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BROXTON W. BIRD MATTHEW E. KIRBY IAN M. HOWAT SLAWEK TULACZYK 《Boreas: An International Journal of Quaternary Research》2010,39(1):131-144
Bird, B. W., Kirby, M. E., Howat, I. M. & Tulaczyk, S. 2009: Geophysical evidence for Holocene lake-level change in southern California (Dry Lake). Boreas , 10.1111/j.1502-3885.2009.00114.x. ISSN 0300-9483.
Ground penetrating radar (GPR) data are used in combination with previously published sediment cores to develop a Holocene history of basin sedimentation in a small, alpine lake in southern California (Dry Lake). The GPR data identify three depositional sequences spanning the past 9000 calendar years before present (cal. yr BP). Sequence I represents the first phase of an early Holocene highstand. A regression between <8320 and >8120 cal. yr BP separates Sequence I from Sequence II, perhaps associated with the 8200 cal. yr BP cold event. Sequence II represents the second phase of the early-to-mid Holocene highstand. Sequence IIIa represents a permanent shift to predominantly low lake stands beginning ∼5550 cal. yr BP. This mid-Holocene shift was accompanied by a dramatic decrease in sedimentation rate as well as a contraction of the basin's area of sedimentation. By ∼1860 cal. yr BP (Sequence IIIb), the lake was restricted to the modern, central basin. Taken together, the GPR and core data indicate a wet early Holocene followed by a long-term Holocene drying trend. The similarity in ages of the early Holocene highstand across the greater southern California region suggests a common external forcing – perhaps modulation of early Holocene storm activity by insolation. However, regional lake level records are less congruous following the initial early Holocene highstand, which may indicate a change in the spatial domain of climate forcing(s) throughout the Holocene in western North America. 相似文献
Ground penetrating radar (GPR) data are used in combination with previously published sediment cores to develop a Holocene history of basin sedimentation in a small, alpine lake in southern California (Dry Lake). The GPR data identify three depositional sequences spanning the past 9000 calendar years before present (cal. yr BP). Sequence I represents the first phase of an early Holocene highstand. A regression between <8320 and >8120 cal. yr BP separates Sequence I from Sequence II, perhaps associated with the 8200 cal. yr BP cold event. Sequence II represents the second phase of the early-to-mid Holocene highstand. Sequence IIIa represents a permanent shift to predominantly low lake stands beginning ∼5550 cal. yr BP. This mid-Holocene shift was accompanied by a dramatic decrease in sedimentation rate as well as a contraction of the basin's area of sedimentation. By ∼1860 cal. yr BP (Sequence IIIb), the lake was restricted to the modern, central basin. Taken together, the GPR and core data indicate a wet early Holocene followed by a long-term Holocene drying trend. The similarity in ages of the early Holocene highstand across the greater southern California region suggests a common external forcing – perhaps modulation of early Holocene storm activity by insolation. However, regional lake level records are less congruous following the initial early Holocene highstand, which may indicate a change in the spatial domain of climate forcing(s) throughout the Holocene in western North America. 相似文献
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MARION BOUGAMONT SLAWEK TULACZYK 《Boreas: An International Journal of Quaternary Research》2003,32(1):178-190
Geologic evidence such as subglacial troughs and grounding zone wedges indicate that soft-bedded, West Antarctic ice streams are capable of eroding, transporting and depositing large volumes of debris at high rates (˜100 m3 yr-1 per meter width). In order to understand the dynamics of ice streams and the geologic effects of their activity, it is important to understand the physical mechanisms that control these high rates of sub-ice-stream sediment generation and transport. Here, we use a numerical model of Ice Stream C run over c. 8500 model years to quantify the effects of a recently proposed, till-ploughing mechanism of till formation and redistribution beneath ice streams (Tulaczyk et al. 2001; Clark et al. in press). Our results show that this 'transport-limited' mechanism, in which till transport rates scale with ice velocity and erosion rates with spatial gradients of velocity, is consistent with existing constraints. For instance, our model results predict significantly higher (˜0.6 mm yr-1 ) average erosion rates beneath ice-stream tributaries, which are underlain by deep subglacial troughs, than beneath ice-stream trunks (˜0.2mm yr-1 ), whose subglacial troughs have a significantly smaller relief. We would not obtain this satisfactory result if subglacial erosion was parametrized in the model using the more traditional approach of scaling erosion rates with ice velocity (what we call the 'production-limited' parametrization). Because of the requirement of ice continuity, the magnitude of ice velocity generally increases downstream in polar ice streams, and so do the production-limited erosion rates. Pending further investigations, we propose that geologic and geomorphic effects of soft-bedded ice streams should be quantified using some form of a 'transport-limited' parametrization of subglacial erosion rates, e.g. the till-ploughing mechanism. 相似文献
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Signature of palaeo-ice-stream stagnation: till consolidation induced by basal freeze-on 总被引：3，自引：0，他引：3
POUL CHRISTOFFERSEN SLAWEK TULACZYK 《Boreas: An International Journal of Quaternary Research》2003,32(1):114-129
A combination of glaciological theory and geological observations was used to investigate the possibility of till consolidation being driven by basal freeze-on beneath a stagnating, mid-latitude palaeo-ice stream. We focused on the case of the Baltic Ice Stream that advanced into Denmark at c. 15ka BP and which left behind a characteristic till sequence consisting of a strong and well-consolidated till crust underlain by weak and poorly consolidated till. Our hypothesis is that basal freezing caused the markedly higher consolidation of the uppermost till layer. The freezing may have either triggered or simply just accompanied ice-stream stoppage. To test the feasibility of this hypothesis, we have developed a numerical model that couples ice-stream dynamics to time-dependent changes of till properties. Despite relatively mild palaeo-climatic conditions in this area during Late Pleistocene deglaciation (˜0°C), the ice-stream model is capable of producing basal freezing when the effect of horizontal advection of cold ice is included. Our simulations of till response to basal freezing are based on thermodynamic concepts adapted from permafrost studies. Dewatering of till by basal freeze-on may lead to overconsolidation (OCR>10). Based on the history of effective pressure changes in the till, we can predict postglacial till strength profiles using the SHANSEP method. In a series of numerical experiments we have examined the response of till strength to basal freeze-on induced beneath a decaying ice sheet. We have come reasonably close to reproducing shear strength profiles for till deposited by the Baltic Ice Stream. These observations are most consistent with palaeo-ice-stream stagnation triggered by basal freezing and followed by abrupt retreat (<100 years) due to high surface ablation rates (>10 ma-1 ). 相似文献
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SLAWEK TULACZYK BARCLAY KAMB HERMANN F. ENGELHARDT 《Boreas: An International Journal of Quaternary Research》2001,30(2):101-114
Preconsolidation stress recorded in subglacial sediments provides important information about subglacial effective stresses. It is commonly used to reconstruct past effective stresses from sediments left after ice retreat. In this article, we use properties of sub‐ice‐stream till samples to estimate effective stresses beneath a modern West Antarctic ice stream. Two previous estimates of sub‐ice‐stream effective stress were derived for the Upstream B (UpB) area of Ice Stream B from shear wave velocities (50 ± 40 kPa, Blankenship et al 1987) and borehole water level measurements (63 ± 24 kPa, Engelhardt & Kamb 1997). However, geotechnical tests performed on samples of the UpB till have shown that if subjected to effective stress of 50–63 kPa this till would have significantly lower porosity (?0.32–0.35) and higher strength (?‐22–28 kPa) than it apparently has in situ (?0.4 and ?2kPa). We derive new estimates of sub‐ice‐stream effective stress using: (1) Casagrande's construction applied to the results of six confined uniaxial tests, and (2) a combination of void‐ratio data for 51 till samples and 3 experimentally constrained equations describing compressibility of the UpB till under normal consolidation, overconsolidation and in the critical state. Casagrande's method yields an upper bound on effective stress of 25 kPa for four till samples and values of 13, and 4.4kPa for two other samples. The void‐ratio approach gives 11.7 ± 2.6 (normal consolidation), 18.3 ± 4.4 (overconsolidation) and 2.0 ± 0.8 kPa (critical state). These new, lower estimates of effective stress are consistent with the low till strength that has been independently measured and inferred from recent theoretical ice‐stream models. Our interpretation of data on till void ratio in terms of sub‐ice‐stream effective stress means that we can qualitatively evaluate the nature of the vertical distribution of this stress in the UpB till layer. We infer that in the sampled top 3 m of till the effective‐stress distribution is non‐hydrostatic, probably close to lithostatic. The results may be useful in future modeling of ice‐stream behavior and may aid efforts to delineate paleo‐ice streams based on their geologic record. 相似文献
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