420,000 year assessment of fault leakage rates shows geological carbon storage is secure

Carbon capture and storage (CCS) technology is routinely cited as a cost effective tool for climate change mitigation. CCS can directly reduce industrial CO2 emissions and is essential for the retention of CO2 extracted from the atmosphere. To be effective as a climate change mitigation tool, CO2 must be securely retained for 10,000 years (10 ka) with a leakage rate of below 0.01% per year of the total amount of CO2 injected. Migration of CO2 back to the atmosphere via leakage through geological faults is a potential high impact risk to CO2 storage integrity. Here, we calculate for the first time natural leakage rates from a 420 ka paleo-record of CO2 leakage above a naturally occurring, faulted, CO2 reservoir in Arizona, USA. Surface travertine (CaCO3) deposits provide evidence of vertical CO2 leakage linked to known faults. U-Th dating of travertine deposits shows leakage varies along a single fault and that individual seeps have lifespans of up to 200 ka. Whilst the total volumes of CO2 required to form the travertine deposits are high, time-averaged leakage equates to a linear rate of less than 0.01%/yr. Hence, even this natural geological storage site, which would be deemed to be of too high risk to be selected for engineered geologic storage, is adequate to store CO2 for climate mitigation purposes

Tracing Atlantic Waters Using 129 I and 236 U in the Fram Strait in 2016

In this study 129I and 236U concentrations in seawater samples collected onboard R/V Polarstern during the PS100 expedition in the Fram Strait in 2016 are presented. The overall aim of the study was to investigate the distribution of these long‐lived radionuclides along the transect located at 79°N. The combination of both radionuclides was used for the first time in the Fram Strait to trace ocean circulation pathways of Atlantic waters. Results show that both 129I and 236U concentrations as well as 236U/238U ratios are about two times higher (> 600 × 107 at kg(−1), > 20 × 106 at kg(−1), and 2.8 × 10−9, respectively) in the cold and fresh outflowing surface waters from the Arctic Ocean (Polar Surface Water, PSW) compared to inflowing Atlantic origin waters (300 × 107 at kg(−1) 129I, 12 × 106 at kg(−1) 236U, and 1.4 × 10−9 236U/238U). A comparison with the different 129I and 236U input functions for the Atlantic branches entering the Arctic Ocean reveals that the middepth Atlantic origin waters outflowing the Arctic Ocean show more influence of the Barents Sea Branch Water than the Fram Strait Branch Water. The high radionuclide concentrations observed in the PSW indicate substantial influence of the Norwegian Coastal Current. This current carries a significantly larger proportion of 129I and 236U releases from European reprocessing plants than the aforementioned Atlantic branches. We estimate surface water transit times from the northern Norwegian Coast through the Arctic to the PSW of 12–19 years, less than for the middepth Barents Sea Branch Water (16–23 years)

The Potential of U-233/U-236 as a Water Mass Tracer in the Arctic Ocean

This study explores for the first time the possibilities that the U-233/U-236 atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, U-233 originates from global fallout while U-236 carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the U-233/U-236 ratio provides a tool to identify water masses with distinct U sources. In this work, U-233 and U-236 were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their U-233/U-236 signature; and (b) Deep and Bottom Waters show extremely low U-233 and U-236 concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high U-233/U-236 ratios are interpreted as a relative increase of naturally occurring U-233 and U-236 and thus for gradually reaching natural U-233/U-236 levels in the deep Arctic Ocean. Our results set the basis for future studies using the U-233/U-236 ratio to distinguish anthropogenic and pre-anthropogenic U in the Arctic Ocean and beyond.Funding Agencies|ETH Zurich Research GrantETH Zurich [ETH-06 16-1]; Swiss National Science FoundationSwiss National Science Foundation (SNSF)European Commission [PRIMA SNF PR00P2_193091]; Spanish Government (Ministerio de Ciencia, Innovacion y Universidades)Spanish Government [PGC2018-094546-B-I00]</p