R.A. del Valle

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.

Holocene environmental change in a marine-estuarine-lacustrine sediment sequence, King George Island, South Shetland Islands

Sedimentological features and cluster analysis of diatom assemblages were used to investigate a local Holocene prograding sequence of marine-estuarine-lacustrine sediments. It consists of upward finning and thinning sediment cycles formed at the mouth of a meltwater stream during regional isostatic uplift, which followed early Holocene deglaciation and marine inundation events. The sequence begins in the lower Holocene sublittoral sand (marine diatoms and abundant molluscs) overlying, with a transgressive base, the deltic (?) clastic sediment marking probably one of the pre-Holocene interglacial periods (index diatom Actinocyclus ingens suggests an age >0.62 Ma). The lower Holocene marine sand was truncated by middle Holocene gravity flows, bearing volcanic ash. They were deposited in a high energy estuarine environment (brackish diatoms). The beach subsequently formed separated the estuary from the sea and changed it into a freshwater lake. Accumulation of moss and gyttja, containing a freshwater diatom assemblage, marks the final late Holocene stage of this coastal sedimentary sequence, which can be considered as typical for deglaciation periods in the maritime Antarctic.

Transpressional deformation along the margin of Larsen Basin: new data from Pedersen Nunatak, Antarctic Peninsula

New structural data from the northern Antarctic Peninsula suggest that reverse faults and folds affecting the Pedersen Nunatak beds of the upper Mesozoic–Lower Cenozoic Larsen Basin succession were produced by transpressional forces acting parallel to the Weddell Sea coast of the Antarctic Peninsula during mid-Cretaceous compression of the Larsen Basin. At Pedersen Nunatak, Larsen Basin rocks are deformed into a series of synclines and anticlines that are cut by reverse faults.

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.

Mid cretaceous contact seen below seymour island and followed off shore by the magnetotelluric method along the ne coast of the Antarctic peninsula

Abstract Magnetotelluric soundings were made on two off-shore islands and on the Larsen ice shelf. Below Seymour Island a thick sedimentary accumulation (6.7 km) is found. The mid Cretaceous contact is seen at a depth of 4.7 km. The first intermediate conductive layer appears at 65 km with a thickness of 20 km and a 10 ohm m resistivity. Two hundred km farther to the SW, from Robertson Island to Cape Fairweather, an anticline of the basement is possible; the depth ranges from 0.9 km at the top, to 2.3 and 3.6 km at the edges. The mid Cretaceous contact is followed off shore. The intermediate conductive layer begins at about 30 km depth on an average.

Methane at the NW of Weddell Sea, Antarctica

The presence of gaseous hydrocarbons (from methane to n-pentane) in the seabed sediments and the bubbling of methane may suggest the presence of gas accumulations in the substrate of the NW Weddell Sea, Antarctica. The release of methane from the frozen ocean substrate adjacent to Seymour Island would be linked to climate instability during Late Cenozoic, when vast areas of the Antarctic continental shelf were flooded during the marine transgression that occurred . 18,000 years ago, after the Last Glacial Maximum (LGM). As the ice melted, the sea again occupied the regions which it had abandoned. As the transgression was relatively rapid, the sub-air relief was not destroyed but was submerged and the ground had frozen (permafrost) along with it. Thus, the heat flow from the sea to the marine substrate, now flooded, would have destabilized frozen gas accumulations, which were originally formed into terrestrial permafrost during the LGM, similarly to what would have happened in the Arctic.