Sedimentary Dynamics in the Estremadura Spur Continental Shelf

• Autor(a) principal: Balsinha, M.
• Data de Publicação: 2020
• Idioma: eng
• Título da fonte: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Texto Completo: http://hdl.handle.net/10451/49747
• topic: Dinâmica sedimentar; Esporão da Estremadura; Proxies paleoambientais; Evolução Holocénica; Análise de tendência textural; Sedimentary dynamics; Estremadura Spur; Paleoenvironmental proxies; Holocene evolution; Grain size trend analysis; Domínio/Área Científica::Ciências Naturais::Ciências da Terra e do Ambiente

The marine environment is affected by multiple natural drivers and anthropogenic stressors that can have a strong impact on the sustainability of marine ecosystems and resources. Understanding the marine environment dynamics, in its various aspects, has therefore become a worldwide priority to the study of the sedimentary cover. Understanding of its spatial and temporal patterns, can be of critical importance for resource management, environmental protection initiatives and effective marine governance. The present work aims at the study of the sedimentary processes affecting the Estremadura Spur in order to understand its evolution of marine systems in an environmental and climate change context. With this goal, a multidisciplinary approach will attain the different aspects of marine processes on the Estremadura Spur continental shelf. With a triangular shape, the Estremadura Spur is a prominent geomorphological unit that stands out from the general morphology of the West Iberian Peninsula. This sector is isolated from the nearby sectors by the northern Nazaré Submarine canyon and the Lisboa Canyon in the south. The Estremadura Spur continental shelf has variable width, ranging from 15 km (in front of Peniche) to 70 km (in Ponta da Lamparoeira parallel) and its external border (shelf break) is located at variable depth, ranging from ~50 m to more than ~300 m. The continental shelf is characterized by large areas of carbonated rocky outcrops and a thin layer of unconsolidated sediments and the coastal zone is predominantly composed by rocky cliffs with few embayed beaches, closer to the mouth of small rivers and creeks. This sector of the Portuguese shelf is affected by a high energetic wave regime that dominates inner and middle shelf sediment dynamics. At the outer shelf and upper slope, oceanographic phenomena, like internal waves, have been recognized to occur, which impact particle resuspension and transport. In the Portuguese continental shelf, during the winter, the predominant current direction is northward; however, during the three months period of in situ observations (winter time), in the outer shelf of the Estremadura Spur, the dominant current direction was oriented southward, inducing a particle transport with the same direction; the observed residual current from current meter data ranged from 0 to 13 cm/s. Two main components were identified, namely cross-shelf and along shelf. As a result of this morphology and wave regime exposition, sedimentary patterns indicate that the contribution of recent sand terrigenous particles is only significant in the inner shelf. In the northern sector, the middle shelf is covered by very coarse deposits in the vicinity of extensive rocky outcrops. Immediately south of those outcrops, sediments become finer, with muddy sand deposits covering the entire Mar da Ericeira. Muddy sands also cover the outer domains of the Estremadura Spur, with a large elongated sandy deposit located along the outer shelf. Sedimentary particles from adjacent beaches and rivers that drain directly into the Estremadura Spur are mainly sand-sized. River sediments also have a significant component of fine particles, while the beach sediments have no fine fraction and gravel is only important at Foz do Arelho beach. There is no clear relationship between minerals present in the fine fraction of river sediments that drain to the Estremadura Spur and those found in the shelf. The high percentage of potassium feldspar and plagioclase near the coast can be explained by the proximity to the terrigenous particle sources (continental origin), while the other minerals seem to be related to the effect of distribution processes that control the distribution of the main mineral species. The riverine environment has clearly different assemblages of heavy minerals than the ones found on beaches and inner shelf. The heavy minerals highlight two distinct coastal sedimentary cells, characterized by distinct associations. The beach sediments present a similar heavy mineral content to inner shelf sediments, but the influence of the riverine/continental input is higher at the beaches, as demonstrated by high percentages of tourmaline and andaluzite in the north, and pyroxenes in the south. The analysis of present-day sedimentary dynamics revealed characteristics of wave/storm dominated continental shelf. Threshold orbital velocity near the bottom computed with SWAN, using 22 years of wave data, for coarse (0.500 mm) and fine particles (0.063 mm), show that coarse particles are remobilized more that 40 % of the time for depths up to 30 m and fine particles can be remobilized 50 % of the time till 40 m depth and 10 % of the time till 100 m. The sediment transport pathways over the Estremadura Spur, deduced from the GSTA (Grain Size Trend Analysis), indicate four different sectors in relation to distinct oceanographic, sediment supply and geomorphologic constraints. Due to lower sediment sampling resolution, GSTA was not able to resolve the sedimentary transport in the inner shelf sector. The study of a vertical sample collected offshore Ericeira (in the center of the largest muddy deposit of the Estremadura Spur) allowed the understanding of the environmental shelf changes during the Holocene, with impact on the sediment record. The oldest sedimentary sequence corresponds to a high energy environment, likely representing a coastal deposit in close connection to the terrigenous particle sources. The middle sedimentary sequence shows an increased presence of biogenic particles and a decrease in coarser particles indicating a deeper and lower energy environment, when compared with the older sequence. In the superficial sequence (which is only 20 cm thick), sediments are compatible with middle shelf dynamic processes very similar to present day characteristics. The sudden increase of heavy metals like mercury (Hg) and lead (Pb) can be explained by the increase of anthropogenic activity, either related to deforestation or mining. These activities increase soil erosion, leading to a characteristic inorganic detrital deposit in most aquatic systems (lakes, rivers and seas). The reconstruction of the evolution of the Estremadura Spur during the recent Quaternary was performed by the integration of morphological and sedimentological characteristics. As an important input to this model and to explain the origin of the three sedimentary sequences, additional radiocarbon analyses of benthic organisms were used as reference, as well as published sea level curves for the Portuguese continental shelf and other several studies on paleoreconstruction of the Portuguese margin. During the Last Glacial Maximum, the coastline was at ~ 130 m depth and the present day rivers were active valleys carrying sediment into the shelf break. During this period, the rocky outcrops at Ponta da Lamparoeira parallel, formed a morphological barrier between the northern and southern areas of Estremadura Spur. The sedimentation over the shelf was very scarce at this time due to the cold and arid conditions. In the beginning of the deglaciation (warmer climate), the coastline started to migrate landward and coastal areas were invaded by the sea up to ~ 110/100 m depth and sediment export increased quite slowly. At the end of the deglaciation the sea level rose very rapidly settling at ~ 50/40 m depth. This sudden sea level rise induced such an imbalance that estuaries became sediment traps and few sediments were able to reach the continental shelf, which resulted in a scarcity of sediments and the coastline maintained its rocky characteristics. During the Younger Dryas, the sea surface temperature dropped below 10 ºC and the relative mean sea level (rmsl) lowered 20 m in 1000 years. By the end of this event the msl was ~ 60 m depth. This new cycle induces the remobilization of coastal and estuarine particles deposited in the previous phase. In the Holocene, the establishment of a warmer period contributed to the multi-phase rise of the msl. These multi-phase events are represented by Early, Middle and Late Holocene. In the Early Holocene, the sea level rose almost 40 m in 2000 years, leading to thick transgressive sequences, due to the high levels of precipitation. At this time, the conditions were similar to those of an inner shelf like environment with high energy conditions. During the Middle Holocene, the overall conditions of sea level rise and rapid environmental conditions were maintained (msl rise ~15 m above present sea level). This period is represented in the sediment core as a ~ 80 cm thick layer, indicating a drastic decrease in energy conditions, concurrent with an increase of the living conditions represented by species diversity. In the Late Holocene, the msl settled at the present level. With the increasing human activity, with impacts on deforestation and mining processes, the presence of heavy metals in the sedimentary record also increases. A high fluvial/continental input was expected, with higher species diversity due to higher availability of nutrients and fine particles. Although the sediment input might have increased, the accumulation rate is still very low when compared to other sectors of the Portuguese continental margin.