Thesis Defense: Carson Miller

The Master’s Thesis Defense of Carson Miller will be presented by the University of North Carolina at Chapel Hill Department of Marine Sciences and Institute of Marine Sciences. This event will be held on Monday, March 2nd at 10:00 am. The main location of this event will be in room 222 of UNC’s Institute of Marine Sciences in Morehead City, NC. The defense will be streamed live to conference room 3204 on the 3rd floor of Murray Hall on UNC main-campus in Chapel Hill, NC and online via Zoom (Use Zoom Meeting ID: 252 726 6841).

Title: Increased storminess and sea-level rise promote rapid saltmarsh transgression across low-gradient uplands.

Abstract: Saltmarsh area is decreasing globally in response to impacts from direct human-induced changes in land cover and use in addition to sea-level rise (SLR) and storms.  Those stressors are projected to increase and accelerate loss of existing saltmarsh area; however, saltmarsh upland migration (transgression) can offset losses, with the degree of offset defined by the rate of transgression. In the absence of coastal upland development, increasing saltmarsh transgression rate is commonly attributed to increasing rates of SLR and decreasing upland slope. Storms are an additional driver of saltmarsh transgression, because they introduce stress to coastal upland forests like defoliation, uprooting, and soil salinization, making upland areas more habitable for saltmarsh plant species. The aim of this study is to elucidate the contribution of storms to saltmarsh transgression by reconstructing transgression rates over the past 1000 years during stormy and non-stormy periods when sea level was rising slowly and rapidly. Transgression rates were measured using the stratigraphic record preserved at three sites in eastern North Carolina where low-gradient Pocosin (landward) grades into expansive saltmarsh (bayward). When sea level was rising < 1 mm yr^-1, saltmarsh transgression rates were an average of 2 and 8 times faster during a stormy period between 1400 and 1675 CE than a non-stormy period.  After 1865 CE when SLR accelerated to 3.2 mm yr^-1, transgression rates were an average of 7 times faster than the period of slower sea-level rise and transgression rates during stormy periods were 7 times faster than non-stormy periods. Similarly, the post 1950 historical record shows an average 28% increase in saltmarsh area during stormy decades as compared with no resolvable change during non-stormy decades.  Predictions of transgression rate, using only the paleo-upland slope and a sea level curve, are ≥ observed transgression rates prior to 1850 CE and < observed transgression rates after 1850 CE.  Furthermore, the thickness of saltmarsh peat that had colonized the upland over the past 100 years is greater than what would be predicted from the sea-level curve.  Mismatches between predicted and observed transgression rates indicates that the elevation of the upland surface is modified during transgression. Salinization and decomposition of upland vegetation from SLR and storms decreases the paleo-upland slope, increases relative sea-level rise near the upland boundary, and increases transgression rates above what would be predicted from passive inundation models.  These results suggest that along low-gradient coastlines with Pocosin upland areas storminess and sea-level rise are tightly coupled in modulating transgression rates.  Changes in storminess need be included in forecasts of saltmarsh response to climate change.