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Sediment mixing and stratigraphic disorder revealed by the age-structure of Tellina shells in Great Barrier Reef sediment

Matthew A. Kosnik^*,,1, Quan Hua^*,2, Geraldine E. Jacobsen^*,2, Darrell S. Kaufman^*,3 and Raphael A. Wüst^*,4

¹ 6330 Sana'a Place, Dulles, Virginia 20189, USA
² Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australia
³ Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA
⁴ School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland 4811, Australia

Radiocarbon-calibrated amino acid racemization ages of 250 individually dated Tellina shells from two sediment cores are used to quantify molluscan time averaging with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contain a distinct, essentially modern assemblage with a median age of only 5 yr. Sediment between 20 and 125 cm are age-homogeneous and significantly older than the surface sediment (median age 189 yr). Shell age distributions within layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a subcentennial scale. Shell size is an important correlate of shell half-life and an important determinant of the inferred age of sedimentary layers. These results illustrate the importance of bioturbation in these environments, indicate that age estimates in this depositional setting are sensitive to specimen choice, and document a size-dependent bias in death assemblage formation.

Key Words: Mollusca • carbonate sediments • size distribution • time averaging • taphonomy • bio-turbation

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