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Snowball Earth cap carbonate sequences provide an archive of what are likely the most dramatic climate transitions in all of Earth history. One approach to gain insight into these events is the detailed observation of sedimentary structures within these post-glacial units. Here, we report on newly discovered radially symmetric sedimentary structures within the Keilberg Member post-Marinoan 'cap dolostone' from the Otavi Group of northwest Namibia. We describe the local expression of over 60 decimeter-scale cymbal or disc structures from a single location. We interpret these features, which we name Zildjian structures, to be of likely abiotic origin. Through morphological comparisons, we suggest that Zildjian structures are most similar to Astropolithon, a pseudofossil that formed as a result of fluid or gas expulsion. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article presents a study that focuses on collecting independent regional cavity geometry representation of sub-ice shelf circulation and freezing/melting rates and turn them into a global data set using S-2004 global 1-min bathymetry and an improved version of the bed topography mapping (BEDMAP). It relates that existing global cavity geometry data sets contain inconsistencies and inaccuracies. However, local or regional simulations were able to use more detailed data sets.

Diatoms are important primary producers in present day Antarctic waters but their relative significance in the past is less clear. In this study we used long-chain diols to reconstruct Proboscia diatom productivity in shelf waters of the western Antarctic Peninsula over the last 8500 yr. Biomarker lipid analysis revealed the presence of a suite of long-chain diols in the sediments, mainly comprising the C28 and C30 1,14-diol isomers derived from Proboscia diatoms and C28 and C30 1,13-diols derived from other unknown algae. The relative importance of Proboscia diatoms was assessed using the relative abundances of 1,14-diols versus 1,13-diols, which showed that Proboscia diatoms were relatively more abundant during the Late Holocene, suggesting that stronger upwelling of circumpolar waters occurred at that time. The variations in the diol index strongly correlate with melt events in the Siple Dome ice core, suggesting that the climatic processes responsible for changes in mean summer temperature, open marine influence and atmospheric cyclonic activity recorded at Siple Dome, also controlled the productivity of Proboscia diatoms on the western Antarctic Peninsula region. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Palmer Deep sediment cores are used to produce the first high-resolution, continuous late Pleistocene to Holocene time-series from the Antarctic marine system. The sedimentary record is dated using accelerator mass spectrometer radiocarbon methods on acid insoluble organic matter and foraminiferal calcite. Fifty-four radiocarbon analyses are utilized in the dating which provides a calibrated timescale back to 13 ka BP. Reliability of resultant ages on organic matter is assured because duplicates produce a standard deviation from the surface age of less than laboratory error (i.e., ±50 years). In addition, surface organic matter ages at the site are in excellent agreement with living calcite ages at the accepted reservoir age of |1260 years for the Antarctic Peninsula. Spectral analyses of the magnetic susceptibility record against the age model reveal unusually strong periodicity in the 400, |200 and 50–70 year frequency bands, similar to other high-resolution records from the Holocene but, so far, unique for the circum-Antarctic. Here we show that comparison to ice-core records of specific climatic events (e.g., the ‘Little Ice Age’, Neoglacial, Hypsithermal, and the Bølling/Allerød to Younger Dryas transition) provides improved focus upon the relative timing of atmosphere/ocean changes between the northern and southern high latitudes. [ABSTRACT FROM AUTHOR]

Several grounding zone wedges were left on the floor and flanks of Prydz Channel in western Prydz Bay by the Lambert Glacier during the last glacial cycle. Seismic profiles indicate that vertical accretion at the glacier bed was the most important depositional process in forming the wedges, rather than progradation by sediment gravity flows. Sidescan sonographs reveal extensive development of flutes on the sea floor inshore from the wedges, indicating deformable bed conditions beneath the ice. The region inshore of the east Prydz Channel wedge features extensive dune fields formed by currents flowing towards the grounding zone. This orientation is consistent with models of circulation beneath ice shelves in which melting at the grounding line generates plumes of fresher water that rise along the base of the ice shelf, entraining sea water into a circulation cell. The Lambert Deep is surrounded by a large composite ridge of glacial sediments. Internal reflectors suggest formation mostly by subglacial accretion. The sea floor in the Lambert Deep lacks dune fields and shows evidence of interspersed subglacial cavities and grounded ice beneath the glacier. The absence of bedforms reflects sea floor topography that would have inhibited the formation of energetic melt water-driven circulation. [ABSTRACT FROM AUTHOR]

Lithodid crabs (and other skeleton-crushing predators) may have been excluded from cold Antarctic continental shelf waters for more than 14 Myr. The west Antarctic Peninsula shelf is warming rapidly and has been hypothesized to be soon invaded by lithodids. A remotely operated vehicle survey in Palmer Deep, a basin 120 km onto the Antarctic shelf, revealed a large, reproductive population of lithodids, providing the first evidence that king crabs have crossed the Antarctic shelf. DNA sequencing and morphology indicate the lithodid is Neolithodes yaldwyni Ahyong & Dawson, previously reported only from Ross Sea waters. We estimate a N. yaldwyni population density of 10 600 km(-2) and a population size of 1.55 × 10(6) in Palmer Deep, a density similar to lithodid populations of commercial interest around Alaska and South Georgia. The lithodid occurred at depths of more than 850 m and temperatures of more than 1.4°C in Palmer Deep, and was not found in extensive surveys of the colder shelf at depths of 430-725 m. Where N. yaldwyni occurred, crab traces were abundant, megafaunal diversity reduced and echinoderms absent, suggesting that the crabs have major ecological impacts. Antarctic Peninsula shelf waters are warming at approximately 0.01°C yr(-1); if N. yaldwyni is currently limited by cold temperatures, it could spread up onto the shelf (400-600 m depths) within 1-2 decades. The Palmer Deep N. yaldwyni population provides an important model for the potential invasive impacts of crushing predators on vulnerable Antarctic shelf ecosystems.

The stability of the Antarctic ice shelves in a warming climate has long been discussed, and the recent collapse of a significant part, over 12,500 km2 in area, of the Larsen ice shelf off the Antarctic Peninsula has led to a refocus toward the implications of ice shelf decay for the stability of Antarctica's grounded ice. Some smaller Antarctic ice shelves have undergone periodic growth and decay over the past 11,000 yr (refs 7-11), but these ice shelves are at the climatic limit of ice shelf viability and are therefore expected to respond rapidly to natural climate variability at century to millennial scales. Here we use records of diatoms, detrital material and geochemical parameters from six marine sediment cores in the vicinity of the Larsen ice shelf to demonstrate that the recent collapse of the Larsen B ice shelf is unprecedented during the Holocene. We infer from our oxygen isotope measurements in planktonic foraminifera that the Larsen B ice shelf has been thinning throughout the Holocene, and we suggest that the recent prolonged period of warming in the Antarctic Peninsula region, in combination with the long-term thinning, has led to collapse of the ice shelf.