Geochemical imprints of genotypic variants of Globigerina bulloides in the Arabian Sea

Abstract

Planktonic foraminifera record oceanic conditions in their shell geochemistry. Many palaeoenvironmental studies have used fossil planktonic foraminifera to constrain past seawater properties by defining species based on their shell morphology. Recent genetic studies, however, have identified ecologically distinct genotypes within traditionally recognized morphospecies, signaling potential repercussions for palaeoclimate reconstructions. Here we demonstrate how the presence of Globigerina bulloides cryptic genotypes in the Arabian Sea may influence geochemical signals of living and fossil assemblages of these morphospecies. We have identified two distinct genotypes of G. bulloides with either cool water (type‐II) or warm water (type‐I) temperature preferences in the Western Arabian Sea. We accompany these genetic studies with analyses of Mg/Ca and stable oxygen isotope (δ18O) compositions of individual G. bulloides shells. Both Mg/Ca and δ18O values display bimodal distribution patterns. The distribution of Mg/Ca values cannot be simply explained by seawater parameters, and we attribute it to genotype‐specific biological controls on the shell geochemistry. The wide range of δ18O values in the fossil assemblage also suggests that similar controls likely influence this proxy in addition to environmental parameters. However, the magnitude of this effect on the δ18O signals is not clear from our data set, and further work is needed to clarify this. We also discuss current evidence of potential genotype‐specific geochemical signals in published data on G. bulloides geochemistry and other planktonic foraminiferal species. We conclude that significant caution should be taken when utilizing G. bulloides geochemistry for paleoclimate reconstruction in the regions with upwelling activity or oceanographic fronts.

A Sediment Trap Evaluation of B/Ca as a Carbonate System Proxy in Asymbiotic and Nondinoflagellate Hosting Planktonic Foraminifera

“…This conundrum poses a challenge to the paleoceanographic researchers that unknowingly aggregate distinct G . bulloides cryptic species, particularly if they exhibit geochemical signatures like those that have been demonstrated for this species and study region (Bemis et al, ; Osborne et al, ; Sadekov et al, ).…”

Calibration and Validation of Environmental Controls on Planktic Foraminifera Mg/Ca Using Global Core‐Top Data

“…bulloides model is only partially consistent with previous work. With respect to size, Friedrich et al () suggested a significant decrease in Mg/Ca of ~0.2 mmol/mol per 100 microns with increasing shell size, while Elderfield et al () found a similar magnitude of change but of opposite sign, and others found no relationship with size (Martínez‐Botí et al, ; Sadekov et al, ). With respect to shallow carbonate parameters, culture studies suggest that G .…”

Decadal resolution record of Oman upwelling indicates solar forcing of the Indian summer monsoon (9–6 ka)

“…Error propagation (Mohtadi et al, 2014) yields an average propagating 1σ error of 0.89 • C. We are aware of a potential bias introduced by a genotype-specific fractionation of Mg / Ca ratios, which was recently shown to occur between two species of G. bulloides that prefer warm water (type I) and cool water (type II) in the Arabian Sea WA-PLS = weighted-averaging partial-least-squares method. (Sadekov et al, 2016). However, the high numbers of G. bulloides observed throughout the studied interval of core SL163 suggest intense upwelling conditions and we therefore assume that we mainly sampled specimens of the coolwater genotype, type IIf, which is restricted to upwelling stations of the northern Oman margin (Sadekov et al, 2016).…”

“…(Sadekov et al, 2016). However, the high numbers of G. bulloides observed throughout the studied interval of core SL163 suggest intense upwelling conditions and we therefore assume that we mainly sampled specimens of the coolwater genotype, type IIf, which is restricted to upwelling stations of the northern Oman margin (Sadekov et al, 2016).…”

“…In addition, Mg / Ca measurements of G. bulloides at 8.3 ka BP are > 7 mmol mol −1 and inferred SSTs are > 25.5 • C. A similar feature of a very short warm excursion of Mg / Ca temperatures from G. bulloides with concomitantly high abundances of this species around 8 ka BP was observed by Anand et al (2008) in their record from the Somali upwelling system. It may be speculated here whether a potential intrusion of the warm-water genotype of G. bulloides (Sadekov et al, 2016) during the short-lived period around 8.3 ka BP had occurred, leading to erroneously warm Mg / Ca temperature estimates during this period.…”