The δ 18 O value of meteoric water varies with elevation, providing a means to reconstruct paleoelevation if the δ 18 O values of paleowater are known. In this study, we determined the δ 18 O values of water (δ 18 O mw ) from small tributaries along the Seti River and Kali Gandaki in the Nepal Himalaya. We found that δ 18 O mw values decrease with increasing altitude for both transects. δ 18 O mw vs. altitude along the Kali Gandaki in west-central Nepal fit a second order polynomial curve, consistent with increasing depletion of 18 O with increasing elevation, as predicted by a Rayleigh-type fractionation process. This modern δ 18 O mw vs. altitude relationship can be used to constrain paleoelevation from the δ 18 O values of Miocene–Pliocene carbonate (δ 18 O c ) deposited in the Thakkhola graben in the southern Tibetan Plateau. Paleoelevations of 3800±480 m to 5900±350 are predicted for the older Tetang Formation and 4500±430 m to 6300±330 m for the younger Thakkhola Formation. These paleoelevation estimates suggest that by ∼11 Ma the southern Tibetan Plateau was at a similar elevation to modern.

• The influences of prior monsoon-season drought (PMSD) and the seasonal timing of episodic rainfall (‘pulses’) on carbon and water exchange in water-limited ecosystems are poorly quantified. • In the present study, we estimated net ecosystem exchange of CO₂ (NEE) and evapotranspiration (ET) before, and for 15 d following, experimental irrigation in a semi-arid grassland during June and August 2003. Rainout shelters near Tucson, Arizona, USA, were positioned on contrasting soils (clay and sand) and planted with native (Heteropogon contortus) or non-native invasive (Eragrostis lehmanniana) C4 bunchgrasses. Plots received increased (‘wet’) or decreased (‘dry’) monsoon-season (July–September) rainfall during 2002 and 2003. • Following a June 2003 39-mm pulse, species treatments had similar NEE and ET dynamics including 15-d integrated NEE (NEE_pulse). Contrary to predictions, PMSD increased net C uptake during June in plots of both species. Greater flux rates after an August 2003 39-mm pulse reflected biotic activity associated with the North American Monsoon. Furthermore, August NEE_pulse and ecosystem pulse-use efficiency (PUE_e = NEE_pulse/ET_pulse) was greatest in Heteropogon plots. • PMSD and rainfall seasonal timing may interact with bunchgrass invasions to alter NEE and ET dynamics with consequences for PUE_e in water-limited ecosystems.

Between A.D. 900 and 1150, more than 200,000 conifer trees were used to build the prehistoric great houses of Chaco Canyon, New Mexico, in what is now a treeless landscape. More than one-fifth of these timbers were spruce (Picea) or fir (Abies) that were hand-carried from isolated mountaintops 75–100 km away. Because strontium from local dust, water, and underlying bedrock is incorporated by trees, specific logging sites can be identified by comparing⁸⁷Sr/⁸⁶Sr ratios in construction beams from different ruins and building periods to ratios in living trees from the surrounding mountains.⁸⁷Sr/⁸⁶Sr ratios show that the beams came from both the Chuska and San Mateo (Mount Taylor) mountains, but not from the San Pedro Mountains, which are equally close. Incorporation of logs from two sources in the same room, great house, and year suggest stockpiling and intercommunity collaboration at Chaco Canyon. The use of trees from both the Chuska and San Mateo mountains, but not from the San Pedro Mountains, as early as A.D. 974 suggests that selection of timber sources was driven more by regional socioeconomic ties than by a simple model of resource depletion with distance and time.