The disintegration of ice shelves, reduced sea-ice and glacier extent, and shifting ecological zones observed around Antarctica highlight the impact of recent atmospheric and oceanic warming on the ...cryosphere. Observations and models suggest that oceanic and atmospheric temperature variations at Antarctica's margins affect global cryosphere stability, ocean circulation, sea levels and carbon cycling. In particular, recent climate changes on the Antarctic Peninsula have been dramatic, yet the Holocene climate variability of this region is largely unknown, limiting our ability to evaluate ongoing changes within the context of historical variability and underlying forcing mechanisms. Here we show that surface ocean temperatures at the continental margin of the western Antarctic Peninsula cooled by 3-4 °C over the past 12,000 years, tracking the Holocene decline of local (65° S) spring insolation. Our results, based on TEX(86) sea surface temperature (SST) proxy evidence from a marine sediment core, indicate the importance of regional summer duration as a driver of Antarctic seasonal sea-ice fluctuations. On millennial timescales, abrupt SST fluctuations of 2-4 °C coincide with globally recognized climate variability. Similarities between our SSTs, Southern Hemisphere westerly wind reconstructions and El Niño/Southern Oscillation variability indicate that present climate teleconnections between the tropical Pacific Ocean and the western Antarctic Peninsula strengthened late in the Holocene epoch. We conclude that during the Holocene, Southern Ocean temperatures at the western Antarctic Peninsula margin were tied to changes in the position of the westerlies, which have a critical role in global carbon cycling.
The Antarctic Peninsula is one of the three fastest warming regions on Earth. Here we review Holocene proxy records of marine and terrestrial palaeoclimate in the region, and discuss possible forcing ...mechanisms underlying past change, with a specific focus on past warm periods. Our aim is to critically evaluate the mechanisms by which palaeoclimate changes might have occurred, in order to provide a longer-term context for assessing the drivers of recent warming. Two warm events are well recorded in the Holocene palaeoclimate record, namely the early Holocene warm period, and the `Mid Holocene Hypsithermal' (MHH), whereas there are fewer proxy data for the `Mediaeval Warm Period' (MWP) and the `Recent Rapid Regional' (RRR) warming. We show that the early Holocene warm period and MHH might be explained by relatively abrupt shifts in position of the Southern Westerlies, superimposed on slower solar insolation changes. A key finding of our synthesis is that the marine and terrestrial records in the AP appear to show markedly different behaviour during the MHH. This might be partly explained by contrasts in the seasonal insolation forcing between these records. Circumpolar Deep Water (CDW) has been implicated in several of the prominent changes through the Holocene but there are still differences in interpretation of the proxy record that make its influence difficult to assess. Further work is required to investigate contrasts between marine and terrestrial proxy records, east—west contrasts in palaeoclimate, the history of CDW, to retrieve a long onshore high resolution record of the Holocene, and determine the role of sea ice in driving or modulating palaeoclimate change, along with further efforts to study the proxy record of the RRR and the MWP.
Sub‐ice shelf sediments near Larsen C ice shelf (LIS‐C) show fine‐scale rhythmic laminations that could provide a near‐continuous seasonal‐resolution record of regional ice mass changes. Despite the ...great potential of these sediments, a dependable Late Quaternary chronology is difficult to generate, rendering the record incomplete. As with many marginal Antarctic sediments, in the absence of preserved carbonate microfossils, the reliability of radiocarbon chronologies depends on presence of high proportions of autochthonous organic carbon with minimized detrital organic carbon. Consequently, acid insoluble organic (AIO) 14C dating works best where high productivity drives high sediment accumulation rates, but can be problematic in condensed sequences with high proportions of detrital organic carbon. Ramped PyrOx 14C dating has progressively been shown to improve upon AIO 14C dates, to the point of matching foraminiferal carbonate 14C dates, through differential thermochemical degradation of organic components within samples. But in highly detrital sediments, proportions of contemporaneously deposited material are too low to fully separate autochthonous organic carbon from detrital carbon in samples large enough to 14C date. We introduce two modifications of the Ramped PyrOx 14C approach applied to highly detrital sediments near LIS‐C to maximize accuracy by utilizing ultra‐small fractions of the highly detrital AIO material. With minimization of the uncertainty cost, these techniques allow us to generate chronologies for cores that would otherwise go undated, pushing the limits of radiocarbon dating to regions and facies with high proportions of pre‐aged detritus. Wider use of these techniques will enable more coordinated a priori coring efforts to constrain regional glacial responses to rapid warming where sediments had previously been thought too difficult to date.
Key Points
We 14C date highly detrital sediment from the Antarctic margin containing very little autochthonous carbon
We employ composite and isotope dilution Ramped PyrOx 14C dating, considering blank contamination for each
Both approaches sacrifice analytical precision for gains in accuracy; application to other detrital systems depends on uncertainty limits
We present a new seafloor map for the northern Antarctic Peninsula (AP), including swath multibeam data sets from five national programs. Our map allows for the examination and interpretation of Last ...Glacial Maximum (LGM) paleo-ice-flow paths developed on the seafloor from the preservation of mega-scale glacial lineations, drumlinized features, and selective linear erosion. We combine this with terrestrial observations of flow direction to place constraints on ice divides and ice domes on the AP continental shelf during the LGM time interval. The results show a flow bifurcation as ice exits the Larsen B embayment. Flow emanating off the Seal Nunataks (including Robertson Island) is directed toward the southeast, then eastward as the flow transits toward the Robertson Trough. A second, stronger "streaming flow" is directed toward the southeast, then southward as ice overflowed the tip of the Jason Peninsula to reach the southern perimeter of the embayment. Our reconstruction also refines the extent of at least five other distinct paleo-ice-stream systems that, in turn, serve to delineate seven broad regions where contemporaneous ice domes must have been centered on the continental shelf at LGM. Our reconstruction is more detailed than other recent compilations because we followed specific ice-flow indicators and have kept tributary flow paths parallel.
Since 1995 several ice shelves in the Northern Antarctic Peninsula have collapsed and triggered ice-mass unloading, invoking a solid Earth response that has been recorded at continuous GPS (cGPS) ...stations. A previous attempt to model the observation of rapid uplift following the 2002 breakup of Larsen B Ice Shelf was limited by incomplete knowledge of the pattern of ice unloading and possibly the assumption of an elastic-only mechanism. We make use of a new high resolution dataset of ice elevation change that captures ice-mass loss north of 66°S to first show that non-linear uplift of the Palmer cGPS station since 2002 cannot be explained by elastic deformation alone. We apply a viscoelastic model with linear Maxwell rheology to predict uplift since 1995 and test the fit to the Palmer cGPS time series, finding a well constrained upper mantle viscosity but less sensitivity to lithospheric thickness. We further constrain the best fitting Earth model by including six cGPS stations deployed after 2009 (the LARISSA network), with vertical velocities in the range 1.7 to 14.9 mm/yr. This results in a best fitting Earth model with lithospheric thickness of 100–140 km and upper mantle viscosity of 6×1017–2×1018 Pas – much lower than previously suggested for this region. Combining the LARISSA time series with the Palmer cGPS time series offers a rare opportunity to study the time-evolution of the low-viscosity solid Earth response to a well-captured ice unloading event.
•cGPS stations record uplift due to Northern Antarctic Peninsula ice-mass loss.•A high resolution model rules out an elastic-only Earth response.•cGPS data constrain viscoelastic modelling to determine the Earth structure.•An upper mantle viscosity of 2×1018 Pas or less is needed to fit cGPS data.•Results are less sensitive to lithosphere thickness.
The palaeoceanography and climate history of the East Antarctic Margin (EAM) are less well understood than those of West Antarctica. Yet, the EAM plays an important role in deep ocean circulation and ...the global ocean system and has likely done so in the past. Deglacial-age marine sediments from the EAM provide clues about its past role during this critical period of rapid climate change. Several deep basins across the EAM such as Iceberg Alley (∼67°S, 63°E) on the Mac.Robertson Shelf (MRS) accommodate thick marine sequences that archive the deglaciation in the form of diatom-rich, continuously laminated (varved) sediments. These laminated sediments are pristinely preserved and contain seasonal and long-term information on the cryospheric and palaeoceanographic changes associated with the rapid retreat of the glacial ice sheet across the MRS. We present results of microfabric analysis of the lower ∼2 m of deglacial varves from jumbo piston core JPC43B (Iceberg Alley). Backscattered electron imagery (BSEI) of polished thin sections and scanning electron microscope secondary electron imagery (SEI) of lamina-parallel fracture surfaces are used to analyze the varves. One hundred and ninety-two laminations are investigated and their nature and temporal significance are discussed in terms of seasonal deposition and cyclicity of diatom species. Our high-resolution palaeodata record exceptionally high diatom production and silica flux associated with the retreat of the East Antarctic Ice Sheet, and seasonal sea-ice changes along the EAM. This information is invaluable for assessing cryospheric-oceanographic variation and, therefore, the local and regional response to this period of rapid climate change. Varves are made up of lamina couplets comprising (i) thickly laminated to thinly bedded orange/orange-brown very pure diatom ooze dominated by
Hyalochaete Chaetoceros spp. vegetative cells and resting spores, and (ii) brown/blue-grey terrigenous angular quartz sand, silt and clay with an abundant mixed diatom flora. The colour variation between these two types of lamination is striking. Using floristic and textural information we interpret the diatom oozes as spring flux and the terrigenous laminae as summer flux. Each couplet pair represents one annual cycle and reflects seasonal changes in nutrient availability and stratification associated with the cyclical advance and retreat of seasonal sea-ice. The diatom oozes can reach up to ∼7.5 cm in thickness indicating enormous silica flux to the sea floor associated with ice sheet retreat.
A layer of shallow-water dolostone (“cap dolostone”) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball ...Earth. The dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap dolostones and their isotopic signatures, and therefore for their origin.
In northern Namibia, we studied the Keilberg cap dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ∼
0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope.
Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in
13C by 2–3‰ compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal
δ
13C curve over time with a net decline of 4.4‰. In addition, a lateral gradient of 1.0‰/100 km existed from the inner to outer bank.
If the flooding was rapid (<
10 kyr), as suggested by ice-melting models, the
δ
13C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (>
100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap dolostones, the
δ
13C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO
2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ∼
0.5 km implies a glacioeustatic origin, meaning that cap dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap dolostones in conflict with ice-melting models.
To develop an alternative dating tool for the Antarctic Peninsula (where the 14C method requires large, spatially variable reservoir corrections), we tested the clock‐zeroing assumption of ...photon‐stimulated luminescence (PSL) dating using a variety of PSL procedures. At ice shelf edges around the Antarctic Peninsula, sediment‐water‐interface (“zero‐age” analogs), silt‐rich short cores were collected in 2001–2003, originally only for fine silt dating tests. Later access to suitable instrumentation also permitted testing the potential of single‐grain quartz (SGQ) dating of sand grains from these cores. For the fine silt grains we employed multiple‐aliquot and single‐aliquot methods to obtain last daylight exposure age estimates from near‐core‐top material. With the sand fraction we employed automated SGQ PSL methods to isolate the youngest grains. Five of six fine silt samples gave unreasonably large age estimates (>20 ka), with the sixth sample yielding a multiple‐aliquot short‐bleach age estimate of 1.1 ± 0.6 ka. In contrast, five of seven sand samples yielded geologically reasonable last daylight exposure ages of 0.2–0.6 ka. These SGQ results are also remarkable when compared to published 14C ages of 1 ka to 9.7 ka from core top living calcite and acid‐insoluble organic matter. These SGQ results establish the likely utility of this single‐grain dating approach in such settings to provide chronologies for calving line histories of ice shelves. To take advantage of this utility, core collection should employ large‐diameter coring devices (e.g., Kasten and multicorers). A caveat is that large numbers (e.g., ∼10,000) of quartz grains may need analysis to provide acceptable statistics for useful age calculations.
This study analyses acoustic profiles of two mid-shelf troughs, the JOIDES Basin and Pennell Trough in the western Ross Sea, Antarctica. These troughs are subglacial erosion features formed by ...repeated advance of streaming ice onto the Ross Sea continental shelf. Sediment wedge formations, interpreted to have been deposited at the most seaward grounding zone during the Last Glacial Maximum (LGM), are observed within the mid-shelf region of these troughs. By correlating high-resolution acoustic profiles with core samples, we resolve the stratigraphy of these wedge formations to assess the relationships between bathymetry, subglacial sediment distribution and palaeo-ice-stream configuration. The grounding zone wedge geometries were controlled by the pre-existing topography. The JOIDES and Pennell Troughs were only partially infilled during the LGM. Axial diamict progradation from the landward margin of the troughs indicates enhanced flow (debris supply) along the axis of ice flow. Differences in grounding zone geometries indicate a decrease in basal debris deposition and/or supply during ice recession. The lack of a recessional ice-shelf facies indicates that either the ice shelf was absent during retreat, or that there was no melt-out of basal debris.
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