The record of the Middle Eocene Climate Optimum in the carbonate platforms of the South Pyrenean Basin (Santo Domingo, External Sierras)

The Middle Eocene Climate Optimum (MECO) was a 0.5 Myr warming interval that punctuated the long-term cooling trend of the middle to late Eocene. It has been identified worldwide by a gradual negative shift of the dO18 followed by a sharp return to the cooling trend. The peak warming at about 40 Ma coincides in some records with a sharp negative excursion of the d13C, which suggested a relation of the warming event with a transient increase of pCO₂. Results from various records also point to increased seasonality and chemical weathering of the source area in coincidence with the MECO.

The interval of the MECO is stratigraphically well constrained in the south-central Pyrenees, from the Graus-Trempto the Aínsa and Jaca basins thanks to previous biostratigraphic and magnetostratigraphic investigations. During this period, the south-Pyrenean foreland formed a narrow and shallow elongated basin connected to the west to the Atlantic Ocean. Here we present a record of the MECO in the carbonate platform succession (the Santo Domingo Member of the Arguis Fm.), that accumulated on the Iberian foreland margin, currently outcropping along the External Sierras (the frontal thrust sheet of the southern Pyrenees). Polished samples were micro-drilled to analyse the d18O and d13C isotopes ratios separately from the mud fraction and shells of different species of larger benthic foraminifera. The results show trends of the d18O from the mud fraction that parallel the global ocean isotope signature but with values that are offset towards more negative values (-4‰). There, the influence of the continental waters in the isotopic signal was possibly amplified by the restricted marine paleogeographic context.  Results from fossil shells gave values significantly different from the mud fraction: d18O from Nummulites shells were consistently offset towards more positive values compared to mud, which could indicate a different fractionation pathway or, alternatively, a preferred diagenetic alteration of fossil fragments. A sharp negative excursion of the d13C was identified in coincidence with the negative d18O peak, marking the location of the MECO. Noticeably, a short-lived entry of siliciclastics that replaced carbonate deposition is recorded at the time of the MECO peak, a feature comparable to what is documented in other sections of the eastern Jaca Basin, where a rapid pulse of deltaic sediments (Sabiñánigo sandstone) are embedded within a thick marine marl succession. Our observations agree with a scenario of perturbation of the hydrological cycle and transient increase of sediment discharge from the continent in coincidence with the MECO.