THE ISLAND UPLIFT PARADOX AND OTHER MANIFESTATIONS OF ISOSTATIC ADJUSTMENT ON GEOMORPHIC SYSTEMS

Geologists and geomorphologists back to at least G.K. Gilbert have observed the counterintuitive impacts of isostatic response to crustal loading on earth-surface systems. Ongoing research – especially the proliferation of permanent GPS and other satellite-based geodetic data – has revolutionized our understanding of the magnitudes, wavelengths, and time scales of isostatic response. For example, regional modeling of glacio-isostatic adjustment (GIA) provides a new quantitative paradigm for questions ranging from geodynamics, to geomorphic processes, to sea level. Here, we focused on isostatic impacts on coastal geomorphology, including (1) uplifted coastal terraces, (2) vertical land motion (VLM) from permanent GPS stations, and (3) testing mechanisms of isostatic response using a new raster dataset of post-Last Glacial Maximum (LGM) transgressive ocean mass (TOM). We focused on islands and coasts worldwide containing terrace staircases, which record long-term uplift, punctuated by glacio-climatic oscillations in sea level. VLM from permanent GPS on islands worldwide were assessed for data duration and quality. Predictably, locations under Pleistocene ice-sheet footprints and near convergent plate boundaries predominantly record modern uplift. But counterintuitively, 90.9% of the remaining islands worldwide (n=179) record modern subsidence, at rates averaging –1.31 mm/yr. Modern subsidence at locations of documented long-term uplift is consistent with either: long-term isostatic response to post-glacial TOM, elastic response to modern sea-level rise, or possibly pervasive anthropogenic effects such as groundwater withdrawal. We attempted to test local elastic response vs. regional isostasy such as due to asthenospheric flow. VLM from islands and other coastal sites worldwide show no statistically significant correlation with the distribution of TOM, suggesting that long-term regional isostatic compensation is ongoing. We conclude that geomorphologists and other geoscientists should recognize the large and pervasive, but often counterintuitive, impacts of isostatic response to mass redistribution at the earth surface.