Paleosecular Variation of the Geomagnetic Field in Alaska, Revisited: New Measurements from the Aleutian Islands.

As part of a cooperative program to obtain high-resolution records of the paleomagnetic field over the last 5 Ma, we have been re-investigating the paleomagnetic record in flow sequences in the Aleutian Islands. New data have been obtained from Umnak and Unalaska islands. On Umnak island Crater creek cut a continuous cliff section exposing 21 individual flows. These are all normally magnetized, and preliminary 40Ar/39Ar measurements give ages of less than 100,000 years. Towards the coast from Crater Creek is another cliff section that gives good exposure of 14 reversely magnetized flows with ages of about 1.5 Ma. Near Driftwood bay on Unalaska Island the sea has cut into the lower flanks of Makushin volcano, exposing another normally magnetized sequence of 21 flows. Most of the flows sampled give very clean demagnetization plots, with the individual samples within any given flow grouping well. A few of the paleomagnetic directions are more scattered, but in each case there is a clear correlation with low blocking temperatures as seen in the thermal demagnetization curves. The age span represented by these flow sequences is unclear. In each of them there are rubble beds between flows, but the key areas where fossil soils or other evidence for the passage of time might have been found were always buried or inaccessible. As a guide to estimating the time represented we compared the distribution of the paleomagnetic directions with the distribution that would be expected from measurements of the secular variation of the geomagnetic field over the last few centuries (assuming that the Pacific secular variation low is a transient feature). The scatter found in the two normally magnetized sections is low enough to suggest that the time-span is less than the time needed for one cycle of westward drift, i.e. less than about 1800 years. These data are part of a larger ongoing study involving similar sequences of volcanic flow units from other Aleutian sites, the Wrangel Mountains in south-central Alaska and limited flows from St. Lawrence Island in the northern Bering Sea.