Reconstructing last interglacial sea level to understand how ice sheets behave in a warmer world

The last interglacial (125 ka) marks a time during which global mean temperatures were 1-2º warmer than pre-industrial values. This time period has therefore been used as a natural laboratory for studying ice sheet stability and sea level rise in a warmer world. Local sea level during the last interglacial can be reconstructed using sea level indicators such as fossil corals. In order to infer global mean sea level, or equivalent ice volume, one needs to correct local sea level estimates for post-depositional deformation. In this presentation I will explain what solid Earth deformation needs to be accounted for in these reconstructions and how we can model these processes. I will further show newly obtained last interglacial sea level data from the Bahamas, use them to infer last interglacial global mean sea level and provide an outlook of how these findings can affect predictions of future sea level change

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

Glacial isostatic adjustment reduces past and future Arctic subsea permafrost

Sea-level rise submerges terrestrial permafrost in the Arctic, turning it into subsea permafrost. Subsea permafrost underlies ~ 1.8 million km2 of Arctic continental shelf, with thicknesses in places exceeding 700 m. Sea-level variations over glacial-interglacial cycles control subsea permafrost distribution and thickness, yet no permafrost model has accounted for glacial isostatic adjustment (GIA), which deviates local sea level from the global mean due to changes in ice and ocean loading. Here we incorporate GIA into a pan-Arctic model of subsea permafrost over the last 400,000 years. Including GIA significantly reduces present-day subsea permafrost thickness, chiefly because of hydro-isostatic effects as well as deformation related to Northern Hemisphere ice sheets. Additionally, we extend the simulation 1000 years into the future for emissions scenarios outlined in the Intergovernmental Panel on Climate Change’s sixth assessment report. We find that subsea permafrost is preserved under a low emissions scenario but mostly disappears under a high emissions scenario

Quantifying the sensitivity of post-glacial sea level change to laterally varying viscosity

We present a method for calculating the derivatives of measurements of glacial isostatic adjustment (GIA) with respect to the viscosity structure of the Earth and the ice sheet history. These derivatives, or kernels, quantify the linearised sensitivity of measurements to the underlying model parameters. The adjoint method is used to enable efficient calculation of theoretically exact sensitivity kernels within laterally heterogeneous earth models that can have a range of linear or non-linear viscoelastic rheologies. We first present a new approach to calculate GIA in the time domain, which, in contrast to the more usual formulation in the Laplace domain, is well suited to continuously varying earth models and to the use of the adjoint method. Benchmarking results show excellent agreement between our formulation and previous methods. We illustrate the potential applications of the kernels calculated in this way through a range of numerical calculations relative to a spherically symmetric background model. The complex spatial patterns of the sensitivities are not intuitive, and this is the first time that such effects are quantified in an efficient and accurate manner.NERC studentship for the first autho

Revisiting tectonic corrections applied to Pleistocene sea-level highstands

Tectonic displacement contaminates estimates of peak eustatic sea level (and, equivalently, minimum continental ice volumes) determined from the elevation of Quaternary interglacial highstand markers. For sites at which a stratigraphic or geomorphic marker of peak Marine Isotope Stage (MIS) 5e sea level exists, the standard approach for estimating local tectonic uplift (or subsidence) rates takes the difference between the elevation of the local highstand marker and a reference MIS 5e eustatic value, commonly chosen as +6 m, and divides by the age of the marker. The resulting rate is then applied to correct the elevation of all other local observed sea-level markers for tectonic displacement, including peak highstands of different ages (e.g., MIS 5a, MIS 5c and MIS 11), under the assumption that the tectonic rate remained constant over those periods. This approach introduces two potentially significant errors. First, the peak eustatic value adopted for MIS 5e in most previous studies (i.e., +6 m) is likely incorrect. Second, local peak sea level during MIS 5e is characterized by significant departures from eustasy due to glacial isostatic adjustment in response to both successive glacial–interglacial cycles and excess polar ice-sheet melt relative to present day values. We use numerical models of glacial isostatic adjustment that incorporate both of these effects to quantify the plausible range of the combined error and show that, even at sites far from melting ice sheets, local peak sea level during MIS 5e may depart from eustasy by 2–4 m, or more. We also demonstrate that the associated error in the estimated tectonic rates can significantly alter previous estimates of peak eustatic sea level during Quaternary highstands, notably those associated with earlier interglacials (e.g., MIS 11)

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

Quantifying the sensitivity of post-glacial sea level change to laterally varying viscosity

We present a method for calculating the derivatives of measurements of glacial isostatic adjustment (GIA) with respect to the viscosity structure of the Earth and the ice sheet history. These derivatives, or kernels, quantify the linearised sensitivity of measurements to the underlying model parameters. The adjoint method is used to enable efficient calculation of theoretically exact sensitivity kernels within laterally heterogeneous earth models that can have a range of linear or non-linear viscoelastic rheologies. We first present a new approach to calculate GIA in the time domain, which, in contrast to the more usual formulation in the Laplace domain, is well suited to continuously varying earth models and to the use of the adjoint method. Benchmarking results show excellent agreement between our formulation and previous methods. We illustrate the potential applications of the kernels calculated in this way through a range of numerical calculations relative to a spherically symmetric background model. The complex spatial patterns of the sensitivities are not intuitive, and this is the first time that such effects are quantified in an efficient and accurate manner

Higher than present global mean sea level recorded by an Early Pliocene intertidal unit in Patagonia (Argentina)

Reconstructions of global mean sea level from earlier warm periods in Earth?s history can helpconstrain future projections of sea level rise. Here we report on the sedimentology and age ofa geological unit in central Patagonia, Argentina, that we dated to the Early Pliocene(4.69?5.23 Ma, 2σ) with strontium isotope stratigraphy. The unit was interpreted as representativeof an intertidal environment, and its elevation was measured with differential GPS atca. 36m above present-day sea level. Considering modern tidal ranges, it was possible toconstrain paleo relative sea level within ±2.7m (1σ). We use glacial isostatic adjustmentmodels and estimates of vertical land movement to calculate that, when the Camaronesintertidal sequence was deposited, global mean sea level was 28.4 ± 11.7m (1σ) above present.This estimate matches those derived from analogous Early Pliocene sea level proxies inthe Mediterranean Sea and South Africa. Evidence from these three locations indicates thatEarly Pliocene sea level may have exceeded 20m above its present level. Such high globalmean sea level values imply an ice-free Greenland, a significant melting of West Antarctica,and a contribution of marine-based sectors of East Antarctica to global mean sea level.Fil: Rovere, Alessio. Universitat Bremen; AlemaniaFil: Pappalardo, Marta. Universidad de Pisa; ItaliaFil: Richiano, Sebastián Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto Patagónico de Geología y Paleontología; ArgentinaFil: Aguirre, Marina Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; ArgentinaFil: Sandstrom, Michael R.. Columbia University; Estados UnidosFil: Hearty, Paul J.. University of Texas at Austin; Estados UnidosFil: Austermann, Jacqueline. Columbia University; Estados UnidosFil: Castellanos, Ignacio. Universidad Nacional de La Plata; ArgentinaFil: Raymo, Maureen E.. Columbia University; Estados Unido

An Early Pliocene relative sea level record from Patagonia (Argentina)

We report ciao a geological unit surveyed and dated in central Patagonia, Argentina (Camarones town, San Jorge Gulf). The unit was interpreted as representative of an intertidal environment and dated to the Early Pliocene (4.69-5.23 Ma) with strontium isotope stratigraphy. The elevation of this unit was measured with differential GPS at ca. 36 m above present-day sea level. Considering modern tidal ranges, it was possible to constrain paleo relative sea level within ±2.5m (1s). We use glacial isostatic adjustment models and estimates of vertical land movement to calculate that, when the Camarones intertidal sequence was deposited, global mean sea level was 28.4 ± 11.7m above present. This estimate matches those derived from analogous Early Pliocene sea level proxies in the Mediterranean Sea and South Africa. Evidence from these three locations indicates that Early Pliocene sea level may have exceeded 20m above its present level. Such high global mean sea level values imply an ice-free Greenland, a significant melting of West Antarctica, and a contribution of East Antarctica to global mean sea level.Los datos utilizados para este trabajo pueden accederse haciendo clic en "Documentos relacionados".Facultad de Ciencias Naturales y Muse