ORBITAL FORCING OF LATE MIOCENE-EARLY PLEISTOCENE ENVIRONMENTAL CHANGE IN THE ZHADA BASIN, SOUTHWESTERN TIBETAN PLATEAU

Paleoenvironmental changes in the high-elevation Tibetan Plateau are variably attributed to tectonically-induced circulation changes, as well as global or local climate change. Previous research in the Zhada Basin highlights the dichotomy between these two end members, as the stable isotope record has been attributed to both elevation change and long- or short-term climate change. However, the driving mechanisms underlying the climatic change remains unclear. Here, we present a high-resolution record of ISM precipitation from late Miocene-early Pleistocene fluvio-lacustrine sediments in the Zhada Basin, southwestern Tibetan Plateau. The Zhada Basin lies near the northern extent of strong ISM precipitation where moisture-bearing ISM air masses mix with dry westerly air masses, and thus the basin is sensitive to strengthening or weakening of the ISM. Spectral analysis reveals that variations in the Zhada Basin’s δ¹⁸O_carb record are dominated by 100 kyr cyclicity throughout the late Miocene-early Pleistocene. To further study the orbital signals, we focus on our most densely sampled (1 sample/3 kyr) δ¹⁸Ocarb record for the 4.2-3.6 Ma time interval, where our sampling resolution is high enough to resolve precession, obliquity, and eccentricity cycles. The δ¹⁸O_carb signal closely follows variations in short eccentricity (~100 kyr) and precession (~20 kyr), mimicking insolation forcing. In addition, the δ¹⁸O_carb signal closely follows changes in modeled pCO₂ concentrations. We examine this relationship by comparing the dominant frequencies in the Zhada Basin’s δ¹⁸O_carb record to global records known to be driven by changes in pCO₂. If the Zhada Basin is driven by local changes in daily insolation, we expect to find high coherency between the δ¹⁸O_carb record and the record of daily insolation (35°N). Conversely, if the Zhada Basin is responding to global changes in pCO2 concentrations, we would expect to find high coherency between the two records, as well as a strong correlation with declining sea surface temperatures. Although 100 kyr cycles during the late Quaternary are generally considered anomalous in the marine record, they appear to have been the dominant cycle since at least the late Miocene in this high-elevation, non-marine record.