Foraminíferos modernos de ambientes costeros en el norte de Argentina: implicaciones para la interpretación paleoambiental de depósitos litorales de Holoceno medio

Cecilia Laprida^1,*, Damián Diego Enrique Chandler^1,2, Josefina Ramón Mercau¹, Rubén Alvaro López¹ y Silvia Marcomini¹

¹ Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina.

² Consejo Nacional de Investigaciones Científicas y Técnicas, Rivadavia 1917, Ciudad Autónoma de Buenos Aires, Argentina, *chechu@gl.fcen.uba.ar

ABSTRACT

In Buenos Aires coasts, sedimentary processes were particularly active during the Quaternary owing to eustatic fluctuations in sea level. As a result, during the late Quaternary transgressions, marine and marginal marine sediments were deposited in the coastal plain. In order to interpret these Holocene littoral sequences, we analyzed the distribution, diversity, species composition and taphonomic features of total benthic foraminifera assemblages from modern littoral settings, from the top of the dune to the lower shoreface, in two close but geomorphologically different transects located in the Atlantic coast of Northern Buenos Aires Province (Argentina, South America). Total benthic foraminiferal assemblages from subtidal and supratidal environments are distinguishable in terms of composition, diversity and taphonomic features. In upper shoreface, foreshore, backshore and foredune environments, assemblages are clearly dominated by three species: Buccella peruviana, Ammonia beccarii and Elphidium discoidale (the BAE group). This feature is the result of taphonomic processes that favor the selective preservation of such species. The study of taphonomic modifications of shells in modern assemblages allows a better discrimination between subenvironments than the analysis of taxonomic composition. Although Holocene assemblages have no strict counterparts between total modern assemblages, taxonomic composition and taphonomic modification of shells allow us to infer that the Holocene sequence was deposited between the upper shoreface and the backshore.

Key words: benthic foraminifera, modern beaches, paleoenvironmental reconstruction, Holocene, Argentina.

RESUMEN

En las costas de la provincia de Buenos Aires, los procesos sedimentarios fueron especialmente activos durante el Cuaternario, debido a las fluctuaciones eustáticas en el nivel del mar. Como resultado, durante las transgresiones del Cuaternario tardío se depositaron en la planicie costera sedimentos marinos y marino–marginales. Con el objetivo de interpretar estas secuencias litorales holocenas, se analizó la distribución, diversidad, composición específica y características tafonómicas de las asociaciones totales de foraminíferos bentónicos en ambientes litorales actuales, desde el tope de la duna hasta la playa, en dos transectas cercanas pero geomorfológicamente diferentes, ubicadas en las costas atlánticas del norte de la Provincia de Buenos Aires (Argentina, Sudamérica). Las asociaciones totales de ambientes submareales y supramareales son diferenciables en términos de composición específica, diversidad y características tafonómicas. En los ambientes de playa distal, playa frontal, cara de playa y duna, las asociaciones están dominadas por tres especies: Buccella peruviana, Ammonia beccarii y Elphidium discoidale (grupo BAE). Esta dominancia es el resultado de procesos tafonómicos que favorecen la preservación selectiva de dichas especies. Las características tafonómicas de las asociaciones modernas permiten una mejor discriminación de los subambientes que la composición taxonómica. Si bien las asociaciones holocenas no poseen homólogos estrictos entre las asociaciones totales de ambientes actuales, la composición taxonómica y los rasgos tafonómicos de las asociaciones permiten inferir que la sucesión holocena se depositó entre la parte superior de la anteplaya y la playa distal.

INTRODUCTION

In the low–relief northern Buenos Aires coasts (Argentina, southwestern South Atlantic), sedimentary processes were particularly active during the Quaternary owing to eustatic fluctuations in sea level. As a result, marine and marginal marine sediments were deposited in the coastal plain especially during the late Quaternary transgressions. In the coastal plain of the low, gently sloping Samborombón Bay area, the Holocene transgression is well preserved. There, the postglacial sea reached its maximum level at +4–6 m above sea level (asl) at around 6000 year BP, and middle to late Holocene coastal sedimentation was particularly effective. The area comprises prograding and regressive marine sequences deposited in shallow environments corresponding to barrier islands, salt marshes and beaches (Codignotto and Aguirre, 1993; Aguirre and Whatley, 1995). In central Samborombón Bay, Holocene barrier island complexes are represented mainly by sandy and shelly deposits up to +4.5–5 m asl. These barrier island complexes include barrier islands and beach ridges, and constitute the Cerro de la Gloria Member of the Las Escobas Formation. Detailed sedimentological and paleoecological analyses have led some authors to postulate that the Cerro de la Gloria Member was deposited in a shallow infralittoral to intertidal high–energy, soft–bottom environment (Spalletti et al., 1987; Codignotto and Aguirre, 1993). Even when it is clear that these deposits were originated under a combination of waves, coastal currents and tides (Codignotto and Aguirre, 1993), the identity of the driving process and its precise location within the beach are still doubtful. Did the deposits form as shoals or longshore bars? Did they form as beach berms originated by swash during moderate (non–storm) wave conditions in intertidal settings? Were they originated in the emerged beach during extraordinary stormy conditions?

The objective of this study is to provide further insight into the origin of the Holocene shelly beach ridges present in central Samborombón Bay, northern Buenos Aires coastal margin, by considering micropaleontological and taphonomic data, namely the species composition and preservation state of benthic foraminifer fossil assemblages. The foraminifera have the potential to be used as proxies of environmental factors and therefore are useful in paleoenvironmental reconstructions including Holocene coastal sand bodies (Rine et al., 1991; Murray–Wallace et al., 1999; Hippensteel and Martin, 1999). However, their proxy value depends on the extent to which their modern ecological relationships can be extrapolated to interpret the fossil record in a particular area (Diz and Frances, 2009). The occurrence of benthic foraminifera in Holocene marine sediments of the northern Buenos Aires coastal plain is relatively well documented (Laprida, 1997, 1999; Laprida and Bertels–Psotka, 2003), but modern littoral assemblages have received little attention. The most detailed studies published to date are those of Boltovskoy (1970, 1976), which deal with distribution of littoral foraminifera in Agentina, Ururguay and Southern Brazil, but the author fails to take quantitative analysis of the microfauna into account.

Accurate application of foraminifera to resolve paleoenvironmental problems requires a taphonomic evaluation of fossil assemblages (Barbieri, 1996; Goldstein and Watkins, 1999) since biostratinomic/taphonomic data reflect conditions of depositional environments, representing a valuable additional source of information on coastal dynamics (Brandt, 1989; Laprida and Bertels–Psotka, 2003). Taphonomic signatures (sensu Fürsich and Oschmann, 1993) may vary in a predictable fashion along an environmental gradient (e.g., shallow to deep water) and hence taphonomic data can be powerful tools in paleoenvironmental studies especially when sedimentologic data are lacking or dubious.

In this study we characterize total (live + dead) assemblages recovered from superficial sediments of selected environments of northern Buenos Aires Atlantic littoral from both a taxonomic and a taphonomic point of view, and we compare them with fossil assemblages recovered from a mid–Holocene shelly beach ridge belonging to the Cerro de la Gloria Member of Las Escobas Formation, southern Samborombón Bay. Total populations present a more homogeneous spatial distribution compared to living ones as a consequence of lateral and vertical mixing of tests by biostratinomic processes (Scott and Medioli, 1980; Scott et al., 2001), and hence they are more adequate to compare with fossil assemblages. Consequently, the present study provides insight not only into the present distribution of total littoral foraminiferal assemblages of northeastern Buenos Aires Province but also into the origin and evolution of mid–Holocene beach ridges of the bonaerensian coastal plain.

STUDY AREA

The study area is located along the Atlantic Ocean, in the eastern coastal plain of northeastern Buenos Aires province, Argentina (Figure 1). This coastal area is an accresive coast characterized by superficial mid–Holocene marine sediments overlying Pliocene to late Pleistocene "pampean sediments" (Fidalgo et al., 1975). In the Samborombón Bay area, mid–Holocene sediments are represented by the Las Escobas Formation (Fidago et al., 1975), which includes two members: Cerro de la Gloria and Canal 18. The Cerro de la Gloria Member comprises a well–stratified succession of white, fine to coarse–grained shelly gravels and light brown, fine–grained siliciclastic sands (Spalletti et al., 1987). The gravel fraction is mainly composed of unarticulated and broken bivalve shells. Calcareous microfossils, especially benthic foraminifera and ostracods, are abundant in both the shelly gravel and siliciclastic sand beds (Laprida, 1997; Laprida and Bertels–Psotka, 2003). These deposits were interpreted as beach ridges and barrier islands related to high energy coastal environments (Tonni and Fidalgo, 1978; Bertels–Psotka and Laprida, 1998). To the south, between Punta Rasa and Punta Médanos, mid–Holocene coastal marine sediments are represented by the Mar de Ajó Member of the Pozo N° 17 Formation (Parker, 1979).

The modern littoral area adjacent to the Holocene deposits corresponds to tidal flats in the northern Samborombón Bay area –between Punta Piedras and Punta Rasa– and to sandy beaches of the Argentine Sea, south of Punta Rasa. Buenos Aires coasts between Punta Rasa and Punta Médanos (Figure 1) are exposed to wave action, and they are storm and wave dominated. It is an open coast, straight, with North–South shoreline orientation. Prevailing directions of incident waves are from the S and SSE, producing a northward littoral drift current. The tidal regime is semidiurnal with diurnal inequalities with mean amplitude ranging from 1.37 m (spring tides) to 0.78 m (neap tides) (Servicio de Hidrografía Naval, 2001). Extreme values range from 0 to 2.40 m for high tides and from 0.40 to 1.60 m for low tides (Perillo, 1979). When occurring jointly with spring tides, storms reach the dune base and cause erosional escarpments. The beach varies in width from 40 to 140 m and has a mean slope of 2°. The beach corresponds to the intermediate type with bars and channels (Sunamura, 1988), where stable and non stable (seasonal) berms were observed. Swash bars are frequently observed at the foreshore, and range from 15 to 25 m in width and 0.25 to 0.50 m in height. Two breaker lines can be distinguished next to the shore, between 40 and 100 m and between 140 and 160 m from the mean tide line, respectively. The mean wave height is 0.70 m, while the maximum wave height is 2 m. The mean wave period is 8.4 seconds (Speroni et al., 1999).

MATERIALS AND METHODS

For modern foraminiferal analysis, twenty six superficial sediment samples were collected from two different littoral localities of Mar de Ajó, Municipality of La Costa (Figure 1), during (austral) summer of 2002 and 2007. Samples were taken along measured west–east transects perpendicular to the coastline and spaced about 2 km, at 36°43'44.1"S – 56 °40'24.4"W (Southern transect, 12 samples) and at 36°42'37.4"S – 56°40'30.7"W (Northern transect, 14 samples). Transects were defined from the top of the dune in the foredune to the lower shoreface at around –10 m depth. Samples were collected manually while walking and wading along the intertidal zone and the backshore. A Van Veen grab sampler was used to recover sediments at the sediment/water interface in subtidal settings. From each sample, only the uppermost layer of the sediment (about 5–10 cm, according to Denne and Sen Gupta, 1989) was scraped off and kept in alcohol 70 %. A solution of 2 ppm Rose Bengal in ethanol was used for staining live specimens.

The northern transect (Table 1) lacks seasonal berms, but a stable berm approximately 30 m in width and 0.25 m in height is observed. The beach width varies between 90 and 140 m and the average grain size is medium to fine sand, while in the intertidal sector the average is medium to coarse sand. In the southern transect (Table 1) no stable or seasonal berms are observed. The beach width varies between 55 and 65 m and the average grain size is medium to fine sand.

For Holocene foraminiferal analysis, twenty nine sediments samples were collected from an outcrop, an artificial quarry of 3.90 m located at the left margin of the provincial Route N° 11, in the milestone numbered km 187 (Profile KM187). The outcrop belongs to the Cerro de la Gloria Member of Las Escobas Formation (Fidalgo et al., 1975).Twenty nine samples of 500 g were taken from each identified bed (Figure 2) irrespective of its grain size, geometry and internal structure. KM187 profile comprises a well stratified succession of bioclastic deposits with subordinated light brown siliciclastic sands, which are well defined by sharp changes in grain size. Bioclasts are mainly composed of unarticulated and fragmented shells of bivalves and pebbles of caliche. Subhorizontal stratification is dominant in shelly gravels, whereas hummocky and trough stratification are also present, the latter as isolated sets intercalated in the subhorizontal stratificated facies.

Abundance of tests of modern and fossil assemblages was calculated from the extrapolation of the number of individuals picked out of each sample to 10 g of dry, untreated sediment. Multivariate statistical analyses were applied to both sets of samples using PAST software (version 1.71) (Hammer et al., 2001) in order to identify patterns and correlations among the different parameters measured (species composition, taphonomical modifications of tests, water depth and grain size). Rare species that occur at <1% of the whole assemblage were excluded from the statistical analysis. Sample KM187–1, which only yielded two individuals of Buccella peruviana (D'Orbigny, 1839a), was excluded from the database because of its little reliability. An unconstrained cluster analysis based on the unweighted pair–group average algorithm and the Bray–Curtis similarity index was performed on the taxonomic composition (species relative contribution) of modern and fossil assemblages considered jointly. Likewise, an unconstrained cluster analysis based on the unweighted pair–group average algorithm and the Euclidean distance similarity index was performed on the taphonomic signatures of the whole set of samples. Clusters were defined by the branches cut by a line drawn perpendicularly to the mid–length of the longest distance between successive nodes of the dendogram. When a node fell very close to this line, it was redrawn so as to include both groups, since in view of our objectives and the nature of this investigation underestimation due to the loss of information was deemed worse than overestimation due to acceptance of a somewhat feeble but, in our understanding, meaningful grouping. A total of six linear discriminant analysis (LDA) were performed on both sets of recent samples, in order to evaluate whether subtidal and supratidal beach samples, and northern and southern transect samples, could be distinguished from each other with respect to their species composition, taphonomic signatures and grain size. A leave–one–out validation was performed on each analysis by excluding each sample in turn from the calculation of the discriminant function and classifying with the resulting algorithm. Finally, the correlation between grain size, preservational state and species composition in each transect was assessed by means of the Mantel test in order to evaluate the possibility that both sites were undergoing different processes. The species composition similarity matrices were computed with the Bray–Curtis index, while the others were calculated using the Euclidean distance as a similarity index. Since three Mantel tests were performed on each set of samples, a Bonferroni correction was applied to the significance level, so that the p–value of each test was compared with an adjusted alpha α_ad = α / 3 = 0.017. PAST software was also used to calculate ecological parameters such as species number (S) and Shannon–Weaver diversity index (H) in both modern and fossil assemblages.

Samples were named according to: NST (Northern Submerged Transect) for submerged (subtidal) sites of the Northern transect; NET (Northern Emerged Transect) for emerged (intertidal and supratidal) sites of the Northern transect; SST (Southern Submerged Transect) for submerged (subtidal) sites of the Southern transect; SET (Southern Emerged Transect) for emerged (intertidal and supratidal) sites of Southern transect; and KM187 for KM187 profile. Ordinals indicate relative position in each transect and stratigraphical level in the Mid–Holocene profile.

RESULTS

Foraminifera from modern environments

All samples bore benthic foraminifera, even those of the foredune (base and top of the dune). A total of 78 foraminifer species and 13 genera left in open nomenclature were identified, of which only 13 species showed a relative abundance higher than 1% (Table 2). The number of tests in 10 g of sediment varied from about 95 to 3,559, the number of species in each sample varied from 5 to 38 (Table 3), and the Shannon–Weaver diversity index, H(S), varied between 2.67 and 0.85. Assemblages in the Northern transect tend to be more diverse and abundant than in the Southern transect. As Boltovskoy (1970) stated previously for littoral settings, very few living specimens were present and it was impossible to use them for statistical analysis. Thus, the following study is based on total (live + dead) assemblages. Hyaline calcareous species are dominant. Even though the majority of tests are moderately well preserved, some showed different types and degrees of alteration. In some samples several unidentifiable quinqueloculinid miliolids were mostly broken or fragmented, and hence were lumped in Quinqueloculina spp.

Modern littoral samples are dominated by a few prolific species of benthic foraminifera, particularly Buccella peruviana, Quinqueloculina seminula (Linné, 1758), Ammonia beccarii (D'Orbigny, 1839a), Discorbis williamsoni (Chapman and Parr, in Parr, 1932), and Elphidium discoidale (D'Orbigny, 1839a). Species that are less common, but are nevertheless consistent elements of the fauna, include Quinqueloculina patagonica D'Orbigny, 1839b, Textularia gramen D'Orbigny, 1846, Elphidium gunteri Cole, 1931, Quinqueloculina milleti (Wiesner, 1912), Quinqueloculina sp. cf. Q. implexa Terquem, in Terquem and Terquem, 1886, Trochammina ochracea (Williamson, 1858), Miliolinella subrotunda (Montagu, 1803) and Cibicidoides sp. cf. C. fletcheri (Galloway and Wissler, 1927). Bolivina striatula Cushman, 1922 is locally abundant in the Southern transect in samples adjacent to the shore and confined to subtidal settings.

Table 3 shows the data obtained for the Norhern transect. The benthic foraminiferal number (specimens/10 g dry sediment), species number and the Shannon–Weaver diversity index H(S) are related with water depth and decrease onshore. Lower number of specimens, species and diversity occur at the backshore and the foredune. Considering the total number of species and the total assemblages identified in the Northern transect, only Buccella peruviana, Ammonia becarii and Elphidium discoidale (the BAE group) can be considered as widely distributed along the transect, with Buccella peruviana being by far the dominant species (relative contribution between 63–36 %). The only exception is a lower shoreface samples below –7.5 m where Quinqueloculina seminula dominates, accounting for 21–35 % of the assemblages. Discorbis williamsoni is distributed in all subtidal samples but it is virtually absent in supratidal samples. Ammonia beccarii and Elphidium discoidale, which are also distributed throughout, become dominant only in the upper shoreface and in the backshore and foredune, where the BAE group represents more than 50 % of assemblages. Species such as Trochammina ochracea, Quinqueloculina milletti and Quinqueloculina sp. cf Q. implexa are locally abundant in water depth greater than –7m but they can be categorized as sporadic, and they do not represent more than 12 % of each assemblage.

The following species are classified as accessory species: Quinqueloculina patagonica, Quinqueloculina milletti, Elphidium gunteri, Cibicidoides sp. cf. C. fletcheri and Miliolinella subrotunda. Other species occur in relative abundance lower than 1% of the assemblage and are considered to be rare or accidental species.

Tables 2 and 4 shows the data concerning the Southern transect. Highest species number, abundance and diversity are found in the shoreface and decrease in the foreshore, and are lowest in the backshore and the foredune. Considering the total number of species and the total assemblages identified in the Southern transect, only Buccella peruviana, Quinqueloculina seminula, Ammonia becarii and Elphidium discoidale can be considered as widely distributed throughout the transect. Buccella peruviana is by far the dominant species in supratidal settings (relative contribution per sample >36 %) and in almost all subtidal areas. Ammonia beccarii, Bolivina striatula, Quinqueloculina seminula, and Textularia gramen can be locally abundant in subtidal settings. D. williamsoni is distributed in all subtidal samples but it is absent in samples from the backshore. In the foreshore and the backshore, the species of the BAE group represents more than 70 % of assemblages.

The following species are classified as accessory: Quinqueloculina patagonica, Elphidium gunteri, Elphidium galvestonense Kornfeld, 1931, Cibicidoides sp. cf. C. fletcheri, Cribrorotalia meridionalis (Cushman and Kellett, 1929), Pyrgo ringens (Lamarck, 1804), and Miliolinella subrotunda. Other species occur in relative abundance lower than 1 % of the assemblage and are considered to be rare or accidental species.

Taphomomic modifications are directly related with water depth in subtidal settings (Figure 3b). Well preserved tests represent more than 70 % of assemblages in the lower shoreface, and vary between 60–44 % in the upper shoreface. Fragmented tests vary between 29–15 % in the lower shoreface and clearly increase onshore. In the upper shoreface, fragmented tests represent 39–55 %, and in the foreshore, fragmented shells attain 63–41 % and abraded shells increase to 8–10 %. In the backshore, well preserved tests vary between 22–38 %, but fragmentation is intense (40–43 %). Abraded and recrystalized shells are present almost exclusively in the emerged beach and are relatively abundant in the backshore, and especially in the foredune, where they attain 17 %. The number of test left in open nomenclature due to taphonomic modification is highest in the intertidal zone (40 %) and decreases offshore, whereas in the foreshore and backshore tends to be rather constant (~18 %).

Foraminifera from Holocene environments

Tables 2 and 5 show data obtained from the KM187 profile, where a total of 7,340 individuals were picked out. Species number in each sample varies between 5 and 13, the Shannon–Weaver diversity index varies between 0.73 and 1.48, and absolute abundance varies between 2 (in the topmost sample) and 485 specimens in 10 g dry sediment. Only Buccella peruviana, Ammonia becarii and Elphidium discoidale can be considered as widely distributed in all samples. In fact, the BAE group represents near 95 % of the assemblage in some samples. Buccella peruviana is by far the dominant species (48 % of the whole assemblage). Elphidium discoidale represents 24 % and Ammonia beccarii 21 % of the total assemblage, and only Elphidium gunteri can be considered as an accessory species. All other species occur in relative abundance lower than 1 % of each assemblage and are considered to be rare or accidental species.

Assemblages from KM187 are moderately well to poorly preserved. In the base (KM187–29/22), the majority of shells are well preserved (Figure 4), while in the middle of the section (KM187–21/12) shells with chamber fillings and/or dark polished coatings tend to increase upward (KM187–11/9) and finally constitute more than 70 % of the assemblages. In the topmost samples (KM187–8/1), well preserved shells and shells with chamber fillings and/or dark polished coatings are equally represented, while fragmented shells tend to increase and abraded shells to decrease upward. Remarkably, taphonomic condition of assemblages does not seem to be related with lithology (Figure 2).

Statistical analyses

The cluster analysis performed on the whole set with regard to taphonomic modifications (cofenetic correlation coefficient: 0.8611) (Figure 6) led to the recognition of six clusters and two independent entities (Table 7). Most samples from the base of KM187 are grouped in Cluster 1, characterized by yielding the greatest amount of well–preserved tests (almost 50 % in average) of the whole set of Holocene samples. In Cluster 2, which includes several samples mostly from the top and middle layers of KM187, filled or coated shells are nearly twice more abundant in average than in Cluster 1. The four remaining samples of the profile consist of almost 70 % of filled or coated shells and are grouped in Cluster 3. Samples from modern environments, which lack filled or coated shells, fall entirely apart from Holocene ones and are grouped in account of their position along the foreshore and the upper shoreface. Some of the lower shoreface samples fall together in Cluster 4, which shows the highest proportion of well preserved tests (75 % on average) of the complete set. Cluster 7 comprises most of the upper shoreface of the northern transect and some foreshore samples, all of which present a relatively high amount of fragmented shells. Finally, most emerged and uppermost shoreface samples are grouped in Cluster 8 where 50 % of tests are fragmented. Samples SET–1 and SET–4 are independent entities; the former, located at the base of the foreshore, because of the high proportion of broken shells –the highest in the whole set–; and the latter, located at the base of the foredune, because of the relatively high proportion of recrystallized and abraded tests.

Cluster analysis suggests that there are some meaningful differences among samples from different locations, which led us to perform a series of linear discriminant analyses on the modern dataset in order to evaluate the consistence of these differences. The results are shown in Tables 8 and 9. The test successfully discriminates samples from both transects: however, it doesn't perform as well in distinguishing supralittoral from sublittoral samples, except with regard to taxonomic composition. Additionally, the correlation between grain size, preservation state and species composition was assessed by means of the Mantel test. The results (Table 9) show that species composition is marginally but significantly correlated with taphonomic signature in both sites, whereas species composition and taphonomic signature are significantly correlated with grain size only in the northern transect.

DISCUSION

Littoral foraminifera from modern beaches

The benthic foraminifera from coastal settings of northern Buenos Aires Atlantic coast are moderately diverse. Shannon–Wiener index values obtained in this study are consistent with those typical of marginal marine environments, according to Murray (2006). The number of benthic foraminifera in subtidal settings is similar to those obtained from the inner shelf elsewhere (Murray, 2006; Mendes et al., 2004). A few samples have a modest contribution of miliolids with porcelaneous walls, as Murray (2006) pointed out for the whole southwestern South Atlantic. Agglutinated shells tend to be destroyed differentially even in subtidal settings (Denne and Sen Gupta, 1989; Murray, 2006; Barbieri, 1996), and as a consequence, hyaline calcareous shells dominate the whole modern benthic assemblage.

Species recorded in northern Buenos Aires littoral follow the large regional trend of the littoral benthic foraminifera from the Southwest South Atlantic, but clearly differ from the inner shelf (>40 m depth) assemblages defined by Boltovskoy and Totah (1985) in terms of taxonomic composition, dominance and diversity. All identified species dwell in southern Brazil and Uruguay neighboring littoral areas (Closs, 1963; Closs and Barberena, 1962; Closs and Lopes–Madeira, 1968; Boltovskoy, 1970). Among the dominant species, Buccella peruviana is the most constant and common foraminifera in the South Atlantic littoral area and dominates throughout the whole Argentine shelf (Boltovskoy, 1970). Ammonia beccarii, Quinqueloculina seminula and Discorbis williamsoni are widely distributed all along Argentine littoral and inner shelf (Boltovskoy 1970; Boltovskoy and Wright, 1976; Boltovskoy et al., 1980). Elphidium discoidale is typically an eurytopic, nearshore species inhabiting South American warm waters north of 41°S (Boltovskoy, 1970; Hippensteel and Martin, 1999).

Living foraminifera usually show a pronouncedly patchy geographic distribution in shallow waters (Schaffer, 1971; Boltovskoy and Wright, 1976). Consequently, detailed taxonomic composition of modern total assemblages is frequently determined by local (small scale) environmental parameters. Strong dominance of Ammonia beccarii and Bolivina striatula in the lower shoreface of the southern transect seems to be related to muddy substrates, since these species prefer organic–rich, fine–grained sediments (Alve and Murray, 1999). Abundance and taxonomic composition of total littoral assemblages reflect biological preferences of species but also taphonomic processes operating in the coastal areas (Barbieri, 1996; Murray and Alve, 1999). This could account for the fact that, although our findings confirm the general taxonomic composition consigned by Boltoskoy (1970), small, fragile and/or scarce and rare species have not been found in our littoral samples. Most inner continental shelf assemblages are parauthochtonous (Callender et al., 1992), and potential taphonomic modifications observed are transport of tests from the position of life to be deposited elsewhere, damage of tests and, finally, total destruction of tests (Murray, 2006). This is evident in the backshore and foredune where the number of specimens and diversity of assemblages are strongly reduced as a consequence of biostratinomic processes (mainly shell destruction related with storm waves, winds and subaerial exposure) due to preferential preservation/destruction of more resistant/fragile species.

The Mantel Test indicates that in the Northern transect, taphonomic signatures and taxonomic composition of total assemblages are highly correlated to grain size, and hence, with sedimentary processes in the littoral. In turn taphonomic signatures of assemblages are correlated to taxonomic composition, indicating that taphonomic (mainly biostratinomic) processes are crucial in defining the specific composition of assemblages in the northern transect. Total abundance and diversity confirm this hypothesis since they are related to water depth in subtidal settings and with position from the shoreline in the backshore. However, these relationships are not as straightforward in the Southern transect, where only taphonomic modifications are related with water depth and position from the shoreline. For the Southern transect, the Mantel test does not show any significant correlation of the taphonomic signatures or taxonomic composition with grain size, although taphonomic signatures and taxonomic composition show a weak (marginal) correlation. A plausible explanation would be that taphonomic processes other than purely biostratinomics are crucial in defining the specific compositions of assemblages in the southern transect, since variations in shell preservation are not linked only with bathymetrically–dependent environmental variables (Loubere et al., 1993; Murray and Alve, 1999). However, additional environmental data is needed to identify which processes are operating in the generation of death (and hence total) assemblages in the Southern transect.

Although location of samples can be a major factor affecting faunal trends even in a small scale, relative abundance, abundance and taphonomic modification of shells seem to be rather similar in both transects. Cluster analyses suggest that there are some meaningful differences in species composition among modern samples from similar depths, especially in subtidal settings. In fact, taxonomic composition does not allow bathymetrical classification of samples when all samples are considered together. This lends weight to Boltovskoy's (1970) statement that foraminiferal assemblages between the intertidal zone and –15 m depth "are very similar" (sic.). However, there are some bathymetric tendencies in taxonomic composition especially in the lower shoreface (Table 6). There, Buccella peruviana represents "only" 20–26 % of assemblages and Quinqueloculina seminula represents between 10–39 %. In the upper shoreface, the BAE group increases its dominance, and upper shoreface, foreshore, backshore and foredune assemblages are rather similar in terms of taxonomic composition due to the outstanding dominance of Buccella peruviana. There is no doubt that biological and taphonomical processes determine the benthic foraminiferal assemblages in the lower shoreface, but taphonomic processes dominate not only in the emerged beach, but also in upper shoreface settings. In terms of wave energy, emerged beach is only influenced by storm waves and winds, and sediments can be periodically remobilized. Shells derived from shoreface settings accumulate on backshore and foredune (Murray, 2006), and ultimately they return to the foreshore and upper shoreface by offshore transport of sand during episodes of coastal erosion (Albertzart and Wilkinson, 1990). This coastal dynamic strengthens the argument that supratidal and upper shoreface assemblages are taxonomically similar due to taphonomic processes operating in the beach.

Some species show patchy distribution, a major factor in distinguishing between transects as assessed by linear discriminant analysis (LDA) performed on taxonomic composition. Trochammina ochracea, Quinqueloculina milletti, Bolivina striatula, and Textularia gramen are locally abundant in submerged sites, probably related to local environmental parameters. A similar patchy distribution of foraminifera off Argentine coasts was cited by Boltovskoy and Lena (1969). Additionally, LDA also distinguished successfully between sublittoral and supralittoral assemblages. In other words, even when taxonomic composition of modern assemblages differ in both transects, subtidal and supratidal assemblages could be easily discriminated. This point is particularly relevant because it implies that modern benthic foraminiferal assemblages from sandy beaches of northern Buenos Aires Province are potentially an adequate general baseline to discriminate Quaternary beach subenvironments if both taxonomic and taphonomic features are considered together.

Taphonomic signatures are not uniformly distributed along the bathymetric gradient. Notably, taphonomic modifications allow the recognition between lower shoreface, upper shoreface and foreshore/backshore/foredune assemblages, indicating that taphonomic processes are mainly determined by water depth, and depend mainly on water energy, aerial exposure, and beach dynamics, thus showing high potential to be used in paleoenvironmental analyses.

Morphological features of the species involved can explain their selective preservation in the different subsettings. The BAE group dominates from the upper shoreface to the foredune. Elphidium discoidale has a very heavy, strong, biconvex planospiral shell, while Buccella peruviana and Ammonia beccarii have strong, biconvex, trochospiral shell. According to Peebles and Lewis (1991), large tests are more resistant to abrasion than smaller tests. These species are hence preferentially preserved in the sediments of the foreshore and the foredune. Beaches in the northeast of Buenos Aires Province are exposed to strong wave action, and they are storm and wave dominated, thus it is not surprising that this three large species dominate total assemblages especially at high energy settings, where fragile species are preferentially removed and/or destroyed. In the lower shoreface, fragile species such as Quinqueloculina seminula, Discorbis williamsoni and Textularia gramen are moderately well represented, but they are selectively destroyed in high energy and supratidal settings. In the upper shoreface and in the emerged beach these species represent in average less than 4 % of total assemblages, whereas in subtidal settings they can attain more than 10 %. Hyaline shells are less susceptible to dissolution than arenaceous shells (Peebles and Lewis, 1991), and according to Corliss and Honjo (1981), hyaline trochospiral taxa are more resistant to dissolution than porcelanaceous species such as Quinqueloculina seminula. Textularia gramen has a biserial test with agglutinated wall with relative coarse grains, traversed by canaliculi that may open as perforations or be closed externally by a thin agglutinated layer (Loeblich and Tappan, 1988). In turn, thin–walled foraminifera with a high density of pores are more susceptible to dissolution (Corliss and Honjo, 1981). Discorbis williamsoni has a small test with calcareous, thin wall coarsely perforated in the umbilical side, with prominent umbilical flaps and chamberlets beneath the flaps.

Comparison between Holocene and modern assemblages

Taxonomic lists are rather similar between Holocene and supratidal modern assemblages, but they differ with respect to diversity, numerical dominance of taxa and taphonomic modifications. In general, diversity is strongly reduced in Holocene samples, where only the three species of the BAE group occur consistently and abundantly. The BAE group is also common in present day habitats and they are especially abundant in the emerged beach, but they tend to be comparatively less dominant. Additionally, some key species locally important in defining groups of modern total assemblages are virtually absent from the Holocene samples. For example, Discorbis williamsoni is nearly absent from both emerged beach and Holocene samples, while in subtidal settings it usually represents more than 3 % in average (Table 6). Quinqueloculina seminula is well represented in subtidal settings of both transects (>10 % average, Table 6), but it is nearly absent from Holocene and supratidal beach samples. These species, which can be locally important constituents of the benthic assemblages in subtidal settings, are fragile enough to be destroyed by biostratinomic and/or fossildiagenetic processes that take place in the emerged beach. To sum up, the dominance of the BAE group and the absence of Discorbis williamsoni and Quinqueloculina seminula in Holocene assemblages seem to indicate that KM187 section represents deposition in a backshore, wave–dominated setting. Resemblance between Holocene and emerged modern–beach total assemblages seems to reflect the operation of some common taphonomic processes. In KM187 sediments, the increase of abraded and recrystalized shells and the concomitant diminution of well preserved tests towards the top of the profile indicate the transition of an upper foreshore to backshore environments. Taxonomic composition and taphonomic signatures of Holocene assemblages seem to indicate that the Holocene beaches of the southern Samborombón Bay were storm and wave dominated. Fossil assemblages seem to be in fact strongly affected by taphonomic processes related to waves, tides and wind, which have destroyed preferentially little and/or fragile species. Strong specimens were displaced and concentrated upshore by storm surges, which carried to the upper beach a taphonomically and taxonomically distinctive assemblage, the BAE group, moderately to poorly preserved, and strongly dominated by Buccella peruviana. Moreover, from the cluster analysis, it is clear that the Holocene samples are more similar to each other than are assemblages from modern environments (both subtidal and supratidal). This seems to indicate that taphonomic processes have been particularly intense during the Mid Holocene regression, and they strongly influenced not only the preservational states of shells, but also determined the taxonomic composition and diversity of fossil assemblages.

Although there are some similarities between the taxonomic composition of Holocene and modern assemblages from emerged beaches, they are not strong enough to draw a perfect parallel between Holocene and modern beaches. As Holocene assemblages are overwhelmingly dominated by the BAE group, taphonomic analysis is a more efficient tool than taxonomic composition to analyze paleoenvironmental evolution in Holocene samples. Additionally, Holocene and modern assemblages are not lumped when taphonomic modification of tests is considered, indicating that taphonomic processes prevailing in the Holocene and modern environments are quite different. The main difference between them is the presence of dark shells with chamber fillings and/or with polished coatings in Holocene sediments. Dark shells reflect diagenetic modifications occurred probably in reducing, dysaerobic environments (Martin, 1999) such as salt marsh and swamps. Thus, these shells are allochthonous, and they were probably eroded from Late Pleistocene–Early Holocene? dark deposits such as those described by Nábel (1987) from the adjacent shelf. A similar taphonomic modification has been observed previously in within–habitat time–averaged assemblages from the type section of the Cerro de la Gloria Member of Las Escobas Formation (Laprida and Bertels–Psotka, 2003). In the base, shells with chamber fillings and/or with polished coatings represent in average 28 % of the assemblage (Cluster 1,Table 7), while in the upper part they account for more than 47 % (Clusters 2 and 3, Table 7). The reduction in the rate of sea level rise during the Mid Holocene allowed onshore transport processes to dominate the littoral system along a low gradient inner shelf such as the bonaerensian (Taylor and Stone, 1996). In this context, the preservational states of shells from the top of the KM187 section may indicate that erosion of relictic coastal sediment was more intense, probably due to the relative fall in sea level. Thus, taphonomic signatures of both modern and Holocene assemblages indicate that the source material and coastal dynamics of modern and Holocene beaches are noticeably different. Sediment sources of Holocene beaches of the southern Samborombón (proto)Bay included Late Pleistocene–Early Holocene coastal and marginal marine environments strongly subjected to reworking during the Mid–late Holocene sea level fall. Grain size distribution of Holocene beach ridges suggest that they were built under conditions that are not operative today. For example, Holocene beach deposits are significantly more shell rich than modern beaches, and we can conclude that either shell abundance was higher or sand supply was lower during the Mid–to–Late Holocene than today.

CONCLUSION

Even when modern beaches from Northern Buenos Aires Province are heterogeneous in terms of geomorphology and taxonomic composition of benthic foraminiferal assemblages, discrimination between subtidal and supratidal modern assemblages is accurate enough to use them in paleoenvironmental reconstruction of Holocene littoral environments when taxonomic and taphonomic data are considered together. Abundance and taxonomic composition of total littoral assemblages reflect biological preferences of species but also taphonomic processes operating in littoral environments. Biological and taphonomical processes determine the benthic foraminiferal assemblages in the lower shoreface, but taphonomic processes largely dominate not only in the emerged beach, but also in upper shoreface settings. As a consequence, supratidal and upper shoreface assemblages are taxonomically similar due to taphonomic processes operating in the beach. Small and fragile species are differentially destroyed, thus strong shells such as those of Buccella peruviana, Ammonia beccarii and Elphidium discoidale become dominant in upper shoreface and backshore facies. Taphonomic modifications of test are not uniformly distributed along the bathymetric gradient, which allows the recognition of lower shoreface, upper shoreface and foreshore/backshore/foredune assemblages, indicating that taphonomic processes are mainly determined by water depth, and depend mainly on water energy, aerial exposure, and beach dynamics, thus showing high potential to be used in paleoenvironmental analyses.

Taxonomic composition and taphonomic signatures of Holocene assemblages do not have strict homologous between modern samples, indicating that Holocene beach dynamics were radically different from those operating in the present. However, based on taxonomic composition and some taphonomic signatures, we can assert that the KM187 succession was deposited in upper shoreface–backshore environments, probably related to storm surges in a wave–dominated coast. In addition, taphonomic analysis shows that sediment sources of Holocene beaches of the southern Samborombón (proto)bay included Late Pleistocene–Early Holocene coastal and marginal marine environments strongly subjected to reworking during the Mid–to–Late Holocene sea level fall.

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