The study of an unusual temperate latitude beachrock formation. Characterization of the azkorri beach and tunelboka cove locations

Resumo

EXTRACTO/RESUMENBeachrocks are complex coastal solid sedimentary formations resulting from the in-situ precipitation of CaCO3 in the intertidal zone. The inured outcrops develop prevailingly at tropical and subtropical climate regions, thus beachrock occurrences at higher latitudes (cold-temperate zones), where the seawater tends to be subsaturated with respect to CaCO3 and low rates of evaporation are registered, have received less attention and are even less well known. Therefore, new research seems to be required in order to better understand the early carbonate diagenesis processes in temperate marine environments. The beachrock formations studied in the present research work are located eastwards the mouth of the Nerbioi-Ibaizabal estuary (Bilbao, Basque Country, North of Spain) in the emplacements of Azkorri beach and Tunelboka cove which represent an unprecedented research area for such temperate climate sedimentary outcrops due to their exceptional state of conservation. Historically, the areas have been affected by several impact factors including: a) a high metallic stress coming from the estuary plume, b) high amount of slag and anthropogenic wastes coming from blast furnace of old steel factories that were thrown away to nearby submarine disposal zones, reached the coast by the predominating marine currents and cemented within the sedimentary outcrops and c)Tunelboka cove was the emission point of the outfalls from the first sewage treatment plant of Bilbao . The formation processes of these beachrocks have not been clarified till present days, as a consequence this PhD project has contributed on the clarification of this early carbonate diagenetic phenomenon. Particularly, based on the investigation of the mineral phases of the cements using petrographic examinations and advanced non-destructive spectroscopic techniques such as micro-Raman spectroscopy, SEM-EDS and SCA, the diagenetic context in which the beachrock formations developed has been defined. In addition, destructive analyses carried out over the cement forms as Stable Isotopic Analyses of ¿18O and ¿13C, as well as XRD analyses have also confirmed the origin of the cementation process. Based on those data, 4 cement generations have been identified and among them, the main cement generations were high magnesium calcite (HMC) and aragonite, metastable CaCO3 polymorphs related to marine-phreatic diagenetic environments with a first stage of cementation related to biomineralization activities (due to the presence of HMC forms). The other two, mixtures of iron oxides and silicates with aragonite cement forms, are related to vadose-marine diagenetic context. Furthermore, sporadically calcite cements ascribed to meteoric vadose environs of the latest stages of the diagenetic process have been also identified.Through the analysis of the composition of the amalgamated sediments, the anthropogenic aspects that have altered the coastal environments have been researched, together with the interaction of such pollutants with the sedimentary phenomenon. The elemental quantification was carried out by ICP-MS in different granulometric fractions of the sediments. Based on those analyses, a high metal concentration has been ascertained trapped within the beachrock units (mainly Fe) and the background values of the original sand from Azkorri beach (obtained by test drilling) have also been established. Moreover, the calculation of certain standard environmental indexes verified that the system presents serious anthropogenic pollution evidenced by a moderate-high metallic enrichment presumably related to particulate slag.Finally, the current disintegration and dissolution processes affecting the beachrock units have been examined, since the materials trapped within the beachrock units are releasing to the close environments with the subsequent blackening of the adjacent coastal landscapes, impacts on the aesthetical and recreational uses of the beach, as well as the increase concern of people in this subject.