Geology of the Sirius Group at Mount Feather and Table Mountain, South Victoria Land, Antarctica

<p>The Sirius Group comprises of wet based glacial and related deposits found at high elevations throughout the Transantarctic Mountains. The discovery of marine Pliocene diatoms from within glacial till by Harwood (1983) led Webb et al. (1984) to propose that they were sourced from diatom bearing sediment eroded by glaciers from middle Pliocene marine basins inland of the Transantarctic Mountains. Others consider that temperatures during middle Pliocene times were not high enough to melt back the Antarctic ice sheet and expose these inland basins. They support the long held view of a stable Antarctic ice sheet since middle Miocene times, and insist that the Sirius Group is much older, explaining the diatoms as wind blown. This study was undertaken in order to determine whether the diatoms were incorporated into Sirius Group tills during or after their deposition. Sites were sampled at Mount Feather and Table Mountain in South Victoria Land. The distribution of diatoms through the upper 37 cm of the till were documented. Samples were also taken in snow and from other non Sirius Group surfaces (regolith) for comparison purposes. The geomorphic setting of the Sirius Group tills at Mt. Feather and Table Mountain suggests that their deposition predated the deep valleys that now run through the Transantarctic Mountains. Diatom abundances from within the tills are low (averaging about 1 diatom diatom per gram) and highly variable from site to site. Initially 184 diatoms were recovered from 10 samples at Mt. Feather and less than 7 diatoms were found from 4 samples of till at Table Mountain. At Mount Feather diatoms are concentrated in the surface few centimetres of the till and numbers generally decrease with depth. The pore size within the tills is highly variable but is on average ten times the size of the average diatom (10-15 microns) from within the deposits, allowing at least some diatoms to work their way into the tills from the surface. Diatoms from the snow and regolith from other rock surfaces have a similar diatom assemblage to the Sirius tills, containing many of the same common forms. Some non Sirius Group regolith samples have much larger concentrations of diatoms suggesting they have a much better trapping ability than the Sirius Group tills. These data indicate that most diatoms from the Sirius Group tills have been introduced from the atmosphere and have worked their way into the till. Thus the Sirius diatoms record not Pliocene marine basins of the Antarctic interior and subsequent extensive over riding glaciation, but the atmospheric transport and collection of both modern and ancient diatom bearing dust from within and beyond the continent. The Sirius Group tills do however have a phytolith (siliceous clasts from the cells of plant tissue) flora of glaciogenic origin, indicated by the lack of a vertical trend in abundance and very low levels of phytoliths in nearby snow and regolith samples.</p>

Geology of the Sirius Group at Mount Feather and Table Mountain, South Victoria Land, Antarctica

<p>The Sirius Group comprises of wet based glacial and related deposits found at high elevations throughout the Transantarctic Mountains. The discovery of marine Pliocene diatoms from within glacial till by Harwood (1983) led Webb et al. (1984) to propose that they were sourced from diatom bearing sediment eroded by glaciers from middle Pliocene marine basins inland of the Transantarctic Mountains. Others consider that temperatures during middle Pliocene times were not high enough to melt back the Antarctic ice sheet and expose these inland basins. They support the long held view of a stable Antarctic ice sheet since middle Miocene times, and insist that the Sirius Group is much older, explaining the diatoms as wind blown. This study was undertaken in order to determine whether the diatoms were incorporated into Sirius Group tills during or after their deposition. Sites were sampled at Mount Feather and Table Mountain in South Victoria Land. The distribution of diatoms through the upper 37 cm of the till were documented. Samples were also taken in snow and from other non Sirius Group surfaces (regolith) for comparison purposes. The geomorphic setting of the Sirius Group tills at Mt. Feather and Table Mountain suggests that their deposition predated the deep valleys that now run through the Transantarctic Mountains. Diatom abundances from within the tills are low (averaging about 1 diatom diatom per gram) and highly variable from site to site. Initially 184 diatoms were recovered from 10 samples at Mt. Feather and less than 7 diatoms were found from 4 samples of till at Table Mountain. At Mount Feather diatoms are concentrated in the surface few centimetres of the till and numbers generally decrease with depth. The pore size within the tills is highly variable but is on average ten times the size of the average diatom (10-15 microns) from within the deposits, allowing at least some diatoms to work their way into the tills from the surface. Diatoms from the snow and regolith from other rock surfaces have a similar diatom assemblage to the Sirius tills, containing many of the same common forms. Some non Sirius Group regolith samples have much larger concentrations of diatoms suggesting they have a much better trapping ability than the Sirius Group tills. These data indicate that most diatoms from the Sirius Group tills have been introduced from the atmosphere and have worked their way into the till. Thus the Sirius diatoms record not Pliocene marine basins of the Antarctic interior and subsequent extensive over riding glaciation, but the atmospheric transport and collection of both modern and ancient diatom bearing dust from within and beyond the continent. The Sirius Group tills do however have a phytolith (siliceous clasts from the cells of plant tissue) flora of glaciogenic origin, indicated by the lack of a vertical trend in abundance and very low levels of phytoliths in nearby snow and regolith samples.</p>

Geology, Mineralogy and Geochemistry of the Late Cenozoic McMurdo Volcanic Group, Victoria Land, Antarctica

<p>Rocks of the McMurdo Volcanic Group occur as stratovolcanoes, shield volcanoes, scoria cones, plugs, flows and volcanic piles up to 4000 m high along the Ross Sea margin of the Transantarctic Mountains and make up the Balleny Islands 300 km north of the Antarctic continental margin. The rocks are predominantly undersaturated and range from alkali basalt and basanite to trachyte and phonolite. Four volcanic provinces are recognised; Balleny, Hallett, Melbourne and Erebus. The Balleny volcanic province is situated along a transform fault in the South Pacific Ocean. The rocks are predominantly basanite. Hallett volcanic province occurs along the coast of northern Victoria Land as four elongate piles formed extensive of hyaloclastites, tuffs, breccias and capped by subaerial eruptive products. The lavas are a basanite/alkali basalt-trachyte-quartz trachyte association, and were extruded over the last 7 m.y. Melbourne volcanic province stretches across the Transantarctic Mountains in northern Victoria Land and ranges in age from 0 to 7 m.y. A Central Suite of intermediate and trachytic lavas form stratovolcanoes, cones and plugs, while many small basanite outcrops constitute a Local Suite. Three lava lineages, resulting from differentiation, are recognised. 1) Lavas at The Pleiades and Mt Overlord consist of a mildly potassic trachyandesite-tristanite-K-trachyte-peralkaline K-trachyte lineage. Major, trace and rare earth element (REE) data suggest evolution by fractional crystallization of olivine, clinopyroxene, magnetite, apatite and feldspar. 2) A basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite lineage, found at The Pleiades is believed to result from fractional crystallization of olivine, clinopyroxene, kaersutite, magnetite, apatite and feldspar. 3) An oversaturated (Q = 0 to 18%) strongly potassic quartz trachyandesite-quartz tristanite-quartz trachyte lineage occurs at only Mt Melbourne. The Erebus volcanic province covers all McMurdo Volcanic Group rocks in south Victoria Land. Mt Erebus itself is still active, but the province includes rocks as old as 15 m.y. Two lava lineages very similar chemically are recognised: 1) The Erebus lineage consists of strongly porphyritic nepheline hawaiite-nepheline benmoreite-anorthoclase phonolite. Phenocrysts of feldspar, clinopyroxene, olivine, magnetite and apatite are characteristic. The chemistry of the lineage is compatible with fractional crystallization of the phenocryst phases. 2) A kaersutite lineage consists of basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite-kaersutite phonolite-pyroxene phonolite. Clinopyroxene (Wo44-48 En40-48 Fs7-14) is ubiquitous, kaersutite is common in all intermediate lavas and primary olivine (Fa12 to Fa26) is confined to the basanites. Major element mass balance models for lavas from Hut Point Peninsula suggest formation by fractional crystallization of olivine, clinopyroxene, spinel (includes magnetite and ilmenite), kaersutite, feldspar and apatite. Middle REE show a marked depletion consistent with kaersutite fractionation. REE abundances were evaluated using the mass balance models and published partition coefficients. Calculated REE abundances show excellent agreement with the measured values. Abundances of "incompatible" elements Pb, Rb, Cs, Th and U are not consistent with the models and "volatile enrichment" processes are invoked to explain their abundances. Intermediate lavas of the kaersutite lineage are rare in the Erebus volcanic province, occurring only at Hut Point Peninsula and Brown Peninsula. At other areas basanite and phonolite lavas predominate. However these are considered to form by fractional crystallization processes similar to Hut Point Peninsula lavas. Erebus lineage lavas differentiated at higher temperatures and, lower PH2O than those of the kaersutite lineage, which characterize the periphery of Ross Island. REE abundances and comparison with experimental melting studies indicate DVDP basanite originated by a low degree of partial melting (1-5%) of a hydrous garnet peridotite mantle at pressures of 25-30 kbars. These data suggest that Ross Island is the site of a mantle plume with a diameter of, about 100 km and centred on Mt Erebus.</p>

Geology, Mineralogy and Geochemistry of the Late Cenozoic McMurdo Volcanic Group, Victoria Land, Antarctica

<p>Rocks of the McMurdo Volcanic Group occur as stratovolcanoes, shield volcanoes, scoria cones, plugs, flows and volcanic piles up to 4000 m high along the Ross Sea margin of the Transantarctic Mountains and make up the Balleny Islands 300 km north of the Antarctic continental margin. The rocks are predominantly undersaturated and range from alkali basalt and basanite to trachyte and phonolite. Four volcanic provinces are recognised; Balleny, Hallett, Melbourne and Erebus. The Balleny volcanic province is situated along a transform fault in the South Pacific Ocean. The rocks are predominantly basanite. Hallett volcanic province occurs along the coast of northern Victoria Land as four elongate piles formed extensive of hyaloclastites, tuffs, breccias and capped by subaerial eruptive products. The lavas are a basanite/alkali basalt-trachyte-quartz trachyte association, and were extruded over the last 7 m.y. Melbourne volcanic province stretches across the Transantarctic Mountains in northern Victoria Land and ranges in age from 0 to 7 m.y. A Central Suite of intermediate and trachytic lavas form stratovolcanoes, cones and plugs, while many small basanite outcrops constitute a Local Suite. Three lava lineages, resulting from differentiation, are recognised. 1) Lavas at The Pleiades and Mt Overlord consist of a mildly potassic trachyandesite-tristanite-K-trachyte-peralkaline K-trachyte lineage. Major, trace and rare earth element (REE) data suggest evolution by fractional crystallization of olivine, clinopyroxene, magnetite, apatite and feldspar. 2) A basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite lineage, found at The Pleiades is believed to result from fractional crystallization of olivine, clinopyroxene, kaersutite, magnetite, apatite and feldspar. 3) An oversaturated (Q = 0 to 18%) strongly potassic quartz trachyandesite-quartz tristanite-quartz trachyte lineage occurs at only Mt Melbourne. The Erebus volcanic province covers all McMurdo Volcanic Group rocks in south Victoria Land. Mt Erebus itself is still active, but the province includes rocks as old as 15 m.y. Two lava lineages very similar chemically are recognised: 1) The Erebus lineage consists of strongly porphyritic nepheline hawaiite-nepheline benmoreite-anorthoclase phonolite. Phenocrysts of feldspar, clinopyroxene, olivine, magnetite and apatite are characteristic. The chemistry of the lineage is compatible with fractional crystallization of the phenocryst phases. 2) A kaersutite lineage consists of basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite-kaersutite phonolite-pyroxene phonolite. Clinopyroxene (Wo44-48 En40-48 Fs7-14) is ubiquitous, kaersutite is common in all intermediate lavas and primary olivine (Fa12 to Fa26) is confined to the basanites. Major element mass balance models for lavas from Hut Point Peninsula suggest formation by fractional crystallization of olivine, clinopyroxene, spinel (includes magnetite and ilmenite), kaersutite, feldspar and apatite. Middle REE show a marked depletion consistent with kaersutite fractionation. REE abundances were evaluated using the mass balance models and published partition coefficients. Calculated REE abundances show excellent agreement with the measured values. Abundances of "incompatible" elements Pb, Rb, Cs, Th and U are not consistent with the models and "volatile enrichment" processes are invoked to explain their abundances. Intermediate lavas of the kaersutite lineage are rare in the Erebus volcanic province, occurring only at Hut Point Peninsula and Brown Peninsula. At other areas basanite and phonolite lavas predominate. However these are considered to form by fractional crystallization processes similar to Hut Point Peninsula lavas. Erebus lineage lavas differentiated at higher temperatures and, lower PH2O than those of the kaersutite lineage, which characterize the periphery of Ross Island. REE abundances and comparison with experimental melting studies indicate DVDP basanite originated by a low degree of partial melting (1-5%) of a hydrous garnet peridotite mantle at pressures of 25-30 kbars. These data suggest that Ross Island is the site of a mantle plume with a diameter of, about 100 km and centred on Mt Erebus.</p>

Cosmogenic nuclide exposure age scatter records glacial history and processes in McMurdo Sound, Antarctica

The preservation of cosmogenic nuclides that accumulated during periods of prior exposure but were not subsequently removed by erosion or radioactive decay complicates interpretation of exposure, erosion, and burial ages used for a variety of geomorphological applications. In glacial settings, cold-based, non-erosive glacier ice may fail to remove inventories of inherited nuclides in glacially transported material. As a result, individual exposure ages can vary widely across a single landform (e.g., moraine) and exceed the expected or true depositional age. The surface processes that contribute to inheritance remain poorly understood, thus limiting interpretations of cosmogenic nuclide datasets in glacial environments. Here, we present a compilation of new and previously published exposure ages of multiple lithologies in local Last Glacial Maximum (LGM) and older Pleistocene glacial sediments in the McMurdo Sound region of Antarctica. Unlike most Antarctic exposure chronologies, we are able to compare exposure ages of local LGM sediments directly against an independent radiocarbon chronology of fossil algae from the same sedimentary unit that brackets the age of the local LGM between 12.3 and 19.6 ka. Cosmogenic exposure ages vary by lithology, suggesting that bedrock source and surface processes prior to, during, and after glacial entrainment explain scatter. 10Be exposure ages of quartz in granite, sourced from the base of the stratigraphic section in the Transantarctic Mountains, are scattered but young, suggesting that clasts entrained by sub-glacial plucking can generate reasonable apparent exposure ages. 3He exposure ages of pyroxene in Ferrar Dolerite, which crops out above outlet glaciers in the Transantarctic Mountains, are older, which suggests that clasts initially exposed on cliff faces and glacially entrained by rock fall carry inherited nuclides. 3He exposure ages of olivine in basalt from local volcanic bedrock in the McMurdo Sound region contain many excessively old ages but also have a bimodal distribution with peak probabilities that slightly pre-date and post-date the local LGM; this suggests that glacial clasts from local bedrock record local landscape exposure. With the magnitude and geological processes contributing to age scatter in mind, we examine exposure ages of older glacial sediments deposited by the most extensive ice sheet to inundate McMurdo Sound during the Pleistocene. These results underscore how surface processes operating in the Transantarctic Mountains are expressed in the cosmogenic nuclide inventories held in Antarctic glacial sediments.

Holocene thinning of Darwin and Hatherton glaciers, Antarctica, and implications for grounding-line retreat in the Ross Sea

Chronologies of glacier deposits in the Transantarctic Mountains provide important constraints on grounding-line retreat during the last deglaciation in the Ross Sea. However, between Beardmore Glacier and Ross Island – a distance of some 600 km – the existing chronologies are generally sparse and far from the modern grounding line, leaving the past dynamics of this vast region largely unconstrained. We present exposure ages of glacial deposits at three locations alongside the Darwin–Hatherton Glacier System – including within 10 km of the modern grounding line – that record several hundred meters of Late Pleistocene to Early Holocene thickening relative to present. As the ice sheet grounding line in the Ross Sea retreated, Hatherton Glacier thinned steadily from about 9 until about 3 ka. Our data are equivocal about the maximum thickness and Mid-Holocene to Early Holocene history at the mouth of Darwin Glacier, allowing for two conflicting deglaciation scenarios: (1) ∼500 m of thinning from 9 to 3 ka, similar to Hatherton Glacier, or (2) ∼950 m of thinning, with a rapid pulse of ∼600 m thinning at around 5 ka. We test these two scenarios using a 1.5-dimensional flowband model, forced by ice thickness changes at the mouth of Darwin Glacier and evaluated by fit to the chronology of deposits at Hatherton Glacier. The constraints from Hatherton Glacier are consistent with the interpretation that the mouth of Darwin Glacier thinned steadily by ∼500 m from 9 to 3 ka. Rapid pulses of thinning at the mouth of Darwin Glacier are ruled out by the data at Hatherton Glacier. This contrasts with some of the available records from the mouths of other outlet glaciers in the Transantarctic Mountains, many of which thinned by hundreds of meters over roughly a 1000-year period in the Early Holocene. The deglaciation histories of Darwin and Hatherton glaciers are best matched by a steady decrease in catchment area through the Holocene, suggesting that Byrd and/or Mulock glaciers may have captured roughly half of the catchment area of Darwin and Hatherton glaciers during the last deglaciation. An ensemble of three-dimensional ice sheet model simulations suggest that Darwin and Hatherton glaciers are strongly buttressed by convergent flow with ice from neighboring Byrd and Mulock glaciers, and by lateral drag past Minna Bluff, which could have led to a pattern of retreat distinct from other glaciers throughout the Transantarctic Mountains.

Cosmogenic nuclide exposure age scatter in McMurdo Sound, Antarctica records Pleistocene glacial history and processes

The preservation of cosmogenic nuclides that accumulated during periods of prior exposure but were not subsequently removed by erosion or radioactive decay, complicates interpretation of exposure, erosion, and burial ages used for a variety of geomorphological applications. In glacial settings, cold-based, non-erosive glacier ice may fail to remove inventories of inherited nuclides in glacially transported material. As a result, individual exposure ages can vary widely across a single landform (e.g. moraine) and exceed the expected or true depositional age. The surface processes that contribute to inheritance remain poorly understood, thus limiting interpretations of cosmogenic nuclide datasets in glacial environments. Here, we present a compilation of new and previously published exposure ages of multiple lithologies in local Last Glacial Maximum (LGM) and older Pleistocene glacial sediments in McMurdo Sound, Antarctica. Unlike most Antarctic exposure chronologies, we are able to compare exposure ages of local LGM sediments directly against an independent radiocarbon chronology of fossil algae from the same sedimentary unit that brackets the age of the local LGM between 12.3 and 19.6 ka. Cosmogenic exposure ages vary by lithology, suggesting that bedrock source and surface processes prior to, during, and after glacial entrainment explain scatter. 10Be exposure ages of quartz in granite, sourced from the base of the stratigraphic section in the Transantarctic Mountains, are scattered but young, suggesting that clasts entrained by sub-glacial plucking can generate reasonable apparent exposure ages. 3He exposure ages of pyroxene in Ferrar Dolerite, which outcrops above outlet glaciers in the Transantarctic Mountains, are older, which suggests that clasts initially exposed on cliff faces and glacially entrained by rock fall carry inherited nuclides. 3He exposure ages of olivine in basalt from local volcanic bedrock in McMurdo Sound contain many excessively old ages, but also have a bimodal distribution with peak probabilities that slightly pre-date and post-date the local LGM; this suggests that glacial clasts from local bedrock record local landscape exposure. With the magnitude and geological processes contributing to age scatter in mind, we examine exposure ages of older glacial deposits and suggest that the most extensive Pleistocene ice sheet inundated McMurdo Sound during Marine Isotope Stage 8. These results underscore how surface processes operating in the Transantarctic Mountains are expressed in the cosmogenic nuclide inventories held in Antarctic glacial sediments.