Geol Soc Aust Symp, (1986), 13, 53-67
Sediment generation by Halimeda meadows throughout the northern Great Barrier Reef Province.
Edward A Drew and Kay M Abel
Australian Institute of Marine Science, PMB No 3, MSO, Townsville,
Queensland 4810, Australia
Abstract
Extensive areas of coarse gravels composed largely of Halimeda fragments have been reported on the outer shelf of the Great Barrier Reef north from Cooktown to the limit of the Province (Maxwell, 1973; Orme et al. 1978; Marshall & Davies, 1985). All these geological studies have hinted at the presence of lush Halimeda meadows growing on the gravels, so we have examined both the inter-reefal sediments and the biota they support throughout that area. Using Van Veen grabs, benthic dredges and SCUBA divers, we have confirmed that the coarse gravels are composed of up to 98 percent Halimeda fragments, and they occur from 15^35'S to 11^35'S, supporting a substantial algal biomass dominated by two species of Halimeda. This 380 km strip is broken only at Princess Charlotte Bay and represents about 2,000 km2 of Halimeda meadows, but they do not extend north of Blackwood Channel, contrary to indications in Maxwell (1973).
We are currently assigning the dominant Halimeda species to H. copiosa and H. opuntia var hederacea but there are taxonomic problems with the latter. These are the same taxa which are dominant on reefal platforms (Drew, 1983) and grow together with ten other Halimeda species in the meadows, as well as the closely related calcareous green algae Udotea and Penicillus. The genus Caulerpa is also conspicuous in some localities. Total Halimeda biomass up to 3.3 kg/m2 has been recorded but a more usual value is about 1 kg/m2. This vegetation could generate up to 2 kg CaCO3/m2/y, representing 2 mm/y vertical accretion. This is quite sufficient to have produced the 10-15 m deep Halimeda gravel deposits, indicated by the seismic and coring studies of Orme et al. (1978) and Davies and Marshall (1985), since the last post glacial transgression no more than 10,000 years ago.
We postulate that these Halimeda meadows are restricted to the outer shelf because they require supplies of plant nutients such as NO3 and PO4 available only by upwelling from the adjacent Coral Sea through the narrow, deep entrance channels in the outer barrier. These upwellings may be driven by tidal jet mechanisms and can probably not lift nutrient rich water from below the thermocline over channel sills shallower than 50-60 m. Sills north of Raine island are not deeper than 40 m, thus offering a possible explanation of the paucity of inter-reefal Halimeda there. The structual similarity of the deposits to hydrocarbon-rich bioherms in older rocks makes it important to understand the biological and oceanographic reasons for their localisation and rapid accumulation.