CARBON ISOTOPE CHANGES THROUGH THE RECENT PAST: F14C AND δ13C VALUES IN SINGLE BARLEY GRAIN FROM 1852 TO 2020

Radiocarbon (F14C) and stable carbon (δ13C) values were measured in single grains of spring barley (Hordeum vulgare L.) from the sample archive from two adjacent sites of the Long-term Experiments (LTEs) Hoosfield Spring Barley at Rothamsted Research (Harpenden, Hertfordshire, UK), covering the growing periods (March to September) of 1852 to 2020. F14C data of the barley grain confirm that recent values are approaching and will decline below the “nominal” F14C value of 1, tracking a similar decrease reported in other studies. Importantly, the measured δ13C values reveal a different temporal decline over the pre-bomb and post-bomb timescale. Detailed statistical analysis of δ13C data along with δ13C analysis of independent, archived barley mash samples, verifies and quantifies the extent and rate of this decline. Evidence presented from the barley grain and barley mash samples suggests a clear breakpoint in δ13C data occurring in 1995, where the rate of change alters, in that the slope in δ13C data for the pre-1995 period is declining at 1.4‰ per century, and the slope in δ13C for the post-1995 period is declining at 3.6‰ per century. Such a consistent shift in δ13C data could be used with F14C values to extend the use of the bomb peak for forensic, ecological, and environmental applications.