Size-Normalized Shell Weights in Foraminifera Indicate Changes in Carbonate Dissolution and in Organism Physiology during Deglacial Deoxygenation

Previous workers in the North Pacific proposed a link between oxygen minimum zone (OMZ) intensification and increased ocean acidification, which could exaggerate these stressors on organisms. Thus, understanding the relationship between oxygen depletion and ocean acidification during past intervals of climate change is crucial for forecasting and mediating the effects that future OMZ expansion will have on marine ecology. Here, we use benthic (Uvigerina peregrina) and planktonic (Neogloboquadrina pachyderma) foraminiferal size-normalized shell weights (SNW) from a Gulf of Alaska sediment core record from the upper OMZ (698 m water depth) as a proxy for carbonate dissolution. We record abrupt changes in SNW associated with the LOE for both species, which suggest a change either in shell growth or in the intensity of carbonate dissolution. Computed tomography (CT) scans of foraminifera provide test thickness and chamber volume data, and suggest that U. peregrina with low SNW had thinner test walls and smaller increases in volume between subsequent chambers, thus indicating changes in organism growth and physiology influence SNW. SNW in U. peregrina increases during the LOE. In contrast, N. pachyderma with the lowest SNW had thinner test walls, open pores, and a loss of internal chamber walls indicating intense dissolution compared to higher SNW shells. Thus, we interpret SNW in N. pachyderma reflecting changes in the intensity of carbonate dissolution. Preliminary results suggest that carbonate dissolution increased abruptly after the start of the LOE, thus supporting a link between deoxygenation and acidification in the Gulf of Alaska. Our results further highlight the need to consider changes in organism growth patterns when interpreting SNW, especially for benthic foraminifera.