Chapter 1: The Beginning of a Radioactive Era

On March 1, 1954, a hydrogen bomb detonated over the Pacific Ocean. John Clark, stationed 20 miles away, ordered his team into a concrete blockhouse on Bikini Atoll. Initially, silence prevailed, leading Clark to wonder if the test had failed. Communication from a Navy ship confirmed otherwise: "It's a good one." Moments later, the ground shook as the shockwave hit them, causing chaos inside the structure. Following a tense wait, Clark decided it was time to go outside.

The sight that greeted him was an enormous mushroom cloud, marking the Castle Bravo explosion, the largest nuclear test conducted at that time. It was intended to assess the first hydrogen bomb that could be air-delivered, with many in Washington believing its success was crucial for global security. Clark, as deputy test director for the Atomic Energy Commission and a seasoned participant in over 40 tests, marveled at the towering cloud. However, his Geiger counter soon began to emit alarming sounds.

"It could mean only one thing," Clark later reflected. "We were already getting fallout." This was unexpected; the team had anticipated that the explosion's radiation would disperse harmlessly into the stratosphere. Instead, the blast was nearly three times more powerful than predicted, producing a yield 1,000 times that of the Hiroshima bomb.

In mere moments, the fireball ejected 10 million tons of radioactive coral debris into the atmosphere. Had Clark's team remained outside, they would have faced lethal radiation exposure. Rushing back inside, they found radiation levels still rising, and rescue efforts were deemed too dangerous for helicopter pilots. The crew, trapped in the sweltering, radioactive blockhouse, anxiously awaited help.

When Navy helicopters eventually arrived, the crew donned makeshift protective gear and made a frantic escape. Meanwhile, debris from Castle Bravo fell across the Pacific, contaminating a Japanese fishing vessel and later reaching neighboring islands. The islanders of Rongelap, believing they were playing in harmless snow, were later evacuated after receiving dangerously high radiation doses. Many suffered severe health consequences, including birth defects and increased cancer rates.

The might of Castle Bravo prompted the Soviet Union to bolster its nuclear arsenal, igniting an arms race that brought humanity to the brink of annihilation. However, the alarming reports of affected fishermen and islanders sparked global protests against nuclear tests. Nine years post-Castle Bravo, a treaty was signed by the United States, Soviet Union, and Great Britain to prohibit aboveground nuclear testing. John Clark lived for another five decades before passing away in 2002 at the age of 98.

Among the isotopes generated by these explosions is carbon-14, a radioactive variant of carbon. The Castle Bravo tests and subsequent detonations released vast amounts of carbon-14 that continue to persist. This isotope found its way into Clark's body, integrating into his blood, fat, and muscle tissue, ultimately leaving a legacy of nuclear testing within him.

The widespread distribution of carbon-14 means that it has become a valuable tool for scientists. Its presence can reveal birth dates, track changes in human physiology, and illuminate the interconnectedness of Earth's biosphere. This artificial spike in carbon-14, known as "the bomb spike," is now fading, but scientists are harnessing it to monitor global warming, another self-inflicted peril facing humanity.

To witness the impact of this radioactive legacy, I traveled to Cape Cod, Massachusetts. Despite being thousands of miles away from Bikini Atoll, the effects of Clark's explosion felt palpable.

At the Woods Hole Oceanographic Institute, I met Mary Gaylord, a senior research assistant who guided me to Maclean Hall's lounge. Outside, dogwoods were in bloom. Next to the coffee station stood a massive tree trunk section, a remnant of a beech tree planted around 1870 by a Boston businessman named Joseph Story Fay. After nearly 150 years, the tree succumbed to disease, prompting a memorial ceremony for the beloved village fixture.

"They had to have a ceremony to say goodbye to it. It was a very sad day," Gaylord recalled. "And I saw an opportunity."

Gaylord specializes in measuring carbon-14 levels. Before nuclear testing, carbon-14 was produced solely by cosmic rays colliding with nitrogen atoms, with just one in a trillion atmospheric carbon atoms being carbon-14. Fay's beech absorbed carbon dioxide from the atmosphere in the same proportions.

When Gaylord learned of the tree's impending removal in 2015, she requested a cross-section. Upon its arrival, she and her students meticulously counted its rings, marking the sections where they extracted wood samples for radiocarbon analysis.

"In the end, we got what I hoped for," she stated. Her aim was to document the nuclear era's history.

For most of its existence, the tree's carbon-14 levels remained stable. However, in 1954, a sharp increase began, corresponding with the fallout from the Castle Bravo test. The new carbon-14 atoms spread globally, and Fay's beech absorbed this radiocarbon during the summer months, incorporating it into its growth rings.

As nuclear tests proliferated, the carbon-14 concentration in the tree's outermost rings nearly doubled in less than a decade. The graph detailing these changes showed a peak in 1964, followed by a steady decline post the Partial Test Ban Treaty in 1963. The radiocarbon levels in the final ring, formed before the tree was cut down, were only slightly elevated compared to pre-Castle Bravo levels. Similar findings have been documented in tree rings across the globe.

In October 2019, Gaylord presented a clear representation of the bomb spike in New England.

When scientists first discovered radiocarbon in 1940, they produced it artificially in a lab. Regular carbon has six protons and six neutrons, but by bombarding carbon with neutrons, Martin Kamen and Sam Ruben created a new form with eight neutrons. This form emitted radiation as it decayed into nitrogen, allowing researchers to estimate its half-life at approximately 5,700 years.

Shortly after, physicist Willard Libby recognized that radiocarbon occurred naturally, generated by cosmic rays colliding with nitrogen in the atmosphere. Plants and algae absorb carbon dioxide, meaning radiocarbon is present in their tissues and in the animals that consume them.

Libby's insight led to the realization that the decay of carbon-14 in organic matter serves as an archaeological clock. It can help determine when organisms lived, revealing significant historical events.

The news of Libby's breakthrough reached New Zealand physicist Athol Rafter, who began utilizing radiocarbon dating on various samples. He soon noticed an alarming trend: carbon-14 levels in the atmosphere were rising, attributable to nuclear tests, which were significantly altering the planet's radiocarbon balance.

In 1957, Rafter published his findings, highlighting that man-made carbon-14 was infiltrating the environment from nuclear test sites. His observations were echoed by scientists worldwide who noted the same increase in carbon-14 levels in diverse ecosystems.

Although bomb radiocarbon posed minimal health risks compared to other nuclear byproducts, it raised concerns about humanity's impact on the environment. Linus Pauling, upon receiving the 1962 Nobel Peace Prize, emphasized the importance of recognizing the implications of carbon-14 for global changes due to nuclear testing.

The Partial Test Ban Treaty signed in 1963 halted aboveground nuclear tests, ceasing the introduction of new bomb radiocarbon. The tests generated approximately 60,000 trillion trillion carbon-14 atoms, an amount that natural cosmic rays would take 250 years to produce. By 1964, Rafter observed a notable decrease in carbon-14 levels following the treaty.

Currently, scientists continue to extract valuable information from bomb radiocarbon. Kristie Boering, an atmospheric chemist at UC Berkeley, expressed mixed feelings about the scientific insights gained from such destructive events. "It’s a huge tragedy that we set off all these bombs. And yet, we get all this interesting scientific information from it," she remarked.

For atmospheric scientists like Boering, bomb radiocarbon serves as a tracer, revealing patterns of atmospheric circulation. Most carbon-14 produced during nuclear tests rose into the stratosphere, and each spring, parcels of this air would descend into the troposphere, carrying carbon-14 with them.

As time passed, bomb radiocarbon made its way to the surface, where it was absorbed by plants, eventually entering the soil and oceans. This isotope has become integral to understanding water movement and ocean chemistry.

Chapter 2: The Ocean's Response to Bomb Radiocarbon

The impact of bomb radiocarbon extended to marine life. In the 1970s, scientists began extensive surveys of ocean chemistry and discovered that bomb radiocarbon permeated the top 1,000 meters of the ocean, albeit less so in deeper layers. This observation helped researchers understand the layered structure of ocean currents.

As bomb radiocarbon settled into the ocean, it influenced marine ecosystems. Researchers found that corals recorded their own version of the bomb spike, albeit with a delay due to the time required for radiocarbon to mix in the ocean. Algae and microbes absorbed carbon from the atmosphere, contributing to a complex food web.

In 2009, a team of researchers investigated deep-sea life in the Mariana Trench and discovered that certain invertebrates contained high levels of bomb radiocarbon, suggesting that they fed on organic material from the surface, despite their deep-sea habitat.

More than six decades have passed since the peak of the bomb spike, yet carbon-14 continues to reveal new insights. At the National Ocean Sciences Accelerator Mass Spectrometry facility (NOSAMS), researchers analyze a variety of samples, including fish eyes, to quantify bomb radiocarbon levels.

During my visit, Gaylord was engaged in analyzing lenses from red snapper, which had been caught in the Gulf of Mexico. By measuring the radiocarbon in these lenses, scientists can determine the age of the fish when caught.

This methodology has produced surprising results, such as the discovery of Greenland sharks with lifespans potentially exceeding 390 years based on the radiocarbon levels in their lenses.

Understanding the age of fish with such precision empowers fisheries managers to monitor populations and ensure sustainable practices. Barnett aims to study the impact of the 2010 Deepwater Horizon oil spill on Gulf fish populations using their lens data.

Humans, like marine organisms, incorporate carbon into our bodies through food. Birth year influences carbon-14 levels in our tissues, with those born during the nuclear testing era displaying higher concentrations. Forensic scientists utilize teeth to determine age based on radiocarbon levels, linking it to historical events.

Even in adulthood, bomb radiocarbon continues to tell our biological stories. Research has shown that fat biopsies can reveal how our bodies manage weight over time, shedding light on the complexities of metabolism and health.

As we confront the environmental challenges posed by fossil fuel consumption, scientists like Heather Graven are studying the decline of bomb radiocarbon as a means to project future climate scenarios. Graven's models indicate that the bomb spike is diminishing faster than expected, potentially linked to human activity.

The ongoing decline of radiocarbon serves as a stark reminder of our environmental footprint. Graven emphasized the importance of recognizing this shift: "We're transitioning from a bomb signal to a fossil-fuel-dilution signal."

Jonathan Weiner poignantly likened fossil fuel consumption to a slow-motion explosion, underscoring the gravity of our actions. The remnants of nuclear testing still echo through our environment, signifying the urgent need for sustainable practices.

Ultimately, bomb radiocarbon has illuminated the global consequences of nuclear weapons. Today, humanity bears the burden of that legacy, as the pulse of carbon-14 transforms into a harbinger of the pressing threat posed by climate change.