Tag Archives: CO2

deep sea carbon cycling: more dynamic than we thought?

For years, scientists have speculated that deep sea carbon may have played an essential role in past climate change episodes. Specifically, it has been suggested that the C bound in seafloor sediment has undergone thermodynamic alterations in the past to due upwellings of molten magma from the mantle. Magma may have triggered the release of CO2 and methane into the upper ocean and eventually the atmosphere. Evidence suggests that the Paleocene-Eocene thermal maximum, which lasted approximately 100,000 years, may have been triggered in part by the release of greenhouse gases from the seaflooor.

Despite these speculations about deep sea carbon influencing past climate, little research has been done on the role of seafloor carbon in the present day C cycle. In some ways, this is surprising given the enormous amount of attention being payed to global C budgets and possible means of C sequestration. It is generally assumed that the deep sea represents a huge “carbon sink”, to which organic C from the upper ocean enters and does not emerge again for thousands to millions of years. This would suggest that whatever carbon-cycling processes are occuring at the seafloor are not powerful enough to cause a net carbon release.

Recent research published in Nature Geoscience suggests otherwise. Several case studies have demonstrated dynamic processes occurring on the ocean floor can in fact lead to a net release of greenhouse gases. Spreading seafloor centers- regions where oceanic plates pull apart-  are a site of magma activity and hydrothermal venting. Hydrothermal vents release a variety of hot, mineral-rich fluids that can support a diverse microbial and invertebrate community. At one such spreading center in the Gulf of California, magma is intruding into thick organic basin sediments. These sediments have long been thought to sequester C, however, it now appears tht their heating is causing the release of methane into the upper ocean.

In the Northeast Pacific, another intriguing deep ocean C cycling system has been discovered. Here, microbes are converting ancient inorganic C into dissolved organic C, which is subsequently released to the overlying ocean. This discovery contradicts the general belief that ancient deep-sea C is highly stable and not accessible to microbes.

Other distinct seafloor C sources are rapidly emerging around the world, as improved technology and a heightened interest in seafloor processes are accelerate the pace of discovery. However, the contribution of such “point sources” to global C budgets is still highly uncertain and far more research is needed to come up with even a rough estimate of global deep sea C sources. Nonetheless, it would seem that we can no longer consider the deep ocean a black box of C sequestration, and that we should think carefully about the ramifications of introducing more carbon- either accidentally through the introduction of dissolved greenhouse gases to the ocean, or intentionally as part of a climate change mitigation strategy- to a system that is clearly more dynamic than we once thought.,

Reference : “Deep Sea Discoveries.” 2011. Nature Geoscience: Letters. Volume 4, Page 1.

Shade leaves: nature’s most efficient light harvesters

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Not all leaves are created equal. Within a single plant, a spectrum of light availability exists- some leaves are positioned to receive maximum sunlight every day, while others, by luck of the draw, are primarily shaded and experience full sun only in short, erratic bursts.  The sun leaves, by virtue of their superior position, are equipped with the capacity to capture the most light and perform the most photosynthesis. They tend to have more cell layers, larger numbers of the enzymes involved in capturing CO2 and converting it to sugar, and a larger volume of space to perform carbon-fixing reactions than their less-exposed neighbors.

Though it may seem that shade leaves get a raw deal, they are in fact uniquely adapted to a low light environment. Seemingly minor biochemical changes allow shade leaves to be far more efficient light harvesters, maximizing the amount of sunlight captured during those rare moments of exposure.

Chlorophyll is the main light-harvesting pigment in most plants, capturing light radiation primarily in the red and blue parts of the visible spectrum. These light waves are between 400 and 700 nanometers in length and are known as photosynthetically active raditaion (PAR). When PAR is absorbed by chlorophyllous tissue, radiation of longer wavelengths in the 680-760 nanometer range is re-emitted in a process known as fluroescence.

Measuring the kinetics of chlorophyll fluorescence has proved a useful tool in discerning the different photosynthetic capabilities of sun and shade leaves. To do so, a plant physiologist may incubate identical leaves in contrasting light conditions and measured fluorescence levels when a “saturating” pulse of white light is applied in each case. It turns out that the dark incubated leaves respond far more dramatically to a brief pulse of light than that already light-exposed leaves.

The pulses of light used in such laboratory experiments are in fact comparable to the erratic “sun flecks” that understory leaves may experience over the course of a day. Much as a starving animal will metabolize and release energy more slowly than a well-fed one, shade leaves take the limited light resources available and make far more out of these brief, shining moments than sun leaves would ever be able to.

Rhea provides clues to the origins of atmosphere

Science this week reports new data from the Cassini probe confirming that Saturn’s moon Rhea has the tenuous beginnings of an atmosphere.  A thin veil of O2 and CO2 gas skirts the moon’s icy surface at one in five trillionth the pressure of Earth’s atmosphere.

Rhea, one of over 60 moons of Saturn, is comprised of 75% ice and 25% rock.

The Cassini probe captured a sample of the atmosphere and used mass spectroscopy to confirm the presence of CO2 and O2. A model produced from this data indicates that O2 was formed at the ice surface by irradiation from solar ultraviolet light and possibly charged particles from deep space. UV rays cause chemical changes in the ice, leading to the spontaneous ejection of O2 gas.

The origin of CO2 in Rhea’s atmosphere is less clear. It is hypothesized that the CO2 is native to Rhea’s ice, or that it formed by interactions between atmospheric O2 and carbonaceous grains on the surface. The authors suggest that carbonaceous grains may have been depsited by micrometeorites from elsewhere in the solar system, possibly even Earth.

O2 and CO2 are believed to have accumulated on Earth during the evolution of life and metabolic processes such as aerobic respiration and photosynthesis. The presence of these gases is considered a criterion for the possible presence of life on a planet. However, the formation of an O2-rich atmosphere on Rhea of entirely photo-driven origin calls this assumption into question. The study provides novel insight into some of the processes that lead to formation of an atmosphere.

A newly released image of Rhea from NASA's Cassini probe


B. D. Teolis et al., Science 330, 1813 (2010); 10.1126/science.1198366.