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Mass mortality of a HoloceneTagelus plebeius (Mollusca, Bivalvia) population in the Bahía Blanca Estuary, Argentina
Marine Geology, 106 (1992) 301-308 Elsevier Science Publishers B.V., Amsterdam
301
Mass mortality of a Holocene Tagelusplebeius (Mollusca, Bivalvia) population in the Bahia Blanca Estuary, Argentina Ester A. FarinatP, Salvador Aliotta a'b and Silvia S. Ginsberg b aDepartamento de Geologia, Universidad Nacional del Sur, San Juan 670, 8000 Bahia Blanca, Argentina blnstituto Argentino de Oceanografia, CONICET, Avenida Alem 53, 8000 Bahia Blanca, Argentina (Received May 22, 1991; revision accepted September 9, 1991)
ABSTRACT Farinati, E.A., Aliotta, S. and Ginsberg, S.S., 1992. Mass mortality of a Holocene Tagelus plebeius (Mollusc,a, Bivaivia) population in the Bahia Blanca Estuary, Argentina. Mar. Geol., 106: 301-308. A mass mortality phenomenon affecting a mollusc fauna on the Holocene tidal flat of the Bahia Bianca Estuary is analyzed. Specimens of Tagelusplebeius are located in a well-defined horizon, with both valves articulated and in life position. The mass mortality of the Tagelusplebeius population is a consequence of its catastrophic burial by sandy sediment of fluvial origin.
Introduction Study of organisms and their relationship with the surrounding sediments helps us to determine the geological processes which took place in the areas where they lived. Much research has been carried out on benthic communities in estuaries throughout the world, particularly on Tagelus plebeius, a species that occurs with great frequency in this type of environment. In the coastal zone of the Bahia Blanca Estuary an important fossiliferous level composed mainly of Tagelus plebeius occurs on the banks of the Napost/t Grande Creek about 3 km from the mouth and extends uniformly 800 m along both banks of the creek. Previous works on this area are scarce. Farinati (1983) mentioned the T. plebeius horizon without referring to its origin, and Gonzalez et al. (1983) carried out a study based on isotopic and micropaleontological data, taking into account paleoenvironmental parameters such as salinity and relative temperature. Correspondence to: E.A. Farinati, Departamento de Geologia, Universidad Nacional del Sur, San Juan 670, 8000 Bahia Blanca, Argentina. 0025-3227/92/$05.00
The purpose of this work is to analyze the horizon of Tagelus plebeius and its associated malacofauna and to gain some insight into the geological history of the area and the mechanism responsible for the mass mortality seen in the T. plebeius horizon.
Methods Different types of sediment were recognized on the banks near the mouth of the Napost/t Grande Creek and samples were collected for fossil studies. An undisturbed sample (30 x 20 x 8 cm) containing I". plebeius was taken from the horizon in order to determine the biogenic structures, which were then analyzed by means of X-ray radiography (Siemens equipment) and computer tomography (General Electric 9000 equipment). In order to analyze the base of the exposure, a 30 cm long core was taken and a Helix drill was used to drill down to a depth of 2.55 m.
Bank profile Near its mouth the Napostfi Grande Creek erodes the wide Holocene tidal flat and develops
© 1992 - - Elsevier Science Publishers B.V. All rights reserved
302
E.A. FARINATI ET AL.
banks as high as !.50 m. A sedimentary sequer, ce that is characteristic of the fluvial-coastal area east of Ingeniero White Port occurs (Fig. !). At the top of the exposure is a 0.15 m thick bed made up of light brown fining-upward sand (medium to fine sand). Between 0.15 and 0.27 m the sand has a low percentage of silt and differs from the underlying bed by virtue of its darker brown colour. The bed directly beneath at 0.28 m (Fig. 2) is made up of light brown fining-upward sand (medium to fine sand) with some levels of wavy bedding. In this bed, as in the beds above, no mollusc remains were found, although ostracod taxa of clearly non-marine affinity such as Heterocypris salina, Darwinula sp. and llyocipris gibba were present (Berteis and Martinez, 1990). We assigned a fluvial origin to these sediments. The sandy material rests on light brown sandy silt, with no visible sedimentary structures. The Tagelus plebeius horizon with its articulated valves
and individuals still in life position (Fig. 3) is situated at this last level, between 0.70 and 0.90 m from the surface. Radiocarbon dating carried out by Gonzalez et al. (1983) on T. plebeius valves indicated ages of 3850 and 3560 yrs B.P. Farinati (1985) established a similar approximate age of 3373 yrs B.P. At a depth of 1.60 m from the top of the bank, the sediment changes transitionally to a light brown silty sand, with small subangular pebbles ( < 0.5 cm in diameter) made up of a well-consolidated siltstone rich in calcium carbonate. Between 1.95 and 2.15 m there is a light brown sand (fine to medium) with a low percentage of silt. In this bed, the rate and size of pebbles increases; in some cases they have diameters of 1.5 cm. At approximately 2.15 m (Fig. 2) there is fine sand with some silt, no change of colour with respect to the overlying level, and large amounts of fine particulate calcium carbonate. Grain size increases downwards, and at the limit of the drill
62~20 ' HORIZON ~r
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MASS MORTALITY OF TAGELUSPLEBEIUSIN THE BAHIA BLANCA ESTUARY, ARGENTINA 111
the additional presence of bryozoa, cirripedia, calcareous microfossils such as ostracodes and foraminifera, and remains of Stylatula darwini (Anthozoa, Pennatulacea). The following molluscs were identified:
o
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Nucula semiornata d'Orbigny Brachidontes rodriguezi (d'Orbigny) Phlyctiderma semiaspera (Philippi) Raeta plicatella (Lamarck) Darina solenoides (King) Macoma uruguayensis (Smith) Semele proficua (Pulteney) Tagelus plebeius (Lightfoot) Pitar rostratus (Koch) Sphaenia hatcheri (Pilsbry) Erodona mactroides Daudin Corbula patagonica d ' O r b i g n y Gastropods:
Littoridina australis (d'Orbigny) Natica isabelleana d'Orbigny Buccinanops deformis (King) Oliveila tehuelchana (d'Orbigny)
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.
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Fig. 2. Study profile on left bank Napost/~ Grande Creek.
penetration there is a medium to fine brown sand with small subangular 'caliche' pebbles less than 1 cm in diameter and a few fragments of very small shells. It was impossible to continue drilling beyond about 2.5 m due to the compacted nature of the deeper material. Horizon of
Tagelus plebeius and associated fauna
The faunal association in the horizon studied involves mainly pelecypods and gastropods, with
The paleoecological reconstruction of the fossiliferous horizon is shown in Fig. 4. Most of the organisms in the horizon are infaunal and are thus highly dependent on the sediment/water interface. Some are deposit feeders (Nucula semiornata, Semele proficua), but most are suspension feeders. Among the epifaunal organisms are 8rachidontes rodriguezi, which attaches to the substrate by its byssus, and the gastropods Littoridina australis and Buccinanops deformis. Organisms of this type were found at different levels in the sandy silt bed. All the fauna in the fossiliferous horizon are very well preserved. The most outstanding feature in the faunal association is the presence of Tagelus plebeius in life position (Fig. 3). We also recorded the existence of some loose valves in a horizontal position, and none of these showed any sign of wear through transport. Tagelus plebeius is a bivalve of the family Solecurtidae, whose wide distribution extends from North Carolina (USA), Surinam, Brasil, Uruguay, and down to the San Matias Gulf in Argentina. This bivalve has an elongate shell with rounded ends and its outer surface shows a growth line and a deep pallial sinus. The valves may be
304
E.A. FARINATIET AL.
Fig. 3. Horizon of Tagelus plebeius in life position and articulated valves.
i
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i
1
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4
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iii
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8
6
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Fig. 4. Paleoecologic reconstruction of horizon of Tagelus plebeius and associate fauna. 1 = Brachidontes rodriguezi; 2 = Littoridina australis; 3 = Tagelus plebeius: 4= Raela plicatella: 5= Nucula semiornata: 6= Semele proficua; 7= Pitar rostratus; 8 = Buccinanops deformis.
MASS MORTALITY OF TAGELUS PLEBEIUS IN THE BAHIA BLANCA ESTUARY, ARGENTINA
up to 90 mm long (Holland and Dean, 1977b). On the basis of 110 measured valves it was determined that 93% of them in the studied exposure fell within the 33-55 mm range (Fig. 5). The average length was 46 mm and the standard deviation 8.5 ram. Tagelus plebeius is a euryhaline species which lives in the river mouth in a vertical position with its foot downwards and the long exhalant and inhalant siphons stretching upwards to the sediment/water interface (Fig. 4). These siphons, which reach 30 cm in length (Carcelles, 1944), are extendable structures. The foot is perfectly adapted to excavation. According to Coscaron (1959) T. plebeius replaces Mesodesma mactroides (Deshayes) (the yellow clam), which lives in strictly marine environments, because riverine influences are unfavourable for the latter mollusc. Holland and Dean (1977a,b) established that T. plebeius does not live in unstable, mobile sandy areas where it cannot maintain the integrity of its siphon tubes. Instead T. plebeius lives in tidal flat areas between the mean low tide level and 90 cm above this level, where it inhabits sediments composed of more than 2% silt and clay. This explains why, in the Napost/L Grande Creek, the sediment that contains T. plebeius is a cohesive sandy silt. This sediment is found on the Holocene tidal flat that developed during the transgressive-regressive process which affected the Bahia Blanca area (Aliotta et al., 1987; Aliotta and Farinati, 1990) and which is being eroded by the creek today. It would appear that a mass mortality phenome-
30"
,%
"!
20-
I0-
33
39
45
51
II i
57
I
I
63
69
mm
Length of vatve
Fig. 5. Frequency distribution of the length of Tagelusplebeius, Napost~ Grande Creek.
305
non affecting an entire living population of the old tidal flat took place and was responsible for the sudden death of the organisms.
Mass mortality and its relationship with the geological history of the estuary Brongersma Sanders (1957) defined catastrophes of limited scope that produce mass mortality in the sea and investigated their cause and significance in paleontology. At the sea bottom, near the sediment/water interface, burial of organisms over small areas is a fairly common occurrence, generally taking place over brief periods. This is the cause of mass mortality of the resident populations. However, there are relatively few recognized cases of catastrophic burials in the paleontological literature. For animals as small as T. plebeius, sudden burial under an influx of sediment several centimetres deep is a major catastrophe that is capable of killing them. Not all bivalves are able to escape burial because such ability is related to life habits, size, and the type of sediment in which the organisms are buried (Kranz, 1974). Using sand and silt Shulenberger (1970) has demonstrated the ability of the bivalve Gemma gemma to burrow upwards in response to catastrophic burial. Tagelus plebeius is a deep-burrowing bivalve whose behaviour when faced with sudden burial is changeable. When young, it is a rapid burrower capable of escaping from depths of between 15 and 50 cm. With age, however, it develops a boring behaviour and cannot escape once buried; it therefore survives to maturity only when sedimentation is feeble (Kranz, 1974). Sedimentary processes such as erosion or deposition acting in estuarine environments (tidal flats) may induce imbalance in the ecosystem. The organisms react to the new conditions and try to find their ideal habitat. Thus, where sedimentation is rapid and excessive, they must migrate from their living horizon to new levels near the sediment/ water interface, thus producing escape structures (Reineck, 1958). To study the subsurface behaviour of T. plebeius in the Bahia Blanca Estuary conventional X-ray
306
E.A. FARINATI ET A L
equipment supplemented with computer tomography was used. It was thus possible to analyze the sedimentary structures produced by the organisms and to infer through them the sedimentary processes affecting their habitats.
B I
I
Specimens of T. plebeius in life position and the excavation burrows (a) of two of them are shown in Fig. 6, I. The absence of sedimentary structures in the material is evident. Transverse slices through the core using tom-
A
B
III
.... I V
VI 0
5 cm
O
loan
Fig. 6. X-ray radiography(I) and computed tomography images (II-VI). ,4 and B= right and left limits of the X-ray radiography. For explanation, see text.
MASS MORTALITY OF TAGELUS PLEBEIUS IN THE BAHIA BLANCA ESTUARY, ARGENTINA
puter tomography were carried out to supplement the limited information provided by the X-ray radiography. These slices take the shape of a 10 mm thick plane in which different material shows variations according to its absorption coefficient: the iightest shades correspond to the most dense materiJ. On the right-hand side of Fig. 6, II, two small semicircular areas (b) correspond to the disturbed material of the excavation trace formed by the organisms. These are the same structures as shown in the radiograph, but in a vertieai position. Valves of T. plebeius (c) and the inner part of the organisms (black) without sediment are shown in Fig. 6, III. Below these specimens (Fig. 6, IV and V) is a darker zone (d) which was not observed in the slice (Fig. 6, VI), where the sediment shows almost no disturbance. The biogenic structr~e observed in Fig. 6, IV and V demonstrates that in this area the sediment was disturbed by organisms, probably during a period in which the bivalves inhabited a lower level. This level was later abandoned and escape structures were excavated during migration towards the surface. The structures reflect the reaction of the bivalves to the imbalance produced at the sediment/water interface. The process described above is undoubtedly related to the geological evolution of the estuary. The last marine transgression in the Bahia Blanca Estuary took place during the middle Holocene (Gonzalez et al., 1983). The sandy material in the lower part of the profile pertains to a transgressive phase, and is also widely distributed north of the profile, lying discordantly upon a sandy silt bed rich in calcium carbonate (caliche) (Aliotta and Farinati, 1990) corresponding to the Pampeano Formation (Late Pleistocene). Due to the very slight slope of the flooded coastal area the tidal flat was formed slowly in the littoral zone. These sandy silt sediments formed the habitat of T. plebeius and other associated marine organisms. The deposition of fluvial sand over the sandy silt (i.e. over the habitat of T. plebeius) occurred in the late Holocene. Simultaneously with the marine regression this process was probably responsible for the mass mortality of the T. plebeius
307
population. The velocity of sedimentation must have been very high, and the bivalves were unable to survive in the new environmental conditions. The sandy burial material probably came from the old area of the Napostfi Grande Creek mouth. The fluvial sediment must have been deposited during the regressive period, where the influence of marine dynamics was either very reduced or non-existent, because no marine molluscs have been found.
Conclusions Near the mouth of the Napost/l Grande Creek abundant remains of a Tagelus plebeius may be found. These bivalves are articulated and in life position and represent mass mortality on the Holocene tidal fiat. The phenomenon is due to the sudden catastrophic influx of sandy fluvial sediment, probably from the old mouth of the Napost/L Grande Creek (late Holocene regressive period), that buried the molluscs. The T. plebeius specimens attempted to escape burial--although without success--and moved up their burrows leaving behind a network of small escape structures.
Acknowledgements The authors wish to express their thanks to the Image Diagnosis Service of the Dr. Leonidas Lucero Health Centre, and particularly to Dr. Carlos Bauni for the radiography and computer tomography. Thanks also go to Mr. E. Giolitti for processing the film.
References Aliotta, S. and Farinati, E., 1990. Stratigraphy of Holocene sand-shell ridges in the Bahia Bianca Estuary, Argentina. Mar. Geol., 94: 353-360. Aliotta, S., Farinati, E. and Ginsberg, S., 1987. Un nivel conchilifero en el subsuelo de Ingeniero White, Provincia de Buenos Aires, Argentina. Congr. Geol. Argent., X (Tueuman), Actas, pp. 123-117. Bertels, A. and Martinez, D., 1990. Quaternary ostracodes of continental and transitional littoral-shallowmarine environments. Eur. Ostracodologist'sMeet. (Frankfurt). Brongersma Sanders, M., 1957. Mass mortality in the sea. Geol. Soc. Am. Mere., 67(1): 941-1010.
308
Carceiles, A., 1944. Cat,logo de los moluscos marines de Puerto Quequ~n. Rev. Mus. Plata (NS) Secc. Zool., 3: 233-309. Coscaron, S., 1959. La almeja amarilla de la costa de la Provincia de Buenos Aires. Agro Publ. Tec. 1(3): 1-66. Farinati, E., 1983. Pa:eontologia, Paleoecologia y Paleogeografia de los Sedimentos Marines de los Alrededores de Bahia Blanca. Doct. Thesis, Univ. Nac. del Sur, 179 pp. (Unpubl.). Farinati, E., 1985. Radiocarbon dating of Holocene marine deposits, Bahia Blanca area, Buenos Aires Province, Argentina. In: J. Rabassa (Editor), Quaternary of South America and Antarctic Peninsula. Balkema, Rotterdam, Vol. 3, pp. 197-206. Gonzalez, M., Panarello, H., Marino, H. and Valencio, S., 1983. Niveles marinos del Holoceno en el Estuario de Bahia Blanca (Argentina). Isbtopos estables y microfbsiles calc~treos
E.A. FARINAT! ET AL.
come indicadores paleoambientales. In: Simp. Oscilaciones Niveles Mar Durante el dltimo Hemicicio Deglacial en Argentina (Mar del Plata). Actas, pp. 48-68. Holland, A. and Dean. J., 1977a. The biology of the stout razor clam Tagelus plebeius. 1. Animal-sediment relationships, feeding mechanism and community biology. Chesapeake Sci., 18(1): 58-66. Holland, A. and Dean, J., 1977b. The biology of the stout razor clam Tagelus plebeius. 2. Some aspects of the population dynamics. Chesapeake Sci., 18(2): 188-196. Kranz, P., 1974. The anastrophic burial of bivalves and its paleoecological significance. J. Geol., 82: 237-265. Reineck, H.E., 1958. Wuhlbau-Gefuge in Abhangigkeit yon Sediment Umlagerungen. Senckenbergiana Lethaea, 39: 1-23. Shulenberger, E., 1970. Responses of Gemma gemma to a catastrophic burial. Veliger, 13(2): 163-168.
301
Mass mortality of a Holocene Tagelusplebeius (Mollusca, Bivalvia) population in the Bahia Blanca Estuary, Argentina Ester A. FarinatP, Salvador Aliotta a'b and Silvia S. Ginsberg b aDepartamento de Geologia, Universidad Nacional del Sur, San Juan 670, 8000 Bahia Blanca, Argentina blnstituto Argentino de Oceanografia, CONICET, Avenida Alem 53, 8000 Bahia Blanca, Argentina (Received May 22, 1991; revision accepted September 9, 1991)
ABSTRACT Farinati, E.A., Aliotta, S. and Ginsberg, S.S., 1992. Mass mortality of a Holocene Tagelus plebeius (Mollusc,a, Bivaivia) population in the Bahia Blanca Estuary, Argentina. Mar. Geol., 106: 301-308. A mass mortality phenomenon affecting a mollusc fauna on the Holocene tidal flat of the Bahia Bianca Estuary is analyzed. Specimens of Tagelusplebeius are located in a well-defined horizon, with both valves articulated and in life position. The mass mortality of the Tagelusplebeius population is a consequence of its catastrophic burial by sandy sediment of fluvial origin.
Introduction Study of organisms and their relationship with the surrounding sediments helps us to determine the geological processes which took place in the areas where they lived. Much research has been carried out on benthic communities in estuaries throughout the world, particularly on Tagelus plebeius, a species that occurs with great frequency in this type of environment. In the coastal zone of the Bahia Blanca Estuary an important fossiliferous level composed mainly of Tagelus plebeius occurs on the banks of the Napost/t Grande Creek about 3 km from the mouth and extends uniformly 800 m along both banks of the creek. Previous works on this area are scarce. Farinati (1983) mentioned the T. plebeius horizon without referring to its origin, and Gonzalez et al. (1983) carried out a study based on isotopic and micropaleontological data, taking into account paleoenvironmental parameters such as salinity and relative temperature. Correspondence to: E.A. Farinati, Departamento de Geologia, Universidad Nacional del Sur, San Juan 670, 8000 Bahia Blanca, Argentina. 0025-3227/92/$05.00
The purpose of this work is to analyze the horizon of Tagelus plebeius and its associated malacofauna and to gain some insight into the geological history of the area and the mechanism responsible for the mass mortality seen in the T. plebeius horizon.
Methods Different types of sediment were recognized on the banks near the mouth of the Napost/t Grande Creek and samples were collected for fossil studies. An undisturbed sample (30 x 20 x 8 cm) containing I". plebeius was taken from the horizon in order to determine the biogenic structures, which were then analyzed by means of X-ray radiography (Siemens equipment) and computer tomography (General Electric 9000 equipment). In order to analyze the base of the exposure, a 30 cm long core was taken and a Helix drill was used to drill down to a depth of 2.55 m.
Bank profile Near its mouth the Napostfi Grande Creek erodes the wide Holocene tidal flat and develops
© 1992 - - Elsevier Science Publishers B.V. All rights reserved
302
E.A. FARINATI ET AL.
banks as high as !.50 m. A sedimentary sequer, ce that is characteristic of the fluvial-coastal area east of Ingeniero White Port occurs (Fig. !). At the top of the exposure is a 0.15 m thick bed made up of light brown fining-upward sand (medium to fine sand). Between 0.15 and 0.27 m the sand has a low percentage of silt and differs from the underlying bed by virtue of its darker brown colour. The bed directly beneath at 0.28 m (Fig. 2) is made up of light brown fining-upward sand (medium to fine sand) with some levels of wavy bedding. In this bed, as in the beds above, no mollusc remains were found, although ostracod taxa of clearly non-marine affinity such as Heterocypris salina, Darwinula sp. and llyocipris gibba were present (Berteis and Martinez, 1990). We assigned a fluvial origin to these sediments. The sandy material rests on light brown sandy silt, with no visible sedimentary structures. The Tagelus plebeius horizon with its articulated valves
and individuals still in life position (Fig. 3) is situated at this last level, between 0.70 and 0.90 m from the surface. Radiocarbon dating carried out by Gonzalez et al. (1983) on T. plebeius valves indicated ages of 3850 and 3560 yrs B.P. Farinati (1985) established a similar approximate age of 3373 yrs B.P. At a depth of 1.60 m from the top of the bank, the sediment changes transitionally to a light brown silty sand, with small subangular pebbles ( < 0.5 cm in diameter) made up of a well-consolidated siltstone rich in calcium carbonate. Between 1.95 and 2.15 m there is a light brown sand (fine to medium) with a low percentage of silt. In this bed, the rate and size of pebbles increases; in some cases they have diameters of 1.5 cm. At approximately 2.15 m (Fig. 2) there is fine sand with some silt, no change of colour with respect to the overlying level, and large amounts of fine particulate calcium carbonate. Grain size increases downwards, and at the limit of the drill
62~20 ' HORIZON ~r
f
02ol "r. PLEBEIUS
OF
l ,=
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IA BLAN(
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:'.'i °- • • °• "
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PROFILE
•
• °"
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o
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. . . D
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°
Fig. 1, Location o f the study site.
.
•
.
.
•
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2-'m
MASS MORTALITY OF TAGELUSPLEBEIUSIN THE BAHIA BLANCA ESTUARY, ARGENTINA 111
the additional presence of bryozoa, cirripedia, calcareous microfossils such as ostracodes and foraminifera, and remains of Stylatula darwini (Anthozoa, Pennatulacea). The following molluscs were identified:
o
sand
.?. :..::?-:?-j
sand with silt ~.. ; .'..,.
303
-.
i ."?."~-:."
Peleeyimds: 0,5 ¸
t
decreasing grainsize
O Tagelus plebeius l 1,0"
~l
sandy silt
u ndlefurbed sample
~e pebbles
Nucula semiornata d'Orbigny Brachidontes rodriguezi (d'Orbigny) Phlyctiderma semiaspera (Philippi) Raeta plicatella (Lamarck) Darina solenoides (King) Macoma uruguayensis (Smith) Semele proficua (Pulteney) Tagelus plebeius (Lightfoot) Pitar rostratus (Koch) Sphaenia hatcheri (Pilsbry) Erodona mactroides Daudin Corbula patagonica d ' O r b i g n y Gastropods:
Littoridina australis (d'Orbigny) Natica isabelleana d'Orbigny Buccinanops deformis (King) Oliveila tehuelchana (d'Orbigny)
1,5, • •
.
.
°
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•
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.
°
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Fig. 2. Study profile on left bank Napost/~ Grande Creek.
penetration there is a medium to fine brown sand with small subangular 'caliche' pebbles less than 1 cm in diameter and a few fragments of very small shells. It was impossible to continue drilling beyond about 2.5 m due to the compacted nature of the deeper material. Horizon of
Tagelus plebeius and associated fauna
The faunal association in the horizon studied involves mainly pelecypods and gastropods, with
The paleoecological reconstruction of the fossiliferous horizon is shown in Fig. 4. Most of the organisms in the horizon are infaunal and are thus highly dependent on the sediment/water interface. Some are deposit feeders (Nucula semiornata, Semele proficua), but most are suspension feeders. Among the epifaunal organisms are 8rachidontes rodriguezi, which attaches to the substrate by its byssus, and the gastropods Littoridina australis and Buccinanops deformis. Organisms of this type were found at different levels in the sandy silt bed. All the fauna in the fossiliferous horizon are very well preserved. The most outstanding feature in the faunal association is the presence of Tagelus plebeius in life position (Fig. 3). We also recorded the existence of some loose valves in a horizontal position, and none of these showed any sign of wear through transport. Tagelus plebeius is a bivalve of the family Solecurtidae, whose wide distribution extends from North Carolina (USA), Surinam, Brasil, Uruguay, and down to the San Matias Gulf in Argentina. This bivalve has an elongate shell with rounded ends and its outer surface shows a growth line and a deep pallial sinus. The valves may be
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Fig. 3. Horizon of Tagelus plebeius in life position and articulated valves.
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Fig. 4. Paleoecologic reconstruction of horizon of Tagelus plebeius and associate fauna. 1 = Brachidontes rodriguezi; 2 = Littoridina australis; 3 = Tagelus plebeius: 4= Raela plicatella: 5= Nucula semiornata: 6= Semele proficua; 7= Pitar rostratus; 8 = Buccinanops deformis.
MASS MORTALITY OF TAGELUS PLEBEIUS IN THE BAHIA BLANCA ESTUARY, ARGENTINA
up to 90 mm long (Holland and Dean, 1977b). On the basis of 110 measured valves it was determined that 93% of them in the studied exposure fell within the 33-55 mm range (Fig. 5). The average length was 46 mm and the standard deviation 8.5 ram. Tagelus plebeius is a euryhaline species which lives in the river mouth in a vertical position with its foot downwards and the long exhalant and inhalant siphons stretching upwards to the sediment/water interface (Fig. 4). These siphons, which reach 30 cm in length (Carcelles, 1944), are extendable structures. The foot is perfectly adapted to excavation. According to Coscaron (1959) T. plebeius replaces Mesodesma mactroides (Deshayes) (the yellow clam), which lives in strictly marine environments, because riverine influences are unfavourable for the latter mollusc. Holland and Dean (1977a,b) established that T. plebeius does not live in unstable, mobile sandy areas where it cannot maintain the integrity of its siphon tubes. Instead T. plebeius lives in tidal flat areas between the mean low tide level and 90 cm above this level, where it inhabits sediments composed of more than 2% silt and clay. This explains why, in the Napost/L Grande Creek, the sediment that contains T. plebeius is a cohesive sandy silt. This sediment is found on the Holocene tidal flat that developed during the transgressive-regressive process which affected the Bahia Blanca area (Aliotta et al., 1987; Aliotta and Farinati, 1990) and which is being eroded by the creek today. It would appear that a mass mortality phenome-
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Fig. 5. Frequency distribution of the length of Tagelusplebeius, Napost~ Grande Creek.
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non affecting an entire living population of the old tidal flat took place and was responsible for the sudden death of the organisms.
Mass mortality and its relationship with the geological history of the estuary Brongersma Sanders (1957) defined catastrophes of limited scope that produce mass mortality in the sea and investigated their cause and significance in paleontology. At the sea bottom, near the sediment/water interface, burial of organisms over small areas is a fairly common occurrence, generally taking place over brief periods. This is the cause of mass mortality of the resident populations. However, there are relatively few recognized cases of catastrophic burials in the paleontological literature. For animals as small as T. plebeius, sudden burial under an influx of sediment several centimetres deep is a major catastrophe that is capable of killing them. Not all bivalves are able to escape burial because such ability is related to life habits, size, and the type of sediment in which the organisms are buried (Kranz, 1974). Using sand and silt Shulenberger (1970) has demonstrated the ability of the bivalve Gemma gemma to burrow upwards in response to catastrophic burial. Tagelus plebeius is a deep-burrowing bivalve whose behaviour when faced with sudden burial is changeable. When young, it is a rapid burrower capable of escaping from depths of between 15 and 50 cm. With age, however, it develops a boring behaviour and cannot escape once buried; it therefore survives to maturity only when sedimentation is feeble (Kranz, 1974). Sedimentary processes such as erosion or deposition acting in estuarine environments (tidal flats) may induce imbalance in the ecosystem. The organisms react to the new conditions and try to find their ideal habitat. Thus, where sedimentation is rapid and excessive, they must migrate from their living horizon to new levels near the sediment/ water interface, thus producing escape structures (Reineck, 1958). To study the subsurface behaviour of T. plebeius in the Bahia Blanca Estuary conventional X-ray
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equipment supplemented with computer tomography was used. It was thus possible to analyze the sedimentary structures produced by the organisms and to infer through them the sedimentary processes affecting their habitats.
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Specimens of T. plebeius in life position and the excavation burrows (a) of two of them are shown in Fig. 6, I. The absence of sedimentary structures in the material is evident. Transverse slices through the core using tom-
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Fig. 6. X-ray radiography(I) and computed tomography images (II-VI). ,4 and B= right and left limits of the X-ray radiography. For explanation, see text.
MASS MORTALITY OF TAGELUS PLEBEIUS IN THE BAHIA BLANCA ESTUARY, ARGENTINA
puter tomography were carried out to supplement the limited information provided by the X-ray radiography. These slices take the shape of a 10 mm thick plane in which different material shows variations according to its absorption coefficient: the iightest shades correspond to the most dense materiJ. On the right-hand side of Fig. 6, II, two small semicircular areas (b) correspond to the disturbed material of the excavation trace formed by the organisms. These are the same structures as shown in the radiograph, but in a vertieai position. Valves of T. plebeius (c) and the inner part of the organisms (black) without sediment are shown in Fig. 6, III. Below these specimens (Fig. 6, IV and V) is a darker zone (d) which was not observed in the slice (Fig. 6, VI), where the sediment shows almost no disturbance. The biogenic structr~e observed in Fig. 6, IV and V demonstrates that in this area the sediment was disturbed by organisms, probably during a period in which the bivalves inhabited a lower level. This level was later abandoned and escape structures were excavated during migration towards the surface. The structures reflect the reaction of the bivalves to the imbalance produced at the sediment/water interface. The process described above is undoubtedly related to the geological evolution of the estuary. The last marine transgression in the Bahia Blanca Estuary took place during the middle Holocene (Gonzalez et al., 1983). The sandy material in the lower part of the profile pertains to a transgressive phase, and is also widely distributed north of the profile, lying discordantly upon a sandy silt bed rich in calcium carbonate (caliche) (Aliotta and Farinati, 1990) corresponding to the Pampeano Formation (Late Pleistocene). Due to the very slight slope of the flooded coastal area the tidal flat was formed slowly in the littoral zone. These sandy silt sediments formed the habitat of T. plebeius and other associated marine organisms. The deposition of fluvial sand over the sandy silt (i.e. over the habitat of T. plebeius) occurred in the late Holocene. Simultaneously with the marine regression this process was probably responsible for the mass mortality of the T. plebeius
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population. The velocity of sedimentation must have been very high, and the bivalves were unable to survive in the new environmental conditions. The sandy burial material probably came from the old area of the Napostfi Grande Creek mouth. The fluvial sediment must have been deposited during the regressive period, where the influence of marine dynamics was either very reduced or non-existent, because no marine molluscs have been found.
Conclusions Near the mouth of the Napost/l Grande Creek abundant remains of a Tagelus plebeius may be found. These bivalves are articulated and in life position and represent mass mortality on the Holocene tidal fiat. The phenomenon is due to the sudden catastrophic influx of sandy fluvial sediment, probably from the old mouth of the Napost/L Grande Creek (late Holocene regressive period), that buried the molluscs. The T. plebeius specimens attempted to escape burial--although without success--and moved up their burrows leaving behind a network of small escape structures.
Acknowledgements The authors wish to express their thanks to the Image Diagnosis Service of the Dr. Leonidas Lucero Health Centre, and particularly to Dr. Carlos Bauni for the radiography and computer tomography. Thanks also go to Mr. E. Giolitti for processing the film.
References Aliotta, S. and Farinati, E., 1990. Stratigraphy of Holocene sand-shell ridges in the Bahia Bianca Estuary, Argentina. Mar. Geol., 94: 353-360. Aliotta, S., Farinati, E. and Ginsberg, S., 1987. Un nivel conchilifero en el subsuelo de Ingeniero White, Provincia de Buenos Aires, Argentina. Congr. Geol. Argent., X (Tueuman), Actas, pp. 123-117. Bertels, A. and Martinez, D., 1990. Quaternary ostracodes of continental and transitional littoral-shallowmarine environments. Eur. Ostracodologist'sMeet. (Frankfurt). Brongersma Sanders, M., 1957. Mass mortality in the sea. Geol. Soc. Am. Mere., 67(1): 941-1010.
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Carceiles, A., 1944. Cat,logo de los moluscos marines de Puerto Quequ~n. Rev. Mus. Plata (NS) Secc. Zool., 3: 233-309. Coscaron, S., 1959. La almeja amarilla de la costa de la Provincia de Buenos Aires. Agro Publ. Tec. 1(3): 1-66. Farinati, E., 1983. Pa:eontologia, Paleoecologia y Paleogeografia de los Sedimentos Marines de los Alrededores de Bahia Blanca. Doct. Thesis, Univ. Nac. del Sur, 179 pp. (Unpubl.). Farinati, E., 1985. Radiocarbon dating of Holocene marine deposits, Bahia Blanca area, Buenos Aires Province, Argentina. In: J. Rabassa (Editor), Quaternary of South America and Antarctic Peninsula. Balkema, Rotterdam, Vol. 3, pp. 197-206. Gonzalez, M., Panarello, H., Marino, H. and Valencio, S., 1983. Niveles marinos del Holoceno en el Estuario de Bahia Blanca (Argentina). Isbtopos estables y microfbsiles calc~treos
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come indicadores paleoambientales. In: Simp. Oscilaciones Niveles Mar Durante el dltimo Hemicicio Deglacial en Argentina (Mar del Plata). Actas, pp. 48-68. Holland, A. and Dean. J., 1977a. The biology of the stout razor clam Tagelus plebeius. 1. Animal-sediment relationships, feeding mechanism and community biology. Chesapeake Sci., 18(1): 58-66. Holland, A. and Dean, J., 1977b. The biology of the stout razor clam Tagelus plebeius. 2. Some aspects of the population dynamics. Chesapeake Sci., 18(2): 188-196. Kranz, P., 1974. The anastrophic burial of bivalves and its paleoecological significance. J. Geol., 82: 237-265. Reineck, H.E., 1958. Wuhlbau-Gefuge in Abhangigkeit yon Sediment Umlagerungen. Senckenbergiana Lethaea, 39: 1-23. Shulenberger, E., 1970. Responses of Gemma gemma to a catastrophic burial. Veliger, 13(2): 163-168.