Accelerator Mass spectrometry: revealing subtle signals in ice sheets

• Main Author: Smith, AM
• Format: Conference Object
• Language: English
• Published: Past Global Changes 2015
• Subjects: Accelerators; Mass spectroscopy; Isotopes; Isotope dating; ANSTO; Antarctica; Greenland; Antarc*; ice core
• Online Access: http://apo.ansto.gov.au/dspace/handle/10238/6459

Accelerator Mass Spectrometry (AMS) determines the ratio of a rare isotope, normally radioactive and of intermediate half-life, to a stable isotope. AMS permits the detection of individual atoms in a sample and so is an inherently sensitive analytical technique. A well-known example is radiocarbon dating (14C, t1/2 = 5730 a), where measurement of the 14C/12C ratio permits determination of the age of an artifact. Such AMS measurements can be performed rapidly (~ 20 min), at good precision (~ 0.3 ‰), with high sensitivity (< 10-15) and on very small samples (as little a few μg of carbon). Radiometric measurements, by contrast, require much larger sample masses and much longer measurement times in order to obtain good precision. Besides its use as a chronometer, 14C is increasingly used as a tracer in geophysical studies as the amount of carbon required for a measurement has decreased.At ANSTO we routinely measure 14C, 10Be, 26Al and the Actinides by AMS and in 2010 we added 7Be to the list. Here I give some examples from the ice sheets in Greenland and Antarctica of palaeoclimate research I have been involved in. In each case AMS has provided the unique key to unlock these important climate archives. I will discuss 14C studies of atmospheric gases from firn air and ice core bubbles, with the objective of learning more about the natural and anthropogenic sources of the important greenhouse gas methane. Additionally, I will discuss studies of the beryllium isotopes, 7Be (t1/2 = 53 d) and 10Be (t1/2 = 1.4 _ 106 a) in snow and ice, with the objective of improving the use of 10Be as a proxy for Solar variability.