A geochemical comparison of modern and holocene mangrove sediments, Townsville region, North Queensland

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The geochemistry of coastal sediment is important in terms of potential soil acidification and the resultant mobilisation of metals. This paper reports geochemical analyses for mangrove sediments from the Townsville region, North Queensland. Vibrocores were taken from modern swamp (ca. 200 B.P.), and from buried Holocene mangroves of the inner shelf of Cleveland Bay. Although the average pH (ca. 5) and Eh (ca. -250mV) is similar for both the modern and early Holocene mangrove sediments, the pH and Eh are more variable in the younger mangrove sediments. Strongly acidic conditions (ca. pH 2.5) exist in the recently buried mangrove sediments, mainly caused by the degradation of organic matter. the porewater chemistry of early Holocene mangrove sediments suggests that acid-sulphate material may be preserved under marine and anoxic conditions because of limited exchange with seawater. The clay-rich mature of the mangrove sediments in the central Great Barrier Reef region enhances their chemical buffering capacity, the main buffers being clay (silicate) minerals, carbonates, iron-oxyhydroxides and organic matter. This leads to a reduced potential for acid-sulphate soil development and metal mobilisation.