Temporal variability of nepheloid layer structures on continental slope environments: Natural and anthropogenic processes
- Autores: Marta Arjona
- Directores de la Tesis: Pere Puig Alenya (dir. tes.), Albert Palanques Monteys (codir. tes.), Antoni Calafat Frau (tut. tes.)
- Lectura: En la Universitat de Barcelona ( España ) en 2022
- Idioma: inglés
- Tribunal Calificador de la Tesis: Xavier Durrieu de Madron (presid.), Anna Sanchez Vidal (secret.), Henko de Stigter (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencias de la Tierra por la Universidad de Barcelona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
Resumen de la Tesis: This Thesis aims to investigate the contribution of the deep-sea trawling activities, besides that of natural processes, to the sediment dynamics in different continental slope areas, particularly in the NW Mediterranean and the NE Pacific basins.
The temporal and spatial variability of the water column turbidity structure in the Foix Canyon (NW Mediterranean) are studied by analysing two months of hydrographic profiles collected by an autonomous hydrographic profiler. The water column turbidity structure consists in intermediate nepheloid layers (INLS) developed between 300 and 500 m water depth and above the canyon rims, and INLs and near-bottom nepheloid layers (BNLs) confined inside the canyon between 650 and 800 m water depth. Data from fishing vessels positioning during the deployment period reveals that the presence and depths of the observed INLs and BNLs depend on the operating depths of bottom trawlers and on the specific fishing grounds being exploited by the local trawling fleet. This data strongly suggests that bottom trawling activities in the study area are the main driver of the turbidity increases observed in the water column.
The spatial and temporal variations on the water column turbidity structure and near-bottom suspended sediment transport associated to both natural processes (storms, river floods, and dense shelf water cascading, DSWC) and bottom trawling activities are also analysed for three months in the Palamós Canyon (NW Mediterranean) using an autonomous hydrographic profiler and near-bottom instrumentation installed in a mooring, covering a trawling closure period, and an active trawling period. Periods of enhanced water turbidity during the trawling closure period are associated with storms and DSWC events, transporting turbid dense waters into the canyon. In absence of such energetic events, the water column remains unchanged, displaying the lowest suspended sediment concentrations of the recording period, until the trawling season began. INLs (250 and 350 m depth) and BNLs (> 500 m depth) are found at the water depths where trawling grounds from the Palamós Canyon are located. High near-bottom suspended sediment fluxes are sporadically registered during the trawling closure associated with a major wet eastern storm and a DSWC event, whereas smaller but more frequent increases in near-bottom suspended sediment fluxes are recorded during trawling activities. However, the continued bottom trawling activity, at least for 30 days of monitoring, produces an accumulated suspended sediment transport similar to that generated by a major DSWC event. Considering that bottom trawling in Palamós Canyon is practised on a daily basis throughout the year, except for the 2-month trawling closure period, a much larger contribution of anthropogenically derived suspended sediment transport can be expected.
The temporal evolution of the hydrographic and nepheloid layer structure in the upper slope of Vancouver Island (British Columbia, Cascadia margin, NE Canadian Pacific) is also assessed by analysing four months of sensor data from the NEPTUNE observatory. The water column turbidity structure during the study period consists in surface nepheloid layers (SNLs) (<100 m depth) associated to primary and secondary productivity, INLs between 150 and 300 m depth at the shelf-break and upper slope domain, and BNLs developed at deeper continental slope regions (~400 m depth). Strong fall storms occurring during the monitored months seem to generate the more intense INLs found at shelf-break depths. However, not all the recorded INLs coincide with the more intense storms, indicating that other sediment resuspension mechanisms might be modulating these INL detachments. Results from this study suggest that, in combination with the regional currents, the presence of continuous fishing along the continental slope off Vancouver Island, contributes to the advection of suspended sediment particles, playing a major role in their transference along the margin as nepheloid layers.
