A model-based interpretation of low-frequency changes in the carbon cycle during the last 120,000 years and its implications for the reconstruction of atmospheric Δ14C



A main caveat in the interpretation of observed changes in atmospheric Δ14C during the last 50,000 years is the unknown variability of the carbon cycle, which together with changes in the 14C production rates determines the 14C dynamics. A plausible scenario explaining glacial/interglacial dynamics seen in atmospheric CO2 and δ13C was proposed recently (Köhler et al., 2005a). A similar approach that expands its interpretation to the 14C cycle is an important step toward a deeper understanding of Δ14C variability. This approach is based on an ocean/atmosphere/biosphere box model of the global carbon cycle (BICYCLE) to reproduce low-frequency changes in atmospheric CO2 as seen in Antarctic ice cores. The model is forced forward in time by various paleoclimatic records derived from ice and sediment cores. The simulation results of our proposed scenario match a compiled CO2 record from various ice cores during the last 120,000 years with high accuracy (r2 = 0.89). We analyze scenarios with different 14C production rates, which are either constant or based on 10Be measured in Greenland ice cores or the recent high-resolution geomagnetic field reconstruction GLOPIS-75 and compare them with the available Δ14C data covering the last 50,000 years. Our results suggest that during the last glacial cycle in general less than 110‰ of the increased atmospheric Δ14C is based on variations in the carbon cycle, while the largest part (5/6) of the variations has to be explained by other factors. Glacial atmospheric Δ14C larger than 700‰ cannot not be explained within our framework, neither through carbon cycle-based changes nor through variable 14C production. Superimposed on these general trends might lie positive anomalies in atmospheric Δ14C of ∼50‰ caused by millennial-scale variability of the northern deep water production during Heinrich events and Dansgaard/Oeschger climate fluctuations. According to our model, the dominant processes that increase glacial Δ14C are a reduced glacial ocean circulation (+∼40‰), a restricted glacial gas exchange between the atmosphere and the surface ocean through sea ice coverage (+∼20‰), and the enrichment of dissolved inorganic carbon with 14C in the surface waters through isotopic fractionation during higher glacial marine export production caused by iron fertilization (+∼10‰).