Modelling δ13C in benthic foraminifera: Insights from model sensitivity experiments

Abstract

The δ 13 C value measured on benthic foraminiferal tests is widely used by palaeoceanographers to reconstruct the distribution of past water masses. The biogeochemical processes involved in forming the benthic foraminiferal δ 13 C signal (δ 13 C foram ), however, are not fully understood and a sound mechanistic description is still lacking. We use a reaction-diffusion model for calcification developed by Wolf-Gladrow et al. (1999) and Zeebe et al. (1999) in order to quantify the effects of different physical, chemical, and biological processes on δ 13 C foram of an idealised benthic foraminiferal shell. Changes in the δ 13 C value of dissolved inorganic carbon (δ 13 C DIC ) cause equal changes in δ 13 C foram in the model. The results further indicate that temperature, respiration rate, and pH have a significant impact on δ 13 C foram . In contrast, salinity, pressure, the δ 13 C value of particulate organic carbon (δ 13 C POC ), total alkalinity, and calcification rate show only a limited influence. In sensitivity experiments we assess how combining these effects can influence δ 13 C foram . We can potentially explain 33 to 47% of the interglacial-to-glacial decrease in δ 13 C foram by changes in temperature and pH, without invoking changes in δ 13 C DIC . Furthermore, about a quarter of the −0.4‰ change in δ 13 C foram observed in phytodetritus layers can be accounted for by an increase in respiration rate and a reduction in pH.

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

“…We therefore recommend using equation with parameters LA1 (Table ) rather than a simple one‐to‐one relationship between δ 13 C DIC ≈ δ 13 C Cib . The regression coefficients with respect to carbonate ion and pressure are qualitatively consistent with previous theoretical [ Hesse et al , ] and experimental [ Spero et al , ] studies, but quantitatively, the carbonate ion effects are smaller than those of Spero et al [] and larger than those of Hesse et al [], respectively. Hence, these secondary effects are relatively small and would in practical applications modify reconstructed δ 13 C DIC changes by typically less than about 15% compared to the often used one‐to‐one relationship.…”

“…Slightly shallower slopes with respect to [CO 3 2− ] result from using All data than using only LH data. The values are of the same order of magnitude but smaller than those measured on planktic foraminifera, which vary between −6 and −14 [10 −3 ‰/(μmol kg −1 )] for two different species [ Spero et al , ], but they are larger than theoretical estimates [ Hesse et al , ], which are around −1 [10 −3 ‰/(μmol kg −1 )]. To our knowledge, these results provide the first observational, albeit indirect, evidence to suggest that benthic foraminiferal δ 13 C is also affected by carbonate ion concentrations.…”

“…Implications of pH and temperature effects on glacial‐interglacial δ 13 C DIC reconstructions have been discussed previously in Hesse et al [] who found a strong temperature effect that suggests that 0.11‰ of the observed total whole ocean ~0.35‰ δ 13 C Cib drop from the interglacial to the glacial ocean could be explained by a cooling of bottom waters by 2.5°C. Our analysis did not result in a consistent significant temperature effect, thus questioning their conclusion.…”

“…Color symbols as in Figure . Black lines, which are the slopes from our regression analyses LA3, LA4, LL3, and LL4, are compared to results from Spero et al [] (light blue) and Hesse et al [] (light green).…”

“…Although such studies have not yet been conducted on benthic foraminifera because they are difficult to grow in the laboratory [ Wollenburg et al , ], observations of modern individual foraminifera suggest that benthics are also affected [ Ishimura et al , ], which could significantly impact reconstructions of past δ 13 C DIC . Recent theoretical work using diffusion‐reaction modeling of carbon isotopes in single foraminifera not only confirms the postulated carbonate ion effect but also indicates significant temperature and pressure effects on isotopic fractionation between surrounding bottom waters and benthic foraminiferal shells [ Hesse et al , ].…”