Age determination and grain size distribution of sediment core GeoB3375-1@en

The palaeoclimatic conditions during the Last Glacial Maximum (LGM) of southern South America and especially latitudinal shifts of the southern westerly wind belt are still discussed controversially. Longer palaeoclimatic records covering the Late Quaternary are rare. A particularly sensitive area to Late Quaternary climatic changes is the Norte Chico, northern Chile, because of its extreme climatic gradients. Small shifts of the present climatic zonation could cause significant variations of the terrestrial sedimentary environment which would be recorded in marine terrigenous sediments. To unveil the history of shifting climatic zones in northern Chile, we present a sedimentological study of a marine sediment core (GeoB 3375-1) from the continental slope off the Norte Chico (27.5°S). Sedimentological investigations include bulk- and silt grain-size determinations by sieving, Atterberg separation, and detailed SediGraph analyses. Additionally, clay mineralogical parameters were obtained by X-ray diffraction methods. The 14C-dated core, covering the time span from approximately 10,000 to 120,000 cal. yr B.P., consists of hemipelagic sediments. Terrigenous sedimentological parameters reveal a strong cyclicity, which is interpreted in terms of variations of sediment provenance, modifications of the terrestrial weathering regimes, and modes of sediment input to the ocean. These interpretations imply cyclic variations between comparatively arid climates and more humid conditions with seasonal precipitation for northern Chile (27.5°S) through the Late Quaternary. The cyclicity of the terrigenous sediment parameters is strongly dominated by precessional cycles. For the palaeoclimatic signal, this means that more humid conditions coincide with maxima of the precession index, as e.g. during the LGM. Higher seasonal precipitation for this part of Chile is most likely derived from frontal winter rain of the Southern Westerlies. Thus, the data presented here favour not only an equatorward shift of this atmospheric circulation system during the LGM, but also precession-controlled latitudinal movements throughout the Late Quaternary. Precessional forcing of latitudinal movements of the westerly atmospheric circulation system may be conceivable through teleconnections to the Northern Hemisphere monsoonal system in the Atlantic Ocean region.南美南部末次冰盛期（Last Glacial Maximum，LGM）的古气候背景，尤其是南半球西风带的纬向迁移问题，至今仍存在较大学术争议。目前，覆盖晚第四纪的长序列古气候记录仍较为稀缺。智利北部的北奇科（Norte Chico）地区因极端的气候梯度，成为对晚第四纪气候变化尤为敏感的区域。当前气候分带的微小迁移，便可引发陆相沉积环境的显著变化，而此类变化会被保存在海相陆源沉积物中。为揭示智利北部气候分带的演化历史，本文对采自北奇科海域外大陆坡（南纬27.5°）的海洋沉积岩芯（GeoB 3375-1）开展了沉积学研究。沉积学分析方法包括通过筛分法、阿特伯格分离法（Atterberg separation）开展总质量与粉砂粒度测定，以及高精度SediGraph粒度分析。此外，通过X射线衍射（X-ray diffraction）方法获取了黏土矿物学参数。该经14C测年的岩芯覆盖了约10000至120000校正年距今年代（cal. yr B.P.）的沉积序列，岩性以半远洋沉积物（hemipelagic sediments）为主。陆源沉积学参数显示出显著的周期性，该周期性的成因可归结为沉积物物源变化、陆相风化过程改造以及海洋沉积物输入模式的改变。上述解释表明，晚第四纪期间智利北部（南纬27.5°）的气候存在周期性干湿交替：一端为相对干旱的气候，另一端为具有季节性降水的湿润环境。陆源沉积参数的周期性主要受岁差周期（precessional cycles）控制。就古气候信号而言，这意味着湿润环境与岁差指数的峰值相吻合，例如在末次冰盛期（LGM）期间。智利该区域的季节性降水增多，最可能源自南半球西风带的冬季锋面降雨。因此，本文所呈现的数据不仅支持末次冰盛期期间该大气环流系统向赤道方向迁移，同时也证明晚第四纪期间其纬向迁移受岁差周期调控。西风带大气环流系统纬向迁移的岁差强迫机制，可通过大西洋区域与北半球季风系统的遥相关得到合理解释。