ALBERT

Description: The stable isotopic composition of buried soil carbonate and organic matter from northern Pakistan and Nepal can be used to reconstruct aspects of the paleoecology of riverine floodplain ecosystems over the past 17 Myr. Probable dry woodland dominated the floodplain biomass of large rivers ancestral to the modern Indus and Ganges up to 7.3 Myr. Between 7.3 and about 6 Myr, tropical grasses gradually displaced woodland and have dominated floodplain biomasses to the present. The paleovegetational transition beginning about 7.3 Myr likely signals the onset of the strongly seasonal precipitation pattern that typifies the monsoonal climate of the region today. One possible analog to the dry woodland soils of the Miocene are found under the Sal woodlands of the northern Indian subcontinent, while undisturbed modern analogs to the Plio-Pleistocene floodplain grasslands can still be found in the Chitwan area of southern Nepal.

Description: Interactions between midlatitude westerlies and the Pamir–Tian Shan mountains significantly impact hydroclimate patterns in Central Asia today, and they played an important role in driving Asian aridification during the Cenozoic. We show that distinct westeast hydroclimate differences were established over Central Asia during the late Oligocene (ca. 25 Ma), as recorded by stable oxygen isotopic values of soil carbonates. Our climate simulations show that these differences are present when relief of the Pamir–Tian Shan is higher than 75% of modern elevation (∼3000 m). Integrated with geological evidence, we suggest that a significant portion of the Pamir–Tian Shan orogen had reached elevations of ∼3 km and acted as a moisture barrier for the westerlies since ca. 25 Ma.

Description: Shallow subduction of the Farallon plate beneath the western United States has been commonly accepted as the tectonic cause for the Laramide deformation during Late Cretaceous through Eocene time. However, it remains unclear how shallow subduction would produce the individual Laramide structures. Critical information about the timing of individual Laramide uplifts, their paleoelevations at the time of uplift, and the temporal relationships among Laramide uplifts have yet to be documented at regional scale to address the question and evaluate competing tectonic models. The Wind River Basin in central Wyoming is filled with sedimentary strata that record changes of paleogeography and paleoelevation during Laramide deformation. We conducted a multidisciplinary study of the sedimentology, detrital zircon geochronology, and stable isotopic geochemistry of the lower Eocene Indian Meadows and Wind River formations in the northwestern corner of the Wind River Basin in order to improve understanding of the timing and process of basin evolution, source terrane unroofing, and changes in paleoelevation and paleoclimate. Depositional environments changed from alluvial fans during deposition of the Indian Meadows Formation to low-sinuosity braided river systems during deposition of the Wind River Formation. Paleocurrent directions changed from southwestward to mainly eastward through time. Conglomerate and sandstone compositions suggest that the Washakie and/or western Owl Creek ranges to the north of the basin experienced rapid unroofing ca. 55.5-54.5 Ma, producing a trend of predominantly Mesozoic clasts giving way to Precambrian basement clasts upsection. Rapid source terrane unroofing is also suggested by the upsection changes in detrital zircon U-Pb ages. Detrital zircons in the upper Wind River Formation show age distributions similar to those of modern sands derived from the Wind River Range, with up to [~]20% of zircons derived from the Archean basement rocks in the Wind River Range, indicating that the range was largely exhumed by ca. 53-51 Ma. The rise of these ranges by 51 Ma formed a confined valley in the northwestern part of the basin, and promoted development of a meandering fluvial system in the center of the basin. The modern paleodrainage configuration was essentially established by early Eocene time. Carbon isotope data from paleosols and modern soil carbonate show that the soil CO2 respiration rate during the early Eocene was higher than at present, from which a more humid Eocene paleoclimate is inferred. Atmosphere pCO2 estimated from paleosol carbon isotope values decreased from 2050 {+/-} 450 ppmV to 900 {+/-} 450 ppmV in the early Eocene, consistent with results from previous studies. Oxygen isotope data from paleosol and fluvial cement carbonates show that the paleoelevation of the Wind River Basin was comparable to that of the modern Great Plains ([~]500 m above sea level), and that local relief between the Washakie and Wind River ranges and the basin floor was 2.3 {+/-} 0.8 km. Up to 1 km of post-Laramide regional net uplift is required to form the present landscape in central Wyoming.

Description: Stable isotopes provide a valuable perspective on the timing of elevation change of the Tibetan Plateau. We begin our paper by looking in depth at isotopic patterns in modern Tibet. We show that the {delta}18O value of surface waters decreases systematically up the Himalayan front in central Nepal by about -2.8{per thousand}/km, in agreement with the patterns documented and modeled by previous research. On the Tibetan plateau itself there is no apparent correlation between elevation and the {delta}18O value of flowing surface waters. Both surface waters and soil carbonates display a northward increase in {delta}18O values, of about 1.5{per thousand}/{degrees} north of the Himalayan crest, even though elevation increases modestly. The isotopic increase with latitude reduces the isotope-elevation gradient for water in the northernmost plateau to -1 to -2{per thousand}/km. Carbonates in both soils and lakes form at higher temperatures than assumed by previous studies on the plateau. Temperature estimates from clumped-isotope ({Delta}47) analyses of modern soil carbonates significantly exceed mean annual air T and modeled maximum summer soil temperatures by 15.8{+/-}2.8{degrees} and 9.7{+/-}2.5 {degrees}C, respectively. Similarly elevated temperatures best account for the {delta}18O values observed in modern soil and lake carbonates. We recalculated paleoelevations from previous studies on the plateau using both higher formation temperatures and latitude-corrected isotopic values. With one notable exception, our revised model produces paleoelevation estimates very close to previous estimates. The exception is the reconstruction from late Eocene age deposits at Xoh Xil, for which we calculate elevations that are higher and much closer to the current elevation than previously reconstructed. Therefore, there is no evidence for northward progression through time of Tibetan elevation change. Instead, the available--but admittedly very scanty--evidence suggests that much of Tibet attained its modern elevation by the mid-Eocene. A truly robust test of the various geodynamic models of uplift await expansion and replication of isotopic records all across Tibet, especially in the center and north and for 〉15 Ma.