Northeastern Atlantic benthic foraminifera during the last 45,000 years: Changes in productivity seen from the bottom up

Abstract

We studied benthic foraminifera from the last 45 kyr in the >63 µm size fraction in Biogeochemical Ocean Flux Studies (BOFS) cores 5K (50°41.3′N, 21°51.9′W depth 3547 m) and 14K (58°37.2′N, 19°26.2′W, depth 1756 m), at a time resolution of several hundreds to a thousand years. The deepest site showed the largest fluctuations in faunal composition, species richness, and benthic foraminiferal accumulation rates; the fluctuations resulted from changes in abundance of Epistominella exigua and Alabaminella weddellensis. In the present oceans, these species bloom opportunistically when a spring plankton bloom results in seasonal deposition of phytodetritus on the seafloor. The “phytodetritus species” had very low relative abundances and accumulation rates during the last glacial maximum. A strong increase in absolute and relative abundance of E. exigua and A weddellensis during deglaciation paralleled the decrease in abundance of the polar planktonic foraminifer Neogloboquadrina pachyderma (s), and the increase in abundance of warmer water planktonic species such as Globigerina bulloides. This strong increase in relative abundance of the “phytodetritus species” and the coeval increase in benthic foraminiferal accumulation rate were thus probably caused by an increase in the deposition of phytodetritus to the seafloor (and thus probably of surface productivity) when the polar front retreated to higher latitudes. The abundance of “phytodetritus species” decreased during the Younger Dryas, but not to the low levels of fully glacial conditions. During Heinrich events (periods of excessive melt‐water formation and ice rafting) benthic accumulation rates were very low, as were the absolute and relative abundances of the “phytodetritus species”, supporting suggestions that surface productivity was very low during these events. In both cores Pullenia and Cassidulina species were common during isotope stages 2,3 and 4, as were bolivinid, buliminid and uvigerinid species. High relative abundances of these species have been interpreted as indicative either of sluggish deep water circulation or of high organic carbon fluxes to the seafloor. In our cores, relative abundances of these species are negatively correlated with benthic foraminiferal accumulation rates, and we can thus not interpret them as indicative of increased productivity during glacials. The percentage of these “low oxygen” species calculated on a “phytodetritus species”—free basis decreased slightly at deglaciation at 5K, but not at 14K. This suggests that decreased production of North Atlantic Deep Water during the last glacial might have slightly affected benthic foraminiferal faunas in the eastern North Atlantic at 3547 m depth, but not at 1756 m. In conclusion, major changes in deep‐sea benthic foraminiferal faunas over the last 45,000 years in our cores from the northeastern Atlantic were the result of changes in surface water productivity, not of changes in deep water circulation; productivity was lower during the glacial, probably because of extensive ice cover.