Tectonic and climate controls on Neogene environmental change in the Zhada Basin, southwestern Tibetan Plateau

Abstract

Prior to stable isotope analysis, sediment samples were crushed and treated with 30% 2 H 2 O 2 to remove organic material. Stable isotopes of oxygen and carbon in bulk carbonate samples (expressed as δ 18 O c and δ 13 C c in units ‰, respectively, and referenced to Vienna Pee 4 Dee Belemnite (VPDB) were measured using a Finnigan MAT 252 gas-ratio mass spectrometer 5 at the University of Texas at Austin. Sample ages were determined by linear interpolation 6 between paleomagnetic ties of the preferred magnetostratigraphic correlation of Saylor et al. (2009). For grain size analysis, samples were disaggregated by soaking in an aqueous solution of hexametaphosphate for at least 3 days. They were subsequently placed in an ultrasonic bath for four hours to deflocculate clays. Samples were removed from the ultrasonic bath and immediately analyzed in a Cilas 1190 laser-diffraction particle size analyzer at the University of Houston which analyzes grain sizes between 4 and 2,500 μm. All samples were analyzed in triplicate to ensure that deflocculation was successful. DISCUSSION Relationship between onset of lacustrine conditions and late Miocene ISM intensification The emergence of paleo-Lake Zhada at ~ 6.0 Ma postdates the evidence for the onset of regional intensification of the monsoon system (

Orbital Forcing of Late Miocene–Early Pleistocene Environmental Change in the Zhada Basin, SW Tibetan Plateau

“…The increase in mean δ 18 O carb and δ 13 C carb values, δ 18 O carb and δ 13 C carb correlation coefficients (Interval B: r 2 = 0.57; Figure 11), and the decrease in mean grain size values at~225 m (6.0 Ma) reflect flooding of the basin and an increase in residence time (Bohacs et al, 2000;Li & Ku, 1997;Talbot, 1990), consistent with the development of a hydraulically closed or balance-filled paleo-Lake Zhada at that time (Saylor et al, 2010b;Saylor et al, 2016). Expansion and contraction of paleo-Lake Zhada are recorded by high-frequency variations in δ 18 O carb values and rapid progradation of lake margin lithofacies resulting in juxtaposition of lithofacies representing littoral depositional environments over profundal depositional environments (Saylor et al, 2010b).…”

“…Changes in δ 18 O carb , δ 13 C carb , and the mean grain size record between intervals A, B, and C reflect major basin-wide changes in basin hydrology (Saylor et al, 2016), which are also evident in changes in depositional environments and sequence stratigraphic stacking patterns (Saylor et al, 2010b;Saylor et al, 2013). Low δ 18 O carb values from the fluvial facies in interval A (9.0-6.0 Ma) are consistent with unevaporated fluvial water on the southern Tibetan Plateau (Li & Garzione, 2017;Quade et al, 2011;Saylor et al, 2009).…”

“…Cane and Molnar (2001) suggested that closure of the Indonesian seaway may have changed the source of flow through the seaway from warm south Pacific water to cold north Pacific water, cooling the Indian Ocean. However, the shift in mean δ 18 O carb values at the transition between interval B and C (at 3.4-3.5 Ma) (Figure 13a) predates significant cooling in the Indian Ocean (at~3.3 Ma) (Figure 13c), suggesting that the closure of the Indonesian seaway may have had less of an influence on weakening of the ISM than previously suggested (Karas et al, 2009;Saylor et al, 2016). This conclusion is tempered by the fact that only a minor revision of either chronology would be needed to synchronize these events.…”

“…Following the work of Saylor et al (2016), the Zhada Basin stratigraphy is separated into three intervals (A, B, and C) based on differences in mean δ 18 O carb values ( Figure 3). The upper portion of interval B (4.2-3.5 Ma; 370-510 m) was isolated for further analysis based on differences in sampling frequency (Figure 4).…”

“…The decrease in δ 18 O carb and δ 13 C carb covariance (r 2 = 0.42; Figure 11), mean δ 18 O carb and δ 13 C carb values, and increase in mean grain size between interval B and C reflects a decrease in lake size such that the sampling location was occupied by lake-margin depositional environments (Saylor et al, 2016). Saylor et al (2016) attribute this transition to a drop in base level, which they attributed to a long-term decrease in ISM strength and an increase in arid conditions (Su et al, 2016). Interval C (3.5-2.2 Ma) is characterized by progradation of deltaic depositional environments with intermittent lacustrine carbonate deposition (Saylor et al, 2010b).…”