South Pacific hydroclimate: Caving in the Cook Islands

Greetings,

It’s been a week since I returned from an amazing field trip to the small island of Atiu in the Cook Islands, an archipelago nation comprising some 15 islands in the middle of the South Pacific. This trip was part of the ARC Discovery research project “South Pacific and Australian hydroclimatic history recorded by stalagmite calcite fabrics“, being led by Silvia Frisia (University of Newcastle, NSW). I’m a co-investigator on this project with Silvia Frisia and John Hellstrom (University of Melbourne). This latest field trip was led by Dr Andrea Borsato (University of Newcastle) with myself and John Hellstrom providing field support.

Atiu is a small island about 190 kilometers northeast of Rarotonga (the Capital of the Cook’s) and is also known as Enuamanu (land of the birds). Atiu has a small population of some 472 people (2013 census) and a rather larger population of wild chickens (as well as some very large herds of goats).

Life proceeds at a predictably sedate rate on Atiu, and most people reside in one of five villages located in the central volcanic plateau. This is separated from the fringing coral platform by deeply developed tropical oxisols. Swamps are typical of the contact between the central volcanic cone and the limestone, which is very heavily weathered and is referred to as makatea (the term coming from the island of Makatea, a raised coral atoll in French Polynesia). The often razor-sharp exposed limestone hides extensive caves which developed at the water table, leading to horizontally developed caverns that have been infilled by speleothem deposition. Navigating the makatea was difficult and at times painful, dubbed by Andrea ‘the trial by makatea!’, so much so that the caves were a refuge from the harsh surface topography. Perhaps for people of earlier times the caves could have served as refugia when the heavy wet season rains arrived, along with plagues of mosquitoes!

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Dappled light close to one of the skylights of Puatea Cave

Weathering rates in the tropics are substantially higher than in the sub-tropics (like here in the Waikato). One measure of the rate of weathering is the degree of mineralisation (i.e. the amount of dissolved limestone) of cave drip waters– in the caves of Atiu we typically find electrical conductivity values in the order of 3-6 times those of cave waters in the Waitomo. High weathering rates, heavy wet season rainfall and rapid infiltration through the poorly-lithified makatea have lead to the development of a confusing maze of chambers in these caves. This was particularly the case in Puatea Cave, in which we found multiple paths of ingress and egress between what at first hand appeared to be distinct and separate passages.

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The amazing maze that is Puatea

Our trip was focused on exploring, surveying and monitoring the caves previously identified by Andrea and Silvia some three years previous, but we also identified samples of stalagmites suitable for developing high resolution records of recent hydroclimate (i.e. last few kyrs). While our days were spent underground we did take the opportunity to visit the beach in search of less weathered coral from the cliffs in order to date the coral terraces. Another approach to dating the present day surface is through dating old speleothem (known as palaeokarst) which litters many parts of the present day forest floor. If dated, this would of course provide a minimum age for the original limestone in which the cave was hosted. It’s a really remarkable feeling to be wondering along a well-worn track, and in a forest glade finding at your feet an ancient stalagmite root, block of flowstone or cave wall– clear markers of what were once subterranean passages. Such things bring to focus the depths of time and the narrow slice which we inhabit.

While we can’t share all of our findings at present, it’s clear that a great deal of interesting science will come from our work on Atiu. For now I’ll leave you with some of my pictures from this fascinating location.

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Cheers, Adam

October, 2017, Cambridge, NZ

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