Cold climatic snaps during the Eocene-Oligocene transition in the central Tibetan Plateau: Implications for ice-induced sedimentary structures and isotope geochemistry

Abstract

The Eocene-Oligocene transition was a time of major climate instability and a shift from an ice-free world to a predominant icehouse climate. Identifying the ancient glacial deposits and their associated sedimentary structures are of paramount significance for understanding the Earth's climate in the Cryogenian Period. Ice-induced sedimentary structures (IISS) are considered as types of primary structures preserved on surfaces and in upper parts of unconsolidated sediments by physical cryospheric processes of seasonal ice freezing and thawing, and therefore sensitive not only to climate, but also to paleogeography, hydrology, and type of environment. Despite the common occurrence of modern IISS in a wide variety of environments, from high-latitude regions to low-latitude areas of high altitude, available records from ancient geologic archives are scarce. Herein, we integrated sedimentological and geochemical proxies to study IISS and geochemical signals as reliable indicators of cold climate in an outstanding example of the Eocene-Oligocene interval from the Niubao Formation in the Lunpola Basin, central Tibet. Detailed field sedimentological, petrographic microscopic investigations, and bulk carbonate oxygen (δ18Ocarb) and carbon (δ13Ccarb) isotope geochemistry were conducted in this lacustrine succession. Seven sets of IISS are represented by ice crystal marks, frozen cracks, ice-frozen bubbles, ice-expanding laminae, ice-rafted debris (dropstones), ice water pea-like, and ice water pits. They are classified into three groups based on major deriving mechanisms: ice-related crystal marks, deformation-related ice structures, and gravitational fall IISS. Positive δ18Ocarb excursions at the Eocene-Oligocene transition provide evidence of a shift toward cold or near-freezing climatic snaps in response to global cooling episodes at this time. Integrating the paleogeomorphic scenario of central Tibet with the stratigraphically reported IISS and glendonite clusters, the results support that the Lunpola lake sustained cold or near-freezing conditions at Eocene-Oligocene transition. These findings provide one of the most comprehensive sedimentological and isotopic analyses of ancient IISS and a more direct spatial-temporal constraint for the regional climate system of central Tibet.