Juan M. Lirio

Holocene glacial history and sea-level changes on James Ross Island, Antarctic Peninsula

A reconstruction of deglaciation and associated sea-level changes on northern James Ross Island, Antarctic Peninsula, based on lithostratigraphical and geomorphological studies, shows that the initial deglaciation of presently ice-free areas occurred slightly before 7400 14 C yr BP. Sea-level in connection with the deglaciation was around 30 m a.s.l. A glacier readvance in Brandy Bay, of at least 7 km, with the initial 3 km over land, reached a position off the present coast at ca. 4600 yr BP. The culmination of the advance was of short duration, and by 4300 yr BP the coastal lowlands again were ice-free. A distinct marine level at 16- 18 m a.s.l. was contemporaneous with or slightly post-dates the Brandy Bay advance, thus indicating the relative sea-level around 4600-4500 yr BP. Our results from James Ross Island confirm that over large areas in this part of Antarctica the last deglaciation occurred late.

Age and environment of Miocene-Pliocene glaciomarine deposits, James Ross Island, Antarctica

Knowledge of the late Miocene–Pliocene climate of West Antarctica, recorded by sedimentary units within the James Ross Island Volcanic Group, is still fragmentary. Late Miocene glaciomarine deposits at the base of the group in eastern James Ross Island (Hobbs Glacier Formation) and Late Pliocene (3 Ma) interglacial strata at its local top on Cockburn Island (Cockburn Island Formation) have been studied extensively, but other Neogene sedimentary rocks on James Ross Island have thus far not been considered in great detail. Here, we document two further occurrences of glaciomarine strata, included in an expanded Hobbs Glacier Formation, which demonstrate the stratigraphic complexity of the James Ross Island Volcanic Group: reworked diamictites intercalated within the volcanic sequence at Fiordo Belen, northern James Ross Island, are dated by 40Ar/39Ar and 87Sr/86Sr at c. 7 Ma (Late Miocene), but massive diamictites which underlie volcanic rocks near Cape Gage, on eastern James Ross Island, yielded an Ar–Ar age of < 3.1 Ma (Late Pliocene). These age assignments are confirmed by benthic foraminiferal index species of the genus Ammoelphidiella. The geological setting and Cassidulina -dominated foraminiferal biofacies of the rocks at Fiordo Belen suggest deposition in water depths of 150–200 m. The periglacial deposits and waterlain tills at Cape Gage were deposited at shallower depths (< 100 m), as indicated by an abundance of the pectinid bivalve ‘Zygochlamys’ anderssoni and the epibiotic foram Cibicides lobatulus. Macrofaunal and foraminiferal biofacies of glaciomarine and interglacial deposits share many similarities, which suggests that temperature is not the dominant factor in the distribution of late Neogene Antarctic biota. Approximately 10 m.y. of Miocene–Pliocene climatic record is preserved within the rock sequence of the James Ross Island Volcanic Group. Prevailing glacial conditions were punctuated by interglacial conditions around 3 Ma.

Magnetic screening and heavy metal pollution studies in soils from Marambio Station, Antarctica

Abstract Magnetic screening as a cost-effective and non-time-consuming approach has been successfully carried out in Marambio soils and seems to be a suitable method for contamination assessment in Antarctic areas. Ferromagnetic minerals are found in most samples, and magnetite-like carriers are especially dominant in samples collected near pollution sources. Lead and zinc are the main trace elements that have adversely impacted on some areas in this station, both are end products from different pollution sources derived from fuel combustion and residues, solid waste, paints, etc. The correlation results between magnetic and chemical variables show moderate relationships varying from 0.409 to 0.663; this fact supports the use of magnetic parameters, such as magnetic susceptibility and bivariate ratios, for pollution monitoring. The magnetic properties, heavy metal contents, and related maps of these soils can be considered as a reference in the area for future temporal and/or spatial work.

Stratigraphy of the Pliocene Sørsdal Formation, Marine Plain, Vestfold Hills, East Antarctica

The Sørsdal Formation and one member, Graveyard Sandstone Member constitute a sedimentary sequence covering approximately 10 km2 of Marine Plain, Vestfold Hills, East Antarctica. The new Formation consists dominantly of friable diatomaceous siltstone and sandstone with dark limestone lenses. It is in situ, essentially horizontal, 7.2 m thick in its type section and lies less than 25 m a.s.l. Graveyard Sandstone Member occurs near the top of the formation, is highly lithified sandy diamictite, 30–50 cm thick and widespread through the Marine Plain region. Using diatoms, the Formation is Early Pliocene in age (Fragilariopsis barronii, 4.5–4.1Ma). The Graveyard Sandstone Member probably was deposited during the Gilbert Chron interval (lower Chron 2Ar or C3n. 1r) of reversed magnetic polarity. The Sørsdal Formation contains fossil cetaceans and a diverse and well-preserved invertebrate fauna. Foraminifera are rare partly because of diagenesis, but include Ammoelphidiella antarctica. No evidence of coeval terrestrial vegetation has been recovered. The deposit accumulated in a series of small bays probably in an environment warmer than exists in the region today. There is no lithological evidence of glacial influence except in Graveyard Sandstone Member that may represent local glacial influence in a shallow marine to intertidal environment.

Relative paleointensity of the geomagnetic field over the last 21,000 years BP from sediment cores, Lake El Trébol (Patagonia, Argentina)

Four cores from Lake El Trébol (Patagonia, Argentina) have been used to estimate regional geomagnetic paleointensity. The rock magnetic studies indicate that the magnetic mineralogy of the clay-rich sediments is dominated by pseudo-single domain magnetite in a range of grain sizes and concentration which are suitable for paleointensity studies. The remanent magnetisation at 20 mT (NRM20 mT) has been normalised using the an-hysteric remanent magnetisation at 20 mT (ARM20 mT), the saturation of the isothermal remanent magnetisation at 20 mT (SIRM20 mT) and the low field magnetic susceptibility (k). Coherence function analysis indicates that the normalised records are free of environmental influences. Our paleointensity (NRM20 mT/ARM20 mT) versus age curve shows good agreement with published record from another lake in Argentina and with records from other parts of the world, suggesting that, in suitable sediments, paleointensity of the geomagnetic field can give a globally coherent, dominantly dipolar signal.

Sedimentary Cores from Mascardi Lake, Argentina: A Key Site to Study Elpalafquen Paleolake

Sedimentary cores from Mascardi Lake, as well as outcrops, have been studied to reconstruct late Pleistocene and Holocene environmental conditions in northern Patagonia, Argentina. The Mascardi Lake sequence is a key-site for understanding such conditions, providing evidences of ice retreat, volcanic activity and important sedimentation changes. A significant environmental change occurred around 13 ky BP when the great lake named Elpalafquen became several small basins and some of the present lakes from northern Patagonia started to get their current features. The Mascardi Lake occupied a marginal position, close to the ice front at the western side of the paleolake.

A high-resolution palaeoclimate record for the last 4800 years from lake la Brava, SE pampas plains, Argentina

Resumen Los cambios climaticos son reflejados en las variaciones de diferentes parametros. Las secuencias sedimentarias de lagos son buena fuente de esta informacion debido a que proveen grabaciones continuas y detalladas de cambios paleoclimaticos.Para determinar los cambios en el clima al SE de la llanura Pampeana, se presentan estudios de magnetismo de rocas realizados en un testigo colectado del fondo de la Laguna La Brava (Argentina). Tambien se midieron contenidos totales de sulfuro, carbon organico e inorganico (TS, TOC y TIC), elementos alcalinos, metales pesados y livianos, y cambios en las comunidades de vegetacion. Se realizaron cinco determinaciones de edades radiocarbonicas y se calcularon las edades calibradas. La tasa promedio de acumulacion de sedimento es 1.3 mm/ano y la secuencia representa los ultimos 4800 anos en edades calibradas (cal. BP).El objetivo principal fue reconstruir el balance M. A. Irurzun * C. S. G. GogorzaA. M. SinitoM. A. E. ChaparroInstituto de Fisica Arroyo SecoUniversidad Nacional del Centro de la Provincia de Buenos AiresPinto 399, 7000 Tandil, Argentina

Current distribution of Branchinecta gaini on James Ross Island and Vega Island

paper describing current distribution of branchinecta gaini in the area of eastern Antarctic Peninsula