Calcification of the planktonic foraminifera Globigerina bulloides and carbonate ion concentration: Results from the Santa Barbara Basin

Abstract Planktonic foraminiferal calcification intensity, reflected by shell wall thickness, has been hypothesized to covary with the carbonate chemistry of seawater. Here we use both sediment trap and box core samples from the Santa Barbara Basin to evaluate the relationship between the calcification intensity of the planktonic foraminifera species Globigerina bulloides , measured by area density (µg/µm 2 ), and the carbonate ion concentration of seawater ([CO 3 2− ]). We also evaluate the influence of both temperature and nutrient concentration ([PO 4 3− ]) on foraminiferal calcification and growth. The presence of two G .  bulloides morphospecies with systematically different calcification properties and offset stable isotopic compositions was identified within sampling populations using distinguishing morphometric characteristics. The calcification temperature and by extension calcification depth of the more abundant “normal” G .  bulloides morphospecies was determined using δ 18 O temperature estimates. Calcification depths vary seasonally with upwelling and were used to select the appropriate [CO 3 2− ], temperature, and [PO 4 3− ] depth measurements for comparison with area density. Seasonal upwelling in the study region also results in collinearity between independent variables complicating a straightforward statistical analysis. To address this issue, we use additional statistical diagnostics and a down core record to disentangle the respective roles of each parameter on G .  bulloides calcification. Our results indicate that [CO 3 2− ] is the primary variable controlling calcification intensity while temperature influences shell size. We report a modern calibration for the normal G .  bulloides morphospecies that can be used in down core studies of well‐preserved sediments to estimate past [CO 3 2− ].