Growth Characterization of Mesophotic Rhodoliths in the Northern Gulf of Mexico Using Radiocarbon Dating

Boxwork rhodoliths are characterized by a multitaxonomic composition and excessive void spaces, and they are distributed from the intertidal zone to the lower limits of the photic zone. While growth rates and patterns in rhodoliths from shallower depths and simpler morphologies have been assessed using incremental and geochemical analyses, rhodoliths from mesophotic depths have rarely been studied. Furthermore, boxwork rhodoliths have proved too structurally complex for age and growth rate assessment techniques such as stable oxygen isotopes profiling or elemental mapping. Therefore, we utilized sequential radiocarbon analysis for growth characterization of mesophotic boxwork rhodoliths. Rhodoliths were collected from the Flower Garden Banks National Marine Sanctuary in the northern Gulf of Mexico from one location at 48 – 72 m and another at 200 m, spanning the range of the mesophotic zone. Specimens were extensively sampled for radiocarbon concentrations from predicted nucleation points and along growth axes. From depths of 48 – 72 m, nucleation ages ranged from 795 ± 20 to 3270 ± 25 14C ybp. From depths of 200 m, nucleation ages ranged from 8960 ± 110 to 13050 ± 150 14C ybp, dating back to the end of the Pleistocene in some cases. Several rhodoliths contained evidence of growth hiatuses, with deeper samples displaying age reversals that complicated the apparent growth history. We define age reversals to be non-sequential ages within a sampling sequence, typically a singular age before the return to normal age sequence. The research presented here includes the oldest known living rhodoliths, with the oldest rhodoliths grown before and after Holocene sea-level stabilization. These results have implications for the use of rhodoliths as paleoenvironmental indicators, and provide better understanding of the range and volume of carbonate production in these habitats.