the florida state university college of arts and sciences evolution of ...

emoved without an additional input of energy (Aitken, 1998). If the defect is large enough,
electrons can be held for long periods of geologic time. Therefore, over time, defect sites
accumulate electrons. The magnitude of the accumulation is therefore a function of time and the
production rate of the trapped electrons. This then forms the basis of a geochronologic method.
The trapped electrons are released from the defects by exposure to light or heat. Not all electron
traps have an equal probability of being emptied (Huntley, 1985). Such differences could be a
result of attenuation of light while passing through a mineral grain or a result of differences in
the physical properties of the traps (Huntley, 1985). The de-trapping process produces light, or
luminescence, representing the release of photons as the electrons return to the ground state. The
amount of light emitted is proportional to the number of trapped electrons, and thus the radiation
dose accumulated since the sediment was last exposed to light (Huntley and Lian, 1999). If the
de-trapping mechanism is light energy, the method is referred to as optically stimulated
luminescence (OSL). At the time of sedimentation, the luminescence signal that was acquired in
the past is reset to zero. This zeroing is a response to the sunlight exposure that occurs during
erosion and transport. After deposition, exposure to ambient radiation then repopulates the traps
(Huntley and Lian, 1999). This repopulation continues until the sediment is heated in the lab, or
exposed to sunlight during another sedimentation cycle.
OSL was developed in 1985 (Huntley et al., 1985) as a method of determining when
sediments were last exposed to sunlight. The method involves exciting the trapped electrons in
the sediment by shining laser light on grains isolated from a sample and measuring the
wavelength and intensity of light emitted in response.
To calculate OSL ages, the paleodose is divided by the annual dose rate. The dose rate is
the rate at which energy is absorbed by the mineral grain from the incoming flux of radiation
(Aitken, 1998). Its components are environmental beta and cosmic radiation, external and
internal beta radiation from K and Rb in the crystal lattice and internal alpha radiation from U
and Th embedded in the grains (Mejdahl and Christiansen, 1994). This is shown schematically
in Figure 3.4. There are also minor contributions to the dose rate from gamma radiation. The
dose rate is determined by measuring the radioactivity of the surrounding sediment. The
paleodose is the amount of radiation received since the traps were last emptied during a
bleaching, or zeroing, event (Smith et al., 1986). The laboratory equivalent of the paleodose is
the equivalent dose (DE) which is the laboratory dose of radiation that is needed to induce a
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