TESTING THE APPLICATION OF PIR-IRSL LUMINESCENCE DATING FOR MARINE TERRACES ALONG THE SAN DIEGO COAST, CALIFORNIA

Recent luminescence dating of marine terraces in California (e.g., Kelty and Onderdonk, 2022, Morel et al., 2021) has resulted in younger terrace ages than previous age estimates based on other dating techniques. To test the usage of post-infrared infrared stimulated luminescence (pIR-IRSL) for marine terrace ages along the California coast we sampled three sites in the San Diego area, where previous U-series dating of solitary corals constrain the ages of the two youngest marine terraces (Muhs et al., 2002). At La Jolla, two samples of paleo beach deposits from the Bird Rock Terrace yielded ages of 75.5 ± 10.1 ka and 50.7 ± 6.7 ka. The older (stratigraphically lower) sample was collected 20 cm above the Bird Rock Terrace wave-cut platform and agrees with previous U-series dates of ~80 ka. A fading test on the lower sample showed no fading. However, the younger (higher) sample collected approximately 70 centimeters above the paleo wave-cut platform disagrees with previous U-series dating. At Sunset Cliffs we sampled paleo-beach sediment approximately 30 centimeters above the Bird Rock Terrace wave-cut platform. This sample yielded an age of 43.5 ± 5.7 ka, which is also significantly younger than the 80 ka U-series dates. We are evaluating possible causes for these younger pIR-IRSL ages including fading of the feldspar signal and/or influence from soil development on the Bird Rock Terrace. To evaluate the ~120 ka Nestor Terrace, we dated two samples from a tombolo at Crown Point that was previously correlated to the Nestor Terrace. We obtained dates of 75.2 ± 9.6 ka and 55.7 ± 7.9 ka. The younger (higher) sample showed strong evidence of bioturbation, and we therefore disregard the ~55 ka age. The older (lower) sample does not show any evidence of bioturbation, but a fading test showed fading at a rate of 1.8% per decade. We are currently evaluating this fading to see if it can account for the age discrepancy. Although one pIR-IRSL date from the Bird Rock terrace at La Jolla was in agreement with previous dating, our pIR-IRSL dates from Crown Point and Sunset Cliffs do not agree with previous U-series ages. Ongoing work will focus on evaluating these discrepancies in terrace ages from different dating methods to help refine the ages of California marine terraces.