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

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.

澳大利亚昆士兰州中部凯珀尔湾的海滩脊系保存了自现代历史时期追溯至全新世中期的沉积物堆积记录。该脊系由分选良好、富长石的细粒石英砂组成，这些砂体源自菲茨罗伊流域的侵蚀产物，在菲茨罗伊河洪水期沉积于凯珀尔湾，后经盛行流、波浪与风力的沿岸改造，形成海滩与前丘堆积体。此类洪水的平均重现期至少为7年，由登陆进入大型菲茨罗伊流域的气旋活动引发。最新一期的海滩脊系与现代海滩近乎平行，包含6个加积单元；每个单元由一组脊体构成，并以显著的洼地作为分界。该单元内海滩脊的光释光（Optically Stimulated Luminescence, OSL）测年结果显示，其形成于约1500、1000、450与230 yr BP的快速进积时期，彼时菲茨罗伊河经凯珀尔湾向海滩输送的沉积物通量显著升高。对海滩脊滨岸平原存储沉积物总量的估算表明，该平原是一处重要的沉积物储库，其截留的沉积物量约相当于菲茨罗伊河经凯珀尔湾沉积的长期（100年）平均年推移质总量的79%。自约1000 yr BP以来，滨岸平原的沉积物堆积速率有所下降，这与澳大利亚东部其他海岸记录相符——全新世晚期气候相较现今更为湿润。最年轻的海滩脊（OSL测年结果<100 yr BP）为高大的残留前丘，反映出较低的沉积物堆积速率。此类脊体具有独特的微量元素组成，这源于其沉积物更多来自发育玄武质土壤的流域区域。流域物源的变化很可能是该区域原生植被被清除后土壤侵蚀的结果。本研究结果表明，紧邻河流沉积物源的海滩脊堆积体，可为热带海岸沉积系统的沉积物堆积过程及其对气候变化的响应提供重要研究视角。