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 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 δD), in two coastal lakes on seasonal or much shorter time scales throughout a 3-year period. This work 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 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, generally 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 δD 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 multiple OGPs in coastal lake sediments. Applying this understanding to a sediment core from one of the study lakes (Mullet Pond, Florida), we reconstructed a centuries-long record of storms based on the variation patterns in OGPs together with 210Pb ages. The OGP-based storm record suggests that 30 flooding events, including 15 seawater flooding and 15 freshwater flooding incidents, occurred over the last 165 years. Almost all of these flooding events can be matched (within the dating uncertainty) to the historic hurricanes that are known to have passed within 150 km of the study site. This further confirms that OGPs preserved in sediments in coastal lakes of Florida or similar regions may be used as reliable recorders of past storm activity.