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

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 813C vs 818O values suggest that 813C retained its original marine signatures during diagenesis. The 813C curve has several significant negative isotopic excursions in upper Aptian-lower Albian strata. The variations in 813C values and their negative excursions, fossil evidence, positions of 813C 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.

Automated summary

The study on limestones from the Agua Salada and Lampazos formations in Mexico revealed different REE+Y patterns in different stratigraphic layers, indicating variations in seawater precipitation and non-seawater components. Changes in Ce anomalies were influenced by paleo-redox conditions, while the original marine signatures of 813C values were retained during diagenesis. Significant negative isotopic excursions in the 813C curve documented OAE 1b and associated sub-events such as Jacob, Kilian, Paquier, and Leenhardt sub-events.