HOLOCENE SUBPOLAR GYRE DYNAMICS AND IMPLICATIONS FOR PALEONUTRIENT TRANSPORT TO HIGH LATITUDES

Substantial evidence suggests that observed changes in the water mass properties of the northern North Atlantic over the last decades are caused by exchanges between polar water masses and subtropical water masses in the North Atlantic subpolar gyre (SPG). It has been proposed that changes in SPG dynamics affect phytoplankton abundances. During intervals of strong SPG modes more polar water is entrained into the gyre and phytoplankton abundances decrease, affecting higher trophic levels as well. Higher phytoplankton abundances have been associated with weak SPG modes during which more subtropical water is entrained. By looking at Holocene SPG evolution we can add to our understanding of the role the SPG plays on the nutrient transport to high latitudes.

The sediment core NEAP4K from Björn Drift in the northern Iceland Basin (61° 29.91 N, 24° 10.33 W, 1627 m water depth) lies in an ideal position to monitor SPG fluctuations at the eastern boundary. We reconstruct sea surface temperature (SST) from the Mg/Ca ratios in Globigerina bulloides, a planktonic foraminifer, and combine it with δ¹⁸O_calcite to extract the δ¹⁸O_seawater contribution, which covaries with salinity in the upper oceans. We reconstruct surface paleonutrients from the Cd/Ca ratios in G. bulloides. Cd/Ca ratios have been used to reconstruct paleonutrients because in modern sediments they correlate well with dissolved Cd in seawater, which resembles the nutrient PO₄. However, a potential temperature-dependence of the Cd/Ca of G. bulloides has been suggested. We present the Cd/Ca record and the calculated PO₄ record, which is corrected for temperature.

Results show that, in general, the SPG contracted during the mid Holocene and oscillated between extended and contracted modes in the early Holocene with several millennial-scale oscillations superimposed. Results show that although, in general, higher nutrient levels occur when more subtropical Atlantic water is present at the site, the relationship is not straightforward. For example, millennial-scale regime shifts in the average Cd/Ca are not coincident with similar shifts observed in the average SST. Differences between these records suggest other mechanisms in addition to SPG dynamics must play an important role in controlling the paleonutrient distribution in the sub-polar North Atlantic.