Chlorine 36 dating method

130 000 years ago which predates the age of the oldest erratic by about 80 000 years.The deposition of these erratics clearly occurred during the last glaciation of the region.Samples were collected from nine erratics over a distance of about 200 km (Table 1).Samples were taken from upper 4 cm of the rock as fragments from hammer and chisel, from thin beds that were pried loose, or closely spaced diamond drill cores.The chlorine-36/Cl ratio was measured by accelerator mass spectrometry at PRIME Lab, . Uncertainties related to analytical factors and parameter values are probably on the order of 10%.

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The two erratics that yielded ages in excess of the Late Wisconsinan have ages that fall within a period during which there was no glaciation known as the Middle Wisconsinan.Laboratory methods followed those described by Zedra and Phillips (2000).In brief, the samples were dissolved in a mixture of HF and HNO3 and the Cl was extracted as Ag Cl.The low relief and gentle slopes of the region also make it unlikely that the erratic boulders have rolled over, except at the time of initial emplacement as ice melted beneath them.The quartzite that composes the erratics is extremely tough and resistant with little evidence of spalling along bedding planes.2 m) of these blocks make it unlikely that the present surface of an erratic was exposed to cosmic radiation prior to falling from a mountain side onto passing ice.If one side of a block had been exposed in the cliff face, then the chances range between about 0.17 and 0.50 (for blocky and equant erratics and tabular erratics, respectively) that the pre-exposed side would be the present upper surface.The rate of chlorine-36 production by cosmic rays (thermal and fast neutrons) depends on the concentrations of potassium (K), Calcium (Ca) and chlorine (Cl), the elevation of the rock, surface orientation, and geomagnetic latitude.The rate of accumulation of chlorine-36 depends on the balance between the production rate of chlorine-36 and on the rate of erosion of the rock surface.The surface one to two metres of rock shield underlying rock from most types of cosmic radiation.When rock is plucked by a glacier from below this depth range and subsequently deposited on the surface, it begins to accumulate chlorine-36 produced by the cosmic radiation.

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