RELATIVE SEA LEVEL CHANGE AND QUANTIFYING CRUSTAL RESPONSE DURING THE LITTLE ICE AGE IN UPPER COOK INLET, SOUTH CENTRAL ALASKA, USA

Unravelling driving mechanisms of RSL and their individual contribution at tectonically active margins helps us understand tectonic process and crustal response to changes in load, define ranges of isostatic correction to apply to tide gauge and satellite data and estimates of future RSL change. We present a history of Little Ice Age RSL change in upper Cook Inlet, combining established forms of RSL reconstruction with newly developing approaches to dating recent sediments. Tectonics at subduction zones produce a unique signature of relative sea level change (RSL), termed an earthquake deformation cycle model. The dynamics of former ice sheets and glaciers that occupy coastal mountains common modify the pattern of crustal deformation, resulting in a dynamic and complex RSL history.

South central Alaska, USA, is a tectonically active region with a history of great (M_w = 8) earthquakes during the late Holocene. Glaciers in 11 mountain ranges cover approximately 75,000 km², with ~33% within the coastal Chugach and Kenai Mountains. During the Little Ice Age (LIA) temperatures fell to an estimated 2^oC below present with many glaciers reaching their maximum Holocene position at the end of the nineteenth century. Subsequent mass loss in southern Alaska and British Columbia is estimated to supply over half the contribution to global sea-level from sources outside Greenland and Antarctica during the past 50 years, and 0.27 ± 0.10 mm yr^-1 over the past decade. The LIA and the coastal margins of south-central Alaska provide an excellent context in which to test models of crustal response to short-term ice load changes in a tectonically active location.

Using a diatom based transfer function with an age model combining AMS radiocarbon wiggle match dating, ¹³⁷Cs and pollutants associated with gold mining in the region, our results show deviations away from the established earthquake deformation cycle model. When combined with geophysical model results they allow quantification of the crustal response during the Little Ice Age around upper Cook Inlet. The RSL reconstruction allows correlation with LIA ice mass change in the nearby Kenai and Chugach Mountains. These results further understanding of RSL changes in a tectonically active location.