Research challenges models of sea level change during ice-age cycles

<IMG SRC="/Images/114125549.jpg" WIDTH="299" HEIGHT="200" BORDER="0" ALT="Theories about the rates of ice accumulation and melting during the Quaternary Period — the time interval ranging from 2.6 million years ago to the present — may need to be revised, thanks to research findings published by a University of Iowa researcher and his colleagues in the February 12 issue of the journal Science.”>
Theories about the rates of ice accumulation and melting during the Quaternary Period — the time interval ranging from 2.6 million years ago to the present — may need to be revised, thanks to research findings published by a University of Iowa researcher and his colleagues in the February 12 issue of the journal Science.

Theories about the rates of ice accumulation and melting during the Quaternary Period — the time interval ranging from 2.6 million years ago to the present — may need to be revised, thanks to research findings published by a University of Iowa researcher and his colleagues in the 12 February issue of the journal Science.

Jeffrey Dorale, assistant professor of geoscience in the UI College of Liberal Arts and Sciences, writes that global sea level and Earth’s climate are closely linked. Data he and colleagues collected on speleothem encrustations (see photo right), a type of mineral deposit, in coastal caves on the Mediterranean island of Mallorca indicate that sea level was about one meter above present-day levels around 81,000 years ago. The finding challenges other data that indicate sea level was as low as 30 meters — the ice equivalent of four Greenland ice sheets — below present-day levels.

He said the sea level high stand of 81,000 years ago was preceded by rapid ice melting, on the order of 20 meters of sea level change per thousand years and the sea level drop following the high water mark, accompanied by ice formation, was equally rapid.

“Twenty meters per thousand years equates to one meter of sea level change in a 50-year period,” Dorale said. “Today, over one-third of the world’s population lives within 60 miles of the coastline. Many of these areas are low-lying and would be significantly altered — devastated — by a meter of sea level rise. Our findings demonstrate that changes of this magnitude can happen naturally on the timescale of a human lifetime. Sea level change is a very big deal.”

Dorale also noted that although their findings disagree with some sea level estimates, such as those from Barbados and New Guinea that come from ancient coral reefs, they are in agreement with data gathered from other sites such as the Bahamas, the U.S. Atlantic coastal plain, Bermuda, the Cayman Islands and California.

“There has been a long-standing debate on this issue, but our data is pretty robust,” he said. “The key to our research is two-fold. First, the speleothem approach we employed is novel and extremely precise compared to other methods of sea-level reconstruction. Second, Mallorca appears to be particularly well suited to the task, because neither tectonics nor isostasy — geological forces of crustal motion — over-complicate the record. It’s really close to the ideal scenario. It’s also a heck of a nice place to do fieldwork.”

Dorale’s colleagues include Bogdan Onac of the University of South Florida, Tampa; Joan Fornos, Joaquin Gines and Angel Gines, all of the Universitat de les Illes Balears, Mallorca, Spain; Paola Tuccimei of the University of Rome III, Italy; and UI associate professor of geoscience David Peate.

The research was supported by the National Science Foundation in a grant to Dorale and Onac.

New picture of ancient ocean chemistry argues for chemically layered water

Joint UC Riverside, Caltech and Chinese Academy of Sciences field team. Front row, from left to right: J Huang, L. Feng, C. Li, Q. Zhang; the middle row: X. Chu; the back row, from left to right: H. Chang, G. Love, A. Sessions and T. Lyons. -  Chao Li, UC Riverside
Joint UC Riverside, Caltech and Chinese Academy of Sciences field team. Front row, from left to right: J Huang, L. Feng, C. Li, Q. Zhang; the middle row: X. Chu; the back row, from left to right: H. Chang, G. Love, A. Sessions and T. Lyons. – Chao Li, UC Riverside

A research team led by biogeochemists at the University of California, Riverside has developed a detailed and dynamic three-dimensional model of Earth’s early ocean chemistry that can significantly advance our understanding of how early animal life evolved on the planet.

Working on rock samples from the Doushantuo Formation of South China, one of the oldest fossil beds and long viewed by paleontologists to be a window to early animal evolution, the research team is the first to show that Earth’s early ocean chemistry during a large portion of the Ediacaran Period (635-551 million years ago) was far more complex than previously imagined.

Their work is the first comprehensive geochemical study of the Doushantuo Formation to investigate the structure of the ocean going from shallow to deep water environments. It is also one of the most comprehensive studies for any Precambrian interval. (The Precambrian refers to a stretch of time spanning from the inception of the Earth approximately 4.5 billion years ago to about 540 million years ago. It was in the Precambrian when the first single-celled microbes evolved 3.5 billion years ago or earlier, followed by the first multicellular animals much later, around 700 million years ago.)

The researchers’ model for the ancient ocean argues for a stratified marine basin, one with a chemically layered water column. While the surface ocean was oxygen-rich, the deep ocean was ferruginous – oxygen-deprived and iron-dominated. Further, sandwiched in this deep ocean was a dynamic wedge of sulfidic water, highly toxic to animal life, that impinged against the continental shelf.

Dominated by dissolved hydrogen sulfide, the sulfidic wedge was in a state of flux, varying in size and capable of encroaching on previously oxygenated areas of the continental shelf – killing all animal life there. The overall picture is a marine basin with co-existing oxygen-rich, sulfidic and ferruginous water layers.

Study results appear Feb. 11 in Science Express.

In the modern sulfur-rich ocean, hydrogen sulfide in oxygen-poor waters reacts with iron to form the mineral pyrite, thus stripping the dissolved iron from the water column. But the researchers’ results show that under specific geochemical conditions in the early ocean, when levels of dissolved sulfate (the source of hydrogen sulfide in the ocean) and oxygen were particularly low compared to the modern ocean, layers of sulfidic waters could coexist with ferruginous water masses, and even persist for long periods of time.

“This is an entirely new interpretation of ancient ocean chemistry,” said Chao Li, a research specialist in UC Riverside’s Department of Earth Sciences and the first/lead author of the research paper. “Our model provides a brand-new backdrop for the earliest evolution of animal life on the planet. We show that the sulfidic ocean wedge, along with an absence of oxygen, can hinder the colonization of early animals on the shallow seafloor and influence their evolution as they take a foothold. In other words, we cannot ignore hydrogen sulfide when piecing together how animals and other eukaryotes such as algae evolved on our planet.”

The researchers posit that their robust pattern of a stratified marine basin is the best example of a new paradigm in studies of Precambrian ocean chemistry. They predict the record of much of the early ocean elsewhere will show similarities to the complex chemical layering seen in South China.

“This new world order asks that we take into account co-occurring spatial variations in water chemistry in an ocean basin, specifically when moving from near the shallow shoreline along continental shelves to progressively outwards into deeper waters, and when applying a diverse range of complementary geochemical analyses to elucidate these changes in ocean chemistry,” said Gordon Love, an assistant professor of biogeochemistry, who collaborated on the study and in whose lab Li works.

Li explained that in the scientific literature the generally patchy fossil record of early animals observed through the Ediacaran has largely been attributed to poor preservation of fossils. The new research shows, however, that changes in environmental conditions, in this case variations in distribution of hydrogen sulfide, may explain gaps seen in the Ediacaran fossil record.

“Our model points to early animal life having to cope with changing chemical environments even in the shallow waters of the continental shelf,” said Love, the principal investigator on the National Science Foundation (NSF) grant that funded the study. “At times, movement of toxic sulfide-rich waters into the shallow water would be calamitous to animal life. This well explains the patchy time record of animal fossils in most Ediacaran basins.”

Timothy Lyons, a professor of biogeochemistry and a co-principal investigator on the NSF grant, explained that only an incomplete temporal record of animal microfossils has been unearthed in the Doushantuo Formation despite considerable efforts.

“Much of the unequivocal fossil evidence for animals is in the form of microfossil cysts found in only a few sedimentary layers, suggesting that the early animals were environmentally stressed,” he said. “An explanation for this pattern is certain to lie in our model.”

According to the researchers, a stratified marine basin was favored by an overall deficiency of dissolved sulfate in seawater following a long history of oxygen deficiency in the ocean. Ordinarily, sulfate gets introduced into the ocean from the weathering of continental sulfide minerals exposed to an atmosphere with photosynthetically produced oxygen. But the researchers argue that major glaciation events predating Doushantuo time exacerbated the scarcity of sulfate. They note that if glaciation was globally extensive, gas and chemical interactions between the oceans and atmosphere would be suppressed by a layer of ice cover in many areas.

“Ocean chemistry changes as the ice coverage acts like a pie crust sealing off the ocean interior from the atmosphere,” Love said. “The effects of such ice coverage are a reduction of sulfate inputs into the ocean brought in by rivers and a buildup of dissolved iron in the deep ocean sourced by volcanic activity along the mid-ocean ridges. Later, as the ice cover abated, sulfate inputs from rivers localized the animal-inhibiting wedge of hydrogen sulfide along the shallow basin margins.”

Strongest evidence to date shows link between exploration well and Lusi mud volcano

Aeriel view of the Lusi mud volcano crater and the dikes and dams constructed to contain the still-oozing mud.(Courtesy of Channel 9 Australia)
Aeriel view of the Lusi mud volcano crater and the dikes and dams constructed to contain the still-oozing mud.(Courtesy of Channel 9 Australia)

New data provides the strongest evidence to date that the world’s biggest mud volcano, which killed 13 people in 2006 and displaced thirty thousand people in East Java, Indonesia, was not caused by an earthquake, according to an international scientific team that includes researchers from Durham University and the University of California, Berkeley.

Drilling firm Lapindo Brantas has denied that a nearby gas exploration well was the trigger for the volcano, instead blaming an earthquake that occurred 280 kilometers (174 miles) away. They backed up their claims in an article accepted this week for publication in the journal Marine and Petroleum Geology, by lead author Nurrochmat Sawolo, senior drilling adviser for Lapindo Brantas, and colleagues.

In response, a group of scientists from the United Kingdom, United States, Australia and Indonesia led by Richard Davies, director of the Durham Energy Institute, have written a discussion paper in which they refute the main arguments made by Nurrochmat Sawolo and document new data that provides the strongest evidence to date of a link between the well and the volcano. That paper has been accepted for publication in the same journal.

“The disaster was caused by pulling the drill string and drill bit out of the hole while the hole was unstable,” Davies said. “This triggered a very large ‘kick’ in the well, where there is a large influx of water and gas from surrounding rock formations that could not be controlled.

“We found that one of the on-site daily drilling reports states that Lapindo Brantas pumped heavy drilling mud into the well to try to stop the mud volcano. This was partially successful and the eruption of the mud volcano slowed down. The fact that the eruption slowed provides the first conclusive evidence that the bore hole was connected to the volcano at the time of eruption.”

The Lusi volcano, which first erupted on May 29, 2006, in the Porong sub-district of Sidoarjo, close to Indonesia’s second city of Surabaya, East Java, now covers seven square kilometers – nearly three square milesand is 20 meters (65 feet) thick. The mud flow has razed four villages and 25 factories. Thirteen people have died as a result of a rupture in a natural gas pipeline underneath one of the holding dams. The Lusi crater has been oozing enough mud to fill 50 Olympic size swimming pools every day. All efforts to stem the mud flow have failed, including the construction of dams, levees, drainage channels, and even plugging the crater with concrete balls. Lusi may continue to erupt for decades, scientists believe.

Arguments over the causes of the Lusi volcano have stalled the establishment of liability for the disaster and delayed compensation to thousands of people affected by the mud. The Yogyakarta earthquake that occurred at the time of the volcano was cited by some as a possible cause of the eruption, but the research team rejected this explanation.

The Durham University-led group of scientists believe that their analysis resolves the cause beyond all reasonable doubt. According to their discussion paper, ‘The pumping of heavy mud caused a reduction in the rate of flow to the surface. The reason for pumping the mud was to stop the flow by increasing the pressure exerted by the mud column in the well and slowing the rate of flux of fluid from surrounding formations.’

“An earthquake trigger can be ruled out because the earthquake was too small given its distance, and the stresses produced by the earthquake were minutesmaller than those created by tides and weather,” said co-author Michael Manga, professor of earth and planetary science at the University of California, Berkeley.

The group of scientists has identified five critical drilling errors as the causes of the Lusi mud volcano eruption:

  • having a significant open hole section with no protective casing
  • overestimating the pressure the well could tolerate
  • after complete loss of returns, the decision to pull the drill string out of an extremely unstable hole
  • pulling the bit out of the hole while losses were occurring
  • not identifying the kick more rapidly

“This is the clearest evidence uncovered so far that the Lusi mud volcano was triggered by drilling,” Davies said. “We have detailed data collected over two years that show the events that led to the creation of the Lusi volcano.”

“The observation that pumping mud into the hole caused a reduction in eruption rate indicates a direct link between the wellbore and the eruption,” he added. “The decision was made to pull the drill bit out of the hole without verifying that a stable mud column was in place and it was done while severe circulating mud losses were in progress. This procedure caused the kick.”

Supra-glacial lakes focus of study

Image of subglacial lake in the midst of the ice. -  Derrick Lampkin, Penn State
Image of subglacial lake in the midst of the ice. – Derrick Lampkin, Penn State

Rising temperatures on the Greenland ice sheet cause the creation of large surface lakes called supra-glacial lakes. Now a Penn State geographer will investigate why these lakes form and their implications.

NASA awarded Derrick Lampkin, assistant professor of geography, almost $300,000 over three years to look at these lakes.

“Learning where lakes are, how they form, and how that changes through the melt season can help us really understand a lot about important processes that control how the Greenland ice sheet responds to warming,” Lampkin said.

Supra-glacial lakes form when melting water collects in pools in the lower levels of the ice sheet in melt or ablation zones. These lakes drain rapidly through cracks in the ice channeling water to beneath the ice sheet, affecting how ice sheets move and how pieces calve off into the ocean.

Researchers assumed that the influence of basal structure — the structure under the ice at the base — controls where lakes form on the surface, but the magnitude and degree of this influence are not well known, according to Lampkin. It is important to determine how surface processes and basal conditions interact to shape the ice sheet topography.

Lampkin’s work will complement other research by glaciologists at Penn State, such as Richard Alley and Sridhar Anandakrishan, in understanding how ice sheets work and contribute to sea level. He will look at a variety of existing information, including altimeter data, to create surface topography. He will model the temperatures under the ice and, using existing ice-penetrating radar data, create the basal topography. He will also look at ten years worth of high-resolution LandSat images to map lake features.

“This is an exciting time for the study of the world of ice, but unfortunately the public is not always aware of why this type of work is important,” Lampkin said.

In an effort to involve the public in the investigation of ice sheets, Lampkin has proposed an outreach program to create Facebook and iPhone applications that will allow users to map the locations of supra-glacial lakes using high-resolution satellite imagery.

The Facebook and iPhone applications will present users with pre-selected satellite imagery and a tutorial on how to spot the supra-glacial lakes. Lampkin said users who map the locations could receive some sort of incentive through points or rewards for another Facebook game.

According to Lampkin, it is important to track the development of the supra-glacial lakes, because they form and drain quickly. More people mapping these lakes will give researchers more data to learn about them. In addition, if members of the public are able to map the lakes, they might feel they have a personal stake in the study of climate change science.

“The more the public is involved and informed, the more they will understand how climate science is conducted and may be more willing to support these research efforts,” he said. Additionally, participation of this type may be the very spark to encourage a young mind to one day become an ice scientist.

‘Fingerprinting’ method reveals fate of mercury in Arctic snow

A study by University of Michigan researchers offers new insight into what happens to mercury deposited onto Arctic snow from the atmosphere.

The work also provides a new approach to tracking mercury’s movement through Arctic ecosystems.

Mercury is a naturally occurring element, but some 2000 tons of it enter the global environment each year from human-generated sources such as coal-burning power plants, incinerators and chlorine-producing plants.

“When released into the atmosphere in its reduced form, mercury is not very reactive. It can float around in the atmosphere as a gas for a year or more, and it’s not really an environmental problem at the concentrations at which it occurs,” said Joel Blum, the John D. MacArthur Professor of Geological Sciences.

But once mercury is oxidized, through a process that involves sunlight and often the element bromine, it becomes very reactive. Deposited onto land or into water, the mercury is picked up by microorganisms, which convert some of it to methylmercury, a highly toxic form that builds up in fish and the animals that eat them.

As bigger animals eat smaller ones, the methylmercury is concentrated. In wildlife, exposure to methylmercury can interfere with reproduction, growth, development and behavior and may even cause death. Effects on humans include damage to the central nervous system, heart and immune system. The developing brains of young and unborn children are especially vulnerable.

The research is described in a paper published online Feb. 7 in the journal Nature Geoscience.

In the Arctic, mercury remains in its benign gaseous form through the dark winter, because there’s no sunlight to drive oxidation and little bromine to catalyze the process. But in polar springtime, that all changes. As sea ice breaks up, water vapor rises in great clouds through the openings in the ice, bringing with it bromine from the sea water. The bromine enters the atmosphere, where it conspires with sunlight to convert mercury gas into the reactive form. The activated mercury sticks to snowflakes and ice crystals in the air and travels with them onto the surface of the snow.

This leads to what’s known as a mercury depletion event. The normally steady levels of mercury in the atmosphere quickly drop to near zero, as concentrations of mercury on the surface of the snow rise to extremely high levels.

“When we first started observing these events, we didn’t know how much of that mercury returned back to the atmosphere, so the high level of mercury in snow was a great concern,” Blum said. “But the more we learned, the more we realized that the sunlight shining on the snow typically will cause much of the oxidized mercury to become reduced and return to the atmosphere as a gas. And it turns out that its re-release to the atmosphere has a striking “fingerprint’ that we can use to study the progress of this reaction through time.”

The fingerprint is the result of a natural phenomenon called isotopic fractionation, in which different isotopes (atoms with different numbers of neutrons) of mercury react to form new compounds at slightly different rates. In one type of isotopic fractionation, mass-dependent fractionation (MDF), the differing rates depend on the masses of the isotopes. In mass-independent fractionation (MIF), the behavior of the isotopes depends not on their absolute masses but on whether their masses are odd or even.

In the work described in the Nature Geoscience paper, the researchers confirmed, through sample collection and experiments, that MIF occurs during the sunlight-driven reactions in snow, resulting in a characteristic MIF fingerprint that is absent in atmospheric mercury.

“This finding allowed us to use the MIF fingerprint to estimate how much mercury was lost from the snowpack and how much remained behind, with the potential to enter Arctic ecosystems,” said U-M graduate student Laura Sherman, the paper’s first author. “Our experiments showed that a significant portion of mercury deposited to snow was re-emitted. Any mercury that is not re-emitted is likely to retain the unique fingerprint, so we hope future researchers will be able to use our discovery to track mercury through Arctic ecosystems.”

Black carbon a significant factor in melting of Himalayan glaciers

This map of the change in annual linear snow cover from 1990 to 2001 shows a thick band (blue) across the Himalayas with decreases of at least 16 percent while a few smaller patches (red) saw increases. The data was collected by the National Snow and Ice Data Center. -  courtesy of NSCIDC
This map of the change in annual linear snow cover from 1990 to 2001 shows a thick band (blue) across the Himalayas with decreases of at least 16 percent while a few smaller patches (red) saw increases. The data was collected by the National Snow and Ice Data Center. – courtesy of NSCIDC

The fact that glaciers in the Himalayan mountains are thinning is not disputed. However, few researchers have attempted to rigorously examine and quantify the causes. Lawrence Berkeley National Laboratory scientist Surabi Menon set out to isolate the impacts of the most commonly blamed culprit-greenhouse gases, such as carbon dioxide-from other particles in the air that may be causing the melting. Menon and her collaborators found that airborne black carbon aerosols, or soot, from India is a major contributor to the decline in snow and ice cover on the glaciers.

“Our simulations showed greenhouse gases alone are not nearly enough to be responsible for the snow melt,” says Menon, a physicist and staff scientist in Berkeley Lab’s Environmental Energy Technologies Division. “Most of the change in snow and ice cover-about 90 percent-is from aerosols. Black carbon alone contributes at least 30 percent of this sum.”

Menon and her collaborators used two sets of aerosol inventories by Indian researchers to run their simulations; their results were published online in the journal Atmospheric Chemistry and Physics.

The actual contribution of black carbon, emitted largely as a result of burning fossil fuels and biomass, may be even higher than 30 percent because the inventories report less black carbon than what has been measured by observations at several stations in India. (However, these observations are too incomplete to be used in climate models.) “We may be underestimating the amount of black carbon by as much as a factor of four,” she says.

The findings are significant because they point to a simple way to make a swift impact on the snow melt. “Carbon dioxide stays in the atmosphere for 100 years, but black carbon doesn’t stay in the atmosphere for more than a few weeks, so the effects of controlling black carbon are much faster,” Menon says. “If you control black carbon now, you’re going to see an immediate effect.”

The Himalayan glaciers are often referred to as the third polar ice cap because of the large amount of ice mass they hold. The glacial melt feeds rivers in China and throughout the Indian subcontinent and provide fresh water to more than one billion people.

Atmospheric aerosols are tiny particles containing nitrates, sulfates, carbon and other matter, and can influence the climate. Unlike other aerosols, black carbon absorbs sunlight, similar to greenhouse gases. But unlike greenhouse gases, black carbon does not heat up the surface; it warms only the atmosphere.

This warming is one of two ways in which black carbon melts snow and ice. The second effect results from the deposition of the black carbon on a white surface, which produces an albedo effect that accelerates melting. Put another way, dirty snow absorbs far more sunlight-and gets warmer faster-than pure white snow.

Previous studies have shown that black carbon can have a powerful effect on local atmospheric temperature. “Black carbon can be very strong,” Menon says. “A small amount of black carbon tends to be more potent than the same mass of sulfate or other aerosols.”

Black carbon, which is caused by incomplete combustion, is especially prevalent in India and China; satellite images clearly show that its levels there have climbed dramatically in the last few decades. The main reason for the increase is the accelerated economic activity in India and China over the last 20 years; top sources of black carbon include shipping, vehicle emissions, coal burning and inefficient stoves. According to Menon’s data, black carbon emitted in India increased by 46 percent from 1990 to 2000 and by another 51 percent from 2000 to 2010.

However, black carbon’s effect on snow is not linear. Menon’s simulations show that snow and ice cover over the Himalayas declined an average of about one percent from 1990 to 2000 due to aerosols that originated from India. Her study did not include particles that may have originated from China, also known to be a large source of black carbon. (See “Black soot and the survival of the Tibetan glaciers,” by James Hansen, et al., published last year in the Proceedings of the National Academy of Sciences.) Also the figure is an average for the entire region, which saw increases and decreases in snow cover. As seen in the figure, while a large swath of the Himalayas saw snow cover decrease by at least 16 percent over this period, as reported by the National Snow and Ice Data Center, a few smaller patches saw increases.

Menon’s study also found that black carbon affects precipitation and is a major factor in triggering extreme weather in eastern India and Bangladesh, where cyclones, hurricanes and flooding are common. It also contributes to the decrease in rainfall over central India. Because black carbon heats the atmosphere, it changes the local heating profile, which increases convection, one of the primary causes of precipitation. While this results in more intense rainfall in some regions, it leads to less in other regions. The pattern is very similar to a study Menon led in 2002, which found that black carbon led to droughts in northern China and extreme floods in southern China.

“The black carbon from India is contributing to the melting of the glaciers, it’s contributing to extreme precipitation, and if black carbon can be controlled more easily than greenhouse gases like CO2, then it makes sense for India to regulate black carbon emissions,” says Menon.

Carbonate veins reveal chemistry of ancient seawater

Calcium carbonate veins are common in upper ocean crust, where they precipitate from low temperature ( -  Christopher Smith-Duque (NOCS)
Calcium carbonate veins are common in upper ocean crust, where they precipitate from low temperature ( – Christopher Smith-Duque (NOCS)

The chemical composition of our oceans is not constant but has varied significantly over geological time. In a study published this week in Science, researchers describe a novel method for reconstructing past ocean chemistry using calcium carbonate veins that precipitate from seawater-derived fluids in rocks beneath the seafloor. The research was led by scientists from the University of Southampton’s School of Ocean and Earth Science (SOES) hosted at the National Oceanography Centre, Southampton (NOCS).

“Records of ancient seawater chemistry allow us to unravel past changes in climate, plate tectonics and evolution of life in the oceans. These processes affect ocean chemistry and have shaped our planet over millions of years,” said Dr Rosalind Coggon, formerly of NOCS now at Imperial College London.

“Reconstructing past ocean chemistry remains a major challenge for Earth scientists, but small calcium carbonate veins formed from warm seawater when it reacts with basalts from the oceanic crust provide a unique opportunity to develop such records,” added co-author Professor Damon Teagle from SOES.

Calcium carbonate veins record the chemical evolution of seawater as it flows through the ocean crust and reacts with the rock. The composition of past seawater can therefore be determined from suites of calcium carbonate veins that precipitated millions of years ago in ancient ocean crust.

The researchers reconstructed records of the ratios of strontium to calcium (Sr/Ca) and magnesium to calcium (Mg/Ca) over the last 170 million years. To do this, they analysed calcium carbonate veins from basaltic rocks recovered by several decades of scientific deep-ocean drilling by the Integrated Ocean Drilling Program (IODP) and its predecessors.

“The carbonate veins indicate that both the Sr/Ca and Mg/Ca ratios of seawater were significantly lower than at present prior to about 25 million years ago. We attribute the increases in seawater Sr/Ca and Mg/Ca since then to the long-term effects of decreased seafloor volcanism and the consequent reduction in chemical exchange between seawater and the ocean crust,” said Professor Teagle.