Tectonic and orbital forcing of the South Asian monsoon in central Tibet during the late Oligocene

Abstract

The modern pattern of the Asian monsoon is thought to have formed around the Oligocene/Miocene transition and is generally attributed to Himalaya–Tibetan Plateau (H–TP) uplift. However, the timing of the ancient Asian monsoon over the TP and its response to astronomical forcing and TP uplift remains poorly known because of the paucity of well-dated high-resolution geological records from the TP interior. Here, we present a precession-scale cyclostratigraphic sedimentary section of 27.32 to 23.24 million years ago (Ma) during the late Oligocene epoch from the Nima Basin to show that the South Asian monsoon (SAM) had already advanced to the central TP (32°N) at least by 27.3 Ma, which is indicated by cyclic arid–humid fluctuations based on environmental magnetism proxies. A shift of lithology and astronomically orbital periods and amplified amplitude of proxy measurements as well as a hydroclimate transition around 25.8 Ma suggest that the SAM intensified at ~25.8 Ma and that the TP reached a paleoelevation threshold for enhancing the coupling between the uplifted plateau and the SAM. Orbital short eccentricity-paced precipitation variability is argued to be mainly driven by orbital eccentricity-modulated low-latitude summer insolation rather than glacial-interglacial Antarctic ice sheet fluctuations. The monsoon data from the TP interior provide key evidence to link the greatly enhanced tropical SAM at 25.8 Ma with TP uplift rather than global climate change and suggest that SAM’s northward expansion to the boreal subtropics was dominated by a combination of tectonic and astronomical forcing at multiple timescales in the late Oligocene epoch.

Eccentricity Forcing of the Hydrological Cycle in East Asia During the Early Eocene Climatic Optimum (EECO)

“…In lake sediments, the color of sediments is strongly associated with the oxidation state of iron. These changes in turn can be related to variations in sedimentary environment tied to hydroclimate, notably paleolake level (e.g., Ao et al., 2020; Jin et al., 2023; Zhang et al., 2023a). In the Milai section, the brownish red color mainly occurs in mudstones and siltstones layers (Figures 2 and 3), reflecting a low water table and oxidizing conditions at the margin of a paleolake or perhaps in a distal alluvial fan or dry distal mudflat setting (e.g., Ao et al., 2020; DeCelles et al., 2007; Jin et al., 2023; Wang et al., 2021; Zhang et al., 2023a).…”

“…These changes in turn can be related to variations in sedimentary environment tied to hydroclimate, notably paleolake level (e.g., Ao et al., 2020; Jin et al., 2023; Zhang et al., 2023a). In the Milai section, the brownish red color mainly occurs in mudstones and siltstones layers (Figures 2 and 3), reflecting a low water table and oxidizing conditions at the margin of a paleolake or perhaps in a distal alluvial fan or dry distal mudflat setting (e.g., Ao et al., 2020; DeCelles et al., 2007; Jin et al., 2023; Wang et al., 2021; Zhang et al., 2023a). Such conditions likely promote relatively intense pedogenic processes and inhibit the dissolution of magnetic mineral, resulting in the accumulation of magnetic minerals (especially hematite), yielding the reddish‐brown color (Fe 3+ ‐rich) sediments (e.g., Ao et al., 2020; Jin et al., 2023).…”

“…In the Milai section, the brownish red color mainly occurs in mudstones and siltstones layers (Figures 2 and 3), reflecting a low water table and oxidizing conditions at the margin of a paleolake or perhaps in a distal alluvial fan or dry distal mudflat setting (e.g., Ao et al., 2020; DeCelles et al., 2007; Jin et al., 2023; Wang et al., 2021; Zhang et al., 2023a). Such conditions likely promote relatively intense pedogenic processes and inhibit the dissolution of magnetic mineral, resulting in the accumulation of magnetic minerals (especially hematite), yielding the reddish‐brown color (Fe 3+ ‐rich) sediments (e.g., Ao et al., 2020; Jin et al., 2023). The grayish green color of the mudstone layers (Figures 2 and 3) represents relatively wetter conditions, reflecting a higher water table and shallow lake and/or subaqueous alluvial environments (e.g., Ao et al., 2020; Zhang et al., 2023a).…”

“…The grayish green color of the mudstone layers (Figures 2 and 3) represents relatively wetter conditions, reflecting a higher water table and shallow lake and/or subaqueous alluvial environments (e.g., Ao et al., 2020; Zhang et al., 2023a). High rates of organic matter deposition and burial during wetter intervals would facilitate oxygen consumption and reducing conditions (e.g., Jin et al., 2023). Such environments are conducive to accelerated dissolution of magnetic minerals and inhibited pedogenesis, resulting in lower magnetic mineral abundance (especially hematite), thus yielding grayish green (Fe 2+ ‐rich) sediments.…”

“…By contrast, grayish green sediments are interpreted to have been deposited in subaqueous alluvial and/or shallow lake conditions, while brownish yellow sandstone strata were likely to have been deposited in alluvial or very shallow ephemeral lacustrine environments (e.g., Ao et al., 2020; Wang et al., 2021). As such, the variations in sediment color through the studied section likely reflect environmental conditions related to basin water levels (e.g., Jin et al., 2023; Zhang et al., 2023a). Overall, we consider that the sedimentary setting of the Milai section was a distal alluvial‐lacustrine environment (Li et al., 2020; Studnicki‐Gizbert et al., 2008; Xiao et al., 2021).…”