Paleowave

Tropical Field Season Part II

Tropical sunsets are the best. Shot at Tulaghi.

Tropical sunsets are the best. Shot at Tulaghi.

I apologize for the big blogging slump over the last month and a half, when I returned to Austin after a two month field season in the Solomon Islands. An extremely tight schedule, including visa appointments, a paper submission, three AGU abstracts and much progress on my Gulf of Mexico sediment cores, is to blame for this hiatus (apart from procrastination). It's been one hectic July for me! So, it is quite ironic that my subsequent blog post is from back in the Solomon Islands!

We (Fred Taylor and I) left the Solomons in the last week of June after collecting many uplifted (dead) coral samples on land (with rock hammer and chisel) that were indicative of previous uplift events (earthquakes). We also found several huge (living) coral microatolls in shallow water that hold the keys to the amounts of vertical deformation pre-earthquake and post-earthquake. However, in order to read these microatolls, we need to cut them open - which takes more than a hammer and chisel. In fact, it requires a hydraulic/gas-powered drill/saw system - in other words, a lot of equipment (it clocked in at just over one tonne). Unfortunately, as things go awry when you're in the field, our equipment did not make it here last time and we were told that the shipment would arrive only two months later (it was sitting at the Brisbane dock waiting for a ship - the Kopoko Chief came to the rescue!) This is why we are back in the Western Province, Solomon Islands - and now, we have our equipment. Corals beware, we are armed!

Jumping right into it, a week ago, we were in the Florida Islands, a short boat ride away from Honiara, and cut open a bunch of microatolls. As these islands are relatively devoid of neotectonic activity, these corals will aid us in seperating the oceanographic signal (ENSO, sea-level rise etc.) from the tectonic signal (deformation of the land, earthquakes) that we see in the corals of the Western Province. Hauling the hydraulic chain-saw around (the selected weapon of choice) was quite an ordeal but it was no match for the logistical mess that we would endure while leaving for the Western Province.

Originally, we were scheduled to head west aboard the Pelican Express 2, a slick passenger vessel that would reach Gizo, our destination in the Western Province, in a mean 12 hours. This ship was to leave the Honiara dock at 7AM sharp on Sunday. All of our equipment (including the hydraulic chainsaw/drill, gas-powered chainsaw/drill and assorted tools) was at the Solomon Islands Geology Dept., a good 3km away from the dock. We were staying at the United Church Rest House, close to the dock. So, all we needed to do was: wake up early, wait for our truck (which we had already 'booked'), head over to the dept. and haul our equipment to the dock. Simple, right? On our part, we were awake and ready to go at 4:45AM. Our driver who presumably woke up late after a long night, reached the United Rest House at 6:15AM, after much pandemonium from our side while we had already tried to flag down several passing trucks (mind you, loading a tonne of equipment on a truck and unloading it takes a lot of time). But, finally, he had arrived! Crisis averted, right? Well, on the way to the Geology Dept., the truck spluttered, hemmed and hawed and finally came to an ominous stop a good kilometer away - it had run out of diesel! We were stranded on the way up a hill without fuel and without any passing traffic. It was 6:25AM. We still had a lot of work to do and there was only 35 mins for the Pelican Express to leave!

Slabbing a huge ~8m microatoll at Olasana Lagoon, southeast Ghizo

Slabbing a huge ~8m microatoll at Olasana Lagoon, southeast Ghizo

Standing on that hill, forlorn and distraught, ultimately, we hypothesized about the worst-case scenario (not a bad thing to do when one does field work): we don't find a truck to the department in the next 15 mins, go get diesel from the nearest petrol bunk (a cool 5km away), drive to the Geology Dept., load our gear and get it to the dock, forget about the Pelican Express and look to board a later ship. And this was exactly how it played out, resulting in a loss of money and time. The Fair Glory took off from the Honiara dock at 10:00AM, giving us plenty of time to load our equipment and a long 27 hrs later, we made it to Gizo.

At Gizo, we've been staying at the PT 109 restaurant - named after JFK's destroyer boat that was wrecked not far away, 70 years ago. We've been here for four days, cutting and slabbing coral microatolls and it already seems like a month! Each day is a battle against the sun and those tough, tough corals. However, looking at a cut slab is like opening a present - you never know what you're going to get! Depending on internet availability, I'll try to periodically update the blog on the interesting geology we chance upon.

Corals and Earthquakes

Standing on the largest uplifted Porites head we’ve found thus far (Fred says it ranks in the top 5 he’s ever seen in his career!) – spanning a cool 11m. This was on a lagoon at an uninhabited island, Ghoi at northern Ranongga

Standing on the largest uplifted Porites head we’ve found thus far (Fred says it ranks in the top 5 he’s ever seen in his career!) – spanning a cool 11m. This was on a lagoon at an uninhabited island, Ghoi at northern Ranongga

Fred, Alison and I have been in the Western Province of the Solomon Islands for three weeks now (and the Solomon Islands for five). The Western Province is unique as it is one of the only pieces of land in the world that lies so close to a subduction zone, where oceanic lithosphere is devoured. However, this process of subduction is not smooth. At times, the down-going plate gets locked with the upper plate and when the stress is too much, the result is a rupture: an earthquake. Close to the trench where this subduction occurs, the upper plate gets uplifted during an earthquake (coseismic uplift) and further out, past the hingeline, the land subsides (coseismic subsidence). The last big earthquake in this region was a magnitude 8.1 in 2007. This was associated with a tsunami which reached heights up to 15m in certain locations. There were around 60 causalities. What about before 2007? Nobody knows. There are no known large earthquakes in the instrumental record, which spans a hundred odd years (the Brits were keeping record). 2007 was the first of its sort. Now, this is a scary thought! How frequently do earthquakes occur here? How large can they get? Has 2007 been the biggest, baddest one ever? One must look to the past to obtain perspective for the future. Corals that grow near shallow water are fantastic tools that give us clues to these questions.

Ancient coral reef that has become part of the island

Ancient coral reef that has become part of the island

The Solomon Islands is located in the Western Pacific Warm Pool where the climate is amiable for corals and coral reefs. They need warm water to thrive. Corals are found in deep and shallow water. They start from a point on suitable ground and grow outward, leaving behind secreted calcium carbonate. The colored outer portion of the coral is the only ‘level’ where they are alive (note: the color comes from the symbiotic zooxanthella algae and not the coral itself) – everything inside is aragonitic rock. If bioerosion is not too brutal on the corals, the corals can build microatolls and live for many centuries, becoming bigger and bigger from that point source. However, depending on the tides, there is a particular highest level of survival (HLS, as christened by Fred) for the corals. They can only grow in water so shallow. If the water is too shallow and the tide is too low during the day, the corals can die from being exposed under the sun. Therefore, sea-level changes (for example those associated with the El Niño Southern Oscillation) can kill corals. When they are killed there is no outer ‘level’ which is alive – CaCO3 rock is left behind. This too disappears through erosion depending on the environment (so, hunting for well-preserved ones is tough!) However, there is another way corals can die – earthquakes. With the latter, they are lifted out of the water, above their HLS and soon die out. Nevertheless, they can continue to stay alive and grow on the sides and bottom ends of the coral mound if they are still underwater i.e. the uplift wasn’t enough to thwart the entire coral head outside the HLS. There are many complications apart from the two-dimensional sea-level/seismicity issue such as repeated rapid subsidence and subsequent coseismic uplift, global sea-level rise, local effects and so on. In any case, by seeking out microatolls at various places we can piece together the clues they offer and learn more about earthquakes and eventually, subduction related processes.

Hiking through pristine coastal forest

Hiking through pristine coastal forest

Our main purpose in the Western Province is two-fold. One is to find coral microatolls that uplifted or subsided during the 2007 earthquake and understand how permanent deformation is retained (how much is coseismic, aseismic etc.) This mainly consists of offshore field work: we haul our 40HP outboard motor with Alison’s ≈3m canoe and go around looking for coral heads (be it via snorkeling or paddling) with the tropical sun beating down on us. We benefit from this as we can measure net post- (or pre-) seismic subsidence, understand ‘permanent’ anelastic uplift and so on. Our other intention is to find paleo-uplifted corals or corals uplifted by large earthquakes in the geologically recent past (≈2000 yrs ago) which (due to uplift) become part of the land itself. This gives us insights into the inner workings of the earthquake cycle through its frequency and magnitude. As you can imagine, this entails field work on land: hiking through thick, pristine, coastal rainforest in all its glory (bugs, thorns and topography included), we search (and search) for suitable, intact and preferentially in-situ paleo-coral reefs. By dating these (preferably with precise uranium/thorium dating) intact corals, we can glean information about paleo-earthquakes. Over the last three weeks, gradually, we’re working our way through this project: lagoon by lagoon, forest by forest, reef by reef and island by island, one day at a time!

Dispatch from Tulaghi

Tulaghi was the erstwhile capital of the British Solomon Islands Protectorate (pre-WW2). It’s a small constituent of the so-called Florida Islands suite, about 30 km north of Guadalcanal. As the British had already established a solid base, Tulaghi seemed a lucrative target in the eyes of the Japanese during their WW2 conquest. They took over on May 3rd 1942. The historical records reveal the atrocities committed by the Japanese in the Solomon Islands. To put it nicely, they were merciless and apathetic. The Americans got involved post-Pearl Harbor on December 7th, 1941. They arrived eight months later and pretty much obliterated the Japanese as a part of Operation Watchtower.

We reached Tulaghi on the morning of 16th May riding for about an hour due north from Honiara on a small ~7m boat equipped with a 40 w Yamaha outboard motor. The sea was dead calm save for the occasional flying fish (which were quite amazing). The crew: my colleague Fred, the senior seismologist at the Dept. of Mines and long-time collaborator of Fred, Alison K. Papabtu, our driver Joseph from Savo and myself. We disembarked at the southern coast of Tulaghi riding through warm, reef-laden waters of every hue of blue. This was my first sight of the reefs of the Solomon Islands and boy, were they something! The Central Mother’s Union is a small lodge on the southern coast, initially started with some Australian church’s help. This was where we were to stay. It was a nice, quiet place with a set of ~10 rooms, a small stove, a fridge and a bathroom. There was no running water and we had to rely on a big tank situated in the yard surrounded by oddly colored toads, fast moving skinks, and wide-eyed geckos. As expected, the people at Tulaghi were very friendly and inquisitive. They speak English, Pidgin and Nggela, the local language. I met a couple of fisherman who gave us a few pointers on where to find good reefs. They (along with most of the Solomon Islanders I have met) thought that I was from Fiji, which has a substantial Indian-origin population. I had to correct their notion with ‘Me blong India-india not Fiji-India yeah!’

On this trip, we are investigating the paleoseismology (or earthquake history, if you will) of the Solomon Islands using corals. Coral atolls, depending on the species, can only grow to a certain level in the water column i.e. their highest level of survival. Therefore, the shallowest corals respond to sea-level changes. In the Western Province, our field area, where earthquakes occur, vertical uplift can kill corals by thwarting them out of their highest level of survival. However, so can sea-level changes. So, how do we separate the signal of sea-level change and tectonic activity in corals? - by obtaining a coral from an area that responds only to sea-level change and does not undergo the same tectonic activity as the Western Province (a tide-gauge can do the trick, but the local record is too short). There isn’t much going on in the Florida Islands by way of tectonics. They are sheltered by the island of Malaita to the north and Guadalcanal to the south. Hence, it was an ideal location for us to scout out coral microatolls that only record sea-level change.

It was an intense three days of field work. The first day was pretty glum – we didn’t find any large microatolls and I got stung all over my left ankle as I fell into a whole colony of sea-urchins and nearly destroyed my DSLR (which I will henceforth never carry into the water – noob move). Spirits were lowered when all we had to eat were hard tack Navy biscuits, canned tuna, and canned corned beef three times a day (yummy!) Subsequently, we managed to fully cover around 8 islands and partially searched the west coast of Nggela Sule (or Big Nggela) Island, heading well into the Sandfly passage. Paddling around near the shallow portion of the reef and snorkeling around an island for hours and hours, with the hot tropical sun beating down on you is no vacation – it’s tough work. Further, there is always the ominously hovering question of ‘what if we don’t find that ideal sample?’ In the end, there was no ideal sample – as there never is, but we did manage to find some good looking, big Porites and Goniastrea microatolls that will do us good.

Fauna-wise I managed to catch sight of species of birds I never imagined in my wildest dreams that I would one day see. To name a few: the notoriously vocal Willie Wagtail, the elegant Ducorps Cockatoo, the brilliantly colored, endemic Lories, and the skilled Beack Kingfisher amongst others. I also glimpsed birds which I saw in Surathkal, India – the White-Bellied Sea Eagle and the Brahminy Kite, both of which sat pretty atop the food chain. I also saw numerous skinks, geckos, crabs and mudskippers. Oh, plenty of bed bugs and mosquitos too!In the marine life department, apart from the many species of coral, I saw reef fish of many sizes and colors (I was quite taken with the parrotfish), sea-urchins (see above) and sea slugs. While snorkeling, I gulped when I saw a moray eel/sea snake (couldn’t see clearly – quite sure it was the former) about 2.5m below me. It seemed to be snarling at me, reminding me that the ocean was not my natural domain. Fortunately or unfortunately, we didn’t manage to see any sharks, rays or saltwater crocodiles.Tomorrow, the 20th, Fred, Alison and I leave for the Western Province by the Pelican Express ferry. We head to Ghizo, a 12-hr journey away. We will set up our base of operation and continue to conduct field work all over the region with its many islands. Wish us luck! (posted from Ghizo - we already made it safely!)