3 research outputs found

    U-Th dating of travertines on the Colorado Plateau: implications for the leakage of geologically stored CO2

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    In order to avoid the damaging climatic consequences of rising atmospheric CO2, and reduce current atmospheric CO2 concentrations to pre-industrial levels, anthropogenic CO2 emissions must be mitigated by capturing CO2 at power plants and storing it for thousands of years. Underground storage within deep geological formations, such as depleted gas and oil fields or deep saline aquifers, is the best understood solution for storage of CO2. In order for this method to gain more public and political acceptance it is important to characterise the potential causes, quantities and rates of CO2 release that could result if leakage were to occur from anthropogenic storage projects. This study examines two sites in the Colorado Plateau where faulted and actively leaking CO2 reservoirs provide natural analogues for failed anthropogenic storage sites. The two sites in question, the Little Grand Wash and northern Salt Wash graben faults are situated at the northern end of the Paradox Basin in Utah and represent classic three way traps due to juxtaposition of the shallow, north plunging Green River anticline against a set of east-west trending normal faults. In addition to active leakage sites in each area there are numerous fossilised travertine deposits. Along the Little Grand Wash fault the ancient mounds are restricted to the fault trace whereas ancient travertine mounds associated with the northern fault of the Salt Wash graben are far more numerous and occur up to ~530 m into the footwall of the fault. This more diffuse pattern of flow is due to the outcropping of unconfined aquifer units at the surface. A total of 45 U-Th dates from the majority of these travertine mounds provides a unique data set. The oldest deposits from the Little Grand Wash and northern Salt Wash graben faults produced ages of 113,912 ± 604 and 413,474 ± 15,127 years respectively. Repeat ages show reasonable reproducibility and analytical errors on results are of the order of 1% of the ages. The coupling of travertine elevation measurements with their radiometric ages gives an incision rate for each site. A rate of 0.342 m/ka for the Little Grand Wash fault relates directly to Green River incision and agrees with previous work on the Colorado Plateau, providing a further data point for characterisation of uplift of the province. For the northern fault of the Salt Wash graben a rate of 0.168 m/ka for the tributaries running through the area gives a robust method with which to estimate ages for un-dated mounds. The results of radiometric dating and incision rate age estimation of travertine mounds shows that leakage can last for timescales of 100,000’s of years, while high resolution U-Th dating of an individual mound demonstrated that leakage from a single point can last for a minimum of ~11,000 years. A range of travertine ages show that leakage to the surface has constantly switched location through time, while the presence of three mounds of distinct age at one location demonstrate that pathways can become repeatedly re-used over periods of ~45,000 years. There is no evidence of temporal periodicity in travertine deposition but there is a distinct spatial pattern of leakage as shown by localised similarities in the initial uranium chemistries of travertine mounds. Initial leakage is proximally located to the axial trace of the Green River anticline and subsequent leakage spreads from this central point along the fault plane in both east and west directions. The switching of fluid flow pathways to the surface can be explained by three main mechanisms: mineralisation, 3-phase interference of CO2 related fluid flow and seismically triggered alteration in dynamic strain acting upon the hydrology of the faults. These mechanisms have differing influences in each area - demonstrating that the behaviour of fluid flow switching in a system confined to damage zone fractures (Little Grand Wash fault) is different to a system leaking through an unconfined aquifer (northern fault of the Salt Wash graben). Coupling of travertine ages with estimates of their volumes provided a total worse case scenario for quantity of CO2 leakage of 6.2 x 10^6 ± 1.7 x 10^6 tonnes for the Little Grand Wash fault and 7.4 x 10^6 ± 2 x 10^6 tonnes for the northern fault of the Salt Wash graben. From these totals time averaged leakage rates of 55 ± 15 and 47 ± 13 tonnes/year were estimated for each fault. The leakage rate for the actively precipitating Crystal Geyser travertine (which is the result of anthropogenic exploration drilling) is estimated to be 3,153 ± 851 tonnes/year. These total and modern rates provide analogues for leakage via caprock failure and catastrophic wellbore failure. Applying them to large scale storage sites such as Weyburn and Gorgon revealed that for caprock failure complete leakage of these reservoirs will take place over timescales of 10^5-10^6 years, while for catastrophic failure of a single well complete leakage of these reservoirs could occur over as little as 10^3 – 10^4 years. This finding has important implications for the successful monitoring of anthropogenic storage sites

    WTO must ban harmful fisheries subsidies

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    Sustainably managed wild fisheries support food and nutritional security, livelihoods, and cultures (1). Harmful fisheries subsidies—government payments that incentivize overcapacity and lead to overfishing—undermine these benefits yet are increasing globally (2). World Trade Organization (WTO) members have a unique opportunity at their ministerial meeting in November to reach an agreement that eliminates harmful subsidies (3). We—a group of scientists spanning 46 countries and 6 continents—urge the WTO to make this commitment..

    Appendicitis risk prediction models in children presenting with right iliac fossa pain (RIFT study): a prospective, multicentre validation study.

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    Background Acute appendicitis is the most common surgical emergency in children. Differentiation of acute appendicitis from conditions that do not require operative management can be challenging in children. This study aimed to identify the optimum risk prediction model to stratify acute appendicitis risk in children. Methods We did a rapid review to identify acute appendicitis risk prediction models. A prospective, multicentre cohort study was then done to evaluate performance of these models. Children (aged 5\u201315 years) presenting with acute right iliac fossa pain in the UK and Ireland were included. For each model, score cutoff thresholds were systematically varied to identify the best achievable specificity while maintaining a failure rate (ie, proportion of patients identified as low risk who had acute appendicitis) less than 5%. The normal appendicectomy rate was the proportion of resected appendixes found to be normal on histopathological examination. Findings 15 risk prediction models were identified that could be assessed. The cohort study enrolled 1827 children from 139 centres, of whom 630 (34\ub75%) underwent appendicectomy. The normal appendicectomy rate was 15\ub79% (100 of 630 patients). The Shera score was the best performing model, with an area under the curve of 0\ub784 (95% CI 0\ub782\u20130\ub786). Applying score cutoffs of 3 points or lower for children aged 5\u201310 years and girls aged 11\u201315 years, and 2 points or lower for boys aged 11\u201315 years, the failure rate was 3\ub73% (95% CI 2\ub70\u20135\ub72; 18 of 539 patients), specificity was 44\ub73% (95% CI 41\ub74\u201347\ub72; 521 of 1176), and positive predictive value was 41\ub74% (38\ub75\u201344\ub74; 463 of 1118). Positive predictive value for the Shera score with a cutoff of 6 points or lower (72\ub76%, 67\ub74\u201377\ub74) was similar to that of ultrasound scan (75\ub70%, 65\ub73\u201383\ub71). Interpretation The Shera score has the potential to identify a large group of children at low risk of acute appendicitis who could be considered for early discharge. Risk scoring does not identify children who should proceed directly to surgery. Medium-risk and high-risk children should undergo routine preoperative ultrasound imaging by operators trained to assess for acute appendicitis, and MRI or low-dose CT if uncertainty remains. Funding None
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