Tales from the Green Sands (5)


By now we had sampled the basics of our field site: seawater of the surface and along the bottom, olivine sand from the beach and the surf, and even the pore water from the space in between the sand grains (see previous post). Now it was time to move into newer territory. Sampling-wise, that is… Our next objects of attention were located in the deeper parts of the bay. And by deeper, I don’t mean hundreds of meters, but a mere 4 to 9 meters deep. It is fairly easy to train yourself to draw a deep breath and snorkel to such depths, especially using diving fins. However, to be doing actual sampling work while down there, without getting too stressed by the lack of oxygen is another sport. Simply put, we would have to go sampling, doing SCUBA diving. But for SCUBA diving, one needs quite some gear. For two persons, mind you, because one does not dive alone. Not to mention that one would need to transport said gear to the actual field site. And recall that our particular field site was located rather off the beaten track.

We have been extremely lucky in finding a diving company in Kailua-Kona willing to help us with our mission. The manager-on-duty at Kona Diving Company, Katie, is a trained marine biologist herself. After explaining our project to her, she was more than willing to make us a very nice deal. In the name of science, her colleague Ian, set us up with the much-needed gear, ready to dive Mahana Bay. Thanks Katie ! Thanks Ian ! Thank you, Kona Diving Company !


As said before, the diving gear obviously enabled us to stay under for much longer. We were able to plant a pressure meter in the center of the bay, and left it there for an entire day. Because there is more water on top of you under the crest of a wave, and again less water when the wave passes by, the water pressure builds up and decreases again as waves come and go. By measuring the water pressure, we could get an idea of the number and the height of the waves passing through the bay throughout the day. In the movie below you see my dive buddy, dr. Diana Vasquez, diving down in the center of the bay and anchoring the pressure meter (inside the bag she is holding) in the sediment, next to a small rocky outcrop with a coral colony.

Upon reaching our underwater sampling stations, Diana provides the tools to extract both sediment and pore water. In the video below, she hands me a large PVC tube, with a diameter of ca. 25 cm. This tube will be inserted into the sand and the sand inside dug out. The big tube has small holes at pre-determined intervals. These holes serve like little windows, through which we can again insert our rhizon samplers to suck out the pore water at a particular depth into the sediment. In this way, we obtain a similar chemistry profile as we did with the extracted cylinders of sand described in the former post. It basically serves like an inverted version of those incubation cores. This particular station is located at about four to five meters deep. Behind Diana, you can see large, dark-coloured sand ripples along the bottom. These are ripples that consist largely of olivine and basalt grains and are rather stable. The lighter-coloured sand around those ripples is mostly old volcanic ash and much lighter than either the olivine of the basalt. Although the waves on the sea surface can really be felt at the bottom, and we really needed to hold on to our equipment for fear of it washing away. Those same waves cause the formation of the sand ripples. As the heavier material is hardly moved by the waves, you can clearly see the lighter volcanic ash being moved over large distances along the bottom.



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