Award: OCE-2401563

We collected particle and sediment samples from the Santa Clara River, California during the El Nio of December 2023 to June 2024. We measured particulate organic carbon (POC), and carbon isotopes (14C and 13C) of the samples. We hypothesized that extreme rain in the Santa Clara River (SCR) watershed mobilizes and delivers a unique pool of aged OC that is different from that mobilized during normal or low rain years. Extreme rain occurred during the 2024 El Nio and we found that the SCR had a similar concentration of POC as the year with the highest rain during the major El Nio year of 1997-1998. The POC appeared to contain approximately 50% petroleum OC (shale, etc.) and 50% recent plant matter OC. This agrees with results from previous rain years that showed the SCR exports ~40% petroleum OC and ~60% plant OC across a single water year. This supports the hypothesis that when rain exceeds a certain threshold, the same mix of OC is mobilized in the watershed. Our findings indicate that the SCR is a major source of both terrestrial plant and petroleum OC to the Pacific Ocean. We now have three years of data for high rain years and find that they transport hundreds of times more OC than that during low rain years. However, the average lifetime of the plant OC in the watershed is a few years. This is similar to the rate of El Nio oscillation (2-7 years) that is a major control on the rain in the region. This may mean that major storms remove most of the OC from the water shed. This complicates the hypothesis that as rain variations in Southern California become more extreme with climate change, more OC will be exported to the ocean. If extreme rain becomes more common, could erosion deplete OC in the region and reduce the overall export of OC to the ocean? Ongoing work continues in our lab group on compound classes of the OC samples from the SCR, separating out the lipid, protein, carbohydrate and acid-insoluble fractions for C isotope analyses. We hypothesize that the lipid and acid-insoluble fractions contain higher petroleum content OC. An undergraduate student at UC Irvine was trained in the preparation and analysis of environmental OC samples and C isotope analyses. This work will contribute to their senior thesis during the following year. A postdoctoral scholar obtained experience in mentorship and project management by coordinating the research of the student. They also attended two professional conferences and presented the work produced with the help of this grant. Last Modified: 04/26/2025 Submitted by: EllenRDruffel