Geological Carbon Storage is necessary for reduction of anthropogenic carbon dioxide (CO2) emissions. CO2 is captured at 'point sources' (i.e. heavy industries, where large, concentrated volumes of CO2 are produced) and subsequently purified, compressed and transported. Some CO2 can be used for other processes, but excess CO2 can be injected into geological formations where it can be securely stored as either a gas, supercritical fluid or dissolved in formation fluids. Understanding the interactions of CO2 within the subsurface is important for risk characterisation. The long-term storage of CO2 at reservoir scales involves fluids close to equilibrium, which is better characterised using natural analogues than lab experiments.

At the Green River CO2 seeps in Utah, CO2 saturated brines migrate up two fault systems: the Little Grand and Salt Wash faults. Modern springs, a man-made cold water geyser (Crystal Geyser), and fluid sampling during drilling at Little Grand in 2012 constrain current fluid compositions from 100m to 10km scales. Tufa/travertine (CaCO3) deposits form at modern springs, and historic deposits form along both faults. Fossil travertine veins can be dated by U-Th chronology and preserve chemical signatures of the fluids they form from: trace metals, δ234Ui and 87Sr/86Sr. Laser ablation methods were developed to analyse these samples, allowing for rapid, high spatial resolution analysis. Accurate and precise show external reproducibility of ±42 x10-6 (2σ) for 87Sr/86Sr, 7 x10-7 (2σ) for 230Th/238U and ±1.3 x10-6 (2σ) for 234U/238U. These uncertainties are propagated using an excess variance approach, giving typical age uncertainties of 20±1.8kyr and 120±4kyr, and ±40‰ for δ234Ui. Individual veins grow over <5kyr, and vein systems may be active for upto 15kyr.

87Sr/86Sr & δ234Ui show limited variability within samples, and much larger variability between sample localities. At Salt Wash, these trends can be interpreted using an analytical reactive transport model for flow in the Navajo sandstone. This model is calibrated using spring fluid samples, and applied to interpret vein samples. Trends in δ234Ui are more easily interpreted spatially, due to increased residence times inferred from α-recoil inputs. This is counter to previous interpretations at this field area, where trends were interpreted as purely temporal. However, samples on the Little Grand fault do not fit either model well. Next to Crystal Geyser, one vein has clear annual layering. Trace metal trends show similarity to mixing patterns observed during geyser eruptions. There is a tentative link that the draining of the Navajo responds to seasonal changes in crustal stress, prior to man-made geyser activity.