Late Holocene sediment in Nara Inlet, central Great Barrier Reef platform,Australia: sediment accumulation on the middle shelf of a tropical mixed clastic/carbonate system

Nara Inlet in the Whitsunday Islands is a natural depocenter for elastic an d carbonate sediment on the middle shelf of the central Great Barrier Reef (GBR) platform. Three sediment cores were collected from the inlet to under stand sediment accumulation in this tropical mixed clastic/carbonate system over time. Surface and subsurface sediment is comprised of two main compon ents: terrigenous clay and shallow marine carbonate. Similar to other regio ns on the middle shelf of the GBR platform, the carbonate component, 25-80% by weight and dominated by Sr-poor foraminifera tests and mollusc shells, generally increases towards the surface. Radiocarbon ages derived from test s of benthic foraminifera indicate that the top 3 m of sediment accumulated within the last 3000 years, but that the rate has slowed toward present-da y. The change in elastic and carbonate abundance therefore indicates a syst em where both elastic and carbonate accumulation has decreased over time, b ut where elastic accumulation has decreased faster. Reduced elastic accumul ation may reflect progressive impedance of tenigenous input by a laterally growing fringing reef at the mouth of Nara Inlet. Likewise, reduced carbona te accumulation may reflect preferential reef growth and carbonate depositi on outside of the inlet. Even though elastic accumulation has decreased sig nificantly over time, the late Holocene mass, composition, and distribution of elastic material in Nara Inlet implies a terrigenous source previously unrecognized in models for the evolution of the GBR platform. Large amounts of terrigenous sediment probably were stored around topographic highs on t he middle shelf during lowstand and early transgression. This elastic mater ial was then removed over time, mixed with autochthonous carbonate, and dep osited in embayments on the middle shelf. The accumulation of sediment on t he middle shelf of a tropical mixed clastic/carbonate margin is not straigh tforward. (C) 2001 Elsevier Science B.V. All rights reserved.