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Take a few noughts off those fossil ages

Saturday April 25, 2015 Published in Letters to the Editor

Dear Editor,

The letter "Arorangi beach fascinates visiting fossil-hunters" (April 17) referred to intertidal fossils on a beach in Arorangi said by a scientist to be up to two million years old. 

The cartoon by Kata interpreted the fossils as being more than one million years old.

The intertidal beach location and the photographs both indicate that the fossils were in beach rock, locally known as papa, and such rock is less than 5,000 years old.

 The next older group of macro-fossils on Rarotonga are about 125,000 years old and these are found in the fossil reef or makatea on the beach at the Met Office and at the entrance to Ngatangiia Harbour.

 I have not seen any evidence or references to any older macro-fossils on Rarotonga.

A brief geology of Rarotonga

The 1998 "Volcanic geology of Rarotonga, southern Pacific Ocean" notes that the primary basaltic volcanism occurred 2.3-1.6Ma (million years) ago and, after a gap, the secondary phonolite volcanism occurred 1.4-1.1Ma ago. 

Rarotonga grew as a volcanic seamount through about 5,000 metres of water and after it emerged above the surface about two million years ago it formed the inland mountains to about 1,000 metres through two large vents: one in the upper Takuvaine valley and one in the upper Avatiu valley.

After the two vents became dormant, secondary vents opened on various parts of the island to make two types of phonolite rock: (1) the fine-grained phonolite rock of Black Rock and motu Taakoka; and (2) the cemented rubble or breccia phonolite of the Needle and top of Maungatea.

Even before the main basaltic eruptions ceased the island was slowly sinking and corals were growing an encircling reef of limestone.

 As the whole volcano continued to slowly sink the encircling corals grew upward to remain near the ocean surface and the limestone beneath became thicker and thicker. The reef limestone on the north side of Rarotonga is estimated at 400 metres thick, which means the whole island has sunk by this much in the last two million years.

While the volcanic rocks make the bulk of the island, it is the coral limestone that forms the outer coastal plain, beaches and reef system. 

The main controller of coral growth is temperature. We are now in the Present Interglacial Period and the sea-level has been within a metre of its present level for the last 5,000 years or so. Before 5,000 years ago, it was colder and sea-level was lower than present for about 120,000 years. 

About 125,000 years ago, during the Last Interglacial Period it was warmer than today and sea level was about six metres above the present level. 

At that time the Rarotonga reef system was 6m higher than today and we see the remnants of that higher reef as the fossil limestone or makatea exposed at the Met Office, at the entrance of Ngatangiia Passage, and in a few places around the swamplands in Matavera.

 Within the limestone there are a few recognisable fossil corals.

After that 6m high sea level, the cooling climate gradually lowered the sea level until it reached its lowest level of 120 metres-below-present about 20,000 years ago during the coldest time of the Last Glaciation or Ice Age. During that time Rarotonga would have been surrounded by a 120 metres-high fossil reef or makatea with a reef platform at its base.

After this dramatic makatea stage, temperatures rose, ice melted and sea level rose to drown most of the makatea and reach its present level about 5,000 years ago. Actually it initially overshot the present level by about a metre and it took about 1,500 years to settle down to the level of today.

The present coastal plains, beaches and reef system have all developed in the last 5,000 years. The beaches consist of loose sand and cemented sand.

 The cemented sand or beachrock, locally known as papa, was used to build some of the early churches. 

Although the exact mechanism of cementing the sand is debated, it is agreed that it usually involves an interaction of seeping freshwater within the sandy beach interacting with seawater.

 The beachrock forms within the beach and it typically has a similar seaward slope to the beach.

 When the sandy beach erodes the beachrock is exposed, often revealing a series of cemented layers separated by uncemented layers.

Human artefacts found in beachrock show that the youngest cemented rock is about a hundred years old.

 The beachrock with the shells and corals photographed by the fossil hunters has a maximum age of about 5,000 years and it could be as young as a few hundred years old.

Gerald McCormack.

Director,

CI Natural Heritage Trust