The response of benthic meiofauna to hydrothermal emissions in the Pontine Archipelago, Tyrrhenian Sea (central Mediterranean Basin)

Journal of Marine Systems 164 (2016) 53–66

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The response of benthic meiofauna to hydrothermal emissions in the Pontine Archipelago, Tyrrhenian Sea (central Mediterranean Basin) Letizia Di Bella a,⁎, Michela Ingrassia b, Virgilio Frezza a, Francesco Latino Chiocci a,b, Eleonora Martorelli b a b

Department of Earth Science, Sapienza University of Rome, P.le A. Moro, 5, 00185 Rome, Italy CNR-IGAG (Istituto di Geologia Ambientale e Geoingegneria), UOS Roma, P.le A. Moro, 5, 00185 Rome, Italy

a r t i c l e

i n f o

Article history: Received 1 April 2016 Received in revised form 26 July 2016 Accepted 3 August 2016 Available online 07 August 2016 Keywords: Benthic foraminiferal assemblages Seafloor venting CO2 emissions Giant depression Pontine Archipelago central Mediterranean Sea

a b s t r a c t Recent investigations highlighted the occurrence of a giant depression related to hydrothermal activity off the Pontine Archipelago (central Mediterranean Sea, Italy). The new record of a giant seeping depression (Zannone Giant Pockmark, ZGP) in shallow-water provides the opportunity to study fluid vent impact on meiobenthic communities. The micropaleontological analyses on living (Rose Bengal stained) and dead assemblages recorded inside and outside the Zannone Giant Pockmark, allow to highlight changes in the structure and composition of the foraminiferal community that suggest variations of fluid emissions in different sectors of the study area. Inside the ZGP, under the direct influence of venting activity, a very peculiar living foraminiferal assemblage is found. It consists of agglutinated species (Spiculosiphon oceana, Jaculella acuta, Deuterammina rotaliformis) never found or very rare in the Mediterranean Sea. On the contrary dead assemblage testifies the changes on foraminiferal assemblages under carbonate dissolution process. Outside the pockmark in the nearby area of ZGP, the integrated meiofaunal and geochemical data suggest a transitional condition between vent influenced sedimentation and the typical carbonate sedimentation recorded in the rest of the Pontine Archipelago. In particular a possible spread of the venting activity in the northern and southern sectors of the study area, towards the edge of the Zannone insular shelf, is inferred. The impact of fluid emissions on foraminiferal assemblages can be summarized in the following observations: reduced biodiversity, increase of agglutinated species with predominant siliceous component in the test structure, limited distribution of living specimens inside the sediment, disappearance of porcelaneous taxa and presence of carbonate loss tests. As the result, the venting activity is likely to be the main environmental driver on the meiofaunal distribution. We also report, at the emission sites in the Pontine Archipelago, the presence of agglutinated species such as Spiculosiphon oceana, Jaculella acuta, Deuterammina rotaliformis, never found earlier in the Mediterranean Sea. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Underwater hydrothermal vents or CO2 fluid discharges represent seafloor areas characterized by the occurrence of peculiar physical and biological conditions. These submarine sites have been found in different tectonic settings at different depths, from shallow to deep water, and in substrates with various composition (Tivey, 2007). In particular, shallow-water hydrothermal vents have been reported especially in close relation to recent subaerial and submarine volcanic activity (e.g. Dando et al., 1999, 2000; de Ronde et al., 2001; Geptner et al., 2002; Italiano and Nuccio, 1991; Prol-Ledesma et al., 2005; Stoffers et al., 1999; Zhirmunsky and Tarasov, 1990), although some of them were discovered on mid-ocean ridges (Cardigos et al., 2005; Fricke et al., 1989; German et al., 1994) and in continental margin settings undergoing tectonic extension (e.g. Melwani and Kim, 2008; Prol-Ledesma et al., 2004; ⁎ Corresponding author. E-mail address: [email protected] (L. Di Bella).

http://dx.doi.org/10.1016/j.jmarsys.2016.08.002 0924-7963/© 2016 Elsevier B.V. All rights reserved.

Vidal et al., 1978). Since their first discovery in the marine realm, hydrothermal vents attracted an increasing attention of scientific community mainly due to: 1) their importance in commercial resource exploration (e.g. hydrothermal related deposits; Hannington et al., 2011; Hein et al., 2013; Rona, 2008); 2) highly sensitive marine ecosystems they host; 3) possible implications for ocean chemistry, i.e. ocean acidification (e.g. Davis et al., 2003; Hall-Spencer et al., 2008; Jupp and Schultz, 2004; Vance et al., 2009). The environmental conditions in shallow-water vents strongly differ from the surrounding seafloor in terms of both increased temperature and enrichment in reduced chemical compounds such as sulphide and methane (Tarasov et al., 2005). Temperature generally ranges from 10 to 13 °C (Caramanna et al., 2011; Dando et al., 1999 and reference therein; Tarasov et al., 1999, 2005) and fluids formation commonly take place from relatively shallow sources (1–2 km depth). These natural fluid emissions may be able to produce thermal and chemical weathering of sediment substrate as well as alter sea-water geochemistry including reduction in calcifying capacity of marine organisms (Doney et al.,

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2009). Moreover, CO2 fluid emissions escaping from the seafloor can be responsible of the formation of specific morphological features such as pockmark structures (e.g. Hovland, 1992). Regarding the biological features commonly found in shallow-water active pockmarks, several studies were focused on the characterization of microbial communities showing that hydrothermal activity strongly enhances biological activity both within the water column and on the seabed. At shallow water sites, the primary production is based both on chemosynthetic and photosynthesis processes (Namsaraev et al., 1994; Sorokin et al., 1998; Tarasov et al., 2005) leading to the scarce occurrence of vent-obligate taxa, which are mainly represented by bivalves belonging to the Solemyidae, Lucinidae and Thyasiridae. At such environment, Prokaryotes (Bacteria and Archea) represent the main biological component so that they are generally used as optical proxies to recognize dispersed emissions at the ground floor. The most common bacteria found on seafloor areas affected by hydrothermal activity are the genera Thiobacillus, Thiomicrospira and Thiosphaera, or the filamentous sulphur bacteria such as Thiothrix or Beggiatoa (Tarasov et al., 2005). The major biogeochemical processes observed in association to these mats are the oxidation of reduced sulphur compounds (hydrogen sulphide or thiosulphate) to elemental sulphur, thiosulphate or sulphate and organic matter synthesis (Dubinina, 1989; Namsaraev et al., 1991; Sorokin, 1991). Studies focusing on the characterization of meio- and macrofaunal organisms dwelling in such environments are rare (Jones, 1993; Judd and Hovland, 2007; Panieri, 2006a, 2006b; Panieri et al., 2005). The few available studies (see above) show that shallow-water pockmarks can differ in faunal composition from the surroundings and from each other, depending on the degree and effects of the venting activity. Particularly, the distinction is in terms of faunal density, diversity, dominance and infaunal/epifaunal ratio (Jones, 1993; Melwani and Kim, 2008; Panieri, 2006a; Panieri et al., 2005; Wildish et al., 2008). The new record of a giant complex venting pockmark (named Zannone Giant Pockmark – ZGP; Ingrassia et al., 2015a) located in the shallowwater of the central Mediterranean Sea, provides the opportunity to study the impact of fluid vent (mainly regarding the CO2 emission) on benthic communities. Particularly, this study examines the meiofaunal communities of the ZGP located 3 km off the eastern coast of Zannone Island (western Pontine Archipelago, Tyrrhenian Sea). Judging by the previous studies and observations in the area, bottom water topography and sediment characteristics suggest higher vent activity (and hence higher CO2 content) within the Zannone Giant Pockmark. To address this hypothesis and to describe the influence of venting activity on sediment characteristics and meiobenthic (foraminifera) assemblages found on the seafloor sites affected and non-affected by venting activity, this study uses direct observations (through video-imaging), sedimentological, micropaleontological and geochemical analyses.

2. Geological setting and methods 2.1. Geological setting The Pontine Archipelago (Fig. 1a) is located on the eastern Tyrrhenian margin about 30 km off the Latium coastline (central Tyrrhenian Sea, Italy). The Archipelago represents a Plio-Pleistocene volcanic structure formed by two groups of islands. The western group (Palmarola, Ponza and Zannone) and the eastern one (Ventotene and S. Stefano) developed in different geological settings: the western group lies on a structural high where volcanic activity developed along normal faults and ended about 0.9 Ma BP (Bellucci et al., 1997; Cadoux et al., 2005); the eastern group is located in the Ventotene sedimentary basin and represents the summit of a large volcanic edifice, where volcanic activity ended more recently, about 0.3 Ma (Perrotta et al., 1996).

Grab samples studied in this paper come from the seafloor off the western Pontine Archipelago, which is a narrow and steep insular shelf characterized by different morphological features, represented by volcanic and biogenic buildups (Chiocci and Martorelli, 2015; Martorelli et al., 2011), isolated morphological highs and several fluid related features (i.e. pockmarks, giant pockmark, authigenic mounds; Ingrassia et al., 2015a, 2015b) making this environment highly heterogeneous. In particular, the Zannone Giant Pockmark (Fig. 1b–c) recently discovered by Ingrassia et al. (2015a) represents a peculiar submarine area affected by vigorous venting activity and occurrence of peculiar benthic communities (Ingrassia et al., 2015b), related to the occurrence of a hydrothermal field (Martorelli et al., accepted). The seafloor unaffected by venting activity consists of bioclastic sandy sediment mainly composed of foraminifera, coralline algae, bryozoans, ostracods and sponge spicules (Brandano and Civitelli, 2007; Martorelli et al., 2011). Bioclastic sedimentation at the Pontine Archipelago seems to be favored by the low influx and sedimentation rate of continental sediments and high-energy hydrodynamics (Martorelli et al., 2011). The continental slope is characterized by a seabed floored by muddy hemipelagic sediment and occurrence of several canyons, gullies and landslide scars, resulting from widespread mass-wasting phenomena (Chiocci et al., 2003). Two tectonically-controlled intraslope basins (Palmarola and Ventotene) occur adjacent to the western Archipelago, in a water depth ranging from 500 to 800 m and are characterized by high Plio-Quaternary sedimentation rates (Zitellini et al., 1984). 2.2. Materials and methods In June 2014, during the research cruise “Bolle 2014” carried out by R/V Urania, several seafloor sediment samples were collected by means of a 30L Van Veen grab. Although grab sampling is not a very satisfactory method for micro- and meiofaunal analyses, as it does not recover sediment-bottom water interface (Murray, 2006), the occurrence of lithified crusts and coarse-grained sediments prevented us from using a more suitable sampling gear like multiple corer or box corer. On the base of the geomorphological and geochemical features the 6 most representative site stations were chosen at 127–137 m water depth. Four stations are located within three sectors (northern, central and southern) inside the ZGP, whereas two are outside (close to the shelf margin; Fig. 1b–c). For each station 3 grab sediment replicates, are from separate deployments as in Schönfeld et al. (2012), and a total of 18 samples were collected (Table 1). For a comparison with the background condition of the “normal” (undisturbed) seafloor at the Pontine Archipelago, the benthic foraminiferal data reported by Frezza et al. (2010) were considered. Water column data (temperature, salinity, oxygen content) and beam transmission (which provides both an indication of the total concentrations of matter in the water as well as a value of the water clarity) were acquired by mean of CTD SBE 911 and a WET Labs Transmissometer, both inside and outside the Zannone Giant Pockmark (Fig. 2). Geochemical composition data from Martorelli et al. (accepted) were considered. The gas compositions of the most significant stations are summarized in Table 2. 2.2.1. Grain size analysis One replicate for each site station was cleaned with hydrogen peroxide, dried at 60 °C and used to perform the grain size analysis by sieving (grain size N 63 μm) 100 g of unconsolidated sediment sample. The finer fraction (b 63 μm) was analyzed by a high resolution laser diffraction instrument (Helos Sympatec). By means of GranulGraf software the grain size distribution was obtained and the percentage of each sediment fraction (sand, silt and clay) was plotted on ternary granulometric diagram (Tortora, 1999). The information was used to interpret the sediment types occurring in both vent and non-vent seafloor areas.

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Fig. 1. a) Location map of the central Tyrrhenian continental margin. Red box shows the study area (panel c). Black spots: gas samples; Pa: Palmarola Island; Po: Ponza Island; Za: Zannone Island; Ve: Ventotene Island; Ss: Santo Stefano Island. b) Location map of the samples recovered outside Zannone Giant Pockmark (ZGP). c) Location map of the samples recovered inside ZGP. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

2.2.2. Meiofaunal analyses For the meiofaunal analyses all replicates were considered and analyzed. For each grab replicate, small cores (10–15 cm thick, 4 cm in diameter), collected inside the grab, were sampled continuously every 1 cm. The sediment samples were stained and preserved in a solution of 2 g/l of Rose Bengal and ethanol as described by Lutze and Altenbach (1991); Schönfeld et al. (2012) and Walton (1952). After 15 days, the samples were wet-sieved through a 63 μm sieve and then dried at 60 °C. In each sample, Rose Bengal stained foraminifers with well-preserved tests were hand-picked, counted and identified using a

binocular microscope. Non-transparent agglutinated and porcelaneous tests were broken in order to inspect the interior. In the most part of the samples, abundant tubular fragments belonging to different agglutinant taxa (e.g. Hyperammina, Rhabdammina, Spiculosiphon) were found. To avoid an over-estimate of their abundance, only specimens at least 0.5 cm long (with N50–60% of the original test volume preserved) were counted. Some parameters such faunal density and diversity indices were considered in order to investigate the structure of the meiofaunal community at the water-sediment interface and inside the sediment. Faunal density was expressed as specimens/g dry

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Table 1 Geographical coordinates, depth and location of the 6 studied stations and the related replicates. Site stations

Longitude

Latitude

Samples

mwd

Sector

ST1

13° 06′ 11.5568″ E 13° 06′ 11.5819″ E 13° 06′ 11.5595″ E 13° 06′ 6.7319″ E 13° 06′ 4.9981″ E 13° 06′ 6.7319″ E 13° 06′ 7.3451″ E 13° 06′ 8.6093″ E 13° 06′ 5.9526″ E 13° 06′ 5.0241″ E 13° 06′ 5.3114″ E 13° 06′ 5.3123″ E 13° 05′ 43.5137″ E 13° 05′ 43.7181″ E 13° 05′ 43.8529″ E 13° 05′ 12.9495″ E 13° 05′ 12.9495″ E 13° 05′ 13.0731″ E

40° 58′ 20.0338″ N 40° 58′ 19.1587″ N 40° 58′ 18.4449″ N 40° 58′ 21.1866″ N 40° 58′ 20.4771″ N 40° 58′ 21.1866″ N 40° 57′ 56.8114″ N 40° 57′ 57.4808″ N 40° 57′ 56.1399″ N 40° 58′ 15.0946″ N 40° 58′ 15.5209″ N 40° 58′ 15.4885″ N 40° 57′ 13.7763″ N 40° 57′ 14.1040″ N 40° 57′ 13.8792″ N 40° 56′ 56.8619″ N 40° 56′ 56.8619″ N 40° 56′ 57.0261″ N

ST1_BNR1 ST1_BNR2 ST1_BNR3 ST2_BNR1 ST2_BNR2 ST2_BNR3 ST3_BNR1 ST3_BNR2 ST3_BNR3 ST4_BNR1 ST4_BNR2 ST4_BNR3 ST5_BNR1 ST5_BNR2 ST5_BNR3 ST6_BNR1 ST6_BNR2 ST6_BNR3

129 127 130 135 131 131 131 136 136 133 133 131 127 127 127 127 127 127

Inside ZGP - North Inside ZGP - North Inside ZGP - North Inside ZGP - North Inside ZGP - North Inside ZGP - North Inside ZGP - South Inside ZGP - South Inside ZGP - South Inside ZGP - Center Inside ZGP - Center Inside ZGP - Center Outside ZGP - Eastern Zannone Insular shelf Outside ZGP - Eastern Zannone Insular shelf Outside ZGP - Eastern Zannone Insular shelf Outside ZGP - Eastern Zannone Insular shelf Outside ZGP - Eastern Zannone Insular shelf Outside ZGP - Eastern Zannone Insular shelf

ST2

ST3

ST4

ST5

ST6

sediment. Diversity was quantified considering species richness (number of taxa per sample), Shannon-Weaver (H) and α-Fisher indices (Fisher et al., 1943; Shannon, 1948) were calculated using the PAST (PAlaeontological STatistics) version 1.38 data analysis package

(Hammer et al., 2001). The relative abundance of groups distinguished by their basic wall structure (agglutinated, porcelaneous and hyaline) was calculated in order to display their distribution in the samples. In addition, histograms of the foraminiferal density (relative abundance of single wall structure group per gr of dry sediment) of these 3 groups (agglutinated, hyaline and porcelaneous) are shown. The number of living specimens counted in the samples was low, and the differences between the various assemblages collected were visually pronounced, so that a statistical analysis was not possible (Panieri, 2006a). In the samples where living foraminifers were present, dead foraminiferal assemblages were analyzed as well, in order to obtain a more complete picture of the benthic assemblages and to calibrate the microfossil proxies (Fontanier et al., 2014). We divided samples in aliquots using an Otto microsplitter in order to get at least 250–300 individuals per sediment interval. The classification of the species has been made on the base of recent Mediterranean and extra-Mediterranean foraminiferal literature data (Cimerman and Langer, 1991; Frezza et al., 2010; Jorissen, 1987, 1988; Milker and Schmiedl, 2012; Sen Gupta et al., 2009; Sgarrella and Moncharmont-Zei, 1993). Energy Dispersive Spectrometry (EDS) analyses on living and dead foraminiferal tests were performed with FEI-QUANTA 400 scanning electron microscope at the SEM Laboratory of Earth Sciences Department Sapienza University of Rome, in order to examine and evaluate the state of preservation of foraminiferal tests. The complete studied collection was placed on cardboard slides and stored in the Micropaleontological Laboratory of Earth Sciences Department Sapienza University of Rome. 3. Results 3.1. Sea water parameters

Fig. 2. Multi-parameters probe CTD SBE 911 and WET Labs Transmissometer. Patterns of parameters (Salinity, Temperature, Oxygen and Beam Transmission) of the water column above the fluid emission (a) inside the Zannone Giant Pockmark and outside the ZGP (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

The diagrams representing sea-water parameters outside (SVP7) and inside (SVP3) the ZGP are summarized in Fig. 2. Temperature decreases from about 20 to 14 °C near the bottom were recorded both inside and outside the ZGP. Between 10 and 40 m water depth (wd) the temperature depth profiles show the thermocline layer characterized by rapidly decreasing temperatures (between 19.6 °C and 15.2 °C). No significant anomalies of seawater temperatures were recorded at the venting sites but at the ST2 station ST2BNR2 the grab sampled warm sediment. The salinity within and outside the ZGP ranges from 37.5 to 38.2 PSU, increasing with depth. The oxygen content is about 5.3 ml/l at the sea surface, reaching a maximum value of 5.7 ml/l at about 40 m wd and is relatively depleted toward the bottom (5.1 ml/l). Inside the ZGP, below 108 m wd the oxygen content rapidly decreases with an irregular trend, reaching a minimum value of 4.8 ml/l at 116 m wd and a

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Table 2 Geochemical results of the dissolved gases in sea water samples collected by Niskin bottles in vertical casts on the samples. For location of water sampling stations see Fig. 1. ASSW values represent the normal marine conditions. ASSW is the theoretical Air-Saturated Sea Water at 20 °C and 37‰ salinity. In the Table data related to the meiofaunal studied stations are considered; the complete geochemical data set is reported in Martorelli et al. (accepted). While O2 displays value similar to those recorded in balanced sea-water with the atmosphere (see ASSW values), CH4 and CO2 values are higher than ASSW values both outside and inside the ZGP. Location

Sample

Long.

Lat.

Depth

H2 (ml/l)

O2 (ml/l)

N2 (ml/l)

CO (ml/l)

CH4 (ppm)

CO2 (ppm)

Dissolved gases Inside GPZ

BT3

340397

4537387

Inside GPZ

BT4

340293

4537299

Out GPZ

BT6

339010

4534887

Out GPZ ASSW

BT8

364772

4514499

50 m 100 m 129 m 50 m 100 m 127 m 50 m 100 m 129 m 123 m

1,3 × 10−2 1,6 × 10−2 7,0 × 10−3 2,4 × 10−3 4,7 × 10−3 1,2 × 10−3 1,6 × 10−3 7,0 × 10−3 1,5 × 10−2 4,3 × 10−3

4.93 4.21 4.64 4.02 3.96 3.66 4.44 4.41 2.37 4.22 4.8

11.03 9.84 10.96 8.1 9.76 7.99 10.76 10.84 7.77 10.23 9.6

2,5 × 10−5 1,4 × 10−5 1,3 × 10−5 1,3 × 10−5 2,00 × 10−5 2,00 × 10−5 3,00 × 10−5 1,8 × 10−5 1,4 × 10−5 2,3 × 10−5

0.71 0.94 1.8 0.92 1.4 0.75 0.33 0.27 0.81 0.11 1 × 10−3

560 580 720 590 580 700 460 560 720 530 240

value of 4.9 ml/l near the sea bottom. Beam transmission did not show great variations with depth, however, inside the ZGP a markedly lower beam transmission (SVP3 in Fig. 2) was recorded in the basal portion of the water column (below 110 m wd). As a whole, CTD profiler data collected within and outside the ZGP do not vary significantly in almost all the water column. Nevertheless, in the basal portion of the water column (i.e. below 108–110 m wd) both the oxygen content and beam transmission show a marked decrease accompanied by a very slight increase in salinity and temperature. Taking into account the proximity of CTD SVP3 to the main emission point observed by Ingrassia et al. (2015a) the marked decrease of oxygen content and beam transmission might be the result of active fluid emissions producing gas bubbles. 3.2. Zannone Giant Pockmark 3.2.1. Sediments characteristics Zannone Giant Pockmark floor displays great morphological and sedimentological variability. Sedimentological data, according to classification of Tortora (1999), show as the sea bottom within the ZGP mainly consists of sandy sediments except from the sample ST3BNR3 that consists of muddy sand (Fig. 3, Table 3). The sandy sediments recovered inside the ZGP, both along its northern and central sectors, display presence of oxided layers, appreciable sulphur smell and lithified crusts. In particular, the EDS microanalysis performed on this lithified sediments

(i.e. ST2BNR2) highlighted a homogeneous composition constituted exclusively of native sulphur (Fig. 4). Along the north-eastern seafloor sector coarse sediments were recorded. Finally toward the south, video observations show as the seabed is mainly floored by sandy sediment, sometimes covered by bacterial mats (Fig. 5); here there are several small pockmarks with centimetric size (Ingrassia et al., 2015a). Sample ST3BNR3, located along the southern sector of the ZGP is the only characterized by an increase of the mud percentage (about 10%). In all sectors of the ZGP, the sandy fraction is constituted mainly of quartz grains, rare plagioclase and K-feldspar minerals. The organic fraction of the sediment is exclusively constituted of siliceous spicules, diatoms, radiolaria and agglutinated foraminifers. Dead foraminiferal (benthic and planktonic) assemblage is absent in all the samples within the ZGP. No mollusk fragments, bryozoans or other calcareous tests are observed.

3.2.2. Meiofaunal analyses 3.2.2.1. Live (stained) foraminiferal assemblages. Living foraminifers distribution is very irregular and patchy although live foraminifers were recorded in every station (ST) but not in each sample replicates (R) and generally were restricted to the top most layer of the sediment (0–3 cm). Only in ST3BNR3 samples stained foraminifers were found down to 10 cm in the sediments. The foraminiferal frequencies displays

Fig. 3. Ternary diagram showing grain size of sediments recovered within the Zannone Giant Pockmark (ZGP) and outside the ZGP. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Table 3 Summary of the grain size analysis and of the statistical parameters of selected sediment samples. ID sample

ST1BNR2 ST2BNR1 ST3BNR3 ST4BNR1 ST5BNR1 ST6BNR1

Parameters (Folk and Ward, 1957)

Percentile (PHI)

Populations (Wentworth, 1922)

% sand

% silt

% mud

1°

5°

16°

25°

50°

75°

84°

95°

Mode

Sorting

Skewness

Kurtosis

Zone

96.35 96.65 87.92 98.39 88.56 88.73

1.74 2.00 6.61 0.86 7.01 7.25

1.91 1.35 5.47 0.75 4.43 4.02

1.02 −0.62 −0.61 0.98 −0.67 −0.64

1.8 −0.29 −0.10 1.68 −0.50 −0.38

2.17 0.65 1.87 2.08 0.26 0.38

2.31 1.64 2.18 2.24 0.71 0.76

2.6 2.28 2.64 2.56 2.01 1.80

2.91 2.66 3.19 2.83 3.41 3.35

3.11 2.81 3.67 2.97 3.78 3.76

3.75 3.35 8.18 3.36 7.72 7.41

2.63 1.92 2.73 2.53 2.02 1.98

0.53 1.09 1.70 0.48 2.12 2.03

0.13 −0.46 0.24 −0.06 0.20 0.30

1.33 1.46 3.35 1.16 1.25 1.23

ZGP ZGP ZGP ZGP Out ZGP Out ZGP

a regular decreasing trend from the top down to 10 cm (Fig. 6). Minimum values of living specimens frequencies (exclusively agglutinated tests) were recorded inside the ZGP at ST1 (BNR1-R3), ST2 (BNR1-R3), ST3 (BNR1), ST4 (BNR3). The maximum values of faunal density are recorded in the uppermost sample at ST3BNR3 (about 30 ind/g; Table 4). A clear decrease of faunal density is showed from the southern to the northern sector of the ZGP. Species richness (S) varies between 1 and 10 and diversity indices H and α-Fisher are very low (H ranges from 0 to 2.71; α-Fisher ranges from 0.25 to 14.15; Table 4). The southern sector of the ZGP displays the highest values of diversity indices and foraminiferal abundance (ST3) due to a small calcareous tests contribution and a more diversified agglutinated assemblage. On the contrary, the northern sector is characterized by an assemblage constituted of agglutinated species, with low abundance and diversity. Among these agglutinated taxa, Reophax scorpiurus and Hyperammina spp. are the most frequent in all pockmark area. Deuterammina rotaliformis is present exclusively in the northern sector (ST2BNR3, ST4BNR2) associated to rare specimens of Trochammina inflata and Ammoglobigerina globigeriniformis, while Spiculosiphon oceana was found only in the southern one (ST3BNR3), associated to R. scorpiurus, Hyperammina spp., Lagenammina spp., Glomospira spp. (G. gordialis and G. charoides), Rhabdammina abyssorum and Jaculella acuta.

Sand Sand Muddy sand Sand Muddy sand Muddy sand

calcareous species dominate the foraminiferal assemblages at all stations with percentages N 50%, except in ST6BN station where agglutinated species are dominant (Fig. 7). The porcelaneous group is present only in ST6BN station with very low percentages (b10%). Of the perforate species, Lenticulina spp., Hanzawaia boueana, Lobatula lobatula and Valvulineria bradyana are the most common while agglutinated species are represented by Lagenammina spp., Reophax scorpiurus, Spiculosiphon oceana and rare Jaculella acuta. 3.3.2.2. Dead foraminiferal assemblages. Outside ZGP (ST5BN and ST6BN) dead foraminiferal assemblages are present and highly diverse. Species richness (S) varies between 32 and 58, whereas Shannon Index H ranges from 2.96 to 3.36 and α-Fisher from 10.73 to 22.58 (Table 6). Both in the top 1 cm and in the 1–2 cm of sediment layers, perforate calcareous species dominate the foraminiferal assemblages at both stations with percentages ranging between 76 and 87%. Porcelaneous (9–17%) and agglutinated (4–11%) foraminiferal species are subordinate (Fig. 7). On the whole, benthic foraminiferal assemblage is dominated by Cassidulina carinata (9.3–19.1%), Cassidulina crassa (3.5–14.5%), Lobatula lobatula (3.6–14.4%), Asterigerinata mamilla (5.1–13.7%) and Gavelinopsis lobatulus (0–10.2%). Among porcelaneous taxa, Quinqueloculina stelligera is the most frequent (1.5–5.4%), whereas among agglutinated species Textularia spp. prevail (1.4–4.8%).

3.3. Outside the ZGP: Eastern Zannone insular shelf 4. Discussion 3.3.1. Sediments characteristics The seafloor outside the ZGP represents the seabed condition typical for the outer insular shelf of the western Pontine Archipelago, characterized by a flat morphology and a mixed carbonate-siliciclastic sedimentation. The grab samples (ST5BN and ST6BN) were recovered in a water depth varying from 126 to 127 m, about 130 m from the shelf break. These grabs are characterized by a higher percentage of silt (about 7.5%) and clay (4.6%) than those recovered within the ZGP, and a percentage of sand of about 88%. All collected grab samples show evidence of oxidation, testified by orange colour sediments. Macrofaunal organisms (i.e. bivalves) were observed within the sediment. Samples taken outside the ZGP differ from those within the pockmark by the presence of a reach foraminiferal thanatocenosis and abundant carbonate fraction (bioclasts and calcitic grains). The organic fraction is constituted of mollusk fragments, planktonic and benthic foraminifers, radiolaria, bryozoans, ostracods, rare pteropods and sponge spicules; it is dominant above inorganic fraction which is mainly constituted by quartz and calcite. 3.3.2. Meiofaunal analyses 3.3.2.1. Live (stained) foraminiferal assemblages. Outside the ZGP the absolute abundance of living specimens ranges between 3 (generally inside of the sediment: 1–2 cm interval) to 38 tests in the top layer of ST6BNR2. Species richness (S) varies between 1 and 20, Shannon Index H ranges from 1.01 to 2.61 and α-Fisher from 2.39 to 33.82 (Table 5). In the uppermost analyzed sample (0–1 cm), perforate

4.1. Living assemblages Within the ZGP, the ecological interpretation of data results is quite complex because microscale bio-distribution is directly related to the spatial heterogeneity of biogeochemical processes related to venting activity that normally occurs in these extreme marine environments. This feature could add to the patchy distribution of foraminifers and amplify their distribution variability, locally determining the complete absence of living assemblages. The use of three replicates for foraminiferal analyses at each sampling site confirms high patchiness in the community distribution within the ZGP. Despite the high variability of foraminiferal populations, some main meiofaunal considerations can be highlighted. The most significant faunal changes seem to be driven by the variations of fluid emissions reflecting higher influence in the northern than in the southern sector of the ZGP (Table 2). Although the grain size variations is known to be another important factor controlling foraminiferal distribution, in this case we believe that it could be a secondary cause in respect to the vent emission due to the small variations in grain size within the ZGP. In more detail, we observed that, where fauna is present, an increasing trend from north to south in foraminiferal diversity and density is recorded (Table 4). The diversity indices and the faunal density are on average lower in the northern sector than in the southern one. In the northern sector, the species richness records 2–3 taxa while in the southern sector it reaches 20 taxa (Table 4). Moreover toward the south, a very low percentage of calcareous component (both living

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Fig. 5. Sediments covered by bacterial mats inside the Zannone Giant Pockmark (ZGP).

Fig. 4. A) SEM photos on hard lithified sulphur sediment collected in the northern sector of the Zannone Giant Pockmark (ZGP); B) backscattered SEM photos showing the homogeneity of the lithified sediment and C) EDS microanalyses diagram performed on lithified sediments (ST2BN samples) showing the chemical composition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

and dead) occurs while it is completely absent in the northern sector. This would testify a lower dissolution process, due to a not persistent venting activity in the southern sector. Differences between the two sectors of the ZGP involve the assemblage composition too. In the spots of maximum fluid emissions, observed in the northern sector and always associated to native sulphur deposits (ST1BNR1-R3; ST2BNR1-R3; ST3BNR1-R2; ST4BNR3), meiofauna is totally absent or constituted of small specimens (50 μm max diameter) of Trochammina inflata and Deuterammina rotaliformis, with an attached way of life. Rare Hyperammina spp. and Ammoglobigerina globigeriniformis are recorded too. In the Mediterranean Sea, Trochammina inflata is a common detritivore species, which also feeds on bacteria, and is often found in natural or anthropogenically-stressed environments (Murray, 2006). Deuterammina rotaliformis, to our knowledge, is rare in the Mediterranean and its rare findings are related to hydrothermal activity even though not strictly related to the maximum fluid emissions (Aeolian Arc; Panieri et al., 2005). Also outside of the Mediterranean Sea, D. rotaliformis has been recorded in the bathyal/abyssal hydrocarbon seeps of Gulf of Mexico (Sen Gupta et al., 2009) confirming its occurrence in vent emission sites and a good tolerance to acid and sulfidic environments. In the southern sector of the ZGP, an amelioration of benthic life conditions is deducted by the increase in faunal density and diversity (ST3BNR3). However, the almost exclusive development of siliceous microorganisms like radiolarians, diatoms and siliceous agglutinated foraminifers (Plate 1, Figs. 1–6) likely indicates venting activity influence. This particular site is characterized by foraminiferal density values higher than those recorded in other samples not only inside the pockmark, but also outside of it. Likewise to what has been recorded by Panieri (2006b) in the Adriatic Sea, the area under persistent but not strong fluid emissions, can represent a spot of productivity in respect to other sites not influenced by fluid emissions. It is reasonable to think that the higher availability of food offered by the bacterial mats but also the increase of fine grain size fraction in the sediment acting as trap for food supply, promotes the development of the “vent foraminiferal taxa”, although one has to keep in mind that some species nourish by means of other food sources (Goldstein and Corliss, 1994; Schönfeld, 2001). In addition, in ST3BNR3 the foraminiferal distribution inside the sediment resembles that recorded in normal marine conditions, and hence suggests a lower vent activity, in contrast to the northern and the central ZGP sectors (Fig. 6). Regarding the species composition, foraminiferal analyses highlight a peculiar siliceous agglutinated assemblage constituted of species uncommon for the Mediterranean Sea (i.e. Spiculosiphon oceana and Jaculella acuta). The species composition encountered in ours samples does not resemble those previously reported from the Pontine Archipelago, which are dominated by a high calcareous component (Frezza et al., 2010). Substantially the living association is dominated by Reophax scorpiurus a species that can be considered a “successful colonizers” of stressed environments (Bernhard et al., 2009; Hess and Kuhnt, 1996), but the peculiarity of

60 L. Di Bella et al. / Journal of Marine Systems 164 (2016) 53–66 Fig. 6. Test type composition of living foraminiferal assemblage (all data as ind/g) and distribution of the more abundant taxa within the sediment inside the Zannone Giant Pockmark (ZGP). (For interpretation of the references to the colour in this figure legend, the reader is referred to the web version of this article).

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Table 4 Diversity indices and faunal density calculated for the living (stained) foraminiferal faunas inside the Zannone Giant Pockmark (ZGP). Samples

Inside ZGP

Sediment interval (cm) Taxa_S Individuals Shannon_H Evenness_e^H/S Fisher_α Density (ind/g)

0_1 10 132 1.73 0.57 2.51 30.58

ST3BN R3

ST1 MC 1_2 11 74 1.68 0.49 3.57 9.27

2_3 9 63 1.07 0.32 2.87 5.34

3_4 2 38 0.21 0.61 0.45 4.20

4_5 4 44 0.47 0.40 1.07 3.43

5_6 3 44 0.22 0.41 0.73 3.30

6_7 2 33 0.14 0.57 0.47 2.91

the assemblage is due to the presence of S. oceana and J. acuta. Spiculosiphon oceana is a recently instituted species endemic of the Mediterranean Sea, recorded exclusively in a marine cave off the Spanish coast (Maldonado et al., 2013). Not much is known about this taxon, however on the base of our observations, it is a giant (3–4 cm) shallow infaunal species although the morphology of the test suggests a modality of life only partially in the sediments. The only another species belonging to this genus, S. radiata, is known to occur in the North Sea in an area also characterized by pockmarks and fluid emissions (Hovland, 1997). Jaculella acuta has never been found in the Mediterranean Sea although it is a cosmopolitan taxon amply widespread from 120 to 5.800 m depth and with a stratigraphic range from Triassic to Holocene (Gross, 2001; Loeblich and Tappan, 1988). The assemblage is also characterized by other agglutinated taxa (Glomospira spp., Lagenammina spp., Hyperammina spp. and Rhabdammina abyssorum) generally recorded much deeper (outer neritic to abyssal plain) both in the Mediterranean Sea and extra Mediterranean basins (Kaminski and Gradstein, 2005). The finding of these taxa, in the shallow water insular shelf of the western Pontine Archipelago, is probably related to the peculiar environmental conditions at the bottom that are more similar to those found in the abyssal plain below the Calcite Compensation Depth (CCD). In addition, it is noteworthy that the significant frequencies of Glomospira gordialis and G. charoides occur in natural hydrocarbon seepages (Kaminski et al., 1988) and hydrothermal vents (Jonasson et al., 1995). The living foraminiferal assemblages inside the ZGP highlight the occurrence of enhanced different environmental conditions with respect to the typical calcareous assemblages observed off the Pontine Archipelago and elsewhere on mixed carbonate-siliciclastic temperate shelves (Frezza et al., 2010). This change in foraminiferal assemblage can be explained by venting activity within the ZGP, producing a widespread enrichment of dissolved CO2 content, two to four times higher than the values observed in the normal marine conditions (Table 2). The very high CO2 content (N50,000 ppm) measured in the bubbling gases collected within the ZGP (Martorelli et al., accepted), indicates that the origin of CO2 is related to the degassing vents, probably producing acidification effects, as reported around submarine vents releasing CO2 (e.g. Boatta et al., 2013). In addition the anomalously high values of CH4 contribute to making the seafloor more toxic for living organisms (Rathburn et al., 2000; Sen Gupta et al., 2007).

7_8 2 11 0.47 0.80 0.72 0.85

8_9 1 14 0.00 1.00 0.25 0.91

9_10 1 5 0.00 1.00 0.38 0.37

0_1 20 44 2.71 0.75 14.15 11.40

ST4BN R1 1_2 12 24 2.22 0.77 9.55 2.54

2_3 5 9 1.43 0.83 4.63 1.81

0_1 3 17 0.81 0.75 1.06 10.29

ST4BN R2 1_2 2 3 0.64 0.94 2.62 0.73

0_1 6 13 1.74 0.95 4.32 1.87

1_2 6 11 1.77 0.98 5.40 1.00

On the other hand, the almost total lack of the calcareous component (both of organic and sedimentary origin) might be attributed to acidification (Alve and Murray, 1995; Nguyen et al., 2009). Similarly to other studies focused on the effect of hydrothermal vent on meiobenthic community (Panieri, 2006a), also in our case the foraminiferal assemblage includes almost exclusively agglutinated taxa. In situ observations along the Ischia Island coast demonstrated that with pH value of 7.6 only agglutinated tests were recorded (Dias et al., 2010), although studies conducted in natural CO2 vents located in the northern Gulf of California showed some resilience of calcareous benthic foraminifers to low pH conditions (pH 7.55) (Pettit et al., 2013). Within the ZGP, the almost exclusive presence of foraminiferal tests constituted of siliceous particles should highlight acid condition stronger than that recorded off Ischia Island where foraminiferal tests with agglutinated carbonate particles occur (Dias et al., 2010). The siliceous agglutinated fauna might indicate a diffuse and persisting venting of fluids from the bottom for a long time. This evidence is consistent with the venting activity reported by Ingrassia et al. (2015a) and geochemical data (Table 2). In addition, within the ZGP, the total absence of planktonic foraminifers might testify that the bubbling gas inhibited not only the benthic life on the floor but likely also the productivity of planktonic fauna as well due to their high susceptibility to dissolution processes (Fabry et al., 2008; Nguyen et al., 2009). In contrast in non-venting areas of the Pontine Archipelago, the plankton fraction is well represented in the sediments; at same water depth (120–140 m), the P/B ratio ranges between 15 and 20% (Frezza et al., 2010). The typical calcareous foraminiferal assemblages found by Frezza (010) in the Pontine Archipelago, was surprisingly not or only partially recorded outside the ZGP in our study. In fact, here (ST5 and ST6) some of the living agglutinated species observed within the ZGP (e.g. A. globigeriniformis, Lagenammina spp. and S. oceana) seem to live in association with calcareous benthic foraminifers represented by Lobatula lobatula, Lenticulina spp. and Cassidulina spp. These taxa are typical epifaunal species of sandy sediments always associated to intense hydrodynamic conditions (Duchemin et al., 2008; Hayward et al., 2002; Mojtahid et al., 2009; Murray, 2006). Moreover, the low frequencies of porcelaneous tests (miliolids) in respect of the Ponza shelf sites (Frezza et al., 2010), associated to the presence of mainly siliceous agglutinated taxa, suggest the indirect presence of venting activity also outside of the ZGP. The structure of these tests consists of high

Table 5 Diversity indices and faunal density calculated for the living (stained) foraminiferal faunas outside the Zannone Giant Pockmark (ZGP). Samples

Outside ZGP ST5BN R1

Sediment interval (cm) Taxa_S Individuals Shannon_H Evenness_e^H/S Fisher_α Density (ind/g)

0_1 9 13 2.10 0.91 12.94 1.74

ST5BN R2 1_2 3 6 1.01 0.92 2.39 0.35

0_1 12 19 2.30 0.83 13.98 1.71

ST5BN R3 1_2 3 3 1.10 1.00 0.00 0.24

0_1 14 24 2.40 0.79 14.06 1.61

1_2 5 5 1.61 1.00 0.00 0.30

ST6BN R1

ST6BN R2

0_1 17 33 2.51 0.73 14.09 3.41

0_1 20 38 2.61 0.68 17.08 4.35

ST6BN R3 1_2 3 3 1.10 1.00 0.00 0.23

0_1 14 35 1.84 0.45 8.65 2.60

1_2 6 9 1.74 0.94 7.87 0.53

2_3 11 13 2.35 0.95 33.82 0.99

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Fig. 7. Test type composition of living foraminiferal assemblage (all data as ind/g) outside the Zannone Giant Pockmark (ZGP). (For interpretation of the references to the colour in this figure legend, the reader is referred to the web version of this article).

magnesium calcite component that on the base of recent studies could be more susceptible to dissolution than normal calcite or even aragonite (Fabry et al., 2008). The vent activity likely influences foraminiferal population by high dissolved CO2 concentrations, recorded also nearby the ZGP, and the shallow foraminiferal distribution inside the sediment (2 cm top layer), which is similar to the sites under direct influence of fluid emissions. The effect of high pCO2 in bottom water is confirmed by the presence of dissolution signs on the calcareous tests (Plate 1:17, 26, 27). Therefore at the stations ST6BN and ST5BN, located nearby

the ZGP, the integrated meiofaunal and geochemical data suggest a transitional condition between vent influenced sedimentation and the typical carbonate sedimentation. 4.2. Dead assemblages The importance of investigating dead assemblage in the environmental studies is already universally recognized (De Stigter et al., 1999; Duros et al., 2012; Fontanier et al., 2014; Schönfeld et al., 2012).

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Table 6 Foraminiferal (dead) data results of samples outside Zannone Giant Pockmark (ZGP). ST: samples analyzed in this study; b: data from Frezza et al. (2010). Samples

Outside ZGP ST5BN R1

Sediment interval (cm) Taxa_S Counted Foraminifera Shannon_H Evenness_e^H/S Fisher_α Density (ind/g)

0_1 49 274 3.23 0.52 17.38 1066.15

1_2 41 227 3.03 0.50 14.62 683.73

ST5BN R2

ST5BN R3

ST6BN R1

0_1 38 218 3.03 0.54 13.31 654.65

0_1 58 272 3.36 0.50 22.58 254.68

0_1 40 211 3.15 0.58 14.62 1087.63

1_2 42 222 3.12 0.54 15.33 1337.35

1_2 50 303 3.21 0.50 17.05 888.56

ST6BN R2 1_2 35 202 3.09 0.63 12.22 706.29

0_1 32 201 2.96 0.60 10.73 1057.89

ST6BN R3 1_2 42 205 3.15 0.56 15.99 800.78

0_1 34 201 2.95 0.56 11.73 670.00

1_2 46 221 3.19 0.53 17.67 648.09

2_3 49.00 0.06 3.33 0.57 19.41

b117

b126

b131b b132

0_2 53 312 3.13 0.43 18.33 /

0_2 50 305 3.25 0.51 17.00 /

0_2 50 305 3.19 0.49 17.00 /

0_2 54 307 3.33 0.52 19.00 /

Plate 1. Benthic foraminiferal species found inside and outside the Zannone Giant Pockmark. Scale bar = 100 μm. Legend: L - live specimen; D - dead specimen; SDE - shell dissolution evidences in living specimens. 1. Hyperammina sp. General view, ST3BNR3 (L). 2. Jaculella acuta Brady, 1879. General view, ST3BNR3 (L). 3. Reophax scorpiurus de Montfort, 1808. General view, ST3BNR3 (L). 4–6. Spiculosiphon oceana Maldonado, López-Acosta, Sitjà, Aguilar, García & Vacelet, 2013. 4. General view, ST3BNR3 (L). 5–6. Detail of foraminiferal test, ST3BNR3 (L). 7–8. Rhabdammina abyssorum M. Sars, 1869. General view, ST3BNR3 (L). 9. Lagenammina atlantica (Cushman, 1944). Side view, ST4BNR1 (L). 10. Textularia bocki Höglund, 1947. Side view, ST6BNR1 (D). 11. Glomospira gordialis (Jones & Parker, 1860). Side view, ST3BNR3 (L). 12. Deuterammina rotaliformis (Heron-Allen & Earland, 1911). Spiral side of a specimen attached on quartz grain, ST2BNR3 (L). 13. Miliolinella subrotunda (Montagu, 1803). General view, ST6BNR3 (L). 14. Discorbinella bertheloti (d'Orbigny, 1839). Spiral view, ST5BNR3 (D). 15–17. Asterigerinata mamilla (Williamson, 1858). 15. Spiral view, ST5BNR1 (D). 16. Spiral view, ST5BNR1 (SDE). 17. Detail of altered test, ST5BNR1 (SDE). 18–20. Lobatula lobatula (Walker & Jacob, 1798). 18. Spiral side, ST6BNR3 (L). 19. Umbilical side, ST6BNR3 (L). 20. Umbilical side, ST5BNR1 (SDE). 21. Bolivina striatula Cushman, 1922. General view, ST6BNR2 (L). 22. Cassidulina carinata Silvestri, 1896. General view, ST6BNR3 (D). 23. Cassidulina crassa d'Orbigny, 1839. General view, ST6BNR3 (D). 24–26. Eponides concameratus (Montagu, 1808). 24. Spiral view, ST6BNR3 (L). 25. Spiral view, ST6BNR3 (SDE). 26. Detail of altered test, ST6BNR3 (SDE). 27–28. Lenticulina orbicularis (d'Orbigny, 1826). 27. Detail of altered test, STB6BNR1 (SDE). 28. Side view, STB6BNR1 (SDE).

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Fig. 8. Comparison between dead foraminiferal assemblages outside the Zannone Giant Pockmark (ZGP) near Zannone Island (a) and off Ponza Island (b) (data from Frezza et al., 2010). (For interpretation of the references to the colour in this figure legend, the reader is referred to the web version of this article).

The living assemblage represents seasonal environmental conditions (e.g. reproduction, spring blooms), whereas dead fauna records information averaged over a year or several years. However taphonomic processes have to be taken into account. Dissolution, fragmentation, transport and displacement of the tests have to be considered to better understand the benthic population dynamics and the potential environmental instability over time. In this study, the complete absence of dead assemblages within the ZGP can be related to the strong and persistent CO2 emissions. It is possible that the acid/toxic conditions at the sea bottom inhibited life of calcareous foraminifers or caused the complete dissolution of post-mortem calcareous tests. This group, mainly constituted of hyaline specimens, is present in the southern sector of the ZGP and is found in even higher abundances outside the ZGP, where also the porcelaneous group occurs. Outside the ZGP, the major differences between dead and living assemblages consist of a strong abundance increase of the calcareous tests in the dead assemblage, with the dominance of Cassidulina carinata, C. crassa, Lobatula lobatula, Asterigerinata mamilla and Gavelinopsis lobatulus. Porcelaneous group (miliolids) also increases while agglutinated group shows a clear decrease. It is significant to highlight that the dead agglutinated group includes only carbonatic cement taxa (e.g. Textularia spp.) while the living agglutinated species are characterized by exclusively siliceous grains composition. It is possible that this turnover in the agglutinated group is due to a CO2 fluid emissions influence at the time of the sampling also in the area outside of the ZGP although no direct evidences of fluid emissions were recorded. The previous study conducted on recent total assemblages in the western Pontine Archipelago, between Ponza and Zannone islands at the same water depth (119–147 m) (Frezza et al., 2010), showed foraminiferal assemblage (named as C. carinata assemblage) comparable with the dead assemblage recognized in this study outside the ZGP (Fig. 8). This assemblage is very similar in the most abundant species (C. carinata with C. crassa, L. lobatula and A. mamilla) and in the assemblage structure (diversity indices, percentages of dominance, strong abundance of perforate calcareous species) although no siliceous agglutinated taxa were found by Frezza et al. (2010).

5. Conclusions The new record of a giant complex venting pockmark (ZGP) in the shallow-water of the western Pontine Archipelago (central Mediterranean Sea, Italy) provided the opportunity to study fluid vent (mainly regarding the CO2 emission) impact on benthic foraminiferal communities. This study demonstrates that benthic foraminifers represent an important environmentally sensitive group in the marine ecosystem. Meiofaunal analyses performed on foraminiferal assemblages suggest that CO2 emissions are likely to be an important environmental driver. Dead and living assemblages change in the structure and composition related to different venting activity in the northern, central and southern sectors of the ZGP. Moreover results indicate the occurrence of venting effects also outside the ZGP toward the edge of the Zannone insular shelf possibly suggesting a widespread venting influence in the whole study area. This is supported by the following observations: loss of biodiversity related to increasing vent activity, increasing of agglutinated group with predominant siliceous component (Reophax scorpiurus, Spiculosiphon oceana and Jaculella acuta), reduced living specimens in the sediment and disappearance of porcelaneous tests. Moreover, the peculiarity of the ZGP pockmark area lies in compositional features of the living foraminiferal assemblage consisting of agglutinated species, never found or very rare in the Mediterranean Sea (J. acuta and S. oceana). This study allow us to increase the knowledges about the ecological features of these uncommon species and, because of the distinctive environmental context in which they were found, they could represent useful environmental proxies both in recent and fossil record. However, further geochemical data and longterm monitoring of the study area are needed to evaluate more detailed impact of venting activity on the benthic ecosystem. Acknowledgments We would like to thank to all the officers, crew and technicians of the R/V Urania for their precious work. This research was performed in the framework of the “RITMARE Project” and coordinated by the National Research Council and “La risposta della microfauna a stress ambientali indotti da processi geologici attivi: esempi attuali e fossili”

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