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 (

Rock strength evaluation is commonly performed on extracted core samples. The retrieval process of core samples is costly and with high risk, especially in unconventional wells. Besides, mechanical testing carried out on core samples could be time-consuming, particularly for low permeability rocks such as shales. In contrast, tons of shale cuttings are generated and discarded at the end of drilling. Small scale testing, such as instrumented indentation test, could be performed on shale cuttings to obtain the mechanical properties of shale rocks. Unlike core samples, however, the bedding orientation, critical to the definition of mechanical properties of shale cuttings as transversely isotropic material, is indistinguishable on cuttings. A methodology has been developed in the following work to deduce the mechanical properties based on indentation testing of randomly oriented artificial shale cuttings. Estimation of shale cuttings' elastic constants was carried out using microindentation and constrained inverse algorithm developed based on contact mechanics solutions. Microindentation testing was performed on multiple oriented artificial shale cuttings to obtain the indentation modulus as the function of the unknown bedding orientation. The contact mechanics solutions for both transversely isotropic and anisotropic material were utilized to correlate the indentation modulus to the stiffness tensor components of transversely isotropic shale. An inverse problem was formulated with imposed constraints to identify the mean values of the quantities of interest that best fit the data. The constraints represent the physical information about the bounds on elastic properties as well as a mathematical constraint on the structure of elasticity tensor ensuring the accuracy and robustness of the solutions to this optimization problem. Lastly, Ultrasonic Pulse Velocity (UPV) test was performed to validate the modeling results and good agreement was found between the results of the experimental and modeling efforts and results from UPV tests performed on the same material. Mechanical properties of shale rocks hold great importance in the design and implementation of drilling and production programs. Retrieval of traditional core samples is known to be expensive and risky as a failure in the recovery process could lead to well abandonment. By using several cuttings with unknown varying bedding orientations, the elastic constants of shales were inferred based on the microindentation testing and the algorithm developed in this work. Successful implementation of this work would allow for a more efficient and economical mechanical characterization of shales.

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 δ18Ocarb and δ13Ccarb values 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 δ18Ocarb and δ13Ccarb values at ~3.5 Ma in response to tectonically driven long‐term ISM weakening. Spectral analysis reveals that high‐frequency variations in the δ18Ocarb record 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 δ18Ocarb record is coherent with the record of Northern Hemisphere insolation at precession periods but not at obliquity or eccentricity periods. Additionally, the tuned δ18Ocarb record 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.

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