Abstract
The early Cambrian carbonaceous shale and laminated chert-phosphorite assemble (the black rock series) are widespread at the northwest margin of the Tarim Basin, Northwest China. In combination with previously reported data, we present stable molybdenum isotope (δ98/95Mo), TOC, and redox-sensitive trace elements to evaluate the sedimentary conditions in early Cambrian water column during the deposition of the black rock series in the Tarim Basin. Redox variation was documented based on enrichment factors (MoEF, VEF, and UEF) and redox-sensitiv elements ratios (Ni/Co, V/Cr, δU), etc. During the early Cambrian, there was transgressive event, and the sea level continues to rise. In the basal Cambrian, laminated chert-phosphorite assemble with low TOC concentrations suggest the oxic sedimentary condition in a restricted basin. Light Mo isotope values and redox sensitive elements enrichment in the carbonaceous shale layer indicate lack oxygenic sedimentary condition, and was suboxic/anoxic conditions during the transgressive phase. The hydrothermal fluids from the open ocean affected the whole deposition process of the black rock series.
Highlights
In combination with previously reported data, we present stable molybdenum isotope (δ98/95Mo), TOC, and redox-sensitive trace elements to evaluate the sedimentary conditions in early Cambrian water column during the deposition of the black rock series in the Tarim Basin
We investigated black rock series from the Lower Cambrian Sugetbrak section, Akesu-Wushi area, Tarim Basin
This section consists of interbedded chert-phosphorite assemblages (1.04 m) and the carbonaceous shale (1.02 m) of the Early Cambrian Yurtus Formation in an ascending order (Figure 1(c))
Summary
The molybdenum (Mo) isotope system, together with trace metal geochemistry have been taken as the indicator of the redox conditions in ancient paleo-envi-. In euxinic sediments ( ) ([H2S] > 11 μm), Mo is turned into tetrathiomolybdate MoS24− and δ98/95Mo has heavy isotope value closer to that of modern ocean water (2.3‰) [11] [14] Trace elements, such as Mo, U, V, etc., are highly sensitive to redox changes in the water column and are highly enriched in the reducing sediment, potentially making them and their ratios for important proxies for paleo-redox conditions [15] [16] [17]. In China, the Lower Cambrian sedimentary strata, containing black shales and cherts, occur on the Yangtze and Tarim Platforms [7] [23] [24] [25] [26] [27].
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