Sedimentation across the Great Barrier Reef

The continental margin off northeast Australia, comprising the Great Barrier Reef (GBR) platform and Queensland Trough, is the largest tropical mixed siliciclastic/carbonate depositional system in existence. We describe a suite of 35 piston cores and two Ocean Drilling Program (ODP) sites from a 130*240 km rectangular area of the Queensland Trough, the slope and basin setting east of the central GBR platform. Oxygen isotope records, physical property (magnetic susceptibility and greyscale) logs, analyses of bulk carbonate content and radiocarbon ages at these locations are used to construct a high resolution stratigraphy. This information is used to quantify mass accumulation rates (MARs) for siliciclastic and carbonate sediments accumulating in the Queensland Trough over the last 31,000 years. For the slope, highest MARs of siliciclastic sediment occur during transgression (1.0 Million Tonnes per year; MT/yr), and lowest MARs of siliciclastic (<0.1 MT/yr) and carbonate (0.2 MT/yr) sediment occur during sea level lowstand. Carbonate MARs are similar to siliciclastic MARs for transgression and highstand (1.1-1.4 MT/yr). In contrast, for the basin, MARs of siliciclastic (0-0.1 MT/yr) and carbonate sediment (0.2-0.4 MT/yr) are continuously low, and within a factor of two, for lowstand, transgression, and highstand. Generic models for carbonate margins predict that maximum and minimum carbonate MARs on the slope will occur during highstand and lowstand, respectively. Conversely, most models for siliciclastic margins suggest maximum and minimum siliciclastic MARs will occur during lowstand and transgression, respectively. Although carbonate MARs in the Queensland Trough are similar to those predicted for carbonate depositional systems, siliciclastic MARs are the opposite. Given uniform siliciclastic MARs in the basin through time, we conclude that terrigenous material is stored on the shelf during sea level lowstand, and released to the slope during transgression as wave driven currents transport shelf sediment offshore.

Methane discharge in sediments of the Dvurechenskii mud volcano

During the MARGASCH cruise M52/1 in 2001 with RV Meteor we sampled surface sediments from three stations in the crater of the Dvurechenskii mud volcano (DMV, located in the Sorokin Trough of the Black Sea) and one reference station situated 15 km to the northeast of the DMV. We analysed the pore water for sulphide, methane, alkalinity, sulphate, and chloride concentrations and determined the concentrations of particulate organic carbon, carbonate and sulphur in surface sediments. Rates of anaerobic oxidation of methane (AOM) were determined using a radiotracer (14CH4) incubation method. Numerical transport-reaction models were applied to derive the velocity of upward fluid flow through the quiescently dewatering DMV, to calculate rates of AOM in surface sediments, and to determine methane fluxes into the overlying water column. According to the model, AOM consumes 79% of the average methane flux from depth (8.9 x 10**+ 6 mol a**-1), such that the resulting dissolved methane emission from the volcano into the overlying bottom water can be determined as 1.9 x 10**+ 6 mol a**-1. If it is assumed that all submarine mud volcanoes (SMVs) in the Black Sea are at an activity level like the DMV, the resulting seepage represents less than 0.1% of the total methane flux into this anoxic marginal sea. The new data from the DMV and previously published studies indicate that an average SMV emits about 2.0 x 10**+ 6 mol a**-1 into the ocean via quiescent dewatering. The global flux of dissolved methane from SMVs into the ocean is estimated to fall into the order of 10**+10 mol a**-1. Additional methane fluxes arise during periods of active mud expulsion and gas bubbling occurring episodically at the DMV and other SMVs.