Abstract
Rhodoliths, free‐living calcareous red algae, create large and diverse habitats worldwide. Although these plants are abundant and ecologically important, little is known about their growth rate. We determined the growth rate for an individual rhodolith, Lithothamnium crassiusculum , from the southern Gulf of California through 14 C analysis using accelerator mass spectrometry (AMS) to be 0.6 mm yr −1 . This growth rate suggests large L. crassiusculum , which have been found with radii in excess of 6 cm, may live over 100 yr. Declines in the Δ 14 C record associated with the large El Niño events of 1957, 1982, and 1992 indicate 14 C analysis may lead to identification of important climate events in the more distant past. The ability to determine changes in past ocean circulation related to changes in past climatic conditions through AMS 14 C analysis of rhodoliths would increase the geographic range of available climate records from the tropical oceans to the entire global ocean and potentially allow for the determination of past climate conditions from rhodoliths in fossil beds.