Calibration of the carbon isotope composition (δ¹³C) of benthic foraminifera

The carbon isotope composition (δ¹³C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ¹³C of foraminifera tests. Here more than 1700 δ¹³C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ¹³C_Cibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ¹³C of dissolved inorganic carbon (δ¹³C_DICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ¹³C_Cibnat and δ¹³C_DICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10⁻³‰/(μmol kg⁻¹) [CO₃ ²⁻] and pressure (−4.9 ± 1.7) × 10⁻⁵‰ m⁻¹ (depth) effects, which we use to propose a new global calibration for predicting δ¹³C_DICnat from δ¹³C_Cibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ¹³C_DICnat = δ¹³C_Cibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ¹³C record.