Patterns of carbon-isotope excursions recorded in lacustrine sedimentary rocks of Central China during Paleocene–Eocene hyperthermals

Abstract The rapid warming events (hyperthermals) of late Paleocene to early Eocene time—the Paleocene–Eocene Thermal Maximum (PETM), Eocene Thermal Maximum 2 (ETM2/H1), H2, I1, and I2—were recorded in marine and terrestrial facies in the form of negative carbon-isotope excursions (CIEs). Unlike the numerous discoveries of the PETM in terrestrial facies, records of the other hyperthermals (ETM2/H1, H2, I1, and I2) are rare. To better understand these other Paleocene–Eocene hyperthermals and their utility for stratigraphic correlation between marine and terrestrial deposits, we performed carbon- and oxygen-isotope analysis of lacustrine sedimentary rocks from the ZK0303 well in the southwestern Jianghan Basin, Central China. Based on palynological data, the 320–1000 m section of the well represents the stratigraphic period from 65 Ma to 52 Ma. The δ13C records for bulk carbonate (δ13Ccarb) and bulk organic matter (OM; δ13Corg) include three significant negative shifts. Based on a stratigraphic comparison with global marine and continental records from the same period, the three negative CIEs observed were assigned to three hyperthermals: the PETM, the ETM2/H1, and the H2 events. The recorded magnitudes of the CIEs of the δ13Ccarb values of the PETM, ETM2/H1, and H2 (−10.1‰, −6.7‰, and −5.6‰) are greater than those of the δ13Corg values (−4.4‰, −3.6‰, and −2.3‰). In comparison to the vast ocean, which responded more slowly to the increase in humidity and pCO2 concentration and had stronger buffering capacity during the PETM, the smaller Jianghan Basin was more intensely affected by the increase in the pCO2 concentration and especially the increase in humidity. This resulted in a CIE magnitude of the δ13Corg value (−4.40‰) significantly larger than those of marine records. The climate during the PETM was very humid and hot, with flourishing vegetation, enhanced soil respiration, increased OM oxidation, and higher runoff. This resulted in a significant decrease in the δ13C value of dissolved inorganic carbon in the lake. In contrast, the climate was relatively dry during the ETM2/H1 and H2 events, the lake level declined, and groundwater (springs) probably constituted the main water supply to the lake, with runoff playing a lesser role. The discovery of the three hyperthermals in the ZK0303 well reveals that the humid climate of the Paleocene–Eocene in the Jianghan Basin began during the PETM and ended during the ETM2/H1 and H2 events.