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High precision uranium-series geochronology at Soda Dam, New Mexico provides a record of paleohydrology and incision history for the upper reaches of the Jemez River over the last 700 ka. Active travertine-depositing hot springs occur along the intersection of the Soda Dam fault, and the Jemez River; new dates on active and extinct deposits provide improved geochronologic and geologic context with respect to the timing of travertine accumulation. The largest volume-accumulation, Deposit A, yields ages ranging from >700 ka to 259.6 ± 14.5 ka. The oldest ancestral Jemez River gravels are preserved beneath Deposit A, at a maximum elevation of 132 m above the modern river; efforts to date pumice clasts are ongoing, but model ages of ~700 ka provide an estimate of 188 m/Ma over the last 700 ka. This agrees with longer term bedrock incision rates of 200 m/Ma, from the base of the 1.1 Ma Bandelier tuff. Inset into Deposit A, Deposit A1, contains travertine-coated Jemez River gravels with a strath 30 m above the modern river; these yield an age of 200.6 ± 2.1 ka, giving a bedrock incision rate of 150 m/Ma over the last 200 ka. A cross cutting vein, at the same elevation as the 200 ka sample, yields an age of 110.9 ± 1.5 ka, suggesting substantial artesian head during the 5e substage of the Eemian interglacial. Deposit B, at lower elevations, developed on a banded central fissure ridge; combined ages from the fissure and mound accumulation indicate the system was active from 138-78 ka, a 60,000 yr interval spanning the transitional period during termination II. Deposit C, at 23 and 16.5 m above the modern river, respectively, yielded ages of 103.2 ± 0.5 ka and 101.7 ± 0.5 ka, giving a river incision rate of 160 m/Ma over the last 100 ka. Our results suggest semi-steady bedrock river incision since ~1 Ma with episodic travertine deposition along the Soda Dam fault system at 400-700 ka, 260-360 ka, 134-96 ka, and < 5 ka; reflecting the changes in climate, regional volcanic processes and the existence of the Valles Caldera paleolakes. Stable isotope values of the dated travertines range from δ 18 0 = -19 to -6.5 per mil (PDB), reflecting variations in local spring chemistry and temperature. Compared to global travertine data, Soda Dam travertines have relatively positive C isotope values of +1.4 to +11.7, suggesting degassing of CO2 in the hydrothermal system.

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The Rio Grande is well studied as a regionally important surface-water source, but the small, poorly characterized springs that surface within the Rio Grande rift are also a vital water source. Several of these springs have water chemistries that suggest a mixing of larger volume meteoric recharge with small volume, deeply sourced fluids. It has been hypothesized that deep-seated faults within the rift provide conduits for the ascent of deeply derived fluids, while others have proposed that upwelling sedimentary basin brines represent a significant salinity input to the modern river. This study has developed the first hydrochemical data on a comprehensive suite of springs and wells in the Sevilleta National Wildlife Refuge (NWR), and we continue to test and refine existing models for water quality in the rift using hydrochemistry (major and trace elements, Cl/Br, δ 18 O, δD, δ 13 C, δ 34 S, 3 H, δ 234 U, and 87 Sr/ 86 Sr), microbial characterization and geochemical modeling along a series of transects within the rift. This suite of geochemical tracers is being used to analyze the geochemistry of 26 surface samples and 13 wells in and near the Sevilleta NWR. Our goal is to apply hydrochemical and microbiological analyses of these springs and groundwaters to a rift-wide model for subsurface flow paths. Results from major ions, trace elements, stable isotopes of H and O, and 87 Sr/ 86 Sr indicate the interaction of five distinct hydrochemical facies, several of which suggest that deeply derived fluids are mixing in this system. Continued analyses of major ions for temporal variation, and the addition of δ 234 U, δ 13 C, δ 18 O and δD analyses, will allow for a high-resolution hydrochemical image of the sources of these waters and their impact on the water quality of the Rio Grande.