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Glacio and hydro-isostasy in the Mediterranean Sea: Clarks zones and role of remote ice sheets
Full text linkSolving the sea-level equation for a spherically symmetric Earth we study the relative sea-level curves in the Mediterranean Sea in terms of Clarks zones and we explore their sensitivity to the time-history of Late-Pleistocene ice aggregates. Since the Mediterranean is an intermediate field region with respect to the former ice sheets, glacio- and hydro-isostasy both contribute to sea-level variations throughout the Holocene. In the bulk of the basin, subsidence of the sea floor results in a monotonous sea-level rise, whereas along continental margins water loading produces the effect of «continental levering», which locally originates marked highstands followed by a sea-level fall. To describe such peculiar pattern of relative sea-level in this and other mid-latitude closed basins we introduce a new Clarks zone (namely, Clarks zone VII). Using a suite of publicly available ice sheet chronologies, we identify for the first time a distinct sensitivity of predictions to the Antarctic ice sheet. In particular, we show that the history of mid to Late Holocene sea-level variations along the coasts of SE Tunisia may mainly reflect the melting of Antarctica, by a consequence of a mutual cancellation of the effects from the Northern Hemisphere ice-sheets at this specific site. Ice models incorporating a delayed melting of Antarctica may account for the observations across the Mediterranean, but fail to reproduce the SE Tunisia highstand
Solving the sea level equation, part I, theory
No full textPreface
This booklet is devoted to the study of some theoretical and practical aspects
of the so–called ”Sea Level Equation” (SLE), an integral equation that predicts the time–dependent shape of the equipotential surface of a deformable
body subject to surface forces. In the field of global geodynamics, the SLE
serves as a tool for computing the postglacial sealevel variations and other
observable quantities, taking as an input the shapes and chronology of the
Pleistocene ice–sheets.
Our first purpose was simply to collect various sparse notes and to translate in simple words the theory of the SLE for the PhD students attending
my lessons of ”Global Geodynamics” at the University of Bologna. However,
in Part II we also provide details on the numerical discretization of the SLE
and a freely available Fortran 90 code (SELEN) that anyone can use to solve
the SLE on his own computer. We hope that the material presented will
facilitate the work of colleagues at their first approach to Glacial Isostatic
Adjustment (GIA), and perhaps also more experienced geophysicists willing
to benchmark their own codes. As far as we know, this is the first time that
a sealevel equation solver is made freely publically available.
The development of the theory of the SLE is based on a number of approximations. First, the Earth is assumed to be radially stratified and incompressible, and the various layers are characterized by a linear viscoelastic
rheology. This is a widely diffused approximation, but recent work has been
done to include non–Newtonian rheologies and lateral viscosity variations in
spherical Earth models (see e. g. [4, 26]). Second, it is assumed that the
ocean function is constant, that implies fixed shorelines. Third, we totally
neglect the effects of rotation on the GIA–induced sealevel variations. The
reader is referred to [7] for the theoretical details concerning the rotational
feedback and for the numerical evaluation of its consequences. In view of
the approximations listed above, this booklet provides a zeroth–order model
for the postglacial sealevel changes, that can be considerably refined in the
future, hopefully with the aid and the contribution of other investigators.
The future releases of this document will benefit from the feedback of the
readers of this first edition. Please feel free to write to [email protected]
for questions, comments, and suggestions.
GS, February 8, 2005
Post–glacial sea-level in the Mediterranean Sea: Clark's zones and role of remote ice sheets
No full textInfluence of glacial isostatic adjustment upon current sea level variations in the Mediterranean
No full textPresent-day sea level variations in the Mediterranean depend on various factors, including recent climatic
forcing, tectonic activity, anthropogenic effects, and glacio-isostatic adjustment. The latter is governed by
mantle rheology and the spatio–temporal distribution of the late-Pleistocene ice sheets and it is expected to
produce a long-wavelength pattern of sea level variations across the Mediterranean, mostly determined by
the response of the solid earth and of the geoid to loading effects of melt water since the end of deglaciation.
Modeling glacio-isostatic effects in this region is necessary for a correct interpretation of tide gauge and GPS
time-series, and thereby to constrain both the present-day climate-related sea level rise and regional or local
geological, tectonic and human-driven displacements. By an exhaustive exploration of the parameter space of
mantle rheology and ice sheet chronologies, in this work we outline upper and lower bounds on the current
rate of sea level variation associated with glacial isostatic adjustment in the Mediterranean. This may
contribute to a full assessment of coastal vulnerability by sea level rise on a regional and local scale
Glacio-isostatic adjustment in the Po plain and in the northern Adriatic region
No full textVertical movements in the Po plain (northern Italy) are controlled by natural and anthropogenic
effects. Since Italy is located in the far–field of the former late Pleistocene ice sheets, isostatic deformations are
primarily driven by melt water loading and represent a major component of long–term natural movements across
the entire Mediterranean. In addition to far–field sources, here we consider the isostatic effects of melting of the
nearby Wu ̈rm Alpine ice–sheet, suggesting that it is possible to put bounds on its maximum thickness, extent
and chronology by Holocene relative sea level observations from the northern Adriatic. Using various plausible
ice models, and adopting a viscosity profile that matches Holocene relative sea level observations in the
Mediterranean, we find that melting of the Alpine ice sheet is always responsible for upward movements in the
Po plain, currently at rates of * 0.5 mm/yr. When both far– and near–field sources are considered, the rate of
sea level change in the Venetian Lagoon for the most reasonable mantle rheology and melting chronology is
negative, i.e., opposite to that attributed to human activity and recent climatic variations. However, its amplitude
(fractions of mm/yr) is small compared to the secular signal observed by tide gauges (*2 mm/yr), which makes
glacial isostasy a second–order mechanism of sea level variation in this region
Eustatic and Relative Sea Level Changes
No full textSea level changes can be driven by either variations in the masses or volume of the oceans, or by changes of the land with respect to the sea surface. In the first case, a sea level change is defined ‘eustatic’; otherwise, it is defined ‘relative’. Several techniques can be used to observe changes in sea level, from satellite data to tide gauges to geological or archeological proxies. Regardless of the technique used, ‘eustasy’ cannot be measured directly, but only calculated after perturbing factors of different origins are taken into account. In this paper, we review the meaning and main processes that contribute to eustatic and relative sea level changes, and we give an overview of the different techniques used to observe them
Bounds on the time-history and Holocene mass budget of Antarctica from sea-level records in SE Tunisia
No full textSolving the sea–level equation for a Maxwell Earth, we analyze the sensitivity of Holocene sea–level records in SE Tunisia to the time–history of remote ice sheets. Assuming that mantle viscosity increases moderately with depth, we find that in this region the sea–level variations driven by the Northern Hemisphere ice sheets cancel, so that the late–Holocene sea–level high–stand suggested by the geological record merely reflects the melting history of Antarctica. New insight into the history of this ice sheet is obtained analyzing the information contained in a revised set of relative sea–level observations for sites across the Mediterranean covering the last 8 kyrs. From a trial–and–error misfit analysis, it holds true that in this region the match between model predictions and observations improves when the volume of water released from Antarctica is well below the value imposed by the ICE3G chronology and when a sudden meltwater pulse is allowed between 8 and 7 kyrs before present, corresponding to the epoch of the catastrophic rise event known as CRE3
The timescale and spatial extent of vertical tectonic motions in Italy: insights from relative sea-level changes studies
No full textVertical tectonic displacements in Italy since 125 to 1 Kyr BP are drawn from relative sea-level (RSL)
history studies at coastal sites, and, together with instrumental observations, allow to bridge the gap with events
recorded in the geologic (1 Ma) archive. Our analysis aims at establishing the appropriate spatial extent, rate and
duration of vertical tectonic motion within individual crustal segments, and at placing constraints on the contribution
to displacements coming from regional (deep) and local (shallow-crustal) sources. The central and northern
Tyrrhenian Sea and the Ligurian Sea margins show stability at all scales, except for subsidence in coastal basins and
uplift, at places high, at volcanic centers. On the contrary, sustained, large magnitude uplift of Calabria embeds a
deep-seated contribution, highlighted by the spatial coincidence of the uplifting province with a lithospheric slab, and
a contribution from local faults and folds. Holocene uplift was up larger than since Middle-Late Pleistocene, with rate
changes tuned among all sites. The recent increase in uplift rate, detected also in the instrumental record, is related to
clustering of strain release, possibly triggered by isostatic response to deglaciation. A weak deformation signal is
recorded on the central Adriatic coastline, and records slow Apennines thrust belt migration. In the northern Adriatic
Sea, vertical tectonic motions result from opposite displacements in the southern Alps, internal Dinarids and northern
Apennines, but flexure of the Adriatic (micro-) plate beneath the Northern Apennines is the dominating contributor.
Here, rate and spatial extent of displacements are steady over different time-scales, suggesting prevailing control
exercised by plate dynamics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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