Quaternary Science Reviews 119 (2015) 11e21
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A new glacial isostatic adjustment model of the Innuitian Ice Sheet, Arctic Canada K.M. Simon a, b, *, T.S. James b, a, A.S. Dyke c, d a
School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2, Canada Geological Survey of Canada e Paciﬁc, Natural Resources Canada, Sidney, BC V8L 4B2, Canada c Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada d Department of Anthropology, McGill University, Montreal, Quebec H3A 2T4, Canada b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 October 2014 Received in revised form 2 April 2015 Accepted 8 April 2015 Available online 16 May 2015
A reconstruction of the Innuitian Ice Sheet (IIS) is developed that incorporates ﬁrst-order constraints on its spatial extent and history as suggested by regional glacial geology studies. Glacial isostatic adjustment modelling of this ice sheet provides relative sea-level predictions that are in good agreement with measurements of post-glacial sea-level change at 18 locations. The results indicate peak thicknesses of the Innuitian Ice Sheet of approximately 1600 m, up to 400 m thicker than the minimum peak thicknesses estimated from glacial geology studies, but between approximately 1000 to 1500 m thinner than the peak thicknesses present in previous GIA models. The thickness history of the best-ﬁt Innuitian Ice Sheet model developed here, termed SJD15, differs from the ICE-5G reconstruction and provides an improved ﬁt to sea-level measurements from the lowland sector of the ice sheet. Both models provide a similar ﬁt to relative sea-level measurements from the alpine sector. The vertical crustal motion predictions of the best-ﬁt IIS model are in general agreement with limited GPS observations, after correction for a signiﬁcant elastic crustal response to present-day ice mass change. The new model provides approximately 2.7 m equivalent contribution to global sea-level rise, an increase of þ0.6 m compared to the Innuitian portion of ICE-5G. SJD15 is qualitatively more similar to the recent ICE-6G ice sheet reconstruction, which appears to also include more spatially extensive ice cover in the Innuitian region than ICE-5G. Crown Copyright © 2015 Published by Elsevier Ltd. All rights reserved.
Keywords: Innuitian Ice Sheet Glacial isostatic adjustment Relative sea-level GPS
1. Introduction Glacial isostatic adjustment (GIA) models predict the solid Earth's loading and relaxation response to glacial cycles of growth and decay. They typically consist of a spatial-temporal reconstruction of ice sheet extent and thickness applied to an Earth model in which key parameters are the elastic lithospheric thickness and the radial mantle viscosity proﬁle. The ﬁt of GIA model predictions to observations that are typically attributed to the GIA process, such as Holocene relative sea-level (RSL) change, often yields information relating to ice sheet evolution and Earth properties (e.g., Tushingham and Peltier, 1991; Lambeck, 1993; Peltier, 2004; Ivins and James, 2005; James et al., 2009). However, without sufﬁcient
* Corresponding author. Now at Department of Geoscience and Remote Sensing, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands. Tel.: þ31 (0) 15 2788147. E-mail address: [email protected]
(K.M. Simon). http://dx.doi.org/10.1016/j.quascirev.2015.04.007 0277-3791/Crown Copyright © 2015 Published by Elsevier Ltd. All rights reserved.
independent constraints, model parameters may be correlated with each other and GIA model predictions may not distinguish between different parameter combinations. The incorporation of new or revised independent glacialegeological constraints on ice sheet evolution, or geophysical constraints on the Earth's physical and rheological properties, can therefore provide important additional information with which to construct initial model conﬁgurations. In this study, we focus on the GIA response in the Queen Elizabeth Islands (QEI) of the Canadian Arctic Archipelago (Fig. 1). Although the conﬁguration and extent of ice coverage in the QEI during the last glaciation has been debated, the history of Holocene glacial isostatic uplift and relative sea-level fall following deglaciation is well-recorded by radiocarbon-dated material collected from raised beaches and marine sediments throughout the region. GPS-measured rates of crustal deformation are also available from the region, although the record is sparser than the RSL record, as there are continuous GPS sites installed at only three locations within the QEI.
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Fig. 1. Map of the study area showing the Queen Elizabeth Islands (the region of the former Innuitian Ice Sheet) and geographical place names. AHI e Axel Heiberg Island, ARI e Amund Ringnes Island, BI e Bathurst Island, CI e Cornwallis Island, ERI e Ellef Ringnes Island, MI e Melville Island, PPI e Prince Patrick Island.
This study develops a trial model of Innuitian Ice Sheet (IIS) coverage that incorporates current understanding of the glacialegeological constraints on the evolution of the IIS, as summarized by England et al. (2006). A GIA modelling analysis tests how well this model of the IIS can predict measurements of RSL change. The goal of the study is to evaluate whether the development of a revised IIS reconstruction can better explain the history of Holocene sea-level change in the region. A secondary goal is to constrain better ice thickness estimates in the Innuitian region. We compare the predictions of the IIS model developed in this study to those of the ICE-5G reconstruction (Peltier, 2004), which differs substantially in its ice coverage across the QEI because of recent changes in interpretation of glacial history. We also evaluate to what extent GPS-measured present-day uplift rates may be inﬂuenced by nearby ice mass loss. Finally, the global sea-level equivalent contribution of the best-ﬁt IIS reconstruction is quantiﬁed. 2. Background and previous Innuitian GIA studies The spatial extent of the last glaciation in the Queen Elizabeth Islands and even the existence of a coherent IIS were debated for over a century (see Dyke (1999) for a review). Competing interpretations of the regional glacial history were typically divided into two general hypotheses. In the ﬁrst hypothesis, glaciation was not extensive in the QEI, and no continuous ice sheet was present during the Last Glacial Maximum (LGM). Rather, glaciers and ice caps thickened and extended beyond their present-day boundaries, and ice coverage consisted of a series of discontinuous ice caps with peak ice thicknesses likely not exceeding 1000 m (e.g., England, 1976; Dyke and Prest, 1987). The second hypothesis, which was based largely on the observed pattern of Holocene emergence,
featured extensive ice coverage over the QEI and the presence of a spatially-continuous ice sheet during the LGM (Blake, 1970). The two hypotheses were the subject of intense debate throughout the 1970's and onward, with resolution proving elusive, in part because the paucity of observations allowed different interpretations of glacial history. However, a growing number of observations of glacial ﬂow directions, erratic dispersal trains, lateral meltwater channels, and new relative sea-level measurements, as well as reinterpretations of previous data, have converged to support extensive ice coverage at the LGM and to provide improved constraint on IIS chronology (Bednarski, 1998; Cofaigh et al., 2000; Atkinson, 2003; Atkinson and Dyke, 1999; O England, 2004; England et al., 2004). The most recent consensus is that a continuous ice sheet existed in the Innuitian region. It consisted of an alpine sector to the northeast over Ellesmere and Axel Heiberg islands, and a lowland sector to the southwest. The IIS began advancing to its maximum extent after 27 14C ka BP and as late as 19 14C ka BP, and reached its LGM conﬁguration up to 5 14C ka after the Laurentide Ice Sheet (LIS). It did not begin retreating from its margins until 11e10 14C ka BP (Dyke et al., 2002; England et al., 2006). Recently, Tarasov et al. (2012) developed a glaciological model for the North American ice sheets, and this model includes an Innuitian Ice Sheet component that is constrained by regional RSL data. However, Innuitian Ice Sheet evolution was not the primary focus of the Tarasov et al. (2012) study. Sasgen et al. (2012) also assessed North American GIA models, but again the Innuitian region was not the primary focus of this study, and there has been little recent detailed evaluation of Innuitian Ice Sheet evolution within forward GIA models. One of the few forward GIA modelling studies dedicated to the Innuitian region was performed by Tushingham (1991a,b), prior to resolution of the IIS debate
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(England et al., 1991). Tushingham concluded that a model with thick, extensive ice coverage better ﬁt relative sea-level measurements throughout the region than a model with thinner, more limited ice cover. The more successful ice sheet reconstruction used by Tushingham (1991a) was the global ICE-3G model of Tushingham and Peltier (1991). The ICE-3G ice sheet model used by Tushingham (1991a) featured maximum Innuitian ice thicknesses of 2500 m, and was used in conjunction with an Earth model with a 120 km thick elastic lithosphere, an upper mantle (>670 km depth) viscosity of 1021 Pa s, and a lower mantle viscosity of 2 1021 Pa s. This upper mantle viscosity value is somewhat higher than more recent global estimates of upper mantle viscosity (~4 1020 Pa s, Mitrovica and Forte, 2004). A change in the upper mantle viscosity in the ICE-3G calculations would affect the RSL predictions and require changes to the ice sheet thickness history. ICE-5G (Peltier, 2004) is a more recent global ice sheet reconstruction, and relative to ICE-3G, its Innuitian component features discontinuous ice cover in the southwestern lowland sector, thinner ice in much of the alpine sector, and locally thicker ice over Bathurst Island. The ICE-5G model has been used to predict Innuitian sea-level change (Lecavalier et al., 2013), but the study was focussed on the Greenland Ice Sheet. A successor model to ICE-5G, ICE-6G, has also recently been published (Peltier et al., 2015). Relative to ICE-5G, ICE-6G appears to feature thicker and more spatially extensive ice in most of the southwestern lowland sector and locally thinner ice over Bathurst Island (Peltier et al., 2015); however, the study did not discuss the evolution of the IIS or the observational constraints used in this region of the model. 3. Methods and data 3.1. Modelling approach A recent synthesis of the constraints on IIS conﬁguration and chronology (England et al., 2006) guides the development of a revised IIS reconstruction. Its predictions are compared to regional RSL measurements and to available GPS measurements of presentday crustal uplift. Within the revised IIS model, a key modelling parameter that was varied was a thickness scaling factor. We also assess the ﬁt of ICE-5G (Peltier, 2004) through comparison of its predictions to the same observations. We use the ICE-5G model for quantitative comparison with our results because it is the most recent ice sheet reconstruction for which a detailed ice thickness history is publicly available for the time span of the Last Glacial period (from 122 ka to present). Characteristics of the ﬁnal IIS model are also qualitatively compared to the Innuitian region of ICE-6G (Peltier et al., 2015). 3.2. GIA model description The Innuitian Ice Sheet reconstruction is embedded in a global ice-loading model that features an updated Laurentide Ice Sheet thickness history (Simon, 2014) and, for the rest of the globe, the ICE-5G ice sheet history. The revised Laurentide Ice Sheet history of Simon (2014) was derived by scaling the thickness of a starting ice sheet model to obtain improved ﬁts to RSL histories at 24 locations in northern Canada (Fig. 2). The Earth model follows the VM5a proﬁle of Peltier and Drummond (2008), and consists of a 60 km thick elastic lithosphere overlying a 40 km thick layer of high viscosity (1022 Pa s). Below the lithosphere, the VM5a viscosity proﬁle is an approximation of the VM2 proﬁle derived by Peltier (1996), and has viscosities of 5 1020 Pa s from 100 to 660 km depth, 1.6 1021 Pa s between 660 and 1160 km depth, and 3.2 1021 Pa s from 1160 km depth to the core-mantle boundary. The Earth's
elastic and density structure are averaged values from the Preliminary Reference Earth Model (Dziewonski and Anderson, 1981) and the calculations solve for the response to surface loading of a compressible viscoelastic Maxwell rheology following the description of James and Ivins (1998). The calculations include a globally self-consistent solution of the sea-level equation following the methods of Mitrovica and Peltier (1991) and Mitrovica and Milne (2003). 3.3. RSL and GPS data Holocene RSL data from 18 locations within the QEI are the primary constraints on the GIA models. Of the selected RSL data, there are 9 locations from each of the former alpine and lowland sectors of the IIS (Fig. 3). The 18 RSL histories are part of a larger available compilation of North American RSL data. The selected locations span the study area, providing good spatial coverage, and were chosen because their inferred RSL histories are wellconstrained by the data. The RSL measurements consist of the ages and elevations of radiocarbon-dated material, such as marine shells, driftwood and mammal bones. The material type and stratigraphic setting of each sample determines the inferred position of sea level for each point, and thus collectively deﬁne the RSL history at that location (typically the inferred sea-level position is at or above marine samples, and at or below terrestrial samples). At each location, the RSL data are ﬁltered for non-constraining data points through the removal of marine ages that lie well below the position of the inferred sea-level curve, and terrestrial ages that lie well above the curve, following the methods described in Simon (2014). Marine ages from highly migratory species, such as Balaena mysticetus (bowhead whale), are corrected for the reservoir effect using the global average reservoir correction of 400 years. All other marine samples are corrected for the reservoir effect with a regional correction of 740 years, which was determined to be appropriate for samples from the Canadian Arctic Archipelago (McNeely et al., 2006). The radiocarbon ages are calibrated using the program Calib 5.1 (Stuiver and Reimer, 1993). The Marine04 data set (Hughen et al., 2004) is used to calibrate marine ages, and the IntCal04 data set (Reimer et al., 2004) is used to calibrate terrestrial ages. More recent calibration curves exist (Reimer et al., 2013), but the differences between the calibrations during the Holocene are very small, and the use of a more recent calibration curve does not signiﬁcantly impact the conversion of radiocarbon ages to calendar ages. For each observation point ðsoi ; tio Þ, we calculate the minimum distance between the observation coordinates and the modelpredicted curve (to a point with coordinates spi ; tip ), where (s, t) are the respective coordinates in space and time of the ith observed (o) or predicted (p) RSL value. A c2 measure of misﬁt is then used to evaluate the level of ﬁt between the model predictions and RSL measurements according to
2 ! p 2 N soi si 1 X 4 ¼ þ N i¼1 so;s i
tio ti so;t i
!2 3 5;
where so,s and so,t are the respective observational uncertainties in space and time, and N is the number of RSL measurements (e.g., Mitrovica et al., 2000; Paulson et al., 2007). The temporal uncertainties are laboratory-determined in radiocarbon years; the spatial uncertainties are assigned following Simon (2014), and include an uncertainty associated with the elevation measurement, and if applicable, an uncertainty associated with tidal range and the growth depth of marine shells.
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Fig. 2. LGM (26 ka) ice thickness map of the Laurentide portion of the global ice sheet reconstruction used outside the Innuitian region (from Simon, 2014).
GPS-measured vertical uplift rates from three site locations are also compared to model-predicted present-day uplift rates (Fig. 3, Craymer et al., 2011). The GPS data are processed using the Bernese GPS Software Version 5.0 (Dach et al., 2007) and are aligned to the ITRF2008 reference frame (Altamimi et al., 2011). The misﬁt between the observed and predicted vertical uplift rates is calculated as
N Vio Vip 1 X ¼ N i¼1 soi
where Vio and Vip are the observed and predicted rates of vertical crustal motion at the ith site location, respectively, soi is the ith observational uncertainty, and N is the number of GPS sites. Due to the proximity of the GPS sites to present-day ice cover, a component of the GPS-measured vertical land motion rates may be attributable to the present-day ice mass loss of Arctic glaciers and ice caps (GIC) and the Greenland Ice Sheet (e.g., Hager, 1991; Wahr et al., 1995; Wahr and Han, 1997; Jiang et al., 2010). The predicted long-term GIA uplift rates are adjusted for this effect by adding to them the calculated elastic response of the Earth to changes in present-day ice cover. To determine the elastic effect, rates of present-day mass loss are speciﬁed according to recent estimates. Canadian Arctic GIC are currently experiencing an estimated mass loss of 60 ± 8 Gt/yr (Gardner et al., 2011, 2013). The Greenland Ice Sheet experienced an estimated mass loss of 142 ± 49 Gt/yr from 1992 to 2011 and 263 ± 30 Gt/yr from 2005 to 2010 (Shepherd et al., 2012). To assess the sensitivity of the effect to different mass loss scenarios, we compute the elastic effect for both of these Greenland mass loss rates. Spatially uniform mass loss is assumed for both the Arctic GIC and the Greenland Ice Sheet and ice mass loss from other sources is not included. The use of different mass loss scenarios (which are subject to large uncertainty), or the inclusion of spatially non-uniform mass loss would alter the predictions of the elastic contribution. For example, Luthcke et al. (2013) showed that mass loss from Greenland occurs mostly in coastal areas. Revising the
Fig. 3. Site map of the relative sea-level (RSL) and GPS site locations and names. At Eureka (EUR2/EURK), there is some spatial separation between the average location of the RSL data and the GPS site. The black dotted lines enclose the region of the former Innuitian Ice Sheet and also indicate the approximate boundary of the alpine sector to the northeast and the lowland sector to the southwest.
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Fig. 4. Schematic depiction of the Innuitian Ice Sheet (from England et al., 2006) at LGM. Current glacialegeological constraints indicate continuous ice coverage across the QEI. In the alpine sector, local ice divides formed on the topographic high points and ice ﬂow converged within the alpine interior. Ice in the lowland sector was thinner than the alpine sector, but covered the islands and ﬁlled the inter-island channels.
Greenland mass loss scenario to limit mass loss to coastal regions would likely increase the elastic contribution to vertical land motion rates both in the near-ﬁeld and within the QEI. Thus, the analysis of present-day vertical crustal velocities described here is preliminary. 3.4. A new reconstruction of the Innuitian Ice Sheet A representation of the IIS is developed that incorporates the glacialegeological constraints on ice sheet history recently presented by England et al. (2006). Fig. 4 shows England et al.'s (2006) schematic depiction of Innuitian Ice Sheet extent and conﬁguration. The revised IIS model is on a latitude-longitude grid with a spacing of approximately 0.7 0.7, and features 36 time slices from 122 ka BP to present. Although this grid spacing offers only limited spatial resolution of smaller features (e.g., ice caps) within the Innuitian region, the grid is the same as the ICE-5G grid as well as the grid of the updated Laurentide Ice Sheet reconstruction of Simon (2014). The revised IIS reconstruction does not incorporate changes to ice coverage outside of the margins of the QEI, although England et al. (2006) suggest that peak ice coverage in the region was continuous in Lancaster Sound to Bafﬁn Bay, as well as across Nares Strait and Smith Sound to Greenland. Compared to the ICE-
5G model (Peltier, 2004), there are four main changes to the IIS reconstruction (Fig. 5). First, the updated IIS model features continuous ice coverage over the entire Innuitian region, including grounded ice in the inter-island channels of the lowland sector. In contrast, ICE-5G does not include ice in the southwestern lowland sector over Ellef Ringnes and Amund Ringnes islands. Second, absolute ice thicknesses are based on topographically derived minimum peak thickness estimates. Based on the vertical distance between formerly ice-covered topographic high points and the Cofaigh et al. (2000) and England et al. depth of adjacent ﬁords, O (2004) both suggested the minimum peak thickness of the IIS was approximately 1200 m. In contrast, the peak thickness of ICE-5G over Bathurst Island is approximately 3000 m. The thick ice cover on Bathurst Island is however localized to this region, and thinner ice is present in ICE-5G throughout the remaining Innuitian region. Third, the relative thickness distribution of the revised model is designed to correlate with the observed pattern of Holocene emergence. The emergence history suggests that ice was thickest in Eureka Sound and Nansen Sound in the central alpine sector, and that thinner ice existed in the outer alpine sector and throughout the lowland sector. Fourth, the timing history of the revised IIS model features retreat of the lowland sector of the IIS after 13 cal ka BP, retreat of the alpine sector at approximately
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Fig. 5. Peak ice thickness maps at LGM of the Innuitian Ice Sheet for ICE-5G (left) and the models developed in this study (middle, right). The initial model developed in this study (middle, INNUx1.0) has peak thicknesses of approximately 1200 m in Eureka and Nansen sounds (locations given in Fig. 1), while the model that best ﬁts the relative sea-level data has peak thicknesses of approximately 1600 m (right, SJD15). SJD15 has ice thickened by a factor of one-third relative to INNUx1.0, and also has ice locally thickened south of Clements Markam Inlet and locally thinned near Croker Bay.
9.5 cal ka BP, and an assigned LGM time that is delayed relative to the LGM of the LIS by 5 ka. The revised IIS model and ICE-5G are shown at their LGM times (26 ka BP and 21 ka BP, respectively) in Fig. 5. The boundaries of the present-day ice caps on Ellesmere, Axel Heiberg, and Devon islands are approximated from the Randolph
Glacier Inventory (RGI, Arendt et al., 2012), although the resolution of the GIA model grid is much coarser than the spatial resolution of the RGI. Constraints on present-day thickness of the ice caps are sparse, although Dowdeswell et al. (2004) estimated the Devon ice cap has a peak central thickness of approximately 700 m with thinner ice around the margins. For simplicity, we assume the
Fig. 6. Relative sea-level measurements and predictions for the alpine sector of the Innuitian Ice Sheet. Lower observational constraints on the position of sea level are shown by the blue triangles, and upper observational constraints are shown by the red triangles. The triangles point in the direction of the inferred position of sea level (i.e., sea level was located at or above the blue triangles/lower constraints, and was located at or below the red triangles/upper constraints). Model predictions of the ICE-5G reconstruction (solid black line) are compared to those from the minimum IIS model developed in this study (black dotted line, INNUx1.0), and the best-ﬁt IIS model, which has ice thicknesses that are one-third larger than the 1.0 model (grey dashed line, SJD15). At most sites, the ICE-5G and the SJD15 models both provide similar and good ﬁts to the RSL data from the alpine sector. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
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Fig. 7. Relative sea-level measurements and predictions for the lowland sector of the Innuitian Ice Sheet. The sea-level measurements and model predictions are plotted as described in Fig. 6. The SJD15 model provides the best ﬁt to the RSL data from the lowland sector, and compared to ICE-5G has thinner ice near Devon, Cornwallis and Bathurst islands (site locations Bere Bay (BERE), Prince Alfred Bay (ALFR), Bathurst Island (BAIS), Resolute (RESO), Lovell Point (LOVE)), and thicker, more continuous ice coverage in the southwestern regions of the QEI (site locations Sabine Peninsula (SABI) and Ellef Ringnes Island (ELLE)).
Fig. 8. (top panel) Total c2 misﬁt values for the 18 RSL histories from the Innuitian region for both the ICE-5G ice model (black squares) and the best-ﬁt IIS model developed in this study (SJD15; red circles). (bottom panel) Fractional difference in c2 misﬁt values relative to ICE-5G for the best-ﬁt IIS model (red circles). A positive fractional difference represents a decreased RSL misﬁt (i.e., improved agreement) for the revised best-ﬁt IIS model. The sites are identiﬁed by their four-letter abbreviations and their locations are depicted in Fig. 3. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
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Fig. 9. (left) Predicted and GPS-observed vertical uplift rates in the Innuitian region, and (right) the differences between the observed and predicted rates. The ICE-5G model (black squares) is compared to the best-ﬁt IIS model developed in this study with and without a correction for the elastic crustal response to present-day ice mass changes of nearby glaciers and ice caps and the Greenland Ice Sheet. SJD15 (red circles) shows the predictions with no elastic effect included. SJD15-142 (grey diamonds) and SJD15-263 (blue diamonds) show, respectively, the predictions with a 142 Gt/yr and 263 Gt/yr mass loss assumed for Greenland. Both SJD15-142 and SJD15-263 also include Arctic GIC mass loss, ﬁxed at 60 Gt/yr. Al e Alert, E2 e Eureka, R e Resolute. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
thicknesses of the present-day ice caps on Axel Heiberg and Ellesmere islands follow a similar distribution, although locally, some ice caps may be up to 900 m thick (e.g., Hattersley-Smith, 1969). In the next section, we compare the ﬁt of the predictions of the new model and ICE-5G to observations. 4. Results The GIA model predictions are compared to18 sea-level histories from the QEI (Fig. 3). From the former alpine sector, we consider the RSL histories from Eureka (EURK), Greely Fiord (GREE), Mokka Fiord (MOKK), Cape Southwest (CaSW), Schei Point (SCHE), Makinson Inlet (MAKI), Clements Markam Inlet (CLEM), Cape Herschel (HERS), and Cape Storm (STOR). From the former lowland sector, we consider the RSL histories from Bere Bay (BERE), Prince Alfred Bay (ALFR), Bathurst Island (BAIS), Resolute (RESO), Lovell Point (LOVE), Firkin Point (FIRK), Croker Bay (CROK), Sabine Peninsula (SABI), and Ellef Ringnes Island (ELLE). The starting version of the new Innuitian Ice Sheet model has initial peak ice thicknesses of 1200 m in Eureka and Nansen sounds. This model strongly underpredicts RSL throughout both the alpine and lowland sectors (Figs. 6 and 7). To improve the ﬁt of the model to the observed sea-level history, we applied a uniform scaling factor to the IIS reconstruction. Increasing the thickness of the load by a factor of one-third provides a good ﬁt to most of the sea-level histories from both the alpine and lowland sectors. The scaled model underpredicts RSL at Clements Markam Inlet and overpredicts RSL at Croker Bay. The scaled model also underpredicts RSL at Sabine Peninsula at all but the earliest and latest data points, but the misﬁt here is not as signiﬁcant as that at the other two sites. The introduction of locally thicker ice near Clements Markam Inlet and thinner ice near Croker Bay in the thickened IIS model improves, but does not entirely reconcile, the misﬁt of model-predicted sea level at these sites (Figs. 6 and 7). This ﬁnal model is termed SJD15. The ﬁnal SJD15 model was derived from the starting model by thickening it by one-third, and then imposing local additional thinning and thickening to improve the ﬁt at Croker Bay and Clements Markam Inlet. Despite the misﬁts at Clements Markam Inlet and Croker Bay, SJD15 ﬁts the regional RSL data as well as or better than ICE-5G. In
the alpine sector, the c2 misﬁt of SJD15, relative to ICE-5G, is modestly decreased at most sites and increased at Clements Markam Inlet and Greely Fiord (Fig. 8). At Greely Fiord the increase is partly due to scatter on the data points between 8 and 10 cal ka BP. Overall, both ICE-5G and the revised IIS model appear to predict sea-level change in the alpine sector reasonably well. In the lowland sector, the ﬁt of our IIS model is improved relative to ICE-5G (i.e., the c2 misﬁt is decreased, Fig. 8). This improvement is both the result of decreasing the ice thicknesses around Bathurst Island, and including thicker and grounded ice in the southwest near Sabine Peninsula (Melville Island) and Ellef Ringnes Island. At Firkin Point, the c2 RSL misﬁt value is decreased for our model relative to ICE-5G, although both models ﬁt the data at this site very well (Figs. 7 and 8). Croker Bay is the only selected site location in the lowland sector for which the ﬁt of predictions to observations is not improved for SJD15 relative to ICE-5G. The three continuous GPS site locations in the Queen Elizabeth Islands are located at Resolute, Eureka, and Alert (RESO, EUR2, ALRT). When only the long-term GIA response is considered, neither the original ICE-5G model nor the best-ﬁt SJD15 model predict the vertical uplift rates particularly well, and both models have high c2 misﬁt values (Fig. 9). Both models underpredict the rate of vertical uplift at RESO, EUR2 and ALRT from between ~0.5 and 3.5 mm/yr. Addition of the present-day mass loss effect to the predictions from the best-ﬁt IIS model improves the ﬁt between the predicted and observed uplift rates. Relative to the SJD15 model with no elastic effect included, the Greenland mass loss scenario of 142 Gt/yr improves the c2 misﬁt by a factor of 5, while the Greenland mass loss scenario of 263 Gt/yr improves the c2 misﬁt by a factor of approximately 16 (the Arctic GIC mass loss scenario is ﬁxed at 60 Gt/yr, Fig. 9). 5. Discussion The revised Innuitian Ice Sheet reconstruction uses, as a starting basis, the current independent glacialegeological constraints that were summarized in England et al. (2006). The revision features continuous ice coverage throughout both the alpine and lowland sectors of the ice sheet, compared to the discontinuous ice coverage in the lowland sector of ICE-5G. Peak ice thicknesses at LGM are
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approximately one-half of the peak thicknesses in the Innuitian component of ICE-5G. The peak thicknesses in ICE-5G are localized around Bathurst Island, and may have been based on radiocarbon ages of shells that are interpreted to have been ice-transported to higher elevations (Bednarski, personal communication, 2014). The ﬁrst version of the IIS model deﬁned the peak ice thickness to be 1200 m, consistent with the lower bound on peak ice thickness Cofaigh et al. (2000) and England et al. (2004). This indicated by O model consistently underpredicts sea-level change throughout the region. A one-third increase to the load provides a much improved ﬁt of the model predictions to regional RSL measurements, and corresponds to peak ice thicknesses in the central alpine sector of ~1600 m, and peak thicknesses throughout much of the lowland sector of ~1330 m. Additional forcing of local thinning and thickening yielded a best-ﬁt ﬁnal model termed SJD15 which improved the ﬁt at selected RSL sites. The spatial extent of ice in our best-ﬁt model SJD15 more closely resembles the ICE-3G model of Tushingham and Peltier (1991) than the ICE-5G model, although, at 1600 m, the peak ice thicknesses are approximately one-third smaller than those in ICE-3G. The ability of our thinner ice sheet reconstruction to ﬁt the regional RSL data may in part be the result of using an Earth model with a slightly thinner lithosphere and a slightly decreased upper mantle viscosity compared to that used for ICE-3G. The global ICE-6G ice sheet reconstruction has been developed as an update to ICE-5G, and maps of ice surface elevation and ice cover suggest that ICE-6G also includes more continuous ice coverage in the lowland sector than ICE-5G (Peltier et al., 2015). The constraints used in the development of the Innuitian portion of the ICE-6G model, however, are unclear, as it does not appear Innuitian RSL data were used to evaluate the GIA response of ICE-6G, and regional GPS data are both limited and contaminated by present-day ice mass loss effects. The global sea-level equivalent contribution at LGM for SJD15 is approximately 2.7 m, a net change to the LGM global sea-level budget of þ0.6 m compared to ICE-5G. The larger spatial extent of SJD15, compared to the Innuitian portion of ICE-5G, is offset by the smaller peak thicknesses, leading to a relatively small net change in contribution to global sea level. The growth of a continuous, spatially extensive ice sheet in the Innuitian region is also consistent with current interpretations of regional paleoclimatic conditions. In the past, one of the main arguments against extensive Innuitian ice cover was the presumed regional aridity that would result from the precipitation shadow of the Laurentide Ice Sheet (England et al., 2006). However, oxygen isotope records from Greenland, as well as climate modelling analyses, indicate that Greenland may have received considerable precipitation from the North Paciﬁc Ocean during the LGM, owing to a jet-stream split around the Laurentide Ice Sheet (England et al., 2006). This suggestion supports the active advection of moisture through the Innuitian region during the same time period that glacialegeology measurements indicate IIS growth and expansion. Thus, our IIS model of GIA is both consistent with glacialegeological observations, and compatible with assumed paleoclimatic conditions. Although the new ice model reproduces the ﬁrst-order glacialegeological constraints on IIS history summarized by England et al. (2006), the spatial resolution of the gridded ice model is coarse. The coarseness of the grid allows only approximate deﬁnition of the boundaries of certain glacial features, including the present-day ice caps. Future reconstructions of the Innuitian Ice Sheet could beneﬁt from the use of denser grid spacing, in order to better represent the evolution and conﬁguration of the ice caps. As well, recent glaciological models indicate that the Greenland Ice Sheet was larger than the Greenland component of ICE-5G by up to 2e3 m of global sea-level equivalent at LGM (Simpson et al.,
2009; Lecavalier et al., 2014). These models are constrained by both regional RSL data as well as geological and geomorphological indicators of past ice extent. The spatial extent data, as well as the ﬁt of model predictions to the RSL data, support a larger spatial extent for the Greenland Ice Sheet than previously thought (Lecavalier et al., 2014). The best-ﬁt Greenland Ice Sheet model of Lecavalier et al. (2014) includes LGM ice that extends to the margin of the Innuitian region. In contrast, based on macrofossil analyses of sediment cores from Nares Strait, Mudie et al. (2004) suggested Greenland and Innuitian ice did not merge everywhere along the length of Nares Strait. At any rate, use of a larger Greenland Ice Sheet reconstruction, or of a different ice extent and margin chronology along Nares Strait, would likely affect the Innuitian GIA signal, particularly at sites on eastern Ellesmere Island (Clements Markam Inlet, Cape Herschel, and Makinson Inlet). Future reconstructions of the IIS could therefore be extended to consider scenarios with both more continuous and less continuous ice coverage across Nares Strait. More extensive ice margins in Smith and Lancaster sounds could also be incorporated into the model. Although eastward extension of the ice stream in Lancaster Sound would likely exacerbate the misﬁt to RSL data at Croker Bay, it is possible this effect could be balanced by earlier recession of ice in the outer sound. Our results indicate that the Earth's elastic crustal response to present-day ice mass changes is signiﬁcant in the QEI, consistent with studies focussing on other glaciated regions, such as Greenland (Jiang et al., 2010). An improved ﬁt between observed and predicted rates of vertical uplift is obtained when the predicted elastic response to present-day ice mass changes is added to the predicted long-term viscoelastic GIA response. As well, present-day vertical uplift rates may be inﬂuenced signiﬁcantly by recent volume changes to glaciers and ice caps during the retreat of the Little Ice Age (e.g., Ivins and James, 2004; Larsen et al., 2005), but this effect has not yet been explored for the QEI. For example, the GIA model does not attempt to model in detail the regrowth of ice caps from their mid-Holocene minima to Neoglacial maxima. To understand better the vertical uplift signal, a variation of mass loss scenarios as well as spatially non-uniform mass loss, particularly coastal-dominated mass loss from Greenland, should be considered. 6. Conclusion An ice sheet reconstruction is described here that considers current glacialegeological constraints on Innuitian Ice Sheet history, as summarized by England et al. (2006). The revision features continuous ice coverage throughout both the alpine and lowland sectors of the ice sheet, in contrast to the limited spatial coverage in the lowland sector of ICE-5G. As well, at 1600 m, the peak LGM ice thicknesses of the best-ﬁt IIS model are approximately one-half of those in ICE-5G. The peak ice thicknesses of the best-ﬁt IIS model also cover a larger area than the localized peak ice thicknesses of ICE-5G. In our ﬁrst revision of the IIS model, we deﬁned the peak ice thickness to be 1200 m, consistent with the lower bound on peak Cofaigh et al. (2000) and England et al. ice thickness indicated by O (2004). However, in order for model predictions to ﬁt regional RSL measurements, peak thicknesses of ~1600 m are needed. A comparison of our best-ﬁt IIS model to ICE-5G indicates that both models predict RSL histories in good agreement with measurements in the alpine sector of the IIS. The ability of two different models of ice cover to adequately predict the sea-level measurements from the alpine sector underscores the usefulness of incorporating available independent constraints on ice sheet evolution. Conversely, the continuous and thinner ice cover in the lowland sector of the best-ﬁt IIS model signiﬁcantly improves the ﬁt of RSL
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predictions relative to ICE-5G. The growth of a spatially extensive Innuitian Ice Sheet is also consistent with assumed regional paleoclimatic conditions around the LGM. In the Innuitian region, RSL data provide the strongest constraints on GIA models. Measurements of crustal uplift from GPS are sparse, but improved agreement between model predictions and measurements is obtained when the observations are corrected for the Earth's elastic response to present-day ice mass changes. Our best-ﬁt Innuitian Ice Sheet model, at LGM, is equivalent to 2.7 m of global sea-level, an increase of 0.6 m compared to the Innuitian portion of ICE-5G. Acknowledgments This work was supported by the ArcticNet Networks of Centres of Excellence, the Nunavut Climate Change Partnership, and the Earth Sciences Sector (ESS) Climate Change Geoscience Program (CCGP). KMS was supported by the ArcticNet Network of Centres of Excellence, the University of Victoria, and the ESS CCGP. We thank John Andrews, Jan Bednarski, and Erik Ivins for reviews which substantially improved the manuscript. Fig. 4 is reprinted from Quaternary Science Reviews, 25(7-8), J. England, N. Atkinson, J. Cofaigh, The Innuitian Ice Bednarski, A.S. Dyke, D.A. Hodgson, C. O Sheet: conﬁguration, dynamics and chronology, 689e703, Copyright (2006), with permission from Elsevier. This is ESS contribution number 20140325. References tivier, L., 2011. ITRF2008: an improved solution of the Altamimi, Z., Collilieux, X., Me international terrestrial reference frame. J. Geod. 85, 457e473. Arendt, A., Bolch, T., Cogley, J.G., Gardner, A., Hagen, J.-O., Hock, R., Kaser, G., Pfeffer, W.T., Moholdt, G., Paul, F., Radi c, V., Andreassen, L., Bajracharya, S., Beedle, M., Berthier, E., Bhambri, R., Bliss, A., Brown, I., Burgess, E., Burgess, D., Cawkwell, F., Chinn, T., Copland, L., Davies, B., de Angelis, H., Dolgova, E., Filbert, K., Forester, R., Fountain, A., Frey, H., Giffen, B., Glasser, N., Gurney, S., Hagg, W., Hall, D., Haritashya, U.K., Hartmann, G., Helm, C., Herreid, S., Howat, I., Kapustin, G., Khromova, T., Kienholz, C., Koenig, M., Kohler, J., Kriegel, D., Kutuzov, S., Lavrentiev, I., LeBris, R., Lund, J., Manley, W., Mayer, C., Miles, E., Li, X., Menounos, B., Mercer, A., Moelg, N., Mool, P., Nosenko, G., Negrete, A., Nuth, C., Pettersson, R., Racoviteanu, A., Ranzi, R., Rastner, P., Rau, F., Rich, J., Rott, H., Schneider, C., Seliverstov, Y., Sharp, M., Sigursson, O., Stokes, C., Wheate, R., Winsvold, S., Wolken, G., Wyatt, F., Zheltyhina, N., 2012. Randolph Glacier Inventory [v2.0]: a Dataset of Global Glacier Outlines. Global Land Ice Measurements from Space. Digital Media, Boulder Colorado, USA. Atkinson, N., 2003. Late Wisconsinan glaciation of Amund and Ellef Ringnes islands, Nunavut: evidence for the conﬁguration, dynamics, and deglacial chronology of the northwest sector of the Innuitian Ice Sheet. Can. J. Earth Sci. 40, 351e363. Atkinson, N., England, J., 2004. Postglacial emergence of Amund and Ellef Ringnes islands, Nunavut: implications for the northwest sector of the Innuitian Ice Sheet. Can. J. Earth Sci. 41, 271e283. Bednarski, J.M., 1998. Quaternary history of Axel Heiberg Island bordering Nansen Sound, Northwest Territories, emphasizing the last glacial maximum. Can. J. Earth Sci. 35, 520e533. Blake Jr., W., 1970. Studies of glacial history in Arctic Canada I: pumice, radiocarbon dates, and differential postglacial uplift in the eastern Queen Elizabeth Islands. Can. J. Earth Sci. 7, 634e664. Craymer, M.R., Henton, J.A., Piraszewski, M., Lapelle, E., 2011. An Updated GPS Velocity Field for Canada. American Geophysical Union Fall Meeting, December 5e9, 2011, G21A-0793. €de, A., Hugentobler, U., J€ Dach, R., Beutler, G., Bock, H., Fridez, P., Ga aggi, A., Meindl, M., Mervart, L., Prange, L., Schaer, S., Springer, T., Urschl, C., Walser, P., 2007. Bernese GPS Software Version 5.0. Astronomical Institute, University of Bern, Bern, Switzerland. Dowdeswell, J.A., Benham, T.J., Gorman, M.R., Burgess, D., Sharp, M.J., 2004. Form and ﬂow of the Devon Island Ice Cap, Canadian Arctic. J. Geophys. Res. 109 http://dx.doi.org/10.1029/2003JF000095. Dyke, A.S., Prest, V.K., 1987. Late Wisconsinan and Holocene history of the Lauogr. Phys. Quat. 41, 237e263. rentide Ice Sheet. Ge Dyke, A.S., 1999. Last Glacial Maximum and deglaciation of Devon Island, Arctic Canada: support for an Innuitian Ice Sheet. Quat. Sci. Rev. 18, 393e420. Dyke, A.S., Andrews, J.T., Clark, P.U., England, J.H., Miller, G.H., Shaw, J., Veillette, J.J., 2002. The Laurentide and Innuitian ice sheets during the Last Glacial Maximum. Quat. Sci. Rev. 21, 9e31. Dziewonski, A.M., Anderson, D.L., 1981. Preliminary reference Earth model. Phys. Earth Planet. Inter. 25, 297e356.
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