Reassessing Foraminiferal Stable Isotope Geochemistry: Impact of the Oceanic Carbonate System (Experimental Results)

Abstract

Abstract:Laboratory experiments with living planktic foraminifers show that the δ 13 C and δ 18 O values of shell calcite decrease with increasing sea water pH and/or carbonate ion concentration. The effect has been quantified in symbiotic (Orbulina universa) and non-symbiotic (Globigerina bulloides) species and is independent of symbiont activity and temperature. It is concluded that a kinetic fractionation process affects both the carbon and oxygen isotopic composition of the shell simultaneously. At present it cannot be determined definitively whether the relationship is controlled by the pH dependent balance between hydration and hydroxylation of CO 2 or by [CO 32 -] related variations in the calcification rate. However, independent of which factor ultimately controls the relationship between the carbonate chemistry and isotopic fractionation, in the real ocean [CO 32 -] and pH covary linearly across the relevant pH range. The true relationship between shell isotopic composition and the bulk carbonate chemistry is masked by the fact that host respiration and symbiont activity locally modify the carbonate system. Respiration lowers and photosynthesis increases ambient pH and [CO 3 -]. This translates into modified absolute shell values but leaves the slope between the shell isotopic composition and the bulk carbonate chemistry unaffected. A second level of shell isotopic modification is introduced by the incorporation of respired carbon, enriched in 12 C, which depletes the shell δ 13 C value. In symbiont bearing species this depletion is partially negated by a shell δ 13 C enrichment in the light. As an alternative to the RUBISCO hypothesis (enrichment via preferential removal of 12 CO 2 ), we propose that scavenging of respired CO 2 during photosynthesis, raises the shell δ 13 C value. Our results have partly been documented before (Spero et al. 1997) and demonstrate that the carbonate chemistry is undoubtedly a major control on temporal geochemical variability in the fossil record. For instance, the sea water carbonate system of the prePhanerozoic world (Berner 1994;Grotzinger and Kasting 1993) or during glacials (Sanyal et al. 1995) was significantly different from today confounding direct interpretation of foraminiferal stable isotope data using existing relationships (see companion paper in this volume by Lea et al.).

Biomineralization in perforate foraminifera

“…Seawater as the direct source for Ca 2+ and DIC Foraminifera calcify by creating a microenvironment supersaturated with respect to CaCO 3 , while overcoming inhibition by crystallization inhibitors such as Mg 2+ . Hence, calcification requires a tight control on the concentration and/or ion activity at the site of calcification, commonly referred to as the "delimited" space (Erez, 2003) (Erez, 1978;Grossman, 1987;Ter Kuile and Erez, 1991;Hemleben and Bijma, 1994;Bijma et al, 1999), the majority of the carbon and the Ca 2+ needed for test formation must be derived from the seawater environment. Calcification requires equal amounts of Ca 2+ and CO 3 2−…”

Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau

“…Discrete measurements of the test size and weight of foraminifer individuals facilitate the calculation of size-normalized weight (SNW), a commonly used descriptor of test density/thickness (Bijma et al, 1999;Beer et al, 2010a;Marshall et al, 2013). The SNW was calculated for each individual by dividing the weight by the minimum test diameter (SNW (mg mm À 1 )¼ W/d min ).…”

“…SNW was originally considered as a proxy for [CO 3 2 À ] (Lohmann, 1995;Bijma et al, 1999;Broecker and Clark, 2001;Barker and Elderfield, 2002;Bijma et al, 2002). Additionally, the comparison of foraminifer tests from modern sediment traps samples and Holocene sediments demonstrated the impact of ocean acidification and the lowering of [CO 3 2 À ] on the reduction of the test weight at high southern latitudes (Moy et al, 2009).…”

Effect of salinity and seawater calcite saturation state on Mg and Sr incorporation in cultured planktonic foraminifera

“…According to Anderson and Faber (1984), Lea et al (1995) and Bijma et al (1999), G. sacculifer has a higher calcification rate than O. universa. The rates at which these two species calcify might thus be responsible for both the difference in Sr/Ca ratios and the different response of foraminiferal Sr incorporation to [CO 3 2− ] changes (Fig.…”