Geologists love zircon for its ability to tell time. They’ve also used these robust, tiny time capsules in a variety of studies ranging from estimating when water first appeared on Earth to exploring the origin of plate tectonics.
Scientists led by Chris Spencer, an assistant professor of tectonics and geochemistry at Queen’s University in Kingston, Ont., Canada, combed through data from hundreds of thousands of zircons culled from numerous studies. In a recent Nature Geoscience paper, they compiled only single crystals with three kinds of analyses—the age of the zircon and two additional measurements that serve as proxies for what the melt that birthed each crystal was like.
With this data set, the authors posit that zircons—perhaps known best for recording magmatic and metamorphic events deep within Earth—might chronicle the radiation of plants with roots, leaves, and stems, a development that occurred about 430 million years ago.
Elements and Isotopes
Zircon contains zirconium, silicon, and oxygen. Other elements, like uranium and hafnium, can also sneak into its structure; uranium isotopes are radioactive and decay to lead, providing geochronologists with a way to date nearly every zircon crystal.
Oxygen—part of zircon’s backbone—has only stable, naturally occurring isotopes. Low-temperature surface processes preferentially sort these isotopes, divvying heavy from light. For example, water with light oxygen tends to evaporate first. Water with heavy oxygen will precipitate more readily as rain. And when water interacts with rock, weathering processes partially separate heavy oxygen from its lighter counterparts, explained Brenhin Keller, an assistant professor and geochronologist at Dartmouth who was not involved with this study.
In particular, as rocks erode, they disintegrate into sands and eventually muds made from clays. Clays tend to incorporate more heavy oxygen, explained Annie Bauer, an assistant professor and geochronologist at the University of Wisconsin–Madison who was also not involved in this study. Subducting mud and mixing it into the mantle would result in melt—and likely zircon—featuring heavier oxygen than a melt that incorporates no crustal material or crust that experienced less weathering.
Therefore, oxygen isotopes can be used as a proxy for whether a zircon crystal’s precursor melt contained rocks that spent time at the surface, explained Spencer.
Zircons also contain plenty of hafnium, some of which is produced by the radioactive decay of lutetium. “To a first order, the lutetium-hafnium system will tell you about the source of a magma and therefore also the source of a zircon…crystallizing from that magma,” said Keller.
If the magma contains melt fresh from the mantle, its hafnium signature will look very different from a melt signature containing old crust that’s been recycled via subduction. In Hawaii, for instance, freshly erupted basalts weather into sediments easily identified as being “from magmas that were extracted from the mantle very, very recently,” said Spencer. The hafnium isotope signatures of these sediments will indicate their youth. Sediments in the Amazon River delta, in contrast, come from several-billion-year-old cratons. “The rocks from which those sediments are derived have a very different [hafnium isotope signature] that goes back billions of years,” he explained.
“At first blush…it just looks like shotgun blasts of data,” said Spencer, referring to the relationship between oxygen and hafnium signatures. There is a general lack of correlation for pre-Paleozoic zircons older than about 540 million years, but hafnium signatures do correlate with oxygen isotopes in younger zircons.
Taken together, these data point to zircons coming from a mantle source containing old crust (from hafnium) that was exposed to liquid water (from oxygen), said Keller.
This relationship is surprising, said Bauer, because “there’s no reason to expect hafnium and oxygen to correlate [in zircons].” Sediments incorporated into a mantle melt might contain heavier oxygen, indicating more weathering, but they need not have a distinct hafnium signature because “it’s just random sedimentary material.”
Pinning down just when the two signatures began to correlate took some statistical sleuthing. Nevertheless, Spencer found a shift between 450 million and 430 million years ago that suggests some rapid, irreversible change in zircon chemistry, he said.
Around 430 million years ago, few mountains were being built, said Spencer, which led him to surmise that something else must have caused the peculiar correlation.
Prior to about 450 million years ago, river deposits tended to have very low proportions of mud, whereas after that, muddy river deposits increased. The cause of this shift to muddy rivers, said Spencer, “is the advent of land plants.” Roots, he explained, help hold mud and other sediment on river banks, which in turn helps rivers meander. Therefore, roots control what sediment eventually arrives in subduction zones to be carried down to the mantle, melted, and returned to the surface, perhaps with zircons transcribing the tale.
Just how land plants changed the sediment cycle, however, is still being debated, Keller pointed out. For instance, plants stabilize banks, but they can also increase the extent of weathering. “It’s a reasonable hypothesis that [plants] should maybe do something to the global cycling of sediments,” he said, “and if so, then maybe you can see it in the geochemical record.”
Ultimately, there are only about 5,000 zircons in Spencer’s database, which he described as “paltry” compared to other zircon data repositories that reach into the hundreds of thousands of analyses. The small sample size is a result of few studies obtaining both oxygen and hafnium information from a single zircon, in addition to age.
“The main challenges are always representativeness,” said Keller, “and preservation bias.”
“I anxiously await the time when we have 10,000 [analyses],” said Spencer. “At this moment, this is what we have.”
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer