Mechanisms controlling the long‐ and short‐term variability of the Indian Summer Monsoon (ISM) and high‐elevation environmental change have largely been examined using low‐elevation or marine records with less emphasis on high‐elevation non‐marine records. We address this using a high‐resolution, long‐term record from upper Miocene–lower Pleistocene (~9.0–2.2 Ma) fluvio‐lacustrine strata in the Zhada Basin, southwestern Tibetan Plateau. Long‐term changes include the onset of lacustrine deposition, a decrease in mean grain size, and an increase in δ18Ocarband δ13Ccarbvalues at ~6.0 Ma in response to basin closure following regional extension. This was followed by a return to palustrine/fluvial deposition, an increase in mean grain size, and a decrease in δ18Ocarband δ13Ccarbvalues at ~3.5 Ma in response to tectonically driven long‐term ISM weakening. Spectral analysis reveals that high‐frequency variations in the δ18Ocarbrecord are dominated by 100 and ~20 kyr cycles from ~6.0–2.2 Ma. Wavelet and spectral analysis of the most densely sampled interval (4.23–3.55 Ma), tuned to the record of daily insolation (21 June at 35°N) confirms and highlights 100 and 20 kyr cycles. The tuned Pliocene δ18Ocarbrecord is coherent with the record of Northern Hemisphere insolation at precession periods but not at obliquity or eccentricity periods. Additionally, the tuned δ18Ocarbrecord is anticorrelated to the insolation record, indicating that stronger Northern Hemisphere insolation correlates with a stronger ISM. These results suggest that variations in daily insolation drove late Miocene–early Pleistocene high‐frequency ISM variability and environmental changes in the high‐elevation southwestern Tibetan Plateau.
Paleosols preserved in the Red Clay depositional sequence of the Chinese Loess Plateau record information about vegetation and regional hydrology responses to global temperature variation throughout the late Miocene and Pliocene. Reconstructing spatial and temporal patterns of environmental change across the Loess Plateau from carbon isotopes of pedogenic carbonate (δ13Ccarb) is complicated because multiple factors affect δ13Ccarbvalues and higher resolution records do not exist along the northern margin of the Loess Plateau. To address these needs, we present paired carbon isotope records of pedogenic carbonate and occluded organic matter (δ13Corg) from 697 discrete nodules sampled from 119 different depths at the Jiaxian section, North Central China. Between 7.6 and 2.4 Ma, δ13Ccarbvalues increase by nearly 5‰, while δ13Corgvalues increase by 2.5‰. These increases are explained by a progressive decline in moisture availability through time, and there is no definitive evidence from these δ13C data for C4vegetation at the Jiaxian site until after 3.6 Ma. Comparison of the Jiaxian record to other Loess Plateau sections reveals a consistent spatial gradient with δ13Ccarbvalues becoming higher and more variable to the N‐NW. Additionally, an independent index of monsoonal precipitation from a southern site corresponds to fluctuations in δ13Ccarbvalues at Jiaxian, while southern δ13Ccarbrecords remain more stable. These spatial patterns are explained by a progressive decline in moisture availability across the Loess Plateau through the Late Miocene and Pliocene, with δ13Ccarbvalues being more sensitive to moisture availability under consistently more arid conditions to the NW.
more » « less- NSF-PAR ID:
- 10361351
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 36
- Issue:
- 12
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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