Big, Bigger, Biggest

In 2020, Watercare Services Ltd and its contractor Ghella Abergeldie Joint Venture dug a big hole. Not just any hole, but one big enough to fit Hiwa-i-te-Rangi, a very large tunnel boring machine. Hiwa-i-te-Rangi was built in Germany and brought to Auckland especially for the Central Interceptor Project. 

We have all heard about Central Auckland’s wastewater woes – overflowing drains, spillages, contaminated streams – which are down to our aging wastewater infrastructure. As the largest, most costly wastewater project ever undertaken in New Zealand, the Central Interceptor Project is designed to significantly reduce these problems. Its aim is to build one large tunnel with a network of smaller linking tunnels, which will run for nearly 15 km from Grey Lynn all the way to the Māngere Wastewater Treatment Plant. 
 

So, back to the hole. The large Central Interceptor tunnel is 4.5m in diameter and runs between 15 and 110 metres below our feet. The first step was for two very big shafts to be created at the Māngere end, into which Hiwa-i-te-Rangi wil be lowered so she can start boring the tunnel. 

But with big projects come big surprises! While digging this enormous shaft, contractors came across a 5m-thick layer of marine shells, about 35m below the surface. This was a hugely exciting discovery, as the layer turned out to be thousands of fossils that are around 3 million years old. 

The fossils are mostly molluscan, meaning they belong to the phylum Mollusca, which includes snails, clams, squid and octopus. Similar molluscan fossils were found in the 1940s in a well sunk by the then Waitemata Brewery at their site at Ōtāhuhu. Its then-manager, Mr Morton Coutts, contacted scientists to evaluate the spoil heaps and, as a result, fossil specimens representing at least 60 new species were discovered and lodged with Auckland Museum, Auckland University and the New Zealand Geological Survey (now GNS Science). This time, Watercare is partnering with Auckland Museum, Mana Whenua and specialist palaeontologists to document these taonga recovered from the Māngere site. 
 

Already the number of fossil species found in the Māngere sediments is more than double the number found at Ōtāhuhu, with more than 150 species identified to date from the shell layer. Many of the fossils are also more complete and better preserved than the Ōtāhuhu fossils. One of these is the huge extinct cockle Maoricardium spatiosum, which will be on display at Auckland Museum during the Sea Monsters Exhibition. 

There are also new species, including land snails, and the sediments continue to yield more discoveries. This Māngere material presents a unique opportunity to substantially enhance our knowledge of the fauna that inhabited seas and lived in forests in the Auckland region around three million years ago. 

Just like humans, molluscs have ecological preferences, such as the conditions provided by the substrate they like to live on or in, the turbulence of the waves, the salinity of seawater and the temperature of the water or air. So fossil molluscs help us to define what the environment looked like at the time these species were alive. 

Dr Alan Beu (emeritus, GNS) and Dr Bruce Hayward (Geomarine Research) have provided expert identifications and both believe that the diversity of species shows the shells were deposited in a shallow near-shore environment. The first-ever discovery of fossil flax snails indicates that there was forested land nearby. There would also have been sandy open ocean beaches, as shown by the presence of fossil tuatua; and rocky shores, as shown by the presence of species such as limpets and cats’ eyes. In that way, the landscape will have looked much like we have around Auckland today, but with one big difference: three million years ago the Firth of Thames and the Hauraki Plains did not exist, so rivers flowing from the Coromandel Ranges would have reached Auckland and brought down much of the sediment that was deposited in this region. This would also have been the source of the fossil flax snails. 

The presence of a cone shell and other subtropical species suggests a very warm temperature at the time of deposition, perhaps like that at Rangitāhua/Kermadec Islands at present. 

The Mangere fossils are unique as they are the biggest number of fossils ever found in one spot near Auckland. And they were discovered as part of the biggest wastewater infrastructure project ever in New Zealand! 

Watercare has generously funded Auckland Museum to employ two part-time specialists who will carry out further field work, help with species identifications and catalogue all our discoveries. Those discoveries will then be housed at Auckland Museum for the community, to help facilitate education projects as well as future research by our teams and the wider scientific community. In this way, we can gain as much knowledge as possible from this once-in-a-blue-moon discovery. 

Acknowledgement 
For their generous support of AM and Māngere fossil project, we would like to thank Watercare/GAJV; Dr Alan Beu and Dr Bruce Hayward.