Oxygen isotope composition of modern and archaeological otoliths from the estuarine hardhead catfish ( Ariopsis felis) and their potential to record low-latitude climate change

Shells and otoliths (fish “ear bones”) from archaeological deposits of Pine Island, southwest Florida, provide valuable archives of ecological patterns, climate change, and associated human responses. Many remains from these deposits that can be used for climate reconstruction are from estuarine animals. The challenge in using oxygen isotope composition of their carbonate hard parts lies in deconvoluting the effects of temperature and salinity (mixing of fresh- and saltwater, each having different isotopic compositions). Otoliths of the hardhead catfish ( Ariopsis felis, Linneaus, 1766) may provide a record of winter temperature without the complication of salinity variation because this fish spends winter months in waters of marine to near-marine salinities. Hence, δ 18O WATER can be constrained when employing published temperature equations. Here, we present geochemical data from modern and archaeological otoliths to evaluate whether they preserve winter temperature. A modern catfish was caught near Pine Island Sound and the otoliths (lapilli) removed. Archaeological otoliths from the 2nd/3rd century AD, falling within the Roman Optimum (RO), and the 13th/14th century AD, falling within the Little Ice Age (LIA), were obtained from the Florida Museum's collections. Oxygen isotope compositions were converted to temperature using published temperature equations and assuming + 1‰ for Gulf water. Isotopic compositions for the modern otolith range from −3.55‰ to + 0.29‰, the LIA otolith ranges from −3.92‰ to + 0.52‰, and the RO otolith ranges from −1.80‰ to + 0.84‰. All three otoliths record winter temperature (∼ 20 °C) similar to modern conditions. Calculated summer temperatures for the modern and LIA otoliths (∼ 40 °C) are overestimated and reflect the combined influence of temperature and salinity when catfish inhabit brackish estuarine waters during their reproductive season. In contrast, summer temperature estimates for the RO otolith are similar to modern conditions indicating estuarine water during the summer was close to + 1‰. This result suggests that seasonal rainfall patterns during the time period represented by the RO otolith were different than today, such that the summer wet season was not prevalent.