See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/250085804 Distribution of living benthic foraminifera in relation with environmental variables within the Aiguillon cove (Atlantic coast, France): Improving knowledge for paleoecological int... ARTICLE in BULLETIN DE LA SOCIETE GEOLOGIQUE DE FRANCE · MAY 2009 Impact Factor: 1.07 · DOI: 10.2113/gssgfbull.180.2.131 CITATIONS 11 READS 53 4 AUTHORS, INCLUDING: Eric Armynot du Châtelet Université des Sciences et Technologies de … 39 PUBLICATIONS 484 CITATIONS SEE PROFILE Pierre-Guy Sauriau Université de La Rochelle 131 PUBLICATIONS 1,963 CITATIONS SEE PROFILE Available from: Eric Armynot du Châtelet Retrieved on: 05 February 2016
Mineralogy of agglutinated benthic foraminifera; implications forpaleo-environmental reconstructions
ERIC ARMYNOT du CHATELET1, PHILIPPE RECOURT1 and VINCENT CHOPIN1
Key-words. – Agglutinated foraminifera, Salt marsh, Test mineralogical composition, Sediment grain size
Abstract. – Benthic foraminifera of recent salt marsh environments are often dominated by species with an agglutinatedtest. The grains used for test construction by these foraminifera are collected from their surrounding environment. Inthis study we investigate the role of sediment grain size and mineralogical composition for richness, population densityand taxonomic composition of agglutinating foraminifera. Foraminifera from 15 stations of the tidal marsh of theCanche estuary (Pas-de-Calais, France) were studied.
The species richness depends on the grain size of the sediment, whereas the density is not related to sedimentgrain size. The distribution of foraminifera species throughout the tidal marsh may depend on many environmental pa-rameters such as OM as well as tidal elevation, already largely discussed in literature. The mineralogical composition ofthe agglutinated grains in Trochammina inflata and Arenoparrella mexicana is very different from that of sediment; thecomposition of Jadammina macrescens is generally different from that of the sediment with some exceptions, and inMiliammina fusca, Paratrochammina haynesi and Remaneica plicata the mineralogical compositions are similar tothose of the sediment. The studied species may be able to select their preferred grains based upon composition even if aparticular mineral is scarce in the sediment.
Minéralogie des tests agglutinés de foraminifères des environnements de marais maritime ;implications pour les reconstitutions paléo-environnementales
Mots-clés. – Foraminifères agglutinés, Marais maritimes, Composition minéralogique du test, Granulométrie du sédiment
Résumé. – Les foraminifères benthiques qui vivent dans les environnements de marais maritimes ont principalement untest agglutiné. Les grains utilisés pour la construction des tests de ces foraminifères sont prélevés dans leur environne-ment proche. L’objectif de l’étude est de déterminer le rôle de la granulométrie des grains et de la composition minéra-logique sur la richesse et la densité de la population ainsi que sur la composition taxinomique de l’assemblage desforaminifères. Les foraminifères de 15 échantillons prélevés sur le schorre de l’estuaire de la Canche (Pas-de-Calais,France), ont été observés.
La richesse spécifique dépend de la taille des grains du sédiment alors que la densité n’y est pas reliée. La distri-bution des espèces de foraminifères dans les marais maritimes dépend de nombreux paramètres tels que le contenu enmatière organique, l’élévation par rapport au niveau de la mer, largement déjà discuté dans la littérature scientifique.Les compositions minéralogiques des grains agglutinés de Trochammina inflata et de Arenoparrella mexicana sont trèsdifférentes du sédiment; celle de Jadammina macrescens est en générale différente avec quelques exceptions et celles deMiliammina fusca, Paratrochammina haynesi et Remaneica plicata sont similaires à celle du sédiment. Les espèces étu-diées sont capables de choisir leurs grains mêmes s’ils sont rares dans le sédiment.
INTRODUCTION
Numerous studies have used benthic foraminifera for thestudy of changes in relative sea-level during the Holocene.Marsh foraminifera are contained in both Recent and fossilsedimentary deposits and are accurate sea-level indicatorsbecause they have narrow ecological tolerances and narrowvertical zones in the tidal marshes [Scott and Medioli, 1978;1980b]. Several authors have investigated the effects ofcm-scale variations in relative sea level within the intertidalzone using recent foraminiferal associations for many areas,including the Atlantic Ocean [Edwards and Horton, 2000;Edwards et al., 2004; Gehrels, 1994; Haslett et al., 2001;
Horton et al., 1999b; Patterson et al., 2004], the NorthAmerican eastern Pacific [Guilbault et al., 1995; Pattersonet al., 1999; Williams, 1999], South American eastern Paci-fic [Jennings et al., 1995], and the southeastern PacificOcean [Hayward et al., 1999, 2004; Horton et al., 2003;Southall et al., 2006; Woodroffe et al., 2005]. Models ofHolocene sea-level change have been based on vertical dis-tribution of foraminiferal associations [Gehrels, 2000; Hor-ton et al., 1999a].
However, the interpretation of microfossil data remainsprone to error. The ecological interpretation based on fora-minifera tests presents some problems. One error comesfrom the ecological conditions for their test construction.
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1. UMR Géosystèmes, U.F.R. des Sciences de la Terre, Bâtiment SN5, Université Lille I, 59655 Villeneuve d'Ascq cedex, [email protected], [email protected], [email protected] déposé le 29 octobre 2007 ; accepté après révision le 2 avril 2008
Many factors control foraminiferal abundance and assem-blage type, including nutrition, dissolved oxygen condi-tions, pH, salinity, substrate and temperature, subaerialexposure, or floral substrates [Alve and Nagy, 1986; Bol-tovskoy et al., 1991; De Rijk, 1995; Debenay et al., 2002;Murray, 1968; Murray, 1991; Murray, 2006]. One largelyunderestimated factor is the sediment grain size and minera-logical composition. The texture of agglutinated tests maybe highly variable. Agglutinated foraminifera have the abi-lity to select grains of different shape and size to make thethree dimensional form of the test. Some species are knownto select the grains even if they are rare in the sediment [He-ron-Allen and Earland, 1909, in : Debenay et al., 1996] andit has been demonstrated that the reticulopodia are able tosort out grains [Tuckwell et al., 1999, in : Murray, 2006].The nature of the agglutinated tests may depend on their se-dimentological environment, which may introduce somemodifications in the structure of assemblages.
The aim of this study is to investigate the grain size andmineralogical structure of the dominant agglutinated fora-minifera present in marshes and to compare them with thesediment composition of their substrate.
MATERIAL AND METHODS
Study area
The Canche estuary (Pas-de-Calais, France) (fig. 1) is si-tuated at the end of a 96 km long river having a relativelysmall drainage basin of 1407 km2. This estuary is submit-ted to macro-tidal influence. Temperature ranges between7 degrees in winter and 21 in summer. Salinity ranges fromnormal sea salinity down to low salinity depending of tideand river runoff. The sampling area is however never
submitted to zero salinity. On the salt marsh, salinity maybecome higher because of the dehydration of the sediment.
We studied a 325 m transect across the shore of theCanche estuary. Fifteen stations, spaced regularly every25 m, were sampled. Four stations were located on the mud-flat (stations 1 to 4), one at the transition between the mud-flat and the salt marsh (station 5), eight were located on thesalt marsh (stations 6 to 13) and two on drainage channels,one across the tidal flat (station 110) and a second one on alarge drainage channel behind a sea wall (station 14). Sta-tion 110 was close to station 11. The elevation gradually in-creases from the sea to the sea wall (fig. 1 and tabl. I).Station 9 is a little lower than the two surrounding stations 8and 10. The mudflat was without vegetation. The transitionbetween the mudflat and the lower salt marsh was not fullycovered by vegetation and colonized by Salicornia herba-cea (32%) and Puccinellia maritima (21%). Rare Spartinamaritima occurred at this transition. The high marsh wascovered mostly by Agropyrum pungens with some patchesof Obiones portulacoides (tab. I).
Sampling method
The study was carried out on sediments collected in Decem-ber 2006. At each station, a constant volume of 150 cm3 ofsediment was sampled from a one square meter zone, follo-wing the pseudo-replication method [Debenay and Guillou,2002; Hurlbert, 1984]. After homogenization, two sub-sam-ples were made, one of 50 cm3, for the manipulation of li-ving benthic foraminifera species and one of 100 cm3 forthe analyses of the sediments. The samples for foraminife-ral analysis were stored in an ethanol-rose bengal solutionat 1g L-1. This staining method of living foraminifera wasproposed by Walton [1952] and was considered as an effi-cient and fast compromise method by Murray and Bowser[2000]. In the laboratory, wet samples were washed through63 and 315 µm mesh sieves. Before observation, the tests ofthe intermediate fraction (63-315 µm) were concentrated byflotation on trichloroethylene.
Foraminifera analyses
The analyses were carried out only on living (stained) fo-raminifera. At each station the density (number of indivi-duals per 50 cm3) and species richness were determined.Correspondence analysis was used for providing a graphicrepresentation of the distribution of the foraminifera spe-cies. A dendrogram classification of samples produced byQ-mode cluster analysis was superimposed. The chemicalcomposition of the minerals comprising the walls of agglu-tinated foraminifera was determined by means of 59 pointchemical analyses on selected mineral grains bigger than2 µm, because of the mediocre precision on smaller parti-cles, using an Environmental Scanning Electron Micros-cope (ESEM) using an Energy Dispersive X-raySpectroscopy (EDS). These minerals were selected be-cause of their chemical contrast caused by different atomicnumber. The chemical compositions were then grouped bymeans of a cluster analysis into mineral poles. This inves-tigation of the chemical composition of the minerals wascarried out on three species with a maximum of mineral di-versity (Deuterammina eddystonensis, Jadammina macres-cens and Arenoparrella mexicana). Stations 4 and 6 wereselected for the analysis.
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FIG. 1. – Study area and localization of the 15 sampling stations.FIG. 1. – Zone d’étude et localisation des 15 stations de prélèvements.
Mineralogical compositions were evaluated on 148 fo-raminiferal specimens of the six agglutinated species selec-ted because of their abundance in the 15 stations. Thecartographical analyses were carried out using theESEM-EDS. The proportion of each mineral was then eva-luated by image analysis. The results were compared withsimilar analyses carried out on the 15 sediments then grou-ped by means of a cluster analysis in order to extract intra-specific and inter-specific mineral links. Correspondenceanalyses and Q-mode cluster analyses were performedusing the R integrated suite of software facilities [Ihaka andGentleman, 1996]. Euclidean distance correlation coeffi-cients were used to measure similarities and Wards’ linkagemethod was used to arrange pairs and groups into hierarchicdendrograms. The only species for which grain size wasanalyzed for all stations was Jadammina macrescens.
Sediment analyses
The sediment grain size was characterized using a MalvernMastersizer 2000 (size range 0.02-2000 µm). Descriptiveparameters of the grain size were determined: mode, sortingand skewness. For comparison with the grains agglutinatedon the benthic foraminifera test, the mineralogy of sedimentgrains with similar size was analyzed (sizes less than36 µm, sampled by sieving). This sedimentary fraction wasthen analyzed using the ESEM-EDS, extracting cartogra-phical output, then analyzed with image analysis. Total or-ganic matter (OM) content was determined from the loss ofweight on ignition method (ignition at 450oC for 3h). The
OM content was expressed as percentage of dry sedimentweight.
RESULTS
Sediment grain size, mineralogical composition and totalorganic matter characteristics
Sediment from stations 1 and 3 display a bimodal distribu-tion with a main mode of 196-214 µm and a second of25-38 µm (tabl. I, fig. 2). The second mode slightly increa-ses along the transect until stations 8 to 10 (92-95 µm), thendecreases towards the upper part of the tidal marsh. Themain mode of station 2 is of 195 µm, similar to the one ofstation 3. The sediment grain mode of station 110 is close tothat of station 11. No clear mode occurred at station 9 witha staggering of the highest frequencies between 30 and50 µm. Only station 2 showed a good granulometric sorting(fig. 2). All other stations were poorly sorted. The skewnessshowed for the majority of them a granulometric symmetry± an asymmetry towards large sizes. Station 1 and 3 showeda strong asymmetry towards large size.
Sediment mineralogical nature is dominated by carbo-nates (up to 60.8% at station 9), except for station 5 wherecarbonate proportion is slightly inferior to the proportion ofquartz and albite, and except for station 6 and 7 wherequartz and albite proportion is dominant. At station 9,quartz and albite are noticeably low (9.7%). Potash feldspardoes not show a clear evolution throughout the transect butabove 10% except at station 12 and 14 (2.9 and 7.9% res-pectively). Rutile does not show a peculiar pattern, always
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MINERALOGY OF AGGLUTINATED BENTHIC FORAMINIFERA 585
FIG. 2. – Skewness vs. sorting of the sediment grain size. Three representative granulometric curves were given : stations 1, 2 and 6.FIG. 2. – Asymétrie en fonction du classement de la taille des grains du sédiment. Trois courbes granulométriques représentatives sont données : stations 1, 2 et 6.
3.2%. Chlorite proportion is generally low as well, except atstation 7 and 110 where it represents 11.1 and 12.2% res-pectively.
Surface sediments were generally characterized by ahigh proportion of organic matter (tabl. I). The total OM in-creases from 1.12% at station 2 to 34.63% at station 10; it is25% on average between stations 6 and 10. The total OMwas variable in stations 11, 12 and 13. Stations 14 and 110display OM levels equivalent to stations 4 and 5.
Foraminifera characteristics: density, species richnessand relative abundance
In the 15 samples, a total of 31 living (stained) species wereobserved. Only 11 of them have an agglutinated test (tabl. I,fig. 3). Species richness is highest in the mudflat sampleswere the sediment grain size is small and OM is high. Spe-cies richness is high in the two transitional stations, situatedbetween the mudflat and the salt marsh (15 and 16 species
respectively in samples 4 and 5) and in the sediments of thedrainage channel (11 species in samples 14 and 110)(fig. 3). Species richness is low in the salt marsh (4 to 7 spe-cies at stations 6 to 13), whereas the density sharply increa-sed after the mudflat/salt marsh threshold with the highestcontent in OM. At station 1 only two porcelaneous speciesoccur: Miliolinella subrotunda and Quinqueloculina semi-nula, and two hyaline species occur in station 3: Cribroel-phidium williamsoni and Haynesina germanica (tabl. I). Noforaminifera were observed at station 2.
Stations 6 and 7 have very close elevations (4 cm diffe-rence in 25 m), and foraminifera proportions, especiallythose of Jadammina macrescens and Trochammina inflata,are different (respectively 48.1 –73.7% and 46.9 –2.2%).Stations 8, 10, 11 and 12 have also very close elevations(6 cm difference in 100 m). These are the stations where Ja-dammina macrescens is largely dominant, with more than81%. However, the next species Trochammina inflata havehighly variable proportions (0.5 to 10.5%). As well as
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TABLE I – Sampling station main characteristics: station elevation measured above the lowest see level (ELSW), flora characteristics; sediment characte-ristics (mineralogical composition, main mode and organic matter proportion); foraminifera characteristics (species richness, density per 50 cm3 and ag-glutinated and calcareous foraminifera relative abundance).TABL. I. – Principales caractéristiques des stations de prélèvement : altitude des stations mesurées au-dessus du niveau des plus basses marées, caracté-ristiques de la flore; caractéristiques du sédiment (composition minéralogique, mode principal et proportion de matière organique); caractéristiques desforaminifères (richesse spécifiques, densité pour 50 cm3 et abondances relatives des espèces de foraminifères à test agglutiné et calcaire).
species proportions, proportions of potash feldspar, quartzand albite in the sediment are different from the station withsimilar elevation; carbonates content are however similar.The OM proportion is positively correlated to the foramini-fera density and the Jadammina macrescens proportion aswell.
Dealing only with the agglutinated forms (tabl. I), thedensities were lowest at stations 1 to 5 and highest at sta-tions 7 and 12 (mainly composed of Jadammina macres-cens). The two main agglutinated species, J. macrescensand Trochammina inflata, are dominant with respect to rela-tive abundance and density followed by Arenoparrellamexicana and Deuterammina eddystonensis. The maximumdensities of J. macrescens and T. inflata were observed atsalt marsh stations 12 and 6, respectively. More than 550 in-dividuals per 50 cm3 occurred at stations 6 to 13, where OMwas above 16.7%. The highest densities of A. mexicana andD. eddystonensis were observed at the mudflat / salt marshthreshold (station 5) and at station 8.
Mineralogical nature of the agglutinated foraminiferaltest and the sediment
Cluster analysis of the results of the analyses on selectedminerals of the agglutinated foraminifera produces fiveclusters corresponding to five mineralogical poles: chlorites,
potash feldspar, rutile, albite and quartz, and carbonates(fig. 4).
Based on a cluster analysis of the mineralogical compo-sition of grains in the test and sediments particles, threegroups were observed (fig. 5). In the first group, almost allJadammina macrescens (108 of 117 specimens) were grou-ped together with a unique individual of Arenoparrellamexicana; in the second group, all other A. mexicana andTrochammina inflata were grouped; in the third one, Para-trochammina haynesi, Miliammina fusca and Remaneicaplicata and some Jadammina macrescens (9 of 117 speci-mens) were grouped with the sediments mineralogical com-position of all the stations. The mean mineralogicalcomposition of foraminifera is given in table II. The mine-ralogical composition of Jadammina macrescens is complexand variable. Some individuals show considerable varia-tions within a single test (e.g., fig. 6). Two groups of cham-bers are observed, some rich in Ca, and others poor in Ca.By the way, the test is mainly impoverished in carbonatesand enriched in chlorites and potash feldspar.
The mineralogical composition of the test of Arenopar-rella mexicana and Trochammina inflata, differs stronglyfrom the sediment. Both are enriched in quartz and albite.At the opposite, Miliamina fusca, Paratrochammina haynesiand Remaneica plicata have a mineralogical composition ofthe test close to that of the sediment (tabl. II) with however
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FIG. 3. – Foraminiferal distribution, on the basis ofthe relative abundances of the species obtained by acorrespondence analysis (the thickness of the co-lumns is proportional to the relative abundance ofeach species). Agglutinated species are underlinedby grey lines. Stations are classified by use of hie-rarchical analysis.FIG. 3. – Distribution des foraminifères, basée surles abondances relatives des espèces au moyend’une analyse des correspondances (l’épaisseur descolonnes est proportionnelle à l’abondance relativede chaque espèce). Les formes agglutinées sont sou-lignées en gris. Les stations sont classées au moyend’une analyse hiérarchique.
some enrichment and impoverishment compared to it. In thedrainage area, Miliammina fusca test composition may beenriched in potash feldspar. In the transition zone betweenthe mudflat and the salt marsh, the test composition of P.haynesi may slightly be impoverished in quartz and albiteand in stations 5 and 11, R. plicata is enriched in potashfeldspar.
For four of the five mineral poles, the mean size of thetest grains was smaller than the mean sediment grain size;only for potash feldspar was it similar (fig. 7). The grainsize variation of grains is similar to that of the test grains.These trends were less evident for chlorite. Remarkably, the
mean sizes of quartz and albite test grains are constant, ir-respectively of the sediment grain size.
DISCUSSION
At least two tidal sub-zones are reported in the majority ofstudies in coastal environments [Scott and Medioli, 1980a;Woodroffe et al., 2005]. A lower zone is often dominated bycalcareous species, typically displaying a higher speciesrichness. In the present study, this zone corresponds to sta-tions 1 to 5. However, in this study the species richness re-mains low until the sediment grain size becomes small
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TABL. II. – Agglutinated test mineralogical composition of the main foraminifera (Arenoparella mexicana, Jadammina macrescens, Miliamina fusca, Pa-ratrochammina haynesi, Remaneica plicata and Trochammina inflata). Values are given in % of surface for the 5 mineralogical poles (chlorites, potashfeldspar, rutile, quartz + albite and carbonates). The standard error is précised.TABL. II. – Composition minéralogique des tests des principales espèces de foraminifères agglutinées (Arenoparella mexicana, Jadammina macrescens,Miliamina fusca, Paratrochammina haynesi, Remaneica plicata and Trochammina inflata). Les valeurs sont données en proportion de la surface pour les 5pôles minéralogiques (chlorites, feldspath-K, rutile, quartz + albite et carbonates). L’écart type est précisé.
FIG. 4. – Oxide composition of selected minerals on foraminifera grouped with a hierarchical analysis in 5 mineral poles. The foraminifera (Jmac = Ja-dammina macrescens, Deddy = Deuterammina eddystonensis, Amex = Arenoparrella mexicana) and the sample source of the analyzed mineral were men-tioned.FIG.4. – Compositions en oxydes de minéraux sélectionnés sur les foraminifères, groupées en pôles minéralogiques à l’aide d’une analyse hiérarchique.Les foraminifères (Jmac = Jadammina macrescens, Deddy = Deuterammina eddystonensis, Amex = Arenoparrella mexicana) et la station source sontmentionnés.
enough. The richness and abundance was low in sampleswith coarse sediment, probably because mechanical stressin the high energy environment of the tidal area restrictedthe foraminifera fauna. At station 4 with finer sediment, ahigher species richness and density was observed.
The upper zone (stations 5 to 13) is typically less di-verse and dominated by agglutinated species [Murray,1973]. In the Canche estuary, it is dominated by two spe-cies, Jadammina macrescens and Trochammina inflata.Both species are typical salt marsh foraminifera [Guilbaultet al., 1995; Ruiz et al., 2005; see review in Scott et al.,2001]. Williams [1999] also observed typical high marshspecies on the mud flat and argued that these species wereprobably transported from the marshes by ebb tides or sur-face run-off. In the Canche estuary, this phenomenon mayalso occur at stations 1 to 4, but agglutinated tests may bepoorly preserved on the mud flat and have therefore a lowerchance to be sampled. Wetmore [1987] maintains that acomplex relationship exists between crushing strength andmorphology and showed that the test architecture is moreimportant than the thickness of the test wall. Test strength isgreater in foraminifera associated with coarse sediment thanwith fine sediment. The chamber structure of Quinquelocu-lina seminula and Miliolinella subrotunda allow these spe-cies to live in a highly dynamic environment such as station1, whereas the three agglutinated trochospiral species J. ma-crescens, T. inflata and Arenoparrella mexicana may liveonly in the calm salt marsh environment. In these species,the grains are glued with organic cement and are not strongenough to resist the highly dynamic mud flat environment[Le Calvez, 1953] and may allow these species to live insalt marsh only. The transition between the low and the highsalt marsh is marked by the replacement of T. inflata by J.macrescens as main dominant species in the assemblages aswell as already observed [Jennings and Nelson, 1992; Ruizet al., 2005].
The increase in species richness and density of forami-nifera may be linked to the increase in organic matter fromthe mud flat towards the salt marsh. The effects of OM onforaminifera are known to be diverse. Organic waste isknown to favor an increase in diversity and density of fora-miniferal populations [e.g., Cearreta, 1988; Debenay et al.,2001; Nagy and Alve, 1987]. On the contrary, other authorshave observed density and species richness to decrease withincreasing levels of OM [e.g., Schafer et al., 1991; Schaferet al., 1995]. A lack of effect of OM contents on foraminife-ral assemblages was observed by Collins et al. [1995], andSetty and Nigam [1982]. OM may be favorable to foramini-fera until it becomes toxic when its proportion in the sedi-ment increases [Alve, 1991]. Hence, OM may not be alimiting factor for the development of agglutinated specieswithin the salt marsh and they may live within a range ofrestricting environmental parameters. Indeed, according toniche theory, distribution patterns of benthic foraminiferaare controlled by critical environmental thresholds [Murray,2001]. Jadammina macrescens is particularly well influen-ced by an increase of OM whatever its tidal elevation
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FIG. 5. – Foraminifera and sediment grouped after their mineralogical composi-tion by means of cluster analysis.FIG. 5. – Regroupement des foraminifères et des sédiments en fonction de leurcomposition minéralogique, au moyen d’une analyse hiérarchique.
position. The amount of organic matter needs to be taken inaccount amongst all parameters often discussed and analy-zed in tidal marshes. For example, Gehrels and van dePlassche [1999] observed that Jadammina macrescens is awidely present indicator of these extreme estuaries condi-tions, and showed significant correlations with tidal eleva-tion. Nevertheless, the distribution of Jadamminamacrescens and Trochammina inflata may also be governedby other parameters. For example, Patterson [1990] sugges-ted that T. inflata increased in abundance where salinity isclose to normal marine levels and Pujos [1984] suggested
that J. macrescens might be more tolerant to low pH,low O2 conditions, and higher subaerial exposure. All envi-ronmental parameters should act together to explain the fo-raminifera distribution.
The sediment characteristics (grain size and mineralogi-cal composition) are similar at stations 11 and 110, but theforaminiferal assemblages are different. As station 11 is lo-cated within the salt marsh and station 110 is located in adrainage channel not covered by vegetation, the differencemay be due to specific ecological settings, such as tidal expo-sure. These settings may be more important than sedimentcharacteristics. This explains that Arenoparrella mexicanawas found in stations 4, 5, 14 and 110, which have similarecological conditions and not in station 11 with similar sedi-ment but different ecological settings. Moreover, at stations5, 6, 13 and 110, the proportion of Trochammina inflata andJadammina macrescens was similar. Their different minera-logical composition differs from species to species indepen-dent from the sediment condition. Trochammina inflata isnoticeably enriched in quartz + albite and J. macrescens inpotash feldspar. Thus, the composition of the tests is inde-pendent of the sediment characteristics and the cell is able tosort sediment grain by size and by mineralogical nature toconstruct their tests, as presented by Heron-Allen [1915].Moreover, as well as other agglutinated species, Trochammi-na inflata, Jadammina macrescens and Arenoparrella mexi-cana might temporally migrate deeper below the sedimentsurface in order to find sediment grains that may be scarce inthe surface sediment. Saffert and Thomas [1998] observedthese three species living 40 cm deep in the sediment even ifbioturbation is suggested as a reason for such a deep distribu-tion. However, Patterson et al. [1999] explained that J. ma-crescens may have an infaunal habitat and may be found upto 10 cm deep in the sediment. In the same way, other speciesof agglutinated foraminifera have been shown to live infau-nally [Kaminski et al., 1988]
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FIG. 6. – Ca-map on a Jadammina macrescens. Chambers are delimited bydotted lines. Two groups of chambers, rich and poor in Ca, are delimitedby continuous lines. Chambers comprising lightened particles are rich inCa (~ carbonates). Other chambers, appearing in darker gray, were compo-sed of mainly potash feldspar and quartz + albite minerals.FIG. 6. – Cartographie du Ca sur Jadammina macrescens. Les loges sontdélimitées par des lignes pointillées. Deux groupes de loges, riches et pau-vres en Ca sont délimités par des lignes continues. Les loges comportantdes minéraux clairs sont riches en Ca (~ carbonates). Les autres loges, ap-paraissant en gris foncé, sont composées principalement de feldspath-K etde quartz + albite.
FIG. 7. – Minerals size and standard error measured both on individual of Jadammina macrescens (continuous line) and on sediment (dotted line) in eachof the 15 stations.FIG. 7. – Taille des minéraux et écart-types mesurés conjointement sur des individus de Jadammina macrescens (lignes continues) et sur les sédiments (li-gnes pointillées) dans chacune des 15 stations.
The peculiar composition of Jadammina macrescens withtwo specific mineralogical compositions could be explainedby the change of the sediment composition during the diffe-rent construction phases of the chambers. Jadammina macres-cens stays however most of the time enriched in potashfeldspar compared to the surrounding sediment. The tests ofParatrochammina haynesi, Miliammina fusca and Remaneicaplicata are similar in mineralogical composition to the sedi-ment with some enrichment and impoverishment. Howeverthese species lived at completely different stations. Again,ecological settings predominate sediment characteristics.When ecological conditions are assembled to allow foramini-fera to live and reproduce, they are able to choose essentiallycarbonate minerals to construct their tests.
CONCLUSION
Agglutinated foraminifera were confined into the salt marsh(mainly Trochammina inflata, Arenoparrella mexicana andJadammina macrescens). The sediment grain size influen-ces foraminiferal species richness whereas foraminiferaldensity is not related to sediment grain size. Environmental
parameters such as OM content and not only tidal elevationmay influence tidal marsh foraminiferal distribution. Themineralogical composition of the grains tests depends onthe species. The compositions of Trochammina inflata andArenoparrella mexicana are very different from that of thesediment; Jadammina macrescens is generally differentfrom the sediment with some exceptions; and Miliamminafusca, Paratrochammina haynesi and Remaneica plicata aresimilar in mineralogical composition to that of the sedi-ment. The studied species may be able to choose their pre-ferred mineral grain even if this is scarce in the sediment.Mineralogical composition does not influence the speciesdensity and species richness of the observed agglutinatedspecies in the Canche Estuary.
Acknowledgments. – The authors are much indebted to François Guillot forhis help on the minerals analysis and Laurence Debeauvais, Sylvie Régnierand Deny Malengros for their technical assistance. The authors are gratefulto Evelyne Goubert and Michael Kaminiski for the thorough revision andTaniel Danelian and Björn Krüger for their help in improving the manus-cript. This study was part of a BQR (Bonus Qualité Recherche) project fi-nanced by the Université de Lille 1. Funding for sampling campaign wasprovided by CPER (Contrat de plan Etat Région) and Fédération de Re-cherche CNRS 1818 “Milieux naturels et anthropisés”.
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