Influence of Climate Fluctuations and Changes in Catchment Land Use on Late Holocene and Modern Beach-ridge Sedimentation on a Tropical Macrotidal Coast: Keppel Bay

Brief description

Beach ridges at Keppel Bay, central Queensland, Australia, preserve a record of sediment accumulation from the historical period back to middle Holocene times. The ridges comprise fine, well-sorted, feldspar-rich quartz sand that was eroded from the Fitzroy River catchment, deposited in Keppel Bay during floods of the Fitzroy River, and reworked onshore into beach and foredune deposits by the prevailing currents, waves and wind. These floods have an average recurrence interval of at least 7 yr and are induced by the passage of cyclones onshore into the large Fitzroy catchment. The youngest series of beach ridges sit sub-parallel to the modern beach and comprise six accretional units, each unit formed by a set of ridges and delineated by prominent swales. Optically stimulated luminescence (OSL) ages of beach ridges in these units indicate they were deposited in periods of rapid progradation approximately 1500, 1000, 450 and 230 yr BP, when there was an enhanced supply of sediment to the beach from the Fitzroy River via Keppel Bay. Estimates of the mass of sediment stored in the beach-ridge strandplain show that it represents a significant sediment store, potentially trapping the equivalent of 79% of the estimated long-term (100 yr) average annual bedload of the Fitzroy River that is deposited in Keppel Bay. There has been a reduction in the rate of sediment accumulation in the strandplain since around 1000 yr BP, which is consistent with other coastal records in eastern Australia of a relatively wetter phase of climate in the late Holocene compared to the present. The youngest beach ridges (OSL ages < 100 yr BP) are tall relict foredunes that reflect a low rate of sediment accumulation. These ridges have a distinctive trace-element composition produced by a greater contribution from catchment areas with basaltic soils. The change in catchment provenance has likely been a consequence of erosion that followed clearing of native vegetation in these areas. Our findings demonstrate the important insights that beach-ridge deposits proximal to a river sediment source can provide into processes of sediment accumulation and the response to variations in climate in tropical coastal sedimentary systems.

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