This study is the geochemical examination of mineral weathering and its path from hinterland, through sediment deposition and pedogenesis, to its dissolution and eventual uptake into plants or precipitation as carbonate minerals. The three papers examine the rate and character of carbonate and silicate mineral weathering over a wide range of climatic and tectonic regimes, time periods, and lithologies, and focus on very different questions. Examination of the 87Sr/86Sr ratios of architectural ponderosa pine in Chaco Canyon, New Mexico confirms a societally complex style of timber procurement from the 10th to the 12th centuries. In El Malpais National Monument, New Mexico, we measured the 87Sr/86Sr ratios in local bedrock and soils and compared them to the leaf/wood cellulose of four conifers (Pinus ponderosa, Pinus edulis, Juniperus monosperma, Juniperus scopulorum), a deciduous tree (Populus tremuloides), three shrubs (Chrysothamus nauseosus, Fallugia paradoxa, Rhus trilobata), and an annual grass (Bouteloua gracilis) and a lichen (Xanthoparmelia lineola). We found that plant 87Sr/86Sr ratios covaried with variations in plant physiognomy, life history, and rooting depth. In addition, the proportion of atmospheric dust and bedrock mineral contributions to soil water 87Sr/86Sr ratios varied predictably with landscape age and bedrock lithology. On the Himalayan floodplain, soils and paleosol silicate weathering intensities were measured along a climatic transect and through time. Overall, carbonate weathering dominates floodplain weathering. But, periods of more intense silicate weathering between 9 - 2 Ma, identified in soil profile and in the 87Sr/86Sr ratios of pedogenic carbonates, appear to be driven by changes in tectonic, rather than climatic, regime. All three papers are good examples of how 87Sr/86Sr isotopic tracer studies can shed light on pedogenic formation rates and internal processes. The complexity of each system warns against generalizations based on just one locale, one species or lithology, or a few isotopic ratios.

This study deploys compound-specific multi-proxy isotopic study of lipid biomarkers to understand Neogene climatic and ecological variabilities in the Himalayan foreland. The investigation of compound-specific carbon and hydrogen isotopes along with glycerol dialkyl glycerol tetraether (GDGT) is the first of its kind for the Nepalese Siwalik. A total of 49 mudstone (and some paleosol) samples were collected from the paleomagnetically age-constrained Siwalik strata in the Surai Khola and Karnali River sections. δ13C results suggest a domination of C3 trees between 12 and 8.5 Ma, and a stepwise expansion of C4 grasses starting gradually at 8.5 Ma and culminating rapidly around 5.5 Ma. δD results show an overall gradual increase in rainfall since 12 Ma, with a rapid intensification around 5.5 Ma. The negative correlation between rainfall and GDGT-derived paleotemperature prior to 5.5 Ma indicates that the region experienced higher rainfalls during colder periods and vice versa. We propose that this negative correlation could be related to the strong presence of mid-latitude westerlies in the region because of the subdued Himalayas, when summer monsoon winds were weaker, that brought enhanced winter-precipitation particularly during colder periods. After 5.5 Ma, our data show a conspicuous positive correlation between rainfall and annual temperature, indicating the onset of modern-style seasonality in rainfall in the Indian subcontinent, which generates more rainfall during summer than during winter. Notably, this initiation of the Indian monsoon around 5.5 Ma favored the dominance of C4 grasses over C3 trees that is reflected in our δ13C data.

Ce travail porte sur l'étude paléo-environnementale de bassins himalayens: le bassin d'avant pays (Miocène sup.) et le bassin transporté de Kathmandu (Plio-pléistocène). Nous nous sommes appuyés sur l'étude du C et de l'O des mollusques et des eaux de la plaine du Gange et des bassins intra-montagneux modernes.<br /> Pour l'actuel, le Δ18Ow des rivières varie de -18 à -4‰. Le Δ18Ocar montre que les carbonates sont proches de l'équilibre isotopique avec les eaux dans lesquels ils sont formés (20 à 25°C). Le Δ13Ccar est appauvri par rapport au DIC des rivières, impliquant qu'une part de carbone organique est utilisée pour la synthèse des coquilles.<br /> Dans les Siwaliks (16-1 Ma), le Δ18Ocar varie de -16.4 à 0.6‰. Les valeurs anté-6Ma sont très inférieures aux valeurs pliocènes et actuelles de la plaine du Gange. La plaine miocène sup. était donc drainée par des rivières de configuration différente des rivières modernes. Le Δ13C augmente vers 5 Ma jusqu'à environ 0‰, évolution qui semble en rapport avec l'explosion de la flore C4. <br />Le bassin de Kathmandu (3-0 Ma) est caractérisé par une sédimentation fluvio-lacustre. La profondeur du lac montre des fluctuations importantes, contrôlées par la tectonique, les glissements et débris-flow. Pour les périodes pos-50 Ka, la géochimie des isotopes de l'O des mollusques montre des périodes pour lesquelles les valeurs Δ18Ocar sont très élevées (-2.4 à 8‰), compatibles avec les eaux de précipitation d'hiver à Kathmandu (-6.5 à 7.5‰). Ces valeurs sont généralement associées à des diatomées. Deux processus semblent se combiner durant ces épisodes: une diminution des précipitations d'été et une forte évaporation, qui augmentent le 18O.