Marina Laura Aguirre

Coastal evolution, changes in sea level and molluscan fauna in northeastern Argentina during the Late Quaternary

Abstract The northern Argentine shoreline underwent a remarkable progradation during the last 6890 years, equivalent to about 4000 km 2 . The shoreline moved approximately 30 km seaward on the average. Two types of accretionary shorelines were formed. High energy environments produced barrier islands and barrier spits which began to form approximately synchronously. Low energy environments, located between those of high energy, consisted of tidal flats and salt marshes. Four groups of molluscan species represent the high-energy palaeoenvironments of the barrier islands and barrier spits. The other fossils represent a low-energy environment of tidal flats. The formation of the barrier islands began 6000 years B.P. It ended 3500 years ago due to a slight fall in relative sea level of 1.5 m above m.s.l. and subsequent formation of barrier spits seaward. As a consequence a very rapid faunal change occurred. In less than 5000–6000 years the dominant high-energy fossil fauna was replaced by a modern low-energy fauna over most of the area. Palaeoenvironmental changes were probably greater and more rapid during the high-energy episode, when the fossils were more varied than those living now in the same area at Samborombon Bay, whilst along the oceanic coastal area the molluscan fauna increased in diversity to the more varied extant one.

Palaeobiogeography of the Holocene molluscan fauna from northeastern Buenos Aires Province, Argentina: its relation to coastal evolution and sea level changes

Abstract In northeastern Argentina the Holocene (ca. 2500–6000 14C yr B.P.) benthic marine molluscan fauna from the Cerro de la Gloria Member of the Las Escobas Formation represents a higher sea-level stand (approximately 4–5 m above m.s.1.) than the present one and belongs to an original shallow water environment. Most of this fauna (48%) belongs to the Argentine Zoogeographic Province while part of it (12%) is shared with the Magellanian Province. The rest of the taxa are cosmopolitan (5%) or belong to the Caribbean, Antillean and Brazilian provinces (35%). The latter are considered stenothermic warm-water indicators which in the present northeastern marine shelf of Argentina are scarcer (6–14%), most of them ranging from the Antilles to southern Brazil or northeastern Uruguay. It is a thermally anomalous molluscan fauna which is a consequence of local coastal palaeogeography and global mid-Holocene climatic change. On that basis a higher sea-water temperature during the mid Holocene is inferred, probably due to a stronger influence of the Brazil Current which could have extended farther south- and westwards during the Hypsithermal. The disagreement between the data presented here and the micropalaeontological data is still a problem to be solved, likely through critical systematic revisions of the foraminiferids and ostracods not included in this paper.

Late Pleistocene and Holocene palaeoenvironments in Golfo San Jorge, Patagonia: molluscan evidence

Palaeoenvironments suggested by molluscan assemblages preserved in Late Pleistocene and Holocene raised marine terraces along the Golfo San Jorge, central Patagonia coastline, Argentina, are reviewed in this work. A better precision of the stratigraphical and geographical ranges of the molluscs through their systematic review shows that all taxa have living representatives, mostly within the Argentine and/or Magellanean Zoogeographical Provinces. In general, no significant taxonomic appearances or disappearances are evident along this coastal area since the last Pleistocene highstand, reinforcing that the modern palaeoceanographical pattern, dominated by the cool Malvinas (Falkland) Current, has not been substantially modified and that the Magellanean Malacological Province has lasted at least since the Pleistocene (although with varying boundaries). The so-called marine terrace V, recently assigned to the Last Interglacial highstand (Oxygen Isotope Substage 5e?), showed in this area a molluscan assemblage characterised by very low diversity, dominantly large shells of Mulinia edulis and by cold-water taxa, in disagreement with higher sea surface temperatures within Oxygen Isotope Substage 5e (Last Interglacial Maximum) and suggesting softer substrates than in the modern adjacent littoral zone.

Taphonomic processes affecting late Quaternary molluscs along the coastal area of Buenos Aires Province (Argentina, Southwestern Atlantic)

Molluscan concentrations were abundantly deposited along the coastal area of Buenos Aires Province (Argentina, Southwestern Atlantic) by different sea-level episodes during the late Quaternary. Most of them represent Holocene ridges (highly energetic depositional environments) subparallel to the modern shore where the main associations preserved are parautochthonous, and others (mainly autochthonous associations) occurring in tidal flats seaward of the ridges or coastal lagoon deposits accumulated behind them (low-energetic). Analysis of three physical taphonomic signatures (disarticulation, fragmentation and abrasion) of the molluscan skeletons of four bivalve species (Tagelus plebeius, Mactra isabelleana, Pitar rostratus, Brachidontes rodriguezi), which exhibit different shell shape and thickness conditions, habitats and life habits and represent the most widely distributed and palaeoecologically significant taxa along the area of study, support the distinction of both palaeoenvironments, characterized by low and high taphonomic grades, respectively. The taphonomic alteration (breakage and abrasion) of most shells seems to have been a direct consequence of the amount of reworking of the material especially during periods of exposure along an extensive beach zone, more than a consequence of transport from a distant original habitat. Abrasion signatures are the only probably correlated with after death time span, shells showing a near modern aspect and original luster being most probably younger than those with chalky or polished appearance. The infaunal species analysed typically living on clays and silts or fine sandy bottoms of the infralittoral or intertidal (T. plebeius, M. isabelleana) are better preserved (less fragmented and abraded) than shells of the epibyssate B. rodriguezi, which lives on hard bottoms of the intertidal and supralittoral areas more exposed to highly unstable conditions with longer episodes of exposure. Although these nearshore concentrations may represent a time-averaging of 1000 yr B.P., taking into account the ecological requirements of the most constant and dominant taxa together with the taphonomic analysis of shells from parautochthonous and autochthonous associations, they still represent valid indicators of past marginal marine conditions in the area, but are only reliable to establish long-term palaeoenvironmental variations.

Holocene Beach Ridges and Coastal Evolution in the Cabo Raso Bay (Atlantic Patagonian Coast, Argentina)

Abstract The Holocene evolution of the Cabo Raso bay (Atlantic Patagonian coast) was reconstructed by means of geomorphological, stratigraphic, and palaeontological analyses, assisted by radiocarbon dating. Six beach ridges were individuated and mapped in the field, as well as some rocky erosional landforms, e.g., inner margins of marine terraces. Thanks to quarry sections, the internal structure of beach ridges, their relationship with continental deposits, and the fossil contents were determined. Two specimens of Aulacomya atra and Brachidontes purpuratus were radiocarbon dated at 6055 and 4500 ± 20 YBP, respectively. The bedrock outcrops at the base of an analysed section allowed us to associate the age of the samples collected to the elevation of the marine transgression surface upon which the entire deposit rests. Because a beach ridge is a regressive form, the elevation of the base of the dated deposit was assumed to be equivalent to or slightly lower than the maximum sea-level stationing, represented by the inner margin of the coheval marine terrace. The altimetric correlation between the base of the beach ridge dated at 6055 ± 20 YBP and the inner margin of the corresponding marine terraces allowed us to constrain the maximum Holocene marine transgression to about 3 to 2 m above sea level. This elevation for the maximum Holocene transgression is lower than that shown by most of the previous data for Patagonian coast, but it shows a crude agreement with recent estimates coming from geophysical models that report, for this area, a departure from the eustatic value of sea level, mainly caused by glacioisostatic process. This means that the employment of marine erosional landforms, associated with other multisource field data, proved to be determinant for reconstructing the sea-level variation in the Patagonian coast.

Morphological variability of Littoridina australis (d’Orbigny, 1835) (Hydrobiidae) in the Bonaerensian marine Holocene (Argentina)

Abstract Fossil and living shells of Littoridina australis (d’Orbigny, 1835) (Mesogastropoda, Hydrobiidae) from the marine Holocene in the Bonaerensian coastal area (Argentina) and the modern SW Atlantic littoral zone (southern Brazil, Uruguay, Argentina) document its wide intraspecific morphological variability and offer implications for the interpretation of paleoenvironmental changes that have occurred since about 7.6 ka BP. Shell morphs previously described as different taxa, i.e. L. australis , Littoridina isabelleana (d’Orb.), Littoridina australis crassa (Gaillard), Littoridina conexa (Gaillard), Littoridina parchappii (d’Orb.), can be linked together in a chain of transitional forms that reflect responses or adaptations to different salinity conditions. A biometrical study of L. australis allows independent testing of the hypothesis of a polymorphic species ranging from medium-sized chubby shells (= L. conexa ) (oligomesohaline environments, 3–18‰) to a smaller elongate morph (= L. isabelleana ) (polyeuhaline habitats, 18–35‰) with the predominant bigger morph (= L. australis lectotype; = L. crassa ) (genuine brackish species in typically mesohaline waters, 8–18‰) in the middle of the morphological gradient. Results of the covariance analysis applied to 10 fossil and eight modern sample groups indicate: (1) no significant discrepancies between L. australis vs. L. crassa , L. australis vs. L. conexa or L. australis vs. L. isabelleana ; (2) significant differences between L. conexa vs. L. isabelleana ; (3) highly significant differences between L. australis and L. parchappii ; (4) slight differences between L. conexa and L. parchappii . Together with paleoecological data and geographical distribution, these results suggest that: (1) L. parchappii represents a separate species; (2) ‘ conexa ’, ‘ isabelleana ’ and ‘ crassa ’ are intraspecific variations (ecomorphs) of L. australis ; (3) ‘ conexa ’ is intermediate between both species; (4) during the time span of the Holocene transgression (ca. 7.6–1.4 ka BP) only two species lived in the area: L. parchappii in oligohaline waters and L. australis with its optimum in brackish (mixohaline) conditions and ecomorphic variations within the estuarine environment; (5) the modern restriction of L. australis is a consequence of salinity changes linked to sea-level fluctuations along the Bonaerensian littoral.

Stable isotope composition of Littoridina australis from the coast of Buenos Aires province, Argentina, during Holocene climatic fluctuations

Abstract Stable isotope (carbon and oxygen) analyses were performed on Littoridina australis shells collected from molluscan concentrations within Holocene littoral deposits along the Bonaerensian coastal area of Argentina (south-western Atlantic). Isotope data allow us to define two very different areas: the Samborombon Bay, where isotope composition of shells was mainly governed by mixing between marine and freshwater, and the Mar Chiquita lagoon, where the original brackish environment was dominated by evaporation of water that originated high isotope shell values. In both areas some isotope profiles show short and quite large oscillations in δ18O. Their origin may be tentatively explained as due to the changes in moisture regime that control freshwater supply. The results suggest that these deposits can represent natural archives potentially useful for palaeoclimate reconstruction.

New insights on the Holocene marine transgression in the Bahía Camarones (Chubut, Argentina)

The stratigraphic reconstruction of the northern sector of the Bahia Camarones (Chubut, Argentina) allowed to improve our understanding of the Holocene marine transgression in the area.The first phase of the maximum of the transgression, is interpreted as dominated by the high rate of eustatic rise of sea level until ca. 6-7 ka BP possibly associated to sedimentary starvation as suggested by fossil accumulation. After this first phase, the general trend indicates a progressive fall of the relative sea level after the Middle Holocene high stand as documented in other parts of south America Atlantic coast. Our data, coupled with the robust radiocarbondata set available for the area from literature, indicate three main local steps of coastal aggradation between ca. 6600 and 5400 yr BP (ca. 7000-5600 yr cal BP), ca. 3300 and 2000 yr BP (ca. 3100-1700 yr cal BP), and ca. 1300-500 yr BP (ca. 1000-300 yr cal BP). A significant age gap in coastal aggradation is present between ca. 5300 and 4400 yr BP (ca. 5600-4500 yr cal BP), and perhaps between ca 2000 and 1300 yr BP (ca. 1700-1000 yr cal BP). These can be linked to phases of local sea level fall and/or phases of sedimentary starvation and/or changes in drift transport which can have produced local coastal cannibalization. However, no conclusive data can be advanced. Data obtained from careful measurements of sea level markers represented by the top of marsh and fluvial terraces indicate lower values for the sea level estimation compared with the data set previously proposed for the area. This stigmatizes the fact that field-oriented works are still the priority in the Patagonia coast along with accurate age measurement, especially for obtaining the fundamental information we need for predicting the environmental impact, in these coastal areas, from accelerate sea level rise as effect of global warming.

Late Cenozoic Invertebrate Paleontology of Patagonia and Tierra del Fuego, with Emphasis on Molluscs

Publisher Summary Studies on Late Cenozoic marine invertebrates recorded in Patagonia have been performed since the nineteenth century. Mollusc shells—mostly gastropods and bivalves—represent the dominant biogenic elements within the fossil assemblages. Some bivalves, like oysters, Mytilidae, Pectinidae, and Veneridae and, among the gastropods, the Patellacea, Muricidae, and Volutidae are among the most outstanding taxa in terms of abundance, size, thickness, and preservation. On the contrary, the associated macroinvertebrate fauna is represented by skeletons of cnidarians, bryozoans, brachiopods, polyplacophors, scaphopods, polychaets, echinoderms, and crabs. It is widely known that molluscs are useful tools as indicators of former sea level and of environmental and climatic changes. Past conditions regarding substrate nature, water energy levels, sea-surface temperature (SST) and oceanic–atmospheric circulation have been deduced from Quaternary gastropods and bivalves from marine or marginal marine sediments, both from the Northern and Southern hemispheres. The chapter focuses on the synthesis of the taxonomy, distribution, paleoecology, and paleoenvironmental implications of the macroinvertebrate fauna preserved along the Patagonian coast, with emphasis on the molluscan assemblages of Quaternary age.

Reading shell shape: implications for palaeoenvironmental reconstructions. A case study for bivalves from the marine Quaternary of Argentina (south-western Atlantic)

Most research on bivalves from the south-western Atlantic used morphological (shell) characters for taxonomic discriminations. Dominant Veneroids from Argentinian Quaternary coastal deposits exhibit wide morphological variation – often making objective discriminations difficult/impossible, which could be objectively described and compared through geometric morphometrics techniques. This work focuses on comparison of geometric morphometrics methods applied to fossil and modern shells, to assess inter- and intra-generic variations. Three approaches were considered: landmarks (L), semi-landmarks (SL) and outlines. Shell shape analyses for different time spans (Pleistocene, fossil Holocene and modern) and areas (Patagonia and Bonaerensian) showed that Elliptic Fourier analysis (EFA), Landmarks and Landmarks plus Semilandmarks (L+SL) can discriminate at generic levels: Mactra, Mulinia (Mactridae) vs. Pitar, Protothaca, Eurhomalea, Clausinella (Veneridae). L and L+SL are powerful for inter/intraspecific distinctions of Mactra. Variability of Mactra isabelleana includes the remaining nominal ‘species’ (transitional morphs). Causal environmental factors of (phenotype) variation could be addressed for modern environments (substrate, salinity and energy). Subtrigonal-inflated shells predominate in muddy, quieter, shallow mixo-polyhaline waters; ovate-elongate-compressed in sandy, poly-euhaline, deeper habitats. Differential spatial distribution (and abundance) across time responds to Late Quaternary high sea-level stands: transgressive maxima allowed higher salinity in marginal-marine areas and optimal conditions for Mactra isabelleana contrasting with scarcer records in the Mar Argentino today.