Rock magnetic properties and radiocarbon dates of samples obtained on Ile de Possession, South Indian Ocean

A 6200 year old peat sequence, cored in a volcanic crater on the sub-Antarctic Ile de la Possession (Iles Crozet), has been investigated, based on a multi-proxy approach. The methods applied are macrobotanical (mosses, seeds and fruits) and diatom analyses, complemented by geochemical (Rock-Eval6) and rock magnetic measurements. The chronology of the core is based on 5 radiocarbon dates. When combining all the proxy data the following changes could be inferred. From the onset of the peat formation (6200 cal yr BP) until ca. 5550 cal yr BP, biological production was high and climatic conditions must have been relatively warm. At ca. 5550 cal yr BP a shift to low biological production occurred, lasting until ca. 4600 cal yr BP. During this period the organic matter is well preserved, pointing to a cold and/or wet environment. At ca. 4600 cal yr BP, biological production increased again. From ca. 4600 cal yr BP until ca. 4100 cal yr BP a 'hollow and hummock' micro topography developed at the peat surface, resulting in the presence of a mixture of wetter and drier species in the macrobotanical record. After ca. 4100 cal yr BP, the wet species disappear and a generally drier, acidic bog came into existence. A major shift in all the proxy data is observed at ca. 2800 cal yr BP, pointing to wetter and especially windier climatic conditions on the island probably caused by an intensification and/or latitudinal shift of the southern westerly belt. Caused by a stronger wind regime, erosion of the peat surface occurred at that time and a lake was formed in the peat deposits of the crater, which is still present today.

本研究针对钻取自亚南极克罗泽群岛（Iles Crozet）波塞申岛（Ile de la Possession）火山口的6200年泥炭序列，采用多代用指标（multi-proxy）方法开展研究。所采用的分析手段包括大植物化石（macrobotanical）分析（涵盖苔藓、种子与果实残体）、硅藻分析，辅以地球化学（Rock-Eval6）测试与岩石磁学测量。该岩芯的年代标尺基于5个放射性碳测年结果。整合所有代用指标数据后，可推断出如下环境演化阶段：自泥炭形成初期（距今6200校准年，cal yr BP）至约5550校准年时段，生物生产力较高，气候条件相对温暖；约5550校准年时，生物生产力出现下降，该状态持续至约4600校准年，此阶段有机质保存完好，指示当时环境寒冷且/或湿润。约4600校准年时，生物生产力再度回升；自4600校准年至约4100校准年时段，泥炭表面形成了“丘洼微地貌（hollow and hummock）”，导致大植物化石记录中同时出现湿生与旱生物种组合。约4100校准年之后，湿生物种消失，整体更为干燥的酸性泥炭沼泽开始形成。约2800校准年时，所有代用指标数据均出现显著突变，指示该岛气候变得更为湿润，且风力显著增强，这一变化可能由南西风带（southern westerly belt）的强化及/或纬向迁移所致。受增强的风力作用影响，当时泥炭表面发生侵蚀，火山口泥炭沉积中形成了一处湖泊，该湖泊至今仍存续。