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    A tweaked yeast can make ethanol from cornstalks and a harvest’s other leftovers

    When corn farmers harvest their crop, they often leave the stalks, leaves and spent cobs to rot in the fields. Now, engineers have fashioned a new strain of yeast that can convert this inedible debris into ethanol, a biofuel. If the process can be scaled up, this largely untapped renewable energy source could help reduce reliance on fossil fuels.

    Previous efforts to convert this fibrous material, called corn stover, into fuel met with limited success. Before yeasts can do their job, corn stover must be broken down, but this process often generates by-products that kill yeasts. But by tweaking a gene in common baker’s yeast, researchers have engineered a strain that can defuse those deadly by-products and get on with the job of turning sugar into ethanol.

    The new yeast was able to produce over 100 grams of ethanol for every liter of treated corn stover, an efficiency comparable to the standard process using corn kernels to make the biofuel, the researchers report June 25 in Science Advances.

    “They’ve produced a more resilient yeast,” says Venkatesh Balan, a chemical engineer at the University of Houston not involved in the research. The new strain may benefit biofuel producers trying to harness materials like corn stover, he says.

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    In the United States, most ethanol is made from corn, the country’s largest crop, and is mixed into most of the gasoline sold at gas stations. Corn ethanol is a renewable energy source, but it has limitations. Diverting corn to make ethanol can detract from the food supply, and expanding cropland just to plant corn for biofuel clears natural habitats (SN: 12/21/20). Converting inedible corn stover into ethanol could increase the biofuel supply without having to plant more crops.

    “Corn can’t really displace petroleum as a raw material for fuels,” says metabolic engineer Felix Lam of MIT. “But we have an alternative.”

    Lam and colleagues started with Saccharomyces cerevisiae, or common baker’s yeast. Like sourdough bakers and brewers, biofuel producers already use yeast: It can convert sugars in corn kernels into ethanol (SN: 9/19/17).

    But unlike corn kernels with easy-access sugars, corn stover contains sugars bound in lignocellulose, a plant compound that yeast can’t break down. Applying harsh acids can free these sugars, but the process generates toxic by-products called aldehydes that can kill yeasts.

    But Lam’s team had an idea — convert the aldehydes into something tolerable to yeast. The researchers already knew that by adjusting the chemistry of the yeast’s growing environment, they could improve its tolerance to alcohol, which is also harmful at high concentrations. With that in mind, Lam and colleagues homed in on a yeast gene called GRE2, which helps convert aldehydes into alcohol. The team randomly generated about 20,000 yeast variants, each with a different, genetically modified version of GRE2. Then, the researchers placed the horde of variants inside a flask that also contained toxic aldehydes to see which yeasts would survive.

    Multiple variants survived the gauntlet, but one dominated. With this battle-tested version of GRE2, the researchers found that the modified baker’s yeast could produce ethanol from treated corn stover almost as efficiently as from corn kernels. What’s more, the yeast could generate ethanol from other woody materials, including wheat straw and switchgrass (SN: 1/14/14). “We have a single strain that can accomplish all this,” Lam says.

    This strain resolves a key challenge in fermenting ethanol from fibrous materials like corn stover, Balan says. But “there are many more improvements that will have to happen to make this technology commercially viable,” he adds, such as logistical challenges in harvesting, transporting and storing large volumes of corn stover.

    “There are so many moving parts to this problem,” Lam acknowledges. But he thinks his team’s findings could help kick-start a “renewable pipeline” that harnesses underused, sustainable fuel sources. The vision, he says, is to challenge the reign of fossil fuels. More

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    Focusing on Asian giant hornets distorts the view of invasive species

    Fingers crossed for finding nothing: July marks the main trapping season to check for Asian giant hornets still infesting Washington state.

    The first of these invasive hornets found in North America in 2021, in June, was probably not from a nest made this year, scientists say. So that find doesn’t say how well, or if, the pests might have survived the winter. Yet that hornet shows quite well the relentless risk of newly arriving insects.

    That initial specimen, a “crispy” dead male insect lying on a lawn in Marysville, Wash., belongs to the hefty species Vespa mandarinia. Nicknamed murder hornets, these were detected flying loose in Canada for the first time in 2019 and in the United States in 2020 (SN: 5/29/20). Yet the “dry, crispy” male is not part of known hornet invasions, said entomologist Sven Spichiger at a news conference on June 16.

    Testing shows the male “is definitely not the same genetic line as the ones we have found,” said Spichiger, of the Washington State Department of Agriculture in Olympia. Neither the U.S. finds, until now all from Washington’s Whatcom County, nor British Columbia’s on the other side of the border are closely related to the newfound hornet. It’s a separate incursion no one had noticed until now.

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    This oddball new specimen may help correct the skewed impression that sneaky invasive arrivals are rare. The hornets’ appearance in North America may have been a shock to some, but in reality, worrisome insects show up often, and will probably keep doing so. Fortunately making a permanent home is harder than getting here, scientists say.

    When news of the Asian giant hornets’ arrival first broke in 2019, one of the people who was not at all surprised at a foreign species was entomologist Doug Yanega of the University of California, Riverside. “It is very fair to say that there are many invasive species,” he emphasizes. “We just got a new African mantis species in California this past year in LA, and the expectation is that it is likely to spread.”

    But even alarming pest arrivals rarely kick up the fuss prompted by Asian giant hornets. At a peak in hornet news during May 2020, Yanega contrasted the new intruders with the South American palm weevil (Rhynchophorus palmarum). That big weevil had reached southern California and could “wipe out every palm tree in the state,” according to Yanega. Yet, “there have been ZERO [national] mainstream media reports about this, an insect that seriously threatens to have a VASTLY greater negative impact on the economy and our way of life than those hornets ever will,” he fumed in an e-mail.

    That relentless influx of invading insects may be one reason so few make it into the general news. For instance, U.S. Customs and Border Protection reported 31,785 incidents detecting some pest just for fiscal year 2020.

    For 2021, to pick just one example of worrisome arrivals that have not gone viral, inspectors at Washington Dulles International Airport in Virginia and later at Baltimore/Washington International noticed little brown pests called Khapra beetles (Trogoderma granarium) in Basmati rice and then in dried cow peas that travelers were trying to bring in from abroad. Officials banned the contaminated foodstuffs.

    The Dulles contraband had the bigger number of living insects: 12 larvae and four adults. Even that tiny number of tiny insects was unacceptable. This is the only insect species that U.S. customs officials act upon even when all specimens are found dead. The beetles nibble stored seeds but will also soil the goods with stray body parts and hairs that can make human babies fed dirty grain quite sick and adults uncomfortable. In 1953, a major California effort started to stamp out infestations of Khapra beetles and eventually preserved crop marketability. But the effort was expensive, costing the equivalent of about $90 million in today’s economy.

    Tiny but destructive Khapra beetles (shown, side and front view), which California eliminated at great expense, almost got into the United States at least twice in 2021 in air passenger luggage. Customs stopped those two incursions.Both: Pest and Diseases Image Library, (CC BY-NC 3.0 US)

    Tiny but destructive Khapra beetles (shown, side and front view), which California eliminated at great expense, almost got into the United States at least twice in 2021 in air passenger luggage. Customs stopped those two incursions.Both: Pest and Diseases Image Library, (CC BY-NC 3.0 US)

    Beetles aside, menacing hornets of other species have shown up before the latest Asian giants, says Paul van Westendorp, an apiculture specialist who now strategizes British Columbia’s fight against V. mandarinia. In May 2019, just months before the discovery of an Asian giant hornet’s arrival, a V. soror hornet appeared in Canada. It was “alive, but not for long,” van Westendorp says. “I had a chance to admire that specimen.” Not a frail beast, this species hunts down other insects and has been reported to catch prey as large as a gecko. V. soror looks very much like a V. mandarinia, he says.

    Even Asian giant hornets themselves have turned up at least once in the United States before 2020. An inspector in 2016 flagged a package coming into the San Francisco airport holding a papery insect nest but not mentioning insects on the label. The nest held Asian giant hornet larvae and pupae, some still alive when discovered. These and other species of hornets, including the ominously named V. bellicosa, accounted for about half of the 50 interceptions of hornets and yellow jackets flagged from 2010 to 2018 at U.S. ports of entry, researchers reported in 2020 in Insect Systematics and Diversity.  

    Only some stowaways will manage to make permanent homes in new territory. Of these, the real troublemakers seem to be a minority. For instance, out of 455 plant-attacking insects that settled into forests in the continental United States, 62 cause noticeable damage, according to a 2011 tally from U.S. Forest Service researcher Juliann Aukema and colleagues. Even a few rampaging invasive pests, though, can get expensive. Biologists are throwing themselves into the fight.

    Relentless as the onslaught of unwanted arrivals is, there’s hope for stamping out the more noticeable invasions if caught early. Vespa hornets are “very large-bodied and obvious, so people will see them,” says entomologist Lynn Kimsey of the University of California, Davis, one of the authors of the 2020 hornet overview. A Vespa affinis nest showed up in San Pedro, in Southern California at least a decade ago. However, she says, “it was killed and there’s been no sighting of the species since, as far as I’ve heard.”

    Catching such intrusions early isn’t always easy, however. The port of Oakland takes in about 1 million shipping containers from overseas a year, but at best U.S. Department of Agriculture inspectors can check maybe only 10 percent for stowaway insects, Kimsey says. Add to this all the cargo coming into Long Beach, San Diego and the other West Coast ports — plus all the cargo jets. “What’s amazing is that we don’t see more invasives,” she says. “I think this tells you how hard it is for exotic species to get established.”

    They’ll keep arriving though. All the more reason to keep an eye out for something funny on the lawn, even if it’s just a withered nugget. More

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    3 things to know about the record-smashing heat wave baking the Pacific Northwest

    Like a lid on a steaming pot, a high-pressure system is sitting over the U.S. Pacific Northwest and British Columbia, Canada, sending temperatures in the region soaring to unprecedented heights.

    From a historic perspective, the event is so rare and extreme as to be a once in a millennium heat wave. But one consequence of Earth’s rapidly changing climate is that such extreme events will become much more common in the region in future, says Larry O’Neill, a climate scientist at Oregon State University in Corvallis.

    Temperatures in Portland, Ore., reached 115° Fahrenheit (46° Celsius) on June 29, the highest temperature recorded there since record-keeping began in 1940; average high temperatures for this time of year are about 73° F (23° C). Similar records were notched across the region and more are expected to be set as the high pressure system slowly slides east.

    The heat was so extreme it melted transit power cables for Portland’s cable cars and caused asphalt and concrete roads in western Washington to expand and crack. Such high temperatures are particularly dangerous in a normally cool region little used to or prepared for it, raising the risk of heat-related deaths and other health hazards (SN: 4/3/18). Ground-level ozone levels, for instance, also reached the highest seen yet in 2021, the chemical reactions that form the gas amped up by a potent mix of high heat and strong ultraviolet light.

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    O’Neill talked to Science News about three things to know about the heat wave.

    1. The heat wave is linked to a stalled kink in the jet stream.

    Jet streams, fast-moving currents of air high in the troposphere, encircle both poles, helping to push weather systems around Earth’s surface.  The current isn’t smooth and straight; it can meander and form large swirls, peaks and troughs surrounding zones of high- and low-pressure.

    Occasionally, these weather patterns stall, becoming stationary “blocking events” that keep a particular spate of weather in place for an extended period of time. One such stalled-out high-pressure zone — basically a large dome of hot, dry weather — is now sitting atop the Pacific Northwest.

    The punishing heatwave has an incredible jet stream pattern.The dome of heat will be encircled by the polar jet and this helps lift a sub-tropical jet branch almost into the Canadian Arctic.— Scott Duncan (@ScottDuncanWX) June 25, 2021
    London-based meteorologist Scott Duncan tweets about the unusual heat (top) and the jet stream pattern (bottom) that created that heat dome over the Pacific Northwest. In the jet stream image, hot, dry air (in orange) swirls around and maintains a high-pressure system over the region from June 24 to June 29, locking that hot, dry air in place.

    Historically, similar high-pressure patterns have brought heat waves to the region, O’Neill says. But this one is different. A typical severe heat wave in the past might lead to temperatures of about 100 °F, he says, “not 115 °F.”

    2. Climate change is making the heat wave more severe.

    Baseline temperatures were already higher than in the past, due to Earth’s changing climate. Globally, Earth’s average temperatures are increasing, with 2016 and 2020 tied for the hottest years on record (SN: 1/14/21).

    Those changes are reflected in what’s now officially considered “normal.” In May, for example, the U.S. National Oceanographic and Atmospheric Administration reported that the country’s new baseline reference temperature, or “climate normal,” will be the period from 1991 to 2020 — also now the hottest 30-year period on record for the country (SN: 5/26/21).

    That changing reference makes it tough to place such an unprecedented heat wave in any kind of historical context. “We have a historical data record that’s 100 years long,” O’Neill says. Saying that the heat wave is a once-in-a-millennium event means that “you would expect that, at random chance, this would occur once every 1,000 years. But we’ve never observed this. We have no basis to say this,” he adds. “This is a climate that we’re not accustomed to.”

    3. Climate change is likely to make such extreme events more common in the future.

    A week before the onset of the heat wave, forecasters were predicting such unprecedented temperatures for the region that many people dismissed those predictions as “being ridiculous,” O’Neill says. “Turns out, [the forecasters] were right.”

    Future climate change attribution studies may shed some more light on the ways in which this particular heat wave may be linked to climate change (SN: 7/15/20). Overall, it’s known that climate change is likely to make such extreme events more common in the future, O’Neill says. “We’re seeing these highs form more frequently, and more persistently.” Extreme heat and extreme drought in the U.S. West, for example, can create a reinforcing cycle that exacerbates both (SN: 4/16/20).  

    And that poses many dangers for the planet, not least for human health (SN: 4/3/18). In May, scientists reported in Nature Climate Change that 37 percent of heat-related deaths between 1991 and 2018 were attributable to human-caused climate change.  

    “When we talk about climate change, often the conversation is a little more abstract,” O’Neill says. “We’re experiencing it right now (SN: 11/25/19). And this question about whether we adapt and mitigate — that’s something we have to figure out right now.” More

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    ‘Fathom’ seeks to unravel humpback whales’ soulful songs

    In an opening scene of the new film Fathom, Michelle Fournet sits at her computer in the dark, headphones on. The marine ecologist at Cornell University is listening to a humpback whale song, her fingers bobbing like a conductor’s to each otherworldly croak and whine. Software converts crooning whale sounds into the visual space of craggy valleys and tall peaks, offering a glimpse at a language millions of years in the making.

    Debuting June 25 on Apple TV+, Fathom follows two scientific teams studying the enigmatic songs of humpbacks. The film captivates, diving into the quest to unveil the inner world of these animals and their ever-changing song culture — one considered far older than our ancestors’ first upright steps.

    On opposite sides of the Pacific Ocean, scientists head out onto the water. In a mountain-fringed bay in Alaska, Fournet makes repeated attempts to talk to the whales, playing them a painstakingly reconstructed rendition of a yelp that she thinks may be a greeting. In French Polynesia, behavioral ecologist Ellen Garland of the University of St. Andrews in Scotland listens to humpback songs, mapping how they are tweaked, learned and shared by whales across the South Pacific. These settings are stark and gorgeous, their isolation artfully shown through silent, foggy mornings and endless cobalt seas. In a film fundamentally about oceans filled with sound, ample quiet rests on the surface.

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    Directed by Drew Xanthopoulos, Fathom portrays humpbacks and other whales as complex, highly social beings without overstated anthropomorphism. In one goose bump–inducing scene, Garland’s narration identifies whales’ social similarities to humans, but set in a totally different environment. Perceiving each other chiefly with sound cast over stupefying distances, “whales evolved to build relationships in the dark,” Garland says.

    Fathom also gives an intimate look at what scientists undertake to find humpbacks in the vast ocean. Equipment breaks. Whales prove unpredictable. Strategies must change on the fly. These moments communicate the tough realities of science and the resilience needed for successful research.

    Much of the film is immersed in scenes like these, between troubleshooting and long waits on boat surveys. At times, the film’s pace languishes; connections to greater perspectives, such as the possibility of a globally interlinked song culture, are touched on but not fully examined.

    Nonetheless, Fournet’s simple distillation of her complex quest lingers: “I’m trying to start a conversation.” Her words remind us that Fathom is inherently seated at the threshold of unfathomable territory.

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    Collapse may not always be inevitable for marine ice cliffs

    When it comes to global warming and sea level rise, scientists have made some dire predictions. One of the most calamitous involves the widespread collapse of ice cliffs along the edges of Greenland and Antarctica, which could raise sea level as much as 4 meters by 2200 (SN: 2/6/19). Now, new simulations suggest that massive glaciers flowing into the sea may not be as vulnerable to such collapses as once believed.

    One hypothesis that projected calamitous sea level rise is called the marine ice cliff instability. It suggests that sea-facing bluffs of ice more than 100 meters tall will fail and then slough off to expose fresh ice. Those new cliffs will in turn disintegrate, fall into the sea and float away, setting off a relatively rapid retreat of the glacier that boosts sea level rise.

    Although discussed for years, the phenomenon hasn’t yet been seen in today’s glaciers, says Jeremy Bassis, a glaciologist at the University of Michigan in Ann Arbor. “But that may not be surprising, due to the relatively short record of observations in the field and by satellites,” he says.

    Because of the dearth of field data, Bassis and colleagues decided to use computer simulations to explore ice-cliff behavior. Unlike previous models, the researchers’ simulations considered how ice flows under pressure as well as how it fractures when highly stressed. This blended model is “a pioneering composite,” says Nicholas Golledge, a glaciologist at the Victoria University of Wellington in New Zealand, who wasn’t involved in the study.

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    First, the researchers simulated the collapse of a 135-meter-tall ice cliff on dry land. Over a virtual period of weeks, the face of the cliff shattered and then slumped down to the base, where the icy rubble helped buttress the cliff against further collapse. Researchers have often seen this result in the field, Bassis says.

    Then, the team simulated a 400-meter-tall glacier flowing into water that was 290 meters deep. These dimensions are typical of some of the massive glaciers in Greenland flowing into deep fjords, Bassis says. When the cybercliff collapsed, ice that fell into the water at the cliff’s base floated away, leading to repeated failures and rapid, runaway collapse of the glacier. But adding even a small amount of back pressure at the base of the cliff — as would happen if icebergs got stuck and couldn’t waft away, or if they froze in place — prevented a runaway collapse, Bassis and his team reports in the June 18 Science. “We didn’t expect this to be the case,” Bassis says. “But if small bergs got stuck in the shallows ahead of the ice cliff, it was enough to buttress the [cliff] face,” he says.

    Simulations of an 800-meter-tall glacier flowing into 690 meters of water, comparable to the dimensions of the Thwaites and Pine Island glaciers in Antarctica, yielded similar results. The researchers also found that in relatively warm ambient temperatures, ice flow upstream of the cliff thins the glacier and reduces the height of the cliff, thus reducing the likelihood of runaway collapses.

    The team’s simulations “capture what I think of as realistic behavior,” says Golledge, who coauthored a commentary on the study in the same issue of Science. Future fieldwork may help validate the group’s results. If the simulations hold, Golledge says, the less dire results may mean slower sea level rise in the short term than otherwise predicted.

    Bassis and his colleagues’ analysis “is an important piece of work,” says Ted Scambos, a glaciologist at the University of Colorado Boulder, who was not involved in the study. The results, he says, “provide a balance between the possibilities for extreme runaway collapse and some that are more realistic.” More

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    A new book uses stories from tsunami survivors to decode deadly waves

    TsunamiJames Goff and Walter DudleyOxford Univ., $34.95

    On March 27, 1964, Ted Pederson was helping load oil onto a tanker in Seward, Alaska, when a magnitude 9.2 quake struck. Within seconds, the waterfront began sliding into the bay. As Pederson ran up the dock toward shore, a tsunami lifted the tanker and rafts of debris onto the dock, knocking him unconscious.

    Pederson survived, but more than 100 others in Alaska did not. His story is just one of more than 400 harrowing eyewitness accounts that bring such disasters to life in Tsunami. Written by geologist James Goff and oceanographer Walter Dudley, the book also weaves in accounts from researchers examining the geologic record to shed light on prehistoric tsunamis.

    Chapter by chapter, Goff and Dudley offer readers a primer on tsunamis: Most are caused by undersea earthquakes, but some are triggered by landslides, the sudden collapse of volcanic islands or meteorites hitting the ocean (SN: 3/6/04, p. 152). Readers may be surprised to learn that tsunamis need not occur on the coast: Lake Tahoe (SN: 6/10/00, p. 378) and New Zealand’s Lake Tarawera are just two of many inland locales mentioned that have experienced freshwater tsunamis.

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    Copiously illustrated and peppered with maps, the book takes readers on a world-spanning tour of ancient and recent tsunamis, from a deep-ocean impact off the coast of South America about 2.5 million years ago to numerous tsunamis of the 21st century. The authors’ somber treatment of the Indian Ocean tsunami of December 2004 stands out (SN: 1/8/05, p. 19). Triggered by a magnitude 9.1 earthquake, the megawave killed more than 130,000 people in Indonesia alone.

    The authors — Goff is a professor at the University of New South Wales in Sydney and Dudley is a researcher at the University of Hawaii at Hilo — help readers understand tsunamis’ power via descriptions of the damage they’ve wrought. For instance, the account of a huge wave in Alaska that scoured mature trees from steep slopes along fjords up to a height of 524 meters — about 100 meters taller than the Empire State Building — may leave readers stunned. But it’s the heart-thumping stories of survivors who ran to high ground, clambered up tall trees or clung to debris after washing out to sea that linger with the reader. They remind us of the human cost of living on the shore when great waves strike.

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    Many cosmetics contain hidden, potentially dangerous ‘forever chemicals’

    A new chemical analysis has revealed an ugly truth about beauty products: Many may contain highly persistent, potentially harmful “forever chemicals” called PFAS.

    PFAS, short for per- and polyfluoroalkyl substances, include thousands of chemicals that are so sturdy they can linger in the body for years and the environment for centuries. The health effects of only a few PFAS are well known, but those compounds have been linked to high cholesterol, thyroid diseases and other problems.

    “There is no known good PFAS,” says chemist and physicist Graham Peaslee of the University of Notre Dame in Indiana.

    In the first large screening of cosmetics for PFAS in the United States and Canada, Peaslee and colleagues found that 52 percent of over 200 tested products had high fluorine concentrations, suggesting the presence of PFAS, the researchers report online June 15 in Environmental Science & Technology Letters.

    The potential health risks of PFAS in makeup are not yet clear, Peaslee says. But besides people ingesting or absorbing PFAS when wearing makeup, cosmetics washed down the drain could get into drinking water (SN: 11/25/18).

    Peaslee’s team measured the amount of fluorine, a key component of PFAS, in 231 cosmetics. Sixty-three percent of foundations, 55 percent of lip products and 82 percent of waterproof mascara contained high levels of fluorine — at least 0.384 micrograms of fluorine per square centimeter of product spread on a piece of paper. Long-lasting or waterproof products were especially likely to contain lots of fluorine. That makes sense, since PFAS are water-resistant.

    Twenty-nine products further tested for specific PFAS all contained at least four of these chemicals, but only one product listed PFAS among its ingredients. In addition to posing their own potential health risks, these compounds can break down in the body into other PFAS, such as perfluorooctanoic acid, which has been linked to cancers and low birth weights (SN: 6/4/19).

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    Something mysteriously wiped out about 90 percent of sharks 19 million years ago

    About 19 million years ago, something terrible happened to sharks.

    Fossils gleaned from sediments in the Pacific Ocean reveal a previously unknown and dramatic shark extinction event, during which populations of the predators abruptly dropped by up to 90 percent, researchers report in the June 4 Science. And scientists don’t know what might have caused the die-off.

    “It’s a great mystery,” says Elizabeth Sibert, a paleobiologist and oceanographer at Yale University. “Sharks have been around for 400 million years. They’ve been through hell and back. And yet this event wiped out [up to] 90 percent of them.”

    Sharks suffered losses of 30 to 40 percent in the aftermath of the asteroid strike that killed off all nonbird dinosaurs 66 million years ago (SN: 8/2/18). But after that, sharks enjoyed about 45 million years of peaceful ocean dominance, sailing through even large climate disruptions such as the Paleocene-Eocene Thermal Maximum — an episode about 56 million years ago marked by a sudden spike in global carbon dioxide and soaring temperatures — without much trouble (SN: 5/7/15).

    Now, clues found in the fine red clay sediments beneath two vast regions of Pacific add a new, surprising chapter to sharks’ story.

    Sibert and Leah Rubin, then an undergraduate student at the College of the Atlantic in Bar Harbor, Maine, sifted through fish teeth and shark scales buried in sediment cores collected during previous research expeditions to the North and South Pacific oceans.

    “The project came out of a desire to better understand the natural background variability of these fossils,” Sibert says. Sharks’ bodies are made of mostly cartilage, which doesn’t tend to fossilize. But their skin is covered in tiny scales, or dermal denticles, each about the width of a human hair follicle. These scales make for an excellent record of past shark abundance: Like shark teeth, the scales are made of the mineral bioapatite, which is readily preserved in sediments. “And we will find several hundred more denticles compared to a tooth,” Sibert says.

    Researchers sorted fossil shark scales, or denticles, into two main types: those with linear striations (left) and those with geometric shapes and with no striations (right). Following the shark extinction event 19 million years ago, the geometric denticles all but disappeared from ocean sediments.E.C. Sibert and L.D. Rubin/Science 2021

    The researchers weren’t expecting to see anything particularly startling. From 66 million years ago to about 19 million years ago, the ratio of fish teeth to shark scales in the sediments held steady at about 5 to 1. But abruptly — the team estimates within 100,000 years, and possibly even faster — that ratio dramatically changed, to 100 fish teeth for every 1 shark scale.

    The sudden disappearance of shark scales coincided with a change in the abundances of shark scale shapes, which give some clues to changes in biodiversity. Most modern sharks have linear striations on their scales, which may offer some boost to their swimming efficiency. But some sharks lack these striations; instead, the scales come in a variety of geometric shapes. By analyzing the change in the different shapes’ abundances before and after 19 million years ago, the researchers estimated a loss of shark biodiversity of between 70 and 90 percent. The extinction event was “selective,” says Rubin, now a marine scientist at the State University of New York College of Environmental Science and Forestry in Syracuse. After the event, the geometric scales “were almost gone, and never really showed up again in the diversity that they [previously] did.”

    There’s no obvious climate event that might explain such a massive shark population shift, Sibert says. “Nineteen million years ago is not known as a formative time in Earth’s history.” Solving the mystery of the die-off is at the top of a long list of questions she hopes to answer. Other questions include better understanding how the different denticles might relate to shark lineages, and what impact the sudden loss of so many big predators might have had on other ocean dwellers.

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    It’s a question with modern implications, as paleobiologist Catalina Pimiento of the University of Zurich and paleobiologist Nicholas Pyenson of the Smithsonian National Museum of Natural History in Washington, D.C., write in a commentary in the same issue of Science. In just the last 50 years, shark abundances in the oceans have dramatically declined by more than 70 percent as a result of overfishing and ocean warming. The loss of sharks — and other top marine predators, such as whales — from the oceans has “profound, complex and irreversible ecological consequences,” the researchers write.

    Indeed, one way to view the study is as a cautionary tale about modern conservation’s limits, says marine conservation biologist Catherine Macdonald of the University of Miami, who was not involved with this study. “Our power to act to protect what remains does not include an ability to fully reverse or undo the effects of the massive environmental changes we have already made.”

    Populations of top ocean predators can be important indicators of those changes — and unraveling how the ocean ecosystem responded to their loss in the past could help researchers anticipate what may happen in the near future, Sibert says. “The sharks are trying to tell us something,” she adds, “and I can’t wait to find out what it is.” More