Response of the Wilkes Subglacial Basin Ice Sheet to Southern Ocean Warming During Late Pleistocene Interglacials

The response of the East Antarctic Ice Sheet to past intervals of oceanic and atmospheric warming is still not well constrained but critical for understanding both past and future sea-level change. Furthermore, the ice sheet in the Wilkes Subglacial Basin, which is characterized by a reverse-sloping bed, appears to have undergone thinning and ice discharge events during recent decades. By combining new glaciological evidence on ice sheet elevation from the TALDICE ice core with offshore sedimentological records and ice sheet modelling experiments, we reconstruct the ice dynamics in the Wilkes Subglacial Basin over the past 350,000 years. Our results indicate that the Wilkes Subglacial Basin experienced an extensive retreat 330,000 years ago and a more limited retreat 125,000 years ago. These changes coincided with warmer Southern Ocean temperatures and elevated global mean sea level during those interglacial periods, confirming the sensitivity of the Wilkes Subglacial Basin ice sheet to ocean warming and its potential role in sea-level change.

Ultrasonic and seismic constraints on crystallographic preferred orientations of the Priestley Glacier shear margin, Antarctica

Crystallographic preferred orientations (CPOs) are particularly important in controlling the mechanical properties of glacial shear margins. Logistical and safety considerations often make direct sampling of shear margins difficult and geophysical measurements are commonly used to constrain the CPOs. We present here the first direct comparison of seismic and ultrasonic data with measured CPOs in a polar shear margin. The measured CPO from ice samples from a 58 m deep borehole in the left lateral shear margin of the Priestley Glacier, Antarctica, is dominated by horizontal c-axes aligned sub-perpendicular to flow. A vertical seismic profile experiment with hammer shots up to 50 m away from the borehole, in four different azimuthal directions, shows velocity anisotropy of both P-waves and S-waves. Matching P-wave data to the anisotropy corresponding to CPO models defined by horizontally aligned c-axes gives two possible solutions for c-axis azimuth, one of which matches the c-axis measurements. If both P-wave and S-wave data are used, there is one best fit for azimuth and intensity of c-axis alignment that matches well the measurements. Azimuthal P-wave and S-wave ultrasonic data recorded in the laboratory on the ice core show clear anisotropy that matches that predicted from the CPO of the samples. With good quality data, azimuthal increments of 30° or less will constrain well the orientation and intensity of c-axis alignment. Our experiments provide a good framework for planning seismic surveys aimed at constraining the anisotropy of shear margins.

Stratigraphic templates for ice core records of the past 1.5 million years

The international ice core community has a target to obtain continuous ice cores stretching back as far as 1.5 million years. This would provide vital data (including a CO2 profile) allowing us to assess ideas about the cause of the Mid-Pleistocene Transition (MPT). The European Beyond EPICA project and the Australian Million Year Ice Core project each plan to drill such a core in the region known as Little Dome C. Dating the cores will be challenging, and one approach will be to match some of the records obtained with existing marine sediment datasets, informed by similarities in the existing 800 kyr period. Water isotopes in Antarctica have been shown to closely mirror deepwater temperature, estimated from Mg / Ca ratios of benthic foraminifera, in a marine core on the Chatham Rise near to New Zealand. The dust record in ice cores resembles very closely a South Atlantic marine record of iron accumulation rate. By assuming these relationships continue beyond 800 ka, our ice core record could be synchronised to dated marine sediments. This could be supplemented, and allow synchronisation at higher resolution, by the identification of rapid millennial scale-events that are observed both in Antarctic methane records and in emerging records of planktic oxygen isotopes and alkenone sea surface temperature (SST) from the Portuguese Margin. Although published data remain quite sparse, it should also be possible to match 10Be from ice cores to records of geomagnetic palaeointensity and authigenic 10Be/9Be in marine sediments. However, there are a number of issues that have to be resolved before the ice core 10Be record can be used. The approach of matching records to a template will be most successful if the new core is in stratigraphic order, but should also provide constraints on disordered records, if used in combination with absolute radiogenic ages.

Physical processes behind interactions of microplastic particles with natural ice

Microplastic particles (MPs, <5 mm) are found in marine ice in larger quantities than in seawater, however, the distribution pattern within the ice cores is not consistent. To get insights into the most general physical processes behind interactions of ice and plastic particles in cool natural environments, information from academic and applied research is integrated and verified against available field observations. Non-polar molecules of common-market plastics are hydrophobic, so MPs are weak ice nucleators, are repelled from water and ice, and concentrate within air bubbles and brine channels. A large difference in thermal properties of ice and plastics favours concentration of MPs at the ice surface during freeze/thaw cycles. Under low environmental temperatures, falling in polar regions below the glass / brittle-ductile transition temperatures of the common-use plastics, they become brittle. This might partially explain the absence of floating macroplastics in polar waters. Freshwater freezes at the temperature well below that of its maximum density, so the water column is stably stratified, and MPs eventually concentrate at the ice surface and in air bubbles. In contrast, below growing sea ice, mechanisms of suspension freezing under conditions of (thermal plus haline) convection should permanently entangle MPs into ice. During further sea ice growth and aging, MPs are repelled from water and ice into air bubbles, brine channels, and to the upper/lower boundaries of the ice column. Sea ice permeability, especially while melting periods, can re-distribute sub-millimeter MPs through the brine channels, thus potentially introducing the variability of contamination with time. In accord with field observations, analysis reveals several competing factors that influence the distribution of MPs in sea ice. A thorough sampling of the upper ice surface, prevention of brine leakage while sampling and handling, considering the ice structure while segmenting the ice core – these steps may be advantageous for further understanding the pattern of plastic contamination in natural ice.

Volcanic stratospheric sulfur injections and aerosol optical depth during the Holocene (past 11,500 years) from a bipolar ice core array

The injection of sulfur into the stratosphere by volcanic eruptions is the dominant driver of natural climate variability on interannual-to-multidecadal timescales. Based on a set of continuous sulfate and sulfur records from a suite of ice cores from Greenland and Antarctica, the HolVol v.1.0 database includes estimates of the magnitudes and approximate source latitudes of major volcanic stratospheric sulfur injection (VSSI) events for the Holocene (from 9500 BCE or 11500 year BP to 1900 CE), constituting an extension of the previous record by 7000 years. The database incorporates new-generation ice-core aerosol records with sub-annual temporal resolution and demonstrated sub-decadal dating accuracy and precision. By tightly aligning and stacking the ice-core records on the WD2014 chronology from Antarctica we resolve long-standing previous inconsistencies in the dating of ancient volcanic eruptions that arise from biased (i.e. dated too old) ice-core chronologies over the Holocene for Greenland. We reconstruct a total of 850 volcanic eruptions with injections in excess of 1 TgS, of which 329 (39 %) are located in the low latitudes with bipolar sulfate deposition, 426 (50 %) are located in the Northern Hemisphere (NH) extratropics and 88 (10 %) are located in the Southern Hemisphere (SH) extratropics. The spatial distribution of reconstructed eruption locations is in agreement with prior reconstructions for the past 2,500 years, and follows the global distribution of landmasses. In total, these eruptions injected 7410 TgS in the stratosphere, for which tropical eruptions accounted for 70 % and NH extratropics for 25 %. A long-term latitudinally and monthly resolved stratospheric aerosol optical depth (SAOD) time series is reconstructed from the HolVol VSSI estimates, representing the first Holocene-scale reconstruction constrained by Greenland and Antarctica ice cores. These new long-term reconstructions of past VSSI and SAOD variability confirm evidence from regional volcanic eruption chronologies (e.g., from Iceland) in showing that the early Holocene (9500–7000 BCE) experienced a higher number of volcanic eruptions (+16 %) and cumulative VSSI (+86 %) compared to the past 2,500 years. This increase coincides with the rapid retreat of ice sheets during deglaciation, providing context for potential future increases of volcanic activity in regions under projected glacier melting in the 21st century. The reconstructed VSSI and SAOD data are available at https://doi.pangaea.de/10.1594/PANGAEA.928646 (Sigl et al., 2021).

Distribution and Driving Mechanism of N2O in Sea Ice and Its Underlying Seawater during Arctic Melt Season

Nitrous oxide (N2O) is the third most important greenhouse gas in the atmosphere, and the ocean is an important source of N2O. As the Arctic Ocean is strongly affected by global warming, rapid ice melting can have a significant impact on the N2O pattern in the Arctic environment. To better understand this impact, N2O concentration in ice core and underlying seawater (USW) was measured during the seventh Chinese National Arctic Research Expedition (CHINARE2016). The results showed that the average N2O concentration in first-year ice (FYI) was 4.5 ± 1.0 nmol kg−1, and that in multi-year ice (MYI) was 4.8 ± 1.9 nmol kg−1. Under the influence of exchange among atmosphere-sea ice-seawater systems, brine dynamics and possible N2O generation processes at the bottom of sea ice, the FYI showed higher N2O concentrations at the bottom and surface, while lower N2O concentrations were seen inside sea ice. Due to the melting of sea ice and biogeochemical processes, USW presented as the sink of N2O, and the saturation varied from 47.2% to 102.2%. However, the observed N2O concentrations in USW were higher than that of T-N2OUSW due to the sea–air exchange, diffusion process, possible N2O generation mechanism, and the influence of precipitation, and a more detailed mechanism is needed to understand this process in the Arctic Ocean.

Concurrent Changepoints in Greenland Ice Core δ18O records and the North Atlantic Oscillation over the Past Millennium

Structural changes, or changepoints, coinciding in multiple ice core records over the Greenland Ice Sheet (GrIS) may reflect a widespread response of the GrIS to atmospheric forcing. Thus, to better understand how atmospheric circulation may regulate sudden changes in δ18O of Greenland precipitation, we seek synchronous changepoints occurring in ice core-derived δ18O time series across the GrIS and in the North Atlantic Oscillation (NAO) over the past millennium. By utilizing a Bayesian changepoint detection method, four changepoint horizons were revealed: at the beginning of the 20th century, in the late-15th century, and around the turn of the 11th and 10th centuries. Although the changepoints in ice core δ18O records exhibited distinctive spatial arrangements in each horizon, all corresponded to changepoints in the NAO, indicative of a consistent atmospheric influence on GrIS surface changes over the past millennium.