Coastal paleogeography of the Pacific Northwest, USA, for the last 12,000 years accounting for three-dimensional earth structure

Predictive modeling of submerged archaeological sites requires accurate sea-level predictions in order to reconstruct coastal paleogeography and associated geographic features that may have influenced the locations of occupation sites such as rivers and embayments. Earlier reconstructions of the paleogeography of parts of the western U.S. coast used an assumption of eustatic sea level, but this neglects the large spatial variations in relative sea level (RSL) associated with glacial isostatic adjustment (GIA) and tectonics. Subsequent work using a one-dimensional (1-D) solid Earth model showed that reconstructions that accounted for GIA result in significant differences from those based on eustatic sea level. However, these analyses neglected the complex three-dimensional (3-D) solid Earth structure associated with the Cascadia subduction zone that has also strongly influenced RSL along the Oregon-Washington (OR-WA) coast, requiring that the paleogeographic reconstructions must also account for this effect. Here we use RSL predictions from a 3-D solid Earth model that have been validated by RSL data to update previous paleogeographic reconstructions of the OR-WA coast for the last 12 kyr based on a 1-D solid Earth model. The large differences in the spatial variations in RSL on the OR-WA continental shelves predicted by the 3-D model relative to eustatic and 1-D models demonstrate that accurate reconstructions of coastal paleogeography for predictive modeling of submerged archaeological sites need to account for 3-D viscoelastic Earth structure in areas of complex tectonics

Akimiski Island, Nunavut, Canada: The Use of Cree Oral History and Sea-Level Retrodiction to Resolve Aboriginal Title

On 1 April 1999, Akimiski Island of the western James Bay region of northern Ontario, Canada, was included in the newly formed territory of Nunavut, Canada—an Inuit-dominated territory—even though the Inuit had never asserted Aboriginal title to the island. By contrast, the Omushkegowuk Cree of the western James Bay region have asserted Aboriginal title to Akimiski Island. The Government of Canada by their action (or inaction) has reversed the onus of responsibility for proof of Aboriginal title from the Inuit to the Cree. In other words, the Government of Canada did not follow their own guidelines and the common-law test for proof of Aboriginal title. In this paper, we documented and employed Cree oral history as well as a sea-level retrodiction (based on state-of-the-art numerical modeling of past sea-level changes in James Bay), which incorporated a modified ICE-6G ice history and a 3-D model of Earth structure, to establish that criterion 2 of the test for Aboriginal title has now been fully met. In other words, Cree traditional use and occupancy of Akimiski Island was considered sufficiently factual at the time of assertion of sovereignty by European nations. As all the criteria of the common-law test for proof of Aboriginal title in Canada, with respect to Akimiski Island, have now been addressed, the Cree have sufficient basis to initiate the process of a formal land claim.Le 1er avril 1999, l’île Akimiski, située dans la région ouest de la baie James, dans le nord de l’Ontario, au Canada, a été intégrée au nouveau territoire du Nunavut, territoire dominé par les Inuits, même si ceux-ci n’avaient jamais revendiqué le titre ancestral de cette île. En revanche, les Cris omushkegowuk de la région ouest de la baie James ont revendiqué leur titre ancestral à l’égard de l’île Akimiski. Le geste (ou l’absence de geste) du gouvernement du Canada a eu pour effet d’inverser la responsabilité de prouver le titre ancestral des Inuits aux Cris. Autrement dit, le gouvernement du Canada n’a pas respecté ses propres directives et les critères de droit commun comme preuve de titre ancestral. Dans cet article, nous avons documenté et employé l’histoire orale crie ainsi qu’une rétrodiction du niveau de la mer (d’après une modélisation numérique perfectionnée d’anciens changements du niveau de la mer de la baie James), contenant un historique modifié de la glace ICE-6G et une modélisation en trois dimensions de la structure de la Terre, afin d’établir que le critère 2 des critères du titre ancestral est maintenant entièrement atteint. Autrement dit, l’usage et l’occupation traditionnels de l’île Akimiski par les Cris ont été considérés comme des faits suffisants au moment de la revendication de la souveraineté par les nations européennes. Puisque tous les critères de droit commun permettant de prouver le titre ancestral de l’île Akimiski au Canada ont maintenant été respectés, les Cris disposent de fondements suffisants pour entreprendre une revendication territoriale officielle

The effect of lateral variations in Earth structure on Last Interglacial sea level

It is generally agreed that the Last Interglacial (LIG; ∼130–115 ka) was a time when global average temperatures and global mean sea level were higher than they are today. However, the exact timing, magnitude and spatial pattern of ice melt is much debated. One difficulty in extracting past global mean sea level from local observations is that their elevations need to be corrected for glacial isostatic adjustment (GIA), which requires knowledge of Earth’s internal viscoelastic structure. While this structure is generally assumed to be radially symmetric, evidence from seismology, geodynamics and mineral physics indicates that large lateral variations in viscosity exist within the mantle. In this study, we construct a new model of Earth’s internal structure by converting shear wave speed into viscosity using parametrizations from mineral physics experiments and geodynamic constraints on Earth’s thermal structure. We use this 3-D Earth structure, which includes both variations in lithospheric thickness and lateral variations in viscosity, to calculate the first 3-D GIA prediction for LIG sea level. We find that the difference between predictions with and without lateral Earth structure can be metres to 10s of metres in the near field of former ice sheets, and up to a few metres in their far field. We demonstrate how forebulge dynamics and continental levering are affected by laterally varying Earth structure, with a particular focus on those sites with prominent LIG sea level records. Results from four 3-D GIA calculations show that accounting for lateral structure can act to increase local sea level by up to ∼1.5 m at the Seychelles and minimally decrease it in Western Australia. We acknowledge that this result is only based on a few simulations, but if robust, this shift brings estimates of global mean sea level from these two sites into closer agreement with each other. We further demonstrate that simulations with a suitable radial viscosity profile can be used to locally approximate the 3-D GIA result, but that these radial profiles cannot be found by simply averaging viscosity below the sea level indicator site

Rapid postglacial rebound amplifies global sea level rise following West Antarctic Ice Sheet collapse

Earth_Model_Data is a zipped folder containing the Earth model data for both the standard model (V3D_SD and V3D_RH). A readme file is in this folder. FPRINT_CODE is a zipped folder containing the fingerprint code. A readme file for the code is also in this folder. WAmask_512.gz is a mask for West/East Antarctica, used for masking out changes in East Antarctica. All other files are sea-level outputs for each of the runs on a degree 512 Gauss-Legendre grid (uniform longitudes but unevenly spaced latitudes, as described in the readme for FPRINT_CODE). Files are named SLt_??? and numbered from 0 (elastic response) to 105 (10 ky). They have a 1D layout, with the first line being the time tag in years followed by 512*1024 row entries. A time array is included (tt_v10.dat). More details are in README.tx

Mapping geodetically inferred Antarctic ice surface height changes into thickness changes: a sensitivity study

Determining recent Antarctic ice volume changes from satellite altimeter measurements of ice surface height requires a correction for contemporaneous vertical crustal deformation. This correction must consider two main sources of crustal deformation: (1) ongoing glacial isostatic adjustment (GIA), that is, the deformational, gravitational, and rotational response to Late Pleistocene and Holocene ice and ocean mass changes and (2) modern ice mass change. In this study, we seek to quantify the uncertainties associated with each of these corrections. Corrections of ice surface height changes for correction 1 have generally involved the adoption of global models of GIA defined by some preferred combination of ice history and mantle viscoelastic structure. We have computed the GIA correction generated from a coupled ice sheet–sea level model and a realistic Earth model incorporating three-dimensional viscoelastic structure. Integrating the difference between this correction and those from recent GIA analyses widely adopted in the literature yields an uncertainty in total present-day ice volume change equivalent to approximately 10 % of Antarctic ice mass loss inferred for the period 2010–2020. This reinforces earlier work indicating that ice histories characterized by relatively high excess ice volume at the Last Glacial Maximum may be introducing a significant error in estimates of modern melt rates. Regarding correction 2, a spatially invariant scaling has commonly been used to convert GIA-corrected ice surface height changes obtained from satellite altimetry to ice volume estimates. We adopt modeling results based on a projection of Antarctic ice mass change over the period 2015–2055 to demonstrate a spatial variability in the scaling of up to 10 % across the ice sheet. Furthermore, using these calculations, we find a systematic error of ∼ 3 % in the projected net ice volume change, with most of the difference arising in areas of West Antarctica above mantle zones of low viscosity.</p

Inferences of Mantle Viscosity Based on Ice Age Data Sets: The Bias in Radial Viscosity Profiles Due to the Neglect of Laterally Heterogeneous Viscosity Structure

Inferences of mantle viscosity using glacial isostatic adjustment (GIA) data are hampered by data sensitivity to the space‐time geometry of ice cover. A subset of GIA data is relatively insensitive to this ice history: the Fennoscandian relaxation spectrum (FRS), postglacial decay times in Canada and Scandinavia, and the rate of change of the degree‐2 zonal harmonic of the geopotential ( urn:x-wiley:21699313:media:jgrb53014:jgrb53014-math-0001). These geographically limited data have been inverted to constrain the radial (one‐dimensional [1D]) mantle viscosity profile. We explore potential biases in these 1D inversions introduced by neglecting a three‐dimensional (3D) viscosity structure. We perform 1D Bayesian inversions of synthetic GIA data generated from Earth models with realistic 3D variations in mantle viscosity and lithospheric thickness and compare results to the 1D viscosity profile associated with the 3D model used to generate the synthetics. Differences between these two 1D profiles reflect GIA data resolution and biasing introduced by neglecting, in the inversions, a 3D viscosity structure. We focus on the second issue, demonstrating that the largest bias occurs within the upper mantle (in particular, the transition zone). This remains consistent when varying inversion parameters (e.g., prior/starting models) and the 1D/3D viscosity fields adopted in generating the synthetics. Inversions of individual data sets show 3D biasing increases for data exhibiting shallower (thus more localized) sensitivity to viscosity. Of the data considered herein, inversions of the FRS are subject to the largest bias followed by decay time data. The bias is minimal for urn:x-wiley:21699313:media:jgrb53014:jgrb53014-math-0002, as its deeper sensitivity is accompanied by broader averaging of structure in radial and lateral directions