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

Abstract

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 (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) 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 δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one‐to‐one relationship (δ13CDICnat = δ13CCibnat). 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.

Deglacial carbon cycle changes observed in a compilation of 127 benthic <i>δ</i><sup>13</sup>C time series (20–6 ka)

“…Measurements of δ 13 C from the calcite tests of epibenthic foraminifera Cibicides wuellerstorfi and related species (Schweizer et al, 2009) are commonly used to trace the spatial distribution of nutrients and deep water masses as well as changes in ocean carbon cycling (e.g., Curry et al, 1988;Duplessy et al, 1988;Curry and Oppo, 2005;Schmittner et al, 2017). Benthic δ 13 C is also slightly influenced (< 15 %) by changes in carbonate ion concentration of sea water .…”

“…iments "LW" and "CW" in Schmittner et al, 2017). Accounting for the carbonate ion concentration of seawater can improve the accuracy of benthic δ 13 C , but estimates of carbonate ion changes throughout the deglaciation are scarce.…”

“…Accounting for the carbonate ion concentration of seawater can improve the accuracy of benthic δ 13 C , but estimates of carbonate ion changes throughout the deglaciation are scarce. In the absence of carbonate ion data, a linear regression can be used to convert between C. wuellerstorfi δ 13 C and DIC δ 13 C (regressions LW6 and CW6 in Schmittner et al, 2017). However, because our study focuses on the timing of δ 13 C change rather than its amplitude, we present all δ 13 C data using the values originally measured in foraminiferal calcite.…”

“…The δ 13 C of benthic foraminiferal calcite is a wellestablished carbon cycle proxy, which records the δ 13 C signature of dissolved inorganic carbon (DIC) in seawater at seafloor depths (e.g., Woodruff and Savin, 1985;Zahn et al, 1986;Lutze and Thiel, 1989;Duplessy et al, 1988;Mackensen, 2008;Gottschalk et al, 2016;Schmittner et al, 2017). Averages of benthic foraminiferal δ 13 C time series, called stacks, can improve the signal-to-noise ratio of regional or global seawater changes (e.g., Lisiecki et al, 2008;.…”

Deglacial carbon cycle changes observed in a compilation of 117 benthic δ&lt;sup&gt;13&lt;/sup&gt;C time series (20–6 ka)

“…However, identifying the biogeochemical mechanisms associated with these carbon transfers is complicated by a variety of carbon cycle feedbacks (e.g., Archer et al (2000); Sigman and Boyle (2000); Peacock et al (2006); Toggweiler et al (2006); Kohfeld and Ridgwell (2009); Brovkin et al (2012); Menviel et al (2012); Galbraith and Jaccard (2015); Buchanan et al (2016)). This study seeks to improve our understanding of glacial- C of benthic foraminiferal calcite is a well-established carbon cycle proxy, which records the δ 13 C signature of the dissolved inorganic carbon (DIC) in seawater at seafloor depths (e.g., Woodruff and Savin (1985); Zahn et al (1986); Lutze and Thiel (1989); Duplessy et al (1988); Mackensen (2008); Gottschalk et al (2016); Schmittner et al (2017)). Averages of benthic foraminiferal δ 13 C time series, called stacks, can improve the signal-to-noise ratio of regional or global seawater changes (e.g., Lisiecki et al (2008);Lisiecki (2014)).…”