STRONTIUM ISOTOPE STRATIGRAPHY FROM OYSTER SHELLS, PLIOCENE, SPAIN

Pliocene heterozoan carbonates in the Agua Amarga basin in southeastern Spain lack accurate age constraints. 87Sr/86Sr-derived ages from relatively stable low-magnesium calcite shells (oysters and barnacles) hold promise for constraining the chronostratigraphy of these Pliocene rocks. Previous ages were based on biostratigraphy from foraminiferal biozones (M-Pl 1 to M-Pl 4: 3.6-3.17 Ma) from the adjacent time-equivalent Carboneras Basin. Methods include X-Ray diffraction analysis to confirm mineralogy and petrographic examination in plane polarized light, trace element (Fe, Mn, Sr), and stable isotope geochemistry for detecting diagenetically altered shell areas. Samples selected for strontium dating have low Fe and Mn and high Sr values, oxygen and carbon stable isotope values similar to those carbonates in equilibrium with seawater, and are considered within the normal ranges for Recent shells that have not undergone significant diagenetic alteration. Resulted 87Sr/86Sr ratios are converted to numerical ages using a nonparametric LOcally WEighted Scatterplot Smoother statistical regression function lookup curve. Sr-isotope ratios between 0.7090-0.7091 indicate Pliocene ages. Constraining ages allows for correlation with other nearby areas (e.g., Carboneras basin) and further aid in understanding the local, regional, and global controls (e.g., sea-level fluctuations, paleotopography, climate, subsidence/uplift, siliciclastic influx, water depth) on deposition of heterozoan systems. For instance, previously constructed sea-level curves from the Agua Amarga and Carboneras basins interpreted glacio-eustatic-related high-amplitude, high-frequency relative sea-level cycles. However, offset of the Agua Amarga’s curve from the global sea-level curve suggests local subsidence and uplift during or since Pliocene deposition.