Exploring the Effects of Residence Time on the Utility of Stable Isotopes and S/C Ratios as Proxies for Ocean Connectivity

Various geochemicalproxies have been developed to determineifancient sedimentary strata were deposited in marine or nonmarine environments.A critical parameter for proxy reliability is the residence time ofaqueous species in seawater, which is rarely considered for proxiesrelying on stable isotopes and elemental abundance ratios. Differencesin residence time may affect our ability to track geologically short-livedalternations between marine and nonmarine conditions. To test thiseffect for sulfur and nitrogen isotopes and sulfur/carbon ratios,we investigated a stratigraphic section in the Miocene OberpullendorfBasin in Austria. Here, previous work revealed typical seawater-likerare earth element and yttrium (REY) systematics transitioning tononmarine-like systematics. This shift was interpreted as a brieftransition from an open marine depositional setting to a restrictedembayment with a reduced level of exchange with the open ocean andpossibly freshwater influence. Our isotopic results show no discernibleresponse in carbonate-associated sulfate sulfur isotopes and carbon/sulfurabundance ratios during the interval of marine restriction inferredfrom the REY data, but nitrogen isotopes show a decrease by severalpermil. This observation is consistent with the much longer residencetime of sulfate in seawater compared with REY and nitrate. Hence,this case study illustrates that the residence time is a key factorfor the utility of seawater proxies. In some cases, it may make geochemicalparameters more sensitive to marine water influx than paleontologicalobservations, as in the Oberpullendorf Basin. Particular care is warrantedin deep time, when marine residence times likely differ markedly fromthe modern.

Automated summary

This study investigates the reliability of proxies for marine and nonmarine environments by analyzing sulfur and nitrogen isotopes and sulfur/carbon ratios in the Miocene Oberpullendorf Basin. The results show that the residence time of isotopes in seawater affects the accuracy of proxies, with nitrogen isotopes being more sensitive than sulfur isotopes. This highlights the importance of considering residence time when using geochemical proxies.