Friday, 25 July 2008 1:37 PM

Lake Tenggano Rising or Island Sinking?

Greetings again editor mi hope mi no mek mess lo column blo iu.

It is encouraging to read Mr Pugeva's exposition on the threat to Lake Tenggano from global warming. It is also great to note that he actually did some research on the subject matter. We should encourage our young people to engage themselves in global issues.

I have some colleagues of mine who are palaeontologists/climatologists & environmental geologists. They could not seem to agree on the fundamentals of global warming.

One school of thought argues that what we are witnessing now is nothing new, its has happened before. We are at the crest of a temperature rise cycle again. Its just that we cannot reconcile geological timeframes with respect to the human lifespan (in other words none of us was around to witness the last global rise in temperature that results in ice cap melting and eventual rise in sea level). The other side of the argument pointed out that global temperature rises is in tandem with the advent of the industrial revolution. The industrial revolution was marked by a remarkable increase in the use of fossil fuels which produced greenhouse gases resulting in global warming. The interesting thing however, is that both sides have sound scientific evidence to substantiate their claims. Most of the evidence is from ice cores on both poles and glaciers. I cannot pretend to know anything about global warming so I leave it here for you to decide.

However my interest is on the issue of rising lake and/or sinking land at Rennel. I note Mr Pugeva made mention of the 'sinking island' (besides Mr Fairfax, East Rennel - Sinking Little Bit, 22/07/08) as well. Is this a prevalent theory in Solomon Islands? Has any work been carried out by the Geology Department or SOPAC or anybody else?

Essentially what we are confronted with are two likely scenarios: either the lake level is rising or the land is subsiding; how about making it more interesting by suggesting that both are happening (I think we will leave that for another discussion). Refer to my previous article on this subject for reasons why lake level may rise (Lake Tenggano Residents Say Lake Level Increasing, 24/07/08). What I want to focus on however is the 'sinking land' notion.

From a geotechnical point of view, as mentioned in my previous article, the land could subside (sink) under dynamic loading such as from earthquakes due to seismotectonic activities like a fault rapture events. Bodies (in this case, Rennel Island) that are under the influence of gravity can still subside under static conditions. This will be brought about by the settlement of soft and compressible material underlying the island. While the island of Rennel may have been uplifted coral (can someone confirm this?), if there are organic rich sediments between the coral and the basement rocks, over time these organics will decay and hence result in the island subsiding, if indeed the island is settling! This is all speculative and needs to be confirmed.

What I would be very interested to know (as previously suggested), is if someone can compare a fixed datum on the island with respect to historic lake levels and a known land mark to ascertain if the lake level did rise or the island had subsided (can the Geology Department or Land & Survey help?)

Why is this important? Well, as the resident and Mr Pugeva had pointed out livelihood of the people and the biodiversity of a World Heritage locality are at risk. Understanding the issues involved allows for proper planning to remedy any possible adverse effects of rising lake level or sinking land.

Disclaimer: The views and opinions expressed in this letter/article are those of Warwick Sitana and do not necessarily reflect the official policy or position of Solomon Times Online.

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