Diagenetic history of lower Pliocene rhodoliths of the Azores Archipelago (NE Atlantic): Application of cathodoluminescence techniques

Micron 80 (2016) 112–121

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Diagenetic history of lower Pliocene rhodoliths of the Azores Archipelago (NE Atlantic): Application of cathodoluminescence techniques A.C. Rebelo a,b,c,d,∗ , R.P. Meireles e , V. Barbin f , A.I. Neto a,g,i , C. Melo c,h , S.P. Ávila a,b,c a

Departamento de Biologia, Universidade dos Ac¸ores, Campus de Ponta Delgada, Apartado 1422-801 Ponta Delgada, Ac¸ores, Portugal CIBIO—Centro de Investigac¸ão em Biodiversidade e Recursos Genéticos, InBIO Laboratório Associado, Pólo dos Ac¸ores—Departamento de Biologia da Universidade dos Ac¸ores, 9501-801 Ponta Delgada, Ac¸ores, Portugal c MPB—Marine Palaeobiogeography Working group, University of Azores, Portugal d SMNS—Staatliches Museum für Naturkunde Stuttgart, Rosenstein 1, D-70191 Stuttgart, Germany e Universidade Federal de Santa Catarina, Laboratório de Oceanografia Costeira, Florianópolis, SC, Brazil f GEGENAA, EA 3795, Université de Reims Champagne-Ardenne, France g cE3c—Centre for Ecology, Evolution and Environmental Changes/Azorean Biodiversity Group, and Universidade dos Ac¸ores, Departamento de Biologia, 9501-801 Ponta Delgada, São Miguel, Azores, Portugal h Departamento de Geociências—Universidade dos Ac¸ores, Apartado 1422, 9501-801 Ponta Delgada, Ac¸ores, Portugal i CIRN—Universidade dos Ac¸ores, Apartado 1422, 9501-801 Ponta Delgada, Ac¸ores, Portugal b

a r t i c l e

i n f o

Article history: Received 6 August 2015 Received in revised form 13 October 2015 Accepted 14 October 2015 Available online 19 October 2015 Keywords: Cathodoluminescence-microscopy Back scatter electron image microscope Energy dispersive X-ray spectrometer Rhodoliths Lower Pliocene Azores Archipelago

a b s t r a c t The diagenetic history of calcareous fossils is required for their application as palaeoenvironmental indicators. In this study, cathodoluminescence-microscopy (CL microscopy) and back scatter electron image–energy dispersive X-ray spectroscopy (BSE–EDS microscopy) were applied to Pliocene rhodoliths from the Azores Archipelago (NE Atlantic) in order to gain additional insight regarding the trace element content distribution throughout the algae thalli, and to ascertain palaeoenvironmental interpretations. Two types of luminescence were obtained: (1) high and (2) low luminescence. Rhodoliths with high luminescence are related with high concentrations of Mn2+ in seawater and low luminescence rhodoliths are related with low concentrations of Mn2+ in seawater. When the rhodoliths were deposited at about 4.0–4.5 Ma, the shoreline configuration of Santa Maria Island was much different than today. The influence of volcanic activity due to the extrusion of lavas and associated products and/or the presence of active shallow-water hydrothermal vents, was reflected in the sea water chemistry, with penecontemporaneous palaeoshores of the island featuring a high sea water concentration of Mn2+ , which mirrored on the rhodolith Mn2+ high concentration. By contrast, rhodoliths located about 2.8 and 2.9 km from the shore, in areas with low seawater Mn2+ concentration, had low luminescence, reflecting the low Mn2+ concentration in seawater. Rhodoliths chemical data and the geological history of the island proved to be congruent with the palaeogeographical reconstruction of Santa Maria Island at the time of the formation of the rhodoliths. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction The reconstruction of the diagenetic history of calcareous fossils is necessary before they can be used as palaeoenvironmental indicators. A reliable tool for the achievement of this goal is

∗ Corresponding author. E-mail addresses: [email protected] (A.C. Rebelo), [email protected] (R.P. Meireles), [email protected] (V. Barbin), [email protected] (A.I. Neto), [email protected] (C. Melo), [email protected] (S.P. Ávila). http://dx.doi.org/10.1016/j.micron.2015.10.004 0968-4328/© 2015 Elsevier Ltd. All rights reserved.

by mean of cathodoluminescence-microscopy (CL microscopy) (Richter and Zinkernagel, 1981; Barbin, 2000; Ries, 2006; Benito and Reoloid, 2012). In a single algal microridge, for example, successive superimposed thallial algae may show different luminescence intensities, which may correspond to changes in physicochemical conditions (Barbin, 2000, 2013). The primary causes for a particular observed pattern under CL may be very complex and controlled by a variety of parameters that are either not known or easily determined (e.g. Marshall, 1988; Machel, 2000; Pagel et al., 2000; Boggs and Krinsley, 2010). Various outcomes appear to be the complex function of Mn2+ activation and both Mn2+

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self-quenching and Fe2+ quenching. Although Mn2+ and Fe2+ are the principal ions that affect CL emissions in carbonates, numerous factors influence the partitioning of these ions into carbonate minerals. Such factors or processes are dependent on activity coefficients, activity of calcium, various chemical species in solution, temperature, crystal-surface structures, crystal-growth rates, pH, as well as redox reactions (Boggs and Krinsley, 2010). The chemical information associated with these changes is of great interest in order to discuss environmental parameters, to understand changes in seawater temperature, salinity, and to interpret results from geochemical investigations related to palaeoenvironmental parameters recorded by biogenic carbonates. Factors such as the concentration of activator (Mn2+ ) and quencher (Fe2+ ) are important, as are physicochemical parameters that control the intensity of CL (Marshall, 1988; Barbin and Schvoerer, 1997). CL, together with back scatter electron image microscopy–energy dispersive X-ray spectroscopy (BSE–EDS microscopy), as employed in this study, offers a novel approach in order to trace changes in the immediate environment. Coralline algae are cosmopolitan in geographic distribution and have an extensive fossil record (Borowitzka and Vesk, 1978; Basso and Tomaselli, 1994; Rasser and Piller, 1999; Rasser, 2000; Harvey et al., 2005; Iryu et al., 2012). Most genera are extant and have relatively long-life spans, thus increasing the confidence of which comparisons can be made between present-day coralline ecology and ancient corallines (Bosence, 1991). For these reasons, coralline algae constitute excellent palaeoenvironmental indicators and their occurrence in the fossil record may facilitate a better understanding of past sedimentary processes. Rhodoliths are nodules formed by unattached crustose coralline algae (Corallinales, Rhodophyta). Their skeletons are composed of calcite (CaCO3 ) resulting from organic precipitation processes that occur within the organic “matrix” of the cell wall. Thus, the initial formation of CaCO3 occurs in a closed or semi-closed space and not at the open outer surface in contact with seawater. For this reason, the precipitated crystals typically show a distinct orientation in relation to the cell (Borowitzka and Vesk, 1978). These facts strongly indicate that a common mechanism in algal calcification may be found in the cellular secretion of organic substances and Ca2+ within a closed or semi-closed space (Miyata et al., 1980). Experimental work shows that changes in the Mn2+ content in seawater during the growth of the algae have a direct influence on the concentration of Mn2+ in algal biogenic carbonates (Barbin, 2000, 2013). It was also shown that encrusting algae from a shallow lagoon exhibit variations in the orange luminescence (Mn2+ activated CL) having a brighter luminescence in the dorsal cell filaments than in the ventral cell filaments. In addition, successive layers in a single microridge may show different luminescence intensities that can be interpreted as the signature of adjustments to changing environmental settings (Barbin et al., 1991; Barbin, 2000). These differences may also be related with the mineral composition of the rhodoliths skeletons. In this work, techniques of CL and BSE–EDS microscopy are employed in the study of lower Pliocene rhodolith assemblages from Santa Maria Island (Azores, Portugal) in order to gain additional insight regarding the trace element content distribution throughout the algal thalli, and to assess the palaeoenvironment of the studied assemblages.

1.1. Study area The Azores is an archipelago consisting of nine volcanic islands and several islets located on the central North Atlantic (Fig. 1), about 1.600 km off the Portuguese mainland. The archipelago lies on a complex convergence zone between three tectonic plates: the

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American, the African (Nubian) and the Eurasian plates (Franc¸a et al., 2003). Santa Maria (Fig. 1) is situated in the most southeastern part of the archipelago, having a present area of 97 km2 and a maximum length of 16.8 km (Franc¸a et al., 2003). It is characterized by two distinct morphologic areas: a flat western side with several extensive, staircase shore platforms at different heights (the topmost with elevations from 200 to 230 m above sea level); and a mountainous eastern half, very irregular, where the highest peaks exceed 450 m (Madeira, 1986; Serralheiro, 2003). The coastline is rugged with high steep cliffs reaching 342 m in height. There are some bays but few sandy coves (Serralheiro and Madeira, 1990). Rock lithologies include limestones, shelly tufts, sandstones and conglomerates (Ferreira, 1955; Mitchell-Thomé, 1974). The lower Pliocene fossils from Santa Maria mainly occur in two stratigraphic units: the Touril Complex (∼5.3 to 4.5 Ma) and the Facho-Pico Alto Complex (4.5 to ∼3.5 Ma) (Serralheiro and Madeira, 1990; Serralheiro, 2003; Ramalho et al., 2014). The Touril Complex corresponds to a phase when the first emergence stage of Santa Maria Island probably completely disappeared under water due to erosion (Ávila et al., 2012; Ramalho et al., 2014). This unit is represented in the outcrops of Figueiral, Pedreira do Campo, Malbusca and Pedra-que-pica. Facho-Pico Alto Complex is younger than the Touril Complex (Serralheiro and Madeira, 1990; Serralheiro, 2003). Facho-Pico Alto lava flows and also some pyroclastic levels regularly follow after the limestone and calcarenite layers within Touril Complex, overlying them on the north and south coasts, and dipping into the sea (Serralheiro et al., 1987; Meireles et al., 2013).

2. Material and methods Fossil rhodoliths were collected from four different outcrops, all belonging to the Touril Complex (Pedreira do Campo, Figueiral, Malbusca and Pedra-que-pica) on the southern shores of Santa Maria, and studied in detail. The composite stratigraphic section of Malbusca outcrop was made following the same methodology as the one used for the remaining outcrops by other authors (Kirby et al., 2007; Habermann, 2010; Ávila et al., 2015) (Fig. 1). Fossil corallines were studied in thin sections using a petrographic optical microscope. Thin section preparation follows the work by Braga et al. (1993). A total of 22 uncovered thin sections (4.5 × 2.9 cm in size) were studied. Back scatter electron images and compositional information were obtained using a tabletop microscope Hitachi model TM-1000. In addition we used an Energy dispersive X-ray spectroscopy instrument manufactured by Oxford Instruments. The results obtained are only relative and the percentages were calculated with regard to the detected elements. Cathodoluminescence emission was examined under a cold cathode luminescence instrument provided by OPEA (Laboratoire d’Optique Electronique Appliquée, Vincennes France) mounted on a petrographic optical microscope (Olympus, BX-50). A cold cathode source electron gun was chosen for its ability to produce a stable electron beam (intensity-current fluctuations <1%). Observations were carried out under accelerating voltage ranging from 14 to 21 keV and beam current density from 100 to 430 ␮A. Images were collected with a QUICAM Imaging Fast1394 digital camera in time integration mode using the Archimed Pro Microvision Instrument program. It was not possible to obtain the same areas in both SEM and CL pictures, because SEM resolution is much greater than that of CL. Cathodoluminescence and back scatter electron image–energy dispersive X-ray spectroscopy microscopy studies were carried out at the «Groupe d’étude sur les géomatériaux et environnements naturels, anthropiques et archéologiques» (GEGENAA), Université de Reims Champagne-Ardenne, France. All the studied samples

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Fig. 1. Geographic and geotectonic framework of the Azores Archipelago. In highlight is Santa Maria Island and composite stratigraphic columns of the four studied outcrops: Pedreira do Campo and Figueiral (after Habermann, 2010), Malbusca and Pedra-que-pica. Stars indicate the location of the studied rhodoliths: “NAP” (North American Plate), “EUP” (Eurasian Plate), “NUP” (Nubian Plate), “Az” (Azores Archipelago), “Mad” (Madeira Archipelago), “Can” (Canary Islands Archipelago), “NA” (North Africa), “Eu” (Europe), ¨ ¨ Complex), “F.-P.A. Comp.(Facho-Pico Alto Complex), “Pleis. deposits” (Pleistocene deposits). “T.C.(Touril

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Fig. 2. BSE microscopy analysis on thin section. (A and B) Pedreira do Campo. (A) Perpendicular view of thallus, cells with polygonal shape. Cell walls seem to be dissolved; thin section DBUA-F 575. B) Thin section is constituted by coralline thallus surrounded with cement; thin section DBUA-F 575. (C and D) Figueiral. (C) Longitudinal view of dorsal cell filaments of the thallus. Cell infilling: with outer radial crystals. Cell walls seem to be dissolved; thin section DBUA-F 564. (D) Thallus including several rotaliid foraminifers (arrows); thin section DBUA-F 564. (E and F) Malbusca. (E) Longitudinal view of a non-coaxial monomerous thallus (coralline thallus = C1), cell walls preserved, cell infilling: outer radial crystals. Growth direction is from base to the top (direction of arrow). Dorsal cell filaments at the left. On the very right-hand side there might be a different thallus (coralline thallus = C2); thin section DBUA-F 269. (F) Transversal view of thallus, polygonal shape cells. Cell walls seem to be dissolved. Cell infilling:

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are housed within the fossil collections of the Department of Biology—University of the Azores, under the acronym DBUA-F. 3. Results 3.1. Rhodoliths and petrographic BSE–EDS description The studied rhodoliths from all four localities were mostly spheroidal and showed a wide variety of growth forms from encrusting to warty, fruticose and lumpy. The nuclei, when present, consisted of either bioclasts or volcanic pebbles. Rhodoliths ranged between 2 and 6 cm in diameter. Rhodoliths from Pedreira do Campo, Figueiral and Malbusca appeared concentrated at the base of the succession, while at Pedraque-pica they were dispersed throughout the outcrop. At Figueiral and Malbusca, the concentration of rhodoliths may exceed 650 m2 and 600 m2 , respectively. The rhodolith layer in Malbusca may locally reach ∼3 m in height. Thin section analyses (Fig. 2A–H), revealed coarse to mediumcoarse sparite as the typical cement in all samples studied, with rare micrite present. Sparite crystal grains were angular to sub-angular. In some thin sections foraminifera were recognised, occurring inside some rhodoliths (Fig. 2D and H). Sedimentary features and depositional patterns of the associated fauna are the object of a different paper (work in progress), and preliminary results indicate a high-energy environment with conditions favourable to the absorption/precipitation of carbonate, and therefore a ready availability of Ca/Mg. No dolomite rhombs within the fossil coralline algae was observed in the studied samples. Although the relative values of chemical elements studied varied throughout the thallus, all fossil rhodoliths analysed are similar in their relative chemical composition, with Ca as the most abundant element (75.9–100.0%) and with minor percentages of Mg (0.1–8.3%) and Fe (0.4–14.7%) (Table 1). 3.2. Cathodoluminescence description In all samples, as expected (cf. Boggs and Krinsley, 2010), the rhodoliths’ nuclei under CL showed variations from dark blue to bright orange and yellow. The high intensity of the luminescence (orange to yellow colours) reflects the high concentration of Mn2+ in the calcite. From the studied outcrops, Pedreira do Campo features the most strongly cemented strata. Under CL microscopy, rhodoliths from this outcrop generally exhibit, dark blue colours, with yellow and orange luminescence mainly concentrated in the dorsal cell filaments (Fig. 3A and B). Rhodoliths from Figueiral predominantly exhibit luminescence with orange fenestral patterns with bluish bands. The bluish bands are interpreted as pristine algal calcite, whereas the orange fenestral pattern represents late fillings in the remaining pores (Fig. 3C and D). Rhodoliths collected from the Malbusca outcrop, like those from Pedra-que-pica, exhibit bluish bands that concentrate on the dorsal cell filaments (Fig. 3E). Rhodoliths seem to exhibit high porosity as it is restricted to the non-luminescence dark areas. This high intragranular porosity may have been caused by incorporation of a bryozoan colony in the thallus of the rhodolith (Fig. 3F) and thus is constituted by their cavities (zooecia). The porosity was occluded by a non-luminescent cal-

cite corresponding to low Mn2+ content in the near-environment seawater. The orange luminescence is extremely high in the core filaments and this colour follows the growth of thallus filaments (Fig. 3G). The calcite occluding the cavities of the thallus near the last growth stage, is richer in Mn2+ and may have precipitated reducing conditions later on. Patterns of high luminescence were found only in rhodoliths collected from the Malbusca and Pedra-que-pica outcrops. The intensity of luminescence in Pedra-que-pica samples is particularly high, with predominance of bright orange and yellow emissions in the nuclei of the rhodoliths, and orange with bluish bands in the dorsal cell filaments (Fig. 3H). It is noteworthy that foraminifera found in the inner part of the rhodolith show the same bright orange luminescence as the surrounding algal thallus (Fig. 3I). It is also worth mentioning that the original structure of the thallus was enhanced by CL imaging (Fig. 3J), showing a calcite-dominated type of skeleton. 4. Discussion 4.1. Rhodoliths and petrographic BSE–EDS The diagenetic history of carbonates may be quite complex. Composition of biogenetic carbonates is more or less linked with fluctuations in the amount of different chemical elements such as Mn, Mg, Sr and Ca in seawater, but biotic influences clearly play an important role in many carbonates particularly during fast juvenile growth stage (Barbin, 2013). Thus, the uptake of cations is linked with variations in the metabolic activity and physicochemical pattern of the surrounding environment. Although the chemistry of fossil red algae is poorly understood, it is known that the thallus is composed of Mg-calcite in a range of 7–32% Mg (Tucker and Wright, 1992). Our relative results show much lower relative percentages of Mg, ranging between 0.1 and 8.3% (Table 1). Experiments performed in artificial seawaters encompassing a range of Mg/Ca ratios and using the crustose coralline algae Neogoniolithon sp., demonstrate that the Mg/Ca molar ratio calcite produced by the alga varies as a function of the Mg/Ca molar ratios of its ambient seawater (Ries, 2006). According to the same author, marine aragonite deposition prevailed during Neogene with Mg/Ca molar ratio of seawater ranging between 2.3 in the beginning of the Miocene and 5.2 in the recent times. In the present study, the values obtained for the molar ratios on the dorsal cell filaments of the thallus near the last growth stage that are in contact with sea water, were above or near 2.0 Mg/Ca (cf. Table 1). This result is in agreement with the values obtained by Ries (2006). Usually, there is no strong vital effect associated with incorporation of Mg into the algal skeleton (Ries, 2006). A similar effect occurred adding Mn2+ in artificial seawater, as this increased the Mn amount in ostrea shells (Barbin et al., 2008) and Corallinaceae (Barbin, 2013). 4.2. Cathodoluminescence According to Machel (2000) it is possible to describe four groups that affect CL emissions in carbonates: changes in redox potential, closed-system element partitioning, organic matter maturation and clay-mineral diagenesis. Exotic sources may also contribute to

outer radial crystals, there is two generations of cement: the inner cell walls are lined by a thin layer of small calcite crystals, then there might have been a time without cementation marked by a black layer (maybe impregnation by organic material), the second generation is the infilling of the remaining cell volumes by the large crystals; thin section DBUA-F 269. (G and H) Pedra-que-pica. (G) Longitudinal view of dorsal cell filaments. Ventral cell filaments at the base growing towards the lower-right portion of the image, the upper part is made up by the dorsal cell filaments (coralline thallus = C1). At the very right side there might be a different alga; it is marked by the diving black line (coralline thallus = C2). Cell walls seem to be dissolved, most cell infilling is preserved; thin section DBUA-F 500. (H) Foraminifera in the interior of a thallus; thin section DBUA-F 498.

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Table 1 Relative values (minimum and maximum%) of the chemical elements present in the rhodolith thalli from the different locations. Inferred sea water Mg/Ca molar ratios were based on readings made on the dorsal cell filaments of the thallus (e), and on readings on the ventral cell filaments (i). Chemical elements Outcrops

Mg (%)

Ca (%)

Fe (%)

Inferred sea water Mg/Ca molar ratios

Pedreira do Campo Sample: DBUA-F 575 Figueiral Sample: DBUA-F 564

0.0–1.2 0.9–6.6

79.2–96.9 87.6–99.1

3.6–9.2 1.5–4.2

0.7 (i) 0.5–1.8 (i) 1.0–2.1 (e)

1.2–7.7

78.8–98.2

0.9–10.9

1.8–5.3

93.5–98.2

0.4–2.6

DBUA-F 577

1.4–8.3

75.9–98.6

2.2–14.7

0.6–1.8 (i) 1.0–2.5 (e) 0.8–1.6 (i) 1.0–1.7 (e) 0.7–2.4 (i) 2.2–2.4 (e)

Pedra-que-pica Samples: DBUA-F 498

0.4–1.1

94.5–97.0

1.4–2.9

DBUA-F 500

1.1–2.8

94.6–100.0

2.5

Malbusca Samples: DBUA-F 171-4 DBUA-F 269

the input of trace elements. Most of the cathodoluminescent studies known from the literature were performed on ancient carbonate rocks with tectonic and burial processes that clearly have important diagenetic effects. The studied lower Pliocene rhodoliths are younger than those presented in literature, but still experienced pre-burial diagenesis. Cathodoluminescence may thus be the result of calcite formation in low-Eh pore-waters in which the Mn2+ activator of luminescence is possible. Even if Mn2+ incorporation in pristine biological materials is partly controlled by vital effects and seawater composition, the degree of crystallite alteration via dissolution or precipitation within an algae segment is controlled by mass-transfer rates with external porewaters (Hover et al., 2001). Ecological parameters such as the life habitat (e.g. a rhodolith living on the sediment surface vs. short-lived rhodoliths within the sediment) have an important influence on luminescence (e.g., Ritcher and Zinkernagel, 1980; Barbin, 2013). Moreover, young marine carbonates either do not luminesce or exhibit very dull luminescence (blue to dark brown). Numerous authors described luminescence in fossil shells but generally attributed luminescence to diagenetic alteration (Barbin and Gaspard, 1995; Barbin, 2000). The early alteration of biogenic allochems within the upper 25 cm of carbonate platform sediments has several significant implications for the study of sediment diagenesis (Hover et al., 2001). The implications are, chiefly, to be cautious in geochemical analysis and their interpretation due to the difficulties to collect materials that are clearly very well preserved. The non-luminescence dark areas of the rhodoliths indicate porosity, which is attributed by Nitsch et al. (2015) to constructional voids (primary voids present within single cells and/or produced by conjoined protuberances of coralline algal thalli) or destructional voids (caused by dissolution and decay of nuclei or of soft-body organisms, or even bioerosion). In the present study we have interpreted all non-luminescence dark areas of Fig. 3 as constructional voids. The majority of the calcitic cements filling skeletal cells within the examined rhodoliths include a bright-yellow-CL phase and two subsequent phases of dull-orange and dull-yellow luminescence (cf. Fig. 3H). This CL pattern is interpreted as resulting from a relative enrichment of Mn2+ and possibly Fe2+ , reflecting the presence of reducing diagenetic fluids. This is the case, because the non-luminescent skeletal walls contrast sharply with the dull-to brightly luminescent calcite cements that fill cells, in the ventral cell filaments as can be seen in Fig. 3G. It is quite remarkable that CL consistently reveals original skeletal fabric in these cores, including within fragmented grains (Moshier and Kirkland, 1993).

0.4–0.6 (i) 0.3 (e) 0.6–1.1 (i) 0.9 (e)

In our opinion, the formation of bands (cf. Fig. 3C and D) can result from two possible situations. One possibility is the occurrence of seasonal differences in the thallial growth. Studies on molluscs by Halfar et al. (1998), Barbin (2013) and Williams et al. (2014) observed a general difference in growth, being rapid in the summer and slow in the winter months. This was interpreted as due to environmental factors or ontogenetic effects. Another possibility is the occurrence of alterations due to the diagenetic processes that depend on the composition of the diagenetic fluid and may be linked to volcanic activities. The presence of iron, detected on all samples (including Figueiral, with relative values ranging from 1.5% to 4.2%; Table 1), and usually related with the proximity of volcanic systems and iron oxydes, strengthens this possibility. Barbin et al. (1991) observed that the thick, indistinctly layered dorsal cell filaments give a higher intensity of luminescence than the ventral filaments. In other samples the internal part of the colonies show similar characteristics to those previously described, but is surrounded by a weaker luminescent or non-luminescent thallus. The outermost parts of the thallus are weakly luminescent. The same was observed in this study for the rhodoliths collected from Malbusca (Fig. 3E–G), where the bluish bands also occur near the surface of the thallus (dorsal cell filaments) and the bright orange colour was concentrated in the ventral filaments, following the growth of the thallus (Fig. 3E). Dissolution of the most abundant and/or most reactive sediment allochem at a given saturation state may buffer the trace element and the isotopic compositions of pore-waters. Precipitation of carbonate from these pore-waters, as overgrowths on individual crystallites in biogenic components, necessarily alters the trace element and isotope composition of those components. Therefore, it cannot be assumed that the trace or isotope composition of allochems harvested from modern platform sediments (even sediments from the water-sediment interface) reflect the composition of living species growing from marine waters (Hover et al., 2001). Because pore-waters in the sediment are often sulfidic and reducing, the portioning of redox-sensitive trace elements such as rare earth elements, Mn and Fe between pore-water and carbonate overgrowths will be different than in oxidizing seawater. Biogenic components in the sediment evolve towards equilibrium with pore-waters, and thus likely contain elevated concentrations of redox sensitive trace elements; they may therefore not reflect the seawater composition in which they grew. Thus, it is critically important to consider the compositions and textures of “pristine” modern compositions and to compare them with diageneticallyaltered ancient sediments (Hover et al., 2001). High Mn content

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Fig. 3. CL microscopy analysis on thin section. (A and B) Pedreira do Campo: dark blue colours in the core filaments and yellow and orange concentrate a lot more in dorsal cell filaments; thin section DBUA-F 575. (C and D) Figueiral: luminescence predominantly orange with bluish bands along the thallus; thin section DBUA –F 564. (E–G) Malbusca. (E) Bluish bands concentrating on the dorsal cell filaments; thin section DBUA-F 171-4. (F) Non-luminescence around a bryozoan colony (arrows) incorporated by the thallus of the rhodolith; thin section DBUA-F 577. (G) Orange colour extremely high in core filaments and it seems to follow the growth of thallus filaments; thin section DBUA- F 577. (H–J) Pedra-que-pica. (H) Bright orange and yellow predominate in the dorsal cell filaments; thin section DBUA-F 500. (I) Foraminifers (arrows) found in the interior of the thallus show the same luminescence as the surrounding alga thallus; thin section DBUA-F 498. (J) Original coralline thallial structure (centre of photo); thin section DBUA-F 500. Scale bar is 500 ␮m (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

in fossil carbonate may not always be the result of alteration of the pristine carbonate material. Calcite precipitation in equilibrium with reducing seawater would contain more Fe and Mn than calcite crystallized from normal seawater (Veizer, 1977). 4.3. Palaeoenvironmental reconstructions The above-mentioned CL and BSE–EDS methods may give invaluable information regarding the palaeoenvironmental depo-

sitional settings. For instance, if rhodoliths collected in the same outcrop yield the same algal species, and if the luminescence in the dorsal thallus is different between the analysed rhodoliths, we may infer that (if no diagenesis occurred) our samples were probably affected by time-averaging (sensu Kidwell, 1998). This is so because, as luminescence is related to the amount of Mn2+ incorporated by the calcareous algae cells, and as this is a proxy of the concentration of Mn2+ on sea-water (which is enhanced by the proximity of spring water flows or of volcanic products extruded

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Fig. 4. Reconstruction of Santa Maria palaeoshore at 3.5 Ma, based on recent Digital Terrain Model (DTM) (data extracted from the Portuguese Military Map—Azores Archipelago, sheet 35; coordinate system: U.T.M.; Datum: WGS84; scale: 1/25000). (A) Vertical projection of the island. (B) Oblique projection of the island. White dashed-line represents the palaeoshore at 3.5 Ma (205 m above present sea level). 1 (Pedreira do Campo), 2 (Figueiral), 3 (Malbusca) and 4 (Pedra-que-pica).

from nearby shallow-water hydrothermal vents), then if the same species appears in different rhodoliths and these have different luminescence, we may conclude that the rhodoliths were probably formed at different times or that part of the rhodoliths was re-sedimented (i.e. formed in another site and removed into the present site). The comparison of rhodoliths collected at different outcrops may also provide interesting results, even when the algal species composition is unknown, as is our case. According to the luminescence intensity of the studied rhodoliths, we may divide these in two groups: high (Pedra-que-pica and Malbusca) and low luminescence (Pedreira do Campo and Figueiral). As stated previously the rhodoliths with low luminescence are related with low concentrations of Mn2+ in seawater. When the rhodoliths were deposited at the latter two localities (Pedreira do Campo and Figueiral) at about 4.0–4.5 Ma, the shoreline configuration of Santa Maria Island was much different than today. At Pedreira do Campo, the common occurrence of limestones with no signs of fine-grained terrigenous material was interpreted by Kirby et al. (2007) as a depositional setting in a submarine bank or shoal, far from any sub-

aerial input of terrigenous sediments. Our results for Pedreira do Campo indicate low luminescence, suggesting that the rhodoliths formed in low Mn2+ seawater, which is in agreement with the interpretation of Kirby et al. (2007). We reconstructed the shoreline of Santa Maria Island at 3.5 Ma, based on a recent Digital Terrain Model (DTM); the inferred uplift rate of the island (60 m/Ma, according to Ramalho et al., 2014) and the global sea-level curve derived from Bintanja and van de Wal (2008). This geographic reconstruction should be accepted with precaution, as it does not take into account volcanic activity (which only ceased ∼2.8 Ma; Ramalho et al., 2014), or the effects of submarine erosion and subsidence of the island. Nevertheless, as can be seen in Fig. 4, Pedreira do Campo and Figueiral are located far from the island’s palaeo-shore (about 2.8 and 2.9 km, respectively), whereas Malbusca and Pedra-que-pica are much closer to the palaeo-shoreline (only 0.4 and 0.1 km, respectively). This outcome further reinforces the earlier interpretations by Kirby et al. (2007) and Ávila (2013).

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Further studies in this area will provide a better understanding of the rhodolith growth dynamics, trace-element composition and diagenesis. 5. Conclusions Application of cathodoluminescence-microscopy and back scatter electron image microscopy–energy dispersive X-ray spectroscopy to the study of lower Pliocene rhodoliths from the Azores Archipelago (NE Atlantic), demonstrates that the two methods are useful tools in the investigation of the diagenetic history of fossil organisms, and that such methods should be used to detect post-depositional chemical exchanges with groundwater and trace changes in seawater composition. The latter information strengthened our palaeoenvironmental reconstruction of the island. In fact, high luminescence is related with high concentrations of Mn2+ in seawater and our chemical data proved to be congruent with the palaeogeographical reconstruction of Santa Maria Island at the time of the formation of the rhodoliths. The Malbusca and Pedraque-pica outcrops were located near the penecontemporaneous palaeoshore of the island, thus the influence of volcanic activity due to the extrusion of lavas and associated products and/or the presence of shallow-water hydrothermal vents should be reflected in the sea water chemistry, featuring a high sea water concentration of Mn2+ which mirrored on the rhodolith Mn2+ concentration. Moreover, the Facho-Pico Alto volcanic edifice grew substantially to the east, closer to Malbusca and Pedra-que-pica outcrops, suggesting that the penecontemporaneous volcanic activity was most likely concentrated on the east, therefore altering the water chemism, whereas Pedreira do Campo and Figueiral outcrops, located west of the volcanic edifice away from the penecontemporaneous palaeoshore, were less affected by the alteration of the water chemistry produced by this volcanic activity, proved by the low luminescence evidenced by the rhodoliths collected from those outcrops. Our results also confirm that: • ecological parameters such as the life habitat and environmental parameters (e.g., seawater composition) have a great influence on the CL of carbonated thallus; • CL is a powerful tool to discriminate diagenetic carbonates, and thus should be used in order to choose the best samples for geochemical analysis; • CL analysis may provide crucial palaeoecological information in order to accomplish a more detailed palaeoenvironmental reconstruction of depositional settings. Cathodoluminescence provides an innovative tool for revealing the skeletal fabric of fossils in thin sections. We hope this study may serve as a reference for future studies on fossil rhodoliths from oceanic islands. Acknowledgements We thank Direcc¸ão Regional da Ciência, Tecnologia e Comunicac¸ões (Regional Government of the Azores), and Fundac¸ão para a Ciência e a Tecnologia (FCT) of the Portuguese government for financial support, Clube Naval de Santa Maria and Câmara Municipal de Vila do Porto for field assistance. We are grateful to the organizers and participants of the International Workshops “Palaeontology in Atlantic Islands” who helped in fieldwork. The first author thanks Didier Delor (Université de Reims Champagne-Ardenne) for the preparation of the thin sections, Vittorio Zanon (University of the Azores) for fruitful discussions and Ricardo Ramalho for reviewing the manuscript and help-

ing improve it. A.C. Rebelo was supported by EURODYSSÉE, an exchange program of the Assembly of European Regions (AER) and by a grant SFRH/BD/77310/2011 of FCT, Portugal. Sérgio Ávila acknowledges his Ciência 2008 research contract funded by FCT. This research was partially supported by (i) the European Regional Development Fund (ERDF) through the COMPETE—Operational Competitiveness Programme and national funds through FCT—Foundation for Science and Technology, under the project “PEst-C/MAR/LA0015/2013”, and (ii) the Strategic Funding UID/Multi/04423/2013 through national funds provided by FCT and European Regional Development Fund (ERDF) in the framework of the programme PT2020, and by (iii) cE3c funding (Ref: UID/BIA/00329/2013). Support was also provided by CIRN/UAc (Centre of Natural Resources of University of the Azores) and by CIIMAR (Interdisciplinaru Centre of Marine and Environmental Research, Porto, Portugal). We are also grateful to two anonymous referees for their helpful comments and suggestions.

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