RECONSTRUCTION OF REDOX AND PRIMARY PRODUCTIVITY OF THE EARLY CAMBRIAN OCEAN DURING THE DEPOSITION OF THE YURTUS BLACK SHALE IN THE TARIM BASIN, NORTHWESTERN CHINA: IMPLICATION ON ORGANIC MATTER ACCUMULATION

The Early Cambrian Yurtus Formation (<541 Ma) in the Tarim basin, northwestern China consists of lower carbonaceous chert-shale unit with high TOC content and upper carbonate unit. The black chert-shale unit has been proven to be high-quality hydrocarbon source rocks. In addition, abundant phytoplankton and small shelly fossils have been reported from the lower Yurtus chert. Nevertheless, the existence of green sulfur bacteria indicated by lipid biomarker of aryl isoprenoid hydrocarbons argues that the photic zone was prevailingly euxinic, which seems contradictory with the diversification of phytoplankton. Therefore, in order to bridge the gap between the ocean redox and biological activities, in this study, we systematically collected samples from the Yurtus Formation and analyzed the nitrogen isotopic (δ¹⁵N) composition and the content of Barium (Ba).

The large positive excursion of δ¹⁵N (>17‰) in the lower and upper parts of the Yurtus Formation probably resulted from denitrification or annamox, both of which occur only when O₂ is depleted. The big negative excursion of δ¹⁵N (~-11‰) in the lower part of the Yurtus Formation is likely attributed to the metabolism of green and purple sulfur bacteria, which is consistent with previous biomarker discovery. It may indicate that the surface ocean contains prolific organic matters which consume dissolved oxidants in the euphotic zone.

Surface ocean primary productivity can also be reconstructed by Barium flux in marine sediments. The extremely high Ba content (>2000ppm) and appearance of BaSO₄ particles in the lower part of the Yurtus Formation implies high primary productivity, which explains the substantive organic matter in the surface ocean and is also consistent with the abundant phytoplankton fossil record and high TOC content. In addition, the high Ba content and presence of BaSO₄ particles coincide with δ¹⁵N perturbations in the lower part of the Yurtus Formation.

In conclusion, high primary productivity of phytoplankton results in the high TOC of the Yurtus Formation. The strong reduced condition in the euxinic zone facilitates the accumulation and preservation of the surplus organic matter. This study provides insights into the interaction between the biosphere and paleoenvironment at the surface ocean before the Cambrian Explosion.