A Millennial-Scale Oscillation in Latitudinal Temperature Gradients Along the Western North Atlantic During the Mid-Holocene

Changes in vegetation in North America indicate Holocene shifts in the latitudinal temperature gradient along the western margin of the North Atlantic. Tree taxa such as oak (Quercus) and birch (Betula) experienced opposing directions of change across different latitudes consistent with changes in temperature gradient steepness. Pollen-inferred temperatures from 34 sites quantify the gradient changes and reconstruct a long-term northward steepening in summer and southward steepening in winter. From 4.8 to 3.8 ka, an oscillation in tree distributions interrupted the long-term trends as a steep temperature gradient developed north of 43.5(degrees)N. The shift likely limited cold outbreaks to the south, producing anomalously high summer temperatures at 42-43.5(degrees)N, and enabling a northward expansion of oak forests. The forest and temperature gradient changes appear consistent with orbital and ice sheet forcing as well as millennial variability in the North Atlantic pressure field analogous to the North Atlantic Oscillation on interannual time scales.