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Quaternary International 301 (2013) 135e149
Contents lists available
Quaternary International
journal homepage: www.elsevier .com/locate/quaint
Paleoecological significance of Late Quaternary molluscan faunas
ofthe Bahia San Blas area, Argentina
M.P. Charó a,*, S. Gordillo b, E.E. Fucks c
a Facultad de Ciencias Naturales y Museo, Universidad Nacional
de La Plata, CONICET, 64 No.3, ArgentinabCentro de investigaciones
en Ciencias de la Tierra (CICTERRA, CONICET-UNC), Córdoba,
Argentinac Facultad de Ciencias Naturales y Museo y Ciencias
Agrarias y Forestales-LATYR-UNLP, Argentina
a r t i c l e i n f o
Article history:Available online 25 December 2012
* Corresponding author.E-mail addresses:
[email protected] (M.
conicet.gov.ar (S. Gordillo), [email protected]
1040-6182/$ e see front matter � 2012 Elsevier Ltd
ahttp://dx.doi.org/10.1016/j.quaint.2012.12.019
a b s t r a c t
Late Quaternary marine deposits in the area of Bahía San Blas
(Buenos Aires, Argentina) form beachridges and tidal plains bearing
remains of organisms, mainly mollusc shells. These mollusc
assemblageswere characterized, and their changes assessed both over
time, and related to local environmentalfactors. Twenty-six sites
were analyzed, eleven Pleistocene, seven Holocene and eight modern
sites. Fiftytaxa (27 bivalves and 23 gastropods) were recorded for
the area, with marine species predominant overestuarine. Within
gastropods, epifaunal species and carnivores are predominant, with
a lower proportionof herbivores and filter feeders, while bivalves
are mostly infaunal and suspension feeders. Pleistoceneassociations
are characterized by the high quantity of molluscs in rocky
environments, and those of theHolocene by the presence of the warm
water Crassostrea rhizophorae, which is currently displaced tolower
latitudes. Within the modern associations four taxa, Mesodesma
mactroides, Solen tehuelchus,Barnea lamellosa and Crassostrea gigas
(an introduced species), are not recorded as fossils, but
Mesodesmamactroides is found in Holocene shell middens from the
area. According to multivariate analysis, the siteswere divided
into two groups: high energy deposits (storm ridges) and low-energy
deposits (tidalplains), regardless of their age. Based on these
results, mollusc variations in the area are thought to berelated in
part to changes in temperature that took place since the
Pleistocene (e.g. C. rhizophorae); butmostly associated with the
presence of sub-environments of different energy levels. This would
favor thepresence of different taxa in accordance to their
ecological requirements.
� 2012 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction
TheQuaternary is characterizedworldwide by important
climateoscillations, with extremes represented by glacial and
interglacialperiods resulting from temperature variations that
caused markedchanges in sea level (e.g. Rohling et al., 2003). In
coastal areas,transgressive events have been registered by a
sequence of erosionforms (coastal terraces and paleocliffs) and
beach deposits that, fordifferent reasons, have been protected from
degradation processesand are therefore an important testimony of
climate changes thathave occurred in most recent geologic times
(Shackleton, 1987).
In the Patagonian coastal region, these deposits can be
foundboth along the coastline, and well inland. This gives evidence
of themagnitude of the transgressive events, and of the
geomorphologiccharacteristics and isostatic variations. The most
extreme sea levels
P. Charó), sgordillo@cicterra-(E.E. Fucks).
nd INQUA. All rights reserved.
are clearly seen at various sites along the coast, although due
to thecontinental rise (Fucks et al., 2009; Pedoja et al., 2011)
lower levelscan also be seen above the current sea level in several
sites. Ona larger scale, Pleistocene coastal fossil deposits are
knownworldwide, from the North American Pacific coast (e.g. Roy et
al.,1998; Parham et al., 2007), to Mexico (e.g. Ortlieb, 1991;
Muhset al., 1992, 2002; De Diego-Forbis et al., 2004), Bermuda
(Hearty,2002), Japan (e.g. Kim et al., 1999; Kitamura et al.,
2000),Australia (Murray-Wallace and Belperio, 1991;
Murray-Wallaceet al., 2000), and Europe (e.g. Lario et al., 1993;
Zazo, 1999; Zazoet al., 2003a; Dumas et al., 2005). Along the South
Americancoast, deposits of different sea levels related to
transgressive eventscan be seen in Brazil (e.g. Caruso et al.,
2000; Barreto et al., 2002),Uruguay (e.g. Martinez et al., 2001;
Goso Aguilar, 2006), Chile(Quezada et al., 2007), and Argentina
(e.g. Cionchi, 1987;Codignotto et al., 1988; Codignotto and
Aguirre, 1993; Schellmann,1998; Isla et al., 2000; Rostami et al.,
2000; Schellmann and Radtke,2000, 2003; Weiler, 2000; Bujalesky and
Isla, 2006; Isla andBujalesky, 2008; Pedoja et al., 2011).
mailto:[email protected]:[email protected]:[email protected]:[email protected]://crossmark.dyndns.org/dialog/?doi=10.1016/j.quaint.2012.12.019&domain=pdfwww.sciencedirect.com/science/journal/10406182http://www.elsevier.com/locate/quainthttp://dx.doi.org/10.1016/j.quaint.2012.12.019http://dx.doi.org/10.1016/j.quaint.2012.12.019http://dx.doi.org/10.1016/j.quaint.2012.12.019
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M.P. Charó et al. / Quaternary International 301 (2013)
135e149136
The most representative fauna of these Quaternary marinedeposits
are the molluscs, in particular gastropods and bivalves,and their
preservation makes them very useful as proxy paleo-climatic and
paleoenvironmental records. Analysis of these taxaprovides
information on different environmental parameters suchas
temperature, salinity and substrate, thus leading to an
inter-pretation of the paleoenvironment and the paleo-communities
thatexisted in the different regions throughout time. Recent
studieshave also used this information as paleoenvironmental
parametersand for paleoclimatic studies such as atmospheric and
oceano-graphic changes (e.g. Ortlieb et al., 1994; Martinez et al.,
1997, 2001;Massch et al., 2001; Ragainia et al., 2002; Aguirre et
al., 2006;Cárdenas and Gordillo, 2009; Rabassa et al., 2009; Jones
et al., 2010;Rojas and Urteaga, 2011).
During the last interglacial the MIS 5e (MIS ¼ Marine
IsotopeStage), which occurred between 130 � 2 and 119 � 2 ka
(Heartyet al., 2007), sea level rose at least twice (e.g. Zazo et
al., 2003a,b;Tuccimei et al., 2006). In this period, surface sea
temperatures (SST)were approximately 2 C� above the present
temperature (McCulloch and Esat, 2000; Murray-Wallace et al., 2000;
Rohlinget al., 2008), and sea level reached approximately five to
sixmeters above the present sea level (Shackleton, 1987; Neumannand
Hearty, 1996). Records worldwide show evidence of warmwater in
benthic marine molluscs, as well as changes in theirgeographical
distribution (Chaar and Farinati, 1988; Cuerda et al.,1991;
Martinez et al., 2001; Muhs et al., 2002; Zazo et al., 2003b,Zazo
et al., 2010; Rojas and Urteaga, 2011, among others).
A warm event was detected in the Mid-Holocene (8700e7800BP) of
the Northern Hemisphere, known as the Climatic Optimumor
Hypsithermal, in which the SST and humidity were higher thantoday
(e.g. Funder and Weidick, 1991; Salvigsen et al., 1991; Hjortet
al., 1995; Rohling and De Rijk, 1999; Yuan et al., 2011).
Someevents have also been recorded in the Southern Hemisphere
(e.g.Aguirre, 1990, 1993b, 2002; Cohen et al., 1992; Lutaenko,
1993;Gordillo et al., 2005). The main purpose of this study is to
charac-terize molluscan associations of Bahía San Blas during this
period,and to assess their changes over time and related to local
envi-ronmental factors.
1.1. Study area
The study area is located in the south of the Buenos
AiresProvince, extending from Isla Jabalí (40�360S; 62�110W) to
FaroSegunda Barranca (40�460S; 62�160W; Lighthouse SegundaBarranca)
and from the coastline to National Route 3 (Fig. 1). Themost
characteristic geomorphologic features are beach ridges andlow,
gravel plains. The beach ridges may be grouped according
todifferent transgressive episodes. The lower ones (3e5 m a.s.l.)
aregrey gravels and sandswith low-angle cross bedding, fourmeters
ofmaximum thickness. They are small active cliffs and
topographicalledges among the different groupings that can even be
consideredas beach strand plains due to their quantity and
disposition (Fuckset al., 2012). Holocene deposits can be seen in
large number at IslaJabalí, where they can be geomorphologically
divided into two welldefined groups. To the east, they are flat and
composed of brown-grey silty sand; the lower section is finely
stratified, while theupper is more homogenous and browner. The
western area hasa significant amount of ridges very close together
and are eitherparallel, transverse or at an oblique angle to each
other (Fucks et al.,2011, 2012).
Inland, behind the Holocene deposits, there is a second level
ofbeach ridges (8e10 m), less pronounced and either parallel to
thecoast or at an oblique angle. These ridges are made up of
friablegravel, with nodules, and correspond to the Late Pleistocene
trans-gressive event (MIS 5e). They extend north to south and are
cut off in
the north by the Jabalí Streamand in the south by theWalker
Stream(Fucks et al., 2012). These ridges are parallel to the coast
to FaroSegunda Barranca,where a gravel and sand beach is observed
on thecoastal platform of the Río Negro Formation. The cliffs are
10 mmaximum height, and consist of sand and silt at the base, and
well-stratified gravel in the middle and upper sections (Fig.
2).
Above these, up to near 35m a.s.l., gentle plains with outcrops
oflong, circular elevations of gravel highly cemented with
calciumcarbonate have been observed. In all of these
morphologies,mollusc remains and/or flattened clasts indicate their
littoral origin(Fucks et al., 2012).
The development of numerous deflation basins occupied by
saltflats (Barrancoso, Grande, La Salinita, Salina del Inglés,
amongothers) is one of the most common features of the continental
area.The highest concentration of salts is normally found in the
lowestareas, with sodium chloride predominating, and clay and
alkalinesulphates in lower quantities (Trebino,1987; Del Blanco et
al., 2005).
1.2. Geological background
Witte (1916) was the first to study the area and to describe
thegeology of the San Blas coastal deposits. He defined five
evolutionstages (IeV) represented in different sections of the
coast, andnoted that the sediments forming the deposits
corresponded toStages III (Pleistocene), IV and V (Holocene).
Ambrosini (1984) and Trebino (1987) analyzed the geomor-phology
of Bahía San Blas area, and described Pleistocene, Holoceneand
modern geofeatures. Trebino (1987) recognized three levels ofmarine
terraces, I and II from the Pleistocene and III for the Holo-cene.
Terraces I and II are located in Rincón deWalker, and III in
IslaJabalí, and are a result of successive accumulation of beach
strandsand spits (Etcheverría et al., 2006). The Holocene deposits,
locatedat an altitude of 3 m a.s.l., were previously dated by
Trebino (1987)at 2170 � 110, 3450 � 110, 3650 � 80 (LP ¼ 2434),
4100 � 95,5370 � 110, 2320 � 80, and 3450 � 80 14C BP.
Pleistocene beach ridges, located to the west of the
JabalíStream, were dated at 28,400 � 800 and 29,120 � 970 14C
BP(Trebino, 1987) and 30,780 � 1650 BP Weiler (2000). These
areconsidered to be the minimum ages, correlating with the
LatestInterglacial event (MIS 5e).
Schnack et al. (2005) considered thatWitte’s (1916) I to III
stageswereequivalent toTrebino’s (1987) level I and II Pleistocene
terraces,whereas the Holocene deposits corresponding to Witte’s
(1916)stage IV would be equivalent to Trebino’s (1987) level III
terrace.
The most recent geochronological research carried out in theFaro
Segunda Barranca area used ESR and amino acid racemizationdating
techniques. The base of the cliff (site 24) was dated at 102and 108
ka, while the upper part of the cliff (site 23) oscillatedbetween
94, 79, and 72 ka. Although all the dating was carried outusing the
ESR method, the D/L ratio of aspartic acid and leucinecould link
this site to an older transgressive event (Rutter et al.,1989,
1990). These last sequences belong to the MIS 5e strati-graphic
unit, which is equivalent to Feruglio’s terrace V (1959) andto the
Belgranense (Ameghino, 1889) or Puente Pascua Formation(Fucks et
al., 2005).
Fucks et al. (2012) reinterpreted the stratigraphic sequences
ofthis area, assigning a minimum of four transgressive cycles.
Beachridges, as well as beach strand plains and tidal plains
withmaximum altitudes of 6 m a.s.l. and very clear morphologies
arepresent from the coast to the present day continent,
particularly inIsla Jabalí. Above them, at altitudes of 8e10 m
a.s.l., clear ridgescould be probably related to MIS 5e, although
less than theprevious ridges, and that increase gradually to
altitudes of over30 m a.s.l. These could have been originated in
two �9transgressive events (Table 1).
-
Fig. 1. Map showing the location of the Bahia San Blas in the
south of the Buenos Aires province, and the Pleistocene, Holocene
and modern sampling.
Table 1Geomorphological characterization of the studied sites
located in the San Blas area.
Marinedeposits
Witte (1916) Trebino (1987) erosivefeatures: Terraces
Accumulation features Schnack et al. (2005) Fucks et al.
(2012)
Bahia SanBlas
Holocene Stage V: Modern beachmarine deposits.Stage IV: Marine
deposits1.50 m higher than currentheight.
Level III: Poorly represented;can be seen in a
discontinuousalong Walker Stream.Corresponds to more modernterraces
with altitudes of 3 ma.s.l.
Section B: All marineaccumulation cut off bymarine erosion
orinterdigitation.
Witte’s (1916) StageIV equivalent toTrebino’s terracelevel
III.
Represented by beachridges, beach strandplains and tidal
plainswith maximum altitudesof 5 m a.s.l.
Pleistocene Stage III: Marine coastalbanks with a sea
levelapprox. 10 m higher thanin modern times.Old coastlines are
clearlyvisible.
Level II: Rows of beach strandscorresponding to old coastlinesat
10 and 11 m a.s.l.Level I: The oldest: located onold boulder
deposits between12 and 14 m a.s.l.
Section A: Located westof Jabali Stream and tothe south of the
GuanacoStream. Long, wide beachridges located between 9and 10 m
a.s.l.
Witte’s (1916) StagesIeIII e equivalent toTrebino’s (1987)
terracelevels I and II.
Beach ridges at 8e10 ma.s.l.
M.P. Charó et al. / Quaternary International 301 (2013) 135e149
137
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M.P. Charó et al. / Quaternary International 301 (2013)
135e149138
1.3. Paleontological background
The first studies on marine molluscs in the Buenos Aires
Prov-ince were mainly of a geological character. Frenguelli (1928,
1950)contributed greatly to the understanding of Quaternary
marinefauna. Pioneer studies from around this period were also
carriedout by D’Orbigny (1834e1847, 1842e1844), Darwin
(1846),Ameghino (1898, 1908), von Ihering (1907) and Wahnish
(1939),among others. The first mention of systematic mollusc
studiesappeared in contemporary research (Carcelles, 1944;
Camacho,1966; Castellanos, 1967, 1979, 1981). Among the most
recentstudies on Quaternary molluscan assemblages in the
littoraldeposits of the Buenos Aires Province, contributions from
Farinatiand Camacho (1980), Farinati (1985, 1994), Chaar et al.
(1992),Aguirre (1990, 1993aec, 2002), Aguirre and Whatley
(1995),Aguirre and Farinati (2000), Aguirre and Fucks (2004), and
Fuckset al. (2005), among others, are noteworthy. Some recent
studiesof Quaternary deposits of Bahía San Blas and Faro Segunda
Barrancamention molluscs, but they are mainly geomorphological
andgeochronological research (Trebino, 1987; Rutter et al., 1989,
1990;Fucks et al., 2012).
2. Materials and methods
Twenty-six sites were analyzed in the study area: eleven
Pleis-tocene, seven Holocene, and eight modern (Table 2). The
samplescollected from the Pleistocene and Holocene deposits (1 dm3)
wereexposed to running water using sieves of three different
size:2.80 mm, 1.40 mm and 0.080 mm. A sequence of washing anddrying
on paper was then followed. The modern samples weretaken in a 1 m �
1 m quadrant on transects perpendicular to thecoastline. The
molluscs were collected by hand and were thensorted using a 2.80 mm
sieve.
Table 2Description of sampled localities.
Bahíasan Blas
Age Sites Coordinates (Lat-Long)
Modern 1 40�31040.8000S; 62�1905.4000O2 40�3209.3200S;
62�18047.1600O3 40�32020.1200S; 62�15022.0800O4 40�33015.6800S;
62�13028.3800O5 40�34011.7000S; 62�11049.8600O6 40�35035.9200S;
62�10015.7800O7 40�46030.8000S; 62�16018.5000O8 40�46032.9100S;
62�16019.9500O
Holocene 9 40�3209.4300S; 62�17022.5500O10 40�34014.3300S;
62�15019.4500O11 40�3409.3000S; 62�13057.8000O
12 40�34023.5900S; 62�14011.7100O13 40�34056.2000S;
62�14011.4300O14 40�34051.9100S; 62�1307.1400O
15 40�3709.0000S; 62�12049.2000OPleistocene 16 40�33058.5000S;
62�1907.3000O
17 40�3406.7000S; 62�18052.1200O18 40�3509.9000S;
62�17028.8000O19 40�35019.4000S; 62�1705.8000O20 40�35047.3500S;
62�19011.9300O21 40�3606.1700S; 62�16010.5500O22 40�40049.9900S;
62�3006.6000O23 40�46024.0200S; 62�16017.2400O
24 40�46034.5600S; 62�16024.5000O25 40�27032.7700S;
62�47057.6600O
26 40�35030.3600S; 62�49052.9100O
Each piece of biogenic content that was captured by the
sieveswas identified and labelled; first, the material was sorted,
usinga magnifying glass when necessary, and then it was divided
intotaxa by comparing it with catalogs and current specific
systematicstudies.
For each sampled site, the Shannon Index (H0) was
calculatedthrough an XeY graph and the ecological parameters of
gastropodsand bivalves were compared for each period. In order to
analyze thedegree of similarity between sites, cluster analyses
were carried outwith R software, version 2.15.0 (vegan package)
(Oksanen, 2011),and the UPGMA method was used to group the faunal
associationsaccording to the BrayeCurtis Index. In this case, a
matrix was usedfor each site to show the abundance of different
species. ACorrespondence Analysis (CA) was then carried out to
observethe relationship between sites. The ecological information
on thetaxa is based on living specimens and also on
bibliographicalsources (Lasta et al., 1998; Bastida et al., 2007;
Balech and Ehrlich,2008; Rosenberg, 2009).
3. Results
3.1. Taxonomic and paleoecological composition
Twenty-six sites were analyzed in Bahía San Blas,
includingPleistocene, Holocene and modern deposits, in which 50
taxa(N ¼ 388 shells) were recorded (27 bivalves and 23
gastropods)(Figs. 3 and 4) (Tables 3 and 4). According to Shannon’s
diversityindex (H0), the modern sites have greater diversity, with
valuesranging between 2.3 and 1.5, compared to Holocene sites
whichshow lower values, between 0.22 and 0.86, with the exception
ofsites 11 (H0 ¼ 2.28) and 14 (H0 ¼ 2.28) that belong to beach
ridges.Pleistocene sites show values between 0.29 and 1.98, with
thehighest values at sites 20 (H0 ¼ 2.09) and 22 (H0 ¼ 2.52) (Fig.
5).
Geomorphology Altitude (m.a.m.s.l.)
Beach 0Beach 0Beach 0Beach 0Beach 0Beach 0Beach 0Beach 0Tidal
plain deposits 1Tidal plain deposits 1Beach ridge made of sandy
sedimentsand containing clasts
4
Tidal plain deposits 3Tidal plain deposits 5Beach ridge made of
sandy sedimentsand containing clasts
4
Tidal plain deposits 5Beach ridge made of sandy sediments 9Beach
ridge made of sandy sediments 7Beach ridge made of sandy sediments
7Beach ridge made of sandy sediments 4Beach ridge made of sandy
sediments 5Beach ridge made of sandy sediments 5Profile 50 cm from
the surface 2Beach ridge with layers of sandy sedimentsand
clasts
8
Fine sediment facies at the base of the profile 10Beach ridge
with sandy sediments containingclasts and rough gravel
32
Quarry with clustered gravel 33
-
Table 3Bivalves from the Quaternary (P ¼ Pleistocene, H ¼
Holocene and M ¼ modern)marine deposits in the Bahía San Blas
area.
Bivalvia P H M
Nucula (N.) nucleus (Linné, 1758) X XEnnucula grayi (d’Orbigny,
1846) XGlycymeris (G.) longior (Sowerby, 1832) XMytilus (M.) edulis
(Linné, 1758) X X XBrachidontes (B.) rodriguezi (d’Orbigny, 1846) X
X XAequipecten tehuelchus (d’Orbigny, 1842) X X XPlicatula gibbosa
(Lamarck, 1801) XOstreola equestris (Say, 1834) X X XCrassostrea
rhizophorae (Guilding, 1828) XOstrea puelchana (d’Orbigny, 1841) X
X XCrassostrea gigas (Thunberg, 1793) XDiplodonta (D.) patagonica
(d’Orbigny, 1842) X XDiplodonta (F.) vilardeboana (d’Orbigny, 1846)
XCarditamera plata (Inhering, 1907) X XTrachycardium muricatum
(Linné, 1758) XMactra isabelleana (d’Orbigny, 1846) XMactra guidoi
(Signorelli and Scarabino) X XMulinia edulis (King and Broderip,
1831) XMesodesma mactroides (Reeve, 1854) XSolen tehuelchus
(Hanley, 1842) XMacoma (P.) uruguayensis (Smith, 1885) X XTagelus
(T.) plebeius (Ligthfood, 1786) XPitar (P.) rostratus (Philippi,
1844) X X XAmiantis purpurata (Lamarck, 1856) X X XCorbula (C.)
patagonica (d’Orbigny, 1846) X X XCorbula (C.) lyoni (Pilsbry,
1897) XBarnea lamellosa (d’Orbigny, 1846) X
Table 4Gastropods from the Quaternary (P ¼ Pleistocene, H ¼
Holocene and M ¼ modern)marine deposits of the San Blas bay
area.
Gastropods P H M
Diodora (D.) patagonica (d’Orbigny, 1841) XLucapinella henseli
(Martens, 1900) XTegula (A.) patagonica (d’Orbigny, 1835) X X
XCalliostoma coppingeri (Smith, 1880) XHeleobia australis
(D’Orbigny, 1835) X X XBostrycapulus odites (Collin, 2005) X X
XCrepidula argentina (Simone Pastorino and
Penchaszadeh, 2000)X X
Notocochlis isabelleana (d’Orbigny, 1840) XTrophon varians
(d’Orbigny, 1841) X X XUrosalpinx cala (Pilsbry, 1897) XZidona
dufresni (Donovan, 1823) X X XAdelomelon (P.) brasiliana (Lamarck,
1811) XOdontocymbiola magallanica (Gmelin, 1791) XOlivella (O.)
tehuelcha (Dúclos, 1835) X XOlivancillaria urceus (Röding, 1798)
XOlivancillaria carcellesi (Klappenbach, 1965) X XMarginella
martini (Petit, 1853) X XBuccinanops moniliferum (Kiener, 1834) X
XBuccinanops cochlidium (Dilwyn, 1817) X XBuccinanops globulosum
(Kiener, 1834) X X XBuccinanops uruguayense (Pilsbry, 1897) X
XParvanachis isabellei (d’Orbigny, 1839) X
Table
5Ec
olog
ical
requ
irem
ents
anddistribution
ofbiva
lves:Ep
¼ep
ifau
nal,I¼
infaunal,Ce¼
Cem
ented;
H¼
hard,
S¼
soft;C¼
carn
ivorou
s,D
¼detritivo
rous,
H¼
herbivo
re,Sf
¼su
spen
sion
feed
er;O
¼oligoh
aline(3e8&
),M
¼mesoh
aline(8e18
&),P¼
polyh
aline(18e
30&),E¼
euhaline(>
30e35
&).
Bivalvia
Salin
ity
Life
hab
itDep
th(m
)Su
bstrate
Trop
hic
type
Distribution
area
Nuc
ula(N
.)nu
cleu
s(Linné,
1758
)E
I0e
200
SD
23� Se53
.5� S
Ennu
cula
gray
i(d’Orbigny,
1846
)E
I5e
1850
SD
22.93�Se
55.5
� SGlycymeris(G
.)long
ior(Sow
erby
,183
2)E
I10
e75
SSf
10� Se42
� SMytilu
s(M
.)ed
ulis(Linné,
1758
)Pe
EEp
0e50
HSf
68� N
e55
.5� S
Brachido
ntes
(B.)rodrigue
zi(d’Orbigny,
1846
)E
Ep0e
25H
Sf34
� Se42
� SAeq
uipe
cten
tehu
elch
us(d’Orbigny,
1842
)E
Ep10
e12
0M
Sf21
� Se53
� SPlicatulagibb
osa(Lam
arck
,180
1)E
Ce
0e12
0H
Sf35
.3� N
e34
� Sa
Ostreolaeq
uestris(Say
,183
4)Pe
ECe
0e80
HC
37� N
e42
� SCrassostrearhizop
horae(G
uild
ing,
1828
)Pe
ECe
0e50
HC
21.4
� Ne35
� SOstreapu
elch
ana(d’Orbigny18
41)
PeE
Ce
0e70
HC
22� Se42
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rg,1
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rditam
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1907
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cardium
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I0e
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� SMactraisab
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next
page)
M.P. Charó et al. / Quaternary International 301 (2013) 135e149
139
The faunal composition of bivalves and gastropods shows
thatmarine species predominate over estuarine species (e.g.
Heleobiaaustralis). They mainly are species with active locomotion
asepifaunal gastropods and infaunal bivalves. Bivalves from
Pleisto-cene deposits are mainly rocky sediment and filter
feeders,whereas for the Holocene and modern, sandy sediment
andsuspension feeders are predominant. Among gastropods,
theepifaunal type, common in sandy sediments, is predominant,
withfilter feeders mainly present in Pleistocene deposits and
carnivoresin Holocene-modern ones (Fig. 6) (Tables 5 and 6) (Fig.
7).
-
Table 5 (continued )
Bivalvia Salinity Life habit Depth (m) Substrate Trophic type
Distribution area
Pitar (P.) rostratus (Philippi, 1844) E I 10e100 S Sf
22�Se38.7�Sa
Amiantis purpurata (Lamarck, 1856) E I 0e20 S Sf
19�Se43�SCorbula (C.) patagonica (d’Orbigny, 1846) E I 15e90 S Sf
23�Se43�SCorbula (C.) lyoni (Pilsbry, 1897) E I 11e67 S Sf
19�Se43�SBarnea lamellosa (d’Orbigny, 1846) E I 15e150 H Sf
34�Se43�S
a Taxa found in the studied area.
Table 6Ecological requirements and distribution of gastropods:
Ep ¼ epifaunal, I ¼ infaunal; H ¼ hard, S ¼ soft; C ¼ carnivorous,
D ¼ detritivorous, He ¼ herbivore, Sf ¼ suspension feeder; O ¼
oligohaline (3e8&), M ¼mesohaline (8e18&); P ¼ polihaline
(18e30&); E ¼ euhaline (>30e35&).
Gastropods Salinity Lifehabit
Depth Substrate Trophictype
Distribution area*
Diodora (D.) patagonica (d’Orbigny, 1841) E Ep 0e15 H He
11�Ne45�SLucapinella henseli (Martens, 1900) E Ep 0e55 H He
23�Se53�STegula (A.) patagonica (d’Orbigny, 1835) E Ep 0e57 H He
23�Se54�SCalliostoma coppingeri (E.A. Smith, 1880) E Ep 13e86 S He
30�Se44.21�SHeleobia australis (d’Orbigny, 1835) O, P, M Ep 0e60 M
He 24�Se41�SBostrycapulus odites (Collin, 2005) E Ep 0e46 H Sf
25�Se45.8�SCrepidula argentina Simone (Pastorino and Penchaszadeh,
2000) E Ep 30e50 H Sf 38�Se41.03�SNotocochlis isabelleana
(d’Orbigny, 1840) E I 0e113 S C 22.4�Se42.58�STrophon varians
(d’Orbigny, 1841) E Ep 0e50 H C 32�Se40�SUrosalpinx cala (Pilsbry,
1897) E Ep 28e28 H C 32�Se41�SZidona dufresni (Donovan, 1823) E Ep
10e90 S C 23�Se42�SAdelomelon (P.) brasiliana (Lamarck, 1811) E Ep
8e70 S C 23�Se52�SOdontocymbiola magallanica (Gmelin, 1791) E Ep
10e200 M C 35�Se55.2�SOlivella (O.) tehuelcha (Dúclos, 1835) E Ep
15e57 S C 23.69�Se43�SOlivancillaria urceus (Röding, 1798) E Ep
5e50 S C 19�Se42�SOlivancillaria carcellesi Klappenbach, 1965 E Ep
0e22 S C 23�Se42.5�SMarginella martini (Petit, 1853) E Ep 10e80 S C
22.93�Se42�SBuccinanops moniliferum (Kiener, 1834) E Ep 0e50 S C
35�Ne42�SBuccinanops cochlidium (Dilwyn, 1817) E Ep 5e66 S C
23�Se42.58�SBuccinanops globulosum (Kiener, 1834) E Ep 0e6 S C
35�Se46�SBuccinanops uruguayense (Pilsbry, 1897) Ep 15e45 S C
24�Se42�SParvanachis isabellei (d’Orbigny, 1839) E Ep 10e65 C C
30�Se54�SSiphonaria lessoni (Blainville, 1824) E Ep 0 H He
32�Se55.22�S
* Range of distribution with the upper limit in higher
latitudes.
M.P.Charó
etal./
Quaternary
International301
(2013)135
e149
140
-
Fig. 2. Distribution of the Pleistocene and Holocene deposits in
the study.
Fig. 3. Bivalvia taxa from Quaternary marine deposits in Bahia
San Blas. a. External view, b. in(L ¼ 9.62) (MLP:33.410, Holocene);
2, Ennucula grayi (d’Orbigny) (L ¼ 8.86) (MLP:33.411, Horodriguezi
(d’Orbigny) (L ¼ 9.68) (MLP:33.994, Holocene); 5, Aequipecten
tehuelchus (d’Orbi(MLP: 33.412, Pleistocene); 7, Crassostrea
rhizophorae (Guilding) (L ¼ 21.80) (MLP: 33.998,Mesodesma
mactroides Reeve (L ¼ 43.54) (MLP: 33.403, Holocene); 10, Ostreola
equestris(MLP:33.403); 12, Diplodonta patagonica (d’Orbigny) (L ¼
14.99) (MLP: 33.402, Holocene); 1Lamarch (L ¼ 22.42) (MLP:33.415,
Pleistocene); 15, Corbula patagonica d’Orbigny (L ¼ 1(MLP:33.400,
Holocene); 17, Mactra guidoi Signorelli and Scarabino (L ¼ 23.05)
(MLP: 33.406Mulinia edulis (King and Broderip) (L ¼ 24.61)
(MLP:33.408, Holocene); 20, Corbula Iyoni P(MLP: 33.995,
Pleistocene); 22, Macoma (P.) uruguayensis (Smith) (L ¼ 27,50)
(MLP:33.404Trachycardium muricatum (Linné) (L ¼ 31.69) (MLP:
33.417, Pleistocene); 25, Crassostrea g(L ¼ 41.25) (MLP: 33.414,
Pleistocene); 27, Amiantis purpurata d’Orbigny (L ¼ 46.98)
(MLP:
M.P. Charó et al. / Quaternary International 301 (2013) 135e149
141
In the cluster analysis, Holocene bivalves were more similar
tomodern than to Pleistocene ones. However, Pleistocene
gastropodswere similar to Holocene ones, and both differ from
modern ones(Fig. 8).
3.2. The Pleistocene of Bahía San Blas
Eleven sites were analyzed for the Pleistocene, in which 33
taxawere recorded (17 bivalves and 16 gastropods). Five of the
elevensites were studied from a lithological and paleoecological
point ofview. Site 25, is located approximately 50 km from the
village ofBahía San Blas. This outcrop is composed of 2.5 m of
gravel sedi-ments with a sandy silt matrix and a homogeneous grey
colour(Fig. 9) (Fucks et al., 2012). The mollusc fauna is composed
of Pitarrostratus, Glycymeris longior (bivalves), Buccinanops
cochlidium, andOlivancillaria urceus (gastropods).
The second Pleistocene deposit is site 22, near Salina del
Inglés,26.6 km from the village of Bahía San Blas. It is one of the
mainevaporite deposits in the area, ovoid-shaped, 5.5 km long
E-Wdirection and 4.8 kmwide, and composed of silty sediments,
brownto green, with gypsum crystals and mollusc shells
(Etcheverríaet al., 2006). At the western end there is a selenite
gypsum layernearly 50 cm thick composed of slightly carbonatic,
brown claymud (Del Blanco et al., 2005). In this site, a 50 cm
unexposed profilewas found, with articulated Pitar rostratus in
life position (Fig. 10).Nineteen taxa were observed (10 bivalves
and 9 gastropods) ina good state of preservation and with gypsum
precipitation on theshells. Among the most abundant were Pitar
rostratus (bivalve),Olivancillaria urceus, Tegula patagonica and
Zidona dufresnei
ternal view. L ¼maximum antero-posterior length in mm.1, Nucula
(N.) nucleus (Linné)locene); 3, Mytilus edulis Linné (L ¼ 25.07)
(MLP:33.409, Modern); 4, Brachidontes (B.)gny) (L ¼ 54.96)
(MLP:33.996, Pleistocene); 6, Ostrea puelchana d’Orbigny (L ¼
29.90)Pleistocene); 8, Tagelus (T.) plebeius (Ligthfoot) (L ¼
61.05) (MLP:33.418, Holocene); 9,(Say) (L ¼ 29.52) (MLP: 33.413,
Holocene); 11, Glycymeris (G.) longior (L ¼ 19.43)
3, Solen tehuelchus d’Orbigny (L ¼ 58.70) (MLP:33.401,
Holocene); 14, Plicatula gibbosa1.80) (MLP: 33.401, Holocene); 16,
Diplodonta vilardeboana (d’Orbigny) (L ¼ 12.33), Pleistocene); 18,
Mactra isabelleana d’Orbigny (L ¼ 28.44) (MLP: 33.405, Modern);
19,ilsbry (L ¼ 11.58) (MLP: 33.999, Holocene); 21, Carditamera
plata (Inhering) (L ¼ 5.30), Holocene); 23, Barnea lamellosa
(d’Orbigny) (L ¼ 32.69) (MLP: 33.993, Modern); 24,igas (Thunberg)
(L ¼ 35.70) (MLP: 33.997, Modern); 26, Pitar (P.) rostratus
(Philippi)33.992, Modern).
-
Fig. 4. Taxa of gastropods from marine Quaternary deposits in
Bahia San Bias. A. Abapertural view; b. Apertural view. H ¼ maximum
dorso-ventral height in mm. 1, Heleobiaaustralis (d’Orbigny) (H ¼
6.51) (ML: 34.038,Holocene); 2, Calliostoma coppingeri (Smith) (H ¼
7.36) (MLP: 34.046, Pleistocene); 3, Bostrycapulus odites (Collin)
(H ¼ 18.84) (MLP:34.025,Holocene); 4, Notocochlis isabelleana
(d’Orbigny) (H ¼ 16.15) (MLP: 34.035, Pleistocene); 5, Diodora (D.)
patagonica (d’Orbigny) (H ¼ 40.11) (MLP: 34.026, Modern);
6,Parvanachis isabellei (d’Orbigny) (H ¼ 10.58) (MLP: 34.045,
Modern); 7, Tegula (A.)patagonica (d’Orbigny) (H ¼ 13.05) (MLP:
34.033); 8, Crepidula argentina Simone, past.& Pen,(H ¼ 15.09)
(MLP: 34.028, Holocene); 9, Lucapinella henseli (Martens) (H ¼
26.60) (MLP; 34.027, Modern); 10, Olivella (O.) tehuelcha (Duclos)
(H ¼ 11.71) (MLP: 34.036, Holocene); 11,Buccinanops moniliferum
(Kiener) (H ¼ 34.46) (MLP: 34.040, Modern); 12, Trophon varians
(d’Orbigny) (H ¼ 41.51) (MLP: 34.037, Modern); 13, Marginella
martini (Petit) (H) (MLP:34.044, Pleistocene); 14, Buccinanops
uruguayense (Pilsbry) (H ¼ 22.53) (MLP: 34.034, Holocene); 15,
Buccinanops cochlidium (Dillwyn) (H ¼ 64.28) (MLP:34.032,
Holocene); 16,Olivancillaria urceus (Röding) (H ¼ 48.37) (MLP:
34.042, Pleistocene); 17, Olivancillaria carcellesi Klappenbach (H
¼ 29.03) (MLP: 34.039, Modern); 18, Urosalpinx cala (Pilsbry)(H ¼
13.21) (MLP: 34.041, Pleistocene); 19, Buccinanops globulosum
(Kiener) (H ¼ 17.84) (MLP: 34.043, Modern); 20, Zidona dufresnei
(Donovan) (H ¼ 90.92) (MLP: 34.029,Holocene); 21, Odontocymbiola
magallanica (Gmelin) (H ¼ 80.40) (MLP: 34.030, Holocene); 22,
Adelomelon (P) brasiliana (Lamarck) (H ¼ 120.24) (MLP: 34.031,
Holocene).
M.P. Charó et al. / Quaternary International 301 (2013)
135e149142
(gastropods), micro-molluscs Nucula nucleus, Carditamera
plata,Corbula patagonica (bivalve), and Heleobia australis
(gastropod).
Sites 23 and 24 correspond to the Pleistocene of Faro
SegundaBarranca. This outcrop has two levels: the lower level (site
24)comprises 2m of yellow-brown sandy silt sediments with
boulders.Mollusc fauna includes T. patagonica, Bostrycapulus
odites, Uro-salpinx cala, Parvanachis isabellei, Buccinanops
cochlidium and Buc-cinanops globulosus (gastropods), and Mytilus
edulis andBrachidontes rodriguezi (bivalves) (Fig. 11). The upper
level (site 23)is composed of clast-supported gravel, with sand
and
Fig. 5. Diversity (Shannon index) of the molluscan fauna in the
localities of differentages.
conglomerated sand strata on the surface. The overall colour is
grey,with clearly defined parallel to low-angle cross bedding,
showinga slight southern inclination and partial clustering in some
strata.Fossil remains include T. patagonica, B. odites, Crepidula
sp., Olivellatehuelcha (gastropods) and Pitar rostratus, M. edulis,
Brachidontesrodriguezi, Aequipecten tehuelchus, Ostreola equestris,
Ostrea puel-chana and Pitar rostratus (bivalves) (Fig. 12).
A Pleistocene ridge at an altitude of 33 m a.s.l., 10 km from
thevillage of Cardenal Cagliero (40�39018.1000 S/62�44053.6600 W),
wasanalyzed (site 26). It is a quarry of clustered gravel probably
olderthat MIS 5e (Fucks et al., 2012). Pitar rostratus (bivalve)
and Zidonadufresnei (gastropod) were found.
3.3. The Holocene of Bahía San Blas
Large quantities of diverse gastropods and bivalves were foundin
the Holocene beach ridges. Seven sites were analyzed and 31taxa
were collected (18 bivalves and 13 gastropods) in a good stateof
preservation, in some cases articulated. All Holocene deposits
arelocated at Isla Jabalí, and are represented by two geofeatures:
beachridges (Fig. 13) and tidal plains (Fig. 14). Beach ridges
(sites 11 and14) deposits are well-stratified gravels (Fig. 10Ba,
c) and sands(Fig. 10Bb, d and C) with shell remains. Among the most
commontaxa are Pitar rostratus, Amiantis purpurata, Glycymeris
longior,
-
Fig. 6. Proportion of bivalves and gastropods according to their
substrate type, trophic type and mode of life.
Fig. 7. Dendrogram of Quaternary deposits based on BrayeCurtis
similarity index inbivalves.
Fig. 8. Dendrogram of Quaternary deposits based on BrayeCurtis
similarity index ingastropods.
M.P. Charó et al. / Quaternary International 301 (2013) 135e149
143
-
Fig. 9. Outcrop of gravel littoral beach in Bahia San Blas (Site
25).
Fig. 10. General view of Pleistocene site Salina del Ingles
(site 22) showing a detailedsector with articulated Pitar
rostratus.
Fig. 11. Lower level of site 24, Faro Segunda Barrance (MIS
5e).
M.P. Charó et al. / Quaternary International 301 (2013)
135e149144
Ostrea equestris (bivalves) and T. patagonica, B. odites,
Buccinanopsglobulosus (gastropods). In the tidal plains (sites 9,
10, 12, 13, and15), which are lower energy environments, Heleobia
australis(gastropod) and Corbula patagonica and Tagelus plebeius
(bivalves)are common, and inmost of these deposits, articulated T.
plebeius inlife position can be observed (Fig. 12).
3.4. Modern fauna
Beaches of Bahía San Blas are made of boulders and have a
verypronounced slope, thus forming a reflective beach (Trebino,
1987;Codignotto, 1997; Cuadrado and Gomez, 2010) with
molluscremains (Fig. 15).
At the Isla Jabalí beaches, six sites were analyzed and 24
taxawere found (10 gastropods and 14 bivalves) including B.
odites(gastropods) and Brachidontes rodriguezi, Ostrea equestris,
Pitarrostratus, Amiantis purpurata and Crassostrea gigas
(bivalves), beingparticularly noteworthy Mesodesma mactroides,
Solen tehuelchus,Barnea lamellosa and C. gigas. At the beaches of
Faro SegundaBarranca, two sites were analyzed and 16 taxa were
recorded (10gastropods and 6 bivalves), being the most abundant B.
odites,Trophon varians and T. patagonica (gastropods), and
Ostreolaequestris (bivalve).
3.5. Mollusc associations
In order to assess the degree of similarity (or
dissimilarity)among the set of samples from both areas, the
UPGMAmethod wasused, together with the BrayeCurtis Index. Two large
samplegroups, A and B, were defined according to the cluster
analysis.Group A mainly contains Pleistocene and modern sites, with
theexception of two Holocene sites. All belong to high energy
envi-ronments. This group is divided into four sub-groups.
Sub-group A1stands out for having three Pleistocene sites and three
modernsites, including all the sites of Faro Segunda Barranca.
Sub-group A2contains three Pleistocene sites that belong to the
continental partof Bahía San Blas, of which two (sites 25 and 26)
are located at thehighest altitudes. Sub-group A3 gathers three
Pleistocene and fourmodern sites from Isla Jabalí, but did not
include site 16. Group Bgathers only Holocene sites, all of which
belong to low-energyenvironments (Fig. 16).
According to the Correspondence Analysis (CA), the sites can
bedivided into groups A and B. Group A is composed of
continentalPleistocene sites and modern sites of Isla Jabalí. Group
B comprisesall Holocene sites in low-energy environments. In the
upper regionof the graph, the sites located at the top, 22 and 15,
correspond tothe two Pleistocene sites considered to be older
thanMIS 5e. Site 17is located in the lower region of the graph,
close to group A and sites7 and 8, which belong to modern Faro
Segunda Barranca sites(Fig. 17).
4. Discussion
For the Pleistocene, 33 taxa were found (17 bivalves and
16gastropods), mostly represented by the gastropods
Brachidontesrodriguezi, M. edulis, Ostreola equestris, Ostrea
puelchana, Pitar ros-tratus, and Amiantis purpurata, and the
gastropod Buccinanopsglobulosus. For the Holocene deposits, 31 taxa
were registered (18bivalves and 13 gastropods), with the
predominant bivalves beingPitar rostratus and Corbula patagonica,
and the gastropods Heleobiaaustralis, T. patagonica, and Zidona
dufresnei. In general, the Pleis-tocene and the Holocene molluscs
of Bahía San Blas show nosignificant differences in composition.
However, the presence ofthe bivalve Crassostrea rhizophorae in the
late Pleistocene (MIS 5e)is a clear sign of warmer conditions. This
is also seen in the
-
Fig. 12. Outcrops of Pleistocene gravels (MIS 5e) in Faro
Segunda Barranca (Site 23).
M.P. Charó et al. / Quaternary International 301 (2013) 135e149
145
Pleistocene deposits of Bahía Blanca, 285 km farther south
(Chaaret al., 1992). In contrast, no warm climate molluscs,
currentlypresent in lower latitudes, were recorded during the
Holocene. Inthis regard, the Holocene fauna from Bahía San Blas
contrasts withthe results of previous studies in the northeast
region of theprovince of Buenos Aires, and in Bahía Blanca
(Farinati, 1985;Aguirre, 1993b; Aguirre and Whatley, 1995), in
which largeramounts of gastropods and bivalves were reported
compared to thePleistocene record (MIS 5e). The abovementioned
studies relatethose differences to the Climatic Optimum or
Hypsithermal (mid-Holocene), a paleoclimatic event that occurred in
the NorthernHemisphere, with higher SST and warmer conditions
compared tothe present (e.g. Lutaenko, 1993; Hjort et al., 1995).
However, theresults of this study area do coincide with other
research carried
Fig. 13. Outcrops of Holocene (MIS 1) b
out along Central Patagonia (Pastorino, 1991, 1994, 2000;
Aguirre,2003; Aguirre et al., 2005, 2006) in which no record of
faunalchanges related to the Hypsithermal were found.
Within the Holocene fauna, the presence of articulated T.
ple-beius in life position is worthy of mention. T. plebeius is an
infaunaleurohaline species, commonly found in large quantities in
Holo-cene deposits in intertidal areas of estuaries on the Atlantic
coast,from North Carolina (34� N) to SanMatías Gulf (41�S) (i.e.
Gutiérrezand Iribarne, 1999; Bushek et al., 2008). In the province
of BuenosAires, episodes of mass mortality have been recorded
(Schnacket al., 1982; Farinati et al., 1992; Golfieri et al., 1998;
Iribarneet al., 1998; De Francesco and Zárate, 2001).
In addition, Mesodesma mactroides, one of the most commonmodern
species in the Bahía San Blas area, that lives today on sandy
each ridges in Isla Jabali (Site 11).
-
Fig. 14. Holocene deposit from Isla Jabali (MIS1) with
articulated Tagelus plebeiusfound in life position (Site 12).
Fig. 15. Beach and cliff at the base covered by dunes rampant in
Faro Segunda Barranca(Site 8).
Fig. 16. Dendrogram of the localities, based on BrayeCurtis
index (cluster analysis).
M.P. Charó et al. / Quaternary International 301 (2013)
135e149146
beaches from 23�S (Rio de Janeiro, Brazil) to near 40�S (Isla
Jabalí)(i.e. Rios, 1994; Fiori and Morsan, 2004; Fiori and Defeo,
2006), wasnot found in the Holocene ridges analyzed in this study,
despitebeing mentioned in an unspecified area for this period by
Aguirreand Farinati (2000). In Holocene archaeological sites of
thePampean littoral, Mesodesma mactroides was not found to
beconsumed by hunteregatherer aborigines (Bonomo,
2007).Nevertheless, in Holocene shell middens of the coastal region
ofBahía San Blas, this species has been mentioned as amongst
themost abundant (Zudimendi, 2007; Aldazabal et al., 2010).
Hence,although M. mactroides was not found in this study, it has
beenpresent in the region, probably with a discontinuous
distributionalong the coast of the Buenos Aires province.
Similarly, Solentehuelchus and B. lamellosa have not been found in
the studiedridges, but have been recorded in other Holocene
deposits of theBuenos Aires Province between Sierra Grande and
Bahia Blanca(Aguirre and Fucks, 2004).
The appearance of C. gigas on the current coast of Isla Jabalí
isthe result of anthropic modification. This Japanese species
wasbrought to Argentina and introduced into the area of Bahía
Anegada(39�500S/40� 400S) in 1981 for farming purposes, and in 2005
themost southern natural population in the country was recorded in
ElCondor beach resort (41�S).
5. Conclusions
For the Quaternary of Bahía San Blas, 50 taxa were recorded
(27bivalves and 23 gastropods) in a good state of preservation, and
insome cases, articulated. The highest diversity indexes
correspondmainly to modern sites, followed by Pleistocene sites.
Holocenesites have the lowest diversity, except for two. Marine
species ofbivalves and gastropods prevail over estuarine species,
beingmainly free locomotion species as epifaunal gastropods
andinfaunal bivalves.
Among bivalves, rocky sediment and filter-feeder bivalvesprevail
in Pleistocene deposits, whereas sandy sediment andsuspension
feeders are predominant in Holocene and moderndeposits. On the
other hand, epifaunal gastropods typical of sandysediments and
filter feeders are predominant in Pleistocenedeposits, whereas
mobile carnivores prevail in Holocene andmodern deposits. It
remains unclear if these changes in thepredominance of different
trophic groups is related to the presenceof different
sub-environments within the study area or are due toregional
changes over time.
Two mollusc associations were defined in Bahía San Blas (A andB)
depending on the type of environment: high energy (stormrange) or
low-energy (tidal plains), and not on their age. Sites 22and 25 are
different from the rest of the Pleistocene sites: theformer due to
its species richness, and the latter for being one of theoldest,
along with site 26. Both sites yielded the largest numbers ofPitar
rostratus. During the Pleistocene, warmer conditions favouredthe
entrance of C. rhizophorae, currently inhabiting lower
latitudes.The large number of articulated T. plebeius in life
position indicatesa low-energy environment of fine sediments and
silty sand, bestseen in Holocene deposits. For the Holocene, no
warm water taxacurrently living in lower latitudes was found.
Modern sites from Faro Segunda cluster with Pleistocene sites
ofBahía San Blas, but separate from modern sites of Isla Jabalí in
theCorrespondence Analysis (CA). This difference is due to the
largenumber of Trophon varians in Faro Segunda Barranca
beaches.
Four modern taxa were not found in either Pleistocene orHolocene
deposits: Mesodesma mactroides, Solen tehuelchus, B.lamellosa and
C. gigas. However, although Mesodesma mactroideswas not found in
this study, it has been reported in archeologicalsites (Holocene
shell middens) in the south of Bahía San Blas.
-
Fig. 17. Correspondence analysis of sites based on the abundance
of taxa. Most sites conform to two groups A and B.
M.P. Charó et al. / Quaternary International 301 (2013) 135e149
147
Meanwhile, C. gigaswas introduced into the Argentine seas of
BahíaAnegada and has proliferated over the last few years along
thesouthern coast of the Buenos Aires Province. From the 50
taxafound in Quaternary deposits in the study area, 14 (8 bivalves
and 6gastropods) are recorded since the Pleistocene.
The last marine transgressive events have been recorded inBahía
San Blas area. MIS 1 and MIS 5e (last interglacial) are
wellrepresented, while other older events, MIS 7 and/or MIS 9, are
notso clearly represented. The �MIS 7 events are represented
byelongated geofeatures, characterized by scarce marine fauna
andmaximum altitudes of 30 m a.s.l. The Late Pleistocene (MIS 5e)
iswell developed inland, where typical outcrops can be found
withinmaximum altitudes of 10 m a.s.l.
MIS 1 is recorded exclusively on Isla Jabalí, with
continuousbeach strands, beach ridges and shallow tidal plains at
maximumaltitudes of 6 m a.s.l. as the most typical geofeatures. For
both MIS 1and MIS 5e, the characteristic marine molluscs are
bivalves andgastropods. Among these, C. rhizophorae, today living
at lowerlatitudes, is an excellent indicator of warmer conditions
during theLate Pleistocene (MIS 5e). Nevertheless, as a whole,
molluscs do notdenote warmer environments in the study area that
could berelated to the global event known as the Climatic Optimum,
orHypsithermal. Based on the quantitative and qualitative
analysischanges of Pleistocene and Holocene molluscs, are mostly
due tothe presence of sub-environments and local, rather than
global,environmental changes that affect ecological parameters.
Acknowledgments
The authors thank Lic. Florencia Pisano (FCNYM, CONICET) forher
collaboration during fieldwork; Prof. Mario Giaconi (FCNYM,UNLP,
CONICET) for logistic support, Dr. Mikel Zudimendi (UNLP,CONICET),
who provided the bibliography which enriched thisstudy in an
interdisciplinary way and Dra. Cecilia Deschamps forhelp in the
editing of the final version.
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Paleoecological significance of Late Quaternary molluscan faunas
of the Bahia San Blas area, Argentina1. Introduction1.1. Study
area1.2. Geological background1.3. Paleontological background
2. Materials and methods3. Results3.1. Taxonomic and
paleoecological composition3.2. The Pleistocene of Bahía San
Blas3.3. The Holocene of Bahía San Blas3.4. Modern fauna3.5.
Mollusc associations
4. Discussion5. ConclusionsAcknowledgmentsReferences