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6. LA RELACIÓN ENTRE EL DERECHO Y EL SERVICIO
The sediment cores were transported to and stored in the cool rooms at Geoscience Australia in Canberra. From previous work on core CB from the southern end of Lake St Clair, it was observed that the basal glacial sediments have high magnetic susceptibility which contrasts strongly with the very low magnetic susceptibility of the overlying more organic lake sediments (Hopf et al, 2000). Based on this information, whole core magnetic susceptibility
measurements were run by Dr Charles Barton on all ten cores (Figures 4-5) collected using a Bartington Susceptibility Bridge and the most promising core, in terms of likelihood of full
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recovery of a late Glacial/Holocene sequence, selected without having to cut the cores to assess the stratigraphy.
As the focus for this study would be the northern end of Lake St Clair, Core CG was selected because:
1. the core was of good length (548 cm),
2. appeared to include both glacial and lake sediments (unlike e.g. cores CE, CN and CK which most likely only include Holocene lake sediments or CI, CJ and CM which only appear to have captured glacial sediments) and, in contrast to core CB,
3. appeared likely to cover a more transitional phase between the high magnetic susceptibility glacial sediments and low magnetic susceptibility lake sediments (between 260cm and 450 cm) suggesting that there would be potential for higher resolution analysis of this transitional time period.
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Figure 3-2 Magnetic susceptibility plots for cores CA-CC.
3.3.2 Description of the core
3.3.2.1 Stratigraphy and sub-sampling
Core CG was split, photographed and the stratigraphy described at Geoscience Australia. The core was sliced at 1 cm intervals, outer edges of sediment in contact with the core tube were discarded and the remaining sample bagged and returned to the ANH laboratories, ANU where they were stored for further analysis.
3.3.2.2 Particle Size Analysis
Particle size analysis of the Core CG sediments was carried out by laser diffraction using a Malvern Mastersizer 2000 with Hydro MU attachment housed at the Fenner School of Environment & Society, ANU. The samples were first treated with 10% HCl to remove carbonates and 30% H2O2 to remove organic matter. Calgon was added to disperse any
aggregates in the sediment and 30 seconds of additional ultrasonic dispersal was applied just prior to measurement. Five repeat measurements were taken for each sample, the average results of which were then assigned to groups of the various size fractions of sand, silt and clay.
3.3.2.3 Loss on Ignition and Carbon/Nitrogen Analysis
Loss on ignition is a method that can be used to estimate the amount of organic matter in sediments. The procedure outlined by Dean (1974) was followed using dried 2.5 cc subsamples which were weighed before and after controlled heating to a temperature of
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550°C to calculate the % of sample lost in the form of carbon dioxide which can be directly correlated with organic matter content.
Figure 3-3 Magnetic susceptibility plots for cores CE-CN.
Total Carbon is another way in which the organic component in a sediment can be measured. An Elementar VarioMAX CNS elemental analyser was used at the Fenner School of
Environment & Society, ANU to measure total carbon and nitrogen using small sub-samples of approximately 0.5 cc. more detail. Whereas total organic matter and total carbon can give a general indication of the productivity of a system, the ratio of carbon to nitrogen in a sample can further inform about the likely source of the sediment in the lake. Low ratios around 4-10 are indicative of sediment that was produced within the lake system itself whereas high ratios
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of > 20 point towards sediment being transported from the surrounding catchment into the lake system (Meyers, 1994).
3.3.3 Radiocarbon Dating
Bulk sediment samples were taken from all cores and submitted for accelerator mass spectrometry (AMS) radiocarbon dating at the Australian Nuclear Science and Technology Organisation (ANSTO) and the Australian National University (ANU) Radiocarbon Dating Laboratory. No suitable terrestrial macrofossil remains were isolated for dating purposes. Initial samples received the standard alkali-acid pretreatment; those submitted to the ANU Radiocarbon Dating Laboratory from 2012 were first split into the humin and humic acid fractions and each fraction dated separately. Individual radiocarbon ages were calibrated using Clam v.2 (Blaauw, 2010) using the Southern Hemisphere calibration dataset SHCal13 (Hogg et al., 2013). A chronology was produced using Bacon v.2.2 (Blaauw and Christen, 2011) by age depth modelling based on Bayesian statistics.
3.3.4 Pollen Analysis
3.3.4.1 Pollen and Microscopic Charcoal Preparation
Pollen processing followed standard HCL, HF, KOH and acetolysis methods (Faegri & Iversen, 1989) using 1cc subsamples and included addition of Lycopodium marker grains to calculate concentrations of pollen, spores and microscopic charcoal (Stockmarr, 1971). Samples were processed at 2 cm intervals between 180-440 cm to cover the LGIT in detail and at 4 cm intervals for the remainder of the core. Pollen residues were mounted on slides in a medium of glycerol and sealed with nail polish.
3.3.4.2 Pollen and Microscopic Charcoal Counting
Pollen, spores and microscopic charcoal (> 10µm) were counted at 400 x magnification using a Zeiss Axiophot microscope and identifications made using the reference collection held at the Department of Archaeology & Natural History, Australian National University (ANU) and the Australian Pollen and Spore Atlas (APSA Members, 2007). At least 300 terrestrial pollen grains were counted in each sample wherever possible.
3.3.5 Macroscopic Charcoal Analysis
Contiguous, 1.25 cc sub-samples were taken from Core CG and processed for macroscopic charcoal analysis according to a modified technique developed by Stevenson and Haberle (2005) based on the method by Rhodes (1998). The samples were dispersed in calgon, bleached and then washed through 125 micron and 250 micron sieves to obtain the macroscopic charcoal fractions that are greater than 125 microns in size (macroscopic
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charcoal). The total number of macroscopic charcoal pieces in each fraction were counted for each sample using a dissecting microscope.