Contemporary beachrock formation at Arrigunaga beach (Bizkaia, Spain) on anthropogenic slag

The large, thick beachrock of Arrigunaga beach (Bizkaia, Spain) is unusual among beachrocks because of (a) Its location, at 43ºN latitude; (b) The substrate that was cemented, largely consisting on smelter slag mixed with natural beach sediment; (c) The timing and amount of slag dumping at sea, with millions of tons of waste dumped in the short interval AD 1902–1966; (d) The sudden cessation of dumping (AD 1966), followed by immediate beachrock exhumation and retrogradation and (e) An aggressive engineering intervention (AD 1999), supposedly aimed at beach regeneration, with imported bioclastic sands, resulting in additional weak cementation of the residual blocks. Thickness of the beachrock and the identification of internal clasts attest for a multiepisodic process with at least three main cementation steps and several CaCO3 polymorphs precipitated from mixed marine and freshwaters. Evidence of bacterial remains suggests that biological activity helped to trigger cementation. C and O isotopic values obtained in the cements confirm the dominance of mixed marine and freshwaters composition. Exact knowledge of the start and finishing dates of slag dumping permits to tightly constrain the rapid cementation, which was already evident at least since AD 1924, that produced the beachrock, as well as its evolution towards its current fast and complete disappearance.


Introduction
Arrigunaga beach (43º 21′ 22.0" N-3º 01′ 10.00" W), in the municipality of Getxo (Bizkaia), is part of the so called "Abra Exterior de Bilbao" (Bilbao's exterior haven), a large embayment of the Nervión estuary ( Fig. 1a-c). The beach, some 630 m long and with variable width of 75-100 m was originally formed by rounded pebbles / boulders, as Fig. 1 a Location of Arrigunaga beach (Getxo, Bizkaia) in "Abra Exterior de Bilbao" (Bilbao's External Haven), with indication of the buoy [*(b)] and the main places mentioned in the text (modified from Google Earth, 2020). b, c Views of Arrigunaga beach at low tide from the NE, at two different seasons. Exposure of beachrock plates or its burial by turbiditic blocks from the cliff behind and/or added bioclastic sands varies with season. Total length of the beach around 630 m. Bathers can be used as scale evidenced by the "Plano de la Barra de Bilbao (1806)" (in Martin-Merás & Rivera, 1990. The landward limit of the beach is a cliff formed on hemipelagic limestone-marl pairs alternating with turbiditic calcarenites of middle Eocene age (Rodríguez-Lázaro et al., 1989). Fallen boulders from the cliff are transported by litoral drift into the beach, where they accumulate. The beach suffers from a chronic bioclastic sand deficit. It is this type of pebble/boulder beach that explains the local name (in Basque language, Arrigun-aga means "place of rounded stones"; Goikoetexea Araluce, 1984).
At the beginning of the twentieth century, industrial activities faced the problem of managing the wastes generated by smelters. The solution at the time was dumping at sea, at a short distance from the coast (4.5-6 km North of Punta Galea), where about 6-12 MTm (amounts calculated by García-Hidalgo & Elorza, 2022;their Fig. 18) of solid residues (metal and fluxing slags, refractory bricks and other smelter wastes) were dumped between 1902 and 1966, generated by "Altos Hornos de Vizcaya" (Martínez Vitores 1999Merino Martínez, 2005). Recent oceanographic surveys have located additional industrial wastes at several points closer to shore (43º 25.5′ N-3º 3′ W; 43º 26.5′ N-3º 1.7′ W, at > 70 m water depth), thus in clear violation of the original authorization (Borja et al., 2008). Littoral drift transported, accumulated and mixed such wastes with natural sediments in the nearby reflective beaches at Barinatxe, Gorrondatxe-Azkorri and Arrigunaga, and in the littoral abrasion platform at Tunelboka and Punta Galea (Fig. 1a). In the high-energy intertidal zone, these mixed materials were rapidly cemented and consolidated, apparently forming a single beachrock restricted to the mesotidal (< 4 m) range. Cementation was enhanced by higher sediment accumulation during summer (beach accretion) and variably eroded away during winter storms (beach retrogradation). However, the uninterrupted supply of large amounts of waste maintained the conditions required for multiepisodic (progradation and agradation) beachrock formation, remarkably thick and wide along the intertidal (foreshore) zone. Historical aerial photographs and geotechnical records from civil buildings in Gorrondatxe-Azkorri beach attest that anthropic sediments (slag wastes) already filled the beach by 1957, with possible cementation below loose sand a few centimetres deep. Sudden halt of dumping in 1966, however, resulted in strong degradation of the beach in the following years, due to high energy tides and wave action during winter storms exhuming part of the beachrock.
As a result, Arrigunaga beach strengthened its reflective (occasionally intermediate towards the W) character, exposing extensive, imbricated beachrock units formed by coarse sand and pebbles, together with large (0.5-1.0 m), loose boulders of turbiditic calcarenite that hindered easy access to the surf for decades (~ 1970 to 1995). Aimed at recovering the beach for recreational use, regional authorities (Diputación de Bizkaia) promoted coastal works that, between 1995 and 1999, included building three groynes and a protective breakwater, as well as partially removing beachrock slabs (~ 16,000 m 3 ), and the reconstruction of a sandy beach with about 213,000 m 3 of natural bioclastic sand dredged from a marine sand bar in the vicinity of Bakio (Bizkaia) following the initial project (Losada & Medina, 1993).
Several decades later, the added bioclastic sand, originally homogeneously spread, has drifted along the beach, having been eroded from the northeastern sector and moved to the SW part of the beach, where it accumulated. As a result, extensive stretches of beachrock, not removed at the time probably due to technical difficulties, are now exposed, together with large, now rounded, blocks fallen from the cliff. In addition, poor maintenance of groynes and breakwater (originally built using Urgonian facies -Aptian-Albian-limestone) has also contributed limestone blocks to the beach (Elorza, 2021).
The beachrock is most conspicuous in the intertidal zone of the NE sector of the beach, currently almost devoid of bioclastic sand. In this area an old trench digged during the 1995-1999 works still exists, and the marks of the excavator's bucket teeth are clearly visible. Beachrock debris then produced has been newly cemented, and the compacted sediment produced by the excavator's footprints is now preserved as nodules easily identifiable due to their lighter colours. The destruction caused by these works hinders the study of sedimentological features, such as the depositional architecture, so in this article we concentrate on the petrographic features of first-generation cements (García-Garmilla, 1990;Iturregui et al., 2014Iturregui et al., , 2016, that are different in the lower foreshore and upper foreshore. But also on the late cements, to the best of our knowledge, so far unpublished, joining together the rubble produced by the late 90's intervention. To ascertain the cement's origin we have studied its C and O isotopic composition, as well as the O and H isotopic ratios of environmental waters, both marine and meteoric, with the aim of characterizing the fluids that precipitated the cement. The possible bacterial mediation of the cementation process was also considered. For reference, small beachrock remains developed on Upper Maastrichtian marl-limestone couplets above Sopelana beach, as well as the microstructure and isotopic features of a carbonate crust formed by small groundwater seeps at Punta Galea cliff have been studied.

Materials and methods
Detailed sedimentary logs, photographic documentation and a large sample set of bulk sediments from the sections studied were collected over several field seasons. We examined more than 50 thin-sections by standard microscopy methods, using Alizarin Red S and potassium ferricyanide staining (Dickson, 1965). All cathodoluminescence (CL) work employed a Technosyn Cold Cathode Luminescence system (model 8200 Mk II) mounted on an Olympus trinocular research microscope with a maximum magnification capability of 400×, using universal stage objectives. Standard operating conditions included an accelerating potential of 12 kV and a 0.5-0.6 mA beam current with a beam diameter of approximately 5 mm.
Fifteen beachrock samples, taken along a vertical section, were collected for C and O isotopic analysis. In addition, two samples of a carbonate crust formed at a groundwater seep well above the surf at Punta Galea cliff were also analysed as representative of purely meteoric groundwater (bi)carbonate, as well as two bioclast fragments, as indicators of the bioclastic sand used for beach restoration. Powder for isotopic analyses was obtained by crushing the bioclasts in a small agate mortar and by means of a handheld microdrill with diamond-coated drill bits from the carbonate crust. For beachrock samples, carbonate cement was scrapped from the surface of slag grains, held under the binocular microscope by means of stainless-steel tweezers, using the tip of a hypodermic needle. To avoid contamination of the mass spectrometer with halogens (NaCl crystals were observed under the binocular microscope in some samples), powders were repeatedly washed with distilled water in an ultrasound bath and dried overnight in an oven held at 60 °C prior to analysis. Isotopic ratios were measured on CO 2 gas produced by reacting c. 50 μg of powder with 103% H 3 PO 4 at 70 °C in a Multicarb device coupled to an Isoprime mass spectrometer via a dual inlet. Results are reported in Table 1 as δ 13 C and δ 18 O relative to V-PDB and V-SMOW. Repeated analyses of both internal and international references indicate reproducibility better than ± 0.02‰ for δ 13 C and ± 0.05‰ for δ 18 O. Waters in and around the beachrock were also collected for isotopic analysis. Collection was spread along the year hoping to record natural seasonal variability, and included three meteoric (rain) waters (collected in winter, spring and summer); groundwater from a natural seep and seven seawater samples collected from holes digged into the beachrock both at high and low tide.
Oxygen and hydrogen isotopic ratios were measured by Cavity Ring-down Spectroscopy (CRDS) using a Picarro L-2130i infrared laser spectrometer at the Stable Isotope Lab of the "Servicio Interdepartamental de Investigación" (SIdI), Universidad Autónoma de Madrid (www. uam. es/ sidi). Normalization of data was achieved by means of several internal references (usually waters remaining after International Atomic Energy Agency IAEA-led round robin tests) interspersed among unknowns. Also, each analyses batch included at least two vials with Vienna Standard Mean Ocean Water (V-SMOW) and one with Standard Light Antarctic Precipitation (SLAP), and usually another one with Greenland Icesheet Precipitation (GISP). Longterm reproducibility (1σ) is better than ± 0.03‰ for oxygen and ± 0.15‰ for hydrogen isotopic ratios. Results are reported in Table 2 as δD and δ 18 O relative to V-SMOW.
A number of samples were selected and examined under a scanning electron microscope (SEM) and qualitatively determined (Al, Si, Fe, P, K, Ca) by energy-dispersive spectrometry (EDX) using a Jeol JSMT6400 at the University of the Basque Country. X-ray diffraction (XRD) analyses were performed with a Phillips PW1710 diffractometer using Cu Kα radiation monochromated by graphite with generator conditions of 40 kV, 20 mA, a step size of 0.02° (°2θ) and a time per step of 1 s.

Climate of the area and characteristics of sea water
From temperature and meteoric precipitation records, climate in the Basque-Cantabrian coast can be assimilated to the Cbf type in the Köppen classification (McKnight & Hess, 2000), which corresponds to a temperate and humid oceanic or Atlantic climate (b: temperate, where the summer is cool because average temperature does not exceed 26 °C in the warmest month and is above 10 °C at least four months per year; f: humid, with 124 days of rain throughout the year, without dry season). Average annual precipitation for the 1981-2010 period at Bilbao airport (9 km away from Arrigunaga beach) was 1134 mm/year, and average relative humidity was 70%. During that period, average annual temperature was 14.7 ºC, with maximum daily averages of 19.5 ºC and minimum of 9.9 ºC. Comparison with equivalent records for the period 1971-2000 indicates lower precipitation and humidity and higher average temperatures for the more recent 1981-2010 period, hinting towards increasing aridity in the area evidenced in a short time span (Online Resource 1). Sea water temperatures and salinities are recorded by several buoys near the beach. Data from "Costera de Bilbao" and "Bilbao-Vizcaya" buoys were provided by "Puertos del Estado" (www. puert os. es/ en-us/ ocean ografi a/ Pages/ portus. aspx) for the period 2004-2020. Considering the year 2010 as reference, average monthly values indicate that the highest temperature (23.9 ºC) is measured in August, while the coldest temperature (10.7 ºC) corresponds to January (Online Resource 2). Mean salinities vary throughout the year by just over 1‰, from 34.5 to 35.7‰, although there have been years when salinities above 36.5‰ were reached. The tidal system is of a semidiurnal mesotidal

Geological context
In the cliff behind Arrigunaga beach, planktonic foraminifera allow differentiating three stretches delimited by faults (Rodrí guez-Lázaro et al., 1989). In the northeastern part appear middle Eocene (Lutetian) turbidite materials consisting of a repeated alternation of calcarenites and coarse calcisiltites with marls. In mechanical contact (considered as a thrust sheet) with the latter there are lower-middle Maastrichtian materials, predominantly marls and marly limestones without calcarenitic levels, whose macropalaeontological content includes ichnofossils, internal moulds of inoceramids and ammonoid impressions. These form a small synclinal structure in mechanical contact to the west with materials from the middle Eocene to the upper lower Eocene. The outcrops at the centre of the cliff are mostly marls, except at some points where turbidite beds with nummulitid fossils occur. Further west, the Algorta-Azkorri sandstones emerge (Payros et al., 2006(Payros et al., , 2009, forming a major development; these rocks are turbiditic quartz-feldspathic in nature. Ostracod associations indicate a deep marine depositional environment that increases from the Maastrichtian to the middle Eocene (Rodríguez-Lázaro et al., 1989).

Location and evolution of the antrophocene beachrock
At low tide it can be seen how the abrasion platform ("1"), developed on folded materials of Maastrichtian-Eocene age, forms the substrate of the anthropic and natural deposits ("2" and "2*") that make up the beachrock ("3") under study (Figs. 2, 3 and 4). The beachrock is best preserved at the northeastern end of the current Arrigunaga beach. In the lower foreshore zone, it is arranged in 20-40 cm thick banks, mostly conglomeratic together with some coarse sand ("3" in Figs. 2e, f, 4). In contrast, the upper part of the foreshore gradually changes into sandy sets with parallel laminations of high flow regime, limited by erosive surfaces, which produces low-angle cross-stratifications inclined 5º-8º towards the sea. The conglomeratic banks (sets) have been fractured and apparently disconnected from the upper sandy part of the foreshore, which is in clear connection with the backshore zone, considered as spray zones, where weak cementation occurs 4). The bottom of each bank is more friable than its top, a common situation in other beachrocks (Hopley, 1986;Turner, 2005). Overall, maximum thickness is up to 6 m. The exhumed surface varies according to season, but it can reach 50 m wide at low tide and when conditions are strong retrogradation, it reaches a maximum length of 120-150 m. In the lower zone of the foreshore the beachrock is covered by filamentous green algae ("3"), while the banks near the supratidal, that undergo continuous abrasion by loose sand, are not colonized (Figs. 2a;4). Nature of polymictic boulders alternating with the sands can either be calcarenites of turbiditic origin or foundry slag, as well as different types of common and refractory bricks (Astibia, 2012) all of which have suffered strong erosion resulting in a tendency towards sphericity (Figs. 2e, f). These conglomeratic lenses wedge towards the upper foreshore and when the pebbles, mostly turbidite, have a dominant major axis, they are imbricated and tend to be parallel to the coastline (Fig. 2e). The upper part of the beachrock, formed by dark sands, reaches the Paleogene rocks of the cliff (Fig. 2g). Usually the top of the beachrock, either covered by green algae or not, shows an erosion pattern of wave-induced fluting, runnels and cavitations (giant's kettle; potholes) as in other beachrocks (Calvet et al., 2003;Howie, 2009;Turner, 2005). Along Punta Galea cliff small remains of beachrock are preserved as basal-conglomerate, with hardly any sand, adhered to the folded beds of the middle Eocene (Lutetian), indicating that in recent times the anthropic deposits, now frankly receding, extended along a greater stretch of the coast (Fig. 2h).

Effect of engineering works
The beachrock that survived 1999's engineering intervention may remain buried by the added bioclastic sand for years (24 months, uninterrupted, have been documented while preparing this manuscript). However, spring tides and large storms can temporarily uncover the traces left by an excavator shovel in the upper area of the foreshore, such as a ditch ("Z"), along with small accumulations of beachrock fragments and drag marks produced by the shovel teeth ( Fig. 2a-d). Beachrock fragments left have been weakly cemented since ("3*" in Figs. 2d; 3a, b). At the end of drag tracks nodules up to 12 cm wide and more than 10 cm high are observed ("3**"). These nodules are individualized from the beachrock ("3") by light-colored, intense cementation that results in recrystallization and loss of porosity, providing greater resistance against weathering/erosion of the beachrock (Fig. 3c). A late cementation is also visible in bioclastic sand, finer and with lighter colours, infilling small cracks ("4*"; Fig. 3d) of the darker beachrock ("3"; Fig. 3d).
For this weak, late cementation to occur, a small protective sheet of unconsolidated beach sand, that preserved enough moisture, must have existed at the time. In this upper foreshore there also are decimetric conglomerate fragments (br1), that correspond to materials already cemented in the lower foreshore, torn out by storms and incorporated into the upper zone (Fig. 3e). The earliest recorded presence of beachrock at Arrigunaga beach dates back at least to AD 1924, through a graphic document (Fig. 3f). Dispersed along Punta Galea cliff there are small surges of underground water supersaturated in carbonate that generates fine laminations of what can be called "lamellar calcrete"? ( Fig. 3g-h). Due to the severe dismantling suffered in 1999, it is difficult to carry out a detailed study on the depositional architecture; however, an attempt has been made to outline the main sedimentological stages in the intertidal zone, with accumulation/gradation, retrogradation, regenerative intervention and strong retrogradation, through which Arrigunaga beach has passed from approximately 1924 to 2020 (Fig. 4). (2*): boulders fallen from the cliff, incorporated to the beach without cementing; (3): beachrock banks (those in the low intertidal zone colonised by green algae); (4) added bioclastic sands used for beach restoration; (z) excavator trench (year 1999). b General appearance of the trench (z) digged in the beachrock (3), covered by bioclastic sands (4). c Detail of a section made up mostly of sands with conglomeratic lenses wedging towards the supratidal, together with large currently uncemented boulders fallen from the cliff (2*). d Detail of the upper part of the trench (z) with bioclastic sands (4) covering broken and cemented fragments (3*) of the beachrock (3). e General view and f detail of the conglomerate (3) with cast edges, refractory bricks and turbidite with traces of lithophages. The white line with arrows indicates the orientation and imbrication of the edges following the coastline. g Subhorizontal beachrock bed (3) at the edge of the cliff, northeastern part of the beach, with debris (b) and loose blocks (2*). h Remains of basal conglomeratic beachrock (3) on the tidal flat (1) along the Punta Galea cliffs. The original boulder beach (see (2) Fig. 4) is not exhumed

Presence of beachrock patches (Sopelana beach)
In an attempt to expand our observations into nearby areas, we have sampled and studied the petrographic characteristics of beachrock patches found adhered onto Upper Maastrichtian marl-limestone pairs from Sopelana beach (Dinarès-Turell et al., 2013;Elorza et al., 2021;Iridoy et al., 2010). There, small remnants of cemented bioclastic sands with subhorizontal laminations formed on the marl-limestone surfaces but not in the marls, probably due to the easy decomposition of the latter. The presence of small leaks of meteoric water that emerge from the cliff and slide through the carbonate series is notable, which suggests that the cementation of the grains of bioclastic sand is related to these leaks ( Fig. 5a-c).

Composition and cements
Materials are heterogeneous, and textures vary accordingly, from poorly ordered conglomerates associated with coarse sands in the lower foreshore, giving way to smaller-sized sands that occupy the upper foreshore and part of the backshore. The conglomeratic blocks (3-30 cm) are polymictic, eroded and rounded, both those from industrial discharges and those formed from calcarenites of turbiditic origin, detached from the cliff (Fig. 2c, e-f). The whitish slag with abundant vacuoles comes from the dolomite-limestone added as a flux in the process of pig iron production in the blast furnace. Its composition, recognized by XRD, is mostly pseudowollastonite (CaSiO 3 ) and Gehlenite-Akermanite Ca 2 (Mg 0.5 Al 0.5 ) (Si 1.5 Al 0.5 O 7 ), with some amorphous glasses recognized under the microscope. Opaque ore grains Fig. 4 General view of the present Arrigunaga beachrock and approximate evolutionary scheme in three stages, from ≈1924-1995; 1995-1999  Wüstite can be found in meteorites and basalts, but it is also produced at high temperatures (> 570 °C) in the reducing environment of anthropogenic slag (Cornell & Schwertmann, 2003;Gil et al., 2008;He et al., 2021). The coarse sandstone (1-1.5 mm) banks and the sandy matrix of the conglomerates have the same composition, made up of blast furnace slag. XRD analysis identified quartz (SiO 2 ), calcite (CaCO 3 ), magnetite, goethite, hematite and wüstite. They stand out for their whitish colour, product of the intense cementation suffered, which contrasts with the darker tone of the clasts (Fig. 2e, f). Under conventional petrographic microscope and SEM, the small carbonate bioclasts (5-10%) are mostly identified as sea equinoid spines, and fragments of gastropods and bivalves. The initially very high porosity is now reduced by cementation, to a greater or lesser degree according to the zones (Figs. 6, 7).
Natural bioclastic and quartz-feldspathic sands of creamy tones ("4" in Figs. 2,4), obtained by suction dredging of a marine sandbar (> 20-30 m deep) near Bakio (Bizkaia) were dumped at the beach for restoration Fig. 5 a Set of Upper Maastrichtian marly limestone-marl couplets from Sopelana beach (Fig. 1a). Small decimetric beachrock (br) fragments hang above the contact with the beach sand, associated with meteoric water seepage. b Appearance of the cemented beach sands with subhorizontal lamination adapted to the irregularities of the marly limestone bed. c-e photomicrographs of the cemented beach sand: opaque ores from the blast furnace slag can be seen in addition to bioclasts and quartz grains. f, g SEM view and detail of bioclast grains cemented by bladed calcite crystals with a tendency to form meniscus contacts characteristic of a vadose meteoric environment as part of 1995-1999 engineering works (Elorza, 2021). Grain size is smaller (0.2-0.5 mm), the grains are loose and this sand is currently remarkably homogenized with the sediments resulting from dismantling of the beachrock itself (Figs. 1b,c;. Occasionally, late cementation occurs, when a small part of the bioclastic sands ("4" in Figs. 2, 4) fall down fractures mechanically produced in the beachrock, leaving them ("4*") and the fragments ("3*" in Fig. 3d) of beachrock produced weakly cemented (Figs. 2d, 3a, b, d, 8).

The aragonite micritic cement
Examining the sandy matrix of the basal conglomerates, using the conventional petrographic microscope and SEM, the presence of a fine fringe-film (< 5 microns) of micritic aragonite is observed, mainly visible in the bioclasts, apparently conditioned by bioerosive activity, as it is recognized and accepted in the literature (Adams & MacKenzie, 1998;Konhauser, 2007;Tucker, 1991;Tucker & Wright, 1990). The micritic aragonite remains in continuity and without a

Fibrous aragonite cement in needles
Long needles of fibrous aragonite cement fill large parts of the intergranular porosity forming isopaque circumgranular rims perpendicular to the clast surface (Figs. 6, 7). Occasionally, it also forms in intragranular pores. In spite of their large volume, the central spaces of the pores remain empty, with visible meniscus cementations but not pendant forms. Pseudospherulitic forms, or spatic habits filling up the pores, common in other beachrocks (Calvet et al., 2003;Vieira & De Ros, 2006), are not produced either. Individual needles can reach up to 90-100 μm in length and 2-5 μm in width at the basal part, and have pointed terminations. Under the microscope the fibers show pseudo-wavy extinction and are not luminescent under CL, as expected due to their formation in an oxidizing environment (Neumeier, 1998). Around bioclasts, a characteristic syntaxial cementation can be observed (Figs. 6c, d). Such intense aragonite cementation results in Fig. 7 a SEM image of anthropic clasts with vacuoles, where a contact zone with small micrite crystals (m) can be seen to pass to the well-developed acicular crystals of aragonite (a). The crust (c) surrounding the clast is an alteration crown, typical of the anthropic material independent of the micrite. b Late development of nodular forms of iron oxides, possibly goethite, on the acicular crystals. c Late development of an irregular network of clays and carbonates on the aragonite crystals. d, e General view and detail of the cementation closest to the grain with clear evidence of Coccus-type (cc) bacterial activity a remarkably compact rock with lighter shade than would be expected given the noticeable presence of slag with iron oxides.
Large unoccupied areas at the centre of pores and cements with meniscus geometry but not reaching pendant forms are apparently indicative of vadose environments, despite the beds corresponding to the low intertidal zone, more prone to present cements typical of the phreatic zone. It could also correspond to a vadose microenvironment within the phreatic zone (Beier, 1985).

Low-Mg calcite cement
This type of cement is restricted to the dark sandstones, formed by slag and bioclasts (Fig. 2b-d), located in the upper foreshore and backshore zones, and to the finer sand infilling the mechanical fractures in the above (Fig. 3d). Petrographic observations and SEM show the same type of cement in both lithologies, although with morphological variations (Fig. 8a-e). Thus, there are low-Mg scalenohedral bladed calcite crystals with staggered shapes and Fig. 8 a Photomicrograph under the binocular microscope of the different types of clasts, poorly cemented, belonging to the mechanically fractured beachrock (3*). b Appearance of the slightly cemented bioclasts (4*) that fill the fractures of the beachrock. c Small magnification photomicrograph of the contact (white line) between the initial beachrock, mechanically fractured, with a second cementation stage (3*), and with the bioclastic sands infilling the cracks, also cemented, forming a new stage (4*) (see Fig. 3d). Note the differences in grain size and composition between both. d Photomicrograph of the clasts of the initial fractured and cemented beachrock (3*), with high porosity and more regular cementation. Crossed nicols. e Photomicrograph of the bioclastic sands, with quartz and bivalve fragments with some grains of opaque ore, slightly cemented, maintaining a high porosity (4*). Crossed nicols crystal defects (step-side, defective crystals), either because they have grown at successive impulses, typical of a vadose environment, or due to possible subsequent surface corrosion ( Fig. 9a-f). Low-Mg calcite crystals (major axis 10-20 µm) are not present on all grain surfaces, so they never become circumgranular, concentrating mainly on the contacts between grains, with clear examples of meniscus fabric, and without the presence of micrite. This fabric produces a poorly cohesive and easily erodible dark rock.
In the beachrock patches of Sopelana beach (Fig. 5a-c), the cementation, examined by polarizing microscope and SEM, consists of the growth of net bladed Mg-poor calcite crystals with a semi-circumgranular arrangement and perpendicular both to bioclasts and quartz-feldspathic clasts. In turn, most of the clasts remain cohesive with each other with geometric shapes of meniscus, with sizes between 30 and 40 µm ( Fig. 5d-g). No early micritic aragonite growth is seen on the surface of the clasts.

Cements in lighter laminations inside the upper foreshore
Although scarce, the presence of a withish cement, observed inside the upper foreshore zone outcrop, allows separating light-toned laminations from the general mass of poorly cemented materials with darker tones of the upper foreshore zone (Fig. 10a). This cement turns out to be larger-sized fibrous-radial aragonite, whose cores (5-6 µm in diameter) are formed by clustered nanospheres (≈100 nm each), from which radiate aragonite crystals that can reach 10-15 µm (Fig. 10b-d). The possibility that these nanospheres were initially fossilized as vaterite, an unstable polymorph of CaCO 3 , could not be confirmed by XRD in the samples analyzed, possibly due to its low overall volume. Around the vitreous flux paste clasts from blast furnaces, a very thin micrite fringe is also generated. More relevant, however, is the formation of a thicker aureole, which can be confused with micrite, but that is produced by alteration of the clast itself, forming concentric lamellae with mamelonar shape that advance towards the interior of the clast. Under SEM the globular forms of advance of the alteration are remarkable in that the presence of Spirillum-type bacteria, now fossilized by an Al, Mg, Ca-silicate composition, typical of slag (Fig. 10e-h), can be confirmed.

Cementation in nodules
The formation of scattered nodules in the upper part of the beachrock by mechanical pressure from an excavator shovel was mentioned earlier (Fig. 3c). Examined under the petrographic microscope and SEM, the clasts of the opaque ores show some orientation and fracturing (Fig. 11a, b), but the most remarkable feature is the strong alteration of flux material (limestone, dolomite) slag clasts, with a high number of vacuoles, now converted into Fig. 11 a, b Fragment and detail of a nodule of beachrock (3* and 3 in Fig. 3c), resulting from mechanical manipulation with fracturing, pressure and subsequent cementation. c, d Appearance of grains of a diverse nature (slag, metallic remains of foundry) affected by an intragranular alteration with parallel laminations that advance towards the interior of the clasts. Fractures and compaction are appreciated. Later cementation by calcite is very irregular. Parallel and crossed nicols. e Detail of clasts of diverse nature affected by the alteration in bands that advances towards the interior. The calcitic cement that supports the ensemble is irregular and difficult to pinpoint with this technique. Parallel nicols a vitreous/amorphous paste. These are stained pink due to the presence of Ca, and surrounded by wide peripheral crowns, some with concentric laminations that advance towards its interior (Fig. 11c-e). Roaming of the excavator affected the cementation as well, producing varied habits, although all of them having in common the presence of bladed calcites, irregularly arranged on the surface of the clasts, and also evidence of prismatic growth of aragonite, typical of the upper foreshore zone conditioned by storm waves (Fig. 12).

Ferruginous cements
The presence of irregular ferruginous cements and late clayey masses in the empty spaces left by the aragonite crystals (Fig. 7b, c), were already highlighted by Arrieta et al. (2017). Applying various tools [Raman imaging, SEM-EDS and Structural and Chemical Analyzer (SCA)], these authors detected mineral species such as hydrated goethite, hematite, magnetite, magnesiumferrite, lepidocrocite and goethite as by-products of hematite degradation triggered by Fig. 12 a-h Different SEM views of a diagenetic core, product of the mechanical action of an excavator shovel, with fracturing and compaction (3* in Fig. 3c), with different habits of cementation, mostly bladed crystals of low Mg calcite and acicular aragonite. These differences in cementation suggest successive variations in the environment with the incorporation of marine and meteoric waters at different stages and intensities in an upper foreshore to backshore zone with clear evidence of Spirillum-type (s) bacterial activity atmospheric/aqueous leaching processes. Additionally, calcite and gypsum minerals also evidenced the action of meteoric waters, dry deposition processes or the attack of SOx acid gases. The presence of such compounds is modifying the cement stratigraphy and suggests that dissolution of carbonates is currently taking place. Those facts influence the erosive susceptibility and the release of the anthropogenic materials trapped originally in the beachrocks, which could act as potential secondary sources of contaminants to the coastal environment (Arrieta et al., 2011(Arrieta et al., , 2017.

Stable isotope geochemistry
Carbonate δ 13 C PDB and δ 18 O SMOW values measured are reported in Table 1, and plotted in Fig. 13. During analysis oxygen isotopes fractionate between the carbonate and the CO 2 actually measured. Although both aragonite and low-Mg calcite cements contributed, in different proportions, to the formation of the beachrock at Arrigunaga, aragonitic cements were earlier shown to dominate the lower foreshore, while calcite is the most abundant cementing phase in the upper backshore as well as in bioclasts and freshwater carbonate crusts, so oxygen isotopic ratios reported have been corrected accordingly (i.e., the fractionation factor for aragonite (Kim et al., 2007) was used for lower foreshore samples, while for upper backshore and freshwater samples that of calcite (O'Neil et al., 1969) was employed instead).
Clear isotopic differences between aragonitic lower foreshore and calcitic upper foreshore cements are evident in Fig. 13, as well as between beachrock cements and both biogenic carbonates or freshwater crusts. Upper foreshore to backshore cements show good internal correlation (r = 0.927 for n = 8, significant to > 99.9% confidence), and plot in an intermediate position between lower foreshore (and therefore dominantly marine) and freshwater carbonates, thus suggesting a mixed origin.
Water δD SMOW and δ 18 O SMOW values measured in water, as well as associated atmospheric parameters at the time of sampling are reported in Table 2, and plotted in Fig. 14. As expected, δD and δ 18 O of precipitation plot in the vicinity of GMWL (Craig, 1961) and vary according to season. Groundwater has values that fit well within the ranges defined by precipitation, plotting in an intermediate position and very close to GMWL, in agreement with the general wisdom that groundwater isotopic ratios record the weighted average values of precipitation in the recharge area.
Most seawaters sampled at the beach have values within analytical error of SMOW, although the winter sample has significantly lower values that trend towards those shown by meteoric water in Fig. 14; particularly so towards the value measured for groundwater, thus rising the possibility of mixing between marine and meteoric waters in the area now occupied by the beachrock. That this may occur is reinforced by the good overall correlation (r = 0.97, for n = 11;

Discussion
Beachrock was defined by Scoffin and Stoddart (1987) as the consolidated deposit that results from lithification by calcium carbonate of sediment in the intertidal and spray zones of mainly tropical coasts.
Subsequently, Mauz et al. (2015) indicated that beachrock is an intertidal deposit forming in the zone where carbonate saturated meteoric and marine water mix and pCO 2 decreases. A significant number of publications in the last few decades have documented the presence of beachrocks in the intertidal zone within latitudes from 35º N to 25º S (see compilations by Vousdoukas et al., 2007;Friedman, 2011;Mauz et al., 2015). Additionally, beachrocks are also cited in humid, arid or semi-arid climates, and even in temperate areas, being scarce or exceptional in cold, paraglacial climates and in lacustrine systems (Avcioğlu et al., 2015;Cooper et al., 2017;Howie, 2009;Öztürk et al., 2015;Rey et al., 2004;Turner, 2005;Vousdoukas et al., 2007 and authors therein).
A large part of the beachrocks recognized and documented in the published literature remain exposed throughout the year (Vousdoukas et al., 2007), although they may be intermittently covered in calm down conditions and be exhumed by the effects of strong storms, as occurs in Corrubedo beach, Galicia (Rey et al., 2004). They may even be destroyed, as it occurred on the beach of Las Zamoras, La Palma, Canary Islands (Calvet et al., 2003). Many authors argue the beneficial implications that the beachrock has for the evolution and preservation of the beach itself by attenuating wave energy during storm events and maintaining the seasonal sand balance (Vousdoukas et al., 2007 and more authors). Also, the presence of beachrocks is invoked as a tool for reconstructing relative sea level in the far-field (Mauz et al., 2015) and as a possible indicator of sea-level change (Kelletat, , 2007Knight, 2007) and palaeoclimate (Friedman, 2005(Friedman, , 2011Mozeley & Burns, 2006). Vött et al. (2010), however, draw attention to the possibility of misleading fossil shorelines in this way, when the shoreline has been frequently affected by tsunamis, with the formation of dislodged beachrock fragments. Beachrock is recommended not to be used as sea level indicator in future studies unless a tsunamigenic formation can be definitely excluded.
There is evidence that suggests that formation of a beachrock is a rapid process, although variable from months to years to decades (Frankel, 1968;Friedman, 2011;Wiles et al., 2018). For diffusive transport of CO 2 through the overlying sediment to occur (Hanor, 1978) and for cement precipitation to function undisturbed, the sediment supply rate must be limited. The rate of cementation must exceed the rate of sedimentation for lithification to occur (Beier, 1985;Mauz et al., 2015;Shinn, 1969).
The Arrigunaga beachrock is a singular case in this regard, since the continuous sediment supply has  Craig (1961), for reference accumulated important thicknesses (≈ 6 m) over a short period of time (AD 1902(AD -1966 and the most intense cementation has occurred in the lower foreshore zone. Since high sedimentation rates can reduce and even prevent lithification (Beier, 1985), Arrigunaga's beachrock thickness must have been achieved in a stepwise (multiepisodic) manner, by progradation with continued aggradation. On the other hand, there is evidence of episodic dismantling of the beachrock, whose fragments have been included within another sediment, itself later transformed into beachrock (Fig. 3e). We are not aware of early citations in the literature mentioning the presence of beachrock at Arrigunaga beach, but we have found a graphic document -a family photograph from AD 1924-where the presence of partially exhumed beachrock is evident (Fig. 3f).
We interpret that the reflective characteristics of the beach, the mesotidal regime (> 4 m) and the usual strong storms (Online Resource 3), widened the area where splash and marine spray could mix with fresh waters, favouring cementation of the sands in the upper foreshore zone, even up to the sands that came into contact with the marly rocks of the middle Eocene at the foot of the cliff, as can be seen in Fig. 2g. In general, it is observed that cementation is more intense in the top part of each conglomerate bed and less so in the bottom part, thus facilitating basal undermining and subsequent subsidence of the parts above, as it occurs elsewhere (Hopley, 1986;Turner, 2005). This form of cementation is in keeping with multiepisodic beachrock formation, as it is difficult to envisage a single cementation event producing close to 6 m thick beachrock if diffusive transport of CO 2 through thin overlying loose sediment is required to occur. In support of this idea of multiepisodic growth, the usual thicknesses of beachrocks vary from 0.5 to 2 m, being thicker in areas with larger sea level fluctuations (Vousdoukas et al., 2007;Calvet et al., 2003 (1.5 m);Mauz et al., 2015 (< 2 m); Howie, 2009 (1.5 m); Vieira & De Ros, 2006 (cm to < 3 m)).
Accepted mechanisms of beachrock formation are fourfold, and can be complementary: (a) physicochemical precipitation of calcite and aragonite with high Mg content from seawater as a result of high temperatures, CaCO 3 supersaturation and/or evaporation. (b) physicochemical precipitation of calcite and aragonite with low Mg content, by mixing meteoric water and fresh groundwater with seawater. (c) physicochemical precipitation of calcite and aragonite with high Mg content by CO 2 degassing of water from the pores of the beach sediment. (d) precipitation of micritic calcium carbonate as a by-product of microbiological activity (Turner, 2005;Vousdoukas et al., 2007 and references herein).
In temperate latitudes, seawater is slightly undersaturated in CaCO 3 and therefore beachrocks seldom form (Rey et al., 2004). According to these authors, the contribution of carbonate-enriched meteoric waters seems to be required (although these authors do not use isotopic analysis to confirm this contribution), producing cements of calcite with low Mg content, (Rey et al., 2004). Initially, in a water mixing zone, the higher the temperature reached, the less % of seawater is required to achieve supersaturation and precipitation of the cement (Mauz et al., 2015).
In the northeastern end of Arrigunaga beach, the beachrock is visible to a greater or lesser extent throughout the year, due to retrogradation enhanced by: (a) mesotidal regime (< 4 m) of high energy, (b) winter storms with 11-9 m maximum wave height and (c) the abrupt cessation of industrial input since 1966. The presence of sandstones associated with conglomerate beds in the low intertidal zone seems to be the product of the usual sequences of seasonal calm and storm, typical of the Cantabrian coast (Flor & Flor-Blanco, 2016;Martos de la Torre & Flor, 2004).
Overall, Arrigunaga's beachrock is somehow comparable to the modern beachrock in Durban (KwaZulu-Natal), in the vicinity of an old whaling station operating between 1908-1975, where human artifacts such as harpoon heads, bricks or even a World War II grenade have been cemented (Cawthra & Uken, 2012). Friedman (2011) quotes Frankel (1968 who described the wreckage of an aircraft in the northern islands of the Great Barrier Reef in which aircraft parts are cemented into the surrounding well lithified beach sediment. Emery et al. (1954) recorded World War II cartridge cases, glass bottles, and shrapnel fragments cemented into beachrock and even a sardine can from a previous visit had already been fixed in place one year later (Friedman, 1998). Turner (2005) considers that the control and influence of bacterial metabolic activity is evidenced by (a) dark, organic-rich micritic rims identified around cemented grains. (b) microbially-mediated precipitation of carbonates in both marine and terrestrial environments. (c) bacterial populations are particularly large and productive in the intertidal zone. Once biologically mediated cryptocrystalline cements are established as nucleation sites, larger crystals precipitated via physicochemical processes can grow and bridge the sediment grains (Neumeier, 1998(Neumeier, , 1999Turner, 1995).

Biologically induced mineralization
In the lower foreshore zone of Arrigunaga beach, where micritic aragonite is detected and circumgranular aragonite is dominant (Figs. 6, 7), the presence of Coccus-type bacteria is confirmed. These are present in large numbers as nanospheres (≈ 100 nm) at the contact of the clasts and at the beginning of cementation (Fig. 7d, e), possibly being covered by aragonite (although vaterite is not confirmed by XRD). Activity of cyanobacteria to generate a mesh of algae does not seem plausible because they need sufficient light and it is known that below a few millimeters the luminosity decays.
The non-preferred orientation and variable carbonate habit is characteristic of a cement precipitated from meteoric waters (Adams & MacKenzie, 1998, p. 107). Periodic inundation of the upper foreshore by seawater (tides) and seawater filtration into backshore sediments (either by storm waves, splashes and marine spray) is common, but the contribution of meteoric water into these environments can also be relevant: average annual rainfall of 1134 mm, with average temperatures ≈ 10 ºC, typical of temperate zones were recorded for the 1981-2010 period in a nearby meteorological station (Bilbao Airport; Online Resource 1). As a result, a zone of water mixing occurs, which functions as an open aquifer, with different contributions of water types according to sea dynamics and season, producing a repeated and conspicuous cementation such as that recognized (Figs. 8, 9a-d) (also see Mauz et al., 2015). Supporting evidence regarding the presence of low-Mg calcite comes from the nearby Sopelana beach, where single bladed calcite crystals, clearly formed in a meteoric input domain, dominate the cementation (Fig. 5).
Dark sands of the upper foreshore have meniscus cementation consisting of bladed calcite, where the presence of bacteria is also visible: occasionally, bladed calcite crystals are covered by a biofilm formed by Exopolysaccharides (EPS) (Fig. 9e, f), but calcite crystals formed by the successive accumulation of nanospheres of Coccus-type bacteria are most commonly recognized (Fig. 9g, h). Similar bacterial globular forms have been obtained experimentally (Neumeier, 1999;Rodriguez-Navarro et al., 2007). Small sheets of limited extension of whiter-shaded sands, generally associated with concave channel shapes also show up in this same area of the upper foreshore. These are interpreted as a greater contribution of seawater by storm waves and/or splashes relative to meteoric waters, which favors net growth of radial aragonite crystals from a lumpy core of small nanospheres (Coccus-type), which possibly were ureolytic bacteria ( Fig. 10b-d). In addition, clasts of the flux slag present an alteration front, shaped as a circular crown, whose surface shows small flexed "worms" of silicate composition (Ca, Mg silicate, according to EDX) suggestive of intense Spirillumtype bacterial activity (Fig. 10e-h).
In the backshore zone, where sands from the blast furnace were mixed with bioclastic sands from a sea bank mechanically dumped for beach restoration (Fig. 2a, b, d), cementation is very weak but there is also evidence of Coccus-type bacterial activity, similar to that found in the lower and upper foreshore. Examination of the nuclei produced mechanically by the teeth of the bulldozer showed evidence of bacterial activity as well (Fig. 12f, g).
The metabolic activity of ureolytic bacteria in the sediment, that do not require light, releases NH 3 and CO 2 , triggering a significant rise in pH due to the transformation of NH 3 to NH 4 + and OH − , which favours carbonate supersaturation and calcification around the bacteria, facilitating the development of radial aragonite crystals (Fig. 10b-d). Possibly these images are the best proof of bacterial activity triggering nucleation and ensuing growth of the aragonitic cement. Analogous calcification has been reproduced in the laboratory where vaterite forms, as a polymorph of unstable CaCO 3 (Dhami et al., 2013;Rodriguez-Navarro et al., 2007). Also, bacteria recognized in karst caves (Altamira, Tito Bustillo, Candamo in Spain; Grotta dei Cervi in Italy; and in catacombs in Malta and volcanic substrates like Saint Callixtus Catacombs, Rome, Italy) are fossilized by hydromagnesite Mg 5 (CO 3 ) 4 (OH) 2 0.4H 2 O, vaterite and aragonite (Cañaveras et al., 1999;Groth et al., 2001;Sánchez-Moral et al., 2003;Zammit et al., 2011).
It must be kept in mind that during the years when the beachrock was being formed , all domestic and industrial wastewater was discharged into the Bilbao estuary, and it was not until 1979 that the Plan Integral de Saneamiento de Bilbao Metropolitano (general sanitation program) was approved, with the Galindo treatment plant handling, from 1990 onwards, the sewage generated by an equivalent population of 850,000 (about 350,000, m 3 /d; https:// cultu racie ntifi ca. com› 2020/06/01).

Isotopic evidence
δD and δ 18 O values measured in water conform to what it might be expected given the nature of the samples analysed: precipitation and groundwater scatter around GMWL (Craig, 1961), and the groundwater values are intermediate relative to those of precipitation, whose δD and δ 18 O vary consistently with sampling season. Most seawater samples collected at the beach are very close to SMOW, and collection at high or low tide doesn't seem to make a difference. Winter samples Arri-2 and -3 (high and low tide, respectively) however have δD and δ 18 O values that are significantly different of typical seawater, and summer samples Arri-9 and -10 show marginally higher δ 18 O, but clearly heavier δD than SMOW.
When plotted together in Fig. 14 seawater δD and δ 18 O correlate, and extrapolation of the correlation line to GMWL intersects very close to the actual value measured for the groundwater sample, thus hinting to a mixing line between seawater, at times slightly evaporated at the beach surface (as indicated by lightly positive δD of Arri-9 and -10) and average meteoric waters in the area as represented by groundwater sample Arri-5. That mixing of waters occurs in the beach is reinforced by the overall positive correlation (r = 0.97, for n = 11; significant at > 99.9% confidence).
The plot of measured δ 13 C vs δ 18 O values of carbonates shows a clear positive trend, with the lowest values corresponding to calcitic crusts sampled on the cliff, well above the level of beachrocks, and higher ones measured in the aragonitic cements of the lower foreshore beachrock. Calcitic cements of the foreshore to backshore plot in intermediate positions. Isotopic values of bioclasts are unrelated, thus indicating that the bioclastic sands employed for beach restoration in the late 90's did not intervene on beachrock formation or evolution.
The positive trend observed for beachrock cements between values of undoubtedly continental carbonates (calcitic crusts formed at groundwater seeps in the cliff) and those typical of marine HCO 3 − suggests a mixing process between an end-member characterized by low δ 13 C and δ 18 O, as would be expected for soil CO 2 , and higher δ 13 C and δ 18 O marine HCO 3 − . When considered within the context of the actual beachrock at Arrigunaga, calcitic upper foreshore to backshore cements have lower δ 13 C and δ 18 O than aragonitic foreshore cements. This contrast is particularly marked for δ 18 O, such that there is no overlap of values at all. Figure 15 has been constructed considering equilibrium conditions. For Fig. 15a we have calculated the δ 18 O values of carbonate (calcite for cements of the upper foreshore to backshore; aragonite for the lower foreshore) in equilibrium with typical seawater measured (δ 18 O V-SMOW = + 0.24‰) or mixed marine-freshwater (δ 18 O V-SMOW = − 2.03‰). Values corresponding to calcite in equilibrium with measured groundwater (δ 18 O V-SMOW = − 5.67‰) are included for comparison. For Fig. 15b the calculated values are those in equilibrium with average aragonite of the lower foreshore (δ 18 O V-SMOW = 31.36‰, having discarded the two analyses with negative δ 13 C) and average calcite from the upper foreshore (δ 18 O V-SMOW = 29.16‰; backshore calcite with extremely negative δ 13 C was not considered). Again, freshwater calcite crusts were included for comparison.
It is remarkable from Fig. 15 that equilibrium temperatures independently calculated from either measured cements or measured waters do coincide in the environ of 15 °C, which is equivalent to the average annual temperature in the area for the period 1981-2010 (14.3 °C; Online Resource 1) and within the range of monthly averages of maximum and minimum temperatures of surface waters measured in 2010 at a nearby buoy (T Max -T min = 14.6-17.8 °C; Online resource 2).

Conclusions
The beachrock at Arrigunaga (43º N latitude) is the vestige of a larger one formed from the end of the 20's of the last Century onwards. It was almost completely dismantled in 1999 by engineering works aimed at restoring the beach for recreational use. For this reason, it has not been possible to study its depositional architecture in its entirety. This beachrock is unique in that it is made up of blast furnace residues (6-12 million tons of slag dumped between 1902 and 1966 in high sea), mixed with natural sediment. Cessation of the discharge, and therefore of the contribution to the coast, immediately triggered an intense stage of retrogradation and exhumation of the early formed beachrock. The severe intervention in 1999, with the destruction and removal of a large part of the beachrock, and the subsequent new and weak cementation of the residual blocks covered with natural bioclastic a b  sands evidenced that beachrock formation is a currently ongoing process. Original thickness of the beachrock (up to 6 m) suggests a cementation process that is not unique but multi-episodic, staggered in time under equivalent physical-chemical conditions, since already cemented fragments of beachrock included within it are recognized. Three stages of cementation and the presence of several polymorphs of CaCO 3 conditioned by the action of seawater and mixed marine-meteoric water, as well as evidence of the role of bacterial activity in formation of the nucleus over which cementation was triggered were described. In the early stage of cementation, the sediments located in the lower foreshore zone were made strongly cohesive by precipitation of aragonite and micritic aragonite, while the nature of cementation in the area from upper foreshore to backshore was of calcite poor in Mg and less intense. The beachrock fragments produced during the regenerative intervention on the beach, found in the upper foreshore, as well as some bioclastic sands that filled fractures, have been joined together by a weak calcite cement poor in Mg. Isotopic ratios of C and O indicate that lower foreshore aragonitic cements are typically of marine origin, while mixing with freshwaters in increasing proportions towards the upper foreshore and backshore was recorded in the isotopic signature of the prevailing calcitic cements of these zones. Knowledge of the initial and final dates of the anthropic discharges and the subsequent regeneration of the beach, allows us to specify in time the rapid cementation processes recognized (already ongoing in AD 1924, merely 22 years after the start of dumping) and the consequent evolution of the beachrock, currently in the process of being completely dismantled.