Multicentury-Scale Records of Carbonate (Hydrographic?) Variability on the Northern Iceland Margin over the Last 5000 Years

The waters off northern Iceland are subjected to extreme hydrographic variability on annual to decadal timescales. In years when cold low-salinity water moves coastward and sea ice is prevalent (i.e., the late 1960s), marine productivity of surface waters is low because the water column is well stratified. In the opposite oceanographic mode, warm, salty Atlantic Water dominates the shelf and vertical mixing results in high productivity. We track these two contrasting modes by measuring the carbonate content of marine sediments, a proxy for productivity, in three cores from northern Iceland. The fjord record (B997-328PC) is anchored by eleven 14C AMS dates and has a resolution of one sample every 50 yr. Thirteen oscillations occurred over the last 4800 cal yr with an average length of 370 yr; these are superimposed on a long-term decrease in net carbonate accumulation. The Little Ice Age is marked by the largest decrease in carbonate content and in flux, with smaller minima at 2300 and 3000 cal yr B.P. Marked peaks in carbonate (optima) occurred at 2000 and 3800 cal yr B.P. The carbonate record from B997-328PC can be correlated with records on the inner shelf (B997-330PC) and midshelf (B997-327PC), indicating that significant regional changes in oceanography occurred at the southern margin of the Norwegian–Greenland Sea. The marine carbonate fluctuations closely track temperature reconstructions from the Greenland Summit site for the last 2000 cal yr and show similar, but slightly offset, oscillations between 2000 and ca. 4800 cal yr B.P.