It is a question seemingly more appropriate for a private eye than for a sedimentologist: How do you find a severed head?
The case is complicated. No one has ever seen the head, its presumed location is thousands of kilometers from its torso, and it may lie beneath a kilometer of sand and mud. And the burial ground has been narrowed down to within a million square kilometers.
But this head isn’t a bloody remnant of a crime scene, and the examiners aren’t crime scene investigators.
Instead, scientists are searching for the “head of the Himalayas,” the Himalayas being the mountain range that with 10 peaks soaring more than 8,000 meters above sea level, more than earns its nickname as part of the “rooftop of the world.”
The missing head, in this case, describes the very first rocks to erode from the Himalayas, themselves uplifted as the Indo-Australian plate collided with and began subducting beneath the Eurasian plate in the early Cenozoic.
Those rocks washed out to sea and, so the thinking goes, ought to be buried somewhere in the Bay of Bengal. Why the Bay of Bengal? It turns out that river transport hasn’t changed too radically over the course of millions of years. Today, the Ganges and Brahmaputra Rivers transport about a billion tons of eroded Himalayan sediment into the Bay of Bengal every year. The sediment forms and reforms the Bengal Fan, the largest submarine fan on the planet.
“No other deep-sea fan receives so much sediment than the Bay of Bengal,” said Hermann Kudrass, a marine sedimentologist at the University of Bremen in Germany.
To unearth a complete sedimentary record of the fan and to find the long-lost head of the Himalayas, a team of scientists joined Expedition 354 on the R/V JOIDES Resolution and set off from Singapore in February 2015.
One of the 33 scientists on Expedition 354 was Yani Najman, a sedimentologist at Lancaster University in the United Kingdom. Najman searches for sediments that can reveal the stories behind the formation of geographic features like mountains and rivers. In 2018, for instance, she and a team of researchers calculated the ages of sediments in the Nile River delta and were able to report that the Nile is at least 31 million years old.
If Najman and other scientists on Expedition 354 were to find the first sediments to erode from the Himalayas, the results could help answer questions about the evolution of a major part of Earth’s climate: the South Asian monsoon. Each summer, the monsoon wind releases between 1 and 5 meters of rain over South Asia.
Kudrass, who was not a part of the expedition, explained that the Himalayas block the movement of moist air that travels over land from the Indian Ocean. This forces the air to rise and then drop its watery load as the monsoon, weathering and eroding the Himalayas in the process.
“But how is that linked to the uplift of the Himalayas?” he asked.
That is the mystery that finding the head of the Himalayas could help crack.
Najman offers an example. “Say we’ve got an increase in sedimentation rates,” she said. “Then you could say that there was an increase in rainfall at the time.”
“The only way to really estimate this on the long term is to look at the sedimentary record of erosion of the Himalayas that is lying in the oceanic basin,” said Christian France-Lanord, a sedimentologist at the Centre de Recherches Pétrographiques et Géochimiques in Vandœuvre-lès-Nancy, France, and co–chief scientist on Expedition 354.
And so Expedition 354 went head-hunting.
Expedition 354 was not the JOIDES Resolution’s first stab at the Bengal Fan. Another expedition, which sailed in 1987, drilled into the fan and recovered sediments that were between 18 million and 20 million years old. That’s far too young to be the earliest fan sediments, which Najman expected to be Eocene in age.
The JOIDES Resolution’s drill can reach depths of more than 8 kilometers, but it can’t reach the base of the thickest parts of the fan, which can be up to 16.5 kilometers deep. To work around this, the team sailed an east–west transect across a series of ocean ridges. The ancient volcanic ridges formed millions of years before the Bengal Fan did, so the fan sediment that sits atop them should contain a complete record of the fan’s sedimentary layers. The ridges rise high enough off the seafloor for the JOIDES Resolution’s drill to reach them.
On 17 February, the JOIDES Resolution cruised to a spot over the western flank of the Ninety East Ridge.
“‘If we don’t reach the Eocene, I’m gonna be so pissed off,’” Najman recalled France-Lanord saying.
The drill snaked down from the derrick, dove more than 3 kilometers into the watery darkness via the ship’s “moon pool” (a gaping hole in the ship’s hull), and finally plunged into the fan itself. The earliest sediments, explained France-Lanord, ought to be between 1,000 and 1,300 meters below the seafloor.
Before reaching 900 meters, though, the drill got stuck. Then loose sand started to fill the drill pipe, jamming the drill.
Finally, the drill went onward and downward until, at last, it reached the base of the fan at a depth of about 1,100 meters.
When the sediment core came in, Najman and other researchers got busy working 12-hour shifts in the ship’s lab.
“It was like an assembly line,” she said.
There were no complaints from Najman, though: “I love the Himalayas on so many levels, and I want to know as much about them as possible.”
Meanwhile, the paleontologists of Expedition 354 were busy deciphering the age of the Ninety East core by looking for microscopic marine fossils that characterize different geological periods.
“The guys doing the dating were still not saying Eocene,” Najman said.
As she listened, it became obvious that the team was not going to find Eocene sediments.
“It was the saddest day on the ship for me,” she said.
The Mystery Deepens?
Questions abounded. Where was the missing head of the Himalayas? Where could those earliest sediments be? Why weren’t they on the flanks of the Ninety East Ridge?
According to France-Lanord, one reason that the earliest Himalayan sediments might not be on Ninety East is because it took time for fan sedimentation to reach such places—sediments had to fill lower-elevation areas first. The ridge’s core “was too high with respect to fan flow to be covered with turbidic sediments,” he said.
The sediments, however, almost certainly do exist, Najman said. But to get to these deep fan sediments, scientists will need “another boat that drills much deeper,” she said. “That would be the dream horizon for another time.”
Expedition 354 ended without a head to mount on a wall, but the voyage was far from a failure. In fact, the team made history by becoming the first expedition to reach the base of the Bengal Fan. It also retrieved a nearly complete record of fan sediment from the late Oligocene onward.
“That’s the longest and oldest record so far,” Najman said.
To boot, the sediment they retrieved is well preserved, France-Lanord explained. The cores contain, among other things, plant remains and geochemical fingerprints that will allow for the reconstruction of past plant communities and provide insights into how the world’s largest submarine fan evolved over time.
“This was not possible with earlier drilling of the fan,” he said.
The cores are also illuminating other shadowy parts of the Himalayas’ history. Najman and others did research, published in March in the Geological Society of America Bulletin, examining sand grains unearthed from the expedition’s sediment cores.
Their work may help answer a long-standing mystery about Himalayan tectonics: Can a river cause the mountains to rise faster than they already do?
Previous work suggests that the Siang River (a section of the Brahmaputra) might have eroded the mountains to the point where all the missing rock caused some regions to rise faster than the rest of the range. The phenomenon is “kind of like a ‘tectonic aneurysm,’ [as] it’s been called,” Najman said. “Incision by that river into the crust weakened the crust, allowing for the rapid exhumation.”
But the new research indicates that the rapid rise happened much later than river incision did. “[The researchers’] finding of exhumation happening 3 million years ago is inconsistent with the model in the tectonic aneurysm paper that proposes that exhumation started 10 million years ago and was driven by river capture,” said Devon Orme, a tectonicist at Montana State University who was not involved in the work. “What this showed is when that fast exhumation started—that’s the big contribution.”
Meanwhile, the head of the Himalayas—and the secrets it may yet reveal—is still buried somewhere beneath the waves of the Bay of Bengal, awaiting its own exhumation.
—Lucas Joel ([email protected]), Freelance Journalist