Origin of palaeo proglacial lake sediments in Taylor Valley, Antarctica

• Main Author: Milicich, Sarah
• Other Authors: Hendy, Chris H.
• Format: Thesis
• Language: English
• Published: The University of Waikato 2019
• Subjects: Bonney; Fryxell; Glacial Lake; Lake Bonney; Lake Fryxell; Lower Lake; McMurdo Sound; Nussbaum Riegel; Pearse Valley; Taylor Glacier; Taylor Valley; Antarc*; Antarctica
• Online Access: https://hdl.handle.net/10289/12870

The origin of carbonates found at three locations in the Taylor Valley and their association with different glacial advances is presented. Samples were collected from three locations through the Taylor Valley, specifically Nussbaum Riegel west of Lake Fryxell, Sollas Bench on the slopes above Lake Bonney, and from Pearse Valley. Carbonates were collected primarily from the surficial glacial sediment, though at one site at Nussbaum Riegel a pit indicated the stratigraphy contained multiple carbonate horizons. The carbonate from Nussbaum Riegel was in situ, while at the other two sites the carbonate was incorporated into moraines. The oxygen isotope values indicate that carbonates were deposited in lakes with two different origins. δ¹⁸ values of approximately -28% to -32% for the Nussbaum Riegel carbonate samples indicate "Glacial Lake Nussbaum" likely formed during glacial conditions when grounded ice expanded in McMurdo Sound pushing a lobe of ice up the mouth of the Taylor Valley. δ¹⁸ values of approximately -33 %to 38% for carbonate samples from Pearse Valley and Sollas bench indicate that "Glacial Lake Pearse" and the proglacial lake in which the Sollas Bench carbonates were deposited would have formed during interglacial conditions. It is proposed that an expanded Taylor Glacier was advancing though its own proglaical lakes, reworking carbonate sediment deposited in the lakes. The carbon isotopes reflect the nature of the carbon dioxide cycling within the lakes. Strongly positive δ¹³C values are interpreted as reflecting low temperature equilibrium with atmospheric carbon dioxide or freezing processes causing precipitation. The trace element chemistry of the carbonates is inferred to indicate that the origin of their composition is primarily controlled by the composition of the precipitating fluid and the mineralogy of the carbonate. Samples containing higher proportions of aragonite have higher concentrations of the larger ions such as barium and strontium, while those with more calcite in the mineral structure contain more of the smaller ions such as magnesium. XRD analyses indicate that the Nussbaum Riegel carbonate samples are a mix of dominantly aragonite with some calcite. SEM images of the carbonate crystals show that they are cementing detrital material. Petrographic analysis of the Nussbaum Riegel carbonates indicate that much of the detrital material is volcanic in nature and was likely sourced from the McMurdo Volcanics. The Sollas Bench and Pearse Valley carbonate samples are predominantly aragonite as shown by XRD analyses. SEM images show the crystals in these samples are a mix of radiating needles and randomly oriented needles, pointing to two different processes during precipitation. SEM images also showed the presence of halite within the carbonate samples. The Nussbaum Riegel carbonates are inferred to have been deposited during periods of lake level fluctuation, where draining of ground water towards a lower lake level resulted in evaporation at the lake margins and cementation of the surficial sediment. The Sollas Bench and Pearse Valley carbonates were likely deposited under a regime of water removal, either via evaporation or freezing, making these carbonates a mix of evaporites and "cryorites". Cryorite is a new term, being defined as deposits precipitated as a result of concentration by removal of water due to freezing. In this case they appear to be primarily "cryorites". Halite was identified within many of the carbonate samples and gypsum was found associated with them. This suggests that there was a strongly concentrated brine present at the time of precipitation. The source of the salt in the western end in the Taylor Valley is likely to be an evaporate sequence in a subglacial depression indicated on subglacial profiles, currently being overridden by Taylor Glacier. The evaporites could have deposited during a warmer period when the Ferrar Valley was a fjord. Seawater would then have overflowed into Taylor Valley at its convergence with the Ferrar Valley. The resulting lakes then evaporating in the deep depression. Salts are currently being reworked into the valley, a similar situation is proposed for the Sollas Bench carbonates