Mobility of land-ocean interface systems in a millennial to centennial time scales

Resumen

The main goal unveiled in this thesis is to provide a better understanding of how a coastal system evolves under changing conditions, namely sea level rise rate. A sub surface dataset retrieved from preserved Holocene sedimentary deposits was used to achieve a broad understanding of the processes that govern the mobility of a land-ocean interface at different time scales, from thousands to hundred years. The Ria Formosa Barrier Island System, southern Portugal, was selected as the case study for the implementation of the selected methodology. The Holocene evolution of the Ria Formosa Barrier Island System was studied through the examination of a large subsurface dataset acquired from 191 boreholes and five seismic refraction profiles. Two mechanical boreholes with total depths of 26m and 16.5m were selected for a detailed multi-proxy laboratory analysis, including mean grain size distribution, organic matter content, colour variation, shell identification, benthic foraminifera assemblages, metal concentration and dating samples. Selected cores were considered to be representative of depositional sub-basins, following the terminal axis of present-day waterlines and their possible extension. An additional dataset retrieved from 136 boreholes from previous campaigns, a detailed topobathymetric surface model, maximum tidal range at the end of 19th century, and geological features of the area were also integrated. A total of 16 AMS 14C dated samples were plotted against depth, resulting in a coherent age model, time constraining the sedimentary infill process. First, a digital palaeosurface model of Ria Formosa Barrier Island System was created, establishing the base surface over which the system developed during the Holocene. It revealed the existence of a complex network of fluvial palaeovalleys cutting through the underlying pre-Holocene topography. Two major sedimentary units could be distinguished, defined as independent sub-basins. Borehole sediment profiles confirmed the existence of changing depositional environments, which are reflected by a heterogeneous sedimentary sequence on the Ludo palaeovalley sub-basin, and a more homogeneous sequence in the central sub-basin. Second, a conceptualization of the system Holocene evolution is proposed, envisioned as largely controlled by sediment availability, accommodation space, and changing sea level rise rate, first at a rapid rate of 7mm/yr from 10 000 cal yr BP to 7 250 cal yr BP, followed by a slow-down to 1.1mm/yr until present. The conceptual model for the origin and Holocene evolution of the Ria Formosa Barrier Island System implies the succession of three main steps, that ultimately resulted in the present day system configuration: a) marine flooding of incised palaeovalleys by the rapid transgression in the early-Holocene; b) development of a proto-barrier island chain perched on Pleistocene detritic headlands and steeper interfluve areas during the early to mid-Holocene; c) full development of the barrier islands chain and enclosing of the coastal lagoon, followed by the maturation of the system with subsequent siltation and salt marsh expansion from the mid-Holocene until present. The onset of the barrier system formation, and subsequent coastal lagoon enclosure, is considered to have started 8 000 cal yr BP, predating previously suggested ages from different authors. Third, the impact of human activities in metal contamination of a natural system was assessed by determining background values preserved in Holocene sediments. Contaminant elements were discriminated in terms of natural and anthropogenic inputs to the Ria Formosa Barrier Island System by comparing normalized geochemical data from 150 samples acquired in six sediment cores with 28 surface samples. Analysed metal content in the subsurface displayed a similar behaviour in all cores, complementing the Holocene reconstruction of the system. Specific background values and enrichment factors were then determined applying multivariate analysis techniques. Obtained results were reproduced as metal enrichment maps which evidenced the prevalence of anthropic contamination in specific locations. As, Pb, Zn and Cu show high enrichment factors indicating punctual sources of pollution, exceeding the natural signal of the system. Human activities related to industrial areas water runoff, boat paints and leaded gasoline residues are considered to be the main contamination sources. Other metal pollutants (Cd, Cr, Hg and Ni) have generally low values, suggesting enrichment by natural processes. Fourth, the sediment infilling process of the Ria Formosa lagoon was simulated using the Hybrid Estuarine Sedimentation Model in response to eustatic sea level rise from 10125 cal yr BP to 1885 AD. Simulations were carried out in two steps: 1) short-term simulations (decadal scale) and 2) long-term simulations (centennial and millennium scale). The respective validations of the model were based on the Ludo sub-basin reconstruction, using the data obtained from the acquired subsurface dataset and comparison of simulated sediment surface elevations with corresponding historical bathymetric maps. According to obtained results, the early stage of Holocene sediment accumulation took place at the palaeovalley deltaic front. Under a sea level rise rate of 7 mm/yr, this sediment body was increasingly submerged, displacing the sedimentation further landward. The upper palaeovalley area started to form an intertidal plain by 7250 cal yr BP as the inundation limit of the sea slowly reached near to present-day position by 3000 cal yr BP at a rate of 1 mm/yr. Simulation results confirm that during the mid- to late Holocene coastal progradation of the Ludo palaeovalley contributed significantly to the formation and growth of the western barrier islands. Sea level transgression combined with the antecedent morphology of local drainage system on the Algarve shelf are key factors in formation and development of Ria Formosa Barrier Island System. Finally, the multi-proxy analysis of the acquired subsurface dataset in the Ria Formosa Barrier Island System, combined with a 3D surface model of the pre-Holocene topography, allowed to create the first comprehensive conceptual sequence of processes leading to the Holocene origin and evolution of the system. The time scale over which these processes operated is considered to be substantiated, replicating similar results from neighbouring environments in the Gulf of Cadiz. A two-fold pattern is observed, stemming from a significant shift in sea level rise rate in the Holocene combined with a limited accommodation space, resulting in a change from transgressive to aggradation dominant processes. This study also allowed to understand the degree of metal contamination in the system surface sediments, spatially identifying sensible zones that require further attention. Enrichment Factor calculation combined with GIS based spatial analysis is a great tool to assess the environmental quality of coastal systems. Model simulations are considered to be fairly accurate, opening the possibility of the model application to management options. Further improvements in the simulation of the evolution of coastal systems during the Holocene as of function of eustatic sea level rise, can ultimately result in the development of realistic management plans and adapting to the consequences of accelerating sea level rise forecasted for the 21st century.