Review

Inherited landscapes and sea level change

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Science  23 Jan 2015:
Vol. 347, Issue 6220, 1258375
DOI: 10.1126/science.1258375

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Volume and shape combine to find a level

How can we understand the geological record of sea level change? Sea level varies on time scales from decades to millions of years. These changes have local, regional, and global components and are caused by a wide variety of earth processes. Cloetingh and Haq review how the views of stratigraphers (who interpret the record of marine sediments) and geodynamicists (who consider changes in the shape of Earth caused by lithospheric and mantle processes) have begun to complement each other and are moving toward a more coherent interpretation of the history of sea level. They focus on cyclic sea-level changes 0.5 to 3.0 million years in duration that occurred in the Cretaceous period, approximately 145 to 65 million years ago.

Science, this issue 10.1126/science.1258375

Structured Abstract

BACKGROUND

Knowledge of past sea level fluctuations is fundamental to geosciences and for exploration of Earth-bound resources. Recent years have seen a convergence of views between stratigraphers (who measure past sea level changes in marine strata) and geodynamicists (who investigate surficial expressions of lithospheric and mantle processes) with the realization that without understanding inherited topographies, their causal mechanisms and operative time scales, palinspastic (pre-diastrophic) reconstructions of the past landscapes and seascapes remain inchoate at best.

Sequestrations of seawater on land or its subduction-related entrainment in the mantle are two direct means of lowering global sea level. Sea level can also be changed by modifying the container capacity of the ocean through numerous interconnected solid-Earth processes. Some of these can only refashion landscapes regionally, thus affecting local measures of sea level change. Disentangling these processes to uncover the likely cause(s) for the resultant topography poses considerable challenges.

ADVANCES

Recent developments in seismic tomography and high-speed computing that allow detailed forward and inverse modeling, combined with new concepts in stratigraphy and geophysics that permit envisioning large-scale transfers of material among depocenters, have brought us closer to understanding factors that influence landscapes and sea levels and the complex feedbacks. As a result, estimates of the amplitude of long-term eustatic changes have converged using different data sets. We have learned that solid-Earth processes operating on decadal to multimillion-year time scales are all responsible for retaining lithospheric memory and its surface expression: On time scales of tens to hundreds of years, glacial isostatic adjustments cause local topographic anomalies, whereas postglacial rebound can be enhanced by viscous mantle flow on time scales of thousands to hundreds of thousands of years; on time scales of more than 1 million years, oceanic crustal production variations, plate reorganizations, and mantle-lithosphere interactions (e.g., dynamic topography) become more influential in altering the longer-wavelength surface response. Additionally, the lithosphere’s rheological heterogeneity, variations in its strength, and changes in intraplate stress fields can also cause regional topographic anomalies, and syn-rift volcanism may be an important determinant of the long-term eustatic change on time scales of 5 to 10 million years.

OUTLOOK

Despite these remarkable advances, we remain far from resolving the causes for third-order quasi-cyclic sea level changes (~500,000 to 3 million years in duration). Ascertaining whether ice volume changes were responsible for these cycles in the Cretaceous will require discerning the potential for extensive glaciation at higher altitudes on Antarctica by modeling topographic elevation involving large-scale mantle processes. Extensive sea floor volcanism, plate reorganizations, and continental breakup events need to be better constrained if causal connections between tectonics and eustasy have to be firmly established. Another promising avenue of inquiry is the leads and lags between entrainment and expulsion of water within the mantle on third-order time scales. Future geodynamic models will also need to consider lateral variations in upper mantle viscosity and lithosphere rheology that require building on current lithospheric strength models and constructing global paleorheological models. Deep-drilling efforts will be of crucial importance for achieving the integrative goals.

Earth’s sea level history is archived in sedimentary sections.

This example is from the Umbrian Apennines near Gubbio, Italy, that was a part of the ancient Tethys Ocean. It preserves a continuous and nearly complete Cretaceous pelagic record, and its contained microfossils and stable isotopes provide valuable clues about paleoceanography, paleoclimatology, and sea level history of the region. The finer-grained sediment near the base marks the boundary between the Cenomanian and Turonian stages, at which time the highest sea levels of the Cretaceous have been documented around the world. Evidence and measure of the amplitude of this eustatic high based on geodynamics and stratigraphic data have converged. PHOTO COURTESY HOWARD SPERO, UNIVERSITY OF CALIFORNIA, DAVIS

Abstract

Enabled by recently gained understanding of deep-seated and surficial Earth processes, a convergence of views between geophysics and sedimentary geology has been quietly taking place over the past several decades. Surface topography resulting from lithospheric memory, retained at various temporal and spatial scales, has become the connective link between these two methodologically diverse geoscience disciplines. Ideas leading to the hypothesis of plate tectonics originated largely with an oceanic focus, where dynamic and mostly horizontal movements of the crust could be envisioned. But when these notions were applied to the landscapes of the supposedly rigid plate interiors, there was less success in explaining the observed anomalies in terrestrial topography. Solid-Earth geophysics has now reached a developmental stage where vertical movements can be measured and modeled at meaningful scales and the deep-seated structures can be imaged with increasing resolution. Concurrently, there have been advances in quantifying mechanical properties of the lithosphere (the solid outer skin of Earth, usually defined to include both the crust and the solid but elastic upper mantle above the asthenosphere). The lithosphere acts as the intermediary that transfers the effects of mantle dynamics to the surface. These developments have allowed us to better understand the previously puzzling topographic features of plate interiors and continental margins. On the sedimentary geology side, new quantitative modeling techniques and holistic approaches to integrating source-to-sink sedimentary systems have led to clearer understanding of basin evolution and sediment budgets that allow the reconstruction of missing sedimentary records and past geological landscapes.

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