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Middle–Late Holocene palynology and marine mollusks from Archipiélago
Cormoranes area, Beagle Channel, southern Tierra del Fuego, Argentina
María Soledad Candel a,⁎, Ana María Borromei a, Marcelo A. Martínez a, Sandra Gordillo b,
Mirta Quattrocchio a, Jorge Rabassa c
a INGEOSUR-CONICET, Departamento de Geología, Universidad Nacional del Sur, San Juan 670, B8000ICN Bahía Blanca, Argentina
b CIPAL-CONICET, Universidad Nacional de Córdoba, Av. Vélez Sársfield 299, X5000JJC Córdoba, Argentina
c Laboratorio de Geología del Cuaternario, CADIC-CONICET, CC 92, V9410CAB Ushuaia, Tierra del Fuego, Argentina
A r t i c l e i n f o
Article history:
Received 15 January 2008
Received in revised form 26 November 2008
Accepted 2 December 2008
Keywords:
Palynology
Marine mollusks
Paleoenvironment
Holocene transgression
Beagle Channel
A b s t r a c t
The palynology and marine mollusks of a marine sequence from Río Ovando (54° 51′ S, 68° 35′ W),
Archipiélago Cormoranes, Beagle Channel, has been studied in order to reconstruct paleoenvironmental
conditions during the Middle–Late Holocene. The dinoflagellate cyst assemblages from Río Ovando sequence
reflect fjord (estuarine) environments close to terrestrial ice field, affected by glacier meltwater discharge,
and characterized by short-term oscillations of sea-surface water parameters. The base of the section, which
is dated at about ca. 4160 14C yr B.P. (4736 cal yr B.P.) (Palynological Subzone RO-2c), is characterized by a
high species diversity of dinocyst and mollusks, and it is immediately followed by an interval (Palynological
Subzone RO-2b) characterized by the presence of the Echinidinium–Islandinium complex and the
monospecific Mytilus mollusk assemblage. This subzone registers an inverse correlation between Nothofagus
dombeyi type and Echinidinium–Islandinium complex concentration values during ca. 4160 14C yr B.P.
(4736 cal yr B.P.)–4064 14C yr B.P. (4540 cal yr B.P.), suggesting a variable climatic condition, probably related
to Neoglacial episodes occurred in the southern Patagonia Andes during this interval. The pollen assemblages
permit direct correlations with the onshore palynostratigraphy from southern of Tierra del Fuego. The high
percentages of Nothofagus dombeyi type recorded throughout most of the profile strongly suggest the
presence of a closed forest, confirming the existence of a variable, cool and wet climate for the Archipiélago
Cormoranes area during the Middle–Late Holocene.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The present Beagle Channel (54° 53′ S; 67° 00′–68° 40′ W), about
200 km long and 5 km wide, links the Atlantic and Pacific Oceans,
separating the Isla Grande de Tierra del Fuego from the southern
islands of the Fuegian archipelago (Fig. 1A). The Beagle Channel
system is an inland passage in a complex web of channels, inlets and
surrounding land masses that characterizes southern South America
(Antezana, 1999). It is a drowned glacial valley, formerly occupied by a
large outlet glacier from the Cordillera Darwin, the “Beagle Glacier”.
This valley was repeatedly glaciated, at least in two major episodes,
during the “Lennox Glaciation” (Oxygen Isotopic Stage 6, N125 ka B.P.)
and during the Last Glaciation named “Moat Glaciation” (Oxygen
Isotopic Stage 2, 20–18 ka B.P.) (Rabassa et al., 2000). The Beagle
Channel opened before 8200 14C yr B.P. and the marine environment
was fully established at least by 7900 14C yr B.P. (Rabassa et al., 1986).
The Holocene marine transgression in southern Tierra del Fuego,according to Gordillo (1993), is represented by several discontinuous raised terraces along the northern Beagle Channel coast. Four informal terrace units have been recognized: Ancient Low Terrace, High Terrace, Middle Terrace and Recent Low Terrace, deposited ca. 8000,
6000, 5000–3000 and after 3000 yr B.P., respectively. These deposits are mostly sandy and gravely in grain-size, although clay-like sediments are found mainly in the westernmost sector of the Beagle Channel. The origin of these raised beaches appears related to tectonic uplift and/or isostatic recovery following deglaciation (Rabassa et al., 2000; Bujalesky et al., 2004).
There have been many contributions on the Holocene history of relative sea level change in Isla Grande de Tierra del Fuego: Codignotto,1984; Porter et al., 1984; Rabassa et al., 1986, 1992, 2000; Isla, 1989; Rutter et al., 1989; Mörner, 1990;Bujalesky and González Bonorino,1990; Bujalesky et al., 2004; Gordillo et al., 1992, 1993, 2005.Occurrences of organic-walled dinoflagellate cysts, other aquatic
palynomorphs and palynofacies from Holocene marine sediments in the northern coast of the Beagle Channel have been mentioned in Borromei et al. 1997; Borromei and Quattrocchio, 2001, 2007 and Grill et al., 2002. Mollusks are the most common remains in Holocene marine raised beaches along the Beagle Channel (Gordillo, 1992). Prior to this work, special emphasis was placed on taphonomy of bivalves (Gordillo, 1992, 1999) and chitons (Gordillo, 2007), to aid paleoenvironmental reconstruction. These previous works analyzed the postmortem mechanical processes(i.e.disarticulation, fragmentation, orientation, abrasion, bioerosion,encrustation) affecting fossil remains, and their paleontological attributes (i.e. taxa composition,abundance, size frequency, mode of life), showing that – despite the
bias in preservation due to taphonomic processes – these assemblages retain useful information pertaining to life habit and habitats of the marine benthos from which they are derived.The aim of this paper is to characterize depositional paleoenvironments in the Archipiélago Cormoranes, Beagle Channel during holocene
transgression, using palynological analysis and macrofossil paleoecology.
We supplement this datawith information onmollusk assemblages from the Río Ovando area, providing more complete knowledge on the environmental changes affecting the marine realm in this region during the Holocene. The comparison of the sediments found in the Río Ovando area with radiocarbon-dated marine deposits from Bahía Lapataia sites (Borromei and Quattrocchio, 2001, 2007) and Río Varela locality (Grill
et al., 2002) will help in the interpretation of the paleoenvironmental conditions which took place during the Holocene relative sea level change into the Beagle Channel area, southern Tierra del Fuego.
⁎ Corresponding author. Tel.: +54 291 4595101x3050; fax: +54 291 4595148.
E-mail addresses: scandel@uns.edu.ar (M.S. Candel), borromei@criba.edu.ar
(A.M. Borromei), martinez@criba.edu.ar (M.A. Martínez), sgordillo@efn.uncor.edu
(S. Gordillo), mquattro@criba.edu.ar (M. Quattrocchio), jrabassa@cadic.gov.ar
(J. Rabassa).
0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2008.12.009
2. Present climate and vegetation of Tierra del Fuego
The climate of Tierra del Fuego is determined by the belt of prevailing humid and cold westerlies. It is highly oceanic in the West and South parts of the archipelago and increasing continental towards the East and North. Mean summer isotherms increase northeastward from 9° to 12 °C. Precipitation decreases to the North and East. Mean annual rainfall in Ushuaia is 570 mm and less than 300 mm in Río Grande to the north (Heusser, 2003). The modern vegetation corresponds to the Fuego–Patagonian Steppe in the north, followed southward successively by the Subantarctic Deciduous Beech Forest and the Evergreen Beech Forest. They are characterized by three species of southern beech, Nothofagus pumilio (lenga), Nothofagus betuloides (guindo) and Nothofagus antarctica (ñire), which grows to an average altitudinal limit of 550–600 m a.s.l. (meters above sea level) and predominates where precipitation reaches between 400 and 800 mm/year. Magellanic Moorland occurs beyond the forest along the exposed outermost coast under conditions of increased precipitation, wind and poor drainage. High Andean Desert vegetation develops above treeline (600 m a.s.l.) in the Fuegian Andes until snowline is reached (Heusser, 2003).
3. General description of the area
3.1. Present physical setting at the Beagle Channel
The Beagle Channel is connected with the Pacific Ocean through Brazo Noroeste and Brazo Sudoeste surrounding Isla Gordon (Fig. 1B). Despite major connection to the Pacific Ocean at the mouth of the Beagle Channel, the shallow depth of its eastern end (west of Isla Gable) apparently restricts the inflow of subsurface Atlantic Ocean
water (Gordillo et al., 2005). The narrowing (sill) of Archipiélago Gable not only modifies morphologically the fjord dynamics, but it also limits the relative effects of the eastern- and western-flowing tidal currents, and the gravity waves originate from the west (Isla et al., 1999).
The Beagle Channel waters are influenced by a strong freshwater discharge from precipitation and glaciers through the rivers during summer. The water column is strongly thermohaline stratified with water mixing at 12 m depth mainly during the summer season. The average sea-surface temperature is 6.5 °C with a maximum of 9 °C in January and a minimum of 4 °C in August. Sea-surface salinity varies from 27 to 33.5 PSU. Minimum values of salinity are obtained during summer depending on the volume of meltwater discharged into the channel. The Beagle Channel has ice-free conditions throughout the whole year (Iturraspe et al., 1989; Isla et al., 1999). The estuarine (fjord) dynamics are controlled by significant and seasonal freshwater
sources, and by tidal flow from both the east (Atlantic) and the west (Pacific) (Isla et al., 1999).
3.2. Setting and stratigraphy of Río Ovando locality, Archipiélago
Cormoranes
The Lago Roca–Lapataia valley (54° 50′ S, 68° 34′ W) is a glacial landscape formed by a series of low, rounded bed-rock hills, a typical ice-scoured terrain, surrounded by interconnected depressions filled with fresh-water lakes and ponds, peat bogs, or both. The entire area was partially submerged under the sea generating deep and narrow fjords and intricate archipelagos during the Holocene marine transgression, around ca. 8000 14C yr B.P. Probably, the marine advance had been started from the Canal Murray, east of Isla Navarino (Rabassa et al.,1986). The marine deposits are scattered along Bahía Lapataia up to the eastern shore of Lago Roca, including the Archipiélago Cormoranes area and, both margins of Río Ovando and Río Lapataia (Gordillo et al., 1992,1993). The oldest marine radiocarbon dates for the Beagle Channel were obtained from shells of Chlamys patagonica from Lago Roca (7518+58 14C yr B.P.) and from shells of Mytilus sp. from Bahía Lapataia (8240+60 14C yr B.P.) (Rabassa et al., 1986, 2000). A reservoir effect somewhere between 630±70 yr at Beagle Channel (Albero et al., 1987) and 380±100 yr at Estrecho de Magallanes
(Angiolini and Fernández, 1984) should be taken into consideration for those radiometric analyses performed on marine shells.
Lago Roca is connected to the Beagle Channel via fresh water branches: the Río Ovando and the Río Lapataia (Fig. 1C). The studied Holocene sequence (2.90 m a.s.l.) is located at the head of Río Ovando (54° 51′ S, 68° 35′ W). Three informal lithological units, separated by unconformities, are recognized (from base to top, Lithological Unit C to Lithological Unit A) (Fig. 2):
3.2.1. Lithological Unit C (80–24 cm)
Massive greenish grey clays. The unit contains marine shells in its
lowermost part (80–45 cm). This unit was calibrated by radiocarbon
dating. One, 4160+45 14C yr B.P. (Pta 7573) (Coronato et al., 1999)
corresponds to the Tawera gayi shells (Coronato, pers. com), found in
growth position, at 70 cm depth; the second, 4064±35 14C yr B.P., was
obtained on organic matter at 58 cm depth. The last radiocarbon date,
3542±38 14C yr B.P. was also obtained on organic matter at 24 cm
depth. The bottom of the profile was not observed because it was
below water level.
3.2.2. Lithological Unit B (24–19 cm)
Greenish grey clayey coarse sand with pebbles.
3.2.3. Lithological Unit A (19–0 cm)
Present soil.
The 14C ages were calibrated using INTCAL98 (Stuiver et al., 1998).
Results span a calibrated age range between 3800 and 4700 years B.P.(Table 1).
4. Material and methods
A total of 21 samples were collected for palynological analysis from
the base at 80 cm depth to the surface. The lithological Unit C was
sampled continuously each 2 cm of sediment. All samples were
processed for palynological analysis according to Heusser and Stock's
techniques (1984). Following the procedure advocated by Dale (1976),
the samples from marine units (Unit B and C) were treated with cold
acids (HCl, HF) to preserve the organic-walled dinoflagellate cysts, and
no oxidation and no acetolysis was applied in order to prevent the loss
of more fragile protoperidiniacean cysts. All samples were stained
with Safranin O, in accord with Stanley’s technique (Stanley, 1966).
Exotic spores (Lycopodium) were added to allow calculation of
palynomorphs concentration per gram of dry weight of sediment
(Stockmarr, 1971). The residue was sieved through a 10 μm mesh to
concentrate the palynomorphs and mounted between a slide and
cover slide in glycerin gel. The material was studied using a
transmitted light microscope at 200× to 1000× magnification. The
palynological slides are housed in the Laboratory of Palynology,
Universidad Nacional del Sur, Bahía Blanca, Argentina, under the name
UNSP followed by the denomination of the study section: RO (Río
Ovando).
To evaluate the biosphere components, the frequencies (%) of trees,
shrubs and herbs were based upon counts between 250 and 450
pollen grains; aquatic and cryptogam frequencies were from counts of
total pollen and spores. To evaluate the terrestrial/marine environmental
relationships and the sea level changes, the aquatic palynomorph
(organic-walled dinoflagellate cysts, acritarchs, Chlorophyta
algae, copepod egg-envelopes and test linings of foraminifera)
frequencies (%) were based upon counts between 200 and 600 of
total palynomorphs.
According to Heusser (1998), Nothofagus betuloides, Nothofagus
pumilio, and Nothofagus antarctica are shown collectively as Nothofagus
dombeyi type due to the impossibility of specific level differentiation.
Another special case is Empetrum rubrum and Gaultheria/
Pernettya (Ericaceae), being morphologically similar. The latter sometimes
is possibly included together with Empetrum when its sculpture
is not distinct.
The dinocyst taxonomical nomenclature used in this study
conforms with the present in Rochon et al. (1999), Head et al.
(2001) and Zonneveld (1997).
In our samples, the round, brownish, spiny dinoflagellate cysts
recovered were grouped into “Echinidinium–Islandinium complex”
because, in some cases, the determination at the species level was
difficult due to the poor preservation and/or bad orientation (Fig. 3).
The determination of fossil dinocyst taxa at specific levelwas made for
comparison with modern forms from surface samples of Beagle
Channel and at the moment they are reason of study by one of the
authors (M.S.C.). In our analysis, specimens of Brigantedinium simplex
Wall 1965 and Brigantedinium cariacoense Wall 1967 are grouped
under the name of Brigantedinium spp. when the archeopyle was not
observed due to orientation.
Algae assemblages recovered in the Río Ovando section include the
groups Prasinophyceae, Zygnemataceae and Chlorococcales, among others. In this paper we make a general mention; the details about
algae associations will be present in a next work.
Cluster analysis using Edwards & Cavalli–Sforza distance (Program
TILIA, E. Grimm, 1991) was applied to the fossil palynological
assemblages. In this analysis, taxa with percentages below 1% were
excluded.
The pollen/spore frequency (%) (Fig. 2) and total palynomorph
frequencies (%) with terrestrial and aquatic palynomorph concentrations
(Fig. 4) at Río Ovando section are represented. From units B and C, the percentage values and the numbers of organic-walled
dinoflagellate cysts, acritarchs and zoomorphs per gram of sediment
recovered throughout the unit is given in Table 2.
Mollusks were taken from exposures of natural cutting at the head
of the Río Ovando, on its left margin at two levels (basal bed and 50 cm
depth). Large specimens were separated from the sediment matrix in
the field. The smaller specimens (b20 mm) were sorted in the
laboratory from a bulk sediment sub-sample of 0.05 m3, under a
stereoscopic microscopy. Mollusks were identified at the lower taxonomic level possible. A preliminary faunal list was presented inGordillo et al. (2005). However, the generic placement or nominal species of some taxa (especially small-sized species) are still under revision. Specimens were figured using a scanning electron microscope (LEO 1450VP, backsattered electron image) or a binocularmicroscope (LEICA MZ). The material examined here is deposited at the Centro de Investigaciones Paleobiológicas, Universidad Nacional de Córdoba (CEGH-UNC).
Fig. 3. Scale bar is 10 μm.1–3. Echinidinium–Islandinium complex. 1. UNSP RO 1968: C17/1. 2. UNSP RO 1966: W59. 3. UNSP RO 1972: X22. 4, 5. Brigantedinium simplex, UNSP RO 2037:
Q28. 4. Apical view, low focus. 5. Apical view, high focus. 6. Selenopemphix quanta, UNSP RO 1972b: M38/2. 7, 8, 10. Polykrikos kofoidii. 7, 8. UNSP RO 1972b: F37/1. 10. UNSP RO 1972c:
J50/1. 9. Halodinium sp., UNSP RO 1972c: R13 11. Foraminiferal test-lining, UNSP RO 1972d: G34/4. 12. Copepod egg-envelope, UNSP RO 1972c: G14/2.
4.1. Ecology of dinoflagellate assemblages
Dinoflagellates, unicellular planktonic organisms, inhabit surfacewaters in a wide range of marine environments. The abundance and distribution of dinoflagellate cysts (hypnozygotes or resting spores) depend on the primary production and the physico-chemical conditions (temperature and salinity) in surface water of the photic zone (de Vernal et al.,1993). During major sea-level changes, however, this steady state of associations in surface waters and associated sediments is exposed to disturbances fromvarious dynamic processes like erosion, winnowing of sediments, intrusion and mixing of different water masses, which affect the regional microplankton associations within the water column and associated sediments
respectively (Prauss, 2000).
In our samples, among the Peridiniales taxa, specimens of Islandinium
minutum and Echinidinium granulatum, E. delicatum and E. spp. were grouped together into Echinidinium–Islandinium complex.
Islandinium minutum is the more specific taxa of shelf assemblages of the Arctic Ocean, including polynyas (Kunz-Pirrung et al., 2001). It is an euryhaline taxon dominating assemblages from the continental margins where summer temperatures rarely exceeding 7 °C ranging from −2 to 5 °C and salinity varies between 17 to 34 PSU. The duration of sea-ice cover in the Arctic Ocean is greater than 8 months per year
(de Vernal et al., 2001; Head et al., 2001). Echinidinium granulatum, E.delicatum and E. spp. occur in subtropical to tropical regions with temperature and salinity ranges of 13 to 29 °C and 25 to 36, respectively (Marret and Zonneveld, 2003). However, recently, Radi and de Vernal (2004) documented E. granulatum for surface sediment samples from the northeastern Pacific, with a temperature range as low as of 4 to 19 °C. Highest relative abundances of Echinidinium are also found in eutrophic environments related with upwelling and river discharged (Zonneveld, 1997).
Brigantedinium is a cosmopolitan taxon especially in epicontinental environments.
It probably is an opportunistic genus (de Vernal et al., 2001), often dominating low-salinity environments and it is presence suggests high concentrations of nutrients in the surface waters owing to freshwater input from glacier meltwater
(Grøsfjeld et al., 1999). Selenopemphix quanta shows a preference for the temperate to subpolar domain and occurs mainly in neritic environments where salinity can be relatively low (de Vernal et al., 2001). It seems to be more adapted to higher summer temperature between 8 and 14 °C and salinity between 23 and 31 PSU (Kunz-Pirrung, 2001).Cysts of Pentapharsodinium dalei occur from tropics to arctic areas
and coastal to deep-sea sites. This species is distributed within a wide range of temperatures between −2.1 and 29.6 °C and salinity between 21.3 and 36.7 PSU (Marret and Zonneveld, 2003). Among the Gymnodiniales, Polykrikos kofoidii is a tropical to
subtropical species adapted to temperatures between 25 and 29 °C and salinity between 31 and 36 PSU. It has been described from surface samples around the Japanese Archipelago (Matsuoka, 1985) and is generally documented from tropical to subtropical coastal regions of the major upwelling areas. Radi et al. (2001)documented this species for the Bering and Chukchi seas, suggesting temperatures
colder than previously recorded. Polykrikos schwartzii is a cold
temperate to subtropical species distributed within a broad temperature
range between −0.9 and 27.5 C and salinities exceeding 28.5 PSU It occurs in a wide range of environments fromcoastal to open oceanic and oligotrophic to eutrophic (Marret and Zonneveld, 2003).
The gonyaulacalean cysts include cosmopolitan taxa such us Operculodinium cf. centrocarpum recorded in coastal and deep-sea environments and tolerant of large fluctuations in temperature and salinity, and Spiniferites spp., which is usually found in neritic environments from tropical to polar regions (Matthiessen, 1995; de
Vernal et al., 2001).
5. Results
5.1. Palynological assemblages
Based on cluster analysis, two palynological zones spanning the full sequence are distinguished at Río Ovando section according to the pollen assemblages. The Palynological Zone RO-1 is restricted to Lithological Unit A, and the Palynological Zone RO-2 spans Lithological Unit B and Lithological Unit C (Fig. 2). Based on dinoflagellate cyst assemblages, the Palynological Zone RO-2 can be divided into three palynological subzones (Subzones RO-2a, RO-2b and RO-2c) (Fig. 4). In
order, from the lower to the upper part of the sequence, they are:
5.1.1. Zone RO-2
Is dominated by Nothofagus dombeyi type (84–97%). Shrubs and herbs (Poaceae, Misodendrum, Empetrum, Gunnera, Asteroideae and Cichorioideae) are found with percentage values of up to 8.3%. Tree pollen concentration varies between 3811–37626 grains/gram and herb and shrub pollen concentration between 160–1390 grains/gram
(Fig. 4).
5.1.1.1. Subzone RO-2c. The microplankton content is low. The dinocyst assemblage represented by Echinidinium–Islandinium complex, Brigantedinium spp., Polykrikos kofoidii Chatton, 1914, Polykrikos schwartzii Bütschli, 1873, Selenopemphix quanta Bradford, 1975 and Operculodinium cf. centrocarpum make up b1% each one. Other
microplanktonic constituents are present with low values, such as acritarchs (Micrhystridium spp. and Halodinium sp.), reaching up to 0.4% and copepod egg-envelopes (up to 1.4%). Chlorophyta algae association are present with values between 0.6 and 5.8%. The highest diversity of dinoflagellate cysts (7 taxa) of this section is registered at sample 21 (Table 2).
5.1.1.2. Subzone RO-2b. The increase of aquatic palynomorph frequencies reaches maximum values at samples 17 and 15 characterized by dinoflagellate cysts of Echinidinium-Islandinium complex (up to 6.3%). Acritarchs (Micrhystridium spp. and Halodinium sp.) and copepod egg-envelopes are recorded with low percentage values
(b1% each one). Algae group is registered with values up to 6.6%. The
highest abundance of marine palynomorphs is recorded in this subzone. Maximum values of dinocysts concentration are registered at sample 17 (592 dinocysts/gram) and sample 15 (965 dinocysts/ gram) (Table 2).
5.1.1.3. Subzone RO-2a. Microplankton frequencies represented by dinoflagellate cysts of Echinidinium–Islandinium complex, Selenopemphix spp., Brigantedinium spp., cf. Pentapharsodinium dalei Indelicato & Loeblich III, 1986 and Spiniferites spp. decrease (b1% each one). Meanwhile, copepod egg-envelopes frequencies increase (near to 4%) and acritarchs, mainly Micrhystridium spp., are present with b1%. The algae association is registered with percentages between 0.3 and 8.1%. The dinoflagellate cysts show more diversity (6 taxa) and lower concentration values (27–251 dinocysts/gram) than those in the former subzone (Subzone RO-2b) (Table 2). The samples 14 and 13 show the highest values of this subzone of marine palynomorphs,
mainly copepod egg-envelopes (855 and 564 specimens/gram, respectively) accompanied by acritarchs (188 acritarchs/gram).
5.1.2. Zone RO-1
Is characterized by increase of Poaceae (7–17%), Asteroideae (7–8%)
and Cyperaceae (3–11%) accompanied by Gunnera (1–2%), Empetrum
(up to 1%) and Cichorioideae (b3%). Acaena, Misodendrum and
Ranunculaceae, among others, are also present (1% each one). Aquatic
palynomorphs are represented by algae group with values up to 25.1%.
Although Nothofagus dombeyi type frequencies decrease (49–64%),
tree pollen concentration values increase to 6353–14045 grains/gram.
The shrub and herb concentration values also increase to 3689–4780
grains/gram (Fig. 4).
5.2. Mollusk marine assemblages
The Río Ovando area is very rich in fossil remains. These marine
deposits contain large proportion of whole, well-preserved shells. A
great number of specimens retain their original color and unaltered
sculpture. Bivalves normally occur as whole joined valves, oriented in
life position (e.g., Laguna Verde site, Río Ovando site; Gordillo, 1999),
or horizontally and randomly oriented within the bed. A minor
proportion of shells show an abraded surface and damaged margins,
indicating that the skeletal assemblage has been transported some
distances. Bivalves contribute most of the biomass, although gastropods
exhibit the highest richness. Chitons are also present in low
numbers.
Mollusks from the head of Río Ovando, left margin sector, yield two
distinct assemblages:
5.2.1. Diverse soft-substrate assemblage
This mollusk assemblage (Assemblage A) is restricted to the basal
bed at Río Ovando, which has been correlated to sections dated to
4160 14C yr B.P (4736 cal yr B.P). It is composed of a great number of
species (Fig. 5), belonging to the venerids (Tawera gayi, Venus antiqua)
and myoids (Hiatella solida). Among gastropods, the more common
taxa are the muricids (Trophon geversianus, Xymenopsis muriciformis)
and the buccinid (Pareuthria plumbea). Within the small sized
mollusks a great number of taxa as rissoid forms, Laevilitorina, Neolepton,
and the carditids Carditella naviformis and Cyclocardia
compresa, among others, characterize this assemblage.
5.2.2. The Mytilus assemblage
Mollusk Assemblage B is derived from a rather homogeneous
massive greenish clay at 50 cm. The assemblage is almost monospecific,
being characterized by abundant epifaunal Mytilus chilensis,
and less common Hiatella solida.
6. Paleoecological and paleoenvironmental interpretations
6.1. Pollen and spore assemblages
The palynological analysis at Río Ovando section shows predominance
of terrestrial palynomorphs (pollen and spores) over aquatic
palynomorphs (dinoflagellate cysts, acritarchs and zoomorphs) (Fig. 4).
Pollen and spores in marine sediments constitute long distance fluvial
and/or atmospheric inputs originating from the terrestrial vegetation of
adjacent lands (in de Vernal et al., 1993). Based on pollen studies of
surface samples from Tierra del Fuego, the Nothofagus pollen has a great
atmospheric dispersion. It is carried, often in large quantities, far from its
point of origin (Heusser, 1989a). In nearshore regions, pollen and spore
records potentially reflect the regional vegetation on littoral at the time of
their deposition, although their records are often overprinted with a
coastal signal (Borromei and Quattrocchio, 2007).
Spectra from Río Ovando record (Fig. 2) are used to identify
regional changes of vegetation and make to cronostratigraphic
correlations with the previous palynostratigraphy based on pollen
assemblages in the area (Heusser, 1989a, 1998). The significant
percentages of Nothofagus dombeyi type recorded throughout most
of the profile strongly suggest the presence of a closed forest,
confirming the existence of a cool and wet climate for the Archipiélago
Cormoranes area during the Middle–Late Holocene.
The identified Palynological Zone RO-2 with highest percentages of
Nothofagus dombeyi type, correlated with the Pollen Zone 1 (5000–
0 yr BP) of Heusser (1989a), resemble the modern Deciduous Beech
Forest with annual precipitation varying between 500 and 800 mm,
and summer temperature averaging 8–9 °C.
The decrease of Nothofagus dombeyi type and increase of herb and
shrub percentages in the Palynological Zone RO-1, along with increase
of brackish chlorophyta (Prasinophyceae) reflect the development of open beech woodland communities with patches of grass and sedges
over adjacent lands associated with a marginal marine influence.
6.2. Marine microplankton assemblages
The dinoflagellate assemblages in the fossil sequence are characterized
by low species diversity (9 identified taxa) and low
concentration values (Table 2). The Peridiniales dominate over
Gonyaulacales taxa suggesting inner neritic environments (de Vernal
and Giroux, 1991).
The RO-2c subzone, at the base of the section, is characterized by the
highest species diversity (7 taxa) and low dinocyst concentrations (36–
212 dinocyst/gram). The dinocyst assemblage shows co-dominance of
Echinidinium–Islandinium complex and Brigantedinium spp. accompanied
by Polykrikos kofoidii, Polykrikos schwartzii, Operculodinium cf. centrocarpumand
sparse occurrence of Selenopemphix quanta. The dinocyst
assemblage suggests marginal marine environments, low to moderate
salinity and reflects high concentrations of nutrients in the surface
waters probably due to freshwater input from glacier meltwater.
The RO-2b subzone shows not only the dominance of Echinidinium–
Islandinium complex but also the greatest cyst abundance and lowest
species diversity. This assemblage might indicate the occurrence of
monospecific dinocyst assemblage, characterized by “opportunistic
species”, suggesting the high freshwater input may be related to glacier
meltwater. The increase of fresh- to brackish water chlorophyta algae
confirms this scenario. The low diversity of microplankton associations
may be indicative of stressed, restricted conditions with often unstable
salinities (in Gorin and Steffen, 1991). Also, sediments deposited under
low-oxygen conditions show reduced cyst diversities and high abundance
of one species (Sluijs, 2006).
The RO-2a subzone is characterized by an increase in species
diversity (6 taxa) and decrease in dinocyst concentrations. The Echinidinium–
Islandinium complex is accompanied by Selenopemphix spp.,
Brigantedinium spp., cf. Pentapharsodinium dalei and Spiniferites spp.
This dinocyst assemblage suggests environmental conditions comparable
to those of subzone RO-2c. The record of copepod egg-envelopes
could reflect geographical variations in the nutrient regime of the
euphotic zone (Van Waveren, 1994).
6.3. Preservation and paleoecology of marine mollusks
The assemblage A is dominated by filter-feeding shallow infaunal
(Venus antiqua) and semi-infaunal (Tawera gayi, Hiatella solida)
burrowers, and vagrante epifaunal elements, with feed on carrion
(i.e., Pareuthria plumbea) or prey upon bivalves (i.e., the predators
Trophon geversianus and Xymenopsis muriciformis).
In the assemblage B, Mytilus chilensis is a suspension feeder that
most probably lived bysally attached to hard substrates in the area. A
great number of these specimens were found articulated, but their
shells exhibit selective dissolution that facilitates its separation in
layers resulting in broken articulated specimens. A less abundant
species, Hiatella solida, is eurytopic (i.e., able to adapt to a wide range
of environmental conditions) living bysally attached as epifauna, or
partially buried as infauna. These shells are found articulated in this
assemblage.
7. Discussion and conclusions
In the hinterland of the Archipiélago Cormoranes area, late Holocene vegetation conforms to the general paleoclimate evolution in southern Tierra del Fuego. The vegetation and climatic setting of southern Tierra del Fuego over the Late Holocene,as expressed by the pollen data (Heusser, 1989a, 1998, 2003; Borromei, 1995; Borromei et al., 2007, this paper), is characterized by the dominance of beech forest under a variable, cooler and more humid climatewith increased storminess and
cloud cover. During the Holocene, a range of shallow benthic paleocommunities occupied the northern coast of the Beagle Channel (Gordillo, 1999; Gordillo et al., 2005). The postglacial mollusks from the Beagle Channel agree in taxa composition and mollusk assemblages with the fauna living today in the region. Thus, climatic conditions maintain stable enough to allow the survival of the same marine
faunistic associations, which have a wide ecological range equivalent to
taxa living today (Gordillo, 1999).
Fig. 5. Most characteristicmollusks and associated fauna collected at the heads of the Río Ovando. A. Bivalves: 1. Venus antiqua King and Broderip, 1832 (L=71mm), left valve, CEGH-UNC
23399. 2. Tawera gayi (Hupé in Gay,1854) (L=35mm), right valve, CEGH-UNC 23401. 3. Aulacomya atra (Molina,1782) (H=62mm), right valve, CEGH-UNC 22674. 4.Mytilus chilensisHupé in
Gay, 1854 (H=32 mm, broken shell, left valve, CEGH-UNC 23404. 5. Hiatella solida (Sowerby, 1834) (L=42 mm), right valve, CEGH-UNC 23408. 6. Carditella naviformis (Reeve, 1843)
(L=4mm), right valve, CEGH-UNC 23296. 7. Cyclocardia compresa Reeve,1843 (L=3mm), left valve, CEGH-UNC 23289. B. Gastropods: 8. Xymenopsis muriciformis (King and Broderip,1832)
(H=20 mm), CEGH-UNC 22714. 9. Pareuthria plumbea (Philippi, 1844) (H=21 mm), CEGH-UNC 23403. 10. Ataxocerithium pullum? (H=4 mm), CEGH-UNC 23307. 11. Onoba sp. (H=3mm),
CEGH-UNC 23305.12. Eatoniella sp. (H=2mm), CEGH-UNC 23304.13. Trophon geversianus (Pallas,1769) (H=58mm), CEGH-UNC 23406. C. Chitons: 14. Plaxiphora aurata (Spalowsky,1795)
(L=5 mm), head valve, CEGH-UNC 23331. 15. Callochiton puniceus (Couthouy MS, Gould, 1846 (L=3 mm), tail valve, CEGH-UNC 23320. 16. Tonicia lebruni de Rochebrune, 1827 (L=6 mm),
intermediate valve, CEGH-UNC 23343.17. Tonicia lebruni de Rochebrune,1827 (L=4mm), head valve, CEGH-UNC 23332. D. Other invertebrates: 18. Cirriped indet. (W=15mm), CEGH-UNC
23402. Dimensions (in mm). L=length; H=height;W=maximum width. SEM photographs (6, 7, 10, 11 and 12).
Most of the present dinocyst taxa have a widespread ecological distribution, but the dinoflagellate cyst assemblages recorded in Río Ovando section (Echinidinium–Islandinium complex, Polykrikos kofoidii, Polykrikos schwartzii, Brigantedinium simplex, Brigantedinium spp.,Selenopemphix quanta, Selenopemphix spp., Operculodinium cf. centrocarpum, cf. Pentapharsodinium dalei and cf. Spiniferites spp.) have a special interest because constitutes the first mention of these
association at high latitudes of the South America. Their composition is comparable to those of modern assemblages from the Laptev Shelf in the eastern Arctic (Kunz-Pirrung, 2001) and from Canadian Arctic Archipelago, including polynyas (Mudie and Rochon, 2001), that is strongly influenced by the freshwater input of the rivers in summer.
The present dinocyst assemblages reflect fjord (estuarine) environments close to terrestrial ice field affected by glacier meltwater dischargewith anomalously lowsalinity in theArchipiélago Cormoranes (Lago Roca–Bahía Lapataia area). These assemblages showvariations in a wide range of temperatures from cold to tropic conditions, so it is difficult determine climatic variability linked to decline of temperature. At the base of the Río Ovando section, the RO-2c subzone is
characterized by the highest species diversity and low dinocyst concentrations. This dinocyst assemblage suggests marginal marine environments with low to moderate salinity and reflects high concentrations of nutrients in the surface waters owing to freshwater input from glacier meltwater. Mollusk data support that during this
interval (under relatively warmer conditions) a major expansion of the fauna took place, and further diversification of mollusk assemblages was characterized by the dominance of venerids and the appearance of other families or groups (e.g., carditids), indicating shift towards present day conditions.
Throughout the Río Ovando section, Nothofagus dombeyi type concentration values show variability (Fig. 4). These forest fluctuations appear to bear a relationship with fluctuations of the main group of dinoflagellate cysts, the Echinidinium–Islandinium complex, as can be seen in the Palynological Subzone RO-2b (Fig. 4; Table 1) after ca. 4160 14C yr B.P. (4736 cal yr B.P.) and before 4064 14C yr B.P. (4540 cal yr B.P.).
We interpret the inverse correlation of Nothofagus concentration and Echinidinium–Islandinium complex concentration as a result of a rapid climatic variability related to changes in temperature and precipitation.
The replacement from a diverse fauna of suspension-feeding mollusks characterized by the presence of venerids and other mollusks -including bivalves, gastropods and chitons (Assemblage A), to an almost monospecific fauna with taxa (i.e., Mytilus and Hiatella) (Assemblage B) tolerant to low or variable salinity suggest a
major seasonal input of freshwater (from rivers discharging into the area and /or the ice melting).
This climatic variability, lasting ca. 100 14C yr B.P., might be correlated with Neoglacial episodes occurred in the southern Patagonia Andes Range (Mercer, 1982). According to Heusser and Streeter (1980) intervals of relatively low temperature appear to have coincided with periods when precipitations were significantly higher
than today during Late Holocene glacier advances (see Rabassa and Clapperton, 1990).
Upward in the profile, between 4064 14C yr B.P. (4540 cal yr B.P.) and 3542 14C yr B.P. (3815 cal yr B.P.), the intervals with low concentration values of trees along with scarce occurrence of marine palynomorphs might be associated with a regressive event.
Evidence for several Neoglacial readvances was observed in the cirques of the hanging lateral valleys in the Eastern Fuegian Andes according to the geomorphological studies, but all of them remain still undated (Rabassa et al., 2000). Nevertheless, the results obtained from pollen (Heusser, 1989a,b, 1998), dendrochronology (Villalba, 1989, 1994) and marine waters isotopic analyses (Obelic et al., 1998) compared with the Vostok ice-core (Jozuel et al., 1987) showed
climatic reversal episodes in the region between 4400 and 3400, 2800–2000, 1800–1400 and 500 14C yr B.P. (Borromei et al., 2007). As pointed previously by Gordillo et al. (2005), most of these species, if not all of them,were able to persist in the area, even during Neoglacial climatic deterioration.
The study of a Holocene marine record in the Archipiélago Cormoranes (Río Ovando site) clearly shows the potential of organic-walled dinoflagellate cysts and mollusk assemblages associated with pollen studies to reconstruct paleoenvironmental conditions during the marine transgression into the Beagle Channel. Further
palynological studies on additional records from other fossil marine sections in Tierra del Fuego will lead to a more comprehensive view of the Holocene paleoclimate development.
Acknowledgements
The first author M.S. Candel thanks Dr. Anne de Vernal and Dr. Taoufik Radi from the Centre de recherché en géochimie et géodynamique (GEOTOP-UQAM, Université du Québec à Montréal)for their help in the systematic identifications of dinoflagellate cyst of
Tierra del Fuego. We are also grateful to Dr. Andrea Coronato (CADIC,Centro Austral de Investigaciones Científicas, Ushuaia, Tierra del Fuego) for field assistance and contributing resources in the field work. This study was supported by CONICET (Consejo Nacional de Investigaciones Científicas y Tecnológicas; PIP 02787/02) and the Agencia Nacional para la Promoción de la Ciencia y Tecnología (PICTRedes
2002-00067), both being Argentine federal government institutions.
The study on mollusks is part of a broader project focused on the Quaternary molluscan faunas from southern South America, between CADIC (Laboratorio de Geología del Cuaternario) and the Centro de Investigaciones Paleobiológicas (CIPAL), Universidad Nacional de Córdoba. The technical assistance of the staff of the
Laboratorio de Microscopía Electrónica y Microanálisis de la Universidad Nacional de San Luis is greatly appreciated.
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