Implications for the history of Cenozoic opal deposition from a quantitative model

In addition to being a time of enhanced crustal activity and a warmer climate, the Eocene also saw prolonged periods of silica accumulation on the ocean floor. The peak of this accumulation occurred at approximately 50 Ma, creating Horizon AC, a layer of silica-rich sediments spanning approximately 4 My. Horizon AC is comparatively isolated in time from known silicic acid addition processes, leading McGowran (1989) [McGowran, B., 1989. Silica burp in the Eocene ocean. Geology, 17 pp. 857-860.] to suggest a novel mechanism (the “silica burp” hypothesis) that centres on the temporal decoupling of silicic acid supply and burial by climatic variation. Here we examine this hypothesis using a quantitative biogeochemical model of the silicon cycle. Our results unequivocally show that McGowran's hypothesis is unable to account for Horizon AC. The model's residence time and ocean capacity for silicic acid are insufficient to permit the degree of temporal decoupling proposed by McGowran. Inverting the model's results suggests that, instead, the sediment record may be a useful proxy for silicic acid additions to the ocean.