New biostratigraphic, magnetostratigraphic and isotopic insights into the Middle Eocene Climatic Optimum in low latitudes

The Middle Eocene Climatic Optimum (MECO) was a warming event that interrupted the long-term Eocene cooling trend. While this event is well documented at high southern and mid-latitudes, it is poorly known from low latitudes and its timing and duration are not well constrained because of problems of hiati, microfossil preservation and weak magnetic polarity in key sedimentary sections. Here, we report the results of a study designed to improve the bio-, magneto- and chemostratigraphy of the MECO interval using high-resolution records from two low-latitude sections in the Atlantic Ocean, Ocean Drilling Program (ODP) Sites 1051 and 1260. We present the first detailed benthic foraminiferal stable isotope records of the MECO from the low latitudes as well as the biostratigraphic counts of Orbulinoides beckmanni and new magnetostratigraphic results. Our data demonstrate a ~750kyr-long duration for the MECO characterized by increasing δ¹³C and decreasing δ¹⁸O, with minimum δ¹⁸O values lasting ~40kyr at 40.1Ma coincident with a short-lived negative δ¹³C excursion. Thereafter, δ¹⁸O and δ¹³C values recover rapidly. The shift to minimum δ¹⁸O values at 40.1Ma is coincident with a marked increase in the abundance of the planktonic foraminifera O. beckmanni, consistent with its inferred warm-water preference. O. beckmanni is an important Eocene biostratigraphic marker, defining planktonic foraminiferal Zone E12 with its lowest and highest occurrences (LO and HOs). Our new records reveal that the LO of O. beckmanni is distinctly diachronous, appearing ~500kyr earlier in the equatorial Atlantic than in the subtropics (40.5 versus 41.0Ma). We also show that, at both sites, the HO of O. beckmanni at 39.5Ma is younger than the published calibrations, increasing the duration of Zone E12 by at least 400kyr. In accordance with the tropical origins of O. beckmanni, this range expansion to higher latitudes may have occurred in response to sea surface warming during the MECO and subsequently disappeared with cooling of surface waters.