The McMurdo dry valleys, Antarctica; a modern analogy to Proterozoic snowball Earth?
4
Citation
0
Reference
20
Related Paper
Keywords:
Snowball Earth
Earth system science
Early Earth
Cite
<p>Understanding the glacial changes that have occurred in southern McMurdo Sound throughout the Neogene makes an important contribution to reconstructing Antarctic ice volume changes during past periods of climatic warmth, and provides insight into future possible response of the ice sheet. Fossiliferous glacimarine deposits previously identified throughout McMurdo Sound have provided inferences on past changes in ice volume and the implications for global sea level. This study investigates new stratigraphic sections comprising fossiliferous glacimarine sediments from two locations on the flanks of Mount Discovery and one on Brown Peninsula at ~150m above present day sea-level. The aim of this thesis is to undertake a sedimentological, facies and glacimarine sequence stratigraphic analysis together with a quantitative assessment of the constituent micro and macrofossils in order to determine depositional processes, changes in environment and implications for glacial variability in the southern McMurdo Sound. Up to four distinct sedimentary cycles are evident in the Mt Discovery sections with each cycle consisting of: 1. A basal glacial surface of erosion (GSE) or its correlative conformity (CC) seaward of the grounding line, displaying an abrupt transition from a more distal facies to a more proximal facies. 2. A sharp-based massive diamictite displaying physical intermixing of subjacent lithologies, intra formational clasts, soft sediment deformation features, clast rotation features, and a lack of bioturbation, interpreted as subglacial, or in very close proximity to a marine grounding line. In some cases stratified diamictites overly correlative conformities displaying clast alignment, graded beds, and weak decimeter scale parallel bedding in the matrix, interpreted as grounding-line proximal sediment gravity flows or rain-out from ice melt. 3. In some cases, the diamictite passes gradationally-upwards or is sharply overlain by a conglomerate representing appearance of glacimarine fluvio-deltaic deposition or debris flows as the glacier grounding line begins to retreat. 4. Conglomerates are overlain, often sharply, by hemipelagic laminated or massive mudstone and represent the most ice distal and marine part of the sequence at the interglacial minima. 5. A proglacial facies succession is sometimes preserved below the GSE or correlative conformity marking the top of the sequence and usually consists of a transition from mudstone facies into grounding zone proximal conglomerates during re-advance of the grounding line. Lithofacies analysis supported by sediment grainsize, the results of a foraminiferal census and macrofaunal identifications conducted on each facies imply deposition on the continental shelf in depths of up to 400 m, during oscillations in the proximity of a marine grounding line. Radiocarbon dating of constituent macrofauna (barnacle plates) reported only background ¹⁴C implying an age of deposition older than the Last Glacial Maximum. The present day elevation of the site and a model developed here using glacio-iostatic loading and unloading implies that the Mt Discovery sites have been above sea-level for approximately the last ~2.7Ma. Given their inferred association with Scallop Hill Formation and faunal similarity to sequences in the nearby ANDRILL drill cores, a Pliocene age is considered most likely. This finding is consistent with previous investigations that indicate a dynamic, sub-polar marine-terminating ice sheet margin in Southern McMurdo Sound during the Pliocene, with periods of open ice shelf free conditions potentially associated with more frequent regional collapse and retreat events of a marine-based ice sheet in the Ross Sea embayment.</p>
Neogene
Cite
Citations (0)
Dry land
Cite
Citations (0)
Concentrations of ammonium, nitrate, and urea decline along a glacial meltwater stream in Taylor Valley, southern Victoria Land, Antarctica. These reductions accompany increasing concentrations of particulate and dissolved organic nitrogen (other than urea), suggesting that benthic microbial mats present in these systems may be responsible for transforming dissolved inorganic nitrogen into dissolved and particulate organic compounds. A mathematical model of primary production of microbial mats was used to estimate nitrogen transformation, assuming that nitrogen uptake balanced carbon fixation. Export of organic nitrogen was set equal to inorganic uptake driven by net primary production, based on the assumption of steady-state biomass for mat communities. Model results were comparable to observations although transformation rates generally were lower than observed. The model was sensitive to water retention time in the stream, illustrating the critical importance of accurate assessments of stream geometry and hydrology. Application of this model to three other streams feeding Lake Fryxell (Taylor Valley) suggest that dry valley streams have a large potential to transform mineral nitrogen into organic forms.
Nitrogen Cycle
Cite
Citations (5)
The McMurdo Dry Valleys, a hyper-arid cold polar desert located within the Transantarctic Mountains was once covered by vegetation. An in depth study of surface samples of various Neogene age, acquired throughout the Valleys, provide insight to the location of one of Antarctica's last vegetation refugia. Boston University's Antarctic Research Group has collected 82 surface samples from paleo lake sediments on 14 expeditions spanning 22 years in the McMurdo Dry Valleys. The ages of the samples are still not fully constrained, but 8 regions where samples were collected have been interpreted to range between 16.95 ± 0.17 to 4 Ma old (Marchant, pers. comm.). Results show the palynomorph assemblage diversity is low with a terrestrial assemblage dominated by southern beech Nothofagus spp. (fusca group), and the nonterrestrial assemblage is dominated by ice-indicative acritarch Leiosphaeridia spp. with some samples containing spores of the fresh water algae Zygnemataceae. Based on palynological concentrations recovered, the Victoria Valley and Beacon Valley may have been the site for vegetation refugia. Samples collected from Victoria Valley have concentrations ranging from 252.1 to 5191.1 gdw-1 and nonterrestrial derived palynomorph concentrations ranging from 735.6 to 2386.1 gdw-1. The Central Beacon Valley has a terrestrial-derived palynomorph concentration of 131.8 to 675.3 gdw-1. These concentration ranges are higher than other sample locations analyzed for this study such as the Olympus Range, Taylor Valley, and valleys located in the Asgard Range, with samples concentrations that are barren to 17.4 gdw-1 for the Olympus Range, barren to 1.4 gdw-1 for Taylor Valley, and barren to 39.6 gdw-1 sites in the Asgard Range. Based on the present study, the Victoria Valley and Beacon Valley regions acted as refugia for isolated pockets of vegetation to survive around the Mid-Miocene and >8.07±0.06 Ma based on their palynomorph assemblages and concentrations. These sites potentially were ideal locations where geological processes such as erosion and transportation did not influence the geologic record, thus preserving these sites. The results in this study provide detailed information to be utilized for future expeditions into the region to precisely time vegetation's demise within the McMurdo Dry Valleys.
Nothofagus
Assemblage (archaeology)
Cite
Citations (1)
The glaciers of the McMurdo Dry Valleys are fundamental to the hydrology and biology of the valleys because they are the only significant source of water. Understanding the controls on the glacial extent and meltwater runoff is fundamental to a process-oriented approach to studying the dry valleys ecosystem. The elevation of the equilibrium-line of the alpine glaciers changes dramatically in the dry valleys, probably a result of large gradients in precipitation. Temporally, they have been relatively constant since the Pliocene (3.5 million years), the furthest extent is not more than a few hundred meters from their present positions. Ablation (all forms of mass loss) from the glaciers is dominated by sublimation, which accounts for more than 70% of the total mass loss. However the magnitude of sublimation is about the same as that of temperate glaciers. The salient difference from temperate glaciers is the relatively small fraction of ablation due to melt and results from a combination of very cold ice and insufficient sensible heat for much melting. The cliff faces are crucial for initiating and maintaining stream flow because they are the first part of the glacier to start melting and the last to stop. During melt periods the distribution of glacier area with altitude can control the response of stream flow to temperature variations.
Meltwater
Ablation zone
Accumulation zone
Glacier ice accumulation
Cite
Citations (77)
Most of us think of ice when we think of the Antarctic, and rightly so, considering that only 5%of it is ice‐free [ Drewry et al. , 1982]. Dry valleys—the ice‐free areas—have sandy, rocky valley floors, ice‐covered lakes, and streams that flow only two months of the year. The McMurdo Dry Valleys (78°S 163°E are the largest of these regions [ Green and Friedmann , 1993].
Dry ice
Cite
Citations (0)
The lowest 501 m (∼1139–638 m) of the AND-2A core from southern McMurdo Sound is the most detailed and complete record of early Miocene sediments in Antarctica and indicates substantial variability in Antarctic ice sheet activity during early Miocene time. There are two main pulses of diamictite accumulation recorded in the core, and three significant intervals with almost no coarse clasts. Each diamictite package comprises several sequences consistent with ice advance-retreat episodes.
Diamictite
Ice core
Antarctic ice sheet
Cite
Citations (3)
This chapter contains sections titled: Introduction Conceptual Model of the Dry Valley System Generic Biomass Model Linkages Conclusions
Cite
Citations (15)
Cite
Citations (0)