Do hydrothermal fluids cause a dilution effect on organic matter in the Early Cambrian marine black shales?

Abstract

Hydrothermal fluids play a significant role in the formation of black shales. However, most previous studies have focused on their positive impacts on the formation and enrichment of organic matter (OM), while overlooking potential dilution effects. This study uses organic geochemistry, elemental geochemistry, and field emission scanning electron microscopy (FE-SEM) to examine the marine black shales of the Lower Cambrian Hetang Formation at the Hongtao (HT) section in southern Anhui Province and the contemporaneous Niutitang Formation at the Yangtiao (YT) section in Guizhou Province, China. We systematically analyze the paleo-environmental conditions and the influence of hydrothermal activity during the formation of black shales with varying OM abundances. Our findings show that black shales with different OM abundances in the HT and YT section exhibit similar paleo-productivity, paleo-redox conditions, and sedimentation rates, and all show evidence of hydrothermal sedimentation. Notably, hydrothermal activity significantly affects the content of non-terrestrial silicon (Si_ex), which strongly correlates negatively with total organic carbon (TOC). This suggests that an increased silicon flux due to hydrothermal processes leads to OM dilution. Additionally, we explore the origin of silicon, suggesting that hydrothermal sources are a critical contributor to silicon in shales. We develop a silicon cycling model for black shale formation, which highlights the dual role of hydrothermal activity: promoting OM formation and enrichment yet also causing dilution. This study emphasizes the importance of considering both nutrient transport, enhanced anoxia, and OM dilution when evaluating the influence of hydrothermal activity on black shale formation. Our findings offer new insights into the complex interactions between hydrothermal processes and OM dynamics, providing significant scientific implications for a deeper understanding of black shale formation.