The Ocean Twilight Zone’s Mysterious ‘False Bottom’



The depths of Earth’s oceans stay, to a big extent, as mysterious because the reaches of outer area. Chilly, darkish and hostile, lower than 10 p.c of their huge space has been explored by know-how, and a fair smaller fraction by human beings themselves.

A lot of what we all know in regards to the sea, then, we all know from deduction. Earlier than the 21st century, for instance, we had no photographic proof of big squid, nor even an intact carcass. However mariners from way back to antiquity surmised their existence by the marks their tentacles left on the flanks of whales.

The story of the deep scattering layer is one other one in all deduction: an anomalous statement — on this case, a sonar glitch — modified the way in which we take into consideration marine ecology eternally.

Sounding the Depths

Sonar works equally to a bat’s pure echolocation. An underwater sound projector produces an everyday sonic pulse, or “ping,” whereas a receiver detects the mirrored sound waves. The amplitude and return time of those echoes point out the scale and relative distance of objects close by.

When sonar turned normal gear on U.S. Navy ships, round 1942, operators had been confounded by an surprising acoustic signature they known as the “phantom backside.” It gave the impression to be a stable airplane some 300 to 500 meters beneath their ships, in what we now name the mesopelagic, or “twilight,” zone. That’s fairly deep — beneath the depth that the majority up to date submarines can journey — however nowhere close to the precise seabed, which sailors had lengthy identified from lead and line soundings to be a minimum of 4,000 meters down in sure locations.

Celebrated creator and marine biologist Rachel Carson wrote in regards to the phenomenon in her best-selling e book The Sea Round Us. In a lecture following the e book’s launch, she stated, “folks at first mistook [these readings] for shoals or sunken islands, and reported submerged land the place none existed.”

The false backside, nevertheless, was a worldwide phenomenon. Sonar operators throughout each ocean reported related findings — indications of one thing substantial, miles large in each course, the place no seafloor ought to be. Extra curious nonetheless, it moved. Throughout daylight, the phantom backside readings remained regular. However as night time fell the ground started to rise, the mirrored sign dispersing in a single day earlier than descending as daybreak approached.

Run Silent, Run Deep

Through the Second World Conflict, the U.S. Navy counted on sonar warnings to guard sailors and submariners from assault. Due to this fact, unraveling the thriller of the phantom backside turned a matter of nationwide safety. A number of explanations had been proposed, together with gear malfunction or some enemy sonar-jamming system, however none bore as much as investigation.

In 1945, Martin Johnson of the Scripps Establishment of Oceanography posited an answer. He argued that this “deep scattering layer,” as he known as it, was composed of residing creatures — trillions of shrimp, jellyfish and bony fish that rose every night time to feed in hotter floor waters earlier than returning to their deeper houses.

Johnson’s subject work, though restricted to 2 days and nights of sampling in a single location, upended typical knowledge.

Marine ecologists describe the ocean as a collection of layers, or zones, every deeper than the final and every with its personal distinctive ecosystem. Waters flow into with the tides and currents, however the plant and animal life in every zone has tailored to a singular salinity, temperature and stress. The deep scattering layer lies within the mesopelagic zone and as Carson famous, “We had at all times assumed that these mid-depths had been a barren, virtually lifeless, Sahara of the ocean. . . . [W]right here there is no such thing as a daylight, no vegetation can stay. So we assumed that meals could be too scarce to assist a really ample animal inhabitants there.”

(Credit score: EreborMountain/Shutterstock)

In Johnson’s mannequin, nevertheless, the mesopelagic zone teemed with life — sufficient to represent a “residing supercontinent” that blanketed the complete ocean and mirrored sound waves. In consequence, our complete understanding of marine ecology shifted and revised estimates of worldwide marine biomass elevated tenfold.

Piercing the Twilight Zone

Within the a long time since Johnson’s preliminary speculation, new applied sciences have made the mesopelagic zone barely extra accessible to scientists. Samples point out that numerous species of lanternfish, starting from two to 10 inches in size, account for almost all of the deep scattering layer (and as much as 65 p.c of the planet’s complete fish biomass). With a worldwide bulk estimated at between 5 and 10 million metric tons, lanternfish are actually believed to be the commonest and broadly distributed number of all marine animals — and possibly of all vertebrates — on Earth.

However there are nonetheless mysteries to be solved. In 2013, marine scientist Kelly Benoit-Chook despatched robotic cameras into the layer’s midst. “We had been restricted in our perspective, wanting down from the floor,” she says. The brand new perspective revealed beforehand unsuspected education patterns: Mesopelagic animals should not randomly intermixed however appear to assemble in tight formations by species. Scientists beforehand assumed the sort of clustering occurred solely within the sunlit shallows, the place it deterred visible predators like dolphins.

“This examine exhibits that sound-scattering layers should not simply unintended aggregations; they’re the results of intentional effort by each particular person animals and teams of animals,” says Benoit-Chook. The reasoning behind these aggregations is one more twilight zone thriller whose reply might finally be introduced out of the deep and into the sunshine.


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