Data from the cosmogenic nuclide dating program following AAS 4318 ‘CADAGIA’ field surveys

Full description

The CADAGIA program was conducted across four field seasons between 2015-2019 to provide regional-scale record of East Antarctic Ice Sheet change, focussing on the postglacial period (i.e. post ~20 ka BP).

Surveys were conducted on nunataks and other ice-free areas between Vestfold Hills and Windmill Islands. Survey and sampling sites were chosen to explore regional ice sheet behaviour. We avoided areas where existing cosmogenic dating programs had been completed (e.g. Bunger Hills, and central-western Vestfold Hills). Detailed mapping and sampling transects were conducted at Windmill Islands, north and north-eastern Vestfold Hills. These were supplemented reconnaissance sampling at remote outcrops across the scatter of ice free areas across the region. Remote locations were accessed via helicopter or fixed wing supported day trips by small field parties. These primarily include sites co-located with deployment of seismic and GPS recording stations (e.g. Snyder Rocks, Mt Strathcona, Mt Brown, Gillies Islands, Carey Nunatak) but were supplemented where feasible to expand geographic coverage (Ravich nunatak, Hippo and Alligator Islands, Mt Sandow) or target key ice-history locations (Cape Jones and Watson Bluff).

Cosmogenic sampling was conducted in conjunction with and guided by in-field observations of bedrock and clast weathering (White, 2021) and glacial landforms. Vertical transects were collected from steep nunataks (Cape Jones, Watson Bluff and Mts Brown and Strathcona), and horizontal transects from the large coastal oases (Windmill Islands and Vestfold Hills). The ‘ideal’ sample site was a local summit or an exposed ridgeline, where bedrock displayed evidence of past glacial erosion and was mantled in a thin scatter of erratics. At each site sampling typically consisted of a single piece of surficial bedrock (with striae or glacial polish where [rarely] available) and three cobble-sized, subrounded, subglacial erratics perched on bedrock. Cobbles were selected to minimise potential for inheritance, post-depositional movement, weathering and exhumation, and to maximise quartz content.

Sample selection and collection was conducted by several different field scientists (Duanne White, Steven Phipps, Marcello Blaxell and David Small). Duanne White provided guidance and training on sampling methodologies through field examples in Australia (Marcello Blaxell and Steven Phipps) or at Windmill Islands (David Small). Sampling was also conducted following the same set of written guidelines.

Sample sites were located with a Garmin e-trex GPS. Elevations were cross-checked with the Reference Elevation Map of Antarctica, accurate to 10 m. Topographic shielding correction was measured at breaks in slope from the perspective of the sample using a sighting compass and clinometer accurate to less than 1 degree. Shielding was often negligible (less than 5 degree). Samples were photographed in the field from several different angles. Photos are available on request.

Sample characteristics (Size, lithology, rounding, and surface weathering) were described in the laboratory on return to Australia. Rock disaggregation and quartz purification was then performed at the University of Canberra and Wollongong (Child et al., 2000; Fink and Smith, 2007; Kohl and Nishiizumi, 1992) Beryllium isotopes were extracted from quartz at University of Wollongong and ANSTO in nine separate batches of 8 to 16 samples by Matt Jeromson and Marcello Blaxell, alongside a full chemistry procedural blank. 10/9Be ratios were measured at the Sirius AMS Facility at ANSTO, Australia.

Measured 10Be/Be ratios were corrected by the batch-specific procedural blank (ratios of 0.8 to 3.2 x 10-15) and referenced to AMS standards KN-5-2 (nominal ratio 8.55 x 1012) and KN-5-3 (6.32 x 1012). Independent repeat sample measurements were combined as weighted means with the larger of the total statistical error or mean standard error. Final analytical error in concentrations (atoms/gram quartz) are derived from a quadrature sum of the standard mean error in an AMS ratio, 2% for AMS standard reproducibility, 1% in Be spike assay and 1% for quartz mass.

Exposure ages are calculated using CRONUS-Earth online calculator, applying the LSDn scaling scheme (Balco et al., 2008). In the absence of a specific Antarctic production rate model, we apply a recent global calibration dataset (Borchers et al., 2016).

This version of the dataset (V1.0) archives samples measured by September 2021.


4318 cosmo field data

This worksheet describes the sample characteristics as measured in the field. This is generally recorded at a site rather than sample level.

Lat    Long    Locality    

All lat/long are recorded relative to the WGS 84 datum.

Site elevation (metres)

Handheld GPS elevation, relative to sea level. 2015/16 used a barometric corrected GPS (Etrex 300), while other seasons used a standard GPS.

SiteID

Name used to identify site in respective field book. Most are a simple 3 digit code representing the site. Some represent individual samples (usually with a letter suffix). The final seasons used a XXXX/XXXX sample code to generate redundancy in the sample name and more easily detect potential laboratory transfer errors.

Erratics/Bedrock

Number of each sample type collected at each site

Position/Landform

Characteristics of the sample location. Most locations reflect exposure from the ice sheet, but some (809-815, 1173/1174-1183/1184) were sampled to test efficacy of wave washed bedrock as a paleo-sea level record.

Shielding
Az, ele, az ele etc. = azimuth and elevation of topographic shielding for the sample or site. Shielding is the fraction of cosmic ray flux, calculated using the Greg Balco’s V2 (April 2018) online shielding tool (http://stoneage.ice-d.org/math/skyline/skyline_in.html)

4318 cosmo field data

This worksheet describes the sample characteristics as measured in the laboratory. Note that some samples were accidentally crushed prior to description, while others may have been described but not yet processed for Be-10 measurement.

Sample ID

Site ID, plus a letter-suffix to indicate the individual sample name. Letters a-g typically indicate erratics, while a br suffix indicates bedrock.

A, B, C axis.

Size of the sample in cm, measured using a ruler to the nearest half-cm.

Lithology

Sample rock type. Minerals visually identified. Q = quartz, F = feldspar, G = garnet, B = biotite, M = muscovite, A = amphibole

Rounding

Powers (1953) roundness scale, A = angular, SA = subangular, SR = subrounded, R = rounded, WR = well rounded.

Clast surface characteristics

A selection of surface characteristics used to measure degree of weathering and relative exposure age. Classified as absent (a) present (p) or common (c). ‘cm lost’ is a subjective estimate of how much material may have been lost from the clast by subaerial crumbling or weathering, based on the surface features and comparison with a typical subglacial clast morphology.

4318 AMS and chem data

This worksheet describes the measurements conducted during Be-10 extraction and Accelerator Mass Spectrometry measurement. Samples are described in measurement batches.

Sample ID

As per cosmo lab sample description sheet. Site ID, plus a letter-suffix to indicate the individual sample name. Letters “a” to “g” typically indicate erratics, while a “br” suffix indicates bedrock.

UOW Lab ID - University of Wollongong lab number

ANSTO Be Cathode ID – ANSTO lab number specific the Be-10 measurement

ANSTO OZ code – ANSTO laboratory number.

Total Qtz (g) – quartz mass dissolved for Be-10 measurement
    

10Be/9Be    d[10Be/9Be] – ‘raw’ measured Be-10/9 ratio measured via AMS (and uncertainty)

Blank ID(s)    Blank 10Be/9Be    Blank d[10Be/9Be] – Be-10/9 ratios for the procedural blank measured in each batch of samples
            
Corr 10Be/9Be    Corr d[10Be/9Be] – corrected Be-10/9 ratio, subtracted for specific procedural blank for that batch.
        

Carrier     Be Carrier (µg/g)    d[Be Carrier]    Carrier (g)    9Be Spike (µg)    9Be (atoms)
                        
Details of the 9Be carrier added prior to quartz dissolution. Carrier name, 9Be concentration (and uncertainty), carrier solution mass, and finally the calculated number of 9Be atoms added to the sample.

10Be SRM-4325 (atoms.g-1)    d[10Be]    d[10Be]%

Calculated Be-10 concentrations and uncertainties in each sample, referenced against NIST SRM-4325 with an assumed isotope ratio of 2.79 x 10-11.
            



AMS standard    AMS std nominal value    

Actual AMS standards used during each sample run, and the assumed value. AMS run date = date Be10/9 oxide was measured.

4318 Cronus entry data and ages
Data used to calculated exposure ages via the Cronus Earth online calculator. See documentation in Balco et al, 2008 and https://hess.ess.washington.edu/ for description of what each column represents. Thickness is a mix of different measurements a best possible for each sample (uncertainties black = measured in lab (0.5 cm), blue = estimate from photos (1 cm), red = estimate from similar samples (2 cm)). Density assumed to be 2.7 in all samples given quartz rich metamorphic lithologies.



Be-10 at/g    Be-10 at/g uncertainty


Sample ages (at last!) and uncertainties. LSDn scaling scheme.

Lineage

Progress Code: completed