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Benthic foraminiferal evidence for the formation of the Holocene mud-belt and bathymetrical evolution in the central Adriatic Sea

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Benthic foraminiferal evidence for the formation of the Holocene mud-belt and bathymetrical evolution in the central Adriatic Sea
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  Benthic foraminiferal evidence for the formation of the Holocenemud-belt and bathymetrical evolution in the central Adriatic Sea Caterina Msrci a, *, Frans J. Jorissen b , Simona Fraticelli a  , Benjamin P. Horton c ,Mirko Principi a  , Anna Sabbatini a  , Lucilla Capotondi d ,Pietro V. Curzi e , Alessandra Negri a  a   Dipartimento di Scienze del Mare, Universita` Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italia  b  Laboratoire des Bio-Indicateurs Actuels et Fossiles, UPRES EA 2644, Universite´ d’Angers, 2 Boulevard Lavoisier, 49045 Angers,and Laboratoire d’Etude des Bio-Indicateurs Marins, Port Joinville, Ile d’Yeu, France c  Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104-6316, USA d  ISMAR–CNR, Via Gobetti 101, Bologna, Italia e  Dip. di Scienze della Terra e Geol.-Ambientali, Universita` di Bologna, Via Zamboni 67, Bologna, Italia Received 24 July 2004; received in revised form 13 June 2005; accepted 14 June 2005 Abstract Detailed analyses of modern and fossil benthic foraminiferal assemblages collected in the central Adriatic Sea are used astools to reconstruct the environmental changes that occurred between the Last Deglaciation and the Present (last 14 Kyrs); in particular we focus on the timing and formation of the mud-belt. The modern benthic foraminiferal assemblages display a parallel zonation to the Italian coast controlled by the interaction between food/oxygen availability and water depth. Cluster analysis of 4 sediment cores separates the fossil foraminiferal assemblages in 6 groups: Cluster A is dominated by three  Ammonia species; Cluster B consists of  Ammonia papillosa , Nonionella turgida , Elphidium advenum and Elphidium decipiens ;Cluster C is composed of two taxa, Hyalinea balthica and Trifarina angulosa ; Cluster D is dominated by 5 species, Cibicideslobatulus , Buccella granulata , Reussella spinulosa , Textularia agglutinans and Elphidium crispum ; Cluster E contains  Bulimina spp., Gavelinopsis praegeri , Bolivina spp., Cassidulina neocarinata and Asterigerinata mamilla ; and Cluster F isdominated by Bulimina marginata , Valvulineria bradyana , Globocassidulina subglobosa and Melonis padanum . The cluster analysis and contemporary distribution patterns of these taxa are used together with ecological preferences of the most frequent species to reconstruct the spatial and temporal distribution of the different biofacies in the past. This reveals information about Holocene palaeoenvironmental changes that are related to water depth fluctuations and the installment of the coast-parallelmud-belt. The benthic assemblage records the transition from a infralitoral environment (Biofacies I) to deeper marine condition(Biofacies III). After that the sea level reached about the modern level (Biofacies IV) the benthic foraminiferal community 0377-8398/$ - see front matter  D 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.marmicro.2005.06.001* Corresponding author. Tel.: +39 071 2204287.  E-mail address: c.msrci@univpm.it (C. Msrci).Marine Micropaleontology 57 (2005) 25–49www.elsevier.com/locate/marmicro  evidences the development of the mud-belt and the subsequent transformation of the ecological niches linked to the trophicevolution of the environment. D 2005 Elsevier B.V. All rights reserved.  Keywords: Benthic foraminifera; Adriatic Sea; Mud-belt; Sea level; Holocene 1. Introduction Many of the world’s coastal areas are extremelyvulnerable, and are strongly menaced by future sealevel rise. The Adriatic Sea is one of the areas wheresea level rise could create important damages at thecoast and at the human activities. In spite of the fact that an accurate knowledge of the sea level history isimportant for environmental studies, the literaturedealing with the Holocene sea level history of theAdriatic Sea is still scarce (Lambeck et al., 2004). Although the transgressive deposits of the last sea-level cycle in the Adriatic Sea are well investigatedfrom a sedimentological point of view (Pigorini,1968; Van Straaten, 1970; Brambati et al., 1983;Ciabatti et al., 1987; Curzi and Tomadin, 1987; Trin-cardi et al., 1994; Correggiari et al., 2001; Cattaneo et al., 2003), there are few studies about the relation between high stand deposits and benthic microfauna(Asioli, 1996; Asioli et al., 2001).Some of the best records of sea-level change have been derived from intertidal microfossil assemblages(diatoms, foraminifera and pollen) contained in arange of post-glacial Holocene sedimentary deposits(Scott and Medioli, 1980; Horton et al., 2003; Hortonand Edwards, 2005). During the past 50 years, theseindicators have been used extensively to providereconstructions of Holocene Relative Sea Level(RSL) change for the UK, Europe and elsewhere,and have been the primary source of data for devel-oping and testing models of RSL (e.g.Lambeck et al.,2002; Peltier, 2002). However, the potential applica-tion of benthic foraminifera from open marine envir-onments is poorly understood. In open marineenvironments, the bathymetrical zonation of benthicforaminifera appears not to be tightly constrained, but to vary as a function of the organic flux to the oceanfloor (e.g.De Rijk et al., 2000; Msrci et al., 2001). In shallow marine areas, on the contrary, severalstudies report a consistent bathymetric distributionof benthic foraminifera (e.g.Jorissen, 1988; Asioliand Borsetti, 1989; De Stigter et al., 1998).Jorissen(1987, 1988)studied in detail the geographical dis-tribution of the benthic foraminifera in Recent sedi-ment samples of the Adriatic Sea. He found that  benthic assemblages show a zonation parallel to theItalian coast. At greater distance from the Po delta,this bathymetrical species zonation becomes clearer.The runoff of the Po delta is indeed responsible for strong environmental changes over short distances.This influence is evident in the upper part of thewater column, but also on the sea floor, where it strongly affects the distribution of the benthic fora-minifera. River runoff causes the input of suspendedclay, organic detritus and dissolved nutrients into themarine system. This can provoke strong primary production events, leading to eutrophic conditionsin the water column and on the sea floor. As aconsequence of the stratification of the water columnin summer (Giordani and Angiolini, 1983; Tahey et al., 1995) the bottom environment may seasonally become oxygen depleted. The quantity and qualityof the changes in the Recent foraminiferal associationclearly reflect the impact of these phenomena.Several studies (Parker, 1958; Jorissen, 1988; DeStigter et al., 1998) investigated the relationship between water depth and the composition of benthicforaminiferal assemblages, and determined a bathy-metrical species succession. Some studies (Carney,1989; De Rijk et al., 2000), argued that the bathy-metrical changes of benthic fauna are not directlyindicative of a specific water depth but are mainlyrelated to a certain level of organic flux exportedfrom the photic zone. In the Adriatic Sea,Jorissen(1987)showed a slight southward deepening of thedepth zones of taxa inhabiting the mud-belt, inresponse to a deepening of the central part of themud-belt, where maximum percentages of organicmatter are found due to focussing of the fine-grained material.In this paper we want to use the Holocenesediment record, and more precisely the benthic C. Msrci et al. / Marine Micropaleontology 57 (2005) 25–49 26  foraminiferal microfossils to reconstruct the AdriaticSea level history. The aim of our study is toreconstruct palaeoenvironmental changes in this epi-continental sea, as documented by the fluctuationof the foraminiferal assemblage, and particularly, todocument the evolution of the mud-belt depositedafter the time of maximum marine transgression.The study of this high stand deposit is important  because it allows a detailed reconstruction of envi-ronmental variability in the recent geological past and because it may serve as an analogue to recent and future situations. This is achieved by the anal-ysis of the modern benthic foraminiferal communi-ty in the Conero area, and the planktonic and benthic foraminiferal assemblages of four gravitycores taken in the central western part of theAdriatic Sea. 2. Study area The Adriatic Sea is an elongated NW/SE oriented basin, located in the central Mediterranean Sea. The bathymetric features allow a threefold subdivision. Itsnorthern section is very shallow and gently slopingwith an average water depth of about 35 m. In thecentral part, water depth moderately increases off SanBenedetto del Tronto, where a shelf-break delimitsthe 270 m deep Middle Adriatic Depression (MDA)(Ciabatti et al., 1987)or Jabuka pit. The southern Adriatic Sea, which extends to the S of Gargano– Lagosta line, reaches a maximum depth of 1250 m.Finally, the basin is limited by a sill, the Otranto strait,with a water depth of about 800 m.There are three principal water masses in theAdriatic Sea: the Adriatic Surface Water (AdSW),the Levantine Intermediate Water (LIW) and theAdriatic Deep Water (AdDW; each sub-basin hasits own characteristic deep water) (Orlic et al.,1992; Artegiani et al., 1997). The general circulationis cyclonic with a flow towards the northwest alongthe eastern side and a return flow towards thesoutheast along the western side (Fig. 1). The cir- culation in the three sub-basins is often dominated by local cyclonic gyres that vary in intensity accord-ing to the season. The sub-gyre of the southernAdriatic tends to persist throughout the year. Thesub-gyre of the middle Adriatic is more pronouncedin summer and autumn. In the north, during theautumn, a cyclonicgyre is evident in front of thePo river mouth (Russo and Artegiani, 1996). A secondary western-descending current is connectedto bathymetric variations and/or to the input of coastal fresh water. This circulation consists of twostable branches, which turn from E to W at thelatitude of Gargano (Drina cell) and Conero (Ner-etva cell) (Mosetti, 1984). In connection with these circulatory features a long shore current is directedfrom S to N along the eastern coast and from N to Salong the western coast. A transverse line of con-vergence (zone of sinking waters) separates thesetwo opposite surface currents. Because of the pres-ence of important annual river discharge (e.g. Poriver) in the western part of the Adriatic Sea, andthe southward direction of the coastal current com-ing from the northern basin, the western coastalareas are nutrient enriched, whereas theeastern part of the basin is rather oligotrophic (Artegianiet al., 1997; Zavatarelli et al., 1998).The circulation pattern described above clearlyaffects the thickness and the distribution of themodern sediment cover. South of the Po delta, a prograding coastal/deltaic system correlates with amuddy offshore wedge that extends continuouslyalong the western Adriatic coast (Trincardi et al.,1994). This modern sedimentary system shows anumber of shore-parallel depocenters of clay-richsediments (less than 1% sand content), which canreach about 40 m of thickness in its southern part (Curzi and Tomadin, 1987; Cattaneo et al., 1997;Van Straaten, 1970). These transgressive depositsspan from about 14.0 Kyr BP in the southern part to 5.0 Kyr BP in the north (Asioli, 1996; Trincardiet al., 1996). A transgressive surface separates ma-rine and deposits typically condensed to a thin lagveneer of shelly and muddy sediments (about 30–40cm with maxima of over 150 cm) (Correggiari et al., 1992; Cattaneo and Trincardi, 1999). Thesesediments correspond to the b ecozone rf   Q  of Asioli(1996), which is dominated by a benthic faunatypical of intertidal environments. Recent works inthe southern part of the mud-belt dated the begin-ning of the formation of the high stand system track to 5.5 Kyr BP (Trincardi et al., 1996; Cattaneo andTrincardi, 1999). This muddy sedimentary structureis delimited to the west by the modern littoral C. Msrci et al. / Marine Micropaleontology 57 (2005) 25–49 27  deposits and to the east by Pleistocene relict sandsthat crop out offshore. 3. Materials and methods The analysis of fossils sediments is based on 4gravity cores (AD 87:16, 17, 20, 21) collected in thecentral Adriatic Sea during oceanographic cruise AD87 (Fig. 1), at 75 to 80 m water depth. Details of thecores are reported inTable 1.The lithological de- scription of the cores is summarised inFig. 2. Twomain lithological units can be recognised in all thecores. The lower unit is characterised by firm sed-iment almost devoid of foraminifera. The upper unit consists of blue clays rich in micro- and macrofos-sils. These two intervals are separated by an ero-sional surface, marked by a concentration of shelldebris and reworked benthic foraminifera. For themicropaleontological studies, 1 cm thick sampleshave been taken every 10 cm. Preparation of fossilsamples followed a standard technique. All sampleswere dried at 50 8 C for 72 h and weighted. Thenthey were re-hydrated with distilled water andwashed over a 63 A m sieve. Quantitative and qual- Table 1Details of the sediment cores: locations, depth and lengthCores Latitude N Longitude E Water depth (m)Length(cm)AD87-20 43 8 43  V 58 W 14 8 07  V 10 W 78.8 216AD87-21 43 8 40  V 23 W 14 8 00  V 43 W 77.3 360AD87-16 43 8 45  V 24 W 13 8 54  V 57 W 75.5 283AD87-17 43 8 49  V 07 W 14 8 01  V 61 W 76.5 214 37.038.034.035.036.0 16172021 12 o 13 o 14 o 15 o 43 o 44 o 45 o 050100 km Ravenna RELICT SANDSCORESALINITY ‰SURFACE CURRENTSIsopach in meters of thethickness of the mud-belt 25202510155 I   T   A  L I   A   Ancona Fig. 1. Principal circulation and currents in the Adriatic Sea (fromMosetti, 1984andCurzi et al., 1988modified). The thickness of the Holocene mud-belt is indicated in meters, considering a sound speed of 1600 m/s. The position of the 4 cores is shown. C. Msrci et al. / Marine Micropaleontology 57 (2005) 25–49 28  itative analyses of planktonic and benthic foraminif-era have been performed. Samples containing abun-dant microfauna have been split in aliquotscontaining at least 300 specimens, which were sub-sequently picked, identified and counted. Planktonicversus benthic foraminifera ratio (P/P+B) has beenused as a first qualitative approximation of paleo- bathymetry (according toGrimsdale and Van Mor-khoven, 1955). R-mode cluster analysis has been performed on the benthic assemblages found in thecores, and we used the 24 species, that showed arelative abundance in the assemblage, higher than5% in at least one sample (Table 2). This is a mathematical technique which clusters variables based on their similarity or dissimilarity (Davis,1986). In our case we use it in order to define the SCCSCSSCSSCCSSCCSSCCSSCSSCCCSCSCCSSCSLLSCSSCSCSCCSCSCSSC Ad 87-16 50 Ad 87-20 Ad 87-17Ad 87-21 100150250 80% 300cm200350ClaySilty clayClayey siltSandy siltSand Silt ClayShelly debrisShellDisappearance of H. balthica  erosional surfaceSilty sandSiltSiltCSCLSCSSSCSLS CSSSC 0 0Clay %0 80%0 90%080% Clay % Clay % Clay % SLCS Fig. 2. Lithology and clay content (%) of the studied cores. Based on the proportions of sand-, silt- and clay-sized particles, the bottomsediments were classified according to Shepard’s diagram (based on unpublished work of Principi, 1997). C. Msrci et al. / Marine Micropaleontology 57 (2005) 25–49 29

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