Modern oceanographic investigations show that surface ocean warming is associated with increased thickness of the mixed layer, deepening of the thermocline, and reduction of upwelling strength. These changes can profoundly affect surface ocean biological productivity. Proxies to measure past changes in upper water mass structure and stability are often poorly constrained. Stable oxygen isotope studies of planktonic foraminifera collected in sediment traps in Santa Barbara Basin, Southern California, demonstrate that the δ18O difference (Δδ18O) between G. bulloides and N. pachyderma (d.) closely monitors changes in the depth of the thermocline and related thickness and stability of the mixed layer. This proxy can be employed in investigations of past changes in sea surface temperature, upwelling and thermocline strength on the California margin, an area of great sensitivity to global change.