New Mexico Geological Society Annual Spring Meeting — Abstracts

Large-volume Quaternary travertine deposits in New Mexico and Arizona occur along the Rio Grande rift and on the southwestern Colorado Plateau. The travertines formed due to the degassing of CO₂ from carbonic groundwater supersaturated in calcium carbonate that migrated up fault systems towards the surface. U-series dating shows that large volumes (2.5 km³) of travertine accumulated episodically at times of both increased magmatic activity (high CO₂ flux) and recharge (high hydraulic head) in confined aquifer systems. Depositional environments of travertine, e.g. spring mound and marsh, produce distinctive travertine facies such as step-pool, paludal (marsh), and vein facies. Stable oxygen and carbon isotope values of the different travertine deposits overlap substantially, with δ¹⁸O and δ¹³C ranging from -14‰ to -3.8‰ and  -4.9‰ to 9.8‰, respectively. The range in oxygen isotopes represents mixing of different types of groundwater and varying water temperature, whereas carbon isotopes are mainly influenced by the degassing of CO₂.  Preliminary results of trace element analyses show similar trends throughout the region, with high (> 1000 mg/kg) concentrations of Fe, Mg, Mn, Na, and Sr. Different travertine facies have different stable isotopic and geochemical compositions. For example, most of the δ¹³C values of the paludal facies vary between -1‰ and -5‰ as opposed to the step-pool facies where most δ¹³C values range from 2‰ to 7‰, and the vein facies shows higher concentrations of certain trace elements (e.g., Fe, Ni, and Sr).Travertines are natural analogues for CO₂ leakage along fault systems that bypassed regional cap rocks, such as shales of the Triassic Chinle Formation. The volume of the travertine can be used to infer the integrated CO₂ leakage along a fault system over geologic time. This leakage is estimated (as a minimum) as: (1) CO₂ that becomes fixed in CaCO₃/travertine (tons of carbon converted into tons of carbonate), (2) the amount of CO₂ that degassed into the atmosphere (twice the amount of (1), based on reaction stoichiometry), (3) dissolved CO₂ that is carried away with the water discharging from a spring (based on modern spring discharge and dissolved carbon content), and (4) CO₂ that escapes through the soil (based on modern soil flux measurements). The total CO₂ leakage (1and 2) calculated for the study areas in New Mexico and Arizona is estimated as ~6 Gt (gigatons), whereas the integrated CO₂ leakage (1-4) is estimated as ~128 Gt. Better understanding of integrated CO₂ leakage and fault-related seal bypass is needed to design CO₂ sequestration sites to effectively store anthropogenic CO₂ in the subsurface.