Sediment sampling to predict rising sea levels

November 20, 2012

“Blow, blow, thou winter wind,
Thou art not so unkind
As man’s ingratitude;
Thy tooth is not so keen,
Because thou art not seen,
Although thy breath be rude.”

As the nights grow colder, the days become shorter, and gas bills start to rise I thought the “Blow, blow thou winter wind” from As You Like It by William Shakespeare was highly appropriate! Winter is starting to makes its ugly presence felt around the place, with sunshine at a premium and moderate days punctuated by torrential down pours. As I write this blog entry, I am slightly jaded or to put more aptly ‘creamed and crackered’ following a weekend of trekking through the sand dunes of Murlough, south County Down. However, the great thing about any kind of fieldtrips with undergraduates is that it encourages one to have to re-examine and re-learn an aspect of research which they either wouldn’t normally have learned or have simply forgotten, which in my case it meant re-examining sea-level change.

The global rise of sea-level during the last 10,000 years in the period known as the Holocene and during subsequent human activity has prompted studies of the short-term responses of shorelines to changes in water level with various work going into the multitude of aspects of sea-level change such as geomorphological analysis of shoreline behaviour and investigations of the marine encroachment onto land surfaces. The factors responsible for changes in the shoreline morphology of coastal environments are as numerous as they are diverse which can include the supply of sediment, wave energy variability, tidal current variability, wind processes, sedimentology, dynamics of tidal inlets etc. Thus, trying to pin down the precise influence of these various factors on sea-level rise is one of the major challenges not only facing coastal scientists but also governments, policymakers, engineers, and indeed landowners.

Thus, trying to predict what might happen in the future with regards sea-level rise is difficult and one approach has been to try and fix sea-level index points (SLIPs), which relate past sea-level through time and space (i.e. seeing how sea-level was historically, might inform what it may be in the future). The key to this method of SLIPs is that alterations between terrestrial and marine sedimentation are reflective of shifts in the balance between land and ocean levels, which are moderated by the site specific sedimentation rates. Thus, SLIPs contain information on; the age, location, altitude along with a quantified vertical relationship within the palaeo tidal frame known as “indicative meaning”. With this, a sample of sediment from a given location can be employed as a sea-level index point when it is dated and the altitude is determined to a fixed geodetic datum. In order to produce a sea-level index point, in which a sea-level envelope is produced and plotted in terms of a time/depth-elevation graph, each indicator (i.e. the sediment compositional features) must possess a relationship that between the local environmental conditions which the sediment has accumulated in regards to a contemporary reference tide level.

Shown in the two figures are sediments that are radiocarbon dated and fixed to a particular height in terms of present day mean sea-level. The first curve (A) is that collected for a site in Bangladesh by Islam and Tooley (1999) that is about 100 miles north of the curve developed for one of my sites in India, shown in the second graph (B). What we have here, although still early days may well represent two ends of the same story. Essentially, in the northern site the sedimentological evidence presented with the curve developed shows that from the early mid-Holocene until ca. 1750 BP in the Bengal Basin, there was a dominant regional sea-level movement taking place (Islam and Tooley, 1999). The interface between the fluvial to estuarine depositional sequence was related to the rates and directions of this sea-level movement (Islam and Tooley, 1999). So what my site might be picking up is this movement of sea-level but at a higher relative elevation to the site shown in Figure (B). Although further investigations will be required in order to constrain this depositional environment.

Sea-level change is one of those phenomena people learn a great deal about but still act surprised when they storm floods overtopping coastal defences. Of course, one has to bear in mind the great story of King Cnut, the 11th century king of England, and really justifying the second figure presented:

“All the inhabitants of the world should know that the power of kings is vain and trivial, and that none is worthy the name of king but He whose command the heaven, earth and sea obey by eternal laws”.

Till next time folks!

Rory Flood, PhD Student, Queen’s University Belfast, School of Geography, Archaeology and Palaeoecology (GAP),

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