Biogeochemical Alteration Effects on U-Th Dating of Pleistocene Corals

Herries, Katherine Elaine
Rubin, Ken H.
Geology and Geophysics
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U-Th geochronology has been used to determine ages of fossilized coral specimens during the late Pleistocene deglaciation. This dating technique allows for a more precise determination of specific relative sea level (RSL) events, such as meltwater pulses. Meltwater Pulse 1a (MWP-1a) has been studied at far-field RSL sites around the globe. This study focuses on a well-preserved fossil coral record from Penguin Bank, a submerged platform off the southwestern coast of Moloka‘i in the Hawaiian Islands. Unpublished Th-U data by Rubin and Fletcher (2012) show that there was a 21±3 mm/yr relative sea level change over the course of the deglaciation at Penguin Bank, off the southwestern coast of Moloka‘i, from 17 to 14.8 ka. This study has provided a more robust determination of RSL and dating fossilized coral reefs at Penguin Bank during one interval of the deglaciation centered around Meltwater Pulse 1a (defined below). The shallow portions of Penguin Bank are currently a thriving mesophotic coral ecosystem with a large coralline algae and deep sea coral population. This study analyzes the effect of these organisms on the geochemistry of fossilized coral specimens. The specimens were observed under an optical microscope and electron microprobe. Three separate textures were found: pristine primary aragonite, secondary aragonite cement, and secondary calcite cement. The abundance of secondary cements increased in areas of alterations. U-Th dates were calculated for specimens with different types of alteration, such as sponge, coralline algae, bioerosion, and discoloration. The compositional difference in pristine sections and altered sections of each coral was measured. Crustose coralline algae, sponges, and bioerosion all have varying effects on precision of calculated ages. Discoloration did not affect the calculated ages in our samples. More data is needed to fully determine the mechanisms causing these changes. The data collected in this study are in agreement with previously dated corals from Penguin Bank. There is a relationship with sea level change and coral growth and depth, until the onset of MWP-1a, where the corals continue to grow in deeper water instead of keeping up with rising seas. This data will be used in the future to refine the timing of MWP-1a at Penguin Bank and around the globe.
Geochemistry, Paleoclimate science, Marine geology, age dating, coral alteration, corals, geochronology, sea level rise, uranium series
97 pages
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