The Effects of Tropical Cyclone-Generated Deposition on the Sustainability of the Pearl River Marsh, Louisiana: The Importance of the Geologic Framework

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.

Holocene environmental history of a freshwater wetland in southern Louisiana: a sedimentary record of delta development, coastal evolution and human activity

“…Elemental concentrations (e.g. metal elements) in marsh generally decrease downstream (McCloskey et al ., 2018), while metals (Zr, Ti), mainly derived from igneous rocks (Dill, 2010), are associated with fluvial processes (Dickinson & Gehrels, 2009). Since Br associates with increased localised organics (Ryu et al ., 2018), a high Zr/Br ratio can determine a terrestrial source of inorganic sediment.…”

Testing XRF Discrimination of Marine and Terrestrial Flood Deposits in Southeastern Texas Coastal Marshes

“…Previous studies have successfully used XRF analysis to identify marine indicators in coastal sediments, e.g., chlorine (Cl), calcium (Ca), strontium (Sr), and zirconium (Zr) (Ercolani et al, 2015;Liu et al, 2014;McCloskey et al, 2018aMcCloskey et al, , 2018bRamírez-Herrera et al, 2012;Yao et al, 2019Yao et al, , 2020, as well as terrestrial runoff (e.g., iron (Fe), titanium (Ti), and potassium (K) in sedimentary records (Haug et al, 2001;van Soelen et al, 2012;You, 2008). In particular, the chlorine/bromine (Cl/Br) ratio has been used as a marine indicator in sediment-stratigraphic studies in coastal and estuarine environments (Liu et al, 2014;McCloskey et al, 2018aMcCloskey et al, , 2018bYao et al, , 2019Yao et al, , 2020. When a solid or liquid sample is excited by a primary X-ray source, elements present in the sample produce a fluorescent X-ray that is unique for that specific element.…”

A 1200-year history of environmental changes in Bay Jimmy area, coastal Louisiana, USA

“…Hurricanes normally deposit anomalous sediments through either overwash or flooding. Observations of such anomalous layers have been made in storm deposits from multiple sites that have been impacted by intense storm events (Bianchette et al, 2017;Braun et al, 2017;Liu et al, 2011;McCloskey et al, 2018;Reese et al, 2008;Ryu et al, 2018). The storms disturbed the vegetation in coastal Louisiana as is seen in the diminished pollen percentages of marsh vegetation such as Poaceae, Typha, and Cyperaceae accompanied by an increase in Cheno/Am, which are disturbance indicators.…”

“…However, some studies suggest that the impact of a storm may not be uniform across marsh environments. For instance, McCloskey et al (2018) found large unsprouted seeds and rhizomes beneath a storm layer that had been deposited by Hurricane Katrina. McCloskey et al (2018) suggest that at their Pearl River Marsh site, in Louisiana, Hurricane Katrina did not erode a significant amount of material, but it deposited sediments.…”

“…For instance, McCloskey et al (2018) found large unsprouted seeds and rhizomes beneath a storm layer that had been deposited by Hurricane Katrina. McCloskey et al (2018) suggest that at their Pearl River Marsh site, in Louisiana, Hurricane Katrina did not erode a significant amount of material, but it deposited sediments. Similar observations have been made by Bianchette et al (2016) who found evidence of a significant increase in the rates of sediment accretion following Hurricane Isaac (2012).…”