Interpreting the role of pH on stable isotopes in large benthic foraminifera

P O Knorr,P Hallock,L L Robbins,Shubha Sathyendranath,J G Wynn,P J Harries

doi:10.1093/icesjms/fsw056

P O Knorr, P Hallock + Show 4 more

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https://doi.org/10.1093/icesjms/fsw056

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Abstract

Large benthic foraminifera (LBF) are prolific producers of calcium carbonate sediments in shallow, tropical environments that are being influenced by ocean acidification (OA). Two LBF species, Amphistegina gibbosa (Order Rotaliida) with low-Mg calcite tests and Archaias angulatus (Order Miliolida) with high-Mg calcite tests, were studied to assess the effects of pH 7.6 on oxygen and carbon isotopic fractionation between test calcite and ambient seawater. The δ18O and δ13C values of terminal chambers and of whole adult tests of both species after 6 weeks were not significantly different between pH treatments of 8.0 and 7.6. However, tests of juveniles produced during the 6-week treatments showed significant differences between δ18O and δ13C values from control (pH 8.0) when compared with the treatment (pH 7.6) for both species. Although each individual's growth was photographed and measured, difficulty in distinguishing and manually extracting newly precipitated calcite from adult specimens likely confounded any differences in isotopic signals. However, juvenile specimens that resulted from asexual reproduction that occurred during the experiments did not contain old carbonate that could confound the new isotopic signals. These data reveal a potential bias in the design of OA experiments if only adults are used to investigate changes in test chemistries. Furthermore, the results reaffirm that different calcification mechanisms in these two foraminiferal orders control the fractionation of stable isotopes in the tests and will reflect decreasing pH in seawater somewhat differently.

Highlights

Carbonate chemistry of surface seawater is changing as it maintains equilibrium with increasing atmospheric carbon dioxide concentration [CO2atm] (Doney et al, 2009)
We report on analyses of the tests of Amphistegina gibbosa d’Orbigny, 1839 (Order Rotaliida), subsequently abbreviated as A. gibbosa, and Archaias angulatus Fichtel and Moll, 1798 (Order Miliolida), subsequently abbreviated as A. angulatus, to assess the effects of low pH and higher CO2 on stable isotopes as proxies of ocean acidification (OA) and the potential implications in paleoenvironmental reconstruction using these species
A. gibbosa displayed highly significant difference (p, 0.001) for mean d13C values between experimental and control tanks (Figure 6b). Both equilibrium and disequilibrium kinetic effects on isotopes of inorganic carbon species have been shown to contribute to the total observed fractionation of carbon and oxygen isotopes between CO2(aq) and the anions involved in carbonate precipitation, [HCO3−] and [CO32−] (Spero et al, 1997; Zeebe, 1999)

Summary

Introduction

Carbonate chemistry of surface seawater is changing as it maintains equilibrium with increasing atmospheric carbon dioxide concentration [CO2atm] (Doney et al, 2009). Representatives of Amphistegina and Archaias were investigated to determine the effects of higher CO2 and lower pH on light stable isotopes in their calcareous tests in part because members of their respective families are among the most prolific sediment producers (Hallock, 1981; Hallock et al, 1986a; Langer et al, 1997). These taxa have quite contrasting methods of calcification. The test wall consists of two layers of high-Mg calcites; the outermost test layer consists of a continuous sheet of rhombohedral calcite tiles; the more voluminous, inner layer consists of randomly oriented calcite needles (Lynts and Pfister, 1967; Reid and Macintyre, 1998)

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