Record of upper Aptian–lower Albian environmental perturbation in northeastern Mexico

•Colomiellid and strontium isotope data from the La Peña Formation (Mexico) reveal an upper Aptian–lower Albian interval.•We identified a long-term negative carbon isotope excursion equivalent to the Oceanic Anoxic Event 1b.•OAE 1b time-equivalent sediments are enriched in both organic matter and detrital minerals.•We suggest an increased runoff as the mechanism responsible for the paleoenvironmental perturbation.

Here we combine colomiellid biostratigraphy and strontium-isotope data of the upper Aptian–lower Albian, pelagic and organic-rich interval of the La Peña Formation in northeastern Mexico (western Gulf of Mexico basin). The studied sediments recorded a long-term negative carbon-isotope excursion during the Aptian–Albian transition recognizable worldwide and related to the Oceanic Anoxic Event (OAE) 1b set interval. Furthermore, punctuated and short-term negative spikes show values and positions similar to those associated with the Jacob, Kilian, Paquier and Leenhardt episodes. The highest total organic carbon (TOC) values and detrital mineral (quartz and phyllosilicate) contents occur during the 13C-depleted trend. An increase in the detrital index and the absence of feldspars suggest warm and humid conditions that may have led to intense biogeochemical weathering and runoff. An increased runoff may have been the cause of the δ13C decrease, resulting in density stratification of the basin that favored the preservation of organic-matter under oxygen-depleted bottom water conditions.