210Pb and 14C as Indicators of Callianassid Bioturbation in Coral Reef Sediment

Abstract

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.