Evidence of Stable Foraminifera Biomineralization During the Last Two Climate Cycles in the Tropical Atlantic Ocean

Abstract

Planktonic foraminiferal biomineralization intensity, reflected by their shell calcite mass, affects global carbonate deposition and is known to follow the climate cycles by being increased during glacial stages and decreased during interglacial ones. Here we measure the dissolution state and the mass of the shells of the planktonic foraminifera species Globigerina bulloides from a Tropical Eastern North Atlantic site over the last two glacial-interglacial climatic transitions and we report no major changes in plankton calcite production with the atmospheric pCO2 variations. We attribute this consistency in foraminifera calcification to the climatic and hydrological stability of the tropical regions. We however recorded increased shell masses midway through the penultimate deglaciation (Termination II). In order to elucidate the cause of the increased shell weights we performed δ18O, Mg/Ca and μCT measurements on the same shells from a number of samples surrounding this event. We find that shells of increased mass are internally contaminated by sediment infilling and that shell weights are responding to local hydrographic changes.

X-ray tomographic data of planktonic foraminifera species Globigerina bulloides from the Eastern Tropical Atlantic across Termination II

“…Planktonic foraminifera are known to considerably alter their shell weight throughout the paleoceanographic record [2] and the degree of this alteration is a function of latitude [3]. In the present study [4] of a tropical Atlantic core we report relatively small variation in the shell weights of the planktonic species Globigerina bulloides (NCBI:txid69025; urn:lsid:marinespecies.org:taxname:113434) during the last two climatic cycles with the exception of an increased weight event during Termination II. In order to identify the cause of these elevated shell mass values prior to their dissolution for geochemical analyses, specimens from samples that surround this event were tomographically analyzed to gain insight of the actual microscopic fossils.…”

“…Apart from studies that uses CT analysis to examine biometrical foraminifera parameters [9,10,16] there are studies that can use the pixel's grey levels to extract geochemical environmental information [6] or information about the shell's corrosion state [17]. Knowledge of the degree of shell dissolution is of key importance in the study of foraminifera shell mass CT information is particularly useful in the study of foraminifera shell mass since in order to conclude about the cause behind shell mass changes in the paleoceanographic record it is important to have foraminifera biometric information (cell volumes) and information about the quantity of calcite mass loss to dissolution [13] as well as the degree of contamination due to sediment infilling of chambers, which as shown in the recent study [4] can produce considerable artifact yields.…”

“…Samples from GeoB cores can be obtained from MARUM GeoB Core Repository. Additional information about the study area, the material and the methods used for the present analyses can be found in the accompanying paper [4]. Nano-CT scanning was conducted with a Zeiss Versa 520 at the X-ray Microscopy (XRM) facility of the Natural History Museum, London.…”

“…The data presented here in detail accompany a recent paper [4], where they are summarized and discussed further. The analysis of the high resolution CT scan images of the foraminifera specimens lead to accurate determination of their cell volumes, which were combined with their weighed masses and allowed for the first time the report of volume normalized test weights (or shell densities).…”

“…Subsequently the background "material" was deleted and the internal void area of the shell was manually segmented as "protoplasm" area. The three different areas were then added together to produce the "cell" area, which is the total area occupied by the foraminifer (Figure 2 in [4]).…”