Fresh arguments against the Shaw megaflood hypothesis. A reply to comments by David Sharpe on “Paleohydraulics of the last outburst flood from glacial Lake Agassiz and the 8200 BP cold event”

Clarke, Garry K. C.; Leverington, David; Teller, James T.; Dyke, Arthur S.; Marshall, Shawn J.

doi:10.1016/j.quascirev.2004.12.003

Cited by 61 publications

(40 citation statements)

References 45 publications

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“…This compares well with modeling investigations and inferences made from deep borehole temperatures (Rolandone et al, 2003). Where warm based, rates of basal melting are estimated at 5-7 mm a À1 due to basal melting alone [Kump and Alley, 1994;Breemer et al, 2002;Clarke et al, 2005]. This value was probably exceeded in ice marginal areas where there was surface melt supply to the bed and a greater degree of frictional melting associated with ice streaming [Breemer et al, 2002].…”

Section: Thermal Conditions At the Ice Sheet Basesupporting

confidence: 72%

Subglacial methanogenesis: A potential climatic amplifier?

Wadham

Tranter

Tulaczyk

et al. 2008

Global Biogeochemical Cycles

114

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[1] Subglacial environments are a previously neglected component of the Earth's global carbon cycle, a reflection of the view held until recently that they are dominated by abiotic and oxic conditions. Here we provide a realistic assessment of the theory that the basal regions of the ice sheets that formed over North America and Europe during glaciations were host to significant populations of anaerobic microorganisms, including methanogens, able to metabolize organic carbon sequestered during interglacials and overridden during Quaternary glacials. In doing so, we review the current evidence for subglacial methane release during deglaciation, estimate the size of the subglacial reservoir of organic carbon (SOC), and assess the amount of SOC available to subglacial microbes and the likely pathways and rates of carbon turnover. We then discuss the fate of subglacial methane and the potential impact of its release on atmospheric methane concentrations. We calculate that the SOC equates to 418-610 Pg C and includes carbon from terrestrial soils/vegetation, peatlands, lake, and marine sediments. The SOC that is potentially available for microbial conversion to methane is smaller than this estimate due to (1) glacial erosion, (2) accumulation of recalcitrant organic carbon compounds over time, (3) conversion to carbon dioxide by aerobic/anaerobic respiration, and (4) incomplete conversion of labile organic matter to methane. We estimate that the total SOC available for conversion to methane is 63 Pg C. Our estimates of methane production potentials span a wide range because of the current uncertainty surrounding subglacial metabolic rates. We believe, however, that there is a strong likelihood that subglacial microbes could convert 63 Pg of SOC to methane during a glacial cycle. If this were the case, release of this methane from the ice sheet margins during retreat would need to be episodic in order to significantly impact atmospheric methane concentrations. Our findings suggest that it may well be important to consider subglacial environments as active components of the Earth's carbon cycle. Conclusive determination of the potential impact of subglacial methane production on atmospheric methane concentrations during deglaciation, however, awaits more precise determination of the ability of subglacial microbes to degrade organic carbon components and their associated rates of metabolism under in situ conditions.

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Section: Thermal Conditions At the Ice Sheet Basesupporting

confidence: 72%

Subglacial methanogenesis: A potential climatic amplifier?

Wadham

Tranter

Tulaczyk

et al. 2008

Global Biogeochemical Cycles

114

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“…This interpretation has been the subject of much debate (e.g., Benn and Evans, 2006;Evans, 2010;Shaw, 2010a, b;Evans et al, 2013;Shaw, 2013), which is yet to be fully resolved, not least because ice streams are typically associated with abundant subglacial meltwater that helps lubricate their flow. However, questions remain, for example, regarding the sources and volume of water required to feed putative mega-flood tracks (Clarke et al, 2005). Thus, spatiallyconfined fast-flowing ice (ice streaming) is the simpler interpretation at present and one which we adopt in this paper.…”

Section: O a L E S C E N C E W I T H T H E C O R D I L L E R A N I Cmentioning

confidence: 97%

Ice streams in the Laurentide Ice Sheet: Identification, characteristics and comparison to modern ice sheets

Margold

Stokes

Clark

2015

Earth-Science Reviews

223

222

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This paper presents a comprehensive review and synthesis of ice streams in the Laurentide Ice Sheet (LIS) based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed. The inventory includes 117 ice streams, which have been identified based on a variety of evidence including their bedform imprint, large-scale geomorphology/ topography, till properties, and ice rafted debris in ocean sediment records. Despite uncertainty in identifying ice streams in hard bedrock areas, it is unlikely that any major ice streams have been missed. During the Last Glacial Maximum, Laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets. Large ice streams had extensive onset zones and were fed by multiple tributaries and, where ice drained through regions of high relief, the spacing of ice streams shows a degree of spatial self-organisation which has hitherto not been recognised. Topography exerted a primary control on the location of ice streams, but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock, and where networks of ice streams switched direction repeatedly and probably over short time scales. As the ice sheet retreated onto its low relief interior, several ice streams show no correspondence with topography or underlying geology, perhaps facilitated by localised build-up of pressurised subglacial meltwater. They differed from most other ice stream tracks in having much lower length-to-width ratios and have no modern analogues. There have been very few attempts to date the initiation and cessation of ice streams, but it is clear that ice streams switched on and off during deglaciation, rather than maintaining the same trajectory as the ice margin retreated. We provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30% of the margin was drained by ice streams at the LGM (similar to that for present day Antarctic ice sheets), but this decreases to 15% and 12% at 12 cal ka BP and 10 cal ka BP, respectively. The extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate, or internal ice dynamical feedbacks unrelated to climate (or both) is largely unknown and represents a key area for future work to address.

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“…drumlins, ribbed moraine) to delineate flood pathways is more controversial (e.g. Clarke et al 2005). The pattern of tunnel valleys, subglacial meltwater channels, and other meltwaterrelated features, are useful for constraining palaeo-subglacial lakes and elucidating how they interact with the subglacial hydrological system.…”

Section: Subglacial Hydrology Of Ice Sheets and Subglacial Lakesmentioning

confidence: 99%