Ocean sediment reveals that release of carbon stored deep in the sea is linked to the rise in atmospheric CO2 that caused the world to warm

by Tim Radford, Climate News Network, February 13, 2015

LONDON − Scientists believe they may have cracked the mystery of the end of the last ice age. The temperatures suddenly soared, and the glaciers went into retreat, because the deep southern ocean released huge quantities of carbon dioxide.

And the convincing answers have been delivered by analysis of the composition of calcium carbonate shells of ancient marine organisms.

The link between human burning of fossil fuels and the steady rise in atmospheric carbon dioxide levels was proposed more than a century ago and firmly established in the last 30 years.

But the ups and downs of planetary temperatures before the emergence of human civilisation are harder to explain. Fossil evidence suggests a link with carbon dioxide levels, but not necessarily a cause.

Bygone climates

Now paleoceanographer Miguel Martínez-Botí, from the University of Southampton, UK, and ocean and climate change researcher Gianluca Marino, from the Australian National University, report in Nature that they found their evidence in sediment cores – in effect, annual records of bygone climates – rich in the shells of tiny foraminifera called Globigerina bulloides.

This is a species that flourishes in conditions of high nutrients, acting as a kind of biological pump, gulping carbon from the atmosphere.

They found that high concentrations of carbon dioxide dissolved in surface waters of the southern Atlantic Ocean and the eastern equatorial Pacific coincided with rises in atmospheric CO2 at the end of the last ice age.

The implication is that these regions were the source of the carbon dioxide to the atmosphere.

At their coldest, during the ice ages, carbon dioxide levels fell to 185 parts per million. During the interglacials, when the world warmed and lions and hyenas roamed the plains of Europe, the carbon dioxide levels rose to 280 ppm.

Right now, thanks to human activity, CO2 levels are rising ominously towards 400 ppm.

The oceans are home to about 60 times more carbon than the atmosphere and can, it seems, surrender it rapidly.

“The magnitude and rapidity of the swings in atmospheric CO2 across the ice age cycles suggest that changes in ocean carbon storage are important drivers of natural atmospheric CO2 variations,” Dr Martínez-Botí says.

“Our findings support the theory that a series of processes operating in the southernmost sector of the Atlantic, Pacific and Indian oceans, a region known as the Southern Ocean, changed the amount of carbon in the deep sea.

Into the abyss

“While a reduction in communication between the deep sea and the atmosphere in this region potentially locks carbon away from the atmosphere into the abyss during ice ages, the opposite occurs during warm interglacial periods.”

To arrive at their conclusion, the scientists had to analyse subtle evidence from the isotopic composition of the carbonate shells, and then use mathematical techniques to reconstruct a story of a great, faraway sigh of carbon dioxide from the ocean to the atmosphere.

The finding, based on calculated probabilities, is incomplete as there may have been other forces also at play.

Gavin Foster, associate professor in isotope geochemistry at the University of Southampton, says: “While our results support a primary role for the Southern Ocean processes in these natural cycles, we don’t yet know the full story. Other processes operating in other parts of the ocean, such as the north Pacific, may have an additional role to play.” 

Carbon stored deep in Antarctic waters ended the last ice age

by Miguel Martinez-Boti, University of Southampton and Gianluca Marino, Australian National University, The Conversation, February 12, 2015

It’s well known that carbon in the atmosphere is causing global warming. What is less well known, outside of scientific circles at least, is the role oceans have to play in this. Our seas contain 60 times more carbon than the atmosphere, and they can release it at sufficiently rapid rates to cause dramatic changes in the climate. In fact, as we describe in research published in Nature, CO₂ released by the oceans brought about the end of the last ice age.

More than 50 million cubic kilometres of ice once covered North America and Scandinavia. It melted away between approximately 19,000 and 10,000 years ago, releasing enough water to raise the sea level by about 130 metres. This came after CO₂ concentrations increased by approximately 50%, from 180 to 280 parts per million between the last ice age and the current interglacial period. To explain such a pronounced increase, we have to look at the ocean.

Scientists have thought for a long time that the southern sectors of the Atlantic, Indian and Pacific Oceans, a region known as the Southern Ocean, may be key to explaining the increase in atmospheric CO₂.

Large volumes of deep water loaded with carbon come to the surface in this area. However, the low concentration of certain nutrients (for example iron) in surface waters limits the metabolism of planktonic organisms, which cannot fully consume all the carbon brought to the surface ocean, resulting in CO₂ being “outgassed” to the atmosphere.

We wanted to assess if the ocean contributed to the atmospheric CO₂ increase during the last deglaciation, so it made sense to look at areas that are important today for the ocean-atmosphere exchange of carbon: the Atlantic Sector of the Southern Ocean and the Eastern Equatorial Pacific, another area where deep, cold water rises to the surface.

But how can we then go back in time and check if these areas were a source of CO₂ in the atmosphere? The answer is buried a few thousand meters below the surface of the oceans.

Deep-sea drilling for sediment samples. William Crawford, IODP/TAMU, CC BY-NC-SA

Research vessels such as the Joides Resolution are capable of drilling the sea floor to recover long sequences of sediments in which the history of the oceans is recorded. The sediments contain, among other things, fossils of tiny organisms that once lived in the upper ocean, called foraminifera. These creatures build chalky shells, and the waters they live in influence their chemical composition.

After death, the shells sink to the bottom of the oceans, where they accumulate. We analysed the sediment cores and looked for the isotopic composition of the element boron present in shells that lived during particular times of interest. Boron tells us pH levels of the waters, which in turn tells us about carbon levels: a high concentration of CO₂ in the waters will make them more acidic (lower pH), and vice versa.

We found a link. When the glaciers of the last ice age were melting, and the atmospheric CO₂ was increasing, the surface waters of the Southern Ocean and the Eastern Equatorial Pacific were also more acidic. This signalled an increased concentration of CO₂ – much higher than those in the atmosphere.

This is the key finding of our research: the ocean was a source of CO₂ to the atmosphere during key intervals of the last deglaciation, which explains the large increase in CO₂ concentrations.

Where did this carbon come from?

It’s the next obvious question. Previous research has found that the last ice age saw much less carbon exchanged between ocean and atmosphere than we see today, mostly because the Southern Ocean was intensely stratified at the time and deep waters rarely made it to the surface. Nutrients and CO₂ were accumulating in the deep Southern Ocean, due to the decay of the organic matter that was being produced in the surface ocean and transported to the abyss.

Microscopic shells like this can reveal oceanic acidity. Mariana T. Horigome, Autonomous University of Barcelona, Author provided

During the deglaciation, the effective communication between deep and upper ocean was re-established, and this carbon “reservoir” was leaked to the atmosphere.

Since the beginning of the industrial revolution the oceans have absorbed an estimated 155 billion tonnes of carbon, about 30% of the total human emissions. The present atmospheric CO₂ concentrations, approximately 400 parts per million, have not been seen on Earth since the Pliocene, around 3 million years ago, and the rate of increase is unprecedented in the period of on-off glaciers we have had since.

Humanity is performing a large scale experiment with the Earth, and the consequences are already being seen in the form of increased atmospheric and oceanic temperatures, raising sea levels and ocean acidification, to name a few. How the oceanic uptake of CO₂ is going to operate in the future remains unknown, but studies like ours advance our understanding of how the ocean works to store and release carbon on timescales of millennia and that therefore are way beyond the reach of the instrumental record.


This article was originally published on The Conversation. Read the original article.

Robert Spielhagen: 2,000 year water temperature high underlines Arctic threat

2,000 year water temperature high underlines Arctic threat

by andyextance, Simple Climate, January 29, 2011

The water flowing from the Atlantic Ocean into the Arctic through the Fram Strait is warmer today than any time in the past 2,000 years. That's what microscopic seabed deposits have told Robert Spielhagen of the Academy of Sciences, Humanities, and Literature in Mainz, Germany, and his colleagues. The scientists have shown that the average temperature of water flowing into the Arctic since 1890 is 2 ºC higher than it has been on average in the previous two millennia. This sends a stark message about the prospects for the Northern polar region. “I am afraid that my children – now 14 and 17 years old – will be able to see a summer ice-free Arctic Ocean,” Spielhagen told Simple Climate.

On August 4, 2007, Spielhagen and his co-workers extracted the key deposits when they drilled a 46-cm-long cylinder of rock from the sea bed. Such “sediment cores” had previously been used to look at temperature changes as far as 12,000 years into the past, but could only provide measurements for periods of a few hundred years at a time. That's down to how much sediment settles to the sea bed, with too little deposition for high-resolution temperature measurements occurring where cores have been taken before. By contrast, Spielhagen's team was able to give temperatures on a scale of 2–3 decades at a time. “We took our core in a place where a lot of fine-grained particles settle, due to diminished bottom currents,” he explained. “We were the first to find such a spot.”

Microscopic photo of the coarse fraction -- particles greater than 0.1 mm -- of a sample from the team's sediment core. White grains are the foraminifers used for the study published in Science. Image courtesy of Kirstin Werner (IFM-GEOMAR, Kiel).

The key element needed to determine temperature within the deposits are the shells of single-celled animals called foraminiferas. Together with researchers from the US, Norway and Germany, Spielhagen investigated the shells in two different ways. “Method one uses the various species of foraminifers in the sediment,” the scientist explained. “There is only one species which prefers very cold water, from –2 ºC to +2 ºC, and several species which live in warmer waters.” Comparing the percentages of different species in the sample can give historical temperature values. The second method uses the ratio of the elements magnesium and calcium in foraminifera shells, which is directly linked to temperature.

Method	Pre-1850 minimum temperature (°C)	Pre-1850 average temperature (°C)	Pre-1850 maximum temperature (°C)	Post 1890 minimum temperature (°C)	Post 1890 average temperature (°C)	Post 1890 maximum temperature (°C)
1	2.8	3.4	4.4	4.1	5.2	6
2	N/A	3.6	N/A	4.4	5.8	7.1

The team found that the proportion of foraminifer species typically found in warmer water in the sample has seen an “unprecedented increase” at the location they sampled in the past 120 years. Spielhagen admitted that the corresponding rate of temperature increase that they reported in top journal Science on Friday was unexpected. “Publications about the dramatic atmospheric temperature increase in the Arctic made me expect that we should find something similar for the ocean waters,” he said. “I was somewhat surprised, however, how strong the temperature increase in the last 100–120 years was, according to our data.”

Bathymetric map of the Norwegian-Greenland Sea and Arctic Ocean (base map: www.ibcao.org). White shading marks average summer sea ice cover. White arrows mark ice drift directions. Red arrows mark the transport path of warm Atlantic water entering the Arctic where it submerges under the cold, ice-covered surface layer. The yellow spot marks the site the sediment core used in the study was taken from. Image courtesy of Robert Spielhagen (IFM-GEOMAR, Kiel)

Historical ocean temperatures at the sample site measured by Spielhagen's team. The upper graph is produced using the SIMMAX method, based on comparing the numbers of different species of foraminifera. The lower graph is produced by comparing the amounts of the elements magnesium (Mg) and calcium (Ca) in the foraminifera shells, which is proportional to temperature. Courtesy of Robert Spielhagen (IFM-GEOMAR, Kiel).

While such efforts to reconstruct historical temperatures can attract a lot of scepticism, Spielhagen asserted that his findings are robust. “The results from both methods are very similar,” Spielhagen underlined. “The very good correspondence make us very confident about the data. The methods we have applied for our records are well established and have been found to give very reliable results in many different ocean basins.”

These findings were published just a week after measurements showing that the proportion of the year in which the Greenland ice sheet was melting rather than freezing in 2010 was a record 50 days longer than average. Spielhagen emphasized that warmer water flowing into the Arctic could be contributing to this. “Warm water releases heat to the atmosphere,” he explained. “The warmer and stronger the transport of Atlantic Water to the Arctic is, the more heat may be released to the Arctic atmosphere and distributed there – potentially also reaching Greenland.”

http://simpleclimate.wordpress.com/2011/01/29/2000-year-water-temperature-high-underlines-arctic-threat/

Modern seawater acidification: the response of foraminifera to high-CO2 conditions in the Mediterranean Sea, JGS 167 (2010), B. B. Dias, M. B. Hart, C. W. Smart & J. M. Hall-Spencer

Journal of the Geological Society (September 2010), 167(5): 843-846; DOI: 10.1144/0016-76492010-050

Modern seawater acidification: the response of foraminifera to high-CO₂ conditions in the Mediterranean Sea

B. B. Dias¹, M. B. Hart^2,*, C. W. Smart² and J. M. Hall-Spencer³

¹ Laboratório de Oceanografia Costeira, Departamento de Geociências, CFH Universidade Federal de Santa Catarina, Florianopólis, SC 88040-900, Brazil
² School of Geography, Earth & Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK
³ School of Marine Science & Engineering, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

Abstract

The seas around the island of Ischia (Italy) have a lowered pH as a result of volcanic gas vents that emit carbon dioxide from the sea floor at ambient seawater temperatures. These areas of acidified seawater provide natural laboratories in which to study the long-term biological response to rising CO₂ levels. Benthic foraminifera (single-celled protists) are particularly interesting as they have short life histories, are environmentally sensitive and have an excellent fossil record. Here, we examine changes in foraminiferal assemblages along pH gradients at CO₂ vents on the coast of Ischia and show that the foraminiferal distribution, diversity and nature of the fauna change markedly in the living assemblages as pH decreases.

*Correspondence e-mail: mhart@plymouth.ac.uk


Link:  http://jgs.geoscienceworld.org/cgi/content/abstract/167/5/843