Ice Age CO2
At the end of the last Ice Age, atmospheric carbon dioxide levels rose rapidly as the planet warmed; scientists have long hypothesized that the source was CO2 released from the deep ocean. But a new study using detailed radiocarbon dating of foraminifera found in a sediment core from the Gorda Ridge off Oregon reveals that the Northeast Pacific was not an important reservoir of carbon during glacial times. The finding may send scientists back to the proverbial drawing board looking for other potential sources of CO2 during glacial periods. The study, which was supported by the National Science Foundation and the University of Michigan, was published online this week in Nature Geoscience.
The causes of ice ages are not fully understood for both the large-scale ice age periods and the smaller ebb and flow of glacialâ€“interglacial periods within an ice age. The consensus is that several factors are important: atmospheric composition, such as the concentrations of carbon dioxide and methane (the specific levels of the previously mentioned gases are now able to be seen with the new ice core samples from the Antarctic shelf over the past 650,000 years); changes in the Earth's orbit around the Sun known as Milankovitch cycles (and possibly the Sun's orbit around the galaxy); the motion of tectonic plates resulting in changes in the relative location and amount of continental and oceanic crust on the Earth's surface, which affect wind and ocean currents; variations in solar output; the orbital dynamics of the Earth-Moon system; and the impact of relatively large meteorites, and volcanism including eruptions of supervolcanoes.
Some of these factors influence each other. For example, changes in Earth's atmospheric composition (especially the concentrations of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which weathering removes CO2).
"Frankly, weâ€™re kind of baffled by the whole thing," said Alan Mix, a professor of oceanography at Oregon State University and an author on the study. "The deep North Pacific was such an obvious source for the carbon, but it just doesnâ€™t match up. At least weâ€™ve shown where the carbon wasnâ€™t; now we just have to find out where it was."
During times of glaciation, global climate was cooler and atmospheric CO2 was lower. Humans didnâ€™t cause that CO2 change, so it implies that the carbon was absorbed by another reservoir. One obvious place to look for the missing carbon is the ocean, where more than 90 percent of the Earthâ€™s readily exchangeable carbon is stored.
The Pacific Ocean is the largest ocean by volume. The deep water mass longest isolated from the atmosphere and most enriched in carbon is found today in the Northeast Pacific, so the researchers focused their efforts there. They hypothesized that the ventilation age in this basin â€“ or the amount of time since deep water was last in contact with the atmosphere â€“ would be older during glacial times, allowing CO2 to accumulate in the abyss.
â€œWe were surprised to find that during the last ice age, the deep Northeast Pacific had a similar ventilation age to today, indicating it was an unlikely place to hide the missing carbon,â€ said David Lund, a paleoceanographer at the University of Michigan, formerly at Oregon State, and lead author on the Nature Geosciences paper.
â€œThis indicates that the deep Pacific was not an important sink of carbon during glacial times,â€ Lund added. â€œEven more intriguing is that we found the ventilation age increased during the deglaciation, at the exact time that atmospheric CO2 levels were rising.â€
The researchers reconstructed the ventilation history of the deep North Pacific, examining the sediments at a site about 75 miles off the coast of southwestern Oregon. There the water is more than a mile-and-a-half deep and is known as the oldest water mass in the modern oceans, Mix said. By radiocarbon dating both the planktonic, or surface-dwelling, and benthic (seafloor-dwelling) foraminifera, the scientists can determine whether the isotopic signatures of the foraminifera match â€œvalues predicted by the assumption of oceanic control of the atmosphere.â€
The organisms that lived on the seafloor have older apparent radiocarbon ages than the organisms that lived at the sea surface, Mix said, even though both come from the same sediment sample and are of the same true age.
The study is important not just in tracing climatic history, scientists say, but in forecasting how the Earth may respond to future climate change. The Earth â€œbreathes carbon in and out,â€ Mix said, inhaling carbon into sediment and soils, while exhaling it via volcanism and a slow exchange between the oceans, soils and plant life with the atmosphere.
When everything is in balance, the Earth is said to be in a steady state. But on numerous occasions in the past, the carbon balance has shifted out of whack.
"Because the ocean is such a huge repository of carbon, a relatively small change in the oceans can have a major impact," Mix said. "We know ocean circulation changed during the ice ages and that is why many scientists assumed the deep Pacific Ocean was the source for rising CO2 levels during the last deglaciation."
"These are volcanically active regions, so the input of carbon from volcanoes, which lacks radiocarbon because of its great age, needs to be looked at. But it is premature to draw any conclusions." Lund pointed out.
The researchersâ€™ next step will be to look for chemical traces of volcanic influence.
Another source of carbon could be from land, though the authors say it would be difficult to account for the magnitude of atmospheric carbon increase and the apparent radiocarbon age of released carbon by pre-industrial terrestrial sources alone.