No abstract

Beachrocks are sedimentary structures derived from the precipitation of carbonates in the intertidal zone.

Raman spectroscopy, including in-situ and laboratory analyses, were carried out in beachrock outcrops located in temperate latitude (Arrigunaga beach, Bilbao Estuary, North of Spain) to characterise the wide variety of sediments and wastes trapped within the cements. In the examined beachrocks (coastal sedimentary structures formed in the intertidal zone through the early precipitation of carbonates) the presence of abundant slag and anthropogenic wastes is noticeable. In-situ Raman analysis revealed the presence of cristobalite (SiO 2 ), tephroite (Mn 2 SiO 4 ), quartz (SiO 2 ), rhodonite (MnSiO 3 ), labradorite (Ca,Na)(Si,Al) 4 O 8 , hematite (α-Fe 2 O 3 ), gypsum (CaSO 4 ·2H 2 O) and amorphous carbon (C) as the main constituents in the trapped materials. Those were also identified off site together with other phases such as limonite (FeO(OH)·nH 2 O), lepidocrocite (γ-FeOOH), arsenopyrite (FeAsS), fayalite (Fe 2 SiO 4 ) and anatase (TiO 2 ). As secondary compounds, portlandite (Ca(OH) 2 ), C-S-H (Ca 3 Si 2 O 7 ·3H 2 O), gypsum and possibly ettringite (3CaO·Al 2 O 3 ·3CaSO 4 ·32H 2 O) were identified only in-situ, and a possible evolution to the more stable thaumasite (3CaO·SiO 2 ·CO 2 ·SO 3 ·15H 2 O) was found in the laboratory analyses. Therefore, the applied methodology and specifically the in-situ Raman spectroscopy analyses resulted particularly valuable, allowing an accurate description of the trapped materials and the compounds formed by hydration and/or sulphate attack (probably from the high level of atmospheric aerosols coming from seawater or industrial activity nearby), giving an insight of the complexity of the examined outcrops.

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Beachrocks are consolidated coastal sedimentary formations resulting mainly from the relative rapid cementation of beach sediments by different calcium carbonate polymorphs. Although previous works have already studied the elemental composition and the mineral phases composing these cements, few of them have focused their attention on the organic matter present therein. This work describes an extraction methodology based on focused ultrasound solid-liquid extraction (FUSLE), followed by analysis using large volume injection (LVI) in a programmable temperature vaporizer (PTV) combined with gas chromatography-mass spectrometry (GC-MS) in order to determine organics such as polycyclic aromatic hydrocarbons (PAHs) and biomarkers (hopanes), which can increase and confirm the information obtained so far. This goal has been achieved after the optimization of the main parameters affecting the extraction procedure, such as, extraction solvent, FUSLE variables (amplitude, extraction time and pulse time) and also variables affecting the LVI-PTV (vent time, injection speed and cryo-focusing temperature). The developed method rendered results comparable to traditional extraction methods in terms of accuracy (77-109%) and repeatability (RSD<23%). Finally, the analyses performed over real beachrock samples from the Bay of Biscay (Northern Spain) revealed the presence of the 16 EPA priority PAHs, as well as some organic biomarkers which could increase the knowledge about such beachrock formation.