Bulk mineralogy, elemental geochemistry, and magnetic properties of Holocene sediments in the Estuary and Gulf of St. Lawrence

Physical properties, grain size, bulk mineralogy, elemental geochemistry, and magnetic parameters of three sediment piston cores recovered in the Laurentian Channel from its head to its mouth were investigated to reconstruct changes in detrital sediment provenance and transport related to climate variability since the last deglaciation. The comparison of the detrital proxies indicates the succession of two sedimentary regimes in the Estuary and Gulf of St. Lawrence (EGSL) during the Holocene, which are associated with the melting history of the Laurentide Ice Sheet (LIS) and relative sea-level changes. During the early Holocene (10-8.5 ka cal BP), high sedimentation rates together with mineralogical, geochemical, and magnetic signatures indicate that sedimentation in the EGSL was mainly controlled by meltwater discharges from the local retreat of the southeastern margin of the LIS on the Canadian Shield. At this time, sediment-laden meltwater plumes caused the accumulation of fine-grained sediments in the ice-distal zones. Since the mid-Holocene, postglacial movements of the continental crust, related to the withdrawal of the LIS (~6 ka cal BP), have triggered significant variations in relative sea level (RSL) in the EGSL. The significant correlation between the RSL curves and the mineralogical, geochemical, magnetic, and grain-size data suggest that the RSL was the dominant force acting on the sedimentary dynamics of the EGSL during the mid-to-late Holocene. Beyond 6 ka cal BP, characteristic mineralogical, geochemical, magnetic signatures and diffuse spectral reflectance data suggest that the Canadian Maritime Provinces and western Newfoundland coast are the primary sources for detrital sediments in the Gulf of St. Lawrence, with the Canadian Shield acting as a secondary source. Conversely, in the lower St. Lawrence Estuary, detrital sediments are mainly supplied by the Canadian Shield province. Finally, our results suggest that the modern sedimentation regime in the EGSL was established during the mid-Holocene.本研究对从劳伦斯水道（Laurentian Channel）头部至口部获取的3个沉积物活塞岩芯（piston cores）的物理性质、粒度、全岩矿物组成、元素地球化学及磁学参数展开了系统分析，旨在重建末次冰消期（last deglaciation）以来与气候波动相关的陆源沉积物物源（detrital sediment provenance）及搬运过程的演化规律。通过对陆源代用指标（detrital proxies）的对比研究，发现圣劳伦斯河口湾及海湾（Estuary and Gulf of St. Lawrence, EGSL）在全新世（Holocene）期间存在两套沉积体系的演替，二者分别与劳伦泰德冰盖（Laurentide Ice Sheet, LIS）的消融历史及相对海平面变化（relative sea-level changes）存在密切关联。在早全新世（early Holocene，10~8.5 ka cal BP），较高的沉积速率配合矿物学、地球化学及磁学特征指示，圣劳伦斯河口湾及海湾的沉积作用主要受控于加拿大地盾（Canadian Shield）上劳伦泰德冰盖东南缘局部退缩所产生的融水径流。此阶段，挟带沉积物的融水羽状流使得冰缘区域发生细粒沉积物的快速堆积。自中全新世起，与劳伦泰德冰盖消退（约6 ka cal BP）相关的大陆地壳冰后运动，引发了圣劳伦斯河口湾及海湾相对海平面（relative sea level, RSL）的显著变化。相对海平面曲线与矿物学、地球化学、磁学及粒度数据间的显著相关性表明，中晚全新世期间，相对海平面是调控圣劳伦斯河口湾及海湾沉积动力学的主导因素。在6 ka cal BP之后，特征性的矿物学、地球化学、磁学信号及漫反射光谱数据（diffuse spectral reflectance data）表明，圣劳伦斯湾的陆源沉积物主要来源于加拿大大西洋沿岸省份（Canadian Maritime Provinces）与纽芬兰岛西部海岸，加拿大地盾仅作为次要物源。与之相反，圣劳伦斯河下游河口的陆源沉积物则主要由加拿大地盾地区供给。最后，本研究结果显示圣劳伦斯河口湾及海湾的现代沉积体系形成于中全新世时期。