1,720,968 research outputs found
Global Choke Points May Link Sea Level And Human Settlement At The Last Glacial Maximum
Full text linkGlobal choke points are preeminent nodes in geographic networks and geopolitical touchpoints subject to control by nations. They appear today as recurring theaters of conflict worldwide and also in archaeological investigations delving thousands of years back in time. How different were today’s global choke points at the Last Glacial Maximum (LGM) ~ 20,000 years ago? For the first time, we map nine of them to visualize their conditions at LGM. The global feature aquaterra—all lands inundated and exposed repeatedly during the Late Pleistocene ice ages—initially was mapped as first approximations of sea level. Here we refine its boundaries using Glacial Isostatic Adjustment (GIA) models to account for the Earth’s deformation and horizontal migrations of shorelines in response to glacial melting. We found three choke points sufficiently open to navigation, but six others presented substantially greater barriers than today. Implications include strategic insights on where to search for submerged evidence of human settlement
Linear and non-linear sea-level variations in the Adriatic Sea from tide gauge records (1872-2012)
Full text linkWe have analyzed tide gauge data from the Adriatic Sea in order to assess
the secular sea-level trend, its acceleration and the existence of
possible cyclic variation. Analyzing the sea-level stack of all Adriatic
tide gauges, we have obtained a trend of (1.25±0.04) mm yr-1, in agreement
with that observed for the last century in the Mediterranean Sea,
and an acceleration that is negligibile compared to the average global
values. By means of the Ensemble Empirical Mode Decomposition technique,
we have evidenced an energetic oscillation with a period of ∼20
years that we relate with the recurrence of opposite phases in the Atlantic
Multi–decadal Oscillation and North Atlantic Oscillation indices.
We suggest that anomalously high sea-level values observed at
all the Adriatic tide gauges during 2010 and 2011 can be explained by
the rising phase of this 20 years cycl
Extent and dynamic evolution of the lost land aquaterra since the Last Glacial Maximum
No full textWe study the evolution of a recently recognised global geographical feature, named aquaterra, enclosing those lands that in previous glacial cycles have been repeatedly exposed and flooded. So far, the geography of aquaterra has been studied as a first approximation neglecting the isostatic effects and assuming globally uniform (i.e. eustatic) sea-level variations. Focussing on the last deglaciation and considering both global and regional aspects, we show that isostatic effects related with mantle dynamics have indeed played a significant role in the evolution of aquaterra. Our analysis is based upon paleogeographic reconstructions in the framework of well-established Glacial Isostatic Adjustment theories
Modeling sea level changes and geodetic variations by glacial isostasy: the improved SELEN code
Full text linkWe describe the basic features of SELEN, an open source Fortran 90 program
for the numerical solution of the so-called "Sea Level Equation" for a
spherical, layered, non-rotating Earth with Maxwell viscoelastic rheology. The
Sea Level Equation was introduced in the 70s to model the sea level variations
in response to the melting of late-Pleistocene ice-sheets, but it can be also
employed for predictions of geodetic quantities such as vertical and horizontal
surface displacements and gravity variations on a global and a regional scale.
SELEN (acronym of SEa Level EquatioN solver) is particularly oriented to
scientists at their first approach to the glacial isostatic adjustment problem
and, according to our experience, it can be successfully used in teaching. The
current release (2.9) considerably improves the previous versions of the code
in terms of computational efficiency, portability and versatility. In this
paper we describe the essentials of the theory behind the Sea Level Equation,
the purposes of SELEN and its implementation, and we provide practical
guidelines for the use of the program. Various examples showing how SELEN can
be configured to solve geodynamical problems involving past and present sea
level changes and current geodetic variations are also presented and discussed
The Bering Transitory Archipelago: stepping stones for the first Americans
Full text linkRetrospective sea-level mapping advances a promising geographic solution to the longstanding mystery about when, where, and how the first Americans crossed over from Asia. A paleotopographic reconstruction accounting for Glacial Isostatic Adjustment digitally explores an archipelago about 1400 km long that likely existed from >30,000 BP to 8000 BP. Here the authors examine this Bering Transitory Archipelago in regard to established hypotheses—Clovis-first, Ice-free (deglaciation) Corridor, Kelp Highway, and Beringian Standstill hypotheses—and a new Stepping-Stones hypothesis. Scores of islands at that time would meet all requirements previously proposed for a viable hypothesis: a source population in Asia, a pathway with abundant sustenance, settlements in North America soon after but not before, and an isolated sanctuary where Beringians could have become genetically distinct
Tide gauge observations in Antarctica (1958–2014) and recent ice loss
No full textSeveral historical sea level time series from Antarctic tide gauges, available from the Permanent Service for Mean Sea Level, are analysed. Two sea level curves, obtained by averaging data from the Antarctic Peninsula and West Antarctica, for 1958–2014, show trends of (2.0±0.1) and (1.8 ± 0.2) mm yr-1, respectively. By empirical mode decomposition, cyclic and non-cyclic components of sea level change were separated. A periodicity of 4–5 years was confirmed and attributed to the effects of the Antarctic Circumpolar Wave. The non-cyclic components were found to show a ‘levelling off’ of ≈ 1 mm yr-1 since c. 2000, which cannot be attributed to the isostatic response to Holocene ice melting. Using assessed mass balance data from the West Antarctic Ice Sheet and the Antarctic Peninsula, we studied the response to current ice loss in the region and found that the levelling off could be partly explained by accelerated melting during the last approximately two decades. This may represent the first evidence of sea level fingerprints of glacial melting in Antarctica
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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