Variations in growth rates of speleothem calcite have been hypothesized to reflect changes in a range of paleoenvironmental variables, including atmospheric temperature and precipitation, drip-water composition, and the rate of soil CO 2 delivery to the subsurface. To test these hypotheses, we quantified growth rates of modern speleothem calcite on artificial substrates and monitored concurrent environmental conditions in three caves across the Edwards Plateau in central Texas. Within each of two caves, different drip sites exhibit similar annual cycles in calcite growth rates, even though there are large differences between the mean growth rates at the sites. The growth-rate cycles inversely correlate to seasonal changes in regional air temperature outside the caves, with near-zero growth rates during the warmest summer months, and peak growth rates in fall through spring. Drip sites from caves 130 km apart exhibit similar temporal patterns in calcite growth rate, indicating a controlling mechanism on at least this distance. The seasonal variations in calcite growth rate can be accounted for by a primary control by regional temperature effects on ventilation of cave-air CO 2 concentrations and/or drip-water CO 2 contents. In contrast, site-to-site differences in the magnitude of calcite growth rates within an individual cave appear to be controlled principally by differences in drip rate. A secondary control by drip rate on the growth rate temporal variations is suggested by interannual variations. No calcite growth was observed in the third cave, which has relatively high values of and small seasonal changes in cave-air CO 2. These results indicate that growth-rate variations in ancient speleothems may serve as a paleoenvironmental proxy with seasonal resolution. By applying this approach of monitoring the modern system, speleothem growth rate and geochemical proxies for paleoenvironmental change may be evaluated and calibrated.

A detailed chronology for four stalagmites from three central Texas caves separated by as much as 130 km provides a 71 000-yr record of temporal changes in hydrology and climate. Mass spectrometric 238 U-230 Th and 235 U-231 Pa analyses have yielded 53 ages. The accuracy of the ages and the closedsystem behavior of the speleothems are indicated by interlaboratory comparisons, concordance of 230 Th and 231 Pa ages, and the result that all ages are in correct stratigraphic order. Over the past 71 000 yr, the stalagmites have similar growth histories with alternating periods of relatively rapid and slow growth. The growth rates vary over more than two orders of magnitude, and there were three periods of rapid growth: 71-60 ka, 39-33 ka, and 24-12 ka. These growth-rate shifts correspond in part with global glacial-interglacial climatic shifts. Paleontological evidence indicates that around the Last Glacial Maximum (20 ka), climate in central Texas was cooler and wetter than at present. This wetter interval corresponds with the most recent period of increased growth rates in the speleothems, which is consistent with conditions necessary for speleothem growth. The temporal shift in wetness has been proposed to result

Cave calcite deposits (speleothems) provide long and continuous records of paleoenvironmental conditions in terrestrial settings. Typical environmental proxy measurements include speleothem growth rate and variations in elemental and isotope geochemistry. Commonly the assumption is made that speleothems grow continuously and at a constant rate throughout the year. However, seasonal variation of growth rate may be the rule in many caves. Here we apply observations of modern calcite growth and cave-air CO 2 concentrations and a model of factors controlling cave ventilation to construct a global model predicting where cave calcite growth may be seasonal. Previous models and measurements of calcite precipitation in caves demonstrate the retardation of speleothem growth by high cave-air CO 2 . Elevated CO 2 is commonly dissipated by ventilation driven by density differences between cave and surface air. Seasonal cycles in atmospheric temperature, pressure, and humidity commonly drive these density contrasts. Modeling these changes latitudinally and globally indicates a geographic control on seasonal cave ventilation and thus on a principal controlling factor of speleothem growth. The model predicts that given constant water, calcium, and CO 2 inputs, speleothems from temperate to boreal continental regions commonly accumulate more calcite in the cool season and less or none in the warm season. These models predict that proxies from temperate to boreal speleothems may be seasonally biased due to seasonal ventilation, whereas tropical and maritime records should not.

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Orientations of dikes, veins, faults, and slickenlines reveal the evolution of stress during Eocene to Miocene magmatism in the southern Cordillera. Where most thoroughly studied by us in Trans‐Pecos Texas, magmatism began at about 48 Ma shortly after the cessation of Laramide folding. Dikes and veins that formed from then until about 32 Ma strike dominantly east‐northeast. This indicates that the least principal stress (σ3) was north‐northwest; additional data suggest that the maximum principal stress (σ1) was east‐northeast. The stress field changed to σ1 vertical and σ3 east‐northeast (i.e., east‐northeast extension) at least by 28 Ma and probably by 31 Ma. Dikes and veins that formed between 31 and 17 Ma, when all magmatism ceased in Texas, strike north‐northwest. This change marks the beginning of regional, Basin and Range extension; however, major normal faulting, exclusively of high‐angle type, did not begin until about 24 Ma. A similar stress change, marked by a similar change in dike and vein orientations, occurred throughout the southwestern United States and northern Mexico. The time of change is not well constrained in Texas, but available information allows it to have occurred at the same time thoughout the southern Cordillera. We suggest the earlier stress field is related to east‐northeast convergence between the Farallon and North American plates. The change in stress is approximately coincident with collision of the East Pacific Rise and paleotrench. Extension may be related to the change from a convergent to a transform margin along the western edge of North America. The changes in the stress field are accompanied by changes in the sources and compositions of magmas erupted in Texas. Contemporaneity of the changes in stress and magmatism indicates that they are related. Combined with regional age patterns, paleostress and geochemical data indicate that pre‐31 Ma magmatism in the southern Cordillera occurred in a subduction‐related, continental volcanic arc. Subsequent magmatism occurred in an environment of intraplate extension of the Basin and Range province.