A SEDIMENTARY FINGERPRINT OF OSCILLATING REDOX THROUGH THE CAMBRIAN PERIOD

The timing and nature of Phanerozoic oxygenation have endured as central but largely unanswered questions in the narrative of early metazoan evolution. Geochemical proxies indicate the signature, at least episodically, of oxygenated deeper waters in Ediacaran successions, but the spatiotemporal patterns and ecological responses of Ediacaran-Cambrian oxygenation have remained elusive. Independently from proxy records, Ediacaran-Cambrian marine sedimentary successions record a conspicuous shift in mineralogy that coincides with the early history of metazoan life. These strata host the earliest significant glauconite accumulations known from the rock record, many of which were deposited in association with phosphorites and phosphatic sediments. Glauconite, an authigenic Fe(II)-Fe(III)-silicate mineral, reaches its Phanerozoic peak in accumulation during the middle and later Cambrian, increasing in frequency across a range of siliciclastic and carbonate lithofacies as discrete grains, cements, and early diagenetic replacement fabrics. In particular, fibroradiating rims precipitated on carbonate and siliciclastic grains indicate that glauconite formed at or near the sea floor prior to carbonate cementation, reflecting a rate and style of glauconitization absent from modern settings but abundant in global Cambrian successions.

The environmental drivers for widespread glauconite deposition are not known, largely because the geochemistry underpinning its crystallization is poorly understood. Here, merging chemical, crystallographic, and sedimentological data, we propose a new crystallization mechanism. We show that the rate at which pore waters oscillate between oxic and anoxic states directly controls the glauconite crystallization rate, providing key environmental context for glauconite distribution in time and space. Our model sheds new light on the widespread, facies-independent peak in glauconite production within the Cambrian Period, which we argue was a result of the elevated susceptibility of Cambrian marine water masses to frequent redox oscillation. This apex in glauconite production was perhaps the first time, but not the last, that the oceans crossed an oxygen threshold triggering redox oscillations that resonated through multiple biogeochemical cycles.