Comparing MICADAS gas source, direct carbonate, and standard graphite ¹⁴C determinations of biogenic carbonate

Northern Arizona University, Flagstaff, Arizona, USA, recently installed a MIni CArbon DAting System (MICADAS) with a gas interface system (GIS) for determining the C-14 content of CO2 gas released by the acid dissolution of biogenic carbonates. We compare 48 paired graphite, GIS, and direct carbonate C-14 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 C-14 yrs to 6.5 ka BP and decreases to approximately +/- 130 C-14 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 C-14 dating of biogenic carbonates is preferable to direct carbonate C-14 dating and is an efficient alternative to standard graphite C-14 dating when the precision of graphite C-14 dating is not required.