Holocene sea level instability in the southern Great Barrier Reef, Australia: high-precision U-Th dating of fossil microatolls

Three emergent subfossil reef flats from the inshore Keppel Islands, Great Barrier Reef (GBR), Australia, were used to reconstruct relative sea level (RSL). Forty-two high-precision uranium-thorium (U-Th) dates obtained from coral microatolls and coral colonies (2 sigma age errors from +/- 8 to 37 yr) in conjunction with elevation surveys provide evidence in support of a nonlinear RSL regression throughout the Holocene. RSL was as least 0.75 m above present from similar to 6500 to 5500 yr before present (yr BP; where "present" is 1950). Following this highstand, two sites indicated a coeval lowering of RSL of at least 0.4 m from 5500 to 5300 yr BP which was maintained for similar to 200 yr. After the lowstand, RSL returned to higher levels before a 2000-yr hiatus in reef flat corals after 4600 yr BP at all three sites. A second possible RSL lowering event of similar to 0.3 m from similar to 2800 to 1600 yr BP was detected before RSL stabilised similar to 0.2 m above present levels by 900 yr BP. While the mechanism of the RSL instability is still uncertain, the alignment with previously reported RSL oscillations, rapid global climate changes and mid-Holocene reef "turn-off" on the GBR are discussed