Demise of the Barra Honda Carbonate Shoal (Costa Rica) at the Paleocene-Eocene Boundary Linked to Climate Change and Forearc Tectonics

The latest Cretaceous(?)–Paleocene Barra Honda Formation represents one of the largest carbonate shoals (>900 km², 350 m thick) of the convergent margin of Costa Rica. Although the mode of formation of the carbonate shoal is well understood, how environmental and tectonic factors interacted to cause its demise near the Paleocene-Eocene boundary remains poorly constrained. Stable isotopic, biostratigraphic, mineralogical, and geochronological analyses from the Barra Honda Formation and overlying siliceous Buenavista Formation provide new constraints on the timing and causes of the demise of the carbonate shoal. We report one new U–Pb zircon chemical abrasion, isotope dilution, and thermal ionization mass spectrometry date (56.30 ± 0.13 Ma, 2σ) obtained from an ash-rich layer at the boundary between the two formations. The sharp transition from Barra Honda massive limestones to Buenavista marl-chert alternations coincides with a negative shift in carbon isotope (δ¹³C_carb) values of about 3–5 ‰ and a 50% decrease in carbonate contents. The timing of the combined lithological-mineralogical-isotopic change is coeval with the Paleocene-Eocene Thermal Maximum (PETM, 56 Ma). The onset of clay-rich sedimentation is consistent with a PETM-related increase in the terrestrial influx of nutrients and detrital particles, which promoted eutrophication and decreased light availability in the photic zone. Combined with seawater acidification and warming, these environmental parameters were fatal to the carbonate-producing benthic communities of Barra Honda. High subsidence rates of the forearc basin and renewed arc volcanic activity must have closely followed the cessation of shallow carbonate production, preventing further formation of the carbonate shoal.