True polar wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars

The areal centroids of the youngest polar deposits on Mars are offset from those of adjacent paleopolar deposits by 5-10 degrees. We test the hypothesis that the offset is the result of true polar wander (TPW), the motion of the solid surface with respect to the spin axis, caused by a mass redistribution within or on the surface of Mars. In particular, we consider TPW driven by late-stage volcanism during the late Hesperian to Amazonian. There is observational and qualitative support for this hypothesis: in both North and South, observed offsets lie close to a great circle 90 degrees from Tharsis, as expected for polar wander after Tharsis formed. We calculate the magnitude and direction of TPW produced by mapped late-stage lavas for a range of lithospheric thicknesses, lava thicknesses, eruption histories, and prior polar wander events. If Tharsis formed close to the equator, the stabilizing effect of a fossil rotational bulge located close to the equator leads to predicted TPW of <2 degrees, too small to account for observed offsets. If, however, Tharsis formed far from the equator, late-stage TPW driven by low-latitude, late-stage volcanism would be 6-33 degrees, similar to that inferred from the location of paleopolar deposits. 4.4+/-1.3x10^19 kg of young erupted lava can account for the offset of the Dorsa Argentea Formation from the present-day south rotation pole. This mass is consistent with prior mapping-based estimates and would imply a mass release of CO2 by volcanic degassing similar to that in the atmosphere at the present time. The South Polar Layered Deposits are offset from the spin axis in the opposite sense to the other paleopolar deposits. This can be explained by an additional contribution from a plume beneath Elysium. We conclude with a list of observational tests of the TPW hypothesis.Comment: Accepted by Earth and Planetary Science Letters. 3 tables, 8 figure

Detection of a dynamic topography signal in last interglacial sea-level records

Estimating minimum ice volume during the last interglacial based on local sea-level indicators requires that these indicators are corrected for processes that alter local sea level relative to the global average. Although glacial isostatic adjustment is generally accounted for, global scale dynamic changes in topography driven by convective mantle flow are generally not considered. We use numerical models of mantle flow to quantify vertical deflections caused by dynamic topography and compare predictions at passive margins to a globally distributed set of last interglacial sea-level markers. The deflections predicted as a result of dynamic topography are significantly correlated with marker elevations (>95% probability) and are consistent with construction and preservation attributes across marker types. We conclude that a dynamic topography signal is present in the elevation of last interglacial sea-level records and that the signal must be accounted for in any effort to determine peak global mean sea level during the last interglacial to within an accuracy of several meters

Collapse of Polar Ice Sheets during the Stage 11 Interglacial

Contentious observations of Pleistocene shoreline features on the tectonically stable islands of Bermuda and the Bahamas have suggested that sea level about 400,000 years ago was more than 20 metres higher than it is today. Geochronologic and geomorphic evidence indicates that these features formed during interglacial marine isotope stage (MIS) 11, an unusually long interval of warmth during the ice age. Previous work has advanced two divergent hypotheses for these shoreline features: first, significant melting of the East Antarctic Ice Sheet, in addition to the collapse of the West Antarctic Ice Sheet and the Greenland Ice Sheet; or second, emplacement by a mega-tsunami during MIS 11 (ref. 4, 5). Here we show that the elevations of these features are corrected downwards by ~10 metres when we account for post-glacial crustal subsidence of these sites over the course of the anomalously long interglacial. On the basis of this correction, we estimate that eustatic sea level rose to ~6–13 m above the present-day value in the second half of MIS 11. This suggests that both the Greenland Ice Sheet and the West Antarctic Ice Sheet collapsed during the protracted warm period while changes in the volume of the East Antarctic Ice Sheet were relatively minor, thereby resolving the long-standing controversy over the stability of the East Antarctic Ice Sheet during MIS 11

Sea Level Change in the Western James Bay Region of Subarctic Ontario: Emergent Land and Implications for Treaty No. 9

In 1905 and 1906, the Cree of the southwestern James Bay region signed Treaty No. 9 whereby they relinquished to the Canadian government their claim to the lands south of the Albany River (the northern boundary of the province of Ontario at the time). The official text of Treaty No. 9 made no mention of land submerged below water cover, and thus the Cree did not relinquish such regions at that time. By contrast, the Cree of the northwestern James Bay and southwestern Hudson Bay region who signed the 1929–30 Adhesions to Treaty No. 9 relinquished their claims to “land covered by water” for the area bounded on the south by the northerly limit of Treaty No. 9, as this clause was specifically included in the text of the adhesion. The issue of “land covered by water” is significant because the western James Bay region has been, and will continue to be, subject to sea level changes associated with ongoing adjustments due to the last ice age and modern global warming signals. In the absence of detailed maps, we used models of these processes, constrained by available geophysical and geodetic data sets, to retrodict shoreline changes and the rate of land emergence over the last two centuries within the boundaries specified by Treaty No. 9. We also project shoreline migration to the end of the 21st century within the same region. The rate of land emergence since 1905 in the area south of the Albany River is estimated as ~3.0 km2/yr. Over the next century, land will continue to emerge in this region at a mean rate of ~1.4 km2/yr. This emergent land should be a subject of consideration within any comprehensive land claim put forward by the Cree; in this regard, it will be interesting to see how the Canadian judicial system and the Comprehensive Claims Branch handle the novel issue of emergent land.En 1905 et 1906, les Cris du sud-ouest de la région de la baie James ont signé le Traité no 9, par le biais duquel ils ont cédé au gouvernement du Canada leur droit de revendication des terres au sud de la rivière Albany (la limite nord de la province de l’Ontario à l’époque). Le texte officiel du Traité no 9 ne faisait aucune mention des terres submergées sous l’eau, si bien que les Cris n’ont pas renoncé à ces régions à ce moment-là. En revanche, les Cris du nord-ouest de la baie James et du sud-ouest de la baie d’Hudson qui ont signé les adhésions au Traité no 9 (1929-1930) ont renoncé à leurs revendications aux « terres recouvertes d’eau » dans la zone délimitée au sud par la limite nord du Traité no 9, puisque cette clause était expressément incluse dans le texte de l’adhésion. La question des « terres recouvertes d’eau » est importante parce que l’ouest de la région de la baie James a été et continuera d’être assujettie aux variations du niveau de la mer liées aux ajustements continus découlant de la dernière période glaciaire et des récents signes de réchauffement planétaire. En l’absence de cartes détaillées, nous avons utilisé des modèles de ces processus, limités par les ensembles de données géophysiques et géodésiques disponibles, pour déterminer de façon rétrospective les changements du littoral et le taux d’émergence des terres au cours des deux derniers siècles dans les limites précisées dans le Traité no 9. Nous faisons également une projection de la migration du littoral jusqu’à la fin du XXIe siècle dans cette même région. Le taux d’émergence des terres depuis 1905 dans la région au sud de la rivière Albany est estimé à ~3,0 km2/année. Au cours du prochain siècle, les terres continueront d’émerger dans cette région au taux moyen de ~1,4 km2/année. Ces terres émergées devraient être prises en compte dans toute revendication territoriale globale présentée par les Cris. À cet égard, il sera intéressant de voir comment le système judiciaire canadien et la Direction générale des revendications globales traiteront cette nouvelle question des terres émergées

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

A statistical analysis of the correlation between large igneous provinces and lower mantle seismic structure

Large igneous provinces (LIPs) lie approximately above the margins of the African and Pacific large low shear velocity provinces (LLSVPs) in the deep mantle. This spatial correlation has been used to argue that plumes are preferentially generated at the margins of LLSVPs. We perform a series of Monte Carlo–based statistical tests to assess the uniqueness of this conclusion. These tests indicate that (1) the reconstructed locations of LIPs are significantly correlated with both slower-than-average shear wave velocity regions, which contain LLSVPs, and the margins of these structures; and (2) these correlations cannot be statistically distinguished. That is, given current constraints, if plumes were generated randomly throughout regions of slower-than-average shear wave velocity in the deep mantle, then statistical tests are expected to show a significant correlation between the locations of LIPs and the margins of LLSVPs. We therefore conclude that it is premature to argue that the margins of LLSVPs represent preferred zones of plume generation. This conclusion is reinforced in our analysis by a demonstration that the expected mean distance of a set of points randomly placed in slower-than-average shear wave velocity regions is consistent with the observed mean distance between LIPs and the margins of LLSVPs. Finally, we also test the correlation between the reconstructed locations of LIPs and the horizontal gradient in deep mantle shear velocity perturbations. We find, given the uncertainty implied by different tomography models, that there is no statistically significant correlation and that being in a slow region (i.e. in the region of LLSVPs) is a stronger geographic requirement for plume generation than being at a specific (high) gradient.Earth and Planetary Science