Bulk Stable Isotopic Analyses of Shells from Seahorse Key Midden, FL

Sampling

All shells from Units 2 and 3 were individually sectioned and polished prior to microanalysis. Bulk shell aragonite powders were collected using the microsampler to drill a 1mm-wide by 150µm deep transect across the polished shell surface. The outer prismatic and inner nacreous layers were not sampled. Approximately 200 µg of powder was collected from each shell transect.

 

Results

The stable isotopic composition of these bulk samples were measured and the results are shown below. Units 2 and 3 from the North Key Midden represent an interval spanning approximately 300-800A.D. The error bars reflect an analytical error of approximately +/- 0.10 per mil.

The average d18O of a modern clam collected from this area was -0.39 permil. The historical interannual SST variability at this site is ±1.0°C (2-sigma) based on Alexei Kaplan's ~150-year Global SST analysis, or about ±0.2 per mil.

Note that most of the bulk shell isotopic values from Seahorse Key Shell Midden (300-800A.D.) fall within the modern (historical) interannual SST variability range, suggesting that latest Holocene SSTs here between 300-800 A.D were not very different from modern values. The mean bulk shell isotopic value is -0.36 per mil.

Many samples have significantly higher or lower values than the 2-sigma range which indicates multi-year to decadal periods of extreme SST and/or salinity conditions. An approximate scale for these anomalies, calculated as purely SST or purely salinity signals, is shown to the right of the figure.

Click on the data points below to see a scanned image of the shell the sample was taken from.