Geo Hazards and Climate Change
Radiocarbon dating helps to reconstruct an 8000-year quake record that improves understanding of Alpine Fault
Radiocarbon, or 14C, is a naturally occurring radioactive carbon isotope with a half-life of 5730 years. Its presence in organic materials is the basis of the radiocarbon dating method. When an organism dies, the amount of 14C within the material slowly decreases at a known rate (through radioactive decay) relative to the non-radioactive, or stable, isotopes of carbon (12C and 13C) in the material. As a result, measuring the proportion of radiocarbon remaining in the sample compared to amount it would have had when alive provides an objective method of determining the time ranges within which the organism died. Radiocarbon dating is generally the most precise and applicable method for determining age of organic materials for the last 50,000 years, and is widely used in archaeology, geology, environmental and atmospheric studies. GNS Science Rafter Radiocarbon Laboratory operates the only Accelerator Mass Spectrometry (AMS) laboratory in New Zealand, which is also the newest AMS in Australasia. The main advantage of AMS is that it allows milligram-sized samples to be dated. This is significantly less material than is required for the conventional decay counting method, making it possible to date samples of extremely small quantity often encountered in the research environment.
In a study of a major plate boundary fault in New Zealand, the Alpine Fault, scientists from GNS Science used radiocarbon dating to determine ages of leaf and seed remains within the peat sediments that were buried by silt each time there was a major earthquake. Radiocarbon dating, combined with observation, enabled the scientists to establish a record of past earthquakes on the fault extending back in time for 8000 years and representing more than 20 fault movements with resulting major earthquakes. The scientists could see striking alternations between peat and silt, and ages of these major alternations exhibits a fairly regular cycle of stress accumulation and rupture. The findings are entirely new for the Alpine Fault, and a record of this length is very rare world-wide.
Cosmogenic nuclides in geological materials provide a valuable tool for determining the time when geological events occurred
Cosmogenic nuclides are rare isotopes that form in surface rocks through reactions induced by highly energetic cosmic rays. Cosmogenic nuclides build up in exposed rock surfaces at predictable rates, therefore the total concentration of these isotopes in a rock surface represents the length of time the surface has been exposed to the atmosphere.
This provides a valuable tool, known as surface exposure dating, for determining the time when geological events occurred, measuring erosion rates, dating landforms such as faults and volcanic sequences, and timing of landslide movements.
Dating with cosmogenic isotopes requires intensive chemistry and careful calibration, however it has significant advantages due to the number of nuclides available (10Be, 14C, 26Al, 21Ne, 3He and 36Cl). This allows almost every mineral and hence almost every lithology to be analysed. An added advantage of cosmogenic nuclide dating is its long period of applicability, since different nuclides can be used for dating different lengths of times covering the past 10 million years.
The GNS Science Cosmogenic Isotope and Radiochemistry Laboratory is a leader in the preparation and dating of geological materials using 10Be, 9Be , 7Be, 137Cs, 210Pb and Th series isotopes. We have over 30 years of experience, a state-of-the-art accelerator mass spectrometer facility, and we have established the highest quality of sample preparation, analysis and method development. We can prepare and analyse a wide range of samples including soil, lake and ocean sediments, submarine ore deposits and glacial ice.
Isotopic analysis of water samples derived from ice cores, and the air trapped in the ice, is improving our understanding of what drives climate change
The vast Antarctic ice sheets resemble detailed diaries of past climate conditions. As the snow accumulates each year it builds up into a sequence of layers that contain a surprising amount of information such as precipitation rate, temperature, wind direction and strength, as well as dust, aerosols and the greenhouse gases such as CO2 and CH4 that are trapped in air bubbles in the ice.
New Zealand’s ice coring programme, based at Victoria University Wellington and GNS Science, has been underway since 1999. Our scientists have used tritium and 32Si to aid ice core sample dating, as the analysis of an ice core is only useful if the ages of the ice layers are known. We have studied the relative amounts of the heavier isotopes of hydrogen and oxygen in the ice, which are related to the temperature at the time the snow fell. This allows scientists to create a graph of temperature against time going right back to the oldest ice in the core. We also study fossil fuel emission and greenhouse gases using black carbon measurement and isotope analysis of carbon, hydrogen and oxygen.
The studies have allowed scientists to discover that global temperatures are always related to the relative proportions of different gases. At present the levels of these gases are much higher than they ever were in the last 800,000 years (and likely much longer) due to the burning of fossil fuels. Another major discovery has been that climate change can occur very rapidly. This information is improving our understanding of what drives climate change. This also provides crucial knowledge to forecast future climate impacts on New Zealand.
For more information, please contact:
Mike Sim, Head of Department, Isotope Biogeoscience, GNS Science