Palaeoenvironmental evolution of the Santo André lagoon (SW Portugal): Palaeobotanical contribution

Depositional systems such as lagoons and estuaries are natural archives of information on environmental changes that occurred in the recent past and on coastline evolution. For the Portuguese coastal fringe, based on geological and paleoecological data from the Santo Andre lagoon (SW Portuguese coast, 80 km south of Lisbon), an evolutionary model has been proposed which comprises a transgressive phase until the middle Holocene, followed by a forced regression, despite the continuous positive eustatic signal. However, so far no palaeobotanical proxies were included in the evolutionary models. To improve the knowledge of the present models, a 4.96m-long core (LSA4) has been collected from the permanently lagoonal flooded domain. The sedimentological analysis and 14C dating of cored material, allowed us to identify four major lithostratigraphical units, accumulated after 6.3 ka cal BP. Presently the palynological analysis focused on the lower units, before 5000 BP, namely LSA4 Unit I (bioclastic muddy peats) and LSA, base of Unit IIA (muds). The presence of Mediterranean trees (evergreen <em>Quercus</em>) and shrubs (<em>Olea</em>, <em>Pistacia</em>), in the Santo André basin during the first half of the Holocene, reflects the development of a local <em>Quercus </em>forest with Mediterranean shrubland. The vegetation reflects the development of a freshwater pond (<em>Botryococcus</em>,<em>Pediastrum</em>,<em>Nymphaea</em>, <em>Myriophyllum</em>) as a consequence of the formation of a costal barrier, but with some sea water influence as reflected by the presence of dinoflagellate cysts of <em>Spiniferites </em>and occurrence of <em>Cymatiosphaera</em>. The presented results represent a contribution, from palynological science, to the evolutionary models already proposed in the literature (and based on other proxies) in what concerns palaeoclimatological and human-induced contributions to modulate coastal responses.

潟湖（lagoon）与河口湾（estuary）这类沉积体系（depositional systems），是记录近代环境变迁与海岸线演化过程的天然信息档案。针对葡萄牙沿海岸带，研究人员基于里斯本以南80公里的葡萄牙西南海岸圣安德烈潟湖的地质与古生态（paleoecological）数据，曾提出一套演化模式：该区域在全新世（Holocene）中期之前处于海侵（transgressive phase）阶段，随后尽管全球海平面持续呈正变化信号（eustatic signal，即持续上升），却出现了强制海退（forced regression）。然而截至目前，相关演化模型尚未纳入古植物代用指标（palaeobotanical proxies）。为完善现有演化模型的认知，团队从该潟湖的永久淹没区域采集了一根长4.96米的岩芯（core，LSA4）。通过对岩芯样品的沉积学分析（sedimentological analysis）与放射性碳十四测年（14C dating），研究人员识别出4个主要岩石地层单元（lithostratigraphical units），其沉积年代均晚于经校准的距今6.3 ka cal BP（校准放射性碳年代）。本次孢粉学分析（palynological analysis）聚焦于距今5000年之前的下层地层单元，即LSA4第一单元（生物碎屑泥质泥炭）以及LSA4第二A单元的底部层位（泥质沉积物）。全新世早期，圣安德烈盆地中发育有地中海型乔木（常绿栎属<em>Quercus</em>）与灌木（油橄榄属<em>Olea</em>、黄连木属<em>Pistacia</em>），反映出当地伴生有地中海灌丛的栎属森林。该植被组合表明，随着海岸障壁坝（coastal barrier）的形成，当地发育了淡水池塘（freshwater pond）环境，其指示藻类包括葡萄藻属<em>Botryococcus</em>、盘星藻属<em>Pediastrum</em>、睡莲属<em>Nymphaea</em>与狐尾藻属<em>Myriophyllum</em>；但同时存在一定海水影响，具体表现为检出刺甲藻属<em>Spiniferites</em>的甲藻孢囊（dinoflagellate cysts）以及膜球藻属<em>Cymatiosphaera</em>。本研究的孢粉学分析结果，可为已有文献中基于其他代用指标（proxies）构建的海岸演化模型提供科学补充，有助于厘清古气候（palaeoclimatological）与人类活动对海岸系统响应的调控作用。