To determine whether stable isotopes can be used for identifying the geographic origins of migratory bird populations, we examined the isotopic composition of hydrogen (deuterium, δD), carbon (δ 13 C), and strontium (δ 87 Sr) in tissues of a migratory passerine, the black-throated blue warbler (Dendroica caerulescens), throughout its breeding range in eastern North America. δD and δ 13 C values in feathers, which are grown in the breeding area, varied systematically along a latitudinal gradient, being highest in samples from the southern end of the species' breeding range in Georgia and lowest in southern Canada. In addition, δD decreased from east to west across the northern part of the breeding range, from New Brunswick to Michigan. δ 87 Sr ratios were highest in the Appalachian Mountains, and decreased towards the west. These patterns are consistent with geographical variation in the isotopic composition of the natural environment, i.e., with that of precipitation, plants, and soils for δD, δ 13 C, and δ 87 Sr, respectively. Preliminary analyses of the δD and δ 13 C composition of feathers collected from warblers in their Caribbean winter grounds indicate that these individuals were mostly from northern breeding populations. Furthermore, variances in isotope ratios in samples from local areas in winter tended to be larger than those in summer, suggesting that individuals from different breeding localities may mix in winter habitats. These isotope markers, therefore, have the potential for locating the breeding origins of migratory species on their winter areas, for quantifying the degree of mixing of breeding populations on migratory and wintering sites, and for documenting other aspects of the population structure migratory animals -information needed for studies of year-round ecology of these species as well as for their conservation. Combining information from several stable isotopes will help to increase the resolution for determining the geographic origins of individuals in such highly vagile populations.

Is erosion important to the structural and petrological evolution of mountain belts? The nature of active metamorphic massifs colocated with deep gorges in the syntaxes at each end of the Himalayan range, together with the magnitude of erosional fluxes that occur in these regions, leads us to concur with suggestions that erosion plays an integral role in collisional dynamics. At multiple scales, erosion exerts an influence on a par with such fundamental phenomena as crustal thickening and extensional collapse. Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.

mere presence of the activation domain because of its small size (about 6% of the total mass) relative to the CRSP complex. Further, we have mapped the VP16 and SREBP-1a binding sites to comparatively small and distinct regions on the CRSP complex. This provides direct evidence that only a limited number (one or perhaps two) of CRSP subunits are targeted by a particular activator. Because VP16-CRSP and SREBP-CRSP are conformationally distinct in regions distal to the activator binding sites, we suggest that activator binding may induce longrange conformational changes. Thus, different protein surfaces in CRSP are likely exposed as a consequence of activator binding.The conformational flexibility of the CRSP coactivator may have important implications for its mechanism of action. CRSP and its related coactivator complexes appear to be generally required for transcription and are targeted by a diverse array of regulatory proteins (22). Interestingly, different transcription activators can target different subunits of the CRSP complex (3, 9,20,21). Thus, despite binding the same coactivator complex, regulatory proteins may impart promoter-specific functions that may be dependent on CRSP conformation. For example, specific activator-induced CRSP conformations may regulate binding and recruitment of additional activators or cofactors to the preinitiation complex (Fig. 5). Furthermore, these conformational changes may trigger other (as yet undiscovered) enzymatic activities within the CRSP coactivator. Indeed, adopting a number of activator-dependent conformations may enable CRSP to perform more specialized roles in transcriptional activation. Elucidation of these roles will be an important subject of future work. We used the natural abundance of stable isotopes (carbon and hydrogen) in the feathers of a neotropical migrant songbird to determine where birds from particular breeding areas spend the winter and the extent to which breeding populations mix in winter quarters. We show that most birds wintering on western Caribbean islands come from the northern portion of the species' North American breeding range, whereas those on more easterly islands are primarily from southern breeding areas. Although segregated by breeding latitude, birds within local wintering areas derive from a wide range of breeding longitudes, indicating considerable population mixing with respect to breeding longitude. These results are useful for assessing the effects of wintering habitat loss on breeding population abundances and for predicting whether the demographic consequences will be concentrated or diffuse.In recent decades, many species of neotropical migrant birds have shown marked changes in abundance-both increases and de-

A negative feedback between silicate weathering rates and climate is hypothesized to play a central role in moderating atmospheric CO 2 concentrations on geologic timescales. However, uncertainty regarding the processes that regulate the operation of the negative feedback limits our ability to interpret past variations in the ocean-atmosphere carbon cycle. In particular, the mechanisms that determine the flux of weathered material for a given climatic state are still poorly understood. Here, we quantify the processes that determine catchment-scale solute fluxes for two lithologic end-members-basalt and granite-by applying a recently developed solute production model that links weathering fluxes to both specific discharge (runoff) and the reactivity of the weathering material. We evaluate the model against long-term monitoring of concentration-discharge relationships from basaltic and granitic catchments to determine the parameters associated with solute production in each catchment. Higher weathering rates in basaltic catchments relative to granitic catchments are driven by differing responses to increases in runoff, with basaltic catchments showing less dilution with increasing runoff. In addition, results from the solute production model suggest that thermodynamic constraints on weathering reactions could explain higher concentrations in basaltic catchments at lower runoff compared to granitic catchments. To understand how the response to changing discharge controls weathering fluxes under different climatic states, we define basalt/granite weatherability as the ratio of the basalt catchment flux to the granite catchment flux. This weatherability is runoff-dependent and increases with increasing runoff. For HCO 3 and SiO 2 (aq) fluxes, for modern global runoff, the derived mean basalt/granite weatherability is 2.2 (1.3-3.7, 2σ) and 1.7 (1.6-2.1, 2σ), respectively. Although we cannot determine the array of individual processes resulting in differences among catchments, the relative differences in certain model

Abstract. Within the syntaxial bends of the India-Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.