Abstract
Shoreline retreat is a tremendously important issue along the coast of the northern Gulf of Mexico, especially in Louisiana. Although this marine transgression results from a variety of causes, the crucial factor is the difference between marsh surface elevation and rising sea levels. In most cases, the primary cause of a marsh’s inability to keep up with sea level is the lack of input of inorganic material. Although tropical cyclones provide an important source of such sediment, little effort has been made to determine the point of origin of the deposited material. In this study we use sedimentary, geochemical and biogeochemical data to identify the bed of the Pearl River and/or Lake Borgne as the source of a ~5 cm thick clastic layer deposited on the surface of the Pearl River marsh on the Louisiana/Mississippi border. Radiochemical chronologies and sedimentary evidence indicate that this layer was associated with the passage of Hurricane Katrina in 2005. As this material would otherwise have been lost to the system, this deposition indicates a net gain to marsh surface elevation. Accretion rates, determined from 137Cs and 14C profiles and the use of the Katrina layer as a stratigraphic marker, indicate that short-term (~50 years) rates are as much as an order of magnitude higher than the long- term (1000s of years) rates. We suggest that the marsh’s geologic setting in an incised river valley with steep vertical constraints and a large fluvial discharge, promotes rapid accretion rates, with rates accelerating as the sea moves inland, due to extended hydroperiods and the input of clastic material from both the marine and terrestrial sides. These rates are especially large when compared to accretion occurring in the more common open marshes fringing the Gulf that lack fluvial input. The difference is particularly large when related to marsh recovery/regrowth following the deposition of thick hurricane-generated clastic layers. Given the number of similar incised river valleys along the Gulf Coast, we believe that understanding the processes controlling marsh accretion in such environments is essential in evaluating marsh sustainability on a regional basis.
Highlights
Sea level variability is a major concern globally
In this study we investigate accretion rates from the Pearl River (Louisiana) bayhead delta and compare recent (∼ last 50 years) accretion rates to rates observed throughout much of the Holocene
Following the guidelines of Swarzenski (2015) and Corbett and Walsh (2015) and references therein, we evaluated the utility of the Constant Rate of Supply (CRS), Constant Flux: Constant Sedimentation (CF:CS), and Constant Initial Concentration (CIC) models to establish chronology from excess 210Pb (210Pbxs) data
Summary
Sea level variability is a major concern globally. Along the northern coast of the Gulf of Mexico relative sea level rise (rSLR) has resulted in severe coastal erosion and rapid land loss (Day et al, 2007). Between 1932 and 2010 the state of Louisiana lost ∼4,800 km of land (Couvillion et al, 2011), with projected loss of up to ∼13,500 km by 2100 (Blum and Roberts, 2009; Moser et al, 2014). This loss of land has already resulted in significant landward migration of the population and infrastructure (Dalbom et al, 2014; Lam et al, 2016; Cai et al, 2018; Colten et al, 2018), including current government-subsidized resettlement of a threatened community Simms, 2018). Modeling studies generally predict an increase in both the number of intense storms and the maximum intensity of future tropical storms under global warming (Bender et al, 2010; Murakami et al, 2012; Emanuel, 2013; Holland and Bruyere, 2014), highlighting a large potential future increase in the societal costs of coastal erosion
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