Nuclear Methods in the Carbon Cycle

Jenolan Caves, New South Wales.

In this project, two significant nuclear techniques are being developed and applied to carbon cycle dynamics. Carbon isotopic dating and tracing techniques enhance existing research in cave system processes for palaeo-climate assessments.

Neutron Activation Soil Analysis (NASA) and ¹⁴C dating of soil and water carbon (H-bomb pulse and beyond) provide researchers with additional information in modelling climate change and other issues.

The storage of carbon in soils, carbon breakdown rates and response to climate change variables and rainfall is an area of significant uncertainty, with potentially large feedbacks in climate change models. Efficient use of water and fertiliser is important as human populations increase and arable land diminishes due to various forms of land degradation.

Land use management strategies, such as reforestation, crop rotation and carbon sequestration are dependent upon accurate assessment of soil hydraulic properties, carbon and nutrient responses to land use change.

Neutron Activation Soil Analysis (NASA)

Soil sampling.

NASA is able to measure the top 50cm of soil for carbon, moisture, elemental composition (including C, H, Si, Fe, Al, N, S, Cl, etc.), and density by neutron activation methods.

Neutron Activation (NA) soil measurements provide:

• Quantitative measurements of soil composition including soil, carbon, water, density and elements
• Greater ground coverage (2D mapping, thousands to 100s of thousands of measurements) compared with existing point sampling and lab analysis, which is expensive and time consuming,
• The ability to directly measure important soil attributes, such as water content, carbon elements and density, rather than rely on empirical relationships which are dependent on the range of the calibration samples
• The ability to measure properties of the soil profile, as opposed to just the surface and large homogenised sample volume ± 1m3
• Links between airborne or satellite data scale to ground-point data
• Rapid, non-destructive, in-situ, repeatable time-series measurements for variable properties such as soil moisture and fertiliser leaching,
• Quantitative proximal sensing methodology allowing automation
• Low cost per measurement.

Data download from weather station.

Neutron Activation (NA) soil measurements do not provide:

• Speciation of the carbon (charcoal, mineral carbonate, reactive organic carbon) or other compounds
• Satisfactory measurement of some elements with poor sensitivity due to small neutron activation cross-section, gamma spectral interferences or low abundance.

Bomb-pulse measurements

Carbon-14 is continually produced in the Earth's upper atmosphere by cosmic ray bombardment. This ^14C replenishment keeps the concentration of 14C in the atmosphere near constant, defined as 100 per cent modern carbon (pMC) .

H-bomb weapons testing in the 1950s and 1960s released ¹⁴C into the atmosphere, causing a spike in 14C to ~160 pMC. Today, atmospheric ¹⁴C has diminished to around 108 pMC because of CO2 exchange with oceans (not because of radioactive decay).

This spike in ¹⁴C can be used as a dating tool for the past 50 years. Dead plant material, such as leaf litter and wood degrade on the soil surface and underground by biological attack (releasing CO₂ and CH₄ gas as well as soluble organic compounds) leaving behind residual carbon that breaks down more slowly. Measuring the ¹⁴C of specific carbon species in this degradation chain, separated by Gas Chromatography, is now possible.

Studies of cave environments

At Jenolan Caves, 100km west of Sydney, an intensive cave research study is underway to quantify the diurnal and seasonally-variable cave decoration (speleothems, or stalagmites and stalagtites) growth pattern and links to external weather and climate.

Long records of past climate are possible to interpret from isotopic and trace element abundances in speleothems. While there are many good qualitative speleothem palaeo-climate records, a universal quantitative transfer function from external weather/climate to speleothem record is elusive.

Drip water sampling for ¹⁴C and ³H.

Studies of cave environments and speleothem growth are an important step towards quantitative speleothem palaeo-climate interpretation. Net accumulation of CaCO3 (speleothem growth) requires a disturbance to Gas-Aqueous-Solid equilibrium conditions in the cave environment (Aqueous chem., T, P, pCO2).

The largest equilibrium change in a ventilated cave environment causing speleothem growth is fluctuating pCO₂ as a response to the cave air exchange, driven by external temperature. Continuous (five minute) cave chamber monitoring of CO₂ concentration illustrates the temperature contrast driven ventilation pattern with external air, while continuous ¹³C of the CO₂ in the cave atmosphere shows the speleothem growth pattern.

Key collaborations and locations

ANSTO works closely with The Commonwealth Scientific and Industrial Organisation (CSIRO) Exploration and Mining division on the development of NASA equipment and CSIRO Land and Water in deploying this new technology.

CSIRO Land and Water is also developing complementary techniques (IR reflectance, XRF, radiometric) for soil parameter mapping. The ANSTO/NASA and CSIRO rover equipment will be trialled at research sites across Australia. The University of Wollongong atmospheric chemistry group provides FTIR isotopic gas analysis expertise for studying cave environments.

Key contact:

Dr Chris Waring Task Leader
Institute for Environmental Research
Australian Nuclear Science & Technology Organisation
PMB 1, Menai, NSW, Australia, 2234
T: +61 2 9717 9045
E Chris.waring@ansto.gov.au

Team members:

Dr Geraldine Jacobsen
Dr Quan Hua
Stuart Hankin
Mark Peterson

Publications

A list of current publications can be found here: