Stratigraphic and Paleoenvironmental Records of the Late Lutetian Thermal Maximum

The gradual cooling trend of the middle Eocene was interrumpted by several short-lived global warming events, called hyperthermals. They are identified in deep-marine sediments through paired negative carbon and oxygen isotope excursions (CIE and OIE) in bulk sediment and benthic foraminifera, and they are associated with high pCO2 levels and carbonate dissolution. In particular, the Late Lutetian Thermal Maximum (LLTM) or C19r event is recorded within the upper part of magnetic polarity Chron C19r, at around 41.52 Ma. Unlike other hyperthermals, the LLTM has been linked to the highest insolation on Earth’s surface of the last 45 million years, along with acceleration of the hydrological cycle. Its identification in deep-sea sediments has been challenging due to its short duration. It only spanned 30 kyrs in the South Atlantic Ocean, with its peak carbonate dissolution phase lasting just 5 kyr, while the deep-sea warmed by 2ºC for 10 kyr. The analysis of the environmental response to this insolation-driven planetary warming offers insight into the ecological dynamics during rapid warming events, but only five global records of the LLTM are currently available due to difficulties in identification of such a short-lived event in deep-sea records. Most of them are from the Atlantic Ocean, from deep-sea Ocean Drilling Program (ODP) Sites 1260, 1263, 702, and the shallow Cape Oyambre section (NE Atlantic Basque-Cantabric basin). There is a single record from the Pacific Ocean (deep-sea International Ocean Discovery Program - IODP Site U1508, in the Tasman Sea). Here we compare two deep-sea records of the LLTM, equatorial western Atlantic Site 1260 and Pacific Site U1508. Multidisciplinary studies, including magnetostratigraphy, inorganic geochemistry, and benthic foraminiferal assemblages, show that the LLTM is consistently marked by a negative CIE and OIE, decreased CaCO3 content, and reduced benthic foraminiferal diversity. Results point to environmental stress during the LLTM at both sites. The benthic foraminiferal response, however, varied significantly depending on the bathymetry and the oceanographic setting, although a shift toward oligotrophic conditions is observed at both sites. Carbonate dissolution at the seafloor has only been identified at Site 1260, evidenced by corrosive-resistant and infaunal taxa, and Site U1508 is the only location reporting warming-induced water-column stratification and bottom-water dysoxia. These variable responses support the hypothesis that local palaeoceanography and Eocene circulation patterns dictated the specific impacts of the LLTM, highlighting the need for further studies to document the effects of warming across the entire water column.