- Email: [email protected]
Eastward Drainage of Glacial Lake Agassiz: The Perspective from the Lake Superior Basin
Abstracts / Quaternary International 279-280 (2012) 9–120
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nature of the relief of the Mediterranean, which gives rise to a mosaic of local climates and vegetation patterns. Thus, it is difficult to make regional generalizations from individual sites, and conversely comparisons with sites using regional-sized model grid boxes. Here we have addressed this problem by greatly increasing the number of sites previously used in midHolocene studies to obtain a more comprehensive analysis of the midHolocene Mediterranean using pollen data. This analysis includes not only data from public databases such as the European Pollen Database (140 sites) used in previous analysis, but also a large amount of published data from other sources (126 sites). Using this more comprehensive dataset, we present examples of reconstructions of climate, tree cover, biomes, plant functional types, and individual taxa for both 6 ka and 0 ka. This dataset is intended as a tool for understanding data-model discrepancies in the region for the mid-Holocene, as well as a benchmark for understanding late Holocene changes in vegetation as a result of climate change and intensification of anthropogenic disturbance.
recent trends in fire frequency and magnitude are within the natural variability of long-term ecosystem dynamics and (2) whether extreme climatic events, documented by independent climate-proxy information, mainly controlled biomass burning and vegetation shifts; or (3) whether humans have altered African forest ecosystems over a longer time scale, by changing forest composition and ignition frequency already long before the 20th century, thus overriding the effects of climate. Our preliminary results show that fire regime greatly varied through time and between biomes, as a consequence of intensifying land use promoting fire or fullscale intensive agriculture with lower biomass. Understanding the past role of fire in tropical ecosystems is highly relevant for today's key issue of biospheric feedbacks to the global climate system, given the impact of continued deforestation on global carbon emissions. Also, understanding the natural mechanisms controlling the savannah-forest ecotone can provide information on the resilience of tropical ecosystems and the rate at which rainforest may recover from previous disturbances.
EASTWARD DRAINAGE OF GLACIAL LAKE AGASSIZ: THE PERSPECTIVE FROM THE LAKE SUPERIOR BASIN
HOW DO LOGGING AND FIRE AFFECT SOIL STABILITY? INSIGHTS FROM A 2000 YEARS CHARCOAL RECORD IN SOUTHWESTERN OREGON, USA
Steve M. Colman. University of Minnesota Duluth, United States
Daniele Colombaroli. University of Bern/Plant Sciences, Switzerland
E-mail address: [email protected]
E-mail address: [email protected]
Glacial Lake Agassiz successively flowed through several different outlets during its history, some thought to be eastward, through the Lake Superior basin. One of the eastward discharge events, at about 12.9 ka, has been implicated in global climate change at the beginning of the Younger Dryas (YD), by flooding the North Atlantic with fresh water and suppressing Atlantic meridional overturning circulation. New high-resolution seismicreflection data from Lake Superior bear on the history of Lake Agassiz discharge into the Superior basin. The data for Thunder Bay and adjacent Lake Superior, the purported route for Lake Agassiz discharge at the beginning of the YD, do not support high discharge flows into the basin then. No evidence of deltaic sedimentation, bedrock incision, erosion of bay-mouth sills, or deposition of deepwater subaqueous fans were observed. Complete removal of all evidence of high-magnitude discharge at the beginning of the YD by the subsequent Marquette readvance of ice into the basin seems unlikely. In contrast, the seismic data provide abundant evidence for younger Lake Agassiz discharge farther to the northeast. These data are consistent with terrestrial evidence of post-Marquette, eastward drainage of Lake Agassiz through the area around Lake Nipigon. Several embayments of Lake Superior are underlain by deep bedrock channels that are not apparent in the modern bathymetry. Lakeward of these incised channels, where bedrock descends into the Superior basin, are distinct deposits that we interpret as subaqueous fans, related to high discharges into the basin. The deposits are laterally discontinuous and distinctly fan shaped. Internally, they are composed of a lower chaotic unit and an upper weakly stratified unit, from which we infer an early, high discharge phase, followed by waning flows. These features, incised bedrock channels and deepwater subaqueous fans, are considered diagnostic of high-magnitude discharge into the Superior basin.
During the last few decades extensive logging altered forest composition and soil stability in the floristically diverse forest of the Siskiyou Mountains. Also, the occurrence of large and severe fires (e.g. the 196,000-ha Biscuit Fire in 2002) has led to the perception of increased risk of high-severity fire, compared with presettlement conditions. Because ecological restoration in this area is rooted in the understanding of the historical baseline, long-term data are necessary to determine the frequency and severity of such disturbances, before the impact of extensive forest management. We analyzed a 10-m, 2,000-y sediment core in a dry mixed-conifer/broadleaved forest in the Siskiyou Mountains to understand: 1) the variability of fire regime since the Medieval Climate Anomaly and its effect on soils and vegetation and 2) the effect of 40 years of logging on the erosion rate of soils. The charcoal record reveals that most of the charcoal peaks before Euro-American settlement are within nine distinct periods, marked by a 15y mean interval. Also, the largest charcoal peaks are significantly related to decadal-scale drought periods and were followed by pulses of minerogenic sediment related to rapid sediment delivery. We conclude that fires were highly episodic in the past and severe enough to affect soil stability. However, this effect has increased fourfold after 1950, when logging commenced in the watershed, so that sediment load by clear-cut logging in the 1950s and 1960s have no precedent in earlier fire events.
RECORDS OF BIOMASS BURNING IN TROPICAL ECOSYSTEMS: LINKING BIOTIC AND CLIMATIC CHANGES IN EAST AFRICA OVER THE LAST 4000 YEARS Daniele Colombaroli. University of Bern/Plant Sciences, Switzerland E-mail address: [email protected]
Fire is an important ecological factor of disturbance in tropical ecosystems, controlling the position of the ecotone between savannah and tropical forest at the landscape-scale and contributing to global carbon emissions worldwide. Recent projections of global warming, which predict more frequent and more intense extremes of natural events are of particular concern for East Africa, where population growth and natural resource exploitation are among the highest in the world. Ecological responses to climatic and to large-magnitude environmental changes require long-term records, since only they allow observing such responses at decadal to millennial time-scales. In this study we use high-resolution sediment charcoal from several lakes in Kenya and Uganda to reconstruct past fire/ climate/human interactions, and in particular to understand: (1) whether
THE BEGINNING OF QUATERNARY IN THE ITALIAN PENINSULA: A SEDIMENTARY RECORD OF CLIMATIC/TECTONIC INTERACTION Mauro Coltorti. University of Siena, Italy E-mail address: [email protected]
The Italian peninsula is an active tectonic area that emerged from the sea during the Pliocene with uplift rates of 0,2-1 mm/yrs. Tectonics created synform basins and antiform ridges across the Peninsula during the Pliocene. Extensional basins bounded by high-angle normal faults were created in the Tyrrhenian side during the late Early Pleistocene, when an increase of vertical uplift rates led to the present-day relief. During the Pliocene coastal deposition was linked to transgressive-regressive cycles. Few coarse sediments are recorded in the mainly clayey successions. Continental conditions recorded in few basins are mostly made of finegrained alluvial plain successions with minor episodes of gravelly sedimentation. The beginning of Quaternary is everywhere marked by coarsegrained sandy and gravelly sediments lying on top of abrupt unconformities both in the marine and continental basins due to the onset of cold climatic conditions. The rest of the Early Pleistocene is mostly characterized by finer-grained sedimentation although vertical uplift and extensional tectonics continuously acted. Tectonics created the relief, the sediments source areas and the accommodation space but lithofacies distribution and architecture of the successions are strictly linked to climate conditions. We present an overview of the major Early Pleistocene sequences across the Apennine.
95
nature of the relief of the Mediterranean, which gives rise to a mosaic of local climates and vegetation patterns. Thus, it is difficult to make regional generalizations from individual sites, and conversely comparisons with sites using regional-sized model grid boxes. Here we have addressed this problem by greatly increasing the number of sites previously used in midHolocene studies to obtain a more comprehensive analysis of the midHolocene Mediterranean using pollen data. This analysis includes not only data from public databases such as the European Pollen Database (140 sites) used in previous analysis, but also a large amount of published data from other sources (126 sites). Using this more comprehensive dataset, we present examples of reconstructions of climate, tree cover, biomes, plant functional types, and individual taxa for both 6 ka and 0 ka. This dataset is intended as a tool for understanding data-model discrepancies in the region for the mid-Holocene, as well as a benchmark for understanding late Holocene changes in vegetation as a result of climate change and intensification of anthropogenic disturbance.
recent trends in fire frequency and magnitude are within the natural variability of long-term ecosystem dynamics and (2) whether extreme climatic events, documented by independent climate-proxy information, mainly controlled biomass burning and vegetation shifts; or (3) whether humans have altered African forest ecosystems over a longer time scale, by changing forest composition and ignition frequency already long before the 20th century, thus overriding the effects of climate. Our preliminary results show that fire regime greatly varied through time and between biomes, as a consequence of intensifying land use promoting fire or fullscale intensive agriculture with lower biomass. Understanding the past role of fire in tropical ecosystems is highly relevant for today's key issue of biospheric feedbacks to the global climate system, given the impact of continued deforestation on global carbon emissions. Also, understanding the natural mechanisms controlling the savannah-forest ecotone can provide information on the resilience of tropical ecosystems and the rate at which rainforest may recover from previous disturbances.
EASTWARD DRAINAGE OF GLACIAL LAKE AGASSIZ: THE PERSPECTIVE FROM THE LAKE SUPERIOR BASIN
HOW DO LOGGING AND FIRE AFFECT SOIL STABILITY? INSIGHTS FROM A 2000 YEARS CHARCOAL RECORD IN SOUTHWESTERN OREGON, USA
Steve M. Colman. University of Minnesota Duluth, United States
Daniele Colombaroli. University of Bern/Plant Sciences, Switzerland
E-mail address: [email protected]
E-mail address: [email protected]
Glacial Lake Agassiz successively flowed through several different outlets during its history, some thought to be eastward, through the Lake Superior basin. One of the eastward discharge events, at about 12.9 ka, has been implicated in global climate change at the beginning of the Younger Dryas (YD), by flooding the North Atlantic with fresh water and suppressing Atlantic meridional overturning circulation. New high-resolution seismicreflection data from Lake Superior bear on the history of Lake Agassiz discharge into the Superior basin. The data for Thunder Bay and adjacent Lake Superior, the purported route for Lake Agassiz discharge at the beginning of the YD, do not support high discharge flows into the basin then. No evidence of deltaic sedimentation, bedrock incision, erosion of bay-mouth sills, or deposition of deepwater subaqueous fans were observed. Complete removal of all evidence of high-magnitude discharge at the beginning of the YD by the subsequent Marquette readvance of ice into the basin seems unlikely. In contrast, the seismic data provide abundant evidence for younger Lake Agassiz discharge farther to the northeast. These data are consistent with terrestrial evidence of post-Marquette, eastward drainage of Lake Agassiz through the area around Lake Nipigon. Several embayments of Lake Superior are underlain by deep bedrock channels that are not apparent in the modern bathymetry. Lakeward of these incised channels, where bedrock descends into the Superior basin, are distinct deposits that we interpret as subaqueous fans, related to high discharges into the basin. The deposits are laterally discontinuous and distinctly fan shaped. Internally, they are composed of a lower chaotic unit and an upper weakly stratified unit, from which we infer an early, high discharge phase, followed by waning flows. These features, incised bedrock channels and deepwater subaqueous fans, are considered diagnostic of high-magnitude discharge into the Superior basin.
During the last few decades extensive logging altered forest composition and soil stability in the floristically diverse forest of the Siskiyou Mountains. Also, the occurrence of large and severe fires (e.g. the 196,000-ha Biscuit Fire in 2002) has led to the perception of increased risk of high-severity fire, compared with presettlement conditions. Because ecological restoration in this area is rooted in the understanding of the historical baseline, long-term data are necessary to determine the frequency and severity of such disturbances, before the impact of extensive forest management. We analyzed a 10-m, 2,000-y sediment core in a dry mixed-conifer/broadleaved forest in the Siskiyou Mountains to understand: 1) the variability of fire regime since the Medieval Climate Anomaly and its effect on soils and vegetation and 2) the effect of 40 years of logging on the erosion rate of soils. The charcoal record reveals that most of the charcoal peaks before Euro-American settlement are within nine distinct periods, marked by a 15y mean interval. Also, the largest charcoal peaks are significantly related to decadal-scale drought periods and were followed by pulses of minerogenic sediment related to rapid sediment delivery. We conclude that fires were highly episodic in the past and severe enough to affect soil stability. However, this effect has increased fourfold after 1950, when logging commenced in the watershed, so that sediment load by clear-cut logging in the 1950s and 1960s have no precedent in earlier fire events.
RECORDS OF BIOMASS BURNING IN TROPICAL ECOSYSTEMS: LINKING BIOTIC AND CLIMATIC CHANGES IN EAST AFRICA OVER THE LAST 4000 YEARS Daniele Colombaroli. University of Bern/Plant Sciences, Switzerland E-mail address: [email protected]
Fire is an important ecological factor of disturbance in tropical ecosystems, controlling the position of the ecotone between savannah and tropical forest at the landscape-scale and contributing to global carbon emissions worldwide. Recent projections of global warming, which predict more frequent and more intense extremes of natural events are of particular concern for East Africa, where population growth and natural resource exploitation are among the highest in the world. Ecological responses to climatic and to large-magnitude environmental changes require long-term records, since only they allow observing such responses at decadal to millennial time-scales. In this study we use high-resolution sediment charcoal from several lakes in Kenya and Uganda to reconstruct past fire/ climate/human interactions, and in particular to understand: (1) whether
THE BEGINNING OF QUATERNARY IN THE ITALIAN PENINSULA: A SEDIMENTARY RECORD OF CLIMATIC/TECTONIC INTERACTION Mauro Coltorti. University of Siena, Italy E-mail address: [email protected]
The Italian peninsula is an active tectonic area that emerged from the sea during the Pliocene with uplift rates of 0,2-1 mm/yrs. Tectonics created synform basins and antiform ridges across the Peninsula during the Pliocene. Extensional basins bounded by high-angle normal faults were created in the Tyrrhenian side during the late Early Pleistocene, when an increase of vertical uplift rates led to the present-day relief. During the Pliocene coastal deposition was linked to transgressive-regressive cycles. Few coarse sediments are recorded in the mainly clayey successions. Continental conditions recorded in few basins are mostly made of finegrained alluvial plain successions with minor episodes of gravelly sedimentation. The beginning of Quaternary is everywhere marked by coarsegrained sandy and gravelly sediments lying on top of abrupt unconformities both in the marine and continental basins due to the onset of cold climatic conditions. The rest of the Early Pleistocene is mostly characterized by finer-grained sedimentation although vertical uplift and extensional tectonics continuously acted. Tectonics created the relief, the sediments source areas and the accommodation space but lithofacies distribution and architecture of the successions are strictly linked to climate conditions. We present an overview of the major Early Pleistocene sequences across the Apennine.