Green flash at sunset over the water
The green flash captured by onboard videographer Fraser Johnston.

Seamount blog logoDay 18: Onboard communicator, Bethany Green

Every good voyage has its mysteries; here are two particularly colourful ones.

Mystery 1. The elusive green flash

On Investigator, astronomic and atmospheric observations are a frequent topic of conversation: second only to whale sightings. Recent conversations have centred around the elusive green flash.

The green flash refers to a brief moment at either sunset or sunrise – but more commonly sunset – when the light emitted from the sun suddenly appears distinctly green. Observation requires clear skies and an unobstructed view of the horizon.

The atmospheric phenomenon, prominent in sea-faring folklore, is caused by the way the light from the sun is scattered and refracted through different layers of the Earth’s atmosphere, separating out into different colours. As the sun dips below the horizon the colours of the spectrum disappear gradually, longest wavelength to shortest, with green towards the shorter end of the spectrum.

After several weeks of overcast skies and rough seas, the weather had significantly improved and people became more optimistic and enthusiastic about a possible sighting. On Sunday afternoon, the conditions finally aligned. The waters were calm, the temperature was pleasant, and, despite a few clouds, the sun was setting over a clear horizon.

At about 8:20 pm, people started emerging on the stern, vying for an unobstructed view. We stared as the sun descended slowly below the horizon, determined to discover for ourselves whether this flash was legend, or reality. And then, just as we thought the sun had set and we had missed it, there it was: an almost unnatural, yet undeniably green, glow. The sight brought loud cheers of excitement and high fives, as the green flash delighted even the most skeptical among us. It was a truly remarkable experience.

Green flash at sunset over water
The green flash captured by seabird and marine mammal observation team leader Eric Woehler.
People on the Investigator bow at sunset
Looking west in a twilight sky. Image: Nic Bax

Mystery 2. The (most-probably) misidentified mineral

Twelve years ago, scientists aboard Investigator discovered a bright orang, needle-like, crystalline structure on top of a basaltic seamount, approximately 740 metres below the sea surface, at the boundary of the Huon Marine Park. Images from the deep-tow camera showed the mineral in incredible detail; the structure and colour proving a convincing likeness to the Tasmania’s state mineral, crocoite.

In the years following, speculation circulated regarding a possible deep-sea crocoite discovery: the first of its kind. Crocoite, a relatively rare mineral is usually found in a number of Tasmanian mines. Some of you may have seen the shining specimens at the Zeehan West Coast Heritage Centre.

So when the mystery mineral was spotted for the second time on Mongrel seamount during a deep camera tow on this survey, biologists onboard sought advice from the mineralogists. They consulted the UTAS School of Natural Sciences, the Metallic Minerals and Geochemistry Mineral Resources Tasmania, and Geoscience Australia. The feedback was that the mystery mineral is unlikely to be crocoite due to the setting. Crocoite contains the elements oxygen chromium and lead, and these elements need to be in abundance under specialised circumstances for crocoite to form. Basaltic seamounts such as Mongrel typically contain prohibitively small quantities. Rather, it is more likely to be the ‘lookalike’ zeolite, Thomsonite.

Thomsonite can form in a variety of ways and places,  including on basaltic seamounts in the Southern Ocean. It contains the elements sodium, calcium, aluminium, and silicon, all of which are plentiful in basaltic seamounts, and requires water to form: an abundant substance under the sea. Until a sample is collected from the seamount, however, which is not within the science objectives of the seamount survey, we can’t be absolutely certain. Maybe we can keep that mystery alive?

Mongrel seamount seafloor showing orange mineral
The spectacularly decorous seafloor of Mongrel seamount that sparked the crocoite mystery 12 years ago. Image: CSIRO
Orange mineral on seafloor of Mongrel seamount
The recent sighting. Not as clear, but almost as orange. Image: CSIRO
crinoids and Chrysogorgia on the seafoor
Above left: an undercut edge on sediment plain with crinoids. Above right: rock features provide stable attachment for octocorals (Chrysogorgia).

Notes on today's activities from Marine Biodiversity Hub Director, Nic Bax . . .

Good conditions prevailed and we continued to deploy the deep-tow camera throughout the day. Nine operations were launched, with one aborted due to currents. The eight wagon wheel transects on each of the five target seamounts were completed, with only St Helens on the East Coast to be completed when we head north on Thursday. Four further baseline transects were completed searching for coral communities away from the seamounts and especially outside what appears to be the centre of their range from about 1000-1250 m, though possibly deeper on the deeper peaks.

We have much good data that should allow us to show how the coral communities, especially Solenosmilia, transition from dense to patchy cover and then to isolated clumps and tufts by about 1400 m with only the rare sighting below this depth, where urchin numbers also increase, but the shallow end of their range is proving harder to define. At the shallow end, another matrix-forming stony coral, the almost unpronounceable Enallopsamma sometimes, but not always seen, and bare substrate is commonly observed. Isolated clumps or tufts of Solenosmilia have been seen as shallow as 900 m, but not consistently.

This is confounded as many of our shallower tows are on the continental margins where the unconsolidated sediments (sand and gravel) would be less suitable for stony corals to establish. Evidence of fishing, primarily as lost trawl wire, was evident in many areas, including some of the shallower slope transects.

Biologists on board were tantalized with the prospect of a Sherman dredge on the north side of Mongrel where a diversity of putative octocoral species had been observed on video. After several attempts, we got to 900 m (100 m above target) before the winch software became non-communicative again and we had to haul back the empty dredge. As the day ended one of the taxonomists on board was able to target a beam trawl transect to an area holding a variety of Chrysogorgiidae (a bottlebrush form of ocotocoral) and the subject of her PhD thesis.

Today we completed our 131st operation, have more than 40,000 pairs of stereo images, identified 40.5 species of marine birds (the 0.5 was unconfirmed), seven whales, one pinniped, and >200 molluscs. The many other groups collected in the beam trawl await further identification by taxonomists back at the museums.


Voyage date: 
Monday, December 10, 2018