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a b s t r a c tSeveral recent studies have used records of the radiocarbon ( 14 C) bomb peak in speleothems to inversely model the soil a 14 CO 2 and the age distribution of soil organic material (SOM) above caves, in part to investigate the potential of speleothems as sensitive records of past SOM dynamics. The results of these modeling studies have suggested that soil CO 2 at karst sites is derived primarily from the decomposition of SOM with turnover times on the order of decades to centuries. This result is in stark contrast with observations of soil a 14 CO 2 at non-karst sites, which indicate that soil CO 2 is derived primarily from root respiration and decomposition of SOM with much shorter turnover times. This discrepancy suggests that SOM in karst settings may have a very different age distribution than sites that have been studied previously and/or that soil CO 2 is not the main source of speleothem carbon. To help resolve this discrepancy, we present an improved inverse model which we use to estimate the age of CO 2 above several caves. We also present results from a detailed case study of soil carbon dynamics at Heshang Cave, China. This work demonstrates that SOM in karst sites may be much older than SOM in non-karst soils that have been studied previously, but that CO 2 produced in the shallow soil zone is unlikely to be the main source of speleothem carbon. A review of the literature suggests that the most likely explanation for the aforementioned discrepancy is that decomposition of down-washed SOM in the vadose zone is the dominant source of speleothem carbon.

Extreme precipitation events, commonly associated with “Atmospheric Rivers,” are projected to increase in frequency and severity in western North America; however, the intensity and landfall position are difficult to forecast accurately. As the isotopic signature of precipitation has been widely utilized as a tracer of the hydrologic cycle and could potentially provide information about key physical processes, we utilize both climate and precipitation isotope data to investigate these events in California from 2001 to 2011. Although individual events have extreme isotopic signatures linked to associated circulation anomalies, the composite across all events unexpectedly resembles the weighted mean for the entire study period, reflecting diverse moisture trajectories and associated teleconnection phases. We document that 90% of events reaching this location occurred during the negative Arctic Oscillation, suggesting a possible link with higher‐latitude warming. We also utilize precipitation data of extreme precipitation events across the entire western U.S. to investigate the relationships between key tropical and Arctic climate modes known to influence precipitation in this region. Results indicate that the wettest conditions occur when the negative Arctic Oscillation, negative Pacific/North American pattern, and positive Southern Oscillation are in sync and that precipitation has increased in the southwestern U.S. and decreased in the northwestern U.S. relative to this phase combination's 1979–2011 climatology. Furthermore, the type of El Niño–Southern Oscillation event, Central Pacific or Eastern Pacific, influences the occurrence, landfall location, and isotopic composition of precipitation.

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