EP029-0001
Strategies to improve geochronologic constraints on coral microatolls in the Philippines

Thursday, 10 December 2020
Poster
Andrew Paul Mitchell1, Joanne TY Lim1, Anandh Gopal1, Aron J Meltzner1,2, Andrew M Chan1,3, Gina Muthia Sarkawi1,2, Xinnan Li1, Ace Cantillep4, Loraine Faye Sarmiento4, Junki Komori5, Tsai-Luen Yu6, Chuan-Chou Shen7, Shou-Yeh Gong8, Jennifer Weil-Accardo1, Kathrine Valdez Maxwell1, Lin Ke1, Yanbin LU1, Xianfeng Wang1,2 and Noelynna Tuazon Ramos4, (1)Nanyang Technological University, Earth Observatory of Singapore, Singapore, Singapore, (2)Nanyang Technological University, Asian School of the Environment, Singapore, Singapore, (3)California Institute of Technology, Pasadena, CA, United States, (4)National Institute of Geological Sciences, University of the Philippines, Quezon City, Philippines, (5)The University of Tokyo, Tokyo, Japan, (6)National Taiwan University, High-precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geoscience, Taipei, Taiwan, (7)National Taiwan University, High‐precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geoscience, Taipei, Taiwan, (8)National Museum of Natural Science, Geology, Taichung, Taiwan
Abstract:
Coral microatolls allow for the reconstruction of relative sea level (RSL) and the inference of tectonic deformation along tropical coastlines over the Holocene. Microatolls track RSL with unparalleled vertical precision, and their annual banding allows us to count years precisely over an individual coral’s lifetime; however, absolute age estimates of fossil corals still suffer from limitations in radiocarbon (14C) and uranium-thorium (230Th) dating techniques.

After extracting vertical slabs through multiple coral microatolls at sites in Ilocos Region, northwestern Luzon, Philippines, we extracted multiple samples for 14C and 230Th dating from each slab. Because the number of annual bands separating any two samples in a coral is precisely countable, we can readily compare the various dates from an individual slab; after adjusting for the numbers of intervening bands, all dates from a coral are treated as independent (redundant) estimates of the outer preserved band on each coral slab. If the stated lab errors are valid and if the assumed corrections (the radiocarbon marine reservoir correction, ∆R, for 14C and the initial 230Th/232Th ratio for 230Th methods) are reasonable, then 95% of the dates should be within 2σ of the true age, and 99.7% should be within 3σ. In contrast, after excluding samples flagged by x-ray diffraction or other indicators as having likely undergone diagenesis, paired 230Th dates do not overlap even within 4σ in 4 of 8 cases. Although calibrated 14C dates are less precise than reported 230Th dates, their 2σ probability distribution functions overlap in 8 of 9 cases, with the simple assumption that ∆R = 0 yr for all samples. Additional analyses are pending, but for mid-Holocene corals of northwestern Luzon, 14C techniques appear more reliable. We are working to determine if any 230Th dates are reliable enough to serve as independent chronometers to constrain ∆R and its variability over time.

Key words:

RSL, coral microatoll, radiocarbon, uranium-thorium

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