Mantle viscosity constraints from U.S. East coast relative sea level histories: Implications for understanding the glacial isostatic adjustment of the North American continent

Abstract:

Models of the glacial isostatic adjustment process, which are designed to explain the influence of the Late Pleistocene cycle of glaciation and deglaciation on planetary shape and sea level, require two fundamental inputs. These consist of a model of mantle viscosity as well as a global history of ice sheet loading and unloading. Various geophysical and geological observables enable the testing and refinement of these global models: among these, geological inferences of sea level history based upon appropriate relative sea level indicators and modern space geodetic observations of uplift of the surface of the solid Earth obtained from Global Positioning System (GPS) instruments have proven to be particularly important.

High-quality relative sea level history reconstructions have been central not only to the development of existing spherically symmetric models of the internal visco-elastic structure of the mantle, but also to the development of current global ice sheet histories for the Late Quaternary, such as the most recently constructed ICE-6G_C (VM5a) model of Peltier et al. (JGR-Solid Earth, in press). In particular, observations of relative sea level evolution along the East coast of the United States (such as provided by the high-quality database of Engelhart et al. (2011, Geology)) are of special interest, given their location along a transect of the forebulge associated with the former Laurentide ice sheet.

In this paper, we examine how the misfits identified in the literature for the southern part of the U.S. East coast can be eliminated by appropriate changes in mantle viscosity structure represented by VM5a and ice sheet loading history variations represented by ICE-6G_C, focusing mainly on the sensitivity of sea level history predictions for sites along the eastern seaboard to depth-dependent mantle viscosity variations. The impact of ice sheet loading history variations upon these misfits is also discussed, and proves to be especially important in the context of maintaining an appropriate fit to the GPS observations of present-day vertical uplift over North America. In this regard, data from the US West coast proves to be especially useful as a means of confirming the validity of the adjustments to the model determined on the basis of the East coast data set.