Bubbles
After instructing scuba for several years, mainly in the Red Sea and Mauritius (lets forget the Baltic Sea), I decided to study marine geology and hoped to eventually turn my passion for the ocean into a career. At first I was immensely intrigued by the so-called "black smokers", which are submarine hot springs associated with volcanic activity found in deep water along the mid-oceanic ridges. However, I soon shifted my interest to the shallower regions of the ocean, in particular, to submarine hot springs on the flanks of active island arc volcanoes. Fortunately, most of the islands in the western Pacific are island arc volcanoes, providing me with a broad region to explore. Hot springs accessible to scuba divers and close to shore in shallow water, are similar to their land-based counterparts, such as the world famous Yellowstone hot springs, except of course that they can't be reached by car.
Presently I am working on the most interesting hot springs I have yet encountered. Ambitle Island, one of the Feni islands in eastern Papua New Guinea, is an active island arc volcano that last erupted 2000 years ago. Just off-shore, along the west coast, shallow-water hydrothermal activity occurs in the fringing coral reef that circles the island. Alone, the coral reefs of Papua New Guinea make for world class diving, but the combination of hot springs and corals creates an unforgettable diving experience.
From Canada, it took two days of rigorous traveling to reach Kavieng on the northern end of New Ireland. After the destruction of Rabaul by several volcanic eruptions in 1994, Kavieng became the main gateway to the Tabar-Feni archipelago. After hauling 100 kg of equipment half around the world my field assistant, Donna and I were relieved to see Robert, our local contact at the airport. After some rest and re-orientation, we chartered the Alma G, an old lobster fishing boat, for our trip to the Feni Islands. The Feni islands are about 200 nautical miles south east of Kavieng and our trip via Tabar Island and Namatanai on the east coast of New Ireland took two days We arrived at Ambitle Island early in the morning and were immediately greeted by heavy geyser activity in the thermal areas along the west side of the island. Every 20-30 minutes clouds of white steam rose above the jungle to guide us to our precise destination during the last part of our trip. The frequent geyser activity and
the many hot springs are a constant reminder that Ambitle island is still an active volcano, although dormant for the last 2000 years. After anchoring the boat I jumped into the water for a quick snorkel and a first look. I had been anxious about the thermal conditions we would find. Would the vents still be active? After all, it had been more than 2 years since my last visit and knowing that hot springs are dynamic, and sometimes ephemeral, I was worried. It would have been a great disappointment to have traveled for so long and to find the springs gone. On this first snorkeling trip I found the underwater springs discharging gas and water at the same rate as when I left them two years ago. Later I would learn from Philip, a local guide, that the vents in Tutum Bay have been active for at least fifty years. It is always a pleasure to come back to this remote island where people live quite happily without running water, telephones, electricity and only two mini trucks whose purpose is to take people to Sunday church service at the Catholic mission. This time our scientific program consisted of several parts in order to study the impact of venting on the coral reef. We had planned to sample waters from the submarine hot springs for chemical analyses in order to compare them to seawater. We wanted to map the locations of all springs and to take coral samples at varying distances from these springs to test their response to the hot waters.
Most aspects of the submarine thermal area in Tutum Bay are quite similar to land-bound thermal springs with the drastic exception that they discharge into water. The discharge of gas from land-bound hot springs goes unnoticed because carbon dioxide, the main constituent, is invisible and odorless. Under water, however, carbon dioxide gas forms bubbles creating the illusion of a giant Jacuzzi. Tutum Bay and other submarine thermal areas provide us with invaluable observations about the distribution of gas vents in such areas. Here it is relatively easy to map their distribution and to observe changes in discharge rates and shifts in location through time. The underwater hot springs in Tutum Bay when compared to other submarine occurrences, however, are unique in that they have by far the highest liquid discharge and more importantly they occur in the middle of a coral reef. Coral reefs are sensitive ecosystems where complex interactions of biological, climatic, and oceanographic factors influence coral health and growth. The skeleton and tissue of reef corals contain a record of their historical environmental conditions. Their use as paleo-environmental indicators began following the discovery of annual density bands which allows for the construction of chronologies that are in many ways similar to annual growth rings in trees. The variation of chemical constituents in corals such as trace elements and stable and radiogenic isotopes along their growth axis can, therefore, be used to reconstruct historical environmental conditions. Ambitle Island is a natural laboratory for evaluating the adaptations of biota to dynamic environmental changes.
The coral reef in Tutum Bay is made up of patches of coral-algal mounds that grow on old lava flows surrounded by sand. We discovered a rule of thumb: wherever the seafloor is sand rather than coral, ground temperatures are relatively high. Wherever the high heat flow from the cooling magma body below the island warms the seafloor to temperatures more than 35 degrees C coral growth is inhibited and you are left with a sandy bottom. Corals are quite temperature sensitive, being most happy when temperatures are 24 to 28 degrees C. A similar phenomenon can be observed in land-based geothermal areas, where the distribution and height of vegetation is closely controlled by ground temperature. In the thermal areas of Yellowstone National Park for example we do not find well developed vegetation where the ground temperature is higher than normal and complete absence of vegetation once temperatures of more than 60 degrees C are reached.
In the thermal areas of Tutum Bay, two types of venting occur. One is the discharge of a clear fluid from discrete orifices which are 10 to 15 cm in diameter. The temperature is approximately 100 degrees C and the fluids come out of the seafloor boiling. It is the boiling that makes them so clearly visible because of the steam that appears white. The force of the discharge is comparable to that of a firehose which discharges 300 to 400 L/min. Whenever close to a vent we could hear it roaring. The other type of venting is visible as diffuse streams of rising carbon dioxide gas bubbles. These bubbles emerge directly from the sandy seafloor and through fractures in volcanic rocks and coral heads. They do not always discharge from the same location, sometimes ceasing in one place only to reappear a few centimeters to the left or right. The hydrothermal fluids contain extremely high arsenic concentrations of more than 400 times seawater concentration and discharge approximately 1500 grams of arsenic per day into Tutum Bay. These values are the highest arsenic concentrationfound in any marine setting including black smoker fluids from mid-ocean ridges. Despite the amount of arsenic released into the bay, corals, clams and fish do not show any direct response to the elevated values. The skeletons of corals and the shells of Tridacna gigas clams do not show elevated concentration of arsenic when compared to specimens collected from outside Tutum Bay.
Two mechanisms seem to efficiently control and buffer the arsenic concentration: one is the dilution by seawater and the other the incorporation of arsenic in iron minerals that precipitate when the hydrothermal fluids mix with ambient seawater. These iron minerals are found throughout the bay but preferably near vent orifices where they coat volcanic rocks and dead corals in a bright orange. To us the corals in Tutum Bay didn't seem vastly different from corals in the neighboring reefs to the north and south. With the exception of the barren areas with high ground temperatures, coral diversity and health seem to be comparable, suggesting that the reef and its inhabitants easily adapt to the hot spring environment. Several times I found the same big old grouper hovering 2 meters above a vent taking a hot bath. We eventually followed his lead because even 30 degrees C water becomes cold if you soak in it long enough.
For the next 12 days we dove 4 to 5 times a day in order to carry out our research program. Our days generally started with sunrise and finished when there was not enough natural light left to continue working. Except for one torrential rainfall that surprised us in the middle of the night, the April weather was quite pleasant, allowing is to dive every day. The one rainfall, however, was so strong that our dingy sank within 10 minutes and we had to launch a nighttime dingy rescue operation. Once we got the outboard engine on board the Alma G, Robert, Danny and Stan, our local crew, immediately started working on it and the next day it was business as usual. After 15 days on the water we were back in Kavieng. While the trip to Ambitle took us 2 days, the way back took us only one; we sailed all night. We were all happy to be back. Five people on a 30 ft boat with only two bunks for 15 days could get unpleasant, but the magical setting kept us all in good spirit and Danny and Stan were some of the best assistants one could wish for. Every request was greeted with a smile and immediate action, no matter if we asked for help underwater or for a coffee. Even in the end, when we started to run out of food and we had to eat canned corn, fried onions and mashed potatoes everyday, the morale was still up. I can't wait to return
