Ocean Fertilization at the Dawn of the Phanerozoic

Tuesday, 16 December 2014
Robert Riepma Gaines, Pomona College, Claremont, CA, United States
A prolonged episode of continental erosion during the latest Precambrian exposed crystalline basement rocks to atmospheric weathering over an area that is unprecedented in the rock record. As a result, an elevated flux of continental weathering products has been suggested as an important influence over seawater chemistry during the Ediacaran-Cambrian transition. The effects of strongly elevated pCO2 and reduced pO2 on the composition of the continental weathering flux to the oceans was investigated by geochemical and petrologic analysis of nine weathering profiles of early-late Cambrian age from Laurentia, Baltica and Gondwana. These included eight weathering horizons developed on basement rocks ranging from felsic to mafic in composition and one profile that developed on muds in an early Cambrian fluvial basin. Patterns of cation depletion and enrichment are comparable to those observed in modern weathering profiles, with the exception of Fe, which was depleted in all nine profiles analyzed. Thus, under Cambrian weathering conditions, Fe appears to have been preferentially leached and ultimately lost to the oceans, rather than accumulated in soils. These results are supported by clay mineralogy, which reveal structural Fe losses from pedogenic clays as well as from parent rocks in the upper 0.5 to 8 m of the profiles. Because the weathering flux of P is strongly coupled to Fe, these data imply a sustained and elevated flux of iron and phosphorous to the oceans from weathering across an exceptionally large area of exposed crystalline basement. This flux may have had several important consequences, including stimulation of primary productivity during the initial Phanerozoic radiation of the Metazoa.