One drip at a time: Unravelling the rainfall history of Niue

Hi everyone! In February 2020, WEG returned to beautiful Niue in the South Pacific for a second round of caving fieldwork.

The small island of Niue (ca. 1700 inhabitants, 2017 census) presents a prime example of a raised coral atoll with a topography characterised by weathered marine limestones and dense tropical vegetation, riddled with sink holes and caves. The karst landscape nowadays rises to up to ca. 70 m (ca. 35 m on average) above sea level, and bears witness to the island’s origin as fringing reefs lining the shallow waters surrounding a basaltic volcano. The latter is now submerged beneath ca. 400 m of accreted and compacted limestone as the result of millions of years of tectonic activity and sea level fluctuations. Unlike most Pacific islands that come to mind, Niue has barely any sandy beaches. The coast primarily consists of steep and rugged cliffs that drop dramatically to a modern intertidal coral reef terrace.

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Caves have always played an important role in Niuean society, whether for the supply of drinking water, shelter from storms, or recreational and ceremonial purposes. Not surprisingly, most of the main tourist attractions on the island are caves or other limestone structures, such as the Avaiki sea cave featuring spectacular rock pools formerly reserved for the king to bathe in.

Our ongoing fieldwork on the island began with a trip in September 2019 together with partners of the QUEST (QUantitative palaeoEnvironments from SpeleoThems) project, namely Sebastian Breitenbach (Northumbria University) who co-led the trip with Adam, Ola Kwiecien (Northumbria University), Maximilian Hansen (Johannes Gutenberg University of Mainz), and Cinthya Nava-Fernandez (Ruhr University Bochum), as well as Warrick Powrie (University of Waikato) who joined us for field support. This reconnaissance trip was primarily focused on exploring various caves to identify suitable cave monitoring locations and speleothems (cave carbonate deposits, such as stalagmites) for high-resolution reconstructions of past climates. The climate on Niue is heavily affected by the El Niño / Southern Oscillation (ENSO), which is often associated with extreme anomalies in annual temperatures and rainfall amounts. Through the study of Niuean caves and stalagmites, we seek to establish novel geochemical records of climate variability to better understand how and why climate (particularly rainfall) and ENSO have changed over past millennia.

The analysis of one such stalagmite for Cinthya’s PhD project defined an additional question to be addressed on this trip – where exactly had former University of Waikato professor Chris Hendy collected the sample from Niue back in 1976? (Long story short: despite our best efforts to retrace Chris’s steps based on little more than a vague recollection, the search for the exact location of the cave continues…)

After days of navigating around the island’s maze of bush and karst, we eventually deployed a comprehensive set of cave monitoring equipment in two caves deemed promising for ongoing studies (Anapala Cave and Ulupaka Cave). Over the next years, these instruments will provide us with information on how sensitive the respective cave climate is to conditions outside of the cave (e.g., temperature, air pressure), and how rainwater is transported through the limestone to ultimately form speleothems inside the cave. An amazing collaborative trip with exciting prospects for the results to come from this!

Our follow-up trip in February 2020 was aimed to retrieve data from our previously deployed monitoring equipment, collect samples, and explore further caves. I was accompanied by Chris Wood and Andy Pearson (both WEG), as well as Jess Roeger for additional field assistance. Chris had broken his thumb shortly before our flight, but luckily Andy could spontaneously join us for extra support.

The trip began with revisiting Anapala Cave, a multi-level system of several extensive chambers interconnected by narrow passages. Large boulders and abundant rubble throughout the cave bear evidence of several historic roof collapses. To our delight, the equipment deployed 6 months earlier in the first study chamber was still in place and seemingly untouched! Needless to say, there are certain risks associated when leaving sensitive scientific instruments in a cave on a remote Pacific island for several months, and I soon found myself nervously following a trail of microscopic pieces of a drip rate logger to eventually find it heavily beaten up (but working!) in a crab’s den far from its original location… But despite a few equipment casualties, including our prototype dripwater auto-sampler (a.k.a., ‘Otto Zampler’), we got very promising first data from this cave.

Ulupaka Cave consists for the most part of one level and contains abundant speleothem formations, both modern and broken material. Interrupted only by the shiny white of recently precipitated calcite, the cave floor and walls are consistently coated with fine black soil, possibly deposited during episodic extreme rain events. To collect fresh calcite we had previously placed several watch glasses on the tops of active stalagmites, many of which had fallen to the ground (or pushed off by our crustacean nemeses?). Better luck next time.

The remaining time was spent travelling across the island and surveying further caves, but to add a little extra excitement to the schedule, the Niue Meteorological Service had issued not one but two cyclone warnings for the days before our return flight to New Zealand… both of which luckily did not come true!

For now, I will leave you with some impressions from these first two trips.

Cheers,

Seb

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