INVESTIGATING THE ORIGIN OF MARINE SR-ISOTOPE CHANGE IN THE ORDOVICIAN: EVIDENCE FROM PARTITIONING OF MAGNESIUM AND STRONTIUM INTO MARINE CALCITE

Late Darriwilian to early Sandbian aged strata record a precipitous drop in the composition of marine ⁸⁷Sr/⁸⁶Sr from values of approximately 0.7088 to values near 0.7078. Riverine and hydrothermal weathering fluxes provide the primary input of Sr into the marine system, with riverine fluxes providing variably radiogenic Sr derived from old continental crust and hydrothermal fluxes providing strongly non-radiogenic Sr derived from young basaltic crust. Marine ⁸⁷Sr/⁸⁶Sr is thus modeled as the balance of these two fluxes. The late Darriwilian Sr-isotope shift, however, occurred within 3-5 my, which is similar to the modern residence time of strontium in the oceans, and suggests the potential for dramatic reorganization of the marine Sr cycle. We suggest that partitioning of magnesium and strontium into marine calcite can be used to test potential mechanisms for marine Sr-isotope change. Hydrothermal weathering of young basalt from submarine superplume activity or seafloor spreading (Qing et al., 1998) should supply Sr to marine waters, but restrict Mg input. By contrast cool-temperature weathering of young, basaltic arc material (Shields et al., 2003; Young et al., 2009) should supply both Sr and Mg to marine waters.

Carbonate strata from Newfoundland and the Argentine Precordillera span the late Darriwilian to early Sandbian and consist of petrographically well-preserved echinoderm-bryozoan packstones and wackestones. Prior to the initiation of Sr-isotope change, Mg and Sr concentrations average 2000 ppm and 250 ppm, respectively. As ⁸⁷Sr/⁸⁶Sr begins to drop, Mg and Sr concentrations rise dramatically to 8000 ppm and 2000 ppm, respectively. Although partitioning of Mg into calcite increases with temperature, stable paleotemperature estimates (Trotter et al., 2008) suggest that elevated Mg concentrations reflect input from low-temperature basaltic weathering. By contrast, partitioning of Sr into calcite is most strongly controlled by Mg content. Strontium concentrations reported here indicate an excess of Sr over that which can be explained by the increase in Mg, suggesting an additional input. We suggest that oceanic ventilation following a prolonged period of stratification (Thompson et al., 2010) may have provided an additional source of non-radiogenic strontium to the shallow oceans.