Biogenic silica and organic carbon fluxes provide evidence of enhanced marine productivity in the Upper Ordovician-Lower Silurian of South China

Abstract

Biogenic silica is a powerful palaeoproductivity proxy in palaeomarine environments and is significant within shale gas reservoirs, demonstrating the association of marine hydrocarbon source rocks with the abundance of micro-siliceous fossils, yet detailed studies that investigate this strong alliance in the Ordovician-Silurian transition are still lacking. The bulk of TOC in marine sediments is commonly assumed to have been derived predominantly from marine phytoplankton, with modern ocean surveys indicating good correspondence between silica content and primary productivity. This study discovers a strong positive correlation between biogenic silica (Bio-Si) and TOC in the Upper Ordovician-Lower Silurian Wufeng and Longmaxi formations of South China in the Jiaoye 41-5 (R = +0.68, p(α) < 0.01, n = 69) and Jiaoye 51-2 (R = +0.68, p(α) < 0.01, n = 48) cores, implying that enhanced marine productivity has a strong effect on organic carbon enrichment. Geochemical and palaeontological analyses revealed that silica within these shales generally had a biogenic origin in Jiaoye 41-5 and Jiaoye 51-2, subsequently modified by silica dissolution and re-crystallization of radiolarians and sponge spicules. Palaeoproductivity proxies, including biogenic silica, barium, phosphorous, copper and nickel, were applied, together with the occurrence of pyritized or “ghost” radiolarians, to confirm high palaeoproductivity in the O-S transition. Synchronous OCAR and Bio-SiAR profiles imply that silica originating from siliceous organisms (Bio-Si) served as a likely vector for organic carbon enrichment in the Palaeozoic Wufeng-Longmaxi marine siliceous source rock, providing two possibilities: (i) the abundance of radiolarians was linked to algal proliferation, and consequently, high productivity resulted in a large organic carbon flux and/or (ii) radiolarians contributed to TOC (supported by studies revealing that modern radiolarians exhibit lipid contents of up to 47%).