A Phanerozoic I/Ca compilation: potential links to ocean oxygenation, carbon cycle and bio-diversification
Abstract:Dissolved iodine in seawater is present as two chemical species: iodide or iodate in anoxic and oxygenated environments, respectively. Because only iodate can be incorporated into the carbonate structure, I/Ca values in marine carbonate and fossils potentially record seawater iodate concentrations. I/Ca has been used as a paleo-proxy for ocean oxygenation across different time scales, ranging from glacial–interglacial cycles to the abrupt warming and/or oceanic anoxic events of the Meso- and Cenozoic to long-term redox evolution during the Precambrian.
Here we present a compilation of new and published I/Ca data for the Phanerozoic Eon showing a major increase of I/Ca at about 200 Ma, close to the Triassic–Jurassic boundary. This major change post-dates the rise of other paleo-oxygenation indicators, specifically increasing Mo-isotope compositions during the Devonian (Dahl et al., 2010) and the modeled increase in seawater sulfate concentrations in Carboniferous-Permian (Algeo et al., 2015).
I/Ca is more sensitive to the level of dissolved O2 because the redox potential of iodate closely resembles that of O2. By contrast, Mo and S proxies are sensitive to more strongly reducing conditions, specifically the global distribution euxinia in the oceans. The increase of I/Ca in our compilation may indicate that the volume of oxygenated seawater expanded globally to near-modern levels around 200 Ma, which is also the time pelagic calcifiers proliferated (Zeebe and Westbroek, 2003). These planktonic organisms might have shifted the O2 consumption pattern and nutrient cycle, leading to the final oxygenation of ocean interiors.
Fundamental changes in global cycling of redox sensitive elements (Mo, S, I) also coincide with diversification of marine invertebrates (Alroy et al., 2008). These observations highlight that stepwise oxygenation of global oceans and the co-evolution of life may have been a protracted process spanning two-thirds of the Phanerozoic.
Algeo, T. J., et al. Biogeosciences 12, 2131-2151, (2015).
Zeebe, R. E. & Westbroek, P. Geochemistry Geophysics Geosystems 4, (2003).
Alroy, J. et al. Science 321, 97-100, (2008).
Dahl, T. W. et al. PNAS 107, 17911-17915, (2010).