between +10‰ and +25‰, with some values exceeding +25‰. In the late Paleoproterozoic (1700–1600 Ma), a >10‰ decrease in δ 34S CAS and ∼3‰ increase in δ 13C carbonate are coincident with, and likely related to, the breakup of Columbia, a supercontinent that predated Rodinia. Carbon and sulfur isotope data from the Mesoproterozoic, when global tectonic activity was comparatively weaker, fall mostly in the ranges of +15 ± 10‰ and 0 ± 1‰, respectively, but fluctuations of >20‰ for δ 34S CAS and >3‰ for the δ 13C carbonate at ca. 1450–1400 Ma may reflect subduction and large-scale magmatic activity in island arcs marking the end of Columbia breakup. From the late Mesoproterozoic (ca. 1300–1100 Ma) to the early Neoproterozoic (ca. 800 Ma), the δ 13C and δ 34S of seawater increased gradually with increasing variability. Most impressive are δ 34S CAS values that exceed +30‰ in two intervals at ca. 1300–1100 Ma and ca. 1000–900 Ma, which may reflect the assembly and early breakup of Rodinia. Although gaps in the record remain, and studies of even higher resolution are warranted, our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of C and S. Furthermore, periods of the Proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity.

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