Testing XRF identification of marine washover sediment beds in a Coastal Lake in Southeastern Texas, USA

This study tests the ability of a novel approach to identifying washover beds in coastal lakes. Combined X-Ray Fluorescence (XRF) and cluster analysis was used to identify hurricane washover beds in sediment cores from Clam Lake on the McFaddin National Wildlife Refuge in southeastern Texas. The lake is known to contain washover beds from recent hurricanes, but the washover sediment has similar microfossil, loss-on-ignition and textural characteristics to non-washover sediment and is not readily distinguishable. Sediment cores taken from marshes surrounding the lake do contain visually-recognizable sandy washover beds of Hurricanes Ike, Rita, Carla and Audrey. XRF analysis of these washover beds, combined with cluster analysis, was used to construct elemental fingerprints with the potential to detect washover beds in the lake. Results are promising: multiple washover beds were detected in the lake and tentatively attributed to recent hurricanes. In some lake cores, washover beds likely to be present were not detected by the XRF/clustering technique; in other lake cores, up to nine washover beds were detected. The variation in the number of washover beds probably resulted from bio-turbation, identification of two or more washover beds in a single washover deposit, and washover beds resulting from smaller storms. Valuable outcomes of this study are; 1) it confirms the presence of washover beds in the lake; 2) it provides greater insight into the number, stratigraphic position and thickness of washover deposits; 3) it identifies periods of heightened and diminished overwash activity, and 4) it provides a means of estimating the contribution of washover deposition to sedimentation in the lake. An additional unexpected finding is that long-term sedimentation rates derived from the lake and marsh cores closely match the rate of local sea-level rise, suggesting that sea-level rise may drive sedimentation in the study area

Use of Lake Sedimentary Records as an Archive for Reconstructing Fluvial Flooding and Storm Surge Events: A Case Study in Lake Charlotte, Texas

This study applies the principles and methods of paleotempestology to a 10 km inland, estuarine lake and adjacent cypress swamp in the floodplain of the Trinity River, eastern Texas. A 3.89 m sediment core (LC-3) and a 0.65 m sediment core (LC-6) were collected in Lake Charlotte, an estuarine lake in Chambers County, Texas, in the lower Trinity River floodplain, after Hurricane Harvey (2017) and in the adjacent cypress swamp after Tropical Storm Imelda (2019), respectively. LC-3, LC-6, and supplementary cores were analyzed using Loss-on-ignition (LOI) analysis, X-ray fluorescence (XRF) analysis, grain size analysis, 7Be dating, 137Cs dating, and radiocarbon dating to establish sediment stratigraphy and core chronology. Lake Charlotte exhibited evidence of delta formation since 2002 (“old delta”) and had formed a new delta after Hurricane Harvey in 2017. The cores extracted from Lake Charlotte and the adjacent cypress swamp captured sedimentary signatures of an extreme rain and surge event (Hurricane Harvey), an extreme rain event (Tropical Storm Imelda), and an extreme surge event (Hurricane Ike). Additionally, Core LC-3 preserved the history of environmental changes from the middle Holocene at Lake Charlotte. LOI analysis, XRF analysis, 137Cs dating, and radiocarbon dating of LC-3 revealed paleoenvironmental changes in Lake Charlotte. The lake was transitioning from an inland, floodplain to a lake more proximal to the Gulf of Mexico due to relative sea level rise about 7000–1200 cal yr BP. Sea level then began falling to present levels. From ~1430–1963 AD, Lake Charlotte returned to being a floodplain lake with little to no influence from the Gulf of Mexico or the Trinity River, as evidenced by very slow sedimentation rates. Between 1956 and 1961, an artificial channel was created to connect the Trinity River and Lake Charlotte. Since then, sedimentation rate in the lake has increased dramatically and grain size analysis suggests that modern deposition is coarser than previous deposition, which suggests higher-energy environment. Hurricane Harvey produced 3–4 cm of deposition and a sedimentary signature, consistent with combined impacts from storm surge and fluvial flooding in cores taken from both locations. Hurricane Ike produced 4–8 cm of deposition and a sedimentary signature consistent with other studies that show Ike to be an extreme surge event. Tropical Storm Imelda produced extensive fluvial flooding in the region as evidenced by LOI and XRF. 7Be dating was used to identify the Imelda layer and to determine a sedimentary signature for tropical cyclone-induced fluvial flooding events, which is characterized by low marine-indicating elements, high terrestrial-indicating elements, and a low Cl/Br ratio. These findings may provide the basis for determining a sedimentary signature of tropical cyclone-induced fluvial flooding events, which can be used to identify similar events in the stratigraphic record