- Email: [email protected]
Subglacial hydrology and sedimentary processes of a palaeo ice stream
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Abstracts / Quaternary International 279-280 (2012) 9–120
Evidence of past climates can be stored within aquifer systems because groundwater contains environmental information acquired during its passage from recharge to the aquifer or during its residence time within a given aquifer. In general, groundwater systems are considered low resolution archives, contrary to other high resolution, such as ice cores, speleothems or corals. The information contained within an aquifer under favourable conditions may offer a regionally integrated proxy of climatic variations. The combination of high resolution proxy records with low resolution palaeogroundwater information can provide an essential long term understanding of the water cycle variations within large geographic areas. An increasing number of regional studies offer the possibility of a global approach to palaeogroundwater information. The G@GPS in its first year aims to: A) compare palaeoclimatic signals from different hemispheres and latitudes in several selected large basins B) to facilitate exchange of information between researchers from any geographical area with interests in palaeogroundwater and its link to palaeoclimate C) As new data sets are refined and groundwater chronologies are available, expand this approach beyond 14C dating range. A number of basins with abundant local studies from China (NCP), Australia (SydB and GAB), USA (HPA), North Africa and South America have been selected, initially, to test some of our hypothesis. These are large enough to allow a palaeogroundwater component to be present and abundant information is already available. Initial comparisons show that basins at equivalent latitudes and proximity to the ocean (i.e. NCP and SydB) have a clear decreasing groundwater-d18O trend towards the LGM, showing temperature depletion, with more stable temperatures past the LGM. However, some inland datasets (west GAB), show opposite trends suggesting a warming up towards the LGM in those inland locations.
subglacial tills of two ice streams. The relationship between particle size and particle fabric was investigated across the Wentworth size ranges. Particles from silt to coarse gravel size were measured and the most notable results were found in the size ranges silt to medium sand. Comparison between the particle sizes silt to medium sand revealed a consistent mean azimuth orientation, but a clear change and progression in fabric strength (and shape), in all samples. Silt had a very strong azimuth fabric (i.e. measured from a horizontal thin section); very fine sand had a strong azimuth fabric; fine sand had a medium/weak azimuth fabric and medium sand had a very weak azimuth fabric. Dip fabrics (i.e. from vertical thin sections) demonstrated that particles silt to medium sand dipped in all directions, with a preferential horizontal and vertical peak. This distinct pattern of particle fabrics, which has not been observed before, displayed remarkable consistency across all the samples suggesting that this arrangement of particles may be common to subglacial tills. The fabrics observed for silt to medium sand sized particles are not entirely consistent with Jeffery rotation and are not at all consistent with March rotation. Overall orientations of samples varied and were both parallel and transverse to palaeo ice flow. Given the large samples sizes and consistent fabric shapes observed this is clear evidence that subglacial stresses are not always manifest parallel to ice flow. This is inconsistent with experimental work on subglacial fabric, which suggests that experiments do not encompass key subglacial processes that affect particle fabric. CLIMATE VARIABILITY IN THE MID-LATITUDE NORTH ATLANTIC DURING THE PREVIOUS TWO INTERGLACIALS Mark Chapman. University of East Anglia, United Kingdom E-mail address: [email protected]
SUBGLACIAL HYDROLOGY AND SEDIMENTARY PROCESSES OF A PALAEO ICE STREAM Heather Channon. Queen Mary, University of London, United Kingdom E-mail address: [email protected]
A detailed investigation of the LGM Tweed Ice Stream (British Ice Sheet) identifies a landform signature characterised by an isochronous flowset of elongated subglacial lineations. A multi-scale approach integrating geomorphological mapping, sedimentology and thin section micromorphology provides insight into the subglacial processes operating in the Tweed Ice Stream. The onset zone of the ice stream is characterised by small, unelongated drumlins and clusters of poorly connected subglacial meltwater (Nye) channels, cut into impermeable bedrock. A large (0.3 – 0.8 km wide, up to 23 km long, and c. 100 m deep) subglacial meltwater channel coincides with the southern lateral margin of the ice stream. Formation of this channel is thought to have been facilitated by subglacial processes at the lateral margin, including focussed shear and melting, and the channel is also thought to have enabled the ice stream lateral margin to remain static through thermomechanical feedbacks. The channel terminates at a large (c. 42 km2) palaeo lake basin, which is thought to have been a subglacial lake. The typical sediment profile for the ice stream was a glaciotectonite overlain by a subglacial traction till, 1 to 4 m thick. Till injection into bedrock fissures and micromorphological evidence for liquefaction indicates high and variable pore water pressures beneath the ice stream. Micromorphology revealed abundant cutans, which appear to have formed subglacially, giving insight into subglacial water content and transport. A suite of micromorphological structures was identified, indicating mixing, rotation, ductile deformation, brittle deformation, shearing, liquefaction and abrasion. This study provides a unique insight into subglacial processes, especially subglacial hydrology, operating at multiple scales in the Tweed Ice Stream. PARTICLE MOVEMENT AND ROTATION IN A SUBGLACIAL TILL
It is well documented that the subpolar North Atlantic Ocean has experienced considerable variability in sea surface conditions during the last glacial-interglacial cycle but, at present, comparatively little is known about the significance or amplitude of climatic fluctuations in previous glacial-interglacial cycles. We have generated a suite of detailed palaeoenvironmental measurements spanning the last w300 ka from cores SU90-03 and MD99-2253, recovered from the northern margin of the subtropical gyre and the subpolar gyre, respectively. The chronology for the cores, derived from d18O measurements made on the foraminifers Cibicidoides wuellerstorfi and Uvigerina spp., together with stratigraphically significant ice rafted detritus (IRD) episodes enable detailed comparisons to be made. Past sea surface temperature (SST) conditions and the history of IRD input are compared for the previous two interglacials – Marine Isotope Stage (MIS) 7 and 5. The impact of the changes in surface conditions also are explored through benthic d13C derived measurements of North Atlantic Deep Water changes. Glacial IRD depositional events in both cores are readily recognisable at both 40 N and 56 N. In general, the pattern of IRD input and short-term SST cooling is equally distinctive through MIS 7 and 5. A striking similarity is evident in the evolution of surface conditions through the MIS 7e and 5e intervals and the transition that occurred during the subsequent onset of glacial conditions. Faunalderived SST changes suggest a well-defined but small reduction in surface water temperatures through the interglacial, with larger amplitude SST variability (w4 C) postdating glacial inception. Although the SST trends are similar, the composition of the typical ‘interglacial fauna’ varies within the core, suggesting a complex ecological pattern most likely related to differences in the strength of mid latitude currents. NORTHERN PATAGONIA LACUSTRINE SEDIMENTARY RECORD OF CLIMATE VARIABILITY OVER THE LAST MILLENNIUM (PROGLACIAL LAKES FRIAS AND NAHUEL HUAPI, 41 S, ARGENTINA) Emmanuel Chapron. Institut des Sciences de la Terre d'Orléns – ISTO, France E-mail address: [email protected]
Heather Channon. Queen Mary, University of London, United Kingdom E-mail address: [email protected]
Macrofabric (clast orientation) and microfabric (particle orientation measured from thin sections) in till has long been studied and a wide range of fabrics have been reported, but no consistent pattern has emerged. This study presents macrofabric and extensive microfabric results from the
A multidisciplinary investigation of proglacial lakes Frias and Nahuel Huapi recent sedimentary records allowed to precise the impact of climate variability on North Patagonian environments. Based on available seismic profiles (3.5 kHz), several gravity cores were realised in 2008 in the main basin of Lake Frias and in the Blest Branch of Lake Nahuel Huapi within the frame of the ANR Escarsel program. The lithologies of these cores were
Abstracts / Quaternary International 279-280 (2012) 9–120
Evidence of past climates can be stored within aquifer systems because groundwater contains environmental information acquired during its passage from recharge to the aquifer or during its residence time within a given aquifer. In general, groundwater systems are considered low resolution archives, contrary to other high resolution, such as ice cores, speleothems or corals. The information contained within an aquifer under favourable conditions may offer a regionally integrated proxy of climatic variations. The combination of high resolution proxy records with low resolution palaeogroundwater information can provide an essential long term understanding of the water cycle variations within large geographic areas. An increasing number of regional studies offer the possibility of a global approach to palaeogroundwater information. The G@GPS in its first year aims to: A) compare palaeoclimatic signals from different hemispheres and latitudes in several selected large basins B) to facilitate exchange of information between researchers from any geographical area with interests in palaeogroundwater and its link to palaeoclimate C) As new data sets are refined and groundwater chronologies are available, expand this approach beyond 14C dating range. A number of basins with abundant local studies from China (NCP), Australia (SydB and GAB), USA (HPA), North Africa and South America have been selected, initially, to test some of our hypothesis. These are large enough to allow a palaeogroundwater component to be present and abundant information is already available. Initial comparisons show that basins at equivalent latitudes and proximity to the ocean (i.e. NCP and SydB) have a clear decreasing groundwater-d18O trend towards the LGM, showing temperature depletion, with more stable temperatures past the LGM. However, some inland datasets (west GAB), show opposite trends suggesting a warming up towards the LGM in those inland locations.
subglacial tills of two ice streams. The relationship between particle size and particle fabric was investigated across the Wentworth size ranges. Particles from silt to coarse gravel size were measured and the most notable results were found in the size ranges silt to medium sand. Comparison between the particle sizes silt to medium sand revealed a consistent mean azimuth orientation, but a clear change and progression in fabric strength (and shape), in all samples. Silt had a very strong azimuth fabric (i.e. measured from a horizontal thin section); very fine sand had a strong azimuth fabric; fine sand had a medium/weak azimuth fabric and medium sand had a very weak azimuth fabric. Dip fabrics (i.e. from vertical thin sections) demonstrated that particles silt to medium sand dipped in all directions, with a preferential horizontal and vertical peak. This distinct pattern of particle fabrics, which has not been observed before, displayed remarkable consistency across all the samples suggesting that this arrangement of particles may be common to subglacial tills. The fabrics observed for silt to medium sand sized particles are not entirely consistent with Jeffery rotation and are not at all consistent with March rotation. Overall orientations of samples varied and were both parallel and transverse to palaeo ice flow. Given the large samples sizes and consistent fabric shapes observed this is clear evidence that subglacial stresses are not always manifest parallel to ice flow. This is inconsistent with experimental work on subglacial fabric, which suggests that experiments do not encompass key subglacial processes that affect particle fabric. CLIMATE VARIABILITY IN THE MID-LATITUDE NORTH ATLANTIC DURING THE PREVIOUS TWO INTERGLACIALS Mark Chapman. University of East Anglia, United Kingdom E-mail address: [email protected]
SUBGLACIAL HYDROLOGY AND SEDIMENTARY PROCESSES OF A PALAEO ICE STREAM Heather Channon. Queen Mary, University of London, United Kingdom E-mail address: [email protected]
A detailed investigation of the LGM Tweed Ice Stream (British Ice Sheet) identifies a landform signature characterised by an isochronous flowset of elongated subglacial lineations. A multi-scale approach integrating geomorphological mapping, sedimentology and thin section micromorphology provides insight into the subglacial processes operating in the Tweed Ice Stream. The onset zone of the ice stream is characterised by small, unelongated drumlins and clusters of poorly connected subglacial meltwater (Nye) channels, cut into impermeable bedrock. A large (0.3 – 0.8 km wide, up to 23 km long, and c. 100 m deep) subglacial meltwater channel coincides with the southern lateral margin of the ice stream. Formation of this channel is thought to have been facilitated by subglacial processes at the lateral margin, including focussed shear and melting, and the channel is also thought to have enabled the ice stream lateral margin to remain static through thermomechanical feedbacks. The channel terminates at a large (c. 42 km2) palaeo lake basin, which is thought to have been a subglacial lake. The typical sediment profile for the ice stream was a glaciotectonite overlain by a subglacial traction till, 1 to 4 m thick. Till injection into bedrock fissures and micromorphological evidence for liquefaction indicates high and variable pore water pressures beneath the ice stream. Micromorphology revealed abundant cutans, which appear to have formed subglacially, giving insight into subglacial water content and transport. A suite of micromorphological structures was identified, indicating mixing, rotation, ductile deformation, brittle deformation, shearing, liquefaction and abrasion. This study provides a unique insight into subglacial processes, especially subglacial hydrology, operating at multiple scales in the Tweed Ice Stream. PARTICLE MOVEMENT AND ROTATION IN A SUBGLACIAL TILL
It is well documented that the subpolar North Atlantic Ocean has experienced considerable variability in sea surface conditions during the last glacial-interglacial cycle but, at present, comparatively little is known about the significance or amplitude of climatic fluctuations in previous glacial-interglacial cycles. We have generated a suite of detailed palaeoenvironmental measurements spanning the last w300 ka from cores SU90-03 and MD99-2253, recovered from the northern margin of the subtropical gyre and the subpolar gyre, respectively. The chronology for the cores, derived from d18O measurements made on the foraminifers Cibicidoides wuellerstorfi and Uvigerina spp., together with stratigraphically significant ice rafted detritus (IRD) episodes enable detailed comparisons to be made. Past sea surface temperature (SST) conditions and the history of IRD input are compared for the previous two interglacials – Marine Isotope Stage (MIS) 7 and 5. The impact of the changes in surface conditions also are explored through benthic d13C derived measurements of North Atlantic Deep Water changes. Glacial IRD depositional events in both cores are readily recognisable at both 40 N and 56 N. In general, the pattern of IRD input and short-term SST cooling is equally distinctive through MIS 7 and 5. A striking similarity is evident in the evolution of surface conditions through the MIS 7e and 5e intervals and the transition that occurred during the subsequent onset of glacial conditions. Faunalderived SST changes suggest a well-defined but small reduction in surface water temperatures through the interglacial, with larger amplitude SST variability (w4 C) postdating glacial inception. Although the SST trends are similar, the composition of the typical ‘interglacial fauna’ varies within the core, suggesting a complex ecological pattern most likely related to differences in the strength of mid latitude currents. NORTHERN PATAGONIA LACUSTRINE SEDIMENTARY RECORD OF CLIMATE VARIABILITY OVER THE LAST MILLENNIUM (PROGLACIAL LAKES FRIAS AND NAHUEL HUAPI, 41 S, ARGENTINA) Emmanuel Chapron. Institut des Sciences de la Terre d'Orléns – ISTO, France E-mail address: [email protected]
Heather Channon. Queen Mary, University of London, United Kingdom E-mail address: [email protected]
Macrofabric (clast orientation) and microfabric (particle orientation measured from thin sections) in till has long been studied and a wide range of fabrics have been reported, but no consistent pattern has emerged. This study presents macrofabric and extensive microfabric results from the
A multidisciplinary investigation of proglacial lakes Frias and Nahuel Huapi recent sedimentary records allowed to precise the impact of climate variability on North Patagonian environments. Based on available seismic profiles (3.5 kHz), several gravity cores were realised in 2008 in the main basin of Lake Frias and in the Blest Branch of Lake Nahuel Huapi within the frame of the ANR Escarsel program. The lithologies of these cores were