GEOCHRONOLOGY, GEOCHEMISTRY, AND TECTONIC CHARACTERIZATION OF QUATERNARY LARGE-VOLUME TRAVERTINE DEPOSITS IN THE SOUTHWESTERN UNITED STATES AND THEIR IMPLICATIONS FOR CO2 SEQUESTRATION

Travertines are freshwater carbonates that precipitate from carbonic groundwater due to the degassing of CO2. Travertine deposits are often situated along faults that serve as conduits for CO2-charged groundwater and their geochemistry often records mixing of deeply-derived fluids and volatiles with shallow meteoric water. Travertines are surface expressions of dynamic mantle processes related to the tectonic setting. This dissertation includes four chapters that focus on different aspects of travertine formation and their scientific value. They are excellent, although underestimated, diagnostic tools for climatology, hydrology, tectonics, geochemistry, geomicrobiology, and they can inform carbon sequestration models. Quaternary Large-volume travertine deposits in New Mexico and Arizona occur in an extensional tectonic stress regime on the southeastern Colorado Plateau and along the Rio Grande rift. They accumulated above fault systems during episodes of high hydraulic head in confined aquifers, increased regional volcanic activity, and high input of mantle-derived volatiles such as CO2 and He. Stable isotope and trace element geochemistry of travertines is controlled by groundwater geochemistry as well as the degassing of CO2. The geochemical composition allows for distinguishing different travertine facies and evaluating past groundwater flow. The travertine deposits in New Mexico are interpreted to be extinct CO2 fields due to the large volumes that accumulated and in analogy to the travertine deposits in Arizona that are associated with an active CO2-gas field. Travertines are natural analogues for CO2 leakage along fault systems that bypassed regional cap rocks and they provide important insight into the migration of CO2 from a reservoir to the surface. The volume of travertine can be used to infer the integrated CO2 leakage along a fault system over geologic time. This leakage is estimated 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). 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

U-series geochronology of large-volume Quaternary travertine deposits of the southeastern Colorado Plateau: Evaluating episodicity and tectonic and paleohydrologic controls

Large-volume travertine deposits in the southeastern Colorado Plateau of New Mexico and Arizona, USA, occur along the Jemez lineament and Rio Grande rift. These groundwater discharge deposits reflect vent locations for mantle-derived CO2 , which was conveyed by deeply sourced hydrothermal fluid input into springs. U-series dating of stratigraphic sections shows that major aggradation and large-volume (2.5 km3 ) deposition took place across the region episodically at 700–500 ka, 350–200 ka, and 100–40 ka. These pulses of travertine formation coincide with the occurrence of regional basaltic volcanism, which implies an association of travertine deposits with underlying low-velocity mantle that could supply the excess CO2 . The calculation of landscape denudation rates based on basalt paleosurfaces shows that travertine platforms developed on local topographic highs that required artesian head and fault conduits. Episodic travertine accumulation that led to the formation of the observed travertine platforms represents conditions when fault conduits, high hydraulic head, and high CO2 flux within confined aquifer systems were all favorable for facilitating large-volume travertine formation, which was therefore controlled by tectonic activity and paleohydrology. By analogy to the active Springerville–St. Johns CO2 gas fi eld, the large volumes and similar platform geometries of travertine occurrences in this study are interpreted to represent extinct CO2 gas reservoirs that were vents for degassing of mantle volatiles into the near-surface system