- Email: [email protected]
Geological constraints on deglaciation and glacio-isostatic adjustment models of relative sea-level change on the northern Antarctic Peninsula
Abstracts / Quaternary International 279-280 (2012) 346–461
water balance over multiple timescales. However, for palaeohydrological data to serve as an aid for policy-making, lake-based reconstructions need to be transformed into quantitative estimates of the hydro-climate parameters that control water resource availability. This study uses a case example from crater lakes in Central Turkey, whose sedimentary records have been analysed for stable isotopes, diatoms, pollen, microcharcoals and geochemistry, as well as by thin sections. They include Nar lake which has annual varves forming today, and where lake monitoring and sediment trap studies have been carried out since 1998. Lake-sediment proxies for individual varve layers have been “calibrated in time” against local 90-year meteorological observation time series; while hydrological and isotope mass balance modelling provides an independent validation test. The resultant d18BeO-inferred drought index has been applied to the late Holocene lake sediment record from Nar, and – more tentatively – to a longer sequence from now-dry Eski Acigöl crater lake. In both cases, inferred mega-drought events can be compared to other proxy data from the same sediment records (diatom-inferred salinity, geochemistry..), to establish the coherence and reliability of the reconstruction. This methodology for hydro-climatic reconstruction has potential applicability to groundwaterfed lakes in many dryland regions. CLIMATE CHANGE AND THE JUSTINIANIC PLAGUE: AN INTERCOMPARISON OF HIGH-RESOLUTION LAKE SEDIMENT AND DOCUMENTARY RECORDS
407
coalescent onset zone formed of smaller ice streams and fjord outlet glaciers which converged into the Uummannaq trough to form a single ice stream which flowed to the continental shelf break at the LGM. This concept is supported by new offshore data which shows streamlined mega-scale glacial lineations along the trough floor. Ice stream surface elevation throughout the onset zone is constrained to a minimum of 1000m asl based on striae, bedform and moraine data, and is further supported by cosmogenic exposure ages on erratics that show warm based ice operating up to 975m asl in both ice stream and inter-stream areas. 14C and exposure dates along a transect from the mid-shelf to the present ice margin record initial ice surface down-wasting between 25 to 10.5 ka BP, though some ice stream marginal moraines show late stage ice re-thickening prior to extremely rapid ice stream collapse through the Uummannaq trough between 10.5 and 10.1 ka BP. We suggest this pattern of deglaciation reflects strong surface ablation associated with increased air temperatures running up to the Bolling Interstadial (GIS1e) at c. 14 ka BP, followed by ice re-thickening during the Younger Dryas, and late stage rapid marine calving driven by peak sea-level and increasing ocean temperatures at the start of the Holocene. GEOLOGICAL CONSTRAINTS ON DEGLACIATION AND GLACIO-ISOSTATIC ADJUSTMENT MODELS OF RELATIVE SEA-LEVEL CHANGE ON THE NORTHERN ANTARCTIC PENINSULA Steve J. Roberts. British Antarctic Survey, United Kingdom E-mail address: [email protected]
C. Neil Roberts. University of Plymouth, United Kingdom E-mail address: [email protected]
The Plague of Justinian was a pandemic that struck the Byzantine Empire in AD 541–542. It affected much of west and south Asia, North Africa and Europe and recurred periodically until AD w750. Its likely cause was bubonic plague, which later caused the Medieval Black Death, and its epidemiology is complex and multi-causal, perhaps including climate change. Human plague is caused by fleas infected with the bacterium Yersinia pestis, which in turn are carried by rodent hosts. Modern studies show that wetter climatic conditions can lead to increased host and flea populations and heightened plague risk (Gage & Kosoy, Ann. Rev. Ent. 2005). Stathakopoulos (2004) used documentary sources to compile a detailed record of epidemics, famines and extreme weather events in the eastern and central Mediterranean between AD 284 and 750. Independently, Nar crater lake provides an exceptionally well-dated proxy record of climate and land use since AD280 from central Anatolia, then in the agrarian heartland of the Byzantine Empire (Jones et al. Geology 2006; England et al. Holocene 2008). Decadal or better dating precision is possible due to its annually-varved sediment record. Analysis of d18O (5-yr interval) and diatom-inferred salinity (10 yr) shows that the largest dry-to-wet climatic shift of the last 1720 years occurred during the 6th century, centred on 530-560 AD, and led to conditions conducive to the rapid spread of plague. Inferred climate remained humid until AD 750, after which drier conditions lowered the plague risk. From AD 350-550, when extended dry climatic conditions are indicated at Nar, 14 drought and/or famine events are documented for Anatolia, compared to only three from AD 550-750, the plague notwithstanding. Palaeoenvironmental and documentary records therefore show good agreement, although the former register primarily decadal or longer climate trends, whereas the latter reflect mainly shorter-term (e.g. seasonal-annual) discrete events.
The recent disintegration of Antarctic Peninsula ice-shelves and the associated accelerated discharge of continental glaciers has highlighted the contribution of actively melting ice from Antarctica to future sea-level rise. At present, increasing ice mass loss in Antarctica is dominated by changes in the Antarctic Peninsula and the Amundsen Sea sector of West Antarctica, but there is relatively little geological data constraining the geometry, volume and melt history of the Antarctic Peninsula Ice Sheet (APIS) after Termination 1, and during the Holocene. To provide field constraints on the timing and rate of APIS deglaciation, and to assess changes in relative sea level (RSL), we reconstructed a new RSL curve for the north-eastern Antarctic Peninsula based on geomorphological evidence of former marine limits, and radiocarbon-dated marine-freshwater transitions from a series of isolation basins at different altitudes on Beak Island. Relative sea level fell from a maximum of c. 15 m above present at c. 8750 cal yr BP, with rates declining from c. 2.2 mmyr-1 c. 70003000 cal yr BP to 1.7 mmyr-1 c. 3000-2000 cal yr BP, and, finally, to 0.3 mmyr-1 during the last 2000 years. The new Beak Island RSL curve improves the spatial coverage of RSL data in the Antarctic, and is in broad agreement with some glacio-isostatic adjustment models for this location. Elsewhere on the Antarctic Peninsula, we have shown that a RSL fall of up to c. 14 m above present occurred since at least c. 5000 years BP on Alexander Island, and that RSL falls of up to 19 m occurred since c. 7500 years BP on some western Antarctic Peninsula islands. Combined the geological constraints highlighted here imply significant thinning of the APIS by the early-Holocene. Further, they provide key data for the glacialisostatic correction required by satellite-gravity derived measurements of contemporary ice mass loss, and can be used to better assess the future contribution of the APIS to rising sea level. PALAEOENVIRONMENTAL HISTORY OF FORMER SUBGLACIAL LAKE HODGSON, ANTARCTIC PENINSULA Steve J. Roberts. British Antarctic Survey, United Kingdom
ICE STREAM EXTENT AND DEGLACIAL HISTORY OF THE UUMMANNAQ ICE STREAM SYSTEM, WEST GREENLAND David Roberts. Durham University, United Kingdom E-mail address: [email protected]
The offshore and coastal geomorphology of the Uummannaq region of West Greenland records evidence for the advance and decay of the Uummannaq Ice Stream system during the Last Glacial Maximum (LGM). Regional ice flow patterns across this region show evidence for a large
E-mail address: [email protected]
At retreating margins of the Antarctic Ice Sheet, there are a number of locations where former subglacial lakes are emerging from under the ice, but remain perennially ice-covered. Here, we summarise the key data that led us to define Hodgson Lake, situated on southern Alexander Island, Antarctic Peninsula (72 00.5490 S, 068 27.7080 W) as a former subglacial lake. Chronological, geomorphological and isotopic methods were used to determine the local ice sheet deglaciation history. We also undertook isotopic analysis of current and relic lake ice and water to
water balance over multiple timescales. However, for palaeohydrological data to serve as an aid for policy-making, lake-based reconstructions need to be transformed into quantitative estimates of the hydro-climate parameters that control water resource availability. This study uses a case example from crater lakes in Central Turkey, whose sedimentary records have been analysed for stable isotopes, diatoms, pollen, microcharcoals and geochemistry, as well as by thin sections. They include Nar lake which has annual varves forming today, and where lake monitoring and sediment trap studies have been carried out since 1998. Lake-sediment proxies for individual varve layers have been “calibrated in time” against local 90-year meteorological observation time series; while hydrological and isotope mass balance modelling provides an independent validation test. The resultant d18BeO-inferred drought index has been applied to the late Holocene lake sediment record from Nar, and – more tentatively – to a longer sequence from now-dry Eski Acigöl crater lake. In both cases, inferred mega-drought events can be compared to other proxy data from the same sediment records (diatom-inferred salinity, geochemistry..), to establish the coherence and reliability of the reconstruction. This methodology for hydro-climatic reconstruction has potential applicability to groundwaterfed lakes in many dryland regions. CLIMATE CHANGE AND THE JUSTINIANIC PLAGUE: AN INTERCOMPARISON OF HIGH-RESOLUTION LAKE SEDIMENT AND DOCUMENTARY RECORDS
407
coalescent onset zone formed of smaller ice streams and fjord outlet glaciers which converged into the Uummannaq trough to form a single ice stream which flowed to the continental shelf break at the LGM. This concept is supported by new offshore data which shows streamlined mega-scale glacial lineations along the trough floor. Ice stream surface elevation throughout the onset zone is constrained to a minimum of 1000m asl based on striae, bedform and moraine data, and is further supported by cosmogenic exposure ages on erratics that show warm based ice operating up to 975m asl in both ice stream and inter-stream areas. 14C and exposure dates along a transect from the mid-shelf to the present ice margin record initial ice surface down-wasting between 25 to 10.5 ka BP, though some ice stream marginal moraines show late stage ice re-thickening prior to extremely rapid ice stream collapse through the Uummannaq trough between 10.5 and 10.1 ka BP. We suggest this pattern of deglaciation reflects strong surface ablation associated with increased air temperatures running up to the Bolling Interstadial (GIS1e) at c. 14 ka BP, followed by ice re-thickening during the Younger Dryas, and late stage rapid marine calving driven by peak sea-level and increasing ocean temperatures at the start of the Holocene. GEOLOGICAL CONSTRAINTS ON DEGLACIATION AND GLACIO-ISOSTATIC ADJUSTMENT MODELS OF RELATIVE SEA-LEVEL CHANGE ON THE NORTHERN ANTARCTIC PENINSULA Steve J. Roberts. British Antarctic Survey, United Kingdom E-mail address: [email protected]
C. Neil Roberts. University of Plymouth, United Kingdom E-mail address: [email protected]
The Plague of Justinian was a pandemic that struck the Byzantine Empire in AD 541–542. It affected much of west and south Asia, North Africa and Europe and recurred periodically until AD w750. Its likely cause was bubonic plague, which later caused the Medieval Black Death, and its epidemiology is complex and multi-causal, perhaps including climate change. Human plague is caused by fleas infected with the bacterium Yersinia pestis, which in turn are carried by rodent hosts. Modern studies show that wetter climatic conditions can lead to increased host and flea populations and heightened plague risk (Gage & Kosoy, Ann. Rev. Ent. 2005). Stathakopoulos (2004) used documentary sources to compile a detailed record of epidemics, famines and extreme weather events in the eastern and central Mediterranean between AD 284 and 750. Independently, Nar crater lake provides an exceptionally well-dated proxy record of climate and land use since AD280 from central Anatolia, then in the agrarian heartland of the Byzantine Empire (Jones et al. Geology 2006; England et al. Holocene 2008). Decadal or better dating precision is possible due to its annually-varved sediment record. Analysis of d18O (5-yr interval) and diatom-inferred salinity (10 yr) shows that the largest dry-to-wet climatic shift of the last 1720 years occurred during the 6th century, centred on 530-560 AD, and led to conditions conducive to the rapid spread of plague. Inferred climate remained humid until AD 750, after which drier conditions lowered the plague risk. From AD 350-550, when extended dry climatic conditions are indicated at Nar, 14 drought and/or famine events are documented for Anatolia, compared to only three from AD 550-750, the plague notwithstanding. Palaeoenvironmental and documentary records therefore show good agreement, although the former register primarily decadal or longer climate trends, whereas the latter reflect mainly shorter-term (e.g. seasonal-annual) discrete events.
The recent disintegration of Antarctic Peninsula ice-shelves and the associated accelerated discharge of continental glaciers has highlighted the contribution of actively melting ice from Antarctica to future sea-level rise. At present, increasing ice mass loss in Antarctica is dominated by changes in the Antarctic Peninsula and the Amundsen Sea sector of West Antarctica, but there is relatively little geological data constraining the geometry, volume and melt history of the Antarctic Peninsula Ice Sheet (APIS) after Termination 1, and during the Holocene. To provide field constraints on the timing and rate of APIS deglaciation, and to assess changes in relative sea level (RSL), we reconstructed a new RSL curve for the north-eastern Antarctic Peninsula based on geomorphological evidence of former marine limits, and radiocarbon-dated marine-freshwater transitions from a series of isolation basins at different altitudes on Beak Island. Relative sea level fell from a maximum of c. 15 m above present at c. 8750 cal yr BP, with rates declining from c. 2.2 mmyr-1 c. 70003000 cal yr BP to 1.7 mmyr-1 c. 3000-2000 cal yr BP, and, finally, to 0.3 mmyr-1 during the last 2000 years. The new Beak Island RSL curve improves the spatial coverage of RSL data in the Antarctic, and is in broad agreement with some glacio-isostatic adjustment models for this location. Elsewhere on the Antarctic Peninsula, we have shown that a RSL fall of up to c. 14 m above present occurred since at least c. 5000 years BP on Alexander Island, and that RSL falls of up to 19 m occurred since c. 7500 years BP on some western Antarctic Peninsula islands. Combined the geological constraints highlighted here imply significant thinning of the APIS by the early-Holocene. Further, they provide key data for the glacialisostatic correction required by satellite-gravity derived measurements of contemporary ice mass loss, and can be used to better assess the future contribution of the APIS to rising sea level. PALAEOENVIRONMENTAL HISTORY OF FORMER SUBGLACIAL LAKE HODGSON, ANTARCTIC PENINSULA Steve J. Roberts. British Antarctic Survey, United Kingdom
ICE STREAM EXTENT AND DEGLACIAL HISTORY OF THE UUMMANNAQ ICE STREAM SYSTEM, WEST GREENLAND David Roberts. Durham University, United Kingdom E-mail address: [email protected]
The offshore and coastal geomorphology of the Uummannaq region of West Greenland records evidence for the advance and decay of the Uummannaq Ice Stream system during the Last Glacial Maximum (LGM). Regional ice flow patterns across this region show evidence for a large
E-mail address: [email protected]
At retreating margins of the Antarctic Ice Sheet, there are a number of locations where former subglacial lakes are emerging from under the ice, but remain perennially ice-covered. Here, we summarise the key data that led us to define Hodgson Lake, situated on southern Alexander Island, Antarctic Peninsula (72 00.5490 S, 068 27.7080 W) as a former subglacial lake. Chronological, geomorphological and isotopic methods were used to determine the local ice sheet deglaciation history. We also undertook isotopic analysis of current and relic lake ice and water to