Franco Maria Talarico

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

The stratigraphic signature of the late Cenozoic Antarctic Ice Sheets in the Ross Embayment

A 1284.87-m-long sediment core (AND-1B) from beneath the McMurdo sector of the Ross Ice Shelf provides the most complete single section record to date of fluctuations of the Antarctic Ice Sheets over the last 13 Ma. The core contains a succession of subglacial, glacimarine, and marine sediments that comprise ∼58 depositional sequences of possible orbital-scale duration. These cycles are constrained by a chronology based on biostratigraphic, magnetostratigraphic, and 40 Ar/ 39 Ar isotopic ages. Each sequence represents a record of a grounded ice-sheet advance and retreat cycle over the AND-1B drill site, and all sediments represent subglacial or marine deposystems with no subaerial exposure surfaces or terrestrial deposits. On the basis of characteristic facies within these sequences, and through comparison with sedimentation in modern glacial environments from various climatic and glacial settings, we identify three facies associations or sequence “motifs” that are linked to major changes in ice-sheet volume, glacial thermal regime, and climate. Sequence motif 1 is documented in the late Pleistocene and in the early Late Miocene intervals of AND-1B, and it is dominated by diamictite of subglacial origin overlain by thin mudstones interpreted as ice-shelf deposits. Motif 1 sequences lack evidence of subglacial meltwater and represent glaciation under cold, “polar”-type conditions. Motif 2 sequences were deposited during the Pliocene and early Pleistocene section of AND-1B and are characterized by subglacial diamictite overlain by a relatively thin proglacial-marine succession of mudstone-rich facies deposited during glacial retreat. Glacial minima are represented by diatom-bearing mudstone, and diatomite. Motif 2 represents glacial retreat and advance under a “subpolar” to “polar” style of glaciation that was warmer than present, but that had limited amounts of subglacial meltwater. Sequence motif 3 consists of subglacial diamictite that grades upward into a 5- to 10-m-thick proglacial retreat succession of stratified diamictite, graded conglomerate and sandstone, graded sandstone, and/or rhythmically stratified mudstone. Thick mudstone intervals, rather than diatomite-dominated deposition during glacial minima, suggest increased input of meltwater from nearby terrestrial sources during glacial minima. Motif 3 represents Late Miocene “subpolar”-style glaciation with significant volumes of glacially derived meltwater.

Ultra-high-pressure metamorphism in felsic rocks: the garnet-phengite gneisses and quarzites from the Lanterman Range, Antarctica

Mesostructural and microstructural relations between eclogitic boudins and country gneisses in the Ross Orogen of the Lanterman Range (Northern Victoria Land, Antarctica) are in some areas characterized by interlayering with sharp contacts on a cm scale, which indicate that the two rock-types underwent a common metamorphic evolution. Contrary to many other UHP felsic rocks that only preserve a poor record of the HP stage, the studied rocks have recorded a metamorphic history ranging from initial prograde amphibolite facies through the eclogite facies to the retrogressive amphibolite facies. The prograde amphibolite stage is documented by garnet relics preserving prograde zoning and bearing biotite, plagioclase, muscovite, phengite and rutile inclusions. The eclogite stage is characterized by the coexistence of phengite with pyrope-grossu-larite rich garnet, the latter containing phengite and paragonite inclusions, and by radial fractures within garnet around quartz pseudomorphs after coesite. Symplectites have formed during the amphibolite-facies retrogression. They consist mainly of biotite + plagioclase around phengite and garnet; muscovite, biotite and plagioclase grew along the main foliation. The reconstructed metamorphic evolution, involves a steep prograde and retrograde PT path as well as a HP-T peak. Along with the geochronological evidence of fast exhumation, this supports a model of arc-continent collision, with the HP rocks belonging to the over-riding plate. Their exhumation is mainly controlled by extension related to renewed “rollback” of subduction in front of the orogenic zone.

Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene

Significance New information from the ANDRILL-2A drill core and a complementary ice sheet modeling study show that polar climate and Antarctic ice sheet (AIS) margins were highly dynamic during the early to mid-Miocene. Changes in extent of the AIS inferred by these studies suggest that high southern latitudes were sensitive to relatively small changes in atmospheric CO2 (between 280 and 500 ppm). Importantly, reconstructions through intervals of peak warmth indicate that the AIS retreated beyond its terrestrial margin under atmospheric CO2 conditions that were similar to those projected for the coming centuries. Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23–14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

Relict granulites in the Ross Orogen of northern Victoria Land (Antarctica), II. Geochemistry and palaeo-tectonic implications

Abstract Relict granulite facies rocks of the region between Priestley and Aviator Glaciers (Wilson Terrane, Ross Orogen) comprise an older supracrustal sequence which has been intruded by meta-igneous mafic and intermediate granulites and by later synmetamorphic intrusive enderbite and charno-enderbite. The intrusive granulites show chemical patterns which are similar to the ∼ 1000-Ma-old Mawson Charnockite of Mac Robertson Land. Despite significant chemical modifications which have affected most granulite rocks during the Ross amphibolite facies metamorphic overprinting, Cpx ± Opx-bearing felsic granulites of the Campbell Glacier still retain similarities to depleted compositions. Whole-rock SmNd model ages ranging between 1.8 and 2.2 Ga and comparison of the meta-igneous granulites with similar rocks of the Proterozoic mobile belts of East Antarctica support the idea that the relict granulite rocks of the Wilson Terrane represent reworked Proterozoic fragments of the East Antarctic Craton in the Early Palaeozoic Ross Orogen.

Eclogite at the Antarctic palaeo-Pacific active margin of Gondwana (Lanterman Range, northern Victoria Land, Antarctica)

Well-preserved eclogites were found for the first time in Antarctica, at the Lanterman Range, northern Victoria Land. They are part of a mafic-ultramafic belt that lies between the Wilson Terrane, representing part of the palaeo-Pacific margin of Gondwana, and the Bowers Terrane, a Cambro-Ordovician volcanic arc and related sediments, accreted to the margin during the Ross Orogeny. The eclogites formed at temperatures in the range 750-850°C and pressures above 15 kbar and subsequently experienced a decompressional path to low pressure amphibolite facies conditions. The formation and exhumation of eclogites and the attainment of the metamorphic peak in adjacent rock units is consistent with a plate convergent setting model at the palaeo-Pacific margin of Gondwana.

Tectonic Model for Development of the Byrd Glacier Discontinuity and Surrounding Regions of the Transantarctic Mountains during the Neoproterozoic — Early Paleozoic

The Byrd Glacier discontinuity is a major tectonic boundary crossing the Ross Orogen, with crystalline rocks to the north and primarily sedimentary rocks to the south. Most models for the tectonic development of the Ross Orogen in the central Transantarctic Mountains consist of two-dimensional transects across the belt, but do not address the major longitudinal contrast at Byrd Glacier. This paper presents a tectonic model centering on the Byrd Glacier discontinuity. Rifting in the Neoproterozoic produced a crustal promontory in the craton margin to the north of Byrd Glacier. Oblique convergence of a terrane (Beardmore microcontinent) during the latest Neoproterozoic and Early Cambrian was accompanied by subduction along the craton margin of East Antarctica. New data presented herein in support of this hypothesis are U-Pb dates of 545.7 ±6.8 Ma and 531.0 ±7.5 Ma on plutonic rocks from the Britannia Range, directly north of Byrd Glacier. After docking of the terrane, subduction stepped out, and Byrd Group was deposited during the Atdabanian-Botomian across the inner margin of the terrane. Beginning in the upper Botomian, reactivation of the sutured boundaries of the terrane resulted in an outpouring of clastic sediment and folding and faulting of the Byrd Group.

Antarctic Ice Sheet variability across the Eocene-Oligocene boundary climate transition.

Sensitive ice sheets Why did the Antarctic Ice Sheet begin to grow 34 million years ago, and what does that have to do with us? Galeotti et al. studied a marine sediment core recovered from just off the coast of Antarctica (see the Perspective by Lear and Lunt). The ice sheet did not begin to grow until atmospheric CO2 concentrations had dropped to below around 600 parts per million. Indeed, the ice sheet was unstable when CO2 was higher. As modern atmospheric CO2 concentrations continue their rise, a shift back to an unstable Antarctic Ice Sheet could increase harmful rises in sea level. Science, this issue p. 76; see also p. 34 The growth of the Antarctic Ice Sheet began only when atmospheric levels of carbon dioxide dropped low enough. [Also see Perspective by Lear and Lunt] About 34 million years ago, Earth’s climate cooled and an ice sheet formed on Antarctica as atmospheric carbon dioxide (CO2) fell below ~750 parts per million (ppm). Sedimentary cycles from a drillcore in the western Ross Sea provide direct evidence of orbitally controlled glacial cycles between 34 million and 31 million years ago. Initially, under atmospheric CO2 levels of ≥600 ppm, a smaller Antarctic Ice Sheet (AIS), restricted to the terrestrial continent, was highly responsive to local insolation forcing. A more stable, continental-scale ice sheet calving at the coastline did not form until ~32.8 million years ago, coincident with the earliest time that atmospheric CO2 levels fell below ~600 ppm. Our results provide insight into the potential of the AIS for threshold behavior and have implications for its sensitivity to atmospheric CO2 concentrations above present-day levels.

Incomplete blueschist re-crytallization in high-grade metamorphics from the Sesia-Lanzo unit (Vasario-Sparone subunit, Western Alps): A case history of metastability

Abstract The Vasario-Sparone (VS) subunit is part of the Sesia-Lanzo unit (Western Alps) and represents a thin and discontinuous slice of pre-Alpine deep continental crust. It is tectonically interposed between the “eclogitic mica schists” and the “gneiss minuti” subunits. Metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the VS subunit. The VS rocks record a pre-Alpine high grade of metamorphism (T=780±50°C, P≈6 kbar) which is comparable to the metamorphism dated around 500 Ma in the Ivrea Zone. During the Alpine evolution the VS rocks sustained an early blueschist stage (T=300–400°C, P⩾8–10 kbar) of probable Cretaceous age and a later low-T, low-P stage of Tertiary age. The blueschist transformations are governed by slow reaction kinetics, by the heterogeneous distribution of pervasive deformations and by syn-metamorphic fluid phase. In particular, transient kyanite formed in the early stages of the blueschist re-crystallization in response to a process controlled by fluid-deficient conditions and by internal buffering of aH2O at low values.