[1] Paleoaltimetry based on stable isotopes (d 18 O and d 2 H) of paleowater from the central and northern Tibetan Plateau is challenged by the lack of a clear relationship between isotopic composition and elevation north of the Himalaya. In order to determine the environmental factor(s) responsible for temporal changes in isotopic composition revealed in the geologic record, an understanding of the modern controls on isotope evolution in the continental interior is necessary. Here, we present new d18 O and deuterium excess (d excess) data from modern surface water along a roughly south-north transect on the eastern part of the Himalaya and Tibetan Plateau. Results corroborate an inverse relationship between d18 O and elevation in the Himalaya. Northward across the plateau, there is a positive trend in meteoric water d18 O that is linear ($1.5‰ per degree latitude) and robust (R 2 = 0.94). A positive trend northward is also observed in d excess of surface water from large rivers. We show that Rayleigh distillation modified by surface water recycling can account for the observed spatial distribution of both d18 O and d excess across the plateau. HYSPLIT modeling of air parcel back trajectories suggests that air mass mixing varies from east to west across the plateau. However, isotopic trends along the plateau's eastern margin are consistent with roughly parallel transects to the west, suggesting that a local process like moisture recycling exerts control over the isotopic evolution across the entire plateau, regardless of origin of air masses. Assuming the northern Tibetan Plateau was equally far from an oceanic source during late Eocene-Miocene time, paleoelevations of the Hoh Xil Basin are recalculated to account for recycling, increasing elevation estimates by 1100-2700 m.Citation: Bershaw, J., S. M. Penny, and C. N. Garzione (2012), Stable isotopes of modern water across the Himalaya and eastern Tibetan Plateau: Implications for estimates of paleoelevation and paleoclimate,

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28Environmental parameters that influence the isotopic composition of meteoric water (δ 18 O and δD) are 29 well characterized up the windward side of mountains, where orographic precipitation results in a 30 predictable relationship between the isotopic composition of precipitation and elevation. The topographic 31 and climatic evolution of the Andean Plateau and surrounding regions has been studied extensively by 32 exploiting this relationship through the use of paleowater proxies. However, interpretation on the plateau 33 itself is challenged by a poor understanding of processes that fractionate isotopes during vapor transport 34 and rainout, and by the relative contribution of unique moisture sources. Here, we present an extensive 35 dataset of modern surface water samples for the northern Andean Plateau and surrounding regions to 36 elucidate patterns and causes of isotope fractionation in this continental environment. These data show a 37 progressive increase in δ 18

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High-precision triple oxygen isotope analysis of water has given rise to a novel second-order parameter, 17 O-excess (often denoted as D 17 O), which describes the deviation from a reference relationship between d 18 O and d 17 O. This tracer, like deuterium excess (d-excess), is affected by kinetic fractionation (diffusion) during phase changes within the hydrologic cycle. However, unlike d-excess, 17 O-excess is present in paleowater proxy minerals and is not thought to vary significantly with temperature. This makes it a promising tool in paleoclimate research, particularly in relatively arid continental regions where traditional approaches have produced equivocal results. We present new d 18 O, d 17 O, and d 2 H data from stream waters along two east-west transects in the Pacific Northwest to explore the sensitivity of 17 O-excess to topography, climate, and moisture source. We find that discrepancies in d-excess and 17 O-excess between the Olympic Mountains and Coast Range are consistent with distinct moisture source meteorology, inferred from air-mass back trajectory analysis. We suggest that vapor d-excess is affected by relative humidity and temperature at its oceanic source, whereas 17 O-excess vapor is controlled by relative humidity at its oceanic source. Like dexcess, 17 O-excess is significantly affected by evaporation in the rain shadow of the Cascade Mountains, supporting its utility as an aridity indicator in paleoclimate studies where d 2 H data are unavailable. We use a raindrop evaporation model and local meteorology to investigate the effects of subcloud evaporation on dexcess and 17 O-excess along altitudinal transects. We find that subcloud evaporation explains much, but not all of observed increases in d-excess with elevation and a minor amount of 17 O-excess variation in the Olympic Mountains and Coast Range of Oregon. KEY POINTS 1. 17 O-excess correlates spatially with relative humidity across the Pacific Northwest, supporting its use as an aridity indicator in paleoclimate studies. 2. Discrepancies in d-excess and 17 O-excess between the Olympic Mountains and Oregon Coast Range suggest that their moisture source is different. 3. Subcloud evaporation explains most of observed increases in d-excess with elevation, and a minor amount of 17 O-excess variation in the Olympic Mountains and Oregon Coast Range.