The U.S. mid-Atlantic sea-level record is sensitive to the history of the Laurentide Ice Sheet as the coastline lies along the ice sheet's peripheral bulge. However, paleo sea-level markers on the ...present-day shoreline of Virginia and North Carolina dated to Marine Isotope Stage (MIS) 3, from 50 to 35 ka, are surprisingly high for this glacial interval, and remain unexplained by previous models of ice age adjustment or other local (for example, tectonic) effects. Here, we reconcile this sea-level record using a revised model of glacial isostatic adjustment characterized by a peak global mean sea level during MIS 3 of approximately -40 m, and far less ice volume within the eastern sector of the Laurentide Ice Sheet than traditional reconstructions for this interval. We conclude that the Laurentide Ice Sheet experienced a phase of very rapid growth in the 15 kyr leading into the Last Glacial Maximum, thus highlighting the potential of mid-field sea-level records to constrain areal extent of ice cover during glacial intervals with sparse geological observables.
We present new profiles of mantle viscosity derived on the basis of non-linear, Occam-style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic ...adjustment (GIA). The convection related observables include satellite-derived free-air gravity harmonics, the geodetically inferred excess ellipticity of the CMB and tectonic plate motions. The GIA constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late Pleistocene ice history: the so-called Fennoscandian relaxation spectrum (FRS) and a set of site-specific decay times determined from the postglacial sea-level history in Hudson Bay and Sweden. The inverted viscosity profiles show a significant, three orders of magnitude, increase from the upper mantle (mean value of ∼4×10
20 Pa s) to a high-viscosity (>10
23 Pa s) peak at 2000 km depth, followed by a reduction toward the core-mantle boundary.
Palaeomagnetic studies of Palaeoproterozoic to Cretaceous rocks propose a suite of large and relatively rapid (tens of degrees over 10 to 100 million years) excursions of the rotation pole relative ...to the surface geography, or true polar wander (TPW). These excursions may be linked in an oscillatory, approximately coaxial succession about the centre of the contemporaneous supercontinent. Within the framework of a standard rotational theory, in which a delayed viscous adjustment of the rotational bulge acts to stabilize the rotation axis, geodynamic models for oscillatory TPW generally appeal to consecutive, opposite loading phases of comparable magnitude. Here we extend a nonlinear rotational stability theory to incorporate the stabilizing effect of TPW-induced elastic stresses in the lithosphere. We demonstrate that convectively driven inertia perturbations acting on a nearly prolate, non-hydrostatic Earth with an effective elastic lithospheric thickness of about 10 kilometres yield oscillatory TPW paths consistent with palaeomagnetic inferences. This estimate of elastic thickness can be reduced, even to zero, if the rotation axis is stabilized by long-term excess ellipticity in the plane of the TPW. We speculate that these sources of stabilization, acting on TPW driven by a time-varying mantle flow field, provide a mechanism for linking the distinct, oscillatory TPW events of the past few billion years.
Determining eustatic sea level during the Mid-Pliocene warm period (∼3.3 to 2.9 Ma) has been a central but elusive goal in the study of past warm climates. Estimates of eustatic sea level based on ...geologic data span a broad range; variation that we now recognize is due in part to geographically varying post-depositional displacement caused by glacial isostatic adjustment and dynamic topography. In this study, we combine field observations and glacial isostatic adjustment modeling to estimate the dynamic topography signal in three areas that are important to paleo-sea level studies of the Mid-Pliocene warm period (South Africa, West Australia and southeastern United States). We show that dynamic topography played a significant role in the post-depositional displacement of Pliocene, and even younger Pleistocene, shorelines. In this regard, we provide a robust paleo-sea level elevation data set, corrected for glacial isostatic adjustment, that can be used to evaluate predictions from mantle flow models of dynamic topography.
•We use geological datasets to estimate Mid-Pliocene relative sea level.•We use GIA modeling and field data to calculate theoretic DT rates in three areas.•We compare the results with scenarios of Mid-Pliocene eustatic sea level.•We provide a robust paleo-sea level elevation data set.•Our results can be used to evaluate predictions from mantle flow models of DT.
Analysis of Global Positioning System (GPS) data demonstrates that ongoing three-dimensional crustal deformation in Fennoscandia is dominated by glacial isostatic adjustment. Our comparison of these ...GPS observations with numerical predictions yields an Earth model that satisfies independent geologic constraints and bounds both the average viscosity in the upper mantle ($5\times10^{20}$to$1\times10^{21}$pascal seconds) and the elastic thickness of the lithosphere (90 to 170 kilometers). We combined GPS-derived radial motions with Fennoscandian tide gauge records to estimate a regional sea surface rise of 2.1 ± 0.3 mm/year. Furthermore, ongoing horizontal tectonic motions greater than ∼1 mm/year are ruled out on the basis of the GPS-derived three-dimensional crustal velocity field.
Dynamic Topography and Ice Age Paleoclimate Mitrovica, J.X; Austermann, J; Coulson, S ...
Annual review of earth and planetary sciences,
05/2020, Letnik:
48, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The connection between the geological record and dynamic topography driven by mantle convective flow has been established over widely varying temporal and spatial scales. As observations of the ...process have increased and numerical modeling of thermochemical convection has improved, a burgeoning direction of research targeting outstanding issues in ice age paleoclimate has emerged. This review focuses on studies of the Plio-Pleistocene ice age, including investigations of the stability of ice sheets during ice age warm periods and the inception of Northern Hemisphere glaciation. However, studies that have revealed nuanced connections of dynamic topography to biodiversity, ecology, ocean chemistry, and circulation since the start of the current ice-house world are also considered. In some cases, a recognition of the importance of dynamic topography resolves enigmatic events and in others it confounds already complex, unanswered questions. All such studies highlight the role of solid Earth geophysics in paleoclimate research and undermine a common assumption, beyond the field of glacial isostatic adjustment, that the solid Earth remains a rigid, passive substrate during the evolution of the ice age climate system.
Dynamic topography is the large-scale, vertical deflection of Earth's crust driven by mantle convective flow.
This review highlights recent research exploring the implications of the process on key issues in ice age paleoclimate.
This research includes studies of ice sheet stability and inception as well as inferences of peak sea levels during periods of relative ice age warmth.
This review also includes studies on longer timescales, continental-scale ecology and biodiversity, the long-term carbon cycle, and water flux across oceanic gateways.
The free-air gravity trend over Canada, derived from the Gravity Recovery and Climate Experiment (GRACE) satellite mission, robustly isolates the gravity signal associated with glacial isostatic ...adjustment (GIA) from the longer-time scale mantle convection process. This trend proves that the ancient Laurentian ice complex was composed of two large domes to the west and east of Hudson Bay, in accord with one of two classes of earlier reconstructions. Moreover, GIA models that reconcile the peak rates contribute ~25 to ~45% to the observed static gravity field, which represents an important boundary condition on the buoyancy of the continental tectosphere.
Late Holocene sea-level highstands of amplitude
∼3
m
are endemic to equatorial ocean basins. These highstands imply an ongoing and moderate, sub-mm/yr, sea-level fall in the far field of the Late ...Pleistocene ice cover that has long been linked to the process of glacial isostatic adjustment (GIA; Clark et al., 1978). Mitrovica and Peltier (1991) coined the term ‘equatorial ocean syphoning’ to describe the GIA-induced sea-level fall and they provided the first physical explanation for the process. They argued that water migrated away from far-field equatorial ocean basins in order to fill space vacated by collapsing forebulges at the periphery of previously glaciated regions. We provide a complete physical explanation for the origin of equatorial ocean syphoning, and the associated development of sea-level highstands, using numerical solutions of the equation that governs meltwater redistribution on spherical, viscoelastic Earth models. In particular, we separate the total predicted sea-level change into contributions associated with ice and meltwater loading effects, and, by doing so, isolate a second mechanism that contributes significantly to the ocean syphoning process. Ocean loading at continental margins induces a ‘levering’ of continents and a subsidence of offshore regions that has also long been recognized within the GIA literature (Walcott, 1972). We show that the influx of water into the volume created by this subsidence produces a sea-level fall at locations distant from these margins—indeed over the major ocean basins—that is comparable in amplitude to the syphoning mechanism isolated by Mitrovica and Peltier (1991).
Meltwater pulse 1A (mwp-1A) was a prominent feature of the last deglaciation, which led to a sea-level rise of ~20 meters in less than 500 years. Concurrent with mwp-1A was the onset of the ...Bolling-Allerød interstadial event (14,600 years before the present), which marked the termination of the last glacial period. Previous studies have been unable to reconcile a warm Northern Hemisphere with mwp-1A originating from the Laurentide or Fennoscandian ice sheets. With the use of a climate model of intermediate complexity, we demonstrate that with mwp-1A originating from the Antarctic Ice Sheet, consistent with recent sea-level fingerprinting inferences, the strength of North Atlantic Deep Water (NADW) formation increases, thereby warming the North Atlantic region and providing an explanation for the onset of the Bølling-Allerøod warm interval. The established mode of active NADW formation is then able to respond to subsequent freshwater forcing from the Laurentide and Fennoscandian ice sheets, setting the stage for the Younger Dryas cold period.
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