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By Richard Hobbs, OSCAR PI

Most scientists that study the ocean crust will have heard of the 504B. I heard about the site during my PhD when on a 6-month visit to Lamont-Doherty Earth Observatory, as it is now called, a research institute just north of New York in the United States. The drilling at site 504B started during my first Post-doctoral post as part of the British Institutions Reflection Profiling Syndicate in Cambridge, and here I am at the twilight of my career doing a geophysical survey over site 504B. 

So what is 504B? 504B is one of the deepest borehole ever drilled into ocean-crust and it is situated in the Panama Basin where we are working. The hole was started in 1988 as part of Leg-111 of the Ocean Drilling Project (ODP), an internationally funded project to explore the deep ocean by direct sampling. Before ODP, and its forerunner DSDP, most people that studied ocean crust worked on ophiolites – deformed fragments of ocean crust that have been pushed up to the surface by tectonic forces. The drill ship, JOIDES Resolution, was capable of sampling rocks under the oceans in water depths of over 8000 m so it was now possible to sample ocean rocks in-situ and answer first-order questions about their age, composition, and properties of the ocean crust. One of the holes, 504B, was selected as the site to attempt to drill to the level at which the ocean-crust had originally formed at the axial magma chamber. The site is 230 km south of the present-day ridge axis and drilled into crust that is 5.9 Ma (million years old). The hole reached a depth of 2111 m below the ocean floor over a series of expeditions from 1986, with two in 1991 and the last in 1993.

Cartoon of well 504B, showing the crustal structure (after Carlson 2011).

So why is 504B important to the OSCAR project? From a geophysical perspective, I use remote sensing to learn about the earth on which we live. Some of these methods have been explained in previous posts. Though I can map the physical properties to the Earth, I need calibration points like 504B so I my measurements can be directly related to actual rock. Also, this hole provides important evidence of alteration of the basalt rocks by the passage of high-temperature hydrothermal fluids shortly after its formation some 5.9 million years ago. In fact there is still some residual water circulation in the cracks and fractures at this present-day even though the ocean crust is now sealed from the ocean by over 250 m of sediment that acts like a lid.

Determining the thickness of this sediment ‘lid’ is a major objective for the OSCAR project as this  controls the type of heatflow from the crust into the ocean. The best method to remotely map the thickness of layers of sedimentary rock is seismic reflection. An acoustic pulse is generated by a sound source behind the vessel. When the pulse reaches the sea-bed a fraction of the energy is reflected back to the surface, the rest continues to propagate deeper into the Earth and at every deeper change in the sediment generates another reflected echo. At the surface we tow an array of highly sensitive hydrophone detectors that record the returning echoes which we can turn in to pictures that give a cross-section through the sediments.

Seismic reflection section of the crust at hole 504B, showing the sediment layers in high resolution overlaying the ocean crust