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.



Tales from the Green Sands (4)


After a couple of intense first days of sampling Mahana bay, we needed to hold our horses somewhat and allow some time to process the material we had gathered. As you can see at the top of this post, Phil is very busy analysing samples in our holiday home laboratory. It is funny, starting so enthusiastically, one always forgets these simple truths of fieldwork life: you have to empty the bottles you use, lest you can take more samples. Until that point, there is no going on.

One of the things we needed to find out, is what compounds are released by the olivine. Recall that the olivine on Mahana / Papakolea beach is thought to take up CO2 while it (slowly) dissolves in the seawater. The olivine itself is what mineralogists would call quite “pure”. That is, it is made up of only a few different elements, and apart from its main constituents, it does not contain many extra elements, sometimes referred to as “contaminations”. Typical (forsteritic) olivine contains three main compounds: mostly magnesium (Mg), some iron (Fe) and lots of silicate (SiO4). “Normal” sand, which is found on your “average” beach is made up of mainly silicium dioxide (SiO2). There are some “extra” metals, that occur in the olivine mineral, but only in very low concentrations. Some of these metals normally set off a lot of alarm bells, like chromium and nickel, but luckily these occur only in very low concentrations. Nonetheless, when olivine dissolves, all those mentioned compounds, including the metals, are supposed to be released from its solid form and go into solution. At the same time, the dissolving olivine should provoke the uptake of CO2 by the seawater. Exactly this is one of the reasons why we want to measure the concentration of dissolved compounds and CO2 in the seawater.


In the picture above, you see me (Francesc Montserrat) preparing to take samples of the seawater we collected from both the surface and along the bottom of the bay, while snorkelling. These samples will be preserved and later analysed for dissolved CO2.


As the beach sand contains a lot of olivine, the real chemistry is of course happening in the water that fills the space between the sand grains, aptly called the pore water. Because pore water is not as diluted as the constantly moving seawater on top of the sediment, it typically has higher concentrations of the dissolved compounds we were after. One way to analyse the dissolved compounds in pore water, is extract a column of sand from its original place, take it to the laboratory and and suck out the water in between the sand. In the picture below, you can see a PVC cylinder, containing sand from Mahana beach. In the wall of this cylinder, we drilled some holes at fixed intervals. In the holes, we stuck little flexible tubes, which are coated with a sort of filter. These filters, called “rhizon samplers”, are connected with little tubes to syringes with which we suck out the pore water. The filter coating on the rhizon samplers filters the pore water we are sampling, and so provides a snapshot of the chemistry at a particular depth in the sediment.



Tales from the Green Sands (3)

On our first real sampling trip, we walked along the Papakolea coast, to Mahana “Greensand” Beach… As you can see, the coast is rugged, and the low vegetation belies strong winds during most of the year. The first time we walked in, we brought only the most necessary field work equipment, including goggles, a snorkel and a pair of fins. The wide-open spaces of this coast are beautiful and wild, with the typical moon-like landscapes of a volcanic desert, while the waves of the mighty Pacific pound the shore.

The walk in, from the last car park to the beach, took about an hour…a bit more on the way back, when we were literally loaded with water and sediment samples from Greensand Beach.

Tales from the Green Sands (2)

forsterite in lava_1

Mahana beach is located at the southern tip of Hawai’i. It is a natural coastal system with an olivine beach, constantly being reworked by waves rolling in from the vast Pacific Ocean. Mahana beach was formed when lava poured from the volcanoes that make up Big Island, into the Pacific, and cooled off in rains of fine cinders. These cinders contained high concentrations of very pure forsteritic olivine crystals, like those crystals in the lava shown in the header picture for this post. The accumulating cinders formed a coastal hill, which eroded into the ocean, forming a green olivine beach.


Because this beach lies in the tropical Pacific it has several advantages for studying its chemistry and biology. First of all, seawater temperatures are relatively high, some 26-27 degrees Celsius, so all chemical reactions are naturally faster. Second, because of its location in the tropics, we expect not only to find your “normal” marine coastal biota, like seaweeds, shellfish and shrimps, but also some more exotic and interesting groups like corals. The interesting part is here is that corals make their calcium skeleton from the seawater. Corals like it when their seawater is less acidic and more alkaline, so an entire beach of dissolving olivine spells good news for them. However, olivine also contains metals, which leach out into the seawater during the dissolution process.

Are the waters in this bay measurably affected by the presence of an olivine beach ? Is the seawater indeed more alkaline ? What is the fate of the dissolving metals from the olivine ? Do they accumulate in the food web ? How do the different marine organisms fare in these waters ? These are some of the questions we hope to answer, and hopefully use this information for when -in a possible future- artificial olivine beaches may be constructed to protect coasts from rising sea levels, while they slowly absorb the CO2 that is the cause of that same sea level rise

Tales from the Green Sands

green sand at mahana

Our uncontrollable hunger for energy has led to unprecedented emissions of carbon dioxide. More, higher, faster, we exhaust, with no end in sight. Recently, the scientific community has reached an equally unprecedented consensus, stating that to avert dangerous climate change, mankind needs to take back its emissions from both atmosphere and ocean.

As long as there have been carbon dioxide, water and rocks, mineral weathering has been the geological control button on Earth’s climate. Mineral weathering constitutes the dissolution of minerals (rocks) by water and dissolved CO2, better known from your soda pop drinks as carbonic acid. One of the fastest dissolving silicate minerals is olivine. Olivine has been shown to consume protons in solution, which pulls down the acidity and effectively draws more CO2 into the solution in which it is dissolving. This is exactly the desired effect of depositing olivine in seawater: the consumption of protons would counteract ocean acidification, while sea-air equilibration processes draw more CO2 into the seawater, which in turn can be neutralised by the olivine. To observe how this would work in reality, is what we had in mind when we planned this field campaign. Together with Dr. Phil Renforth from Cardiff University (UK), we travelled to Hawai’i to investigate enhanced olivine weathering in a marine environment and the effect it has on the surrounding ecosystem.

After a series of flights, crossing the Atlantic, the US mainland and half of the Pacific Ocean, we arrived in Hilo, the capital of Big Island. Mauna Kea and Mauna Loa, the two enormous volcanoes that essentially make up Hawai’i (as Big Island is officially called), separate the island into roughly two parts: a cloudy, rainy side with lush tropical vegetation, and a dry and sunny side, reminiscent of the southern Mediterranean. Hilo is an old colonial town with a quaint atmosphere, located on the wet, eastern side of the island. Here, all the moisture evaporated from the surface ocean and carried by the north-easterly tradewinds, accumulates against the flanks of the volcanoes, draping the capital in clouds. Although the sun can be brutal on tropical Hawai’i, exactly these clouds make Hilo’s micro-climate very pleasant.

In Hilo, our first stop would be the University of Hawai’i at Hilo. Dr. Tracey Wiegner, a faculty member of the Department of Marine Science, had graciously offered local assistance for our field campaign. She provided some equipment that was simply to bulky to bring from Europe, and offered us space in the laboratory, should we need it. Doing research on ocean alkalinity himself, Also at the Marine Science department, Dr. Steven Colbert was able to provide us with much-needed knowledge on the Hawaiian coastal system and its geochemistry. After exchanging ideas and loading our car with borrowed fieldwork equipment, we set course for Naalehu, our home for the weeks to come.

Naalehu is a small community, located a convenient 15 minute-drive from Greensand Beach, known in Hawaiian as Mahana beach. That is, where the normal road to Mahana ends and one needs to walk for about an hour along the coast to reach the olivine beach. That, or get a shuttle service in an old-but-trusty 4WD car, provided by members of five local Hawaiian families. As they have been fishing the waters off this southern tip of Hawai’i for several generations, they have intimate knowledge of the coastal region. They know this rugged, wind-swept coast like the back of their hands, long before any tourist had ever heard of the mysterious Green Sands Beach.

On our first day in, we met up with prof. dr. Jens Hartmann, of the University of Hamburg, who had been doing his own field campaign in the lava tubes around the volcano. Like us, prof. Hartmann is interested in enhanced weathering and its use in CO2 sequestration and together we set off to do a first reconnaissance into our field site along the wild and beautiful Papakolea coast.

What is going on at Green Beach ?


Yes ! It is almost that day that we fly to Hawai’i. We, that is Dr. Phil Renforth from Cardiff University and myself, Dr. Francesc Montserrat of the Netherlands Institute for Sea Research. Why is it interesting for you to read about two researchers going to the Big Island of that famous archipelago in the central Pacific Ocean, you ask ? Obviously, we are not going there to sunbathe…no, we are going there to do a three-week field research on a natural olivine beach. Papakolea Beach on the southern tip of Big Island is a beach that consists mainly of olivine grains. We are very much interested in the incredible capacity of this green mineral to capture CO2 from both atmosphere and seawater and potentially counteract climate change effects, such as ocean acidification. Before ever coming close to trying this out in real life we wanted to investigate how this works in a natural setting, where olivine has been weathering for hundreds, no thousands of years. We are going to Papakolea Beach to try and measure the effect this dissolving mineral has on both the chemistry of the ocean and the state of the surrounding ecosystem. Please keep following this blog for the coming weeks, as we will try and update it regularly.