Tracking Carbon’s Underwater Dive

Art Courtesy of Luna Aguilar.

Carbon: it’s in the atmosphere, the oceans, the solid Earth, and even in us. This element is found all throughout Earth’s environment and exists in numerous chemical forms. One form of carbon, calcium carbonate (the major component of limestone and chalk), is the main form in which carbon in the ocean and atmosphere is returned to the earth. The precipitation pathways of carbonate minerals in the ocean are referred to as the marine carbonate factory and greatly influences abiotic and biotic geochemistry, including ocean acidity. 

Despite its important role in ocean chemistry, there has been little consensus on how the marine carbonate factory has changed throughout Earth’s history. It is challenging to find well-preserved and reliable markers for its status over billions of years. However, Lidya Tarhan, an assistant professor of earth and planetary sciences, and Jiyuan Wang, an Agouron postdoctoral fellow in Tarhan’s research group, along with colleagues from Yale’s Department of Earth and Planetary Sciences, Northwestern University and University of Miami, recently developed a novel technique to trace the history of the marine carbonate factory. By measuring the ratios of different isotopes (atoms of the same element with different masses) of strontium within carbonate samples, they uncovered a more definitive history of carbonate precipitation and mineral saturation states from the ancient Precambrian to the recent Phanerozoic Eon, and published their findings in Nature. During the Precambrian, contrary to previous conjecture, a major proportion of carbonate was buried in the deep sea through abiotic processes, rather than in the shallow marine environment like reefs that dominate most of the Phanerozoic marine carbonate factory.

The Marine Carbonate Cycle

The marine carbonate factory is one piece of a bigger puzzle, the carbon cycle, which encompasses the cycling of carbon through nature. The carbon cycle can be divided into short-term and long-term cycles. The short-term carbon cycle involves living organisms. Photosynthesizers, largely phytoplankton and algae, absorb carbon dioxide into their cells and use it to generate carbohydrates through photosynthesis. The carbohydrates can be broken down later to provide energy, releasing carbon dioxide back into the ocean and atmosphere.

In the long-term carbon cycle, rain combines with atmospheric carbon dioxide to form carbonic acid, which falls to the earth and slowly dissolves bodies of rock. This releases ions, including calcium, that are then carried through rivers to the ocean, where they combine with dissolved carbonate ions to form calcium carbonate, a solid in water. This process is called carbonate precipitation and can occur abiotically (without living organisms).

However, many marine organisms also use calcium and carbonate ions to build their shells. When they die, these shells and other sediments form rock, sequestering carbon within the ocean floor and the geologic record. Some bacteria can also facilitate precipitation of calcium carbonate in their surroundings—for instance, by taking up carbon dioxide during photosynthesis. Regardless of the route, precipitation of dissolved carbonate into solid calcium carbonate incorporates other ions including strontium, an alkaline earth metal. This study exploited the incorporation of strontium into carbonate precipitates to gain insight into the history of the marine carbonate cycle.

Strontium in the Marine Carbonate Factory’s History

The metal strontium has multiple stable isotopes, including strontium-88 and strontium-86. When carbonate precipitation occurs in the ocean, it incorporates trace amounts of strontium, but the two isotopes are incorporated at different ratios depending on environmental conditions. Strontium-88, the heavier isotope, is incorporated into carbonate minerals less often than the lighter strontium-86, but higher saturation of carbonates in the ocean leads to both faster precipitation and an even lower strontium-88 to strontium-86 ratio (δ88/86Sr) in carbonates. The ratio of these strontium isotopes in carbonates, in particular, is a novel proxy for the history of the marine carbonate cycle because this signature is tied closely to precipitation rates, rather than—like many other isotope systems—seawater temperature or the type of carbonate mineral in which strontium is incorporated. Thus, strontium stable isotope ratios can serve as a marker for changes in the marine carbonate factory, specifically the rate of carbonate precipitation which, in turn, reflects marine carbonate saturation state at the time of precipitation. 

However, using stable isotope measurements has not previously been possible due to technical limitations. Luckily, with the resources of the Yale Metal Geochemistry Center, the researchers were able to develop a novel method using mass spectrometry to measure stable strontium isotope ratios with five-decimal place precision, according to Wang. “[Our work] opened the door toward using the stable strontium isotope proxy to reconstruct long-term records in Earth’s history—and will, we hope, allow us and others to tackle new and exciting questions down the road,” Tarhan said.

Unexpected Sources of Carbonate Precipitation

While shallow regions of the ocean like reefs and carbonate platforms are commonly thought of as hotspots for carbon deposition, which is true today, Tarhan and Wang found an unexpected phenomenon during the Precambrian, the interval preceding the explosion of organisms that created calcium carbonate shells around 540 million years ago. They found evidence of an unforeseen level of abiotic contribution to the marine carbonate cycle in deep ocean waters during this time. “A major part of carbonate was buried outside of the shallow marine environment during Earth’s early history, which is radically different than previously envisaged,” Wang said.

When tracing the ratios of strontium isotopes in carbonate samples formed in shallow seafloor sediments, the researchers found a marked decrease in δ88/86Sr values during the transition between the Precambrian era and the Phanerozoic era (the interval in which biomineralizing animals caused skeletal carbonates to become a large contributor to the marine carbonate factory). This decrease implies slower precipitation and a lower carbonate saturation state in Phanerozoic relative to Precambrian oceans, which is consistent with the increase in precipitation due to calcifying animals in the Phanerozoic period. 

Additionally, the Precambrian δ88/86Sr record also suggests the unexpected presence of a large, non-skeletal carbonate sink within the Precambrian deep ocean—one formed as a result of the activities of bacteria living in seafloor sediments devoid of oxygen. This condition was likely characteristic of much of the Precambrian deep seafloor, leading to carbonate precipitation in the spaces between grains of previously deposited sediment.

It is difficult to directly confirm the ancient history of the deep oceans, as the constant shifting of Earth’s tectonic plates leads to subduction, plates layering over each other on the  deep seafloor, which would have contained this missing piece of the Precambrian carbonate record. “The deep oceans have been something of a ‘black box’ for much of Earth’s history, due to the continual loss of deep-sea sediments with seafloor subduction,” Tarhan said. However, this study has unveiled an aspect of this murky history. “The deep sea, among other muddy stretches of the ancient seafloor, may have been an important locus of carbonate sediment accumulation prior to the emergence of any carbonate-biomineralizing organisms, and this was the baseline state until biomineralizing animals evolved in the shallow oceans approximately 540 million years ago,” Tarhan said.

Looking to the Future

This new research challenges past assumptions about the main players in the marine carbonate factory and helps fill in the history of oceanic geochemistry prior to the evolution of marine calcifying animals. This research could also be used to predict the future. As humans continue to pump carbon dioxide into the atmosphere, the increased emissions alter the natural carbon cycle. While this study focused on Earth’s ancient history, the mechanisms underlying the carbon cycle remain the same, so clarifying the mechanism of past changes can help us extrapolate how human actions may now affect the carbon cycle. 
Tarhan’s lab is currently trying to quantify the behavior of strontium isotopes as it relates to changing seawater carbonate chemistry under ongoing climate change. “The discoveries in this study enable us to better understand how carbon cycled among Earth’s different layers and how Earth maintained its habitability under different levels of CO2,” Wang said. “This is crucial information that can be used to perceive and predict the behaviors of our Earth and oceans [during] contemporary climate change.”