Organic matter oxidation and sediment chemistry in mixed terrigenous-carbonate sands of Ningaloo Reef, Western Australia

The oxidation of organic matter in relation to porewater and solid-phase element chemistry was examined in mixed terrigenous-carbonate sediments in sheltered and exposed lagoons of Ningaloo Reef, Western Australia. Rates of O-2 consumption and CO2 release were faster in the very fine sand (41-43% CaCO3 content) of the sheltered lagoon of Mangrove Bay (means = 10.5 mmol m(-2) day(-1) O-2 and 9.4 mmol m(-2) day(-1) CO2) than in the carbonate-rich (73% CaCO3) sand of the exposed back-reef lagoon of Ningaloo Reef (means = 2.1 mmol m(-2) day(-1) O-2 and 3.5 mmol m(-2) day(-1) CO2). Rates of sulfate reduction (Sigma SRR) were similarly faster in the Mangrove Bay sediments (6.1-25.3 mmol m(-2) day(-1) S), sufficient to account for all of the organic matter mineralization. In Ningaloo reef sands, Sigma SRR rates (1.0 mmol m(-2) day(-1) S) accounted for a significant fraction (57%) of total organic carbon oxidation. In Mangrove Bay, in contrast to previous measurements of sulfate reduction in tropical sediments, most (mean = 64%) of the reduced S-35 was incorporated into the acid-volatile sulfide fraction with a buildup of iron sulfides. In contrast to most carbonate-bearing sediments, the production and accumulation of Fe sulfides (most evident in Mangrove Bay) increased pH to levels promoting carbonate precipitation. Higher decomposition rates in Mangrove Bay are attributed to restricted water circulation, a richer benthic community, and geomorphology conducive to greater input and retention of mangrove- and macroalgae-derived detritus. At both sites, the lack of a clear zonational sequence of porewater solutes, discrepancies between depth profiles of solutes and solid-phase elements, and high core-to-core variation in conservative element concentrations and in rates of bacterial activity, suggest non-steady state diagenesis. Non-steady state conditions may be fostered by a combination of factors, such as physical disturbances, temporal changes in rates and quality of organic sedimentation, and tidal advection.