冷水柳珊瑚骨骼化石重建的早全新世南海中层水十年际变化

Interdecadal variation of intermediate water in the South China Sea reconstructed by cold-water gorgonian skeleton fossils during the Early Holocene

冷水珊瑚可以提供其生长期内周围环境变化的信息,有望填补中-深层海洋高分辨率重建材料的空缺。本文利用“深海勇士”号载人深潜器在甘泉海台西南角海山上(16.55°W,110.89°E;水深1119.3m)采集的柳珊瑚样品(SY185-9)开展探索性研究,检验冷水珊瑚高分辨率古环境重建的应用价值。X射线衍射(XRD)分析结果表明SY185-9的矿物成分主要为镁方解石(Mg_(0.06)Ca_(0.94)CO_(3));骨骼横切面的14C测年结果显示SY185-9生长于早全新世,时间跨度为9621±135~8922±114a B.P.;利用环境扫描电子显微镜结合能谱仪和电子探针,分析SY185-9骨骼横切面的元素组成和变化,其中Mg/Ca比值指示了SY185-9生长时期平均海水温度为4.7±0.9℃,较现代相同位置处的平均海水温度高约0.9℃,可能反映早全新世南海中层水温度较现代略高的特征,但也需注意冷水珊瑚Mg/Ca温度计算公式的区域适用性和珊瑚生命效应问题;Mg/Ca记录的频谱分析结果揭示显著的十年-百年际尺度波动,可能反映中层水温度的自然变率,或因局地海山地形而造成的中层海水与海表气候之间的密切联系。

Cold-water corals can provide information of variations in the ambient environment during their lifespans and have been expected to fill the gap in high temporal-resolution paleoceanographic reconstruction of the intermediate and deep waters.Here,we performed explorative studies on a gorgonian sample(SY185-9)collected by the human operating vehicle"ShenhaiYongshi"to examine the application value of cold-water corals in reconstructing paleoenvironment.Located in the Xisha Area of the South China Sea,the Ganquan Plateau is under the influence of the West boundary current of the South China Sea(SCSWBC)and its related warm eddies.The sample was collected on a small seamount at the southwest Ganquan Plateau(16.55°W,110.89°E;water depth 1119.3m).Through the analysis of the experimental data of SY185-9,we draw the conclusions as follows:(1)X-ray diffraction(XRD)analyses show that SY185-9 is mainly composed of magnesium calcite(Mg_(0.06)Ca_(0.94)CO_(3));(2)14C dating demonstrates that it was grown during 9621±135~8922±114a B.P,in the Early Holocene;(3)Variations of elemental compositions on the cross section of SY185-9 were analyzed by electron microscope scanning combined with energy spectrometer and electron probe.The inferred temperature derived from Mg/Ca ratio was 4.7±0.9℃during the growth period of SY185-9,about 0.9℃higher than the modern sea water temperature nearby,likely indicating that the intermediate water temperature of the South China Sea in the Early Holocene was slightly higher than that in the modern.However,the uncertainty caused by the regional applicability and vital effects on the Mg/Ca-temperature calibration equation cannot be excluded.(4)The spectral analysis of Mg/Ca records reveals significant decadal-centennial scale fluctuations.The Mg/Ca records of SY185-9 shows significant 44-a and 127-a cycles during its lifespan,which correspond well with the changes of solar activity during the same period.They display an inverse phase correlation over the 127-a cycle and in-phase correlation over the 44-a cycle.This possibly reflects the natural variability of intermediate water temperature or the connection between cold-water coral and surface climate induced by the seamount topography.Our exploratory experimental studies on the skeleton of gorgonian collected from the South China Sea provide a centennial history of intermediate water temperature at annual resolution during the Early Holocene,by which the potential of cold-water coral for high-resolution paleoceanographic reconstruction is tested.