The relationship between shell size and Mg/Ca, Sr/Ca, δ¹⁸O, and δ¹³C of species of planktonic foraminifera

Abstract

[1] The relationship between shell size and Mg/Ca, Sr/Ca, d18 O, and d 13 C of planktonic foraminiferal calcite has been investigated using seventeen species in six different size fractions. Mg/Ca increases and Sr/ Ca decreases with increasing size, except for two globorotaliid species which show the opposite trend. The changes in Mg/Ca broadly follow d18 O calcification temperatures except that surface and near-surface dwelling species show larger changes in Mg/Ca than can be accounted for by differences in calcification temperature derived from d18 O. The increases in Mg/Ca and decreases in Sr/Ca vary linearly with the wellestablished increase in d 13 C with size. This is consistent with smaller individuals calcifying faster than larger individuals and larger individuals forming calcite that more closely reflects seawater temperature and composition. It appears that variations in calcification rate affect Mg and Sr shell chemistry. The observations may account for part of the temporal variability in foraminiferal Sr/Ca within a single size fraction that has been attributed to changes in seawater Sr/Ca.

Ecological controls on the shell geochemistry of pink and white Globigerinoides ruber in the northern Gulf of Mexico: Implications for paleoceanographic reconstruction

“…One issue that has been explored to some degree is the effect of foraminiferal test size on Mg/Ca (Elderfield et al, 2002;Ni et al, 2007). Elderfield et al (2002) illustrated a positive correlation between Mg/ Ca and test size in a number of species of planktonic foraminifera; however, it is difficult to quantify the relationship in that study due to the small number of individuals analyzed for each size fraction (~20), and insufficient temporal constraint in their sediment sample (their sample represented~800 yrs).…”

“…Elderfield et al (2002) illustrated a positive correlation between Mg/ Ca and test size in a number of species of planktonic foraminifera; however, it is difficult to quantify the relationship in that study due to the small number of individuals analyzed for each size fraction (~20), and insufficient temporal constraint in their sediment sample (their sample represented~800 yrs). Ni et al (2007) found no relationship between test size and Mg/Ca in the white variety of Globigerinoides ruber, but their sample also represented multiple centuries of deposition, with few replicate analyses.…”

“…Berger et al, 1978;Curry and Matthews, 1981;Oppo and Fairbanks, 1989;Ravelo and Fairbanks, 1995;Elderfield et al, 2002). Berger et al (1978) proposed that foraminiferal δ 13 C increases with test size as a function of ontogeny, due to the fact that metabolic rates are highest in the early ontogenetic stages (i.e.…”

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

“…Allison and Austin, 2003). The large variability in Mg/Ca is attributed to a number of factors including variability in environmental conditions (Wit et al, 2012), genetic differences between individuals (Numberger et al, 2009), and ontogenetic effects (Elderfield et al, 2002). Small fluctuations in experimental conditions cannot be responsible for the variability in Mg/Ca, since the dependency of Mg/Ca on temperature and salinity is too small in Ammonia spp.…”

“…Discrepancies between studies could stem from speciesspecific differences in controls on element incorporation, sample size, or both. Field studies of planktonic species show Mg/Ca increases with size in some species and decreases with size in other species (Elderfield et al, 2002;Anand and Elderfield, 2005;Friedrich et al, 2012), while under controlled constant conditions, Mg/Ca of the planktonic Globigerinoides sacculifer decreases with size (Dueñas-Bohórquez et al, 2011a). This implies that part of the relation between planktonic foraminiferal shell size and Mg/Ca is determined by calcification at different water depths (and hence different temperatures), and part of the ontogenetic controls on Mg/Ca are not environment-induced.…”