354-10 Triple Oxygen Isotopes in Pomacea Shells as a Proxy for Seasonal Hydroclimate and Evaporation.

Abstract:

Freshwater mollusk shells have the potential to preserve sub-annual records of hydrologic variability, but the influence of evaporation on triple oxygen isotope signals recorded during shell growth remains poorly constrained. Here, we evaluate δ′¹⁸O and Δ′¹⁷O in Pomacea shell carbonate as a proxy for seasonal changes in surface-water isotope composition, evaporation, and hydroclimate. We combine modern lake-water and shell carbonates from Florida with stream and rain-water measurements and modern to archaeological Pomacea shells from Belize spanning approximately last 10,000 years.

We micro-sampled shell carbonate sequentially along growth increments from multiple modern and archaeological specimens to generate intra-shell δ′¹⁸O and Δ′¹⁷O profiles. The Belize shells exhibit repeated isotopic variability consistent with seasonal changes in hydrologic conditions. Measured shell carbonate Δ′¹⁷O values range from approximately −20 to −130 per meg (VSMOW), while δ′¹⁸O values range from ~ −7.5 to −4.5‰ (VPDB). Reconstructed water compositions based on the shell carbonate measurements range from approximately −6 to −3.5‰ in δ′¹⁸O (VSMOW) and −25 to 55 per meg (VSMOW) in Δ′¹⁷O. Stream and rain-water measurements provide an additional constraint on the modern Belize hydrologic system and allow evaluation of the role of evaporation of meteoric water.

To better constrain the evaporation signal, we analyzed monthly lake waters and a modern Pomacea shell from Florida. Lake-water δ¹⁸O varies seasonally from 0.03 to 5.36‰ (VSMOW), while measured Δ′¹⁷O values range from approximately −43.5 to +8.1 per meg (VSMOW), indicating substantial seasonal evaporative modification of lake water. Preliminary measurements from a modern micro-sampled Florida shell yield δ′¹⁸O values of −0.87 to −0.47‰ (VPDB) and Δ′¹⁷O values of approximately −149 to −69 per meg (VSMOW). Continued micro-sampling of this shell will test how seasonal changes in lake-water evaporation are transferred into shell carbonate.

Paired water–shell observations from Florida provide a modern framework for interpreting the longer Belize shell record. Our results suggest that combining δ′¹⁸O and Δ′¹⁷O in Pomacea shells may help distinguish variations associated with meteoric-water input from lake or stream- water evaporation, providing a new sub-annual proxy for reconstructing seasonal hydroclimate and hydrologic balance in tropical and subtropical environments. Applied to archaeological shells from Belize, this approach has the potential to resolve changes in Holocene hydroclimate seasonality that are difficult to capture using conventional lower-resolution archives.

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