Evolution of the Paleocene-Early Eocene larger benthic foraminifera in the Tethyan Himalaya of Tibet, China

Abstract

The Paleocene-Early Eocene larger benthic foraminifera (LBF) in the far eastern Neo-Tethyan Ocean of Tibet still remain poorly known. Here, we present a novel, high-resolution larger foraminiferal biozonation from the shallow-water limestones in Tibet, which will improve our current understanding of the larger foraminiferal evolution in the eastern Neo-Tethyan Ocean. Based on one continuous section at Tingri and three separate sections at Gamba, ten Shallow Benthic Zones (SBZ 1–10) have been designated in Tibet by following the principle of Oppel Zone. In contrast to those in Europe, the Paleocene LBFs in Tibet are characterized by high diversification of Lockhartia, Kathina, Daviesina, Miscellanea, Ranikothalia, and Operculina and show progressively increasing diversity of genera and species during SBZ 2–5. Adult dimorphism and large shell size of some LBFs as well as differentiation of the diversity between genera and species initiated as early as SBZ 3. It suggests that the occurrence of the Larger Foraminifera Turnover (LFT) was probably not synchronous in the entire Neo-Tethyan Ocean, because in Europe, the LFT was generally thought to occur at the beginning of SBZ 5. During the Early Eocene, the LBFs in Tibet decreased markedly on the generic level and increased on the species level, and some new genera (Alveolina, Orbitolites, Nummulites, Assilina, Discocyclina) have gained predominance in Tibet. It is nearly identical to the evolution of the LBFs in Europe and indicates a high-degree homogenization of the LBFs in the entire Neo-Tethyan Ocean. Furthermore, the Paleocene-Eocene (P-E) boundary in shallow-water environments has been clearly identified by us, and it is situated in the upper part of SBZ 5 and associated with no evident biotic turnover of shallow benthic foraminiferal communities. The possible diachroneity of the LFT in the Neo-Tethyan Ocean and the evident lagging of the Paleocene-Eocene Thermal Maximum (PETM) behind the LFT imply that the LFT could only be the result of a natural evolutionary process and has no linkage with the PETM. Notably, a transient but distinct larger foraminiferal extinction and origination (LFEO) event has been found in Tibet, which is characterized by a sudden disappearance of all Paleocene lamellar-perforate LBFs, such as Lockhartia, Kathina, Daviesina, Miscellanea, Ranikothalia, and Operculina, and the initial dominance of the Early Eocene porcellaneous-walled Alveolina. The LFEO marks the boundary between SBZ 5 and 6, and might only occur in the low-latitude areas of the Neo-Tethyan Ocean. Surprisingly, the LFEO coincides with the initial recovery of the Carbon Isotope Excursion (CIE), and their synchronicity implies that some possible mechanisms causing the rapid recovery of the CIE probably had also led to the LFEO in the shallow-water environments.