The Late Pleistocene-Holocene sedimentary evolution of the Sines Contourite Drift (SW Portuguese Margin): A multiproxy approach

The Sines Contourite Drift (SCD), located in the Alentejo margin, southwest Iberian continental margin, has been through many depositional phases in result of climatic variations and bottom current oscillations, which determined a variable depositional pattern and an irregular sedimentary evolution since the Late Pleistocene. The SCD, being in the main path of the Mediterranean Outflow Water (MOW), which greatly constrains the sedimentary building of this drift, constitutes the distal part of the Gulf of Cadiz Contourite Depositional System and its sedimentary evolution therefore reflects the history of MOW variations. In order to investigate this, we report on a multiproxy analysis of grain-size, carbon content and physical, geochemical, and environmental-magnetic properties on the AMS 14C dated 350-cm long gravity core CO14-GC-07, retrieved in the SCD, at 1425 meters water depth (mwd). The main objective is to reconstruct the evolution of the sedimentary package of the SCD from > 43 cal kyr BP to Present and bring new insights about the impact of bottom currents' activity on the morphosedimentary evolution of the margin. Results show the existence of four distinct depositional phases in response to climate variations and bottom current oscillations during the Late Pleistocene-Holocene. Phase 1 (350-322 cm: >42.9 cal kyr BP) occurred in Marine Isotope Stage (MIS) 3 and presents silty-muddy sediments with presence of shell fragments and moderate bioturbation. Phase 2 (322-176 cm: >42.9 - ~30.5 cal kyr BP), which lasted from middle MIS 3 to the onset of MIS 2, reveals sediment coarsening towards the top limit, suggesting climate cooling and strong bottom current winnowing. Phase 3 (176-144 cm: ~30.5 - 17.1 cal kyr BP) covers most of MIS 2 prior to the last deglaciation and shows the sharpest variations on all sediment properties and the coarsest sediment facies, with Fe-rich layers associated with the increase of terrigenous input and sea-level regression that coincide with the enhancement of MOW. Phase 4 (144-0 cm: <17.1 cal kyr BP) is associated with the deglacial sea-level rise and represents a calmer depositional environment with finer sediments and increasing contributions of biogenic material. […]锡内斯等深流沉积丘（Sines Contourite Drift, SCD）位于伊比利亚西南大陆边缘的阿连特茹陆缘，自晚更新世以来，受气候波动与底流振荡的共同影响，经历了多期沉积演化过程，形成了多变的沉积样式与不规则的沉积演变格局。该沉积丘地处地中海流出水（Mediterranean Outflow Water, MOW）的主通道之上，其沉积构筑过程极大地受此约束，同时它也是加的斯湾等深流沉积系统的远端组成部分，因此其沉积演化历程能够反映地中海流出水的变化历史。为探究这一科学问题，本研究对采自该沉积丘、水深1425米（mwd）处的长350厘米重力岩芯CO14-GC-07开展了多代用指标分析，涵盖粒度、碳含量以及物理、地球化学与环境磁学性质，该岩芯经加速器质谱碳14（AMS 14C）定年。本研究的核心目标是重建距今>43校准千年（cal kyr BP）至今的锡内斯等深流沉积丘沉积包体演化过程，并为底流活动对该陆缘形态-沉积演化的影响提供全新认知。研究结果显示，在晚更新世-全新世时期，该沉积丘存在四期响应气候波动与底流振荡的独立沉积阶段：阶段1（350~322cm，>42.9 cal kyr BP）形成于海洋同位素阶段（Marine Isotope Stage, MIS）3，沉积物以粉砂质泥为主，含有贝壳碎片且生物扰动程度中等；阶段2（322~176cm，>42.9~~30.5 cal kyr BP）持续从中MIS 3至MIS 2起始阶段，沉积物粒度向上逐渐变粗，指示气候变冷与强烈的底流淘洗作用；阶段3（176~144cm，~30.5~17.1 cal kyr BP）覆盖了末次冰消期前的大部分MIS 2时期，所有沉积指标均呈现最为显著的变化，且沉积物粒度最粗，其中富铁层与陆源输入增加、海平面下降相对应，而这一时期恰好对应地中海流出水强度增强；阶段4（144~0cm，<17.1 cal kyr BP）与冰消期海平面上升相关，代表了更为平静的沉积环境，沉积物粒度更细且生物成因物质贡献逐渐增加。[...]