GROWTH RATES OF RHODOLITHS IN THE GULF OF MEXICO USING RADIOCARBON DATING

Rhodoliths are calcitic nodules formed by coralline red algae that grow unattached to the seafloor. They can form reefs, live in all depths of the photic zone, and are found globally. Determining growth rates of rhodoliths can aid in conservation efforts in modern living reefs and is necessary to utilize rhodoliths as paleoclimate proxy data sources. However, depending on internal structure, habitat, and related factors, assessing age and growth patterns is often challenging. Several techniques have been used to assess rhodolith growth rates such as staining, stable isotope profiles, and elemental mapping, but success has been variable. Here, radiocarbon dating is used to determine the age of rhodoliths collected from the Flower Garden Banks in the northern Gulf of Mexico. These rhodoliths display a “boxwork” internal structure, characterized by lack of growth increments and extensive void spaces, making it particularly difficult to assess age and growth. Two rhodoliths were analyzed using accelerator mass spectrometry and yielded radiocarbon ages of 795 and 660 ± 20 years (1537 ± 64 years cal. A.D.; 1645 ± 79 cal. A.D., respectively). With the growth axes of these rhodoliths being 35 mm and 15 – 20 mm, these ages indicate growth rates of 0.07 mm/yr and 0.04 – 0.05 mm/yr (assuming continuous growth). These growth rates are lower than previously published studies using similar methods in branching rhodoliths from the Gulf of California (0.6 ± 0.1 mm/yr) and boxwork rhodoliths from the eastern Mediterranean (0.1 – 0.4 mm/yr). The eastern Mediterranean rhodoliths contain apparent hiatuses in growth revealed by sequential radiocarbon dates. We will report data from similar sequential analyses performed on Flower Garden Banks rhodoliths. Undetected growth interruptions could explain the apparent slower rates measured here thus far. Rhodolith growth rate studies need refinement, especially those utilizing radiocarbon, and further research will aid in establishing a more standard methodology.