Northern Antarctic Peninsula Region Research Articles

The deep seismic structure of the Earth's crust along the Antarctic Peninsula—A summary of the results from Polish geodynamical expeditions

A summary of the results of four Polish geophysical expeditions, which constituted an extensive programme of seismic wide-angle refraction experiments in the northern Antarctic Peninsula region between 1979 and 1991, is analysed here. The results include the interpretation of 20 deep seismic sounding profiles located along the western coast of the Antarctic Peninsula. Additionally, a few shallow seismic profiles in the Deception Island area and a total of 10 reflection profiles from the Bransfield Strait and Drake Passage area were carried out. Crustal velocity models extending across the Antarctic Peninsula continental shelf between the Adelaide Island and the Bransfield Strait show a typical continental crustal structure, with crustal thicknesses of 36–42km near the coast that decreases to 25–28km beneath the outer continental shelf. Farther north in the Bransfield Strait region, the models document a southeastward dip of the Moho discontinuity from a depth of 12km beneath the South Shetland Trench to 40km under the northern tip of the Antarctic Peninsula. Beneath the trough of the Bransfield Strait, a high-velocity body with P-wave velocities exceeding 7.0km/s was detected in a depth range of 6–32km.

Eocene sedimentary facies in a volcanogenic succession on King George Island, South Shetland Islands: a record of pre-ice sheet terrestrial environments in West Antarctica

About 34 Ma ago there was a radical change of climate that led to the formation of Antarctic ice sheet. King George Island, located in the South Shetland Islands volcanic arc (northern Antarctic Peninsula region), is one of a few places in West Antarctica which shows a geological record of sedimentary environments preceding development of the ice sheet. The Eocene sedimentary facies occur in the dominantly volcanogenic succession of King George Island. They have been recognized in the Arctowski Cove and Point Thomas formations (Ezcurra Inlet Group) and in the Mount Wawel Formation (Point Hennequin Group) in Admiralty Bay, and in the Mazurek Point Formation (Chopin Ridge Group) and Lions Cove Formation (Polonia Glacier group) in King George Bay. They record a cooling trend in terrestrial environments that began at termination of the Early/Middle Eocene Climatic Optimum, and was followed by a significant deterioration of climate during Late Eocene and earliest Oligocene, directly preceding glacial conditions in the northern Antarctic Peninsula region. The ongoing research confirms the existence of three preglacial climatic stages (PGS-1 – PGS-3) during Eocene – earliest Oligocene, from humid, warm to moderate climate (PGS-1), through cool and dry climate (PGS-2), up to cold and humid conditions (PGS-3). Studies were carried out on usually fine-grained volcanoclastic sediments, containing Podocarpaceae– Araucaria – Nothofagus plant fossils assembly. Beds of reworked pyroclastic material alternate with lava flows or volcanic agglomerates, as well as ex situ blocks of Eocene volcanogenic sediments on a moraine. Calculated geochemical indices of weathering (CIA, PIA and CIW) confirm moderate to high chemical weathering under warm and humid climate conditions at the beginning and deterioration of conditions in the end of Eocene.

Windows on Antarctic soil–landscape relationships: comparison across selected regions of Antarctica

Abstract This paper brings together topographic cross-section ‘windows’ from across Antarctica to illustrate soil–landscapes from the margins of the polar plateau in the Transantarctic Mountains and McMurdo Dry Valleys, through East Antarctic coastal areas, to the northern Antarctic Peninsula Region. Soils identified range from Gelisols in the Ross Sea Region, through Gelisols and Entisols in coastal East Antarctica, to a mixture of Gelisols, Entisols, Spodosols and Inceptisols in the northern Antarctic Peninsula Region where permafrost is not ubiquitous. The relative impacts of the soil-forming factors are considered. At a continental scale climate is the main driver of the differences observed between soils in different areas. At local scales strong soil–topographic relationships are observed. Organisms, time and parent material are dominant influences on soil properties only in relatively localized situations. Organisms dominate in areas of organic matter or guano accumulation and time is a dominant influence on exceptionally old upland surfaces in the McMurdo Dry Valleys. The US Department of Agriculture’s Soil Taxonomy gives a useful overall appraisal of Antarctic soils; however, for detailed work, there is a need to introduce some new categories at subgroup level to better capture the range of soils described.

Stable water isotopes of precipitation and firn cores from the northern Antarctic Peninsula region as a proxy for climate reconstruction

Abstract. In order to investigate the climate variability in the northern Antarctic Peninsula region, this paper focuses on the relationship between stable isotope content of precipitation and firn, and main meteorological variables (air temperature, relative humidity, sea surface temperature, and sea ice extent). Between 2008 and 2010, we collected precipitation samples and retrieved firn cores from several key sites in this region. We conclude that the deuterium excess oscillation represents a robust indicator of the meteorological variability on a seasonal to sub-seasonal scale. Low absolute deuterium excess values and the synchronous variation of both deuterium excess and air temperature imply that the evaporation of moisture occurs in the adjacent Southern Ocean. The δ18O-air temperature relationship is complicated and significant only at a (multi)seasonal scale. Backward trajectory calculations show that air-parcels arriving at the region during precipitation events predominantly originate at the South Pacific Ocean and Bellingshausen Sea. These investigations will be used as a calibration for ongoing and future research in the area, suggesting that appropriate locations for future ice core research are located above 600 m a.s.l. We selected the Plateau Laclavere, Antarctic Peninsula as the most promising site for a deeper drilling campaign.

Pre-glacial sedimentary facies of the Point Thomas Formation (Eocene) at Cytadela, Admiralty Bay, King George Island, West Antarctica

Pre-glacial sedimentary facies of the Point Thomas Formation (Eocene) at Cytadela, Admiralty Bay, King George Island, West Antarctica New evidence of Eocene preglacial environments has been found on the southern coast of Ezcurra Inlet on King George Island, South Shetland Islands, West Antarctica. Plant remains (trunks, leaves, detritus) and carbonaceous seams and beds occur in sedimentary strata in a 4 km long Cytadela outcrop of the Point Thomas Formation. They are an evidence for the presence and diversity of terrestrial vegetation in the northern Antarctic Peninsula region. The forests were composed mostly of Podocarpaceae-Araucaria-Nothofagus, with an undergrowth of hygrophilous and thermophilous ferns, and grew on volcanic slopes and surrounding lowland areas of King George Island during breaks in volcanic activity. The succession that crops out at Cytadela provides a record of changing climatic conditions from a warm and wet climate with extensive vegetation to a much drier climate with limited vegetation and ubiquitous weathering of volcanic bedrock. The geochemical indices of weathering (CIA, PIA and CIW) have narrow and relatively high value ranges (76-88), suggesting moderate to high chemical weathering under warm and humid climate conditions. The decrease in humidity and the decline in plant life through the succession can be related to the gradually cooling climate preceding development of the Oligocene ice cover across the Antarctic continent.

Late-Holocene changes in character and behaviour of land-terminating glaciers on James Ross Island, Antarctica

AbstractVirtually no information is available on the response of land-terminating Antarctic Peninsula glaciers to climate change on a centennial timescale. This paper analyses the topography, geomorphology and sedimentology of prominent moraines on James Ross Island, Antarctica, to determine geometric changes and to interpret glacier behaviour. The moraines are very likely due to a late-Holocene phase of advance and featured (1) shearing and thrusting within the snout, (2) shearing and deformation of basal sediment, (3) more supraglacial debris than at present and (4) short distances of sediment transport. Retreat of ~100 m and thinning of 15–20 m has produced a loss of 0.1 km3 of ice. The pattern of surface lowering is asymmetric. These geometrical changes are suggested most simply to be due to a net negative mass balance caused by a drier climate. Comparisons of the moraines with the current glaciological surface structure of the glaciers permits speculation of a transition from a polythermal to a cold-based thermal regime. Small land-terminating glaciers in the northern Antarctic Peninsula region could be cooling despite a warming climate.

Periglacial processes and landforms in the South Shetland Islands (northern Antarctic Peninsula region)

Periglacial processes and landforms together with the presence of permafrost are among the most relevant geomorphological elements in the northern Antarctic Peninsula region. Their distribution affects the hydrology and has consequences for the ecosystems of the ice-free areas. In this paper a compilation of the different types of periglacial landforms and processes occurring in the South Shetland Islands is carried out and their spatial distribution is analysed. Furthermore, the relationships of the periglacial landforms with local conditions and permafrost distribution have been taken into account. A total of thirty three types of periglacial landforms were identified and considered in this work. Patterned ground and stone fields are the most common periglacial landforms, which are located within a wide altitudinal range and mainly on platforms. Field studies, aerial photograph and satellite imagery interpretation were implemented to produce detailed maps from ten areas with different geological, geomorphological and relief characteristics, including the largest and most relevant ice-free areas within the archipelago, showing the presence and spatial distribution of periglacial landforms. This work shows that the periglacial environment, primarily conditioned by the regional climatic conditions, has a great diversity in the studied region and that the distribution of the periglacial landforms is also related to local relief and geomorphological characteristics, lithology, hydrogeology, and presence of permafrost where altitude plays an important role. Periglacial phenomena are widespread above 10m a.s.l. and are especially active on slopes and platforms between 30 and 100m a.s.l. The spatial distribution of periglacial landforms helps to identify the presence of permafrost that is dominant above 25–30m a.s.l. and more than 70% of the surface is occupied by active layer-related landforms.

Lu–Hf isotope evidence for the provenance of Permian detritus in accretionary complexes of western Patagonia and the northern Antarctic Peninsula region

Lu–Hf isotope data are presented for dated Permian zircon grains from six samples of the latest Palaeozoic to Jurassic low-grade metasedimentary rocks of western Patagonia and the northern Antarctic Peninsula, as well as from potential source rocks in the North Patagonian Massif. The results for the metasedimentary rocks yield eHf values mostly between −15 and +4 (130 analyses), with a dominant range (more than 85%) of −6 to +1, indicating provenance from Permian magmatic rocks that incorporated continental crust with a significant residence time. Other zircon grains record more negative eHf values indicating derivation from yet more mature crustal sources. Permian subvolcanic granites in the North Patagonian massif appear to be the closest large source area and dated zircon grains from eight samples of these granites yield initial of eHf values of −12 to +4 (45 measurements), 84% of which fall between −6 and +1, the range shown by the metasediments. However, the North Patagonian massif also contains some more juvenile Permian–Carboniferous components not seen in the metasediments, so that this may not be the primary or unique source. These granites are considered to represent the southernmost extension of the Choiyoi igneous province, which contains abundant Permian rhyolites that crop out on the eastern side of the Andes in central Argentina, for which unpublished Hf isotope data yield a very similar range to that of the metasediments. The widespread nature of the Choiyoi volcanic rocks and the predominance of Permian zircon could make this a more favoured source for the detrital grains. Hf isotope data reinforce the uniformity of the provenance of the turbidite detrital zircons and confirm the Choiyoi igneous province and the Permian granitic rocks of the North Patagonian Massif as feasible sources. They further confirm the dominantly crustal origin of the Permian magmas. A source region involving mixing of, for example, crustal materials of Panafrican/Brasiliano and Grenvillian ages, together with a minor but significant subduction-related magmatic input, is an isotopically feasible explanation and is broadly consistent with the provenance of pre-Permian crust in this region, but the proportions of such a mixture must have remained relatively constant. This supports the proposal that recently recognised but widespread Permian magmatism in Patagonia represents voluminous crustal melting in response to subducted slab break-off. The results are also consistent with the premise that the Antarctic Peninsula and southern Patagonia were closely located from Permian to Jurassic times, receiving detritus from the same source.

Penguin response to the Eocene climate and ecosystem change in the northern Antarctic Peninsula region

Eocene Antarctic penguins are known solely from the La Meseta Formation (Seymour Island, James Ross Basin). They are most numerous and taxonomically diverse (at least ten species present) within strata formed at the end of this epoch, which is concomitant with a significant cooling trend and biotic turnover prior to the onset of glaciation. Moreover, all newly appeared taxa were small-bodied, and most probably evolved in situ. Interestingly, some chemical proxies suggest enhanced nutrient upwelling events that coincided with obvious changes in the record of La Meseta penguins.

Authigenic pyrite framboids in sedimentary facies of the Mount Wawel Formation (Eocene), King George Island, West Antarctica

Authigenic pyrite framboids in sedimentary facies of the Mount Wawel Formation (Eocene), King George Island, West AntarcticaPyrite framboids occur in loose blocks of plant-bearing clastic rocks related to volcano-sedimentary succession of the Mount Wawel Formation (Eocene) in the Dragon and Wanda glaciers area at Admiralty Bay, King George Island, West Antarctica. They were investigated by means of optical and scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and isotopic analysis of pyritic sulphur. The results suggest that the pyrite formed as a result of production of hydrogen sulphide by sulphate reducing bacteria in near surface sedimentary environments. Strongly negative Δ34SVCDTvalues of pyrite (-30 - -25 ‰) support its bacterial origin. Perfect shapes of framboids resulted from their growth in the open pore space of clastic sediments. The abundance of framboids at certain sedimentary levels and the lack or negligible content of euhedral pyrite suggest pulses of high supersaturation with respect to iron monosulphides. The dominance of framboids of small sizes (8-16 μm) and their homogeneous distribution at these levels point to recurrent development of a laterally continuous anoxic sulphidic zone below the sediment surface. Sedimentary environments of the Mount Wawel Formation developed on islands of the young magmatic arc in the northern Antarctic Peninsula region. They embraced stagnant and flowing water masses and swamps located in valleys, depressions, and coastal areas that were covered by dense vegetation. Extensive deposition and diagenesis of plant detritus in these environments promoted anoxic conditions in the sediments, and a supply of marine and/or volcanogenic sulphate enabled its bacterial reduction, precipitation of iron monosulphides, and their transformation to pyrite framboids.

Neogene glacigenic debris flows on James Ross Island, northern Antarctic Peninsula, and their implications for regional climate history

Detailed sedimentological and microtextural analyses of newly-discovered late Neogene diamictites and other coarse-grained facies, mostly sandwiched between hyaloclastite of the James Ross Island Volcanic Group and Cretaceous sandstone and mudstone, indicate deposition mainly by glacigenic debris flows. The deposits on James Ross Island (northern Antarctic Peninsula) constrain the depositional setting, ice–bed dynamics and regional palaeoclimate. The sequences on James Ross Island vary in age but are mainly late Miocene and Pliocene. Unlike Neogene sedimentary sequences elsewhere in Antarctica, those on James Ross Island are unusually well-dated by a combination of 40Ar/ 39Ar and 87Sr/ 86Sr analyses on fresh interbedded lavas and pristine bivalve molluscs, respectively. The Sr isotopic ages of the debris flows cluster around 4.74, 4.89, 5.44, 5.78, and 6.31 Ma and probably date relatively warm periods in the northern Antarctic Peninsula region, when the bivalves lived under ice-poor or seasonally ice-free conditions. The bivalves are often remarkably well-preserved, lack adhering lithified sediment and, in at least two locations, are large, mainly unfragmented and sometimes articulated, suggesting that they were alive immediately prior to their incorporation in subaqueous debris flows at the margins of an advancing glacier. These fossiliferous glacigenic debris flows signify episodes of ice expansion during relatively warm periods, or “interglacials”, of the late Miocene and Pliocene. The James Ross Island glacigenic sedimentary successions attain thicknesses of up to 150 m and extend over 4 km laterally. The high volume of glacigenic sediment delivery implicit in the James Ross Island successions indicates that a series of dynamic ice fronts crossed the region during the late Miocene and Pliocene epochs. Associated evidence, in the form of clast abrasion (including striations and faceting) and bedrock erosion, is indicative of basal sliding and subglacial sediment deformation active at the ice–bed interface and wet-based temperate or polythermal regimes, prior to remobilisation. The evidence further suggests two local ice caps on James Ross Island during the warm periods, as well as ice-overriding by the Antarctic Peninsula Ice Sheet from the west and northwest.

Six million years of glacial history recorded in volcanic lithofacies of the James Ross Island Volcanic Group, Antarctic Peninsula

Basaltic volcanism in the James Ross Island region has been persistent over the last 6 million years resulting in at least 50 mainly effusive eruptions that are preserved predominantly as lava-fed deltas and a small number of tuff cones. Most of the eruptions took place during glacial periods, and the deltas have enabled the characteristics of the palaeo-glacier cover to be deduced for the first time, for multiple time slices. The resolution of 40Ar/39Ar dating of young basaltic lavas is relatively poor compared with the duration of glacial–interglacial periods and precludes any Milankovitch-scale cyclicity being identified, a problem that is now becoming acute in palaeoenvironmental investigations of this type. Our results indicate that glacial periods were characterised by a relatively thin glacier cover in this area, typically just 200–350 m. They were interspersed with fewer periods of thicker ice c. 600–750 m in thickness. These are likely maximum estimates and they may be too high by a few tens of metres. The glacier cover increased in thickness toward the present. However, as evidenced by 4.6 myr-old surfaces at c. 620 m a.s.l. that are glacially unmodified other than frost shattering, no evidence has been found for a substantially thicker ice sheet at any time during the last 6 myr. The glacier cover was formed predominantly of ice (sensu stricto) that was wet-based and erosive. Thus it had a temperate or, probably more likely, sub-polar (i.e. polythermal) thermal regime and, if the ice reached the continental shelf edge, it must have had a low profile. After an early history (c. 6.2–4.6 Ma) dominated by an areally extensive Antarctic Peninsula Ice Sheet (APIS), the local glacial morphodynamics were determined by a local ice cap that draped James Ross Island and was presumably confluent with the APIS along its western margin. These results are the first evidence for the morphology, thickness and thermal regime of the glacier cover in the northern Antarctic Peninsula region for the late Neogene–Quaternary period.

Geochemical characteristics and the provenance of sediments in the Bransfield Strait, West Antarctica

The Bransfield Basin is a Quaternary marginal basin separating the South Shetland Islands from the Antarctic Peninsula. We analyzed major, trace, and rare earth element chemistries, and the Nd, Sr, and Pb isotopic compositions for six piston core sediments from the western and eastern Bransfield basins to determine the sediment provenance and factors controlling the composition of the sediments. The Bransfield sediments have chemical characteristics similar to sediments deposited in a tectonically active setting where the source rocks are predominated by igneous rocks of intermediate to mafic composition: low K and high CN in the A–CN–K diagram, low La / Sc, low Th / Sc, and lower LREE / HREE than average shale. The low chemical alteration index of the sediments, about 45 on average, suggests the effect of weathering on the sediment composition was minimal. The source rocks of the Bransfield sediments are mostly composed of Mesozoic to Tertiary arc volcanic and plutonic rocks in the Antarctic Peninsula and the South Shetland Islands. Meanwhile, contributions from other rock types in the northern Antarctic Peninsula region, such as the Trinity Peninsula Group or Scotia Metamorphic Complex, seem to be negligible. The Bransfield sediments can be divided into three distinct compositional groups. Group I sediments have a lower La / Yb and a less prominent negative Eu anomaly than Group II sediments. They are also characterized by negative Ce anomalies. Group I sediments have been derived mainly from the South Shetland Islands, whereas Group II sediments, which are more widespread than Group I sediments in the Bransfield Strait, have been derived mainly from the Antarctic Peninsula. Group III sediments, which are very low in La / Yb with the lowest 87Sr / 86Sr and the highest 143Nd / 144Nd values, have been derived from Deception Island. The composition of the Bransfield sediments is largely controlled by the composition of nearby source rocks; the distribution of sediments is further modified by the current and topography of the basin.

A multi-proxy approach to determine Antarctic terrestrial palaeoclimate during the Late Cretaceous and Early Tertiary

Fossil wood is abundant throughout the Cretaceous and Tertiary sequences of the northern Antarctic Peninsula region. The fossil wood represents the remains of the vegetation that once grew at the southern high palaeolatitudes at 59–62°S through the general decline in climate, from the Late Cretaceous global warmth through to the mid-Eocene cool period prior to the onset of glaciation. This study draws on the largest dataset ever compiled of Antarctic conifer and angiosperm woods in order to derive clearer insights into the palaeoclimate. Parameters including mean annual temperature, mean annual range in temperature, cold month mean, warm month mean, mean annual precipitation are recorded. The fossil wood assemblages have been analysed using anatomical (physiognomic) characteristics to determine the palaeoclimate variables from the Coniacian–Campanian to the middle Eocene. These results are compared with data derived from Coexistence Analysis of the fossil floras (composed of leaves, wood and palynomorphs) and published data based on leaf physiognomic characters. These studies indicate a relatively warm and wet Late Cretaceous that becomes drier and cooler in the Early Paleocene and subsequently returns to warmer, wetter conditions by the latest Early Paleocene. During the Eocene the climate becomes relatively cool and dry once again. The discrepancies obtained from these two methods coupled with other published data are discussed in the context of the fluctuations in the temperatures of the surrounding oceans and global patterns of climate change.

Cretaceous and early Tertiary climates of Antarctica: evidence from fossil wood

Fossil wood is abundant in Cretaceous and early Tertiary sediments of the northern Antarctic Peninsula region. The wood represents the remains of vegetation that once grew in high palaeolatitudes when the polar regions were warmer, during former greenhouse climates. Fossil wood is a unique data store of palaeoclimate information. Analyses of growth rings and anatomical characters in fossil wood provide important information about temperature, rainfall, seasonality and climate trends for this time period in Antarctica. Climate signals from fossil wood, supported by sedimentary and geochemical evidence, indicate a trend of cool climates during the Early Cretaceous, followed by peak warmth during the Coniacian to early Campanian. Narrower growth rings suggest that the climate cooled during the Maastrichtian and Palaeocene. Cool, wet and possibly seasonal climates prevailed at this time, with tentatively estimated mean annual temperatures (MATs) falling from 7°C to 4–8°C respectively, determined from dicotyledonous (dicot) wood anatomy. The Late Palaeocene/Early Eocene was once again warm, with estimated MATs of 7–15°C from dicot wood analysis, but conditions subsequently deteriorated through the latter part of the Eocene, when cold seasonal climates developed, ultimately leading to the onset of Cenozoic ice sheets and the elimination of vegetation from most of Antarctica.

Bransfield Basin fine-grained sediments: late-Holocene sedimentary processes and Antarctic oceanographic conditions

The Antarctic Peninsula is sensitive to climatic change due to its northerly position and to the relatively reduced volume and character of its ice cover. High-resolution palaeoclimatic records from the Ant arctic Peninsula ice cores extend back only 500 years. A climatic record of 2850 years in the Bransfield Basin is investigated through the analysis of sediment gravity cores from the floor of the central subbasin (core GEBRA-1) and the slope of the eastern subbasin (core GEBRA-2). Sedimentological, mineralogical and geo chemical properties have been systematically measured, together with Accelerator Mass Spectrometry (AMS) radiocarbon dating. The fine-grained sediments result from two main processes: hemipelagic settling from resuspensions and primary productivity, and turbidity currents. Hemipelagic sediments were selected to investi gate the oceanographic and climatic conditions of the northern Antarctic Peninsula region during the last three millennia. Cold climatic periods are characterized by millimetric laminations and/or black layers with higher organic carbon, nitrogen and opal contents. Warm periods are recorded as massive to diffuse laminated facies with lower biogenic contents. The results include the ‘Little Ice Age’ (LIA) cold pulse as well as several 200– 300 year long fluctuations within the LIA and before this major climatic event of the Holocene.

The fossil record of Cunoniaceae: new evidence from Late Cretaceous wood of Antarctica?

Fossil angiosperm wood from Upper Cretaceous sediments of Livingston Island and James Ross Island in the northern Antarctic Peninsula region is identified as having the combination of anatomical characters most similar to modern Cunoniaceae. The material is characterised by predominantly solitary vessels, opposite to scalariform intervessel pitting, scalariform perforation plates, heterocellular multiseriate and homocellular uniseriate rays, diffuse axial parenchyma. Anatomically, the specimens conform most closely to the fossil organ genus Weinmannioxylon Petriella which has been placed within the Cunoniaceae. The presence of Weinmannioxylon in Late Cretaceous sediments suggests that taxa within or stem taxa to the Cunoniaceae might have been a notable component of the forest vegetation that covered the Antarctic Peninsula during the Late Mesozoic and may therefore represent the earliest record of this family.

Holocene deglaciation and climate history of the northern Antarctic Peninsula region: a discussion of correlations between the Southern and Northern Hemispheres

The chronology of post-Last Glaciol Maximum deglaciation in the northern Antarctic Peninsula region is discussed. It is concluded that, contrary to what was earlier believed, the deglaciation process here was largely out-of-phase with that in the Northern Hemisphere. Although, for global eustatic reasons, the marine-based glaciers may have retreated simultaneously with ice-melting in the Northern Hemisphere, the land-based glaciers retreated only slowly during the first halfoftlie Holocene, about 9000-5000 BP. This may have been due either to increased precipitation counterweighing ablation or to delayed warming. A distinct but rather brief Glaciol readvancc took place around 5000 BP, probably caused by a period of renewed cooling. It was followed by the Holocene climatic optimum, about 4000-3000 BP. This warm “hypsithermal” period thus came much later than its equivalent in the Northern Hemisphere, but it roughly coincided with the Milankovitchcan Holocene insolation maximum for these southern latitudes.

Seismic facies changes along a nearly continuous 24° latitudinal transect: the fjords of Chile and the northern Antarctic Peninsula

High-resolution seismic reflection profiles and piston cores from bays, fjords, and straits of the inland passage of Chile and the northern Antarctic Peninsula region were used to examine changes in seismic facies along a nearly continuous 24 ° latitudinal transect that encompasses temperate, subpolar, and polar climatic regimes. These climatic regimes are characterized by differences in seismic facies. Eight seismic facies are recognized on the basis of reflection character, intensity of acoustic contrast at bounding surfaces, and external shape of the unit. The northern and central Chilean fjord region is a temperate maritime setting characterized by thick ice-proximal and ice-distal glacial-marine strata and a variety of morainal features. The southern inland passage and the South Shetland Islands of Antarctica represent transitional (sub-polar) glacial-marine settings. These areas are characterized by relict ice-proximal glacialmarine facies and relatively thin ice-distal glacial-marine facies. Morainal ridges occur in bays and fjords of Chile. The northern Antarctic Peninsula is a polar climatic setting. The straits, bays, and fjords of the region have been virtually scraped clean of sediments. A thin, draping glacial-marine unit occurs locally in these areas.

Late Jurassic palaeogeography and anaerobic-dysaerobic sedimentation in the northern Antarctic Peninsula region

Late Jurassic anaerobic-dysaerobic mudstones crop out on both the Weddell Sea (back-arc) and Pacific (fore-are) margins of the northern Antarctic Peninsula. The only known occurrence on the Pacific margin of the Antarctic Peninsula is the Anchorage Formation of Livingston Island. This mudstone dominated unit comprises interbedded volcaniclastic sandstones, pyroclastic/epiclastic tuffs and radiolarian mudstones. The volcaniclastic sandstones are interpreted as representing deposition from turbidity currents. The tuffs represent sedimentation by both primary airfall processes and resedimentation by low concentration turbidity currents. The radiolarian mudstones represent suspen­sion sedimentation, and reveal an upward increase in bioturbation with a transition from anaerobic-dysaerobic conditions to dysaerobic-aerobic conditions. These facies and the observed vertical change in oxygenation conditions are similar to those seen in the Nordenskjöld Formation on the Weddell Sea margin of the Antarctic Peninsula. However, biostratigraphical investigations show that the transition from dysaerobic to aerobic conditions occurred during the late Kimmeridgian-early Tithonian in the Anchorage Formation but late Tithonian or early Berriasian in the Nordenskjold Formation. This diachroneity is related to the palaeogeographical development of the Antarctic Peninsula magmatic arc. A wide epicontinental sea and subdued arc relief in the early Kimmeridgian was followed in the Tithonian by arc uplift, increasing oxygenation in the fore-are basin, and the development of a restricted basin in the hack-arc region. In latest Tithonian-earliest Berriasian times a substantial arc had developed which supplied volcaniclastic sediment to the fore-arc basin; only then was the back-arc basin undergoing the transition from dysaerobic to aerobic conditions. Anaerobic conditions initiated by regional upwelling and expansion of the oxygen minimum zone were perpetuated in a silled basin in the back-arc area, formed by the emergent arc.