Upper Aptian-lower albian seawater composition and OAEs: Geochemistry of Agua Salada and Lampazos Formations, Sonora, Mexico

Abstract

Upper Aptian-lower Albian seawater composition changes and redox conditions in limestones of the Agua Salada and Lampazos formations in Mexico were investigated by data of major oxides, trace elements, rare-earth elements, and stable isotopes. Most limestones of the Agua Salada Formation (Group 1) show REE + Y patterns consistent with precipitation from seawater whereas limestones from the upper part of the Agua Salada and Lampazos formations (Group 2) have non-seawater-like REE + Y patterns. The relationship among REE and major and trace elements indicates that contamination by detrital input is the main source of REEs, followed by Fe-oxyhydroxides. The variations in Ce anomalies were influenced by paleo-redox conditions. The carbon isotope values, low Mn/Sr ratio and absence of correlation between δ13C vs δ18O values suggest that δ13C retained its original marine signatures during diagenesis. The δ13C curve has several significant negative isotopic excursions in upper Aptian-lower Albian strata. The variations in δ13C values and their negative excursions, fossil evidence, positions of δ13C segments, and correlation of this curve with global reference curves permits documentation of OAE 1 b and the associated sub-events such as Jacob, Kilian, Paquier and Leenhardt sub-events.