Enhanced terrestrial biogenic silica flux to platform cherts in marine–continental transition zones: Evidence from the Carboniferous succession of the eastern Qaidam Basin

Abstract

The Carboniferous Period witnessed a pivotal transformation in global silicon cycling, characterized by the expansion of silica-accumulating vascular plants and the proliferation of marine silicifiers. However, the role of terrestrial biogenic silica in shaping platform chert formation within marine–continental transition zones remains poorly constrained. This study investigates elemental geochemistry with δ³⁰Si–δ¹⁸O isotope analyses of Carboniferous cherts and carbonate host rocks from the eastern Qaidam Basin to elucidate their formation mechanisms and silica sources. Our results reveal that bedded and nodular cherts were primarily derived from terrestrial biogenic silica (phytoliths), as evidenced by δ¹⁸O values ranging from 21.39 ‰–25.74 ‰, overlapping with phytolith signatures and δ³⁰Si compositions (−1.44 ‰ to 0.19 ‰) distinct from those of hydrothermal or weathering-derived silica. Reconstructed seawater temperatures 8.82–12.86 °C and highly negative δ¹⁸O_sw values of −12.0 ‰ indicate meltwater influx from late Paleozoic ice age glaciers accompanied by the dissolution of phytoliths. High-frequency glacio-eustatic fluctuations regulated terrestrial-marine silicon transfer: glacial regressions exposed basin margins, fostering silica-rich forest ecosystems, whereas interglacial transgressions inundated these reservoirs, releasing dissolved silica that precipitated as authigenic chert within carbonate platforms. These findings establish terrestrial biogenic silica as a major silicon source for Paleozoic platform cherts, revising conventional models and highlighting coupled land-ocean silicon cycling in icehouse climates.