"Only about half of the CO2 released into the atmosphere by human activities currently resides in the atmosphere, the rest absorbed on land and in the oceans. The period over which the carbon will be sequestered is unclear, and the efficiency of future sinks is unknown."
—US Carbon Cycle Research Plan
"We desire to be able to predict the future spatial and temporal distribution of sources and sinks of atmospheric CO2 and their interaction (forcing and response) with climate variability on seasonal to human timescales."
As captured in the above concepts an over-arching goal of our carbon cycle activities is to understand the mechanistic controls over the fate, transport, and residence time of carbon in the earth system's reservoirs. These mechanistic studies (laboratory and field-based) are then used to inform and ultimately, improve, models of increasing complexity to assist in predicting the role of the carbon cycle in future climate change. A simple question is whether or not the future carbon cycle will be a net source or sink of CO2 to the atmosphere. Many of the modern process and modeling studies focus on seasonal to interannual variability. However, much of the carbon on the landscape is in separate reservoirs with turnover times that are multi-decadal to millennial. It is the controls on these longer term pools or reservoirs that is a critical unknown in the face of rising greenhouse gases and climate change and uncertainties of the terrestrial biosphere as a future global sink or source of atmospheric CO2.
Radiocarbon measurements, in combination with other data, can provide insight into, and constraints on, carbon cycling. Radiocarbon (t1/2 5730yrs) is produced naturally in the stratosphere (Figure 1) when secondary neutrons generated by cosmic rays collide with 14N atoms. Upon formation, 14C is rapidly oxidized to CO and then to CO2, and is incorporated into the carbon cycle.
Due to atmospheric nuclear weapons testing, the amount of 14C in the atmosphere doubled in the mid/late 1950s and early 1960s from its preindustrial value of 14C/14C ratio of 1.18x10-12. Essentially, atmospheric weapons testing initiated a global 14C pulse-chase experiment. Following the atmospheric weapons test ban in 1963, the 14C/14C ratio has decreased due to the net isotopic exchange between the ocean and terrestrial biosphere and a dilution effect due to the burning of 14C-free fossil fuel carbon, known as the "Suess Effect."
The global 14C pulse-chase experiment (Figure 2) can be used to determine (up to multi-decadal) mean turn over times of the reservoirs of carbon in soils. Δ14C content of a homogeneous, steady state carbon reservoir with turnover times of 5, 20, 100, or 500 years is compared with that of the atmosphere through time in figure 3. Respiration of below ground pools with these mean residence times, will release CO2 with a Δ14C signature indicated by each curve. The atmospheric curve would be equivalent to the respiration of recently fixed carbon (i.e., seasonal exchange). 14C analyses of heterotrophically derived CO2 is a powerful tool to understand which reservoirs of carbon are most active and the controls (climatic, edaphic) of the stabilization of organic matter in mineral soils.