Comparing MICADAS Gas Source, Direct Carbonate, and Standard Graphite 14C Determinations of Biogenic Carbonate

Abstract Northern Arizona University, Flagstaff, Arizona, USA, recently installed a MIni CArbon DAting System (MICADAS) with a gas interface system (GIS) for determining the 14 C content of CO 2 gas released by the acid dissolution of biogenic carbonates. We compare 48 paired graphite, GIS, and direct carbonate 14 C determinations of individual mollusk shells and echinoid tests. GIS sample sizes ranged between 0.5 and 1.5 mg and span 0.1 to 45.1 ka BP (n = 42). A reduced major axis regression shows a strong relationship between GIS and graphite percent Modern Carbon (pMC) values (m = 1.011; 95% CI [0.997–1.023], R 2 = 0.999) that is superior to the relationship between the direct carbonate and graphite values (m = 0.978; 95% CI [0.959-0.999], R 2 = 0.997). Sixty percent of GIS pMC values are within ±0.5 pMC of their graphite counterparts, compared to 26% of direct carbonate pMC values. The precision of GIS analyses is approximately ±70 14 C yrs to 6.5 ka BP and decreases to approximately ±130 14 C yrs at 12.5 ka BP. This precision is on par with direct carbonate and is approximately five times larger than for graphite. Six Plio-Pleistocene mollusk and echinoid samples yield finite ages when analyzed as direct carbonate but yield non-finite ages when analyzed as graphite or as GIS. Our results show that GIS 14 C dating of biogenic carbonates is preferable to direct carbonate 14 C dating and is an efficient alternative to standard graphite 14 C dating when the precision of graphite 14 C dating is not required.