The 17 to 14 kyr 14C Decline

Much debate has occurred regarding the cause of the very high 14C to C ratios reconstructed for the period 30 to 40 kyrs. A corollary problem has to do with the decline in atmospheric 14C during the deglacial interval. In particular, the Δ14C drop from +400 to +200% between the time of H1 (~17,700 kyrs) and that of the onset of the Bolling-Allerod (~14,700 kyrs) is particularly hard to explain. The ice core 10Be record suggests that a drop in cosmogenic production can explain only a small part of this decline. The remainder has been attributed to the mixing between an isolated LGM abyssal reservoir and the remainder of the ocean. But 14C measurements on LGM benthic-planktic foraminifera pairs and 13C measurements on benthic foraminifera appear to rule out this scenario. Similarly, a massive methane injection can be ruled out. While tempting, the suggestion that the half-life of 14C is seriously in error is also highly unlikely.