Lower-altitude of the Himalayas before the mid-Pliocene as constrained by hydrological and thermal conditions

Abstract

Contradictory paleoaltitudinal histories of the Himalayas arrived at using isotopic paleoaltimeters and ecological evidences have constrained our further exploration of the geodynamic mechanisms driving the India-Eurasia collision and the resultant interaction between tectonic uplift and climate change. In this study, based on evidence derived from glycerol dialkyl glycerol tetraethers (GDGTs), the hydrogen isotopes of leaf wax n-alkanes (δ2Halk), we estimated the paleoaltitude of the Gyirong Basin (GB), central Himalayas, for the 7.0∼3.2 Ma period. The paleoaltitudinal reconstruction based on δ2Halk values (∼6.3 km above sea level (asl)) was consistent with that of the revised δ18Oc values (∼5.6 km asl), and higher than the present-day altitude (∼4 km asl). The reconstructed paleotemperatures based on brGDGTs (14.2 ± 4.5°C for the lake surface; 7.5 ± 3.3°C for terrestrial organic matter source areas) were consistent with the ecological evidences, indicating a low-altitude paleoenvironment. This proves that a systemic bias exists between the isotopic paleoaltimeters and the ecological evidence, rather than a problem with specific indicators. The reconstructed paleoaltitudinal differences between the δ2Halk and δ18Oc values are similar to those of brGDGT reconstructions, seemingly representing the paleoaltitudinal differences between the source areas of terrestrial organic matter inputs and the lake surface, and suggesting that the isotopic palaeoaltimeters do not specifically record the higher-altitude parts of the basinal topography. Further paleoenvironmental analysis indicated that the warmer and wetter environment present during the Late Miocene to Mid-Pliocene would, on the face of it, be better suited to the application of brGDGT paleothermometers, and this in turn would potentially lead to a systemic overestimation by isotopic paleoaltimeters. We therefore adopted the paleoaltitudinal reconstructions which used brGDGTs and concluded that the lake surface of the GB during the Late Miocene to Mid-Pliocene was 2.5 ± 0.8 km asl, and that the surrounding mountains exceeded 3.6 km asl in height, implying that the central Himalayas underwent a rapid uplift of ∼1.5 km after the Mid-Pliocene.