Radiocarbon (14C) Research at RSES

Radiocarbon is produced in the stratosphere by the collision of nitrogen atoms with thermal neutrons produced naturally by cosmic rays or artificially by atmospheric nuclear bomb testing. Atomic 14 C is rapidly oxidized to 14 CO 2 in the atmosphere and enters plants and animals via photosynthesis and the food chain. When an organism dies 14 C is no longer taken up and the 14 C decays by beta decay back to Nitrogen 14.

The half-life (the time it takes for half of the carbon-14 to decay away) is 5730 years. After about 10 half-lives there is essentially no carbon-14 left in a sample. This results in a limit of this technique of 50-60,000 years, after which other radiometric techniques have to be used to age a sample.

At the Research School of Earth Sciences, The Australian National University we have an active research program using radiocarbon to date fossils, archaeological middens, marine organisms, and various other materials. We use a Single Stage Accelerator Mass Spectrometer to measure the amounts of carbon-14.

For an in depth description of the radiocarbon dating method go to http://www.c14dating.com/

Current Research projects

Air/Sea CO 2 exchange

One of the strongest constraints on the exchange and uptake of anthropogenic carbon dioxide (CO 2 ) by the ocean is derived from systematic and secular variations of the carbon isotopic signature of the dissolved inorganic carbon (DIC) pool as a consequence of the atmospheric release (burning) of 13 C and 14 C depleted fossil fuel.

The time-history of 14 C is complicated by atmospheric nuclear weapons testing in the late 1950s and early 1960s when the amount of radiocarbon ( 14 C) in the atmosphere essentially doubled. The different time-histories of atmospheric 13 CO 2 and 14 CO 2 are a potential tool to study the rates of oceanic CO 2 uptake. Direct observations of spatial and temporal variations in both d 13 C and D 14 C of DIC are sparse, and are primarily from samples taken during GEOSECS in the 1970s, TTO/SAVE (1985) and the World Ocean Circulation Experiment (WOCE) of the 1990s. These discrete snap-shots are augmented by D 14 C time-series in biogenic archives such as coral skeletons that have been shown to be equivalent to DIC when corrected for mass-dependent fractionation using coralline d 13 C.

Unfortunately, the d 13 C time-history in coral skeletal material is complicated by vital-effects such that we can not use a dual-tracer approach to reconstruct both the 13 C and 14 C history from hermatypic reef-building corals. Estimates of the current ocean CO 2 uptake are derived from a combination of data-based estimates and ocean general circulation models. Estimates based upon d 13 C measurements are hampered due to the large natural DIC spatial and seasonal to interannual temporal variability, and the lack of high quality historical data. These measurements can be used to evaluate ocean GCMs used to predict future ocean CO 2 uptake.

We have resolved the time varying d 13 C and D 14 C surface water history recorded in the skeleton of a calcareous sclerosponge Acanthochaetetes Wellsi, from Vanua Lava, Vanuatu in the southwest Pacific (Fallon et al, 2003). We can compare the time varying response of d 13 C and D 14 C resolved in this record with that simulated in models in an effort to constrain the most parsimonious solution utilizing three parameters: air-sea exchange, vertical diffusivity, and upwelling rates.

Radiocarbon (14C) as a Tracer in Oceanography and Climate Studies

The distribution of radiocarbon ( 14 C ) in the surface ocean is a sensitive indicator of ocean circulation and can be used to track ocean currents, vertical mixing, and air-sea CO 2 exchange. Radiocarbon is produced in the stratosphere by the collision of nitrogen atoms with thermal neutrons produced naturally by cosmic rays or artificially by atmospheric nuclear bomb testing. Atomic 14 C is rapidly oxidized to 14 CO 2 in the atmosphere and is introduced into the surface ocean via gas exchange. The flux of radiocarbon to the deep ocean is accomplished by convective processes, and by settling of particulate matter. Because the residence time of water in the deep ocean is long enough to allow for significant radioactive decay ( 14 C half-life = 5730 yr), the deep ocean is depleted in 14 C relative to the surface ocean. This contrast makes the distribution of radiocarbon in the surface ocean particularly sensitive to vertical mixing.