Organic-walled microfossils and geochemical tracers: sedimentary indicators of productivity changes in the North Water and northern Baffin Bay during the last centuries

Analyses performed on 26 surface sediment samples collected with a box corer at 17 stations throughout the North Water and northern Baffin Bay (75–79°N; 68–80°W) revealed abundant organic-walled microfossils, mostly dinoflagellate cysts (103–104 cysts g−1) and organic linings of benthic foraminifers (102–103 OL g−1), as well as high organic carbon concentrations (0.87–2.81% dry weight). These data indicate high productivity in both the pelagic and benthic environments of the North Water and slightly lower productivity in northern Baffin Bay. The data also showed calcium carbonate and biogenic silica dissolution throughout the study area. The dinocyst assemblages were relatively uniform in the North Water and dominated by heterotrophic taxa (Algidasphaeridium? minutum and Brigantedinium spp.), whereas northern Baffin Bay assemblages were dominated by autotrophic taxa, notably Operculodinium centrocarpum and Spiniferites elongatus. The difference between these two assemblages may be related to higher diatomaceous primary production in the North Water than in northern Baffin Bay, since diatoms constitute the principal food source of heterotrophic dinoflagellates. The biogeographical boundary between the North Water and northern Baffin Bay has been maintained for at least the last few centuries as shown by analyses of microfossils and geochemical tracers in two sediment cores, one taken in the southeastern part of the North Water (76°17′N, 72°02′W) and the other in northeastern Baffin Bay (75°35′N, 70°48′W). The analyses of the North Water core revealed relatively uniform microfossil assemblages and organic carbon fluxes ranging from 1.1 to 1.5 mg Corg cm−2 yr−1 for the last few centuries, which corresponded to 4–6% of the present annual primary production in the euphotic zone. These data suggest high productivity and relatively stable conditions in the polynya on a decadal time scale. In the northeastern Baffin Bay core, the analyses indicated generally lower organic carbon fluxes, ranging from 0.3 to 0.6 mg Corg cm−2 yr−1, and (from the microfossil data) significant variations in sea-surface conditions at this lower latitude over the last centuries.