Experimental developments and application of carbon-14 and in situ cosmogenic nuclide dating techniques

摘要

Gastropod shells are commonly preserved in the geologic record, but are often avoided for radiocarbon dating because some taxa incorporate ¹⁴C-deficient carbon during shell formation. To determine the potential of minute (3-10 mm in length) gastropods for ¹⁴C dating, we collected specimens en vivo from alluvium dominated by Paleozoic carbonates and adjacent to springs with ¹⁴C-deficient water present at the surface. We found that at least three taxa, Pupilla blandi, Euconulus fulvus, and Succineidae, do not incorporate ¹⁴C-deficient carbon during shell formation. We ¹⁴C dated fossil shells from these taxa recovered from the Coro Marl, a paleowetland deposit in the San Pedro Valley of southern Arizona. Our results indicate that the timing of high water table conditions in the valley occurred between >28 and 15.5 ka, which is consistent with moist conditions found in other paleoclimate records from the American Southwest. The summit area of Mauna Kea, Hawaii was covered intermittently by ice caps during the Late Pleistocene. The maximum extents of the last two ice caps (older and younger Makanaka) were similar, reaching ∼800 m below the summit. We have developed a new chronology for these glaciations using cosmogenic ³⁶Cl which shows that the ice caps began retreating from their maximum extents at 23.3 ± 2.3 ka and 13.0 ± 0.9 ka, respectively, broadly coincident with the last glacial maximum and the Younger Dryas chronozone. The potential for using in situ cosmogenic ¹⁴C to determine surface exposure ages of Holocene landforms, quantify erosion rates, and decipher complex exposure histories when used in conjunction with other cosmogenic nuclides is well known. Before this potential can be realized, however, protocols for isolating and extracting in situ ¹⁴C must be developed. Analytical techniques have been developed previously to isolate in situ ¹⁴C from quartz and carbonate. Although these minerals can be found in most places on Earth, they are usually absent from basaltic terrains. To fill this gap, we conducted numerous chemical pretreatment experiments and step-heated extractions aimed at isolating and extracting in situ ¹⁴C from olivine. Our results suggest that step-heated extractions alone may be sufficient to isolate in situ ¹⁴C from olivine.