Deforming-bed origin for southern Laurentide till sheets?

Alley, Richard B.

doi:10.3189/s0022143000042817

Cited by 78 publications

(92 citation statements)

References 51 publications

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“…The published values cover a wide range from 10 to 50 m 2 a À1 [Cutler et al, 2001] to 100-400 m 2 a À1 [Alley, 1991;Johnson et al, 1991;Jenson et al, 1995]. It does not appear likely that the upper range of values could be sustained throughout a whole glaciation and may be simply representative of high fluxes during ice sheet decay, when abundant basal meltwater may promote high rates of ice sliding and, thus, sediment transport [Marshall and Clark, 2002].…”

Section: Residence Times Of Carbon Under Icementioning

confidence: 99%

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: Residence Times Of Carbon Under Icementioning

confidence: 99%

Subglacial methanogenesis: A potential climatic amplifier?

Wadham

Tranter

Tulaczyk

et al. 2008

Global Biogeochemical Cycles

114

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“…Early Pleistocene ice sheets may have been as spatially expansive as during the late Pleistocene [Boellstorff, 1978;Fisher et al, 1985;Joyce et al, 1993] (but see Barendregt and Irving [1998] Figures 3c and 3e). A possible resolution is that a deformable bed underlaid the Laurentide during the early Pleistocene and led to faster flowing, more expansive, and thinner ice sheets [Fisher et al, 1985;Beget, 1986;Alley, 1991;Clark and Pollard, 1998]. In this study we assume the ice sheet rests atop a layer of deformable sediment, represented using the model of Jenson et al [1996].…”

Section: Model Descriptionmentioning

confidence: 99%

Integrated summer insolation forcing and 40,000‐year glacial cycles: The perspective from an ice‐sheet/energy‐balance model

2008

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[1] Although the origins of the 40,000-year glacial cycles during the early Pleistocene are readily attributed to changes in Earth's obliquity (also having a 40,000-year period), the lack of ice-volume variability at precession periods (20,000 years) is difficult to reconcile with most parameterizations of the insolation forcing. It was recently proposed that precession's influence on glaciation is muted because variations in the intensity of summer insolation are counterbalanced by changes in the duration of the summertime, but no climate model has yet been shown to generate obliquity period glacial cycles in response to the seasonal insolation forcing. Here we present a coupled ice-sheet/energy-balance model that reproduces the seasonal cycle and, when run over long time periods, generates glacial variability in response to changes in Earth's orbital configuration. The model is forced by the full seasonal cycle in insolation, and its response can be understood within the context of the integrated summer insolation forcing. The simple fact that obliquity's period is roughly twice as long as that of precession results in a larger amplitude glacial response to obliquity. However, for the model to generate almost exclusively obliquity period glacial variability, two other conditions must be met. First, the ice sheet's ablation zone must reside poleward of $60°N because insolation intensity is more sensitive to changes in Earth's obliquity at high latitudes. Second, the ablation season must be long enough for precession's opposing influences on summer and fall insolation intensity to counterbalance one another. These conditions are consistent with a warm climate and a thin ice sheet, where the latter is simulated as a response to subglacial sediment deformation. If a colder climate is prescribed, or in the absence of basal motion, ice sheets tend to be larger and undergo greater precession period variability, in keeping with proxy observations of late Pleistocene glaciation.Citation: Huybers, P., and E. Tziperman (2008), Integrated summer insolation forcing and 40,000-year glacial cycles: The perspective from an ice-sheet/energy-balance model, Paleoceanography, 23, PA1208,

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“…Based on this inference, Alley et al (1986Alley et al ( , 1987aAlley et al ( , 1989 have hypothesized that the fast motion of Ice Stream B is accommodated in this till layer, which behaves as a viscous fluid that deforms throughout its thickness and erodes the underlying substrata. This viscous-deforming-bed hypothesis for the till beneath Ice Stream B has been embraced as a new potential mechanism of ice streaming and used to justify reinterpretation of many Pleistocene tills in North America and Europe as former viscously deforming glacial beds (Alley et al 1987b;Alley 1991;Boulton 1996;Clark 1991;Clark and Walder 1994;and many others). In essence, the subglacial till of Ice Stream B has been used, implicitly or explicitly, as a modern analog for widely distributed till deposits.…”

Section: Introductionmentioning

confidence: 99%

Sedimentary processes at the base of a West Antarctic ice stream; constraints from textural and compositional properties of subglacial debris

Tulaczyk

Kamb

Scherer

et al. 1998

Journal of Sedimentary Research

237

167

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Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clayrich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, subglacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams.

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Deforming-bed origin for southern Laurentide till sheets?

Cited by 78 publications

References 51 publications

Subglacial methanogenesis: A potential climatic amplifier?

Subglacial methanogenesis: A potential climatic amplifier?

Integrated summer insolation forcing and 40,000‐year glacial cycles: The perspective from an ice‐sheet/energy‐balance model

Sedimentary processes at the base of a West Antarctic ice stream; constraints from textural and compositional properties of subglacial debris

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