Methods Of Ocean Drilling In Climate Research

The European Science Foundation's (ESF) European Collaborative Research (EUROCORES) programme EuroMARC is an essential tool to boost European leadership in the planning of international marine coring expeditions and the preparation of IODP (Intergrated Ocean Drilling Program) or IMAGES (International Marine Past Global Change Study) proposals. The programme consists of seven collaborative research projects with principal investigators from nine countries.

The scientific focus is manifold: reconstructions of the meridional overturning circulation in both high and low latitudes and of the spatial and temporal structure of the interglacials peaks and demises are made using thick marine sediment sections. Fossil reef and carbonate mounds cores are extracted to reconstruct sea-level and environmental changes. Current ocean dynamics and sediment fluxes are investigated with the help of sediment traps, and hydrothermal processes of deep biosphere at mid ocean ridges get explored.

But how does coring work and what is actually done on the cruises? International marine coring expeditions are divided into several parts, the pre-, cruise and post-cruise activities. It's crucial to be a hundred percent prepared for the coring, which means to plan way ahead, starting with getting a slot on one of the few coring and drilling ships, obtaining territorial drilling permits, making sure all the required equipment is on board and getting a good scientific team together. "We had organisational meetings even before EuroMARC started" said Catherine Kissel from the French Atomic Energy Commission (CEA) in Gif-sur-Yvette, who was the chief scientist of the AMOCINT (Atlantic Meridional Overturning Circulation during Interglacials) cruise that took place this summer. The chief scientist is in charge of coordinating all the logistics and operations before and on the cruise, which even includes making sure that all the crew members pass their medical exam.

Often, a site survey cruise precedes the main cruise to identify the best spots for the actual coring and to get the drill sites approved by bodies like IODP. In order to map the topography of the seafloor, a multi-beam echo sounder system is used, which is like a fanlike beam covering a huge swath of the seafloor. Additionally, sediment penetrating systems are employed, which shoot signals with varying energy pulses and wave lengths that hit the bottom and are differently reflected depending on the density of the layers, thus giving a detailed impression of the layering of the sediments.

Besides the seismics, autonomous underwater vehicles are often used for more local surveys. "To make sure we won't damage any living ecosystem, we drop an underwater camera to see the nature of the seafloor just around the potential drilling site" explained Gilbert Camoin from CNRS in Aix-en-Provence, who is the chair of EuroMARC's scientific committee as well as the project leader of CHECREEF (The Last Deglacial Sea-Level and Climatic Changes) and investigates coral reefs in both Tahiti and the Great Barrier Reef. Especially in the case of coral reefs, the regulations for drilling are very strict and pictures are taken before and afterwards. "There is no impact at all, when you pull up the pipes, the hole just collapses and it's even impossible to find it again" assured Camoin.

On the main cruise, the coring itself takes place as well as first measurements and part of the sampling, provided the type of ship allows for it. The cores are extracted in different ways, at different water depths and of different lengths, depending on the sediments and the objectives. Short cores of less than 1.5m are often used to drill coral reefs, and if you have high sedimentation rates going back in time you need long cores. Box coring is used for taking surface samples as is the so-called multicores instrument with four short cores, where even the water above the sediment is captured and subsequently the interface analysed. Once a core is on board, its dimensions are measured. Longer cores are cut into segments, then split into two halves, the archive half and the working half, and the first preliminary, non-destructive on-site analyses are made, such as the core description including photos, microbial activity tests and the measurement of the physical properties in a multi-sensor core logger. Then the cores are stored and cooled in containers before undergoing more sampling procedures.

Life on the cruises is rather exceptional: The expeditions are up to two months long and the science is done around the clock - the members of the scientific crew do two four-hour shifts per day. "It's like a small community living and working together, there is a lot of potential to create a good atmosphere from a scientific perspective, but also to find new friends" said Ralph Schneider of Christian-Albrechts-University in Kiel, who is involved in the GLOW (Tropical Temperature History of Global Warming Events) project. Of course, there are also some downsides, as different personalities can clash as well as families and friends need to be left behind for a considerable time. The crew can be as big as a hundred people, comprising of the science crew, technical crew, the crew maintaining the ship - AMOCINT even had five teachers on board who participated in the "Teachers at Sea" program of EGU-IPEV (European Geoscience Union-Institut Paul-Emile Victor).

As not all techniques can be run on board, the sampling is either done in the individual labs or as a joint activity in form of a science sampling party like in Bremen. "Everyone is sampling for everybody. So it's really a collaborative activity with everyone having a different task" explained Camoin. The scientists receive numerous sampling requests from their community and have to divide their labour. If the cores are sampled in different labs, the results are usually shared in order to avoid duplicate sampling.

Marine coring and drilling is a challenging endeavour. Needless to say that a good recovery of the cores is essential - sometimes the sediment is lost when pulling out the core and the so-called core-catcher at the bottom doesn't shut. The prevailing weather conditions are another crucial success factor: storms, for example, make operations nearly impossible. "Depending on the availability of the ship, you might enter into hurricane season and might lose a week of operations" elaborated Schneider. At times, failures of cruises are more man-made and can reach from difficulties in receiving territorial approval before strict deadlines via rescheduling of slots on the coring ships to failed orders of indispensable drilling equipment. All these factors can seriously jeopardise not only the cruise itself, but also entire research projects. Especially newly hired PhD students are highly dependent on the material retrieved from the sea, as they need to obtain data in time.