Assessing the significance of identifying Globigerinoides ruber albus morphotypes in the south Atlantic subtropical gyre during the late Pleistocene

Abstract

Globigerinoides ruber albus is a planktonic foraminiferal species widespread in tropical and subtropical surface ocean waters. Nevertheless, its taxonomy is still subject of debate. Here we classified at the morphotype level a total of 1816 G. ruber albus specimens recovered from marine isotopic stage (MIS) 14 to 9 at Deep Sea Drilling Project (DSDP) Site 517, located in the South Atlantic subtropical gyre. The specimens were taxonomically divided into seven morphotypes of G. ruber (cyclostoma, normal, platys, kummerform, elongatus cf. 1, elongatus pyramidical and twin) and seven morphometric parameters were measured. The morphotypes G. ruber albus cyclostoma, G. ruber albus normal, G. ruber albus platys and G. ruber albus kummerform present significant differences of morphometric measurements, as well as of ratios between individual measurements, occupying separate segments of morphospace. We also demonstrate that the most relevant morphological characters for the classification of G. ruber albus morphotypes are height and width of the last chamber, and height and width of the aperture. Stable oxygen (δ 18O) and carbon (δ 13C) isotope analyses showed that the G. ruber albus cyclostoma morphotype presented the highest δ 18O values and the lowest δ13C values, indicating a preference for relatively deep and cold waters with low symbiont activity. The G. ruber albus normal morphotype showed the lowest δ 18O signature, evidencing its preference for shallow and warm waters, however its δ13C values were lower than those of G. ruber albus platys and G. ruber albus kummerform, also suggesting reduced photosymbiont activity. Considering changes in relative abundances, although minor, the G. ruber albus cyclostoma morphotype was more abundant during interglacial periods, whereas the G. ruber albus kummerform morphotype was more abundant during glacials in the South Atlantic subtropical gyre. Due to the morphometric and isotopic differences between the morphotypes identified here, we suggest that future paleoceanographic studies consider identifying the G. ruber albus cyclostoma, G. ruber albus normal, G. ruber albus platys and G. ruber albus kummerform morphotypes.