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Sedimentation and blue carbon burial

Organic carbon is only sequestered from the oceanic system if it reaches the sea floor and gets covered by a layer of sediment. Reduced oxygen levels in buried environments mean that tiny bacteria who eat organic matter and respire CO₂ can’t decompose the carbon, so it is removed from the system permanently. Organic matter that sinks, but is not buried by a sufficiently deep layer of sediment is subject to re-suspension by changing ocean currents, bioturbation by organisms that live in the top layer of marine sediments, and decomposition by heterotrophic bacteria. If any of these things occur, then the organic carbon is put back in the system. Carbon sequestration takes place only if burial rates by sediment are greater than the long term rates of erosion, bioturbation, and decomposition.

Spatial variability in sedimentation

Sedimentation is the rate at which floating or suspended particulate matter sinks and accumulates on the ocean floor. The faster (more energetic) the current, the more sediment it can pick up. As sediment laden currents slow, the particles fall out of suspension and come to rest on the sea floor. In other words, a fast current can pick up lots of heavy grains, where as a slow current can pick up only tiny pieces. As one can imagine, different places in the ocean vary drastically when it comes to the amount of suspended sediment and rate of deposition.

Open ocean

The open ocean has very low sedimentation rates because most of the terrigenous sediments that make it here are carried by the wind. Aeolian transport accounts for only a small fraction of the total sediment delivery to the oceans. Additionally, there is much less plant and animal life to be buried. Therefore, carbon burial rates are relatively slow in the open ocean.

Coastal margins

Coastal margins have high sedimentation rates because of rivers, which account for the vast majority of sediment delivery to the ocean. In most cases, sediments are deposited near the river mouth or are transported in the alongshore direction due to wave forcing. In some places sediment falls into submarine canyons and is transported off-shelf, if the canyon is sufficiently large or the shelf is narrow. Coastal margins also contain diverse and plentiful marine species, especially in paces that experience periodic upwelling. More marine life plus higher sedimentation rates create hotspots for carbon burial.

Carbon burial in submarine canyons

Marine canyons are magnets for sediment, because as currents carry sediment on the shelf in the alongshore direction, the path of the current crosses canyons perpendicularly. When the same amount of water flow is suddenly in much deeper water it slows down, and deposits sediment. Due to the extreme depositional environment, carbon burial rates in the Nazare canyon near Portugal are 30x greater than the adjacent continental slope! This canyon alone accounts for about 0.03% of global terrestrial OC burial in marine sediments. This may not seem like much, but the Nazarre submarine canyon only makes up 0.0001% of the area of the worlds oceans.

Human changes to global sedimentary systems

Humans have been modifying sediment cycles on a massive scale for thousands of years through a number of mechanisms.

Agriculture/land clearing

When humans started clearing land to grow crops, they ruined the ability of the land to retain sediment. In a natural ecosystem, roots from plants hold sediment in place when it rains. Trees and shrubs reduce the amount of rainfall that impacts the dirt, and create obstacles that forest streams must flow around. When all vegetation is removed rainfall impacts directly on the dirt, there are no roots to hold on to the sediment, and there is nothing to stop the stream from scouring banks as it flows straight downhill. For many thousands of years, the net effect of humans on global sediment cycles was an increase in sedimentation due to this process.

Dams

The first dams date back to 3000 BC and were built to control flood waters for agriculture. When sediment laden river flow reaches a dam’s reservoir, the water slows down as it pools. Since slower water can’t carry as much sediment, virtually all of the sediment falls out of suspension before the water passes through the dam. The result is that most dams are nearly 100% efficient sediment traps. For thousands of years, there were too few dams to have a significant impact on global sedimentary cycles. The popularization of hydroelectric power in the last century has caused an enormous boom in dam building. Currently only a third of the world’s largest rivers flow unimpeded to the ocean.

Channelization

In a natural system, the banks of a river will meander back and forth as different channels erode, accrete, open, or close. Seasonal floods regularly overwhelm riverbanks and deposit nutrients on adjascent flood plains. These services are essential to natural ecosystems, but are quite troublesome for people. Humans love to build houses and developments close to rivers, but hate watching their buildings get flooded or tumble down a bank. In response rivers in populated areas are often channelized, meaning that their banks and sometimes beds are armored with a hard material that prevents erosion and fixes them in place. This prevents erosion because there is no soft substrate left for the river to take downstream.

Impacts

Currently, the net effect of humans on global sedimentary cycling is a drastic reduction in the amount of sediment that makes it to the ocean. If we continue to build dams and channelize rivers, we will start to see a number of problems in coastal areas including sinking deltas, shrinking beaches, and disappearing salt marshes. In addition, it’s possible that we might ruin one of the biggest carbon sinks that we know of. Without sequestration of carbon in coastal marine sediments, we will likely see accelerated global climate change.

References

http://rsta.royalsocietypublishing.org/content/369/1938/957

Stallard, R. F. (1998). Terrestrial sedimentation and the carbon cycle: Coupling weathering and erosion to carbon burial. Global Biogeochemical Cycles, 12(2), p. 231-57.

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