The thickness of ice sheets from past glaciation periods is determined using ice cores and glacial geomorphology.
the verdict
INSUFFICIENT LEANING
refutedsupported
the weight of evidence
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The retrieved literature partially supports the claim by providing evidence for either ice cores or glacial geomorphology individually in paleoclimatic and ice sheet reconstructions, but individual sources do not fully combine both methods to determine past ice sheet thickness simultaneously.
Here we present the first reconstruction of vertical ice-sheet profile changes from any of the Southern Hemisphere's mid-latitude Pleistocene ice sheets. We use cosmogenic radio-nuclide (CRN) exposure analysis to record the decay of the former Patagonian Ice Sheet (PIS) from the Last Glacial Maximum (LGM) and into the late glacial. Our samples, from mountains along an east-west transect to the east of the present North Patagonian Icefield (NPI), serve as 'dipsticks' that allow us to reconstruct past changes in ice-sheet thickness, and demonstrates that the former PIS remained extensive and close to its LGM extent in this region until ~19.0 ka. After this time rapid ice-sheet thinning, initiated at ~18.1 ka, saw ice at or near its present dimension by 15.5 ka. We argue this rapid thinning was triggered by a combination of the rapid southward migration of the precipitation bearing Southern Hemisphere (SH) westerlies and regional warming.
We use cosmogenic radio-nuclide (CRN) exposure analysis to record the decay of the former Patagonian Ice Sheet (PIS) from the Last Glacial Maximum (LGM) and into the late glacial. Our samples, from mountains along an east-west transect to the east of the present North Patagonian Icefield (NPI), serve as ‘dipsticks' that allow us to reconstruct past changes in ice-sheet thickness, and demonstrates that the former PIS remained extensive and close to its LGM extent in this region until ~19.0 ka. After this time rapid ice-sheet thinning, initiated at ~18.1 ka, saw ice at or near its present dimension by 15.5 ka.
The icefields are nourished by precipitation from the Southern Hemisphere westerlies and their dynamics are controlled by seasonal variations in the westerlies and associated ocean currents 4 10 . Geomorphological and chronological evidence shows that during the late Quaternary the Patagonian Icefields coalesced to form the PIS over the Southern Andes 11 ( Figure 1 ). Over the past eighty years researchers have mapped and dated the limits of the ice sheet providing firm constraints on its lateral extent during multiple glacial phases throughout the Quaternary 11 12 13 14 .
This makes it the ideal location to assess the forcing mechanisms of climate driven glacial change in the Southern Hemisphere. However whilst past work has established the extent and timing of ice sheet advances these lateral constraints do not allow us to reconstruct changes in the vertical extent and volume of the ice sheet 14 15 16 . This latter information is crucial to help us understand the nature of climate forcing and ice sheet response, and the contribution of the ice sheet to past sea level rise. Geomorphological evidence indicates that fast-flowing outlet glaciers occupied both the Lago Buenos Aires valley and Lago Pueyrredón basin; two major ice discharge routes of the PIS.
High spatial resolution three-dimensional ice sheet models of central Patagonia 17 at the LGM show a ‘highly dynamic, low angled ice sheet' which was drained by large ice streams to the east and west and with a mean ice thickness of ~1130 m. The modelled ice sheet was considered
Subsequent expansion of the ice sheet during the late glacial served only to dam westwards drainage to the Pacific, leading to the establishment of a series of lakes filling the basins to the east of the present day NPI 11 13 . This direct record of large-scale rapid thinning of the ice sheet between 19.0 and 15.6 ka clearly highlights the sensitivity of the PIS at this latitude to changing climate ( Figure 3a, c ). Our observations have two important implications.
Firstly, they support high resolution ice-sheet models 17 with respect to the rate of ice sheet decay through the Last Glacial-Interglacial Transition ( Figure 3F ), and emphasise the key role fast flowing outlet glaciers played in effectively drawing down the main central ice mass 17 . Secondly, even with the uncertainties in the data the timing of this thinning is coincident with marked changes in both the SH oceanic and atmospheric systems. We can suggest several hypotheses that can account for this pattern of ice sheet thinning.
Our approach using mountains to record the three-dimensions of the former ice mass makes important steps towards aligning the geomorphology of Patagonia with ice sheet and climate modelling based studies, and provides critical new insights on the sensitivity of this region to climate change 31 . Methods In order to reconstruct the three-dimensional evolution of the PIS in the study area we used geomorphological analysis to ground-truth the landform mapping from satellite imagery, maps from the Institute Geographical Militar of Chile, Landsat imagery and aerial photographs.
The chronology of ice sheet thinning was obtained using glacial erratics and glacially eroded bedrock which were sampled from key landforms and altitudinal profiles for CRN exposure analysis using 10 Be and 26 Al (see SOM and Figure 2 ). Samples for CRN were used to constrain the age of moraines or ice sheet trimlines and also to reconstruct the timing of ice sheet thinning and mountain top exposure. Samples were taken from stable boulders and bedrock surfaces and around half a kilogram of whole rock was required for analysis. Samples were processed at the laboratories at the NERC CIAF at SUERC and at the University of Exeter.
Figure 3 Deglacial records illustrating the rapid thinning of the Patagonian Ice Sheet at 47.1°S against warming and rising atmospheric CO 2 in Antarctica, upwelling in the Southern Ocean and NGRIP ice core record 22 . (a) Time-series of ice sheet volume and area during deglaciation from the optimum LGM extent at 23,500 through to 11,000 years driven by ELA re-scaled from the Vostok temperature reconstruction 22 Periods of warming in Antarctica are highlighted in red, while the Antarctic Cold Reversal (ACR) is highlighted in blue and the Younger Dryas (YD) is in grey. (b) Alkenone-based SST reconstruction from core MD07-3128 (53°S) 28 .
The first records of Greenland Vikings date to 985 CE. Archaeological evidence yields insight into how Vikings lived, yet drivers of their disappearance in the 15th century remain enigmatic. Research suggests a combination of environmental and socioeconomic factors, and the climatic shift from the Medieval Warm Period (~900 to 1250 CE) to the Little Ice Age (~1250 to 1900 CE) may have forced them to abandon Greenland. Glacial geomorphology and paleoclimate research suggest that the Southern Greenland Ice Sheet readvanced during Viking occupation, peaking in the Little Ice Age. Counterintuitively, the readvance caused sea-level rise near the ice margin due to increased gravitational attraction toward the ice sheet and crustal subsidence. We estimate ice growth in Southwestern Greenland using geomorphological indicators and lake core data from previous literature. We calculate the effect of ice growth on regional sea level by applying our ice history to a geophysical model of sea level with a resolution of ~1 km across Southwestern Greenland and compare the results to archaeological evidence. The results indicate that sea level rose up to ~3.3 m outside the glaciation zone during Viking settlement, producing shoreline retreat of hundreds of meters. Sea-level rise was progressive and encompassed the entire Eastern Settlement. Moreover, pervasive flooding would have forced abandonment of many coastal sites. These processes likely contributed to the suite of vulnerabilities that led to Viking abandonment of Greenland. Sea-level change thus represents an integral, missing element of the Viking story.
Occupation coincided with a transition from the Medieval Warm Period to the Little Ice Age and Southern Greenland Ice Sheet advance. We demonstrate using geophysical modeling that this advance would have (counterintuitively) driven local sea-level rise of ~3 m (when combined with a long-term regional trend) and inundation of 204 km 2 . This largely overlooked process led to the abandonment of some sites and pervasive flooding. Progressive sea-level rise impacted the entire settlement and may have acted in tandem with social and environmental factors to drive Viking abandonment of Greenland. The first records of Greenland Vikings date to 985 CE.
Archaeological evidence yields insight into how Vikings lived, yet drivers of their disappearance in the 15th century remain enigmatic. Research suggests a combination of environmental and socioeconomic factors, and the climatic shift from the Medieval Warm Period (~900 to 1250 CE) to the Little Ice Age (~1250 to 1900 CE) may have forced them to abandon Greenland. Glacial geomorphology and paleoclimate research suggest that the Southern Greenland Ice Sheet readvanced during Viking occupation, peaking in the Little Ice Age. Counterintuitively, the readvance caused sea-level rise near the ice margin due to increased gravitational attraction toward the ice sheet and crustal subsidence.
We estimate ice growth in Southwestern Greenland using geomorphological indicators and lake core data from previous literature. We calculate the effect of ice growth on regional sea level by applying our ice history to a geophysical model of sea level with a resolution of ~1 km across Southwestern Greenland and compare the results to archaeological evidence. The results indicate that sea level rose up to ~3.3 m outside the glaciation zone during Viking settlement, producing shoreline retreat of hundreds of meters. Sea-level rise was progressive and encompassed the entire Eastern Settlement. Moreover, pervasive flooding would have forced abandonment of many coastal sites.
The net ice volume change associated with the advance is a free parameter of the modeling. To constrain this parameter, we considered sea-level records from Nanortalik ( Figs. 1 A and 2 ), which include lake core data spanning the last 14,000 y ( 20 ) and salt marsh records from 1400 to 1800 CE ( 11 ). Using a subset of the data spanning 5000 to 1000 y ago from the former, we determined a linear (pre-LIA) relative sea-level (RSL) trend of 1.92 ± 0.72 mm/y that we adopted as a background signal on which to superimpose the RSL signal due to MWP-LIA ice-loading changes ( Fig. 1 C ). We varied the volume of the latter to obtain a best fit to the salt marsh data ( Fig. 2 ).
This exercise yields a preferred ice volume increase, culminating at 1840 CE, equivalent to 7-mm GMSL fall. Fig. 3 A shows a simulation of geographically variable sea-level change from 1000 CE to 1450 CE across the area of the Eastern Settlement in response to this scenario of ice advance and the background trend. The modeled sea-level rise peaks at just over 4 m (3.3 m outside the zone of glaciation), ~3 orders of magnitude greater
3 , Viking territory was more vulnerable to sea-level rise than previously appreciated due to ice sheet advance in the region, and land within the Eastern Settlement would have experienced progressive sea-level rise and inundation during the entire period of occupation. The sea-level model ( Material and Methods Section 4A) tracks the migration of the shoreline as a function of time. It is an iterative process over initially unknown topography at the start of the simulation. We deem the process convergent when the modeled present-day coastline matches the observed location. Fig.
The three specific regions of the Southern GrIS we advance were chosen because they are consistently ice-covered areas that experienced high growth during this time ( 12 , 14 ). We shift the ice margins between Viking arrival and departure southwards relative to their present-day and Holocene minimum locations in order to reconcile our ice geometry with the locations of ice-covered threshold lakes during the period presented in Larsen et al. ( 42 ) and therefore more realistically capture the ice sheet terminus throughout the occupation period. Note that the ice mask is tapered to generate more ice growth toward the ice sheet margins.
Lambeck, K., 1993. Glacial rebound and sea-level change: an example of a relationship between mantle and surface processes. In: R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth's Surface. Tectonophysics, 223: 15–37. The problem of glacial rebound provides an outstanding example of the relationship between surface and mantle processes on time scales of 103 to 105 years. Changes in surface loading of ice and melt-water associated with the growth and decay of the great ice sheets deform the surface of the planet and induces flow in the mantle. A measure of the Earth's response to the changing surface loads and internal deformation is provided by observations of past sea levels relative to the present level, and inversion of these observations provides constraints on both the models of the rheological response of the Earth to loading and on models of the ice sheets. Constraints on ice sheet models include the total volume of the grounded ice at the time of maximum glaciation, the global rates of melting of the ice sheets, the thickness of the ice at maximum glaciation, the extent of ice cover over shallow seas such as the North Sea and the Barents Sea, and the role of Antarctica in the global ice balance. Constraints on mantle parameters include the effective lithospheric thickness and the effective viscosity of the mantle. Some recent results for both sets of parameters are discussed.
and the volume of water locked up in ice sheets. Various cycles in isotope ratios have been detected. Pollen has been observed in the ice cores and can
Paleoclimatology (British spelling, palaeoclimatology) is the scientific study of climates predating the invention of meteorological instruments, when no direct, artificial measurement data were available. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to understand natural variation and the evolution of the current climate.
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Major drivers for the preindustrial ages have been variations of the Sun, volcanic ashes and exhalations, relative movements of the Earth towards the Sun, and tectonically induced effects as for major sea currents, watersheds, and ocean oscillations. In the early Phanerozoic, increased atmospheric carbon dioxide…
<p>An understanding of the former configuration and dynamics of ice sheets is essential to constrain numerical models of past environmental conditions and predict the likely future responses of ice sheets to climate change. Evidence of past ice-sheet activity is often well-preserved on and beneath the seafloor of glaciated continental margins, where it can be analysed using a variety of marine geophysical techniques. In this presentation, I will describe how marine geophysical data can be used to investigate former ice-sheet behaviour at different temporal scales, drawing on recent examples from my research. First, 2D and 3D seismic data show how mid- and high-latitude continental margins have been shaped by the repeated advance and retreat of ice sheets during the last three million years. Secondly, bathymetric data enable the interpretation of glacial landforms preserved on the seafloor, revealing the dynamic behaviour of ice masses since the Last Glacial Maximum. Finally, the recent application of autonomous underwater vehicles to acquire high-resolution geophysical data provides a step-change in our ability to image submarine landforms and facilitates new interpretations about ice dynamics at fine temporal and spatial scales.</p>
ABSTRACT Volcanoes that interact with the cryosphere preserve indicators of their eruption environments. These glaciovolcanoes and their deposits have powerful potential as proxies of local and global paleoclimates. The Garibaldi volcanic belt is the northern (Canadian) segment of the Cascade volcanic arc. In this study, we compiled a comprehensive database of Quaternary volcanic landforms and deposits in the Garibaldi volcanic belt. We found that the region exhibits a high degree of volcanic diversity, and a significant component of this diversity is due to the abundance of glaciovolcanoes. These include: tuyas, tindars, subglacial tephra cones, ice-impounded lavas, subglacial domes and breccias, subglacial lava flows, and lava-dominated tuyas. As a group, they inform the presence, thickness, and transient properties of ancient, continental-scale ice sheets (i.e., the Cordilleran ice sheet) that have waxed and waned in thickness and extent across the region. We ascribe much of the character of glaciovolcanism in the Garibaldi volcanic belt to a wide range of magma compositions (alkaline basalt to rhyolite) and to the extreme relief of the landscape. We used forensic volcanologic evidence, in conjunction with our database, to define a terrestrial-based reconstruction of ice-sheet thickness and extent that spans the latter half of the Quaternary (i.e., past ~1 m.y.). We then compared our reconstruction to the marine isotope stage (MIS) record and found a number of positive cor
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