Carbon From The Deeps
Scientists believe that carbon released from the ocean floor played a key role in past episodes of climate change. Around 55 million years ago, the break-up of the northeast Atlantic continents was associated with the injection of large amounts of molten magma into seafloor sediments. Formation of the North Atlantic basalts heated the carbon-rich sediments, triggering the release of large quantities of methane and carbon dioxide into the ocean and atmosphere. It has been suggested that this release of previously sequestered carbon was responsible for a 100,000 year period of rapid temperature rise known as the Paleocene-Eocene Thermal Maximum or PETM. Three letters published in Nature Geoscience suggest that carbon trapped beneath the seabed continues to influence carbon dynamics, at least in the deep ocean.
In an accompanying Nature Geoscience editorial, three types of sea floor carbon release are identified: seafloor spreading centers, where two or more oceanic plates pull apart, releasing molten magma from below that heats organic sediments; hydrothermal fluids circulate through the ocean crust converting ancient inorganic carbon into dissolved organic matter, which is subsequently vented to the overlying ocean; and gas hydrates—ice-like mixtures of methane and water that form under low temperature and high pressure on the seafloor—also serve as a source of dissolved organic carbon.
“Despite the potential importance of seafloor carbon sources in shaping past climate, little is know about their involvement in the present day carbon cycle,” the editorial states. Here is a review of recent research developments regarding carbon from the ocean deeps.
Formation of a black smoker.
One form of seafloor venting is the now familiar black smoker—chimney-like structures that form above marine hydrothermal vents. Black smokers produce a dense plume of hot, black, fine-grained, precipitates. As it turns out, black smokers are not the only heat driven emitters of mineral enriched water located around ocean ridges and spreading rifts. Other, similar mechanisms have been discovered for organic materials, for among the materials being released from the floor of the ocean is carbon.
In “Carbon release by off-axis magmatism in a young sedimented spreading centre,” Daniel Lizarralde, S. Adam Soule, Jeff S. Seewald and Giora Proskurowski, all from the Woods Hole Oceanographic Institution, examine a spreading system where new igneous crust is formed beneath a layer of organic-rich sediment. They found that, under these conditions, carbon from sediments can be released up to 50 km away from the plate boundary. This is a much larger area than than previously recognized, ten times as far as the 5 km distance found at unsedimented mid-ocean ridges.
Bathymetry of the Guaymas basin within the Gulf of California.
The rift system studied for this report was the Guaymas basin in the Gulf of California. The Gulf of California is a narrow sea formed through continental rifting, with the separation of the Baja peninsula from North America beginning 12–15 million years ago. Organic-rich sediments 1-2 km (0.6-1.2 miles) thick overlie the spreading center of the Guaymas basin, a situation common elsewhere in the gulf and in many other geologically young spreading centers. Magma emerging from the spreading rift center comes into contact with the carbon rich sediments, releasing the formerly sequestered carbon.
It is notable that this release of carbon is the reverse of what had been expected. The formation of organic-rich sedimentation is normally a form of carbon sequestration. In addition, topographic gradients promote silicate weathering, which further consumes atmospheric CO2. “Natural carbon sequestration within rifts and young spreading systems may therefore be an inefficient process despite high sedimentation rates,” the authors conclude.
A related but different mechanism of seafloor carbon release is described in “Chemosynthetic origin of 14C-depleted dissolved organic matter in a ridge-flank hydrothermal system,” authored by Matthew D. McCarthy et al. “Circulation of hydrothermal fluids in upper oceanic crust may support one of the most extensive, but least understood, of the Earth’s biogeochemical systems,” they report, “however, little is known about non-living organic matter carried in crustal hydrothermal fluids or its possible impact on the carbon cycle of the overlying ocean.”
In the Juan de Fuca Ridge (JDFR) flank region, off the coast of Washington state and Vancouver Island, British Columbia, samples of hydrothermal circulation fluids were taken. From these samples, the research team found that even small inputs could represent an important unrecognized source of dissolved organic carbon. “The overall impact of many ridge-flank systems may thus be to simultaneously act as a ‘scrubber’ for surface-derived organic carbon, and a source of new, chemosynthetic material,” they conclude.
Finally, we turn to “Methane hydrate-bearing seeps as a source of aged dissolved organic carbon to the oceans,” by John W. Pohlman et al. It has been estimated that marine sediments contain about 500–10,000 Gt of carbon in the form of methane, primarily locked in seafloor gas hydrate or clathrate deposits.. This amount is comparable to all the organic carbon found in land biota, terrestrial soils, the atmosphere and sea water combined. The paper's authors describe their research and findings:
We use Δ14C and δ13C measurements and isotopic mass-balance calculations to evaluate the contribution of methane-derived carbon to seawater dissolved organic carbon overlying gas hydrate-bearing seeps in the northeastern Pacific Ocean. We show that carbon derived from fossil methane accounts for up to 28% of the dissolved organic carbon. This methane-derived material is much older, and more depleted in 13C, than background dissolved organic carbon. We suggest that fossil methane-derived carbon may contribute significantly to the estimated 4,000–6,000 year age of dissolved organic carbon in the deep ocean, and provide reduced organic matter and energy to deep-ocean microbial communities.
Not only do these methane deposits contribute significantly to oceanic dissolved organic carbon, because of fossil methane's isotopic composition it may be throwing off estimates of how much carbon man is releasing into the ecosystem. What's more, the impact of this methane seepage on ocean acidification could be substantial. The impact of this intermixing can be seen in the figure below
The isotopic composition (δ13C and Δ14C) of the sediment DOC affects concentrations of deep-seawater DOC.
At this time we do not know whether processes similar to the ones described above operate in other hydrothermal and gas hydrate systems, or how much carbon is being emitted. And in the case of the dissolved organic carbon released from ocean ridges and hydrates, oceanographers are unsure of how reactive the released carbon is. One thing is certain, the seeping, circulating and other forms of seafloor carbon release have been going on long before oceanographers recently discovered them. Scientists suspect that one or more of these processes have been responsible for sudden climatic and environmental changes in the past.
“For example, methane released from gas hydrate during the Palaeocene–Eocene Thermal Maximum (55.8 Myr BP) has been implicated in ocean acidification that dissolved ~2,000 Gt of sedimentary carbonate,” states a paper by Michael R. Rampino, published in PNAS. “Although water-column methane oxidation is thought to have been the dominant source of CO2 for Palaeocene–Eocene Thermal Maximum acidification, results from this study suggest a potentially significant fraction of the methane carbon could have been released as DOC to the water column, where it would have been subsequently oxidized to CO2.”
The impact of all these scientific discoveries on everyday people is minimal, so far. Only a dramatic event like the PETM would upset our daily lives. The PETM is only the most recent example of rapid global warming thought to have been caused by a rapid release of carbon. Other, older events involving the motion of Earth's tectonic plates have had more dramatic impact on the planet's biota.
In “Mass extinctions of life and catastrophic flood basalt volcanism,” Rampino offers episodic massive continental flood basalt eruptions as a possible cause of mass extinctions, stating “This connection is illustrated by a study by Whiteside et al. that provides evidence that the eruption of the Central Atlantic magmatic province (CAMP) basalts, with a preserved volume greater than 1 × 106 km3 and covering more than 7 × 106 km2, coincided with the end-Triassic extinction event (ETE) (201.4 Mya) on land and in the oceans.”
Flood basalts extruded from mid-ocean rifts often forms pillow lava.
No human has witnessed an event like the PETM—the Younger Dryas might appear similar but it was a sudden cooling, not warming, and it only lasted 1,000 years. Certainly no human has lived through a real mass extinction event like the ETE. Fortunately for our species and ourselves, such cataclysmic events happen infrequently. As Dr. Rampino explaines:
Natural events of various kinds in the real world tend to follow an inverse-power law relationship between frequency F and magnitude M so that F = 1/MD, where D is positive. Thus, small-magnitude events (e.g., earthquakes, volcanic eruptions, impacts) tend to happen much more frequently than potentially catastrophic large-magnitude events. The reasons are variable, but in general, a probabilistic relationship exists between the magnitude and frequency of events.
Of course, improbable events do happen. Mankind does not need to tempt fate by prodding sea floor deposits of methane clathrates or trying to pump massive quantities of CO2 into the ocean depths. The Nature Geoscience editorial commented specifically on efforts to mitigate anthropogenic carbon dioxide emissions using the deep ocean:
To mitigate such effects, the sea floor—where natural sources of carbon are just being discovered—has been proposed as a potential site for carbon sequestration (Nature Geosci. 2, 815–818; 2009; Nature Geosci. 2, 820–822; 2009). As long as the natural carbon cycle in the deep ocean continues to surprise us, it would probably be unwise to go ahead and disturb it with the deposition of carbon captured from the use of fossil fuels.
Once again, science is giving the public mixed messages. Researchers in one area propose “solutions” to global warming that ignore the dangers uncovered by other scientists. As Richard Feynman noted: “In this age of specialization men who thoroughly know one field are often incompetent to discuss another.” Nonetheless, governments are urged to charge ahead with “clean coal” and other carbon sequestration schemes.
Climate scientists and eco-activists, out to rein in human activity and make their personal reputations, form a collection of carbon cycle Don Quixotes. Tilting at global warming windmills, each of them, as Cervantes might have put it, is “spurred on by the conviction that the world needs his immediate presence.” None are more dangerous than the energetically ignorant.
Despite efforts to the contrary, more settled science has been unsettled, more consensus opinion overturned and our ignorance of the world around us revealed for all to see. Some scientists accept the truth—little is know about carbon from the deeps and its involvement in the present day carbon cycle. Being innocent of real understanding, we should look before we leap, rather than risk a major ecological or economic catastrophe in hopes of avoiding the unproven and ill-defined effects of anthropogenic global warming.
Be safe, enjoy the interglacial and stay skeptical.