An official website of the United States government
Glacial isostatic adjustment (GIA) simulations using earth models that vary viscoelastic structure with depth alone cannot simultaneously fit geographic trends in the elevation of marine isotope stage (MIS) 5a relative sea level (RSL) indicators across continental North America and the Caribbean and yield conflicting estimates of global mean sea level (GMSL). We present simulations with a GIA model that incorporates three-dimensional (3-D) variation in North American viscoelastic earth structure constructed by combining high-resolution seismic tomographic imaging with a new method for mapping this imaging into lateral variations in lithospheric thickness and mantle viscosity. We pair this earth model with a global ice history based on updated constraints on ice volume and geometry. The GIA prediction provides the first simultaneous reconciliation of MIS 5a North American and Caribbean RSL highstands and strengthens arguments that MIS 5a peak GMSL reached values close to that of the Last Interglacial. This result highlights the necessity of incorporating realistic 3-D earth structure into GIA predictions with continent-scale RSL data sets.
more »
« less
This content will become publicly available on March 1, 2026
Constraints of Relative Sea Level Change on the Mantle Viscosity and the Late Pleistocene Deglaciation History
Abstract Two commonly used ice models that are constructed using glacial isostatic adjustment (GIA) modeling are the ICE‐6G and ANU ice models. In this study, we examined the relationships among mantle viscosity, ice models and relative sea level (RSL) data through an analytic GIA model. In general, almost all the pairs of RSL data sets and ice models we considered appear to be consistent with a mantle viscosity structure with a factor of 10–20 viscosity increase from the upper to lower mantles. By using the GIA model with the viscosity structure that produces a minimum model‐data misfit, we constructed the temporal and spatial distributions of misfit (i.e., misfit maps) to different RSL data sets, for both the ICE‐6G and ANU ice models. While the misfit maps at different times clearly show that more observations are desired to constrain ice models, we propose that the spatial and temporal misfit maps should be used to revise the existing ice models to further improve the fit to RSL data. In our initial proof of concept attempts to modify ICE‐6G by adding more ice to it, the three modified ICE‐6G ice models we considered all significantly improve the fit to the far‐field RSL data, although additional effort is needed to reduce misfit to near field RSL data. Finally, we emphasize that RSL at different far‐field sites may differ by up to ∼25 m at the Last Glacial Maximum (LGM) (∼26 ka), suggesting the need for a sufficiently large number of far‐field RSL data in determining the total melt ice volume since the LGM.
more »
« less
- Award ID(s):
- 2222115
- PAR ID:
- 10618577
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 130
- Issue:
- 3
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Contemporary crustal uplift and relative sea level (RSL) change in Greenland is caused by the response of the solid Earth to ongoing and historical ice mass change. Glacial isostatic adjustment (GIA) models, which seek to match patterns of land surface displacement and RSL change, typically employ a linear Maxwell viscoelastic model for the Earth's mantle. In Greenland, however, upper mantle viscosities inferred from ice load changes and other geophysical phenomena occurring over a range of timescales vary by up to two orders of magnitude. Here, we use full‐spectrum rheological models to examine the influence of transient deformation within the Greenland upper mantle, which may account for these differing viscosity estimates. We use observations of shear wave velocity combined with constitutive rheological models to self‐consistently calculate mechanical properties including the apparent upper mantle viscosity and lithosphere thickness across a broad spectrum of frequencies. We find that the contribution of transient behavior is most significant over loading timescales of 102–103 years, which corresponds to the timeframe of ice mass loss over recent centuries. Predicted apparent lithosphere thicknesses are also in good agreement with inferences made across seismic, GIA, and flexural timescales. Our results indicate that full‐spectrum constitutive models that more fully capture broadband mantle relaxation provide a means of reconciling seemingly contradictory estimates of Greenland's upper mantle viscosity and lithosphere thickness made from observations spanning a range of timescales.more » « less
-
Abstract Recent ice-mass loss driven by warming along the Antarctic Peninsula has resulted in rapid changes in uplift rates across the region. Are such events only a function of recent warming? If not, does the Earth response to such events last long enough to be preserved in Holocene records of relative sea level (RSL), and thus have a bearing on global-scale glacial isostatic adjustment (GIA) models (e.g. ICE-6G)? Answering such questions in Antarctica is hindered by the scarcity of RSL reconstructions within the region. Here, a new RSL reconstruction for Antarctica is presented based on beach ridges from Joinville Island on the Antarctic Peninsula. We find that RSL has fallen 4.9 ± 0.58 m over the past 3100 yr, and that the island experienced a significant increase in the rate of RSL fall from 1540 ± 125 cal. (calibrated) yr B.P. to 1320 ± 125 cal. yr B.P. This increase in the rate of RSL fall is likely due to the viscoelastic response of the solid Earth to terrestrial ice-mass loss from the Antarctic Peninsula, similar to the Earth response experienced after ice-mass loss following acceleration of glaciers behind the collapsed Larsen B ice shelf in 2002 C.E. Additionally, slower rates of beach-ridge progradation from 695 ± 190 cal. yr B.P. to 235 ± 175 cal. yr B.P. potentially reflect erosion of beach ridges from a RSL rise induced by a local glacial advance. The rapid response of the Earth to minor ice-mass changes recorded in the RSL record further supports recent assertions of a more responsive Earth to glacial unloading and at time scales relevant for GIA of Holocene and Pleistocene sea levels. Thus, current continental and global GIA models may not accurately capture the ice-mass changes of the Antarctic ice sheets at decadal and centennial time scales.more » « less
-
SUMMARY Earth structure beneath the Antarctic exerts an important control on the evolution of the ice sheet. A range of geological and geophysical data sets indicate that this structure is complex, with the western sector characterized by a lithosphere of thickness ∼50–100 km and viscosities within the upper mantle that vary by 2–3 orders of magnitude. Recent analyses of uplift rates estimated using Global Navigation Satellite System (GNSS) observations have inferred 1-D viscosity profiles below West Antarctica discretized into a small set of layers within the upper mantle using forward modelling of glacial isostatic adjustment (GIA). It remains unclear, however, what these 1-D viscosity models represent in an area with complex 3-D mantle structure, and over what geographic length-scale they are applicable. Here, we explore this issue by repeating the same modelling procedure but applied to synthetic uplift rates computed using a realistic model of 3-D viscoelastic Earth structure inferred from seismic tomographic imaging of the region, a finite volume treatment of GIA that captures this complexity, and a loading history of Antarctic ice mass changes inferred over the period 1992–2017. We find differences of up to an order of magnitude between the best-fitting 1-D inferences and regionally averaged depth profiles through the 3-D viscosity field used to generate the synthetics. Additional calculations suggest that this level of disagreement is not systematically improved if one increases the number of observation sites adopted in the analysis. Moreover, the 1-D models inferred from such a procedure are non-unique, that is a broad range of viscosity profiles fit the synthetic uplift rates equally well as a consequence, in part, of correlations between the viscosity values within each layer. While the uplift rate at each GNSS site is sensitive to a unique subspace of the complex, 3-D viscosity field, additional analyses based on rates from subsets of proximal sites showed no consistent improvement in the level of bias in the 1-D inference. We also conclude that the broad, regional-scale uplift field generated with the 3-D model is poorly represented by a prediction based on the best-fitting 1-D Earth model. Future work analysing GNSS data should be extended to include horizontal rates and move towards inversions for 3-D structure that reflect the intrinsic 3-D resolving power of the data.more » « less
-
Past sea levels provide important constraints on global ice volumes, rates of tectonic motion, ice-sheet sea-level feedbacks, and the migration of species through time. Beneath formerly glaciated regions, the marine limit, the maximum extent of sea-levels after glacial retreat, provides some of the oldest post-Last Glacial Maximum (LGM) sea-level constraints. However, although the elevations of marine limits are plentiful, they often remain undated. In this study, we provide new age and elevation constraints on the late Pleistocene relative sea-level (RSL) history at 12 sites along the eastern flanks of the former Minch Ice Stream (MnIS) of northwest Scotland. Optically stimulated luminescence (OSL) was used to date the highest and presumably oldest preserved RSL indicators immediately after ice-sheet retreat. Although slightly older than earlier estimates, our ages confirm the early deglacial age of ~16.2–19.5 ka for the raised shorelines of northwest Scotland with declining marine limits north of the Isle of Skye from 26.2 ± 4.8 m at Ardaneaskan to 12.8 ± 4.8 m elevation at Achiltibuie, the latter of which lies inside the moraines of the Wester Ross Readvance. Our new OSL ages suggest deglaciation of the MnIS may have been slightly earlier than previously thought, although our large error bars highlight the need for additional age constraints. Our new RSL data provide important constraints for Glacial Isostatic Adjustment (GIA) models for Scotland and shed light on the behavior of the former MnIS, thought to be susceptible to marine ice-sheet instability.more » « less
