SEDIMENT SOURCE AREA CHARACTERIZATION THROUGHOUT THE MONTEREY BAY COAST AND WATERSHED

Terrestrial sediment is the dominant sediment type delivered to the coastal ocean. The source areas for this sediment however, can change within coastal watersheds over time. These changes can provide insight into larger environmental changes within the sediment routing system, providing a better understanding of how the system responds to a variety of external forcings. Therefore, distinguishing sediment source area characteristics can be a useful tool in building a better understanding of the evolution of a coastal sediment system, and this project sought to do so within coastal and watershed areas of Monterey Bay, CA. More than 90 surface sediment samples were collected from beaches, dunes, and stream channels throughout the region. These sediments were analyzed using X-Ray Fluorescence (XRF) to establish an elemental fingerprint for both the source area types (e.g. beaches, dunes, fluvial) and specific locations throughout the bay and watershed. The results showed a general distinction between the beaches north and south of the Monterey Submarine Canyon (MSC)., Additionally, the sediment from each of the three main rivers in the region (Salinas, Parajo, and San Lorenzo) also had different elemental compositions. The distinction in the beach sediment north and south of the MSC generally aligned with the fluvial characteristics for each region, although there were some important distinctions. Beach sediment north of the MSC was more similar to the San Lorenzo than the Pajaro River which likely reflects the influence of longshore drift as most beaches are downdrift of the San Lorenzo rather than the Pajaro River. Beach sediment south of the MSC plotted between the majority of the Salinas River samples and Monterey Peninsula rocky headland beach samples. This suggests that these beaches may reflect a mix of both fluvial and longshore drift sources. From this data, we can apply these source characteristics to offshore sediment collected in the bay to assess how sediment sources to the continental shelf may have changed overtime.