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.

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