Deposition, mineralization and storage of carbon and nitrogen in sediments, Great Barrier Reef

Data was collected in order to study the benthic metabolic rates and pathways of carbon and nitrogen cycling at shelf sites proximal to impacted catchments on the northern GBR and to less heavily impacted catchments on the far northern GBR shelf.

On the far northern shelf, 3 stations (HAY, HAN, BUK) were chosen based on their proximity to in shore coastal reefs, and one (PCB) off the largest river drainage systems on the far northern shelf (Normanby River). On the northern shelf 5 stations (LOW, CNS, FIZ, HIG, RUS) were chosen based on their proximity to coastal reefs and to highly cultivated river catchments.

Box core and Kasten core samples were used for analysis of sediment mean grain size (mm); and percent gravel, sand, silt, clay, and water content. Sediments were classified as mud, sand, sandy mud, or muddy sand. Sub-samples were frozen wet and dry weighted to determine water content, and then ground to a fine powder for determination of total carbon and total nitrogen content. Other elements (P, S, Mn, Fe) were determined. Total inorganic carbon was assumed as CaCO3, and was determined by difference between the total carbon and total organic carbon concentrations.

Mean particulate element concentrations and molar ratios in sediments for: TC, TOC, CaCO3, TN, S, Fe (% sediment dry weight); P (µg g-1 sediment dry weight); Mn (Average above mangrove peat layer); C:N, N:P, C:S, S:Fe and C:P (molar ratios).

Mass sediment accumulation rates (MAR) were determined from gamma spectrometric measurements of 210Pb, 137Cs, and other radionuclides.

Solute fluxes across the sediment-water interface were measured using three sets of three closed opaque chambers (volume: 1 l; area: 82 cm2) from which total CO2, O2, and dissolved inorganic nutrient and metal samples were taken. Concentrations of ammonium, nitrite, nitrate and phosphate were determined using automated techniques.

Rates of sulfate reduction were measured.

Rates of net production of dissolved metals, particulate CO2, and ammonium from sediments at each site were measured and carbonate concentration was calculated from the pH and total CO2 data in estimating the second apparent dissociation constant of carbonic acid at the appropriate temperature and salinity.

Nitrification was estimated using the acetylene inhibition technique. Denitrification was measured from replicate sediment samples taken from boxcores using the N2-gas flux technique. Nitrogen fixation in sediments at each site was measured in three to four dark and clear chambers using the acetylene reduction technique.