Planktonic Foraminifer Globorotalia truncatulinoides Coiling Ratio as Recorder of Upper Ocean Conditions in the Subtropical Atlantic Ocean During Marine Isotope Stages 7 and 8

Tracking changes in the geochemistry and morphology of foraminiferal species has been established as a method of reconstructing ocean conditions through time. Here we look at shifts in the ratio of left to right coiling tests of planktonic foraminifer Globorotalia truncatulinoides as a means to reconstruct changes in the depth of the permanent thermocline. Previous research has found that sinistral (left) coiling ratio peaks indicate a deepening thermocline, while dextral (right) coiling ratio peaks indicate a shallowing thermocline [Feldmeijer et al., 2014]. Tracking shifts in coiling ratio of its population over time can shed light on shifts in the depth of the permanent thermocline, and therefore climatic conditions at a given location. We measured coiling direction of G. truncatulinoides over Marine Isotope Stages (MISs) 7 and 8 (~190-260Ka) to investigate changes in upper ocean conditions at northwestern subtropical Atlantic Ocean Drilling Program Leg 172, Site 1059. According to our results, we see a series of maxima of G. truncatulinoides % sinistral coiling that are irregularly spaced 5 to 30 kyr apart, and a series of minima that are spaced 15 to 20 kyr apart. Our record of coiling ratio over the study interval provides evidence for changes in thermocline depth regardless of the glacial to interglacial background climate conditions. Variations occur on the millennial scale, but do not exhibit any quasi periodic oscillations. We speculate that these changes in the thermocline are related to variations in the position of the Gulf Stream and North Atlantic Gyre circulation. Next steps include comparing these data with proxy records from the higher latitude North Atlantic in order to explore relationships between northward heat transport in the Gulf Stream and high latitude climate change.