Person:

Huybers, Peter

We investigate the stability of marine ice sheets by coupling a gravitationally self-consistent sea level model valid for a self-gravitating, viscoelastically deforming Earth to a 1-D marine ice sheet-shelf model. The evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat. We find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat; in simulations with shallow reversed bed slopes and/or small external forcing, the drop in sea level can be sufficient to halt the retreat. The rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry, and a viscous component due to past ice and ocean load changes. When the ice sheet model is forced from steady state, on short timescales (<∼500 years), viscous effects may be ignored and grounding-line migration at a given time will depend on the local bedrock topography and on contemporaneous sea level changes driven by ongoing ice sheet mass flux. On longer timescales, an accurate assessment of the present stability of a marine ice sheet requires knowledge of its past evolution.

Climate change could potentially destabilize marine ice sheets, which would affect projections of future sea-level rise1, 2, 3, 4. Specifically, an instability mechanism5, 6, 7, 8 has been predicted for marine ice sheets such as the West Antarctic ice sheet that rest on reversed bed slopes, whereby ice-sheet thinning or rising sea level leads to irreversible retreat of the grounding line. However, existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea-level fall at the margin of a rapidly shrinking ice sheet9, 10, 11. Here we present a suite of predictions of gravitationally self-consistent sea-level change following grounding-line migration. Our predictions vary the initial ice-sheet size and also consider the contribution to sea-level change from various subregions of the simulated ice sheet. Using these results, we revisit a canonical analysis of marine-ice-sheet stability5 and demonstrate that gravity and deformation-induced sea-level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes. We conclude that accurate treatments of sea-level change should be incorporated into analyses of past and future marine-ice-sheet dynamics.

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.

A rigorous detection of Milankovitch periodicities in volcanic output across the Pleistocene-Holocene ice age has remained elusive. We report on a spectral analysis of a large number of well-preserved ash plume deposits recorded in marine sediments along the Pacific Ring of Fire. Our analysis yields a statistically significant detection of a spectral peak at the obliquity period. We propose that this variability in volcanic activity results from crustal stress changes associated with ice age mass redistribution. In particular, increased volcanism lags behind the highest rate of increasing eustatic sea level (decreasing global ice volume) by 4.0 ± 3.6 k.y. and correlates with numerical predictions of stress changes at volcanically active sites. These results support the presence of a causal link between variations in ice age climate, continental stress field, and volcanism.

The precession and obliquity frequencies of the Earth’s rotational motion are functions of the dynamic ellipticity of the Earth’s gravitational figure, and this connection has provided a novel bridge between studies of palaeoclimate and geodynamics. In particular, analyses of tuned climate proxy records have yielded bounds on the mean relative perturbation in dynamic ellipticity over both the last 3 Myr and 25 Myr that are less than ∼3 per cent of the non-hydrostatic component of the ellipticity. We demonstrate that this apparent consistency actually defines an important geophysical enigma. Over the last 3 Myr, changes in the Earth’s figure are likely dominated by ice age forcings—in this case, a small perturbation to dynamic ellipticity implies significant isostatic compensation of the ice-ocean surface mass loads and, hence, a relatively low mantle viscosity. In contrast, over the last 25 Myr, changes in the Earth’s long-wavelength gravitational form are likely dominated by mantle convective flow, and in this case, the small perturbation to dynamic ellipticity implies sluggish convection and a relatively high mantle viscosity. There are at least four possible routes to resolving this enigma: The viscosity in the Earth’s mantle is transient (i.e. dependent on the timescale of the applied forcing), tidal dissipation changed in a manner between the last 3 Myr and 25 Myr that was sufficient to resolve the issue, the observationally inferred bounds are unrealistically restrictive, or earth models exist in which the ice age and convection effects approximately cancel leading to no net perturbation. In this paper, we compute a suite of numerical predictions of ice age and convection-induced perturbations to the dynamic ellipticity to illustrate the enigma described above.

Identifying the causes of historical trends in relative sea level—the height of the sea surface relative to Earth’s crust—is a prerequisite for predicting future changes. Rates of change along the eastern coast of the USA (the US East Coast) during the past century were spatially variable, and relative sea level rose faster along the Mid-Atlantic Bight than along the South Atlantic Bight and the Gulf of Maine. Past studies suggest that Earth’s ongoing response to the last deglaciation surface redistribution of ice and water and changes in ocean circulation contributed considerably to this large-scale spatial pattern. Here we analyse instrumental data and proxy reconstructions using probabilistic methods to show that vertical motions of Earth’s crust exerted the dominant control on regional spatial differences in relative sea-level trends along the US East Coast during 1900–2017, explaining most of the large-scale spatial variance. Rates of coastal subsidence caused by ongoing relaxation of the peripheral forebulge associated with the last deglaciation are strongest near North Carolina, Maryland and Virginia. Such structure indicates that Earth’s elastic lithosphere is thicker than has been assumed in other models. We also find a substantial coastal gradient in relative sea-level trends over this period that is unrelated to deglaciation and suggests contributions from twentieth-century redistribution of ice and water. Our results indicate that the majority of large-scale spatial variation in long-term rates of relative sea-level rise on the US East Coast is due to geological processes that will persist at similar rates for centuries.

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.

We predict the perturbation to the Earth's length-of-day (LOD) over the Common Era using a recently derived estimate of global sea-level change for this time period. We use this estimate to derive a time series of “clock error”, defined as the difference in timing of two clocks, one based on a theoretically invariant time scale (terrestrial time) and one fixed to Earth rotation (universal time), and compare this time series to millennial scale variability in clock error inferred from ancient eclipse records. Under the assumption that global sea-level change over the Common Era is driven by ice mass flux alone, we find that this flux can reconcile a significant fraction of the discrepancies between clock error computed assuming constant slowing of Earth's rotation and that inferred from eclipse records since 700 CE. In contrast, ice mass flux cannot reconcile the temporal variability prior to 700 CE.

The impact that long-term changes in Earth's rotation axis relative to the surface geography, or true polar wander (TPW), and continental drift have had in driving cooling of high-latitude North America since the Eocene is explored. Recent reanalyses of paleomagnetic pole positions suggest a secular drift in Earth's rotation axis of over the last 40 Myr, in a direction that has brought North America to increasingly higher latitudes. Using modern temperature data in tandem with a simple model, a reduction in the annual sum of positive degree days (PDDs) driven by this polar and plate motion over the last 20 Myr is quantified. At sites in Baffin Island, the TPW- and continental drift-driven decrease in insolation forcing over the last 20 Myr rivals changes in insolation forcing caused by variations in Earth's obliquity and precession. Using conservative PDD scaling factors and an annual snowfall equal to modern station observations, the snowiest location in Baffin Island 20 Myr ago had a mass balance deficit of ∼0.75–2 m yr−1 (water equivalent thickness) relative to its projected mass balance at 2.7 Ma. This mass balance deficit would have continued to increase as one goes back in time until ∼40 Myr ago based on adopted paleopole locations. TPW and continental drift that moved Arctic North America poleward would have strongly promoted glacial inception in Baffin Island at ∼3 Ma, a location where the proto-Laurentide Ice Sheet is thought to have originated.