Thursday, May 14, 2009

Maldives Reef Sea Level Curves: What About Palk Strait....And Ram Sethu?

A paper in the May issue of Geology by P.S. Kench, S.G. Smithers, R.F. McLean, and S.L. Nichol works out the Holocene sea level history of the Maldives:

Holocene reef growth in the Maldives: Evidence of a mid-Holocene sea-level highstand in the central Indian Ocean

Radiometrically calibrated ages from three reef cores are used to develop a Holocene reef growth chronostratigraphy and sea-level history in the Maldives, central Indian Ocean. Last interglacial reef (U-series age 122 ± 7 ka) was encountered at 14.1 m below mean sea level. An age of ca. 8100 calibrated (cal) yr B.P. immediately overlying this Pleistocene surface records the initiation of Holocene reef growth. Massive in situ corals occur throughout the cores and the consistency of the three age-depth plots indicate that the reef grew steadily between 8100 and 6500 cal yr B.P., and at a decreasing rate for the next 2 k.y. The position of modern sea level was first achieved ca. 4500 cal yr B.P. and sea level reached at least 0.50 ± 1 m higher from 4000 to 2100 cal yr B.P. before falling to present level. Emergent fossil microatolls provide evidence of this higher sea level.

Results are significant to two long-standing issues relating to Maldivian sea level history. First, the ambiguity of a late Holocene highstand has been resolved with clear evidence of its existence reported here. Second, the uncertainty of the regional pattern of sea-level change in the central Indian Ocean has been clarified, the Maldivian results broadly agreeing with island records in the eastern, rather than western Indian Ocean. Our results provide the first field evidence confirming geophysical model projections of a highstand 4–2 k.y. ago in the central Indian Ocean, though the observed level (+0.50 ± 0.1 m) is lower than that projected

As sea level rises coral reefs grow upwards. They thus track the rising sea level. Using the relationship between depth at which corals of a particular age are found the researchers calculated that following the end of the last glaciation, sea level rose uninterrupted at a faster rate between 8100-6500 years ago and then at a slower rate between 6000-2000 years ago. There has been a minor drop of around 0.5 metres since. This drop is indicted by emergent coral atolls i.e corals which are now exposed above sea level and are dead. That would mean that in the past sea level was higher.

How can one be sure that sea-level rose continuously? Could it have been interrupted by a sea-level drop? The Holocene corals don't show any evidence of any significant sea-level drop. Discoloration and recrystallization - signs of meteoric diagenesis- are absent and the mineralogy of the corals is 95%-99% aragonite, which again indicates no contact with fresh water. Aragonite is stable in sea-water but will alter to calcite during fresh water incursions.

So this use of coral reef growth as a proxy for sea level change is a common technique in constructing local sea level curves. How local are these curves? It would be hard to extrapolate the Maldives curve as a general statement on the Indian ocean since the Maldives are a relatively isolated island chain. Fortunately we have more data. The paper refers to previous work done in the eastern Indian ocean at Cocos island and on the western Indian ocean sites of Reunion and Mauritius. The eastern sites are in agreement with the Maldives sea-level curves while in the western sites evidence of the late Holocene highstand and later drop in the form of emergent atolls and reefs is not present. But for most of the Holocene they concur with the Maldives.

Look at the figure below for geographic orientation.

This Holocene sea-level curve is beginning to look like it applies to a wide regional extent of the central and northern Indian ocean.

As my post title indicates what does this say about the Palk straits, which is a narrow body of water separating the coasts of south India and Sri Lanka. Well, as it happens the Holocene sea-level history of the straits is of huge interest not just to geologists but to ...say... about 900 million plus Hindus.

The Indian epic Ramayan tells the story of king Ram who crossed the Palk Straits with his army to rescue his wife Sita held prisoner by emperor Ravana of Lanka. Legend has it that the sea-level was low and had exposed most of the Palk strait sea bed. Ram's army build a causeway (Ram Sethu) over the remaining stretches of water and walked across into Sri Lanka (why didn't they take a boat? me!). Supporters say they can see the causeway from satellite images and there is physical evidence too. I have dispelled the many myths regarding this bridge in this post and here.

The Ramayan like many other Indian epics like the Mahabharata existed and was passed down generations as an oral narrative recording Indian society beginning maybe as early as 1000 B.C. (3000 B.P.) until it was compiled as a written document sometime early A.D. So the epic contains early enough memories for us to speculate whether the legend of a low sea-level could have a physical basis and whether a late Holocene sea -level drop could have exposed the Palk strait sea bed.

The central / north Indian ocean sea level curve (Maldives, Cocos, Reunion) says no sea level drop of a few meters in the late Holocene or for that matter the entire Holocene. But basins like the Palk strait which are attached to large continents may not have the same sea-level history as recorded on isolated islands. Tectonic uplift or subsidence and patterns of sedimentation, especially influx of large amounts of clastic sediment from the continental mainland may result in a local sea -level history different from surrounding regions.

Has that happened in the Palk strait? Unfortunately the kind of detailed studies that this paper represents using corals have not been done. Overall the Holocene history looks similar to the Maldives. As the last glaciation ended an exposed Pliocene-Pleistocene surface was flooded by rising sea-levels beginning early Holocene (8000-9000 B.P.) Coral growth was initiated and continues until present day. Like the Maldives there are emergent coral atolls and coral-sand islands (Rameshwaram). The younger exposed corals are dated to around 2600 B.P. which speak to a late Holocene highstand and then a slight drop in sea-level of about one meter or so.

There are geologists and geo-archaeologists who won't agree with this at all. They will bring up the same objections I outlined above regarding the reliability of extrapolating of sea-level curves. I have spoken to some of them and they insist and there are "findings" that show that Holocene sea-level rise and coral growth in the Palk Strait was interrupted by a sea-level drop of several meters sometime several thousand years ago and that would have exposed most of the sea-bed between India and Sri Lanka. I hope they publish these findings and make their data available for others to review.

Fundamentalists engage in a most literal interpretation of scripture. But amongst the many fantasies, exaggerations and story telling there may also be a record of some real event. A late Holocene sea-level drop in the Palk strait is not out of the question, despite what the Maldives sea-level curve is telling us. This is a small basin adjoining a large continent and it may have a peculiar history of its own.

I would love to see this north Indian ocean Holocene puzzle solved.


  1. You have hit the Jackpot there... Wouldnt all of us wish to have this Holocene Puzzle solved.

  2. fascinating and controversial! makes for a perfect blog post topic :)