STABLE SR ISOTOPES AND THE NEOPROTEROZOIC CARBONATE CYCLE

Radiogenic Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) of marine limestones through time reflect the balance between hydrothermal and riverine input fluxes to the ocean. However, ⁸⁷Sr/⁸⁶Sr ratios do not track seawater Sr output fluxes, which occur primarily via marine carbonate burial. Recent work has shown that marine carbonates preferentially incorporate lighter Sr isotopes relative to seawater, with faster precipitation rates yielding larger fractionations. Therefore, unlike ⁸⁷Sr/⁸⁶Sr ratios, the stable Sr isotope composition (δ^88/86Sr) of seawater may be sensitive to carbonate burial. In this study, we present high-precision TIMS ⁸⁷Sr/⁸⁶Sr and δ^88/86Sr results from Tonian-aged carbonate rocks composing the Copper Cap Formation (Mackenzie Mountains, Canada) in order to better understand changes in the balance among chemical weathering, hydrothermal alteration, and carbonate burial/precipitation rates and their relationship to carbon cycle variation prior to the Sturtian glaciation (~717 – 662 Ma).

This locality of the Copper Cap formation hosts ~360 m of mainly carbonate rock, where δ¹³C_carb values increase systematically from -8.2 to 8.0‰. ⁸⁷Sr/⁸⁶Sr and δ^88/86Sr range from 0.706 to 0.714 and 0.30 to 0.51‰, respectively. We focus on the upper portion of the section because very high ⁸⁷Sr/⁸⁶Sr below ~200 m suggests late-stage diagenetic overprinting. Above ~200 m, where ⁸⁷Sr/⁸⁶Sr are relatively low and stable (0.7064 to 0.7068), δ^88/86Sr shifts sympathetically with δ¹³C_carb. The average δ^88/86Sr value (0.37‰) is higher than the average for Phanerozoic carbonates (0.16‰), which suggests that seawater δ^88/86Sr was higher than modern. Large δ^88/86Sr variations expressed in the Copper Cap section may reflect transient changes in global carbonate burial and/or rates of carbonate precipitation. A coupled ⁸⁷Sr/⁸⁶Sr and δ^88/86Sr model will be constructed to interrogate these hypotheses and examine their relationship to carbon cycle and climate instability during this time.