Strategies to improve geochronologic constraints on coral microatolls in the Philippines

TitleStrategies to improve geochronologic constraints on coral microatolls in the Philippines
Publication TypeConference Paper
Year of Publication2020
AuthorsMitchell AP, Lim JTY, Gopal A, Meltzner AJ, Chan AM, Sarkawi GM, Li X, Cantillep A, Sarmiento LF, Komori J, Yu T-L, Shen C-C, Gong S-Y, Weil-Accardo J
Conference NameAmerican Geophysical Union Fall Meeting
Date Published12/2020
Conference LocationOnline
Keywordscoral microatoll, Radiocarbon, RSL, uranium-thorium
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.