Beschreibung: In paleoenvironmental studies, the mineralogical composition of sediments is an important indicator. In combination with other indicators, they contribute to the understanding of changes in sediment sourcing as well as in weathering and depositional processes. Fourier transforminfrared spectroscopy (FTIRS) spectra contain information on mineralogical composition because eachmineral has a unique absorption pattern in the mid-IR range. Although easily obtained, FTIR spectra are often too complex to infermineral concentrations directly. In this study, we use a calibration set of ca. 200 sediment samples conventionally measured using X-ray diffraction (XRD) in order to developmultivariate, partial least squares (PLS) regressionmodels relatingmineral contents to sediment spectra. Good correlations were obtained for the most common minerals (e.g. quartz, K-feldspar, illite, plagioclase, smectite, calcite). Correlation coefficients ranged from 0.85 to 0.92, coefficients for the validation varied from 0.64 to 0.80, the number of latent variables (PLS regression components) in the models ranged between 3 and 7 and the range of variation of the RMSEcv gradient was from 15.28 to 5.7.

Beschreibung: Abstract The Weddell Sea sector is one of the main formation sites for Antarctic Bottom Water and an outlet for about one fifth of Antarctica's continental ice volume. Over the last few decades, studies on glacial–geological records in this sector have provided conflicting reconstructions of changes in ice-sheet extent and ice-sheet thickness since the Last Glacial Maximum (LGM at ca 23–19 calibrated kiloyears before present, cal ka BP). Terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the Weddell Sea, ice-sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding-line advance at the LGM. Other marine geological and geophysical studies concluded that subglacial bedforms mapped on the Weddell Sea continental shelf, subglacial deposits and sediments over-compacted by overriding ice recovered in cores, and the few available radiocarbon ages from marine sediments are consistent with major ice-sheet advance at the LGM. Reflecting the geological interpretations, different ice-sheet models have reconstructed conflicting {LGM} ice-sheet configurations for the Weddell Sea sector. Consequently, the estimated contributions of ice-sheet build-up in the Weddell Sea sector to the {LGM} sea-level low-stand of ~130 m vary considerably. In this paper, we summarise and review the geological records of past ice-sheet margins and past ice-sheet elevations in the Weddell Sea sector. We compile marine and terrestrial chronological data constraining former ice-sheet size, thereby highlighting different levels of certainty, and present two alternative scenarios of the {LGM} ice-sheet configuration, including time-slice reconstructions for post-LGM grounding-line retreat. Moreover, we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research. The aim of our study is to provide two alternative interpretations of glacial–geological datasets on Antarctic Ice-Sheet History for the Weddell Sea sector, which can be utilised to test and improve numerical ice-sheet models.

Beschreibung: Abstract Radiocarbon and uranium-thorium dating results are presented from a genus of calcitic Antarctic cold-water octocorals (family Coralliidae), which were collected from the Marie Byrd Seamounts in the Amundsen Sea (Pacific sector of the Southern Ocean) and which to date have not been investigated geochemically. The geochronological results are set in context with solution and laser ablation-based element/Ca ratios (Li, B, Mg, Mn, Sr, Ba, U, Th). Octocoral radiocarbon ages on living corals are in excellent agreement with modern ambient deep-water �14C, while multiple samples of individual fossil coral specimens yielded reproducible radiocarbon ages. Provided that local radiocarbon reservoir ages can be derived for a given time, fossil Amundsen Sea octocorals should be reliably dateable by means of radiocarbon. In contrast to the encouraging radiocarbon findings, the uranium-series data are more difficult to interpret. The uranium concentration of these calcitic octocorals is an order of magnitude lower than in the aragonitic hexacorals that are conventionally used for geochronological investigations. While modern and Late Holocene octocorals yield initial δ234U in good agreement with modern seawater, our results reveal preferential inward diffusion of dissolved alpha-recoiled 234U and its impact on fossil coral δ234U. Besides alpha-recoil related 234U diffusion, high-resolution sampling of two fossil octocorals further demonstrates that diagenetic uranium mobility has offset apparent coral U-series ages. Combined with the preferential alpha-recoil 234U diffusion, this process has prevented fossil octocorals from preserving a closed system U-series calendar age for longer than a few thousand years. Moreover, several corals investigated contain significant initial thorium, which cannot be adequately corrected for because of an apparently variable initial 232Th/230Th. Our results demonstrate that calcitic cold-water corals are unsuitable for reliable U-series dating. Mg/Ca ratios within single octocoral specimens are internally strikingly homogeneous, and appear promising in terms of their response to ambient temperature. Magnesium/lithium ratios are significantly higher than usually observed in other deep marine calcifiers and for many of our studied corals are remarkably close to seawater compositions. Although this family of octocorals is unsuitable for glacial deep-water �14C reconstructions, our findings highlight some important differences between hexacoral (aragonitic) and octocoral (calcitic) biomineralisation. Calcitic octocorals could still be useful for trace element and some isotopic studies, such as reconstruction of ambient deep water neodymium isotope composition or pH, via boron isotopic measurements.

Beschreibung: The AND-1B drill core (1285 m-long) was recovered, inside the ANDRILL (ANtarctic geological DRILLing) Program, during the austral summer of 2006/07 from beneath the floating McMurdo Ice Shelf. Drilling recovered a stratigraphic succession of alternating diamictites, diatomites and volcaniclastic sediments spanning about the last 14 Ma. A core portion between 350 and 480 mbsf, including a 80 m-thick diatomite interval recording the early Pliocene warming event, was investigated in term of opal biogenic content and element geochemistry. Across the diatomite interval, in spite of the lithological uniformity, a fluctuating biogenic opal profile mirrors the δ18O record, testifying a decrease in productivity when temperature drops as a consequence of small glacial fluctuations. The comparison of biogenic opal data with Chaetoceros spp. abundances from Konfirst et al. (2012) documents alternations between periods of high primary productivity in stratified surface waters and of enhanced terrigenous input in ice-free conditions. Cluster analysis discriminates elements associated to terrigenous input from those subject to biogenic control. Further separation in sub-cluster was interpreted in term of different element response to changes in provenance but also to depositional/early diagenetic conditions at the seafloor. Whilst K and Ti are related to different sediment sources confirming previous studies from the same interval, V, Zn and, to a lesser extent, Fe, document reducing/anoxic conditions during the diatomites deposition (in particular in 400–460 mbsf interval). Mg, Sr and Mn contents are related to authigenic carbonate precipitation whilst Ba is interested by nonsteady-state processes leading to local peaks of barium below the sulphate-rich/sulphate-poor pore water boundary where generally the low degree of barite saturation is responsible for Ba remobilization. Such alteration in depositional dynamics, responsible of the precipitation of an oxygen-depleted barium phase, was probably induced by change in sedimentation rate and/or in palaeoenvironmental conditions.

Beschreibung: Glaciers flowing into the Amundsen Sea Embayment (ASE) account for 〉35% of the total discharge of the West Antarctic Ice Sheet (WAIS) and have thinned and retreated dramatically over the past two decades. Here we present detailed marine geological data and an extensive new radiocarbon dataset from the eastern ASE in order to constrain the retreat of the WAIS since the Last Glacial Maximum (LGM) and assess the significance of these recent changes. Our dating approach, relying mainly on the acid insoluble organic (AIO) fraction, utilises multi-proxy analyses of the sediments to characterise their lithofacies and determine the horizon in each core that would yield the most reliable age for deglaciation. In total, we dated 69 samples and show that deglaciation of the outer shelf was underway before 20,600 calibrated years before present (cal. yr BP), reaching the mid-shelf by 13,575 cal. yr BP and the inner shelf to within c.150 km of the present grounding line by 10,615 cal. yr BP. The timing of retreat is broadly consistent with previously published radiocarbon dates on biogenic carbonate from the eastern ASE as well as AIO 14C ages from the western ASE and provides new constraints for ice sheet models. The overall retreat trajectory – slow on the outer shelf, more rapid from the middle to inner shelf – clearly highlights the importance of reverse bedslopes in controlling phases of accelerated groundling line retreat. Despite revealing these broad scale trends, the current dataset does not capture detailed changes in ice flow, such as stillstands during grounding line retreat (i.e., deposition of grounding zone wedges) and possible readvances as depicted in the geomorphological record.

Beschreibung: The Weddell Sea sector is one of the main formation sites for Antarctic Bottom Water and an outlet for about one fifth of Antarctica’s continental ice volume. Over the last few decades, studies on glacialegeological records in this sector have provided conflicting reconstructions of changes in ice-sheet extent and ice-sheet thickness since the Last Glacial Maximum (LGM at ca 23e19 calibrated kiloyears before present, cal ka BP). Terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the Weddell Sea, ice-sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding-line advance at the LGM. Other marine geological and geophysical studies concluded that subglacial bedforms mapped on theWeddell Sea continental shelf, subglacial deposits and sediments over-compacted by overriding ice recovered in cores, and the few available radiocarbon ages from marine sediments are consistent with major ice-sheet advance at the LGM. Reflecting the geological interpretations, different icesheet models have reconstructed conflicting LGM ice-sheet configurations for the Weddell Sea sector. Consequently, the estimated contributions of ice-sheet build-up in the Weddell Sea sector to the LGM sealevel low-stand of w130 m vary considerably. In this paper, we summarise and review the geological records of past ice-sheet margins and past icesheet elevations in the Weddell Sea sector. We compile marine and terrestrial chronological data constraining former ice-sheet size, thereby highlighting different levels of certainty, and present two alternative scenarios of the LGM ice-sheet configuration, including time-slice reconstructions for post- LGM grounding-line retreat. Moreover, we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research. The aim of our study is to provide two alternative interpretations of glacialegeological datasets on Antarctic Ice- Sheet History for the Weddell Sea sector, which can be utilised to test and improve numerical icesheet models

Beschreibung: The continental margin of the Ross Sea has been consistently sensitive to the advance and retreat of the Ross Ice Sheet (RIS) between the interglacial and glacial periods. This study examines changes of the glaciomarine sedimentation on the continental slope and rise to the eastern side of Hillary Canyon in the central Ross Sea, using three gravity cores collected at increasing water depths. Besides older AMS 14C ages of bulk sediments, based on the analytical results, sediment lithology was divided into units A, B1, and B2, representing Holocene, deglacial, and glacial periods, respectively. The sedimentation rate decreased as the water depth increased, with a higher sedimentation rate in the deglacial period (unit B1) than the Holocene (unit A). Biological productivity proxies were significantly higher in glacial unit B2 than in interglacial unit A, with transitional values observed in deglacial unit B1. Biological productivity generally decreased in the Antarctic continental margin during the glacial period because of extensive sea ice coverage. The higher biogenic contents in unit B2 are primarily attributed to the increased transport of eroded and reworked shelf sediments that contained abundant biogenic components to the continental slope and rise beneath the advancing RIS. Thus, glacial sedimentation on the continental slope and rise of the central Ross Sea was generally governed by the activity of the RIS, which generated melt-water plumes and debris flows at the front of the grounding line, although the continental rise might have experienced seasonally open conditions and lateral effects due to the bottom current.

Beschreibung: In Antarctica, the near-source exposures of volcanic eruption deposits are often limited as they are not well preserved in the dynamic glacial environment, thus making volcanological reconstructions of explosive eruptions extremely challenging. Fortunately, pyroclastic deposits from explosive eruptions are preserved in Southern Ocean sediments surrounding Antarctica, and the tephrostratigraphy of these sequences offers crucial volcanological information including the timing and tempo of past eruptions, their magnitude, and eruption dynamics. Here we report the results of a tephrostratigraphy and tephrochronology study focused on four sediment cores recovered from the Wood Bay area in the western Ross Sea, Antarctica. In all these sedimentary sequences, we found a well-stratified primary tephra of considerable thickness, up to 80 cm, hereafter named the Aviator Tephra (AVT). According to the characteristics of the tephra deposit and its distribution, the AVT was associated with an eruption of considerable intensity, potentially representing one of the largest Holocene eruptions recorded in Antarctica. Based on the major and trace element geochemistry and the mineral assemblage of the tephra, Mount Rittmann was identified as the source of the AVT. A Holocene age of ∼11 ka was determined by radiocarbon dating organic material within the sediments and 40Ar-39Ar dating of alkali-feldspar crystals included in the tephra. Eruption dynamics were initially dominated by hydromagmatic magma fragmentation conditions producing a sustained, relatively wet and ash-rich eruptive cloud. The eruption then evolved into a highly energetic, relatively dry magmatic Plinian eruption. The last phase was characterized by renewed efficient magma-water interaction and/or collapse of the eruptive column producing pyroclastic density currents and associated co-ignimbritic plumes. The distal tephra deposits might be linked to the widespread lag breccia layer previously identified on the rim of the Mount Rittmann caldera which share the same geochemical composition. Diatoms found in the sediments surrounding the AVT and the primary characteristics of the tephra indicate that the Wood Bay area was open sea at the time of the eruption, which is much earlier than previously thought. AVT is also an excellent tephrostratigraphic marker for the Wood Bay area, in the Ross Sea, and a useful marker for future synchronization of continental ice and marine archives in the region.

Beschreibung: Published

Beschreibung: 106629

Beschreibung: 1V. Storia eruttiva

Beschreibung: 3V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici

Beschreibung: 1A. Geomagnetismo e Paleomagnetismo

Beschreibung: 5A. Ricerche polari e paleoclima

Beschreibung: JCR Journal