The Onset of Anthropogenic Carbon Invasion in the Surface Waters of the Southern California Current

High-resolution carbon isotopic records from organic and carbonate carbon reveal an unprecedented multi-decade decline in stable isotopic compositions over the past millennium. These records were obtained from laminated cores collected in the San Lázaro Basin (SLB), a semi-closed basin off the Baja California Peninsula, Mexico. The SLB is influenced by suboxic waters at depth and lies beneath the southern boundary of the California Current System. Isotopic analyses of two planktic foraminifera species, N. dutertrei and G. ruber, show a decreasing trend in carbon isotopic compositions over the past 80 years, mirroring the trend seen in atmospheric CO₂, albeit with slightly lower slopes. These trends are likely driven by the influx of anthropogenic CO₂ into surface waters of the California Current, a manifestation of the Suess effect in the upper ocean. The differences in slope are likely due to the combined influence of vertical mixing driven by dominant northwest winds, which bring nutrient- and inorganic carbon-rich waters with relatively heavier isotopic values, and the equatorial advection of northern waters carrying anthropogenic CO₂ in this highly productive eastern boundary current.