Development of a deep-water carbonate ion concentration proxy based on
preservation of planktonic foraminifera shells quantified by X-ray CT
scanning
Abstract
The quantitative and objective characterization of dissolution intensity
in fossil planktonic foraminiferal shells could be used to reconstruct
past changes in bottom water carbonate ion concentration. Among proxies
measuring the degree of dissolution of planktonic foraminiferal shells,
X-ray micro-Computed Tomography (CT) based characterization of apparent
shell density appears to have good potential to facilitate quantitative
reconstruction of carbonate chemistry. However, unlike the
well-established benthic foraminiferal B/Ca ratio-based proxy, only a
regional calibration of the CT-based proxy exists based on a limited
number of data points covering mainly low-saturation state waters. Here
we determined by CT-based proxy the shell dissolution intensity of
planktonic foraminifera Globigerina bulloides, Globorotalia inflata,
Globigerinoides ruber, and Trilobatus sacculifer from a collection of
core top samples in the Southern Atlantic covering higher saturation
states, and assessed the characteristics and reliability of CT-based
proxy. We observed that the CT-based proxy is generally controlled by
deep-water Δ[CO32–] like the B/Ca proxy, but its effective range of
Δ[CO32–] is between –20 to 10 µmolkg–1. In this range, the
CT-based proxy appears directly and strongly related to deep-water
Δ[CO32–], whereas the B/Ca of benthic foraminifera appears to be
affected by porewater saturation in carbonate-rich substrates. On the
other hand, the CT-based proxy is affected by supralysoclinal
dissolution in areas with high productivity. Like the B/Ca proxy, the
CT-based proxy requires species-specific calibration, but the effect of
species-specific shell difference in susceptibility to dissolution on
the proxy is small.