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Sunday, January 2, 2011

Sea Level Jumps

Possible explanation for a non-linear migration, excess ice being
melted into the sea could provide conditions where either the shore
untraversible or the the ability to use small craft limited.


Global Sea-Level Rise at the End of the Last Ice Age Interrupted by
Rapid 'Jumps'


Coastal erosion in Portugal. (Credit: E. Rohling)


ScienceDaily (Dec. 4, 2010) — Southampton researchers have estimated
that sea-level rose by an average of about 1 metre per century at the
end of the last Ice Age, interrupted by rapid 'jumps' during which it
rose by up to 2.5 metres per century. The findings, published in
Global and Planetary Change, will help unravel the responses of ocean
circulation and climate to large inputs of ice-sheet meltwater to the
world ocean.


Global sea level rose by a total of more than 120 metres as the vast
ice sheets of the last Ice Age melted back. This melt-back lasted from
about 19,000 to about 6,000 years ago, meaning that the average rate
of sea-level rise was roughly 1 metre per century.


Previous studies of sea-level change at individual locations have
suggested that the gradual rise may have been marked by abrupt 'jumps'
of sea-level rise at rates that approached 5 metres per century. These
estimates were based on analyses of the distribution of fossil corals
around Barbados and coastal drowning along the Sunda Shelf, an
extension of the continental shelf of East Asia.


However, uncertainties in fossil dating, scarcity of sea-level
markers, and the specific characteristics of individual sites can make
it difficult to reconstruct global sea level with a high degree of
confidence using evidence from any one site.


"Rather than relying on individual sites that may not be
representative, we have compared large amounts of data from many
different sites, taking into account all potential sources of
uncertainty," said Professor Eelco Rohling of the University of
Southampton's School of Ocean and Earth Science (SOES) based at the
National Oceanography Centre (NOC) in Southampton.


The researchers brought together about 400 high-quality sea-level
markers from study sites around the globe, concentrating on locations
far removed from the distorting effects of the past massive ice
sheets.


Using an extensive series of sophisticated statistical tests, they
then reconstructed sea-level history of the last 21 thousand years
with a high degree of statistical confidence.


Their analyses indicate that the gradual rise at an average rate of 1
metre per century was interrupted by two periods with rates of rise up
to 2.5 metres per century, between 15 and 13 thousand years ago, and
between 11 and 9 thousand years ago.


The first of these jumps in the amount of ice-sheet meltwater entering
the world ocean coincides with the beginning of a period of global
climate warming called the Bølling-Allerød period. The second jump
appears to have happened shortly after the end the 'big freeze' called
the Younger Dryas that brought the Bølling-Allerød period to an abrupt
end.


"Our estimates of rates of sea-level rise are lower than those
estimated from individual study sites, but they are statistically
robust and therefore greatly improve our understanding of loss of ice
volume due to the melting of the ice sheets at the end of the last Ice
Age," said lead author Dr Jennifer Stanford of SOES.


"The new findings will be used to refine models of the Earth climate
system, and will thus help to improve forecasts of future sea-level
responses to global climate change," added Rohling.


The researchers are Jenny Stanford, Rebecca Hemingway, Eelco Rohling
and Martin Medina-Elizalde (SOES), Peter Challenor (NOC) and Adrian
Lester (The Chamber of Shipping, London).


The research was supported by the United Kingdom's Natural Environment
Research Council.


http://www.sciencedaily.com/releases/2010/12/101201120605.htm


Sea-level probability for the last deglaciation: A statistical
analysis of far-field records
Purchase the full-text article


References and further reading may be available for this article. To
view references and further reading you must purchase this article.


J.D. Stanforda, low asterisk, E-mail The Corresponding Author, R.
Hemingwaya, E.J. Rohlinga, P.G. Challenorb, M. Medina-Elizaldea and
A.J. Lesterc


a School of Ocean and Earth Science, University of Southampton,
National Oceanography Centre, Southampton SO14 3ZH, United Kingdom


b National Oceanography Centre, Southampton, SO14 3ZH, United Kingdom


c The Chamber of Shipping, 12 Carthusian Street, London, EC1M 6EZ
Received 4 March 2010;
accepted 8 November 2010.
Available online 26 November 2010.


Abstract


Pulses of ice-sheet meltwater into the world ocean during the last
deglaciation are of great current interest, because these large-scale
events offer important test-beds for numerical models of the responses
of ocean circulation and climate to meltwater addition. The largest
such event has become known as meltwater pulse (mwp) 1a, with
estimates of about 20 m of sea-level rise in about 500 years. A second
meltwater pulse (mwp-1b) has been inferred from some sea-level
records, but its existence has become debated following the
presentation of additional records. Even the use of the more
ubiquitous mwp-1a in modelling studies has been compromised by debate
about its exact age, based upon perceived discrepancies between far-
field sea-level records. It is clear that an objective investigation
is needed to determine to what level inferred similarities and/or
discrepancies between the various deglacial sea-level records are
statistically rigorous (or not). For that purpose, we present a Monte
Carlo style statistical analysis to determine the highest-probability
sea-level history from six key far-field deglacial sea-level records,
which fully accounts for realistic methodological and chronological
uncertainties in all these records, and which is robust with respect
to removal of individual component datasets. We find that sea-level
rise started to accelerate into the deglaciation from around 17 ka BP.
Within the deglacial rise, there were two distinct increases; one at
around the timing of the Bølling warming (14.6 ka BP), and another,
much broader, event that just post-dates the end of the Younger Dryas
(11.3 ka BP). We interpret these as mwp-1a and mwp-1b, respectively.
We find that mwp-1a occurred between 14.3 ka BP and 12.8 ka BP.
Highest rates of sea-level rise occurred at ~ 13.8 ka, probably (67%
confidence) within the range 100-130 cm/century, although values may
have been as high as 260 cm/century (99% confidence limit). Mwp-1b is
robustly expressed as a broad multi-millennial interval of enhanced
rates of sea-level rise between 11.5 ka BP and 8.8 ka BP, with peak
rates of rise of up to 250 cm/century (99 % confidence), but with a
probable rate of 130 -150 cm/century (67 % confidence) at around 9.5
ka BP. When considering the 67 % probability interval for the
deglacial sea-level history, it is clear that both mwp1a and 1b were
relatively subdued in comparison to the previously much higher rate
estimates.

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