The influence of subpolar gyre dynamics on centennial to millennial scale Holocene climate variability in the high-latitude North Atlantic
PhD: University of Colorado Boulder, 2014.
Holocene paleoclimatic reconstructions provide longer time series than instrumental records, allowing us to examine climate variability under more extreme boundary conditions. We reconstructed temperature and ∂18Osw using paired measurements of Mg/Ca ratios and ∂18Ocalcite of planktonic and benthic foraminifera at sites in the subpolar North Atlantic that are sensitive to Subpolar Gyre (SPG) dynamics today. Glacial freshwater from the final stages of the decay of the Laurentide ice sheet influenced SPG dynamics and was routed via the SPG throughout the subpolar basin. Near Iceland we recorded the 8.2 ka cooling event and the freshwater spike from the catastrophic outburst flood of proglacial lakes Agassiz and Ojibway using paired Mg/Ca and ∂18Ocalcite of the benthic foraminifer Cibicides lobatulus. This evidence for a brief cooling and freshening is supported by a coeval increase in Arctic benthic foraminifera and a decrease in biogenic carbonate. Through the Holocene, the SPG warmed and the ∂18Osw became heavier (saltier), suggesting a contracted gyre and a greater entrainment of subtropical waters. This is in sharp contrast to the canonical view of the rest of the North Atlantic, which was warmer during the early-mid Holocene. We attribute the SPG warming trend to a weakening of the NAO-like circulation in response to increasing winter insolation through the Holocene. Overlying this general trend were abrupt shifts that cannot be explained by changes in insolation. Between 10,000 and 8,000 cal yr BP, the SPG was extended and our study sites recorded cold temperature and light ∂18Osw values. Between 8,000 and 6,000 cal yr BP, the SPG changed shape but was still extended. This change coincided with the onset of Labrador Sea water formation, which today is associated with intensified SPG circulation. We see a major shift in the mid-Holocene, which has been documented in many other records. Between 6,000 and 4,000 cal yr BP, we see a strong influence of Atlantic water on the SW Iceland shelf, consistent with a contracted SPG. After 4,000 cal yr BP, freshwater from the Arctic Ocean and/or from glacial advance of the Greenland ice sheet appeared on the SW Iceland shelf. The water column as recorded in benthic and planktonic foraminiferal assemblages went from well mixed in the early Holocene to stratified in the late Holocene. Overall our records show that the SPG played an important and, in some ways, surprising role in the Holocene climate evolution of the North Atlantic.