Variability in calcitic Mg/Ca and Sr/Ca ratios in clones of the benthic foraminifer Ammonia tepida

Abstract

Biological activity introduces variability in element incorporation during calcification and thereby decreases the precision and accuracy when using foraminifera as geochemical proxies in paleoceanography. This so-called 'vital effect' consists of organismal and environmental components. Whereas organismal effects include uptake of ions from seawater and subsequent processing upon calcification, environmental effects include migration-and seasonality-induced differences. Triggering asexual reproduction and culturing juveniles of the benthic foraminifer Ammonia tepida under constant, controlled conditions allow environmental and genetic variability to be removed and the effect of cell-physiological controls on element incorporation to be quantified. Three groups of clones were cultured under constant conditions while determining their growth rates, size-normalized weights and single-chamber Mg/Ca and Sr/Ca using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Results show no detectable ontogenetic control on the incorporation of these elements in the species studied here. Despite constant culturing conditions, Mg/Ca varies by a factor of~4 within an individual foraminifer while intra-individual Sr/Ca varies by only a factor of 1.6. Differences between clone groups were similar to the intra-clone group variability in element composition, suggesting that any genetic differences between the clonegroups studied here do not affect trace element partitioning. Instead, variability in Mg/Ca appears to be inherent to the process of bio-calcification itself. The variability in Mg/Ca between chambers shows that measurements of at least 6 different chambers are required to determine the mean Mg/Ca value for a cultured foraminiferal test with a precision of ≤10%.

Ba incorporation in benthic foraminifera

“…This implies that the observed variability is a reflection of the inhomogeneous distribution in the test and is hence filtered out when averaging. This is similar to the behavior for Mg and Sr (Sadekov et al, 2008;Wit et al, 2012;De Nooijer et al, 2014a) and underscores the power of single-chamber analyses. If present, inhomogeneity in test wall Ba / Ca in combination with different cross sections sampled during the ablation potentially account for the observed variability.…”

“…4 in this paper with Fig. 5 from De Nooijer et al, 2014a), which would mean that at least 20 chambers need to be analyzed to reach a 5 % relative precision (De Nooijer et al, 2014a). This is not limited by the analytical precision, but rather is due to inherent biological interchamber and interspecimen variability.…”

“…This would imply that although large differences are observed within a test wall, the average still reliably reflects seawater concentration (this paper) and for Mg, still reflects seawater temperature (Hathorne et al, 2009). Comparing within-specimen and between-specimen variability, De Nooijer et al (2014a) showed that within-specimen variability does not account for all of the observed variability in Mg / Ca in Ammonia tepida. This seems to be similar for Ba / Ca (compare Fig.…”

“…From both species, seven specimens were ablated twice at the Royal Netherlands Institute for Sea Research (NIOZ) using a NWR193UC (New Wave Research) laser, containing an ArF Excimer laser (Existar) with deep UV 193 nm wavelength and < 4 ns pulse duration. Provided that the same reference material is used, the use of multiple laser systems (see above) is shown not to bias obtained foraminiferal element / Ca ratios (De Nooijer et al, 2014a). Laser ablation was performed with an energy density of 1 J cm −2 at a repetition rate of 6 Hz for calcite samples and an energy density of 5 J cm −2 for the glass (NIST610) standards.…”

Timing and mechanism for intratest Mg/Ca variability in a living planktic foraminifer

“…Given a Mg/Ca-thermometer sensitivity of ∼9% per • C, the 2σ standard error computed here translates to a temperature uncertainty of ±1.0 to 1.3 • C for pooled shell compositions. This is notable for being similar to estimates for the biological and environmental contribution to the uncertainty of temperature estimates obtained from the analysis of 20 individual G. ruber tests (Sadekov et al, 2008), although somewhat smaller than the Mg/Ca temperature uncertainty determined for the benthic foraminifera, Ammonia tepida (de Nooijer et al, 2014).…”