Topics, Tools and Techniques in Paleoclimate Research

Passive Tracers: Isotope Hydrology
Speaker: Tim Jull,  NSF AMS Laboratory/Physics/Geosciences
March 3rd 2004
 
Summary provided by Mike Evans

Radiocarbon is produced in the atmosphere by the interaction of secondary thermal neutrons with nitrogen nuclei, which produces the radiocarbon isotope, a positron, and energy.  Since the half-life of radiocarbon is 5730 +/-30 years, radiocarbon enters all aspects of the global carbon cycle: atmosphere, ocean, sediments, dissolved carbon species in water, and terrestrial, marine, and lacustrine biota.

Accelerator mass spectrometry technology, developed in the late 1970s and available at the Arizona facility since 1981, is a rapid, high precision technique for making radiocarbon measurements on samples as small as 0.5mg.  A graphite target, produced from CO₂ which is extracted from the source material, is bombarded with Cesium ions to produce negatively charged ions.   A magnetic field is used to deflect and therefore separate negative ions by mass.  Molecular ions, which would interfere with the radiocarbon measurement, are removed via a high voltage accelerator.  A charge stripper converts the accelerated ions to positive ions via interaction with an inert gas.  Further ion filtering is done using an electrostatic analyzer and another magnetic sector.  The ratio of current at collectors situated for monitoring carbon-12, 13 and 14 are measured, and the 13C/12C ratio is used to correct the ¹⁴C/¹²C ratio for biologically-mediated fractionation of the sample.

Radiocarbon ages are not equivalent to calendar ages because the assumption of constant radiocarbon concentration in the atmosphere is wrong.  This is due to changes in the Earth's magnetic field strength and the solar wind, and also to changes in the uptake of radiocarbon by the deep ocean circulation.  For pre-nuclear age samples, calendar ages are assigned using calibration to independently-aged materials including tree-rings (to 8Kya), corals (to 22 Kya), varved sediments and speleothems (to 45Kya).  The calibration curve is most reliable through 26Kya.  Before 45Kya, radiocarbon has decayed beyond reliable measurement detection.  Variability in radiocarbon content in the atmosphere means that some radiocarbon measurements correspond non-uniquely in time, creating additional age determination uncertainty beyond precision estimates.  In the atomic era, radiocarbon production due to atmospheric bomb testing created an artificial pulse of radiocarbon, which can be used to date modern samples and to trace its movement within the carbon cycle.

Sample treatment prior to conversion to CO₂ and graphite depend on the type of material, but are designed to isolate the radiocarbon corresponding only to the material for which a date is desired.  The offsets due to sample contamination can be, for example, 10% at 30Kya, and can skew the precise interpretation of archaeological events.  Examples of dating problems included exchange with environmental waters, the holding of ancient carbon by soil clays; and contamination of organic samples with organic compounds made from fossil fuel-age (radiocarbon-free) carbon.

Paleoclimatological applications of radiocarbon dating include

• Fire probability from charcoal ages (Yellowstone; Peace River, B.C.) over time, which can be linked to aridity cycles. There may be a larger link to Bond cycles, which are cyclic changes in ice-rafted debris found in sediments in the north Atlantic, which in turn appear driven by solar and/or deep ocean ocean circulation variability.
  
• Changes in the SE Asian monsoon from ages of Chinese loess (wind-blown sediments).  
  

Archaeological questions related to paleoclimate include:

• Did human migration, climate change, or climate-related changes in food supply cause many of the extinctions observed in the early Holocene (especially ~13Kya) in the Western Hemisphere?  Evidence from the Murray Springs, AZ, Wrangell Island, Russia, and Cuba suggested to me that it was predominantly human activity, but discussion amongst the class favored a balance between the appearance of ice-free corridors in western Canada, rising sea level vegetation changes as well as human activity.  Many of the dates for these events were almost simultaneous, and suggested to us that we might need to bring additional data or tools to bear on the question.
  

Future directions in radiocarbon research were:

• Exposure history through measurement of in-situ cosmogenic isotopes (¹⁴C, ¹⁰Be)
• Miniaturization of the AMS instrument, to expand its availability.
• Carbon cycling on Mars, using samples returned from future landers.
• Many medical and compound-specific applications.
  

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