Quick Dips

Diving into Darkest Depths

Dr Cornel E.J. de Ronde

The JAGO submersible dive to the bottom of Lake Taupo last October has produced some interesting and intriguing information [Exploring Lake Taupo, Sept 98].

The joint New Zealand-German project, involving IGNS, the University of Kiel and the Max Planck Institute, made the first-such dive in Lake Taupo to look at the lake's previously unexplored hydrothermal system. Two successful dives were accomplished, with four others cancelled due to poor weather conditions.

The first dive was about a kilometre southwest of the Horomatangi Reefs, in a known deep-water part of the lake, and reached a maximum depth of 145m. The landing site was largely devoid of any life, with a 1cm layer of organic material covering the pumice-dominated lake floor.

During the three-hour dive, several areas of hydrothermal activity were seen. They largely comprised pock-marked areas on the lake floor, each characterised by small depressions, about 1-3m in diameter, which had small vents at their base from which streams of gas bubbles emanated. In several areas, the pumice was bright orange and white, indicative of clays and iron-staining.

The maximum temperature anomaly recorded during this dive was 0.5^oC above ambient (11.1^oC). A number of rock samples were recovered, together with gas and water samples. The most common animal seen were lobsters and some small, guppy-like fish.

The second dive site was in an area of suspected submarine venting. Here JAGO dove to a depth of 187 metres -- current bathymetric maps show a maximum depth of 165m.

This dive lasted almost five hours, and discovered a number of hydrothermal sites. Small depressions in the lake floor with a plethora of gas bubbles streaming from their centres were noted, as seen during the first dive. In contrast to the first dive, hot water was seen venting from cracks and fissures in two localities, with maximum temperatures of up to 44^oC.

More rocks were seen to crop out in this locality, and a 1m-wide fracture transected the lake floor. A number of small, dead "chimneys", or spires, rising up to a maximum of 15cm, indicated that focused venting had occurred at this site sometime in the recent past.

Also of note was the discovery of colonies of sponges living on the spires, which previously have not been seen at these depths. Gas and water samples were again collected, together with some of the spires, rocks, and sponges.

Initial studies have provided some exciting results. Gas analyses show the bubbles are dominated by carbon dioxide and are similar to gases collected from vents offshore from White Island, while the hot water has compositions similar to those found within subaerial hot springs.

The chimneys are largely composed of skeletons of diatoms and filamentous microorganisms, upon which hydrothermal minerals have precipitated. Geochemical analyses show these chimneys contain significant amounts of metals. Identification of the sponges is still pending.

Dr Cornel E.J. de Ronde, IGNS, Lower Hutt