NOAA/WDS Paleoclimatology - Central Great Barrier Reef, Porites spp. trace elements 1934-2012 CE

Coral trace element proxies of terrestrial runoff are widely applied to document and quantify historical changes in river discharge, constituent loads and land disturbance. However, some studies show poor replication between trace element records where cores from multiple coral colonies have been analyzed. Conflicting interpretations also exist on the environmental variable the coral trace element proxy is recording. Indeed, few studies have examined trace element behavior in the estuarine mixing zone that influence the coral records and only limited data on river discharge and constituent loads are available to validate the proxy records of terrestrial runoff. This study examined the behavior of Barium (Ba), Manganese (Mn) and Yttrium (Y) in the Burdekin River estuarine mixing zone, north-eastern Australia during several flood events and investigated the ability of coral Ba/Ca, Mn/Ca and Y/Ca ratios to record the variability of measured Burdekin River discharge and suspended sediment loads along a transect of replicate coral cores (10 cores from 5 locations). The results show limited evidence for Ba desorption from suspended sediments in the estuarine mixing zone while considerable desorption of Mn over a wide salinity mixing gradient was evident. Y showed evidence of removal in the initial mixing zone of the estuary (0–5 PSU) but displayed a relatively flat pattern thereafter indicating that it was not diluted by seawater mixing. The coral trace element records generally showed poor agreement within sites and the Ba/Ca ratios in some corals did not respond to regional river discharge, although coral luminescent lines were present in all cores and were significantly correlated with each other and Burdekin River discharge. While it was difficult to disentangle the influence of Burdekin River discharge and suspended sediment loads on the coral Ba/Ca proxy, several lines of evidence point towards discharge as the major contributor to this proxy in this region. A combination of factors likely influence the coral Ba/Ca ratio including an as yet undetermined physiological mechanism possibly related to crystallography or species, distance of the coral to the river mouth, variability in river discharge (including both annual totals and peaks, and local influences), variability in sediment load, the behavior of trace elements in the estuarine mixing zone, the hydrodynamics of flood plumes in the marine environment and the water depth of the coral. While the coral Mn/Ca and Y/Ca ratios did not respond to Burdekin River discharge events, the considerable desorption of Mn and the elevated Y throughout the estuarine mixing zone shows some promise in their ability to record long-term changes in suspended sediment loading and large terrestrial disturbances. Future studies examining coral records of terrestrial runoff should examine trace element behavior in the river estuarine mixing zone of influence, establish a correlation between the trace element and river discharge (such as luminescent lines) and demonstrate replication between different coral colonies.

陆地径流的珊瑚微量元素代用指标（coral trace element proxies）被广泛用于记录并量化河流径流量、物质负荷及土地扰动的历史变化。然而，部分研究显示，在对多个珊瑚群体的岩心进行分析时，不同序列间的微量元素记录重现性较差。此外，关于珊瑚微量元素代用指标所响应的环境变量，学界也存在相互矛盾的解读。

事实上，鲜有研究考察过影响珊瑚记录的河口混合区（estuarine mixing zone）内的微量元素行为，且仅有有限的河流径流量与物质负荷数据可用于验证陆地径流的代用指标序列。

本研究针对澳大利亚东北部的伯德金河河口混合区，在数次洪水事件期间考察了钡（Barium, Ba）、锰（Manganese, Mn）与钇（Yttrium, Y）的行为特征，并通过沿5个采样点、10根重复珊瑚岩心的断面，探究了珊瑚Ba/Ca、Mn/Ca及Y/Ca比值对实测伯德金河径流量与悬浮泥沙（suspended sediment）负荷变化的响应能力。

研究结果显示，在河口混合区内，钡从悬浮泥沙中解吸的证据有限；而锰在较宽盐度（salinity）混合梯度下发生了显著解吸。钇在河口（estuary）初始混合区（0~5 PSU）表现出移除现象，但后续呈现相对平稳的分布模式，表明其未因海水混合而被稀释。

珊瑚微量元素序列通常在同一采样点内一致性较差，部分珊瑚的Ba/Ca比值并未响应区域河流径流量变化，尽管所有岩心中均存在珊瑚发光纹层，且这些纹层之间以及与伯德金河径流量均存在显著相关性。

尽管难以厘清伯德金河径流量与悬浮泥沙负荷对珊瑚Ba/Ca代用指标的影响，但多项证据表明，径流量是该区域该代用指标的主要贡献因素。

多种因素可能共同影响珊瑚Ba/Ca比值，包括尚未明确的、可能与晶体学（crystallography）或物种相关的生理机制、珊瑚距河口的距离、河流径流量变化（包括年总量与峰值径流，以及局地影响）、泥沙负荷变化、河口混合区内的微量元素行为、海洋环境中洪水羽流的水动力特征，以及珊瑚的生长水深。

尽管珊瑚Mn/Ca与Y/Ca比值并未响应伯德金河的洪水事件，但锰的显著解吸以及整个河口混合区内钇的浓度升高，表明二者在记录悬浮泥沙负荷与大型陆地扰动的长期变化方面具备一定潜力。未来针对陆地径流的珊瑚记录研究，应考察河流河口混合区的微量元素行为特征，建立微量元素与河流径流量间的关联（如发光纹层），并验证不同珊瑚群体间的记录重现性。