HOT AND SALTY ON THE TIDE FLAT: DETECTING INTERTIDAL POSITION USING HIGH-RESOLUTION OXYGEN ISOTOPE VARIABILITY IN BIVALVE MOLLUSK SHELLS FROM ARID REGIONS

Here we propose a method for detecting the intertidal position of contemporaneous bivalve mollusks. The basic idea is: during each tidal cycle (12 hrs. 50 mins.) high intertidal areas spend more time above sea level than low intertidal areas. As a result, residual water on the tide flat experiences significant evaporative enrichment in ¹⁸O, especially in arid regions with macro-tidal regimes. Clams that precipitate shell in this isotopically enriched water will have more positive oxygen isotope ratios (δ¹⁸O) than clams living lower on the tide flat. Differences in isotopic ratios will be greater for larger differences in intertidal position. In addition to isotopic differences related to intertidal position, the fortnightly tidal cycle will also play a significant role. During neap tides, clams living high on the tide flat will experience long periods of time (i.e., multiple days) above sea level. Thus, two factors—1) absolute intertidal position and, 2) timing within the fortnightly tidal cycle—will contribute to isotopic differences in the shells of clams living at different tidal positions. To constrain the magnitude of evaporative enrichment we conducted a pan-evaporation experiment in the northern Gulf of California. Over the course of four days we collected salinity and water samples from a five-gallon bucket. The salinity/water-δ¹⁸O relationship has a slope of 0.2 permil/PSU. Previous data suggest the actual slope may be lower (0.1 permil/PSU) because of evaporative pumping of subsurface water. Next we calculated hourly and daily water δ¹⁸O values for one complete year for intertidal positions at half-meter intervals along the entire 10-meter tidal range. Then, using these predicted water-δ¹⁸O values and daily temperature records we modeled annual shell δ¹⁸O profiles for the bivalve mollusk Chionista fluctifraga. Our modeling results suggest the approach outlined above is robust, especially for clams living in the upper half of the tidal range. To evaluate our results we collected clams with known intertidal positions from a 2.4 km intertidal transect. Preliminary results suggest actual profiles from clams living on the tide flat reflect their intertidal position. That said, care must be taken to sample at a significantly high-resolution to capture relatively small isotopic differences.