Geochemical and mineralogical characterization of surficial sediments from the Northern Rias: Implications for sediment provenance and impact of the source rocks
Highlights
► Geochemistry and mineralogy of the marine sediment in the Northern Rias is presented. ► Spatial variations are controlled by source rocks cropping out in the hinterland. ► Geological setting and fluvial loads determine the geochemical and mineralogical pattern.
Section snippets
Background and objectives
The chemical composition of marine surficial sediments is determined by the composition of river-derived material, that is dependant of the catchment petrology (Cho et al., 1999) as well as the biogenic contribution (Lackschewitz et al., 1994). Variations in minerals, lithogenic components, organic material and trace elements abundance, therefore, are a tool for deciphering possible sediment sources and for discriminating physico-chemical processes affecting the geological record.
The
Geography and geology of the studied area
The north coast of Galicia shows a rough morphology, with cliffs up to 100 m high made up of igneous, plutonic, mafic, ultramafic and metamorphic rocks. Eastward of 8°W (Cape Ortegal), and along the 43.6°N are located the Northern Galician Rias Ortigueira, Barqueiro, and Viveiro (locally named Northern Rias or Rias Altas; Torre-Enciso, 1958). These Rias are coastal inlets with an external open area dominated by marine processes and a partially enclosed estuarine shallow area. Different
Material and methods
Seventy one samples were recovered on July 10–12, 2007 in the Ortigueira, Barqueiro and Viveiro Rias and on May 21, 2008, fifteen sediment samples were collected from the continental shelf (Fig. 1). Sampling surveys were conducted onboard the R/V Mytilus and R/V Lura for stations located at depths greater than 10 m, using a 30-liter Van Veen grab sampler. Sediment sampling from shallower stations was carried out onboard small boats using a 5-liter Van Veen grab sampler. The uppermost sediment
Sediment composition
The Northern Rias sediments were mainly composed of sandy fractions, only the sedimentary material of the innermost parts was muddy (Fig. 2). Sediments from Ortigueira Ria contained a wide range of CaCO3 (0.4–35 wt.%) being even higher offshore in the continental shelf where values reached 90 wt.% in the Station C12. C/N molar ratios (Fig. 3) were higher landwards and closer to the Mera, Landoi and Baleo River mouths reaching 20.4.
As found in the Ortigueira Ria CaCO3 content in Barqueiro
Discussion
Seabed sediments of the Northern Rias and shelf are predominantly composed by sand (Fig. 2), due to the high energetic hydrodynamic conditions and the NW swell (Lorenzo et al., 2007b). The origin of the sediment is related to fluvial input, being one of the most relevant sources of fine detrital material in the innermost part of the Rias. Another possible source of supplies is related to the eroded material from exposed coastal cliffs located from Cape Ortegal area and adjacent beaches. Organic
Concluding remarks
Mineral and geochemical composition of the Northern Rias and shelf sediments were used to reconstruct the pathways of modern sediment inputs from the continental domain to Rias and inner shelf. Surficial sediments in these Rias integrate several sources (marine/terrigenous), petrological, lithological characteristics of the river basins draining the area and the human activities on land. The influence of the river-borne sediments in only detected in the innermost part of the three rias, whereas
Acknowledgments
The authors sincerely thank the crew and staff of INTERESANTE cruise on board the R.V. Lura and Mytilus for their kind cooperation during the sampling. This paper was supported by the Spanish Ministry of Science and Technology through project ‘INTERESANTE’ Ref. CTM2007-62546-C03-01/MAR. We thank two anonymous reviewers for comments that contributed to greatly improve the manuscript.
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