UC San Diego

The San Pedro Basin (SPB) is a deep borderland basin in the Southern California Bight containing DDT-contaminated sediments, yet the physical processes governing sediment mobilization, transport, and redistribution remain poorly constrained. This study integrates long-term hydrographic observations from the San Pedro Ocean Time-series (SPOT) with near-bottom mooring measurements to characterize deep-ocean circulation, bottom boundary-layer dynamics, and sediment resuspension potential. Hydrographic observations indicate weak stratification below the 740 m sill and minimal seasonal or interannual variability, consistent with prolonged isolation of deep-basin waters. Relationships between deep-water density, dissolved oxygen, and the Niño 3 index are weak, suggesting limited penetration of ENSO-related variability into the deep basin. Near-bottom Acoustic Doppler Current Profiler (ADCP) measurements show that bottom current variability is dominated by semidiurnal tides, with secondary contributions from diurnal and near-inertial motions. Estimated bed shear stresses were generally weak, with intermittent spring tide maxima approaching theoretical erosion thresholds. Comparisons with cohesive sediment erosion models suggest that widespread resuspension is unlikely under typical conditions. Seafloor imagery collected at the sediment core locations shows evidence of biogenic activity, suggesting a potential pathway for sediment suspension and subsequent water column transport, in addition to the potential for contaminant transfer into the marine food web. Together, these results suggest that deep SPB circulation is dominated by weak tidal forcing that intermittently enhances bottom shear stress while promoting long-term retention of contaminated sediments within the basin.