A History of Warming Sea Surface Temperature and Ocean Acidification Recorded by Planktonic Foraminifera Geochemistry from the Santa Barbara Basin, California

Abstract:

The geochemistry of foraminiferal shells has been widely used to reconstruct past conditions of the ocean and climate. Since the onset of the Industrial Revolution, anthropogenically produced CO² has resulted in an increase in global temperatures and a decline in the mean pH of the world’s oceans. The California Current System is a particularly susceptible region to ocean acidification due to natural upwelling processes that also cause a reduction in seawater pH. The trace element concentration of magnesium and boron in planktonic foraminiferal shells are used here as proxies for temperature and carbonate ion concentration ([CO₃^2-]), respectively. Newly developed calibrations relating Mg/Ca ratios to temperature (R² 0.91) and B/Ca ratios to [CO₃^2-] (R² 0.84) for the surface-mixed layer species Globogerina bulloides were generated using material collected in the Santa Barbara Basin sediment trap time-series. Using these empirical relationships, temperature and [CO₃^2-] are reconstructed using a 0.5 meter long multi-core collected within the basin. ²¹⁰Pb activities were used to determine a sedimentation rate for the core to estimate ages for core samples (sedimentation rate: 0.341 cm/yr). A spike in ¹³⁷Cs activity is used as a tie-point to the year 1965 coinciding with the peak of nuclear bomb testing. Our down-core record extends through the mid-19th century to create a history of rising sea surface temperatures and declining [CO₃^2-] as a result of anthropogenic CO² emissions.