CHARACTERIZING SUB-DECADAL SEDIMENTATION WITHIN A SOUTHERN CALIFORNIA COASTAL WETLAND OVER THE PAST CENTURY

Estuaries and coastal wetlands are ecosystems that have increasingly been at risk in recent history due to natural and anthropogenic impacts. These impacts can be made directly to the coastal zone or occur within the watershed. Regardless of where the alterations occur, the impacts are recorded within the sediment records of these coastal environments. Therefore, estuarine sediments can be a valuable archive for natural and anthropogenic alterations within coastal systems. The Tijuana River Estuary (TRE) is a wetland system in Southern California located proximal to the US-Mexico border. Urbanization within the watershed, the majority of which is in Mexico, has increased exponentially over the past several decades resulting in a system that can experience massive, pulsed sedimentation events during wet periods. In this study we looked to characterize changes in sedimentation within the TRE over the past several decades with an emphasis on the impacts of human alterations and natural events. Here we present data from various cores acquired in areas proximal to the active river mouth and areas located adjacent to tidal creeks distal from the river mouth. The data shows an overall peak in sediment accumulation rates in the late 1990s/early 2000s followed by a general decrease in sedimentation rates in the following decades. These peaks in sedimentation coincides with the intense, and relatively frequent El Nino events during the 1980s and 1990, while the decline in rates is consistent with a more quiescent period post-2000. While the amount sediment delivered to the TRE has varied over decadal timescales, changes in sediment characteristics have been more consistent through time, Over the past ~100 years the sediment in the TRE has become consistently finer, and sedimentary organic carbon analyses suggest an increasing input from marine sources (although pulses of higher terrestrial sources are observed during wetter climate periods). Collectively, these data suggest that the TRE has experienced changes in sediment inputs, both in quantity and type, over the past several decades that is likely related to both climate and human alterations to the system. Understanding these changes will be critical to managing these important habitats in the future.