THE CONTRIBUTIONS OF KARST ISOSTASY, GLACIAL ISOSTATIC UPLIFT, AND DYNAMIC TOPOGRAPHY TO THE ELEVATION OF THE TRAIL RIDGE, FL

The present elevation of Trail Ridge, an often-presumed Pleistocene shoreline at ~70 m height, has been explained by isostatic uplift induced by mass loss from karstification of the underlying Florida Platform (Opdyke et al., 1984). However, glacial isostatic adjustment (GIA; Raymo et al., 2011), dynamic topography (DT; Rowley et al., 2013) and elastic flexure (Woo et al., 2017) also contribute to the observed topography. We investigated the combined contribution of karst isostasy (KI), GIA, and DT to the elevation of Trail Ridge by adapting a numerical model of karst-induced Airy isostatic uplift (Adams et al., 2010) to include the topographic change that incorporates gravitationally self-consistent ice-age sea level cycles since 3 Ma (Kendall et al., 2005). We treated DT as a free parameter and explored a spectrum of scenarios from dynamic subsidence (-25 m/Myr) to dynamic uplift (+25 m/Myr), consistent with observational uncertainty. At this site, GIA-induced departures from eustasy do not substantively contribute to the net karst uplift because the local relative sea level history predicted using a realistic sea level theory does not lead to more prolonged emergence (and karstification) of the Florida Platform than one would compute by adopting a eustatic trend. However, GIA contributes to the present-day elevation of the site and, since Florida is on the peripheral bulge of the Laurentide Ice Sheet, this component should be computed with a gravitationally self-consistent theory. The strongly coupled sensitivity in the predictions to geological marker age, karst efficiency and DT rate demonstrates the need for robust estimates of two of these variables in solving for the third. For example, adopting spring-efflux derived estimates of subsurface carbonate dissolution (1m/38,000 yr; Opdyke et al., 1984) and a modeled DT uplift for the northern Florida Platform (+10 m/Myr; Rowley et al., 2013) yields an age for Trail Ridge >~2.9 Ma. If robust, then Trail Ridge may contribute to a growing body of constraints on peak eustatic sea level during the mid-Pliocene warm period. In contrast, assuming a Pleistocene age for Trail Ridge (~1.5 Ma; Adams et al., 2010) and the same DT rate yields a local karst dissolution of 1 m/~17,500 yr. If robust, this can corroborate field-derived estimates of regional karst dissolution rate.