RAPID SR ISOTOPE EXCURSIONS IN THE MARINE RECORD AS A TOOL FOR UNDERSTANDING GLOBAL SEAWATER EVOLUTION (Invited Presentation)

With the recognition that the Sr isotopic composition of the ocean has varied through Phanerozoic time, Sr-isotope stratigraphy has become a crucial component in many paleo-oceanographic, -climatologic, -tectonic, and stratigraphic investigations. The secular Sr-isotope record principally reflects the balance between inputs via riverine transport of radiogenic Sr derived from continental weathering, and transfer of less-radiogenic Sr to seawater by hydrothermal alteration of mid-ocean ridge basalts. While this tectonically driven two-component mixing model explains long-term fluctuations in the Sr-isotope record, it is inconsistent with occasionally observed abrupt 87Sr/86Sr increases. Historically, the lack of reasonable mechanisms to explain such rapid changes in ocean chemistry has resulted in the practice of discounting such data as a consequence of post-depositional alteration, missing stratigraphic section, erroneous stratigraphic correlation or bad analyses. The established protocol has been that the lowest ⁸⁷Sr/⁸⁶Sr ratios for any particular stratigraphic interval correspond most closely to the seawater ⁸⁷Sr/⁸⁶Sr value. This methodology has encouraged the practice of smoothing the global Sr trend through intervals of many m.y.. However, documentation of correlative Paleozoic Sr spikes in sedimentary sections on different continents with variable sedimentation rates supports an alternative argument—that the spikes correctly record rapid changes in the Sr-isotopic composition related to a short-term input from a previously unidentified Sr reservoir with elevated ⁸⁷Sr/⁸⁶Sr.

Precise temporal correlations, combined with mass balance calculations and oceanographic modeling, suggest that the flux of radiogenic Sr-rich brine from massive hydrothermal exhalations, related to sedex Zn-Pb deposits, were sufficient to cause these prominent spikes. Moreover, the apices of these enigmatic 87Sr/86Sr spikes correlate with global δ¹³C and δ¹⁸O spikes, periods of global anoxia, and significant mass extinctions. Evidence that the flux of key biolimiting nutrients and metals contained in sedex brines surpassed the total modern riverine flux to the ocean suggests that these brine exhalations may have caused ocean eutrophication and thus may have been an underlying trigger for global chemical and biological events.