Evaluating organic geochemical proxies for application to coastal lake sediments along the Gulf Coast of Florida for paleotempestology

Abstract:

Organic geochemical proxies (OGPs) preserved in coastal lake sediments have recently been used to reconstruct millennia-long hurricane records. Although OGPs (δ¹³C, δ¹⁵N and C/N) appear to be a more sensitive indicator of past storm events than traditional proxies used in paleotempestology, the method has not been tested in many lakes and with modern data. In this study, we measured the δ¹³C, δ¹⁵N and C/N values of particulate organic matter (POM), along with the salinities and stable isotopic compositions of water (δ¹⁸O and δ²H), in two coastal lakes on seasonal or much shorter time scales throughout a 3-year period. This was intended to develop a better understanding of the dynamics of OGPs in these lakes. The time-series data not only show that geochemical properties of these lakes varied seasonally, reflecting variations in lake biological and environmental conditions, but displayed unique variation patterns in response to large storms that caused either seawater flooding or freshwater flooding of the lakes. The data show that seawater flooding led to higher δ¹³C and δ¹⁵N values, with either lower or no change in the C/N ratios of the POM, consistent with a previously proposed conceptual model for detecting seawater flooding events. The data also show that freshwater flooding reduces δ¹⁵N and increases the C/N values of POM, in addition to lowering the salinity, δ¹⁸O and δ²H of lake water. These time-series modern data demonstrate the feasibility of detecting past storm events that were large enough to cause either seawater or freshwater flooding by examining variation patterns of OGPs in coastal lake sediment. Applying this understanding to a sediment core from one of the study lakes, we reconstructed a centuries-long record of large storms based on the variation patterns in OGPs together with ²¹⁰Pb ages. The OGP-based storm record suggests that 23 flooding events, including 11 seawater flooding and 12 freshwater flooding events, occurred over the last 165 years. All of these flooding events can be matched (within the dating uncertainty) to the historic hurricanes that are known to have made landfalls within 100 km of the study site. This further confirms that OGPs preserved in coastal lake sediments in coastal lakes of Florida or similar regions are a reliable recorder of past storm activity.