New records of paleoenvironmental change from two lakes near Cordova, south central Alaska, combined with analysis of previously reported sediment sequences from the adjacent Copper River Delta, provide quantitative constraints on a range of Earth system processes through their expression in relative sea-level change. Basal sediment ages from Upper Whitshed Lake indicate ice-free conditions by at least 14,140-15,040 cal yr BP. While Upper and Lower Whitshed Lakes provide only upper limits to relative sea-level change, interbedded layers of freshwater peat and intertidal silt extending more than 11 m below present sea level in Copper River Delta indicate net submergence over the last 6000 years and multiple earthquake deformation cycles. In contrast, Lower Whitshed Lake, situated just above present high tide level, records only one episode of marine sedimentation, commencing AD 1120-1500, that we interpret as the result of isostatic subsidence due to Little Ice Age mass accumulation of the Chugach Mountain glaciers. Lower Whitshed Lake also records isostatic uplift at the end of the Little Ice Age before the end of marine sedimentation caused by ~1.5 m coseismic uplift in the great Alaska earthquake of AD 1964. We successfully explain the records of relative sea-level change from both Copper River Delta and the Whitshed Lakes by integrating the effects of eustatic sea-level rise, glacial isostasy, earthquake deformation cycles, sediment loading, sediment compaction and Late Holocene changes in glacier mass into a single model. This approach provides initial quantitative constraints on the individual contributions of these processes. Taking reasonable estimates of eustasy, post-Last Glacial Maximum and Neoglacial glacial isostatic adjustment and a simple earthquake deformation cycle, we demonstrate that sediment loading and sediment compaction are both contributors to relative sea-level rise at Copper River Delta, together producing subsidence averaging approximately 1.2 mm yr-1 over the mid to Late Holocene. Further isolation basin studies have the potential to greatly improve our understanding of the individual contributions of these processes in this highly dynamic region.
Bibliographical noteFunding Information:
Scott Anderson, Hannah Bailey, Katie Detrich and Joe Licciardi assisted in the field, and Scott Anderson, Katherine Cooper, Michael Ketterer and John Southon provided supporting laboratory analysis. Fieldwork and geochronology was funded by the US National Science Foundation ( EAR-0823522 and ARC-0909332 ). Eyak Corporation kindly allowed access to their land. The modelling was developed through U.S. Geological Survey, Department of the Interior, earthquake hazards projects awards G09AP00105, G12AP20084 and G13AP00051 (the views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government). We thank Thomas James and an anonymous reviewer for their constructive comments and Jeff Freymueller for his editorial input, which has greatly improved this manuscript. This paper is a contribution to IGCP project 588 “Preparing for coastal change: A detailed process–response framework for coastal change at different timescales” and we thank participants of the IGCP 588 5th Annual Meeting, Cordova, Alaska, 2014 for their comments on this work.
© 2014 Elsevier Ltd.
Copyright 2015 Elsevier B.V., All rights reserved.
- 1964 alaskan earthquake
- Isolation basin
- Little ice age
- Relative sea level
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