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 and carbonate sediment on the middle shelf of the central Great Barrier Reef (GBR) platform. Three sediment cores were collected from the inlet to understand sediment accumulation in this tropical mixed clastic/carbonate system over time. Surface and subsurface sediment is comprised of two main components: terrigenous clay and shallow marine carbonate. Similar to other regions 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 tests 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-day. The change in elastic and carbonate abundance therefore indicates a system where both elastic and carbonate accumulation has decreased over time, but where elastic accumulation has decreased faster. Reduced elastic accumulation may reflect progressive impedance of tenigenous input by a laterally growing fringing reef at the mouth of Nara Inlet. Likewise, reduced carbonate accumulation may reflect preferential reef growth and carbonate deposition outside of the inlet. Even though elastic accumulation has decreased significantly 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 the middle shelf during lowstand and early transgression. This elastic material was then removed over time, mixed with autochthonous carbonate, and deposited in embayments on the middle shelf. The accumulation of sediment on the middle shelf of a tropical mixed clastic/carbonate margin is not straightforward. (C) 2001 Elsevier Science B.V. All rights reserved.