Deep beneath the southwestern Atlantic, on the Rio Grande Rise, the teeth of ancient sharks have quietly kept a record of how the ocean's metal-rich crusts first began. Within the hair-thin channels of their dentin, researchers have found a sequence of mineral steps, from iron to manganese to the metals a low-carbon future depends on, unfolding in the order that nature once set. It is a reminder that some of the most consequential resources humanity seeks were started in the smallest of spaces, shaped by living things long before anyone thought to look.
Fossil Shark Teeth Reveal How Deep-Sea Critical-Metal Crusts Begin to Form
The first steps in the making of a deep-sea metal crust
Mark
Why start with a shark tooth at all?
Mimi
Because the tooth is a small, closed system. Inside its dentin tubules the researchers could see each mineral layer in order, from goethite to lithiophorite, instead of a finished crust.
Luke
Worth saying that this is a model drawn from fossil teeth from one place, the Rio Grande Rise. The paper doesn't show that every crust starts this way.
Mark
What stands out in the chemistry?
Mimi
The lithiophorite. It carries about 650 micrograms per gram of lithium and about 37,000 of nickel, and its manganese is highly oxidized, around 3.6.
Luke
Those are measurements of specific domains in these samples, not figures for deep-sea crusts overall.
Mark
And the rare earths?
Mimi
They moved into the altered bioapatite, while cerium stayed with the iron-manganese oxides. So different metals end up in different phases.
Luke
One thing is hedged in the paper: the first stage is only described as potentially linked to bioerosion by organisms. That isn't established.
Mark
So what does it point to?
Mimi
That biological material and tiny confined spaces may help start these deposits, and may decide which metals stay put.
Luke
Whether that scales up to whole crusts is still unanswered here.
Le Pouls
- Critical metals essential to a low-carbon economy sit in deep-sea crusts whose origins have remained largely unknown.
- Researchers pried open the microscopic tubules of fossil shark teeth, where the earliest mineral growth appears to be preserved.
- A four-stage sequence unfolds: bioapatite alters, goethite forms, manganese oxidizes into lithiophorite, and metals are redistributed.
- The lithiophorite holds striking concentrations of lithium and nickel, suggesting that tiny confined spaces decide which metals stay and which move on.
- The model is offered as a first step, with the next question being whether this pattern holds for larger crusts and other substrates.
Deep beneath the southwestern Atlantic, on the Rio Grande Rise, the teeth of ancient sharks have quietly kept a record of how the ocean's metal-rich crusts first began. Within the hair-thin channels of their dentin, researchers have found a sequence of mineral steps, from iron to manganese to the metals a low-carbon future depends on, unfolding in the order that nature once set. It is a reminder that some of the most consequential resources humanity seeks were started in the smallest of spaces, shaped by living things long before anyone thought to look.
Inside a fossil shark tooth dredged from the Rio Grande Rise in the southwestern Atlantic, the dentin is laced with tubules so fine they barely register to the eye. Those tiny channels, it turns out, have preserved something rarely glimpsed: the opening chapters of how a deep-sea metal crust is born.
The bigger story concerns the iron-manganese crusts and nodules that coat parts of the seafloor. They hold major reserves of metals considered essential to a low-carbon economy, yet how these deposits first form has been poorly documented. This study set out to close that gap at the smallest scale possible, combining a battery of imaging and chemical techniques with the taxonomic work of researchers at the Federal University of Pernambuco.
What the team found was a four-stage sequence. First, the tooth's natural mineral was altered and its organic matter broke down, possibly through organisms boring into the bone. Next, nanoscale goethite coated the tubule walls. Manganese then oxidized into lithiophorite, which grew over the iron and carried striking amounts of lithium and nickel. Finally, rare earth elements sorted themselves into the altered mineral while cerium stayed with the oxides, and clay filled the remaining gaps.
The authors caution that this is narrow evidence, yet it suggests that living substrates and tiny confined spaces play a real role in deciding which metals are kept and which are released. Whether the pattern holds for other teeth and for the vast crusts themselves is the question the work now leaves open.
Inside a fossil shark tooth recovered from the Rio Grande Rise, in the southwestern Atlantic Ocean, there are channels so small they are called tubules, running through the dentin. A team of researchers has now shown that these channels preserve something rarely seen: the first steps in the making of a deep-sea metal crust.
The larger setting is the ferromanganese crusts and nodules that coat parts of the ocean floor. They are counted among the major reservoirs of metals regarded as essential to a low-carbon economy. Yet the way these deposits first begin to form is poorly documented. The new study goes after that gap at the smallest possible scale.
The team combined a long list of techniques: Micro-CT, SEM-EDS, EPMA, LA-ICP-MS, TEM, Raman spectroscopy and XANES. With them, they reconstructed the early formation of iron-manganese oxides, abbreviated FMO, inside the fossil teeth. The teeth were taxonomically identified by Dr. Otto Fadig and Dr. Alcina Franca of the Federal University of Pernambuco.
What they found is a four-stage sequence preserved within the dentin tubules. First, the tooth's bioapatite, its natural mineral, was altered and the organic matter in it broke down. The authors suggest this may be linked to endolithic bioerosion, meaning organisms boring into the tooth. Second, nanoscale goethite, an iron mineral, precipitated onto the tubule walls. That step was associated with the release of phosphate and with pH buffering.
Third came manganese. It oxidized and precipitated as lithiophorite, a layered Mn(III–IV) oxide, which grew over the goethite. The lithiophorite was notably rich in metals: about 650 micrograms per gram of lithium and about 37,000 micrograms per gram of nickel. The domains containing it showed a high average manganese oxidation state of about 3.6.
The fourth stage involved redistribution. Rare earth elements and yttrium, grouped as REY, moved selectively into the altered bioapatite, while cerium stayed preferentially associated with the iron-manganese oxides. Finally, clay-rich material filled in what space remained.
The authors draw a broad conclusion from this narrow evidence. Biogenic substrates such as teeth, and the confined microenvironments inside dentin tubules, appear to matter in the early nucleation of these oxides. They also appear to shape which metals are redistributed and which are retained, phase by phase. The result is offered as a microscale model of how metals are partitioned at the start of deep-sea FMO mineralization.
The work was carried out largely in Brazil, using facilities including the Brazilian Nanotechnology National Laboratory and the Carnaúba beamline of the Brazilian Synchrotron Light Laboratory, with funding mainly from FAPESP, CNPq, FINEP and CAPES. The paper went through review by an editor, Dr. Peng Du, and four anonymous reviewers. What to watch next is whether this tooth-scale sequence holds for other substrates and for the larger crusts that make these metals a resource question.
Citations marquantes
Biogenic substrates and confined dentin microenvironments are important in early FMO nucleation and phase-specific metal redistribution and retention.— Study authors (paraphrased)
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