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 Δ 14 C

A main caveat in the interpretation of observed changes in atmospheric Δ 14 C during the last 50,000 years is the unknown variability of the carbon cycle, which together with changes in the 14 C production rates determines the 14 C dynamics. A plausible scenario explaining glacial/interglacial dynamics seen in atmospheric CO 2 and δ 13 C was proposed recently (Köhler et al., 2005a). A similar approach that expands its interpretation to the 14 C cycle is an important step toward a deeper understanding of Δ 14 C 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 CO 2 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 CO 2 record from various ice cores during the last 120,000 years with high accuracy ( r 2 = 0.89). We analyze scenarios with different 14 C production rates, which are either constant or based on 10 Be measured in Greenland ice cores or the recent high‐resolution geomagnetic field reconstruction GLOPIS‐75 and compare them with the available Δ 14 C 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 Δ 14 C 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 Δ 14 C larger than 700‰ cannot not be explained within our framework, neither through carbon cycle‐based changes nor through variable 14 C production. Superimposed on these general trends might lie positive anomalies in atmospheric Δ 14 C 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 Δ 14 C 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 14 C in the surface waters through isotopic fractionation during higher glacial marine export production caused by iron fertilization (+∼10‰).