Middle Eocene benthic foraminiferal stable carbon and oxygen isotope data@en

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 ~ 750 kyr-long duration for the MECO characterized by increasing δ13C and decreasing δ18O, with minimum δ18O values lasting ~ 40 kyr at 40.1 Ma coincident with a short-lived negative δ13C excursion. Thereafter, δ18O and δ13C values recover rapidly. The shift to minimum δ18O values at 40.1 Ma 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 ~ 500 kyr earlier in the equatorial Atlantic than in the subtropics (40.5 versus 41.0 Ma). We also show that, at both sites, the HO of O. beckmanni at 39.5 Ma is younger than the published calibrations, increasing the duration of Zone E12 by at least 400 kyr. 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.中始新世气候最佳期（Middle Eocene Climatic Optimum, MECO）是一次打断始新世长期降温趋势的增温事件。该事件在南极高纬度及中纬度地区已有充分研究记录，但低纬度海域的相关研究却较为匮乏；同时由于关键沉积剖面存在沉积间断、微体化石保存不佳以及磁极性信号微弱等问题，MECO的发生时间与持续时长尚未得到准确约束。 本研究依托大西洋海域两个低纬度大洋钻探计划（Ocean Drilling Program, ODP）站位——1051站与1260站的高分辨率沉积记录，开展了旨在完善MECO地层段生物地层、磁地层与化学地层研究的工作，现将研究结果报道如下。 我们首次获取了低纬度海域MECO的高精度底栖有孔虫稳定同位素记录，同时提供了Orbulinoides beckmanni的生物地层计数数据与全新的磁地层研究结果。 数据显示，MECO的持续时长约为75万年，其特征为碳同位素比值（δ¹³C）升高、氧同位素比值（δ¹⁸O）降低；在40.1 Ma时，δ¹⁸O值达到最低并持续约4万年，同期伴随一次短暂的碳同位素负漂移。此后，δ¹⁸O与δ¹³C值快速恢复至背景水平。 在40.1 Ma出现δ¹⁸O值最低值的同时，浮游有孔虫O. beckmanni的丰度显著升高，这与其被推断的暖水偏好性完全一致。 O. beckmanni是重要的始新世生物地层标志种，以其首现（lowest occurrence, LO）与末现（highest occurrence, HO）限定了浮游有孔虫E12带。 新的记录显示，O. beckmanni的首现存在明显穿时性：在赤道大西洋海域，其首现时间比亚热带海域早约50万年（分别为40.5 Ma与41.0 Ma）。 此外，我们发现两个站位的O. beckmanni末现时间均为39.5 Ma，晚于已发表的校准结果，这将E12带的持续时长至少延长了40万年。 结合O. beckmanni的热带起源背景，其向高纬度海域的分布扩张可能是对MECO期间海表增温的响应；而随着后续海表水温降低，该物种最终消亡。