ABSTRACT

Callianassid shrimps are the dominant endofauna of reef-top sediments on John Brewer Reef, a small platform reef in the central section of Australia's Great Barrier Reef Province. Sediment reworking by these shrimps was investigated using the ²¹⁰Pb radioisotope in conjunction with ¹⁴C radiometry and textural data. ²¹⁰Pb analyses of cores from two reef-top sites revealed a classical tripartite subdivision characteristic of depositional settings in which sediment reworking dominates over sediment accumulation: (1) A vertical surface mixed layer (SML) extending to a depth of 50 cm beneath the sediment surface and representing a zone of intensive callianassid reworking; (2) An intermediate region of radioa tive attenuation in which excess ²¹⁰Pb activity decreases logarithmically to a depth of slightly more than one meter beneath the sediment surface, and interpreted as representing a region utilized less frequently by Callianassa for coarse grain storage and predator avoidance; (3) A basal region, beyond callianassid influence, of consistently low background levels of ²¹⁰Pb activity.

Sediment turnover rates in excess of 120 cm² yr^-1 in the SML and up to 32 cm² yr^-1 in the region of radioactive attenuation were high compared to those typical of siliciclastic continental shelf and deep sea deposits, emphasizing the intensity of callianassid reworking in shallow coral reef environments. ²¹⁰Pb geochronology is unsuitable for determining sediment accumulation rates in such settings, because the disruptive impact of Callianassa gives rise to unacceptably high figures. The depth of callianassid influence established by ²¹⁰Pb radiometry corresponds to points of inflection, indicating the bases of vertical surficial sections, in ¹⁴C subsurface age profiles for the two John Brewer Reef sites Inflection points in other ¹⁴C profiles may similarly indicate sediment reworking depths at locations across the reef where ²¹⁰Pb data were not obtained.