The Gulf of Cadiz has always been an odd body of water. It serves as a sort of underwater intersection between the Moroccan coast and the Iberian Peninsula, where the cooler Atlantic meets the warm, salty Mediterranean outflow and vanishes downward in slow, looping plumes that are invisible to satellites. It has been studied for decades by oceanographers. For the most part, they used the traditional method of studying it, lowering probes from research vessels one slow descent at a time.
Therefore, there was no immediate excitement when a team processing eight seismic reflection sections—which had been shot in the summer of 2001 for energy exploration—started to notice faint reflections inside the water column itself. It resembled puzzled rereading more. The purpose of the data collection was to examine what was beneath the seafloor. The water above was meant to be background noise, which is what you filter out before the actual work starts. As it happened, the noise had a backstory.
Years later, the 869 kilometers of seismic line were meticulously reprocessed to uncover oceanic structures that had only been suggested by conventional sampling. Mediterranean water intrusions in layers. sloping surfaces where salinity and temperature changed suddenly enough to reflect sound back to the surface. Tens of kilometers of vertically stacked features that were impossible to map with a single CTD cast. Hydrocarbons were the target of the initial crews. Almost by accident, they had captured what appeared to be an X-ray of the ocean.
That has a slight irony. Physical oceanographers have long bemoaned the fact that the deep ocean below 2,000 meters is one of the planet’s least observed environments, and they have good reason. At sub-mesoscale resolution, less than 2% of the world’s ocean is continuously sampled. Nevertheless, the petroleum industry has covered the continental margins of the world with seismic surveys for more than 40 years; the majority of these surveys are either forgotten or kept in archives as proprietary. The mismatch is difficult to ignore.
Now available to the public, the Gulf of Cadiz data adds useful complexity to the earlier picture. In the seismic sections, Mediterranean outflow water, which is frequently depicted in textbooks as two neat lobes drifting westward, appears much messier. Double-diffusive staircases, mixing layers, eddies, and intrusions. These photos give the impression that the ocean is doing more and at smaller scales than the conventional interpolated maps indicate. The work may have been funded by offshore energy investors. Its true beneficiaries might be climate modelers.
To put it briefly, seismologists were surprised to discover that the signal they had discarded was actually someone else’s missing data. Oceanographic evidence is now being interpreted from the acoustic reflections they once suppressed. For a geologist mapping a Cretaceous fault, a reflection resulting from a half-degree change in temperature at a depth of 1,500 meters is essentially meaningless. It can mean a lot to a physical oceanographer who is attempting to validate a deep-water mixing model.
The extent to which the global seismic archive will eventually be reopened in this manner is still unknown. A large portion of it is protected by business agreements. A portion of it underwent processing that completely eliminated the water column. However, the Cadiz dataset points to a noteworthy precedent. Reexamining data that was previously deemed uninteresting can sometimes yield the most intriguing results. It appears that the ocean was waiting for someone to have a different perspective.
