Expedition Logbook : October

DP Hunter drilling off Maara on the coast of Tahiti.
Thanks to Stewart Frazer of SeaCore for making this photo possible.

 Hunter_at_Maara.pdf

DP Hunter off the coast of Tahiti.
High definition photo
Photo: Ian Pheasant (C) IODP

Day 24 : The Coral Custodians
By Jody Webster, Expedition Scientist
Drilling at Papenoo (NE Tahiti) has finished for now and we are back to our first site at Maraa (S Tahiti). A major advantage for this cruise has been the short transit times ? only a few hours to go from one side of the island to the other. In this way, we have been able to continually optimize our drilling strategy as more results come in. The targets at Maraa are the reefs between about 75 and 100 m below present sea level. This really is a critical interval as previous work done on other fossil reefs and sediments elsewhere in the world (Barbados, Hawaii, South East Asia) indicate that both sea level and climate may have been changing very rapidly between 15,000-13,000 years ago. For example, these controversial data suggest sea levels may have ?jumped? 15-20 m in several hundred years as a result of a catastrophic ice sheet collapse in either the northern and/or southern glacial regions. Climate scientists have hypothesized that this massive dumping of ?melt water? not only raised global sea-levels but also severely disrupted ocean circulation and perhaps global climate. Recovering more coral records from this interval in Tahiti is therefore critical if we are to better understand these events and their consequences.

Returning to our core flow through the various labs on deck ? their final stop is back at the curation lab. Here our two curators (Alex Wuelbers and Holger Kuhlmann) or ?coral custodians? as I like to call them complete the core curation and archiving before their long trip to Germany. This is perhaps the most important job on the ship. As you might imagine, knowing exactly where each core came from as well as their correct orientation (or way up) is crucial!

The procedure goes something like this: the core sections are kept in their plastic liners, the top is then capped with a blue cap to indicate the way up along with a marked arrow. The core ID is also inscribed directly on the core liner and a printed label, with a unique serial number, is also fixed to each core. This serial number, along with critical information (site, hole, and core number, precise depth in the hole and physical dimensions of the core) are entered into the science database. That way the context and fate of every single millimeter of core is known. Finally, the curated cores are stored in a refrigerated (4 degrees C) container. It is quite a process but all this curation data will be invaluable when we begin piecing together the complete history of the fossil reef in Bremen next year.

Day 20 : Imaging the reef
By Jody Webster, Expedition Scientist

Drilling at Papenoo (Northern Tahiti) continued at a good pace. We have recovered excellent fossil coral reef material from the very earliest part of the sea-level rise following the end of the last ice age.

We have now had the opportunity to log several of the boreholes. Basically this means the Physical Properties (PP) team inserts a suite of different geophysical tools directly into the open hole left over from our drilling. These tools allow us to obtain high-resolution images of the borehole surface, characterize the fluid in the borehole, measure the borehole size and measure or derive the porosity, electrical resistivity, acoustic velocities, and natural gamma radioactivity of the rocks. The beauty of this technique is that provides continuous information on physical and geochemical properties of the rocks throughout the fossil reef sequences. It is incredible to be able to image a borehole wall 150 m down and clearly see an in-place coral complete with coralline algal crusts on the branch tips! This information will be particularly useful in intervals where we have poor recovery and are not be exactly sure where the core came from within these intervals. Combined with comparable measurements or observations we have made on the cores, these borehole geophysical logs will enable us to reconstruct the precise depth positioning of the cores as well as provide information on areas where we were unable to recover material.

Day 15 : Great dipsticks
by Jody Webster, Expdition Scientist

Drilling at Maraa (southern Tahiti) has finally finished and we have moved on to Papenoo, our second site (TAH-02A, Prospectus ID) on the northern margin of Tahiti. The initial drilling results at Maraa are very exciting, as we recovered a large portion of the post glacial (ie. < 20 kyrs) fossil reef sequence as well fascinating material from the much deeper and older reefs below. We have high hopes of getting even deeper (and older) post glacial material from Papeeno. Only time will tell!

Papeeno is more exposed than Maraa, with higher winds and frequent squalls of heavy rain. This has made drilling more challenging but we have been treated to beautiful rainbows several times a day. At this site, large rivers have incised deep valleys into the basalt hills. The coastline is also different to Maraa, with basalt cobble rather than white carbonate sands forming the beaches that face the lagoon.

Continuing our journey with the cores? Once through the physical properties (PP) lab the cores make their way back to the original staging area. At last the sedimentologists (Hiroki Matsuda, Tsutomu Yamada) and coral specialists (Kaoru Sugihara and myself) get a chance to look at the cores. Our purpose is to briefly describe the different sediments and coral types in the cores, with the detailed work to be done in Bremen, Germany next year. A primary aim of the cruise is to reconstruct sea-level changes over the last ~ 20 kyrs ? for this, the reef sediments and corals will be crucial. By carefully analyzing the nature of the sediments as well the coral growth forms, species types and their context (ie. are they in place or loose rubble?) we hope to determine what environment the corals were growing in. For example, were the corals growing on the shallow reef crest or the deep fore-reef slope? Using this information, their current depths below present sea-level and subsequent age dating by the onshore geochemists, we will use the fossil corals as "dipsticks" to build a very detailed record of sea-level rise following the end of the last ice age.

Day 10: Fossil reef heaven
By Jody Webster, Expedition Scientist

We are now drilling our second hole (7B) at Site M0007 (TAH03A-#3 - Maraa, southern Tahiti). This site is shallower (about 47 metres water depth ) and somewhat closer to the modern barrier reef than our previous site M0005 (about 59 metres water depth). So close in fact that the Maraa locals were convinced we had run a ground on the modern reef! Apparently a story was circulating in the local Tahitian media that a huge red ship had been "stuck" on the reef for over a week. The captain spent sometime on the radio re-assuring the Tahitians that we were indeed OK and in the right place.

Drilling continues to go very well. Over the last few days we have recovered a very thick sequence of fossil coral reef material. Again in place robust branching and domed coral frameworks dominate, which we believe will allow us to precisely reconstruct both sea-level and climate (e.g. sea surface temperature) changes for the intervals in which they grew. More details on this later.

Once sub-sampled by the geomicrobiologists, the cores make their way to the physical properties (PP) lab. Here the PP team as we affectionately call them (Hendrik Braaksma, Klaas Verwer, Florence Einaudi, Jenny Inwood and Gilles Henry), measure a diverse suite of physical properties within the cores; sonic velocity and magnetic susceptibility are just two examples. Basically, the cores are fed automatically and incrementally through the multi-sensor system, with measurements recorded every 1-2 centimetre by a computer. This allows us to develop a complete picture of the physical properties, such as hardness, density, and volume of internal spaces within each core interval. We are then able to directly compare these properties with our visual rock descriptions and eventually the down-hole logs that record similar (as well as a few more) properties continuously throughout the drill hole. Already we have seen that different rock intervals have very specific physical properties, so these measurements combined with further sample analysis will be very useful in piecing together what is shaping up to be a complex history of reef growth and demise.

Day 7 - Core on deck!
By Jody Webster, Expedition Scientist
We have been drilling at Site M0005D (TAH03A-#4 - Maraa, southern Tahiti) for several days now and are deep into fossil reef section. The cores are coming thick and fast. So fast in fact that the science party has been working very hard to keep up with the flow of cores! The rock and sediment types are incredibly diverse, from in-place domed and robust branching coral frameworks to thick sequences of dark, well laminated sediments between and surrounding the corals. Fascinating stuff.

Once the cores get on deck the real action starts. The first job, done skillfully by the drillers, is to remove the core from inside the core barrel. While this happening, the scientists can see a small, tantalizing piece of the core protruding from the bottom of the core barrel. For the scientists this is a bit like seeing your Christmas presents but not being able to open them! The drillers finally deliver the cores (within clear plastic liners) to the initial core staging area. The sedimentologists and coral specialists get a quick look, but because of time constraints on getting fresh samples, the geomicrobiologists get the first crack at the cores.

Apart from our sea-level and climate objectives we are also trying to understand how the reefs are bound together. Previous work by our co-chief (Gilbert Camoin) and others identified curiously thick sequences of finely laminated dark sediments that in some places completely engulf the Tahitian fossil corals. It has been suggested that microbes play an important role in the deposition of these sediments and ultimately in binding the fossil reefs together. Our two geomicrobiologists on the cruise, Rolf Warthman and Crisogono Vasconcelos, are trying to measure the microbial activity and abundance within the fossil reefs. They have been busy taking swab samples from small cavities within the reef cores. Using different techniques (eg. metabolic activity, fluorescent enzymes and dyes) they have able to indirectly and directly measure microbe activity and abundance as well as isolate samples for more detailed morphologic and taxonomic studies onshore in the lab. Rolf and Cris tell me the initial results look very promising.

Once the microbiologists have had their fun the cores continue on their journey around the rest of science labs but that's a story for another day.

Drilling begins!
by Jody Webster, Expedition Scientist
Weather: Wind from SE, 20 knots, temperature 29°C. Cloudy and clear with good visibility, rain with showers. Sea state: 1 m swell from the SE

After a short transit from Papeete and some time to setup we have started drilling operations. Our first site is off Maraa (southern Tahiti) in about 60 meters of water. This site is incredibly beautiful with our ship holding station ~ 150 m in front of the modern coral reef with the sheer volcanic, forest covered slopes of Tahiti just beyond. The DP Hunter is able to maintain this fixed position (and wonderful view!) using its dynamic positioning system made up of 6 computer directed thrusters distributed around the ship.

After the drillers were ready, our first task was to lower the drilling apparatus through the open ?moonpool? in the bottom of the ship. The entire drill rig is set up on the ship?s back deck over this opening allowing drill pipes and core barrels to be transported back and forth between the ship and the seafloor. To keep a stable connection between the seafloor and drill rig a large device called a ?DART? (Drill And Re-entry Template) is used. The drill pipes and inner core barrel pass through the DART into the seabed. The entire drill rig is fitted with compensators which allow the rig and the drill bit to remain fixed to the seafloor during drilling despite the ship moving up or down. It is very impressive, and somewhat disconcerting to watch the rig moving up and down and realize it is in fact the ship moving and not the rig!

Before actually drilling we lowered a camera through the drill pipe to observe the seafloor. I have to say it was quite a strange sight ? like something out of the Hollywood film ?Inner Space? as the camera made its way through the narrow 70 mm diameter drill pipe. Once through, the seafloor image was very striking ? a rough, rubble strewn seascape of limestones and white sediment patches. No living corals were observed so we began drilling on a prominent rubble patch.

After less than an hour the first core arrived on deck around 8 pm. At this point the entire science party, despite half of them being officially off shift, were on hand to see the first Tahitian fossil reef sediments. Very interesting material, composed of multiple generations of encrusting corals and coralline algae interlayered with coral rubble and spherical coralline algal nodules deposits.

More core is on the way so ciao for now.

Leaving Papeete, Tahiti
by Jody Webster, Expedition Scientist

Hello from Tahiti! This is the first expedition log for the Integrated Ocean Drilling Program (IODP) Leg 310 expedition to drill the fossil coral reefs around Tahiti. My name is Jody Webster and I am a coral reef geologist from Australia and also part of the Japanese scientific team on board the ship DP Hunter. Every 3-4 days I will bring you some of the scientific highlights, introduce the cruise participants and their interests as well as give you a taste of what life on board a scientific drill ship is like.

Our aims and objectives for this cruise are three fold. We hope by drilling the fossil reefs on the fore-reef slopes in front of the modern Tahitian reefs that we can reconstruct sea-level and climate changes since the last ice age 23,000 years ago, as well as understand the impact of these changes on the how the reef grew and then finally drowned.

Activity has been frantic today as the drillers and ship crew prepared the DP Hunter for departure from Papeete and then drilling operations. While this was going on the science and logging parties held health and safety briefings as well as a general science meeting to streamline our work plan.

We are now heading for our first drill site in the Maraa area (TAH03A-#4) on the south side of the Tahiti. We will be drilling in 75 meters of water and expect to drill to a depth of ~ 70m. We hope that this coral reef records sea-level history of the first part of the sea-level rise following the last ice age. There is definite feeling of excitement in the air as we anticipate seeing the first cores on deck later tonight or in early morning. Many of the science party have been involved from the very earlier stages of cruise planning so as you can imagine finally seeing drill core material will be a real thrill!

Ciao from now.

The tropical Island of Tahiti offers fascinating views...
above...