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We know that vegetation sequesters CO2 in its leaves. Much of this carbon sequestration is temporary, since when the plant dies, much of it is released back into the atmosphere as CO2. If it dies in a frozen region, it could be sequestered in permafrost, but even that can be released back.
What if it dies, and some of the carbon gets carried into a river, which could then deposit its carbon into the ocean basins?
This has been in the news in the last week or so here in Canada:
From the Kyoto rules on what counts in this space, this document provides some good details also with respect to Canada: The Ministry protects Ontario’s biodiversity while promoting economic opportunities in the resource sector and supporting outdoor recreation opportunities.
Thoughts on this space:
Getting to your question: the idea that rivers and oceans suck in organic matter and delay the time before which it becomes re-released into the atmosphere is certainly true in the same type of way, but is something else which has a steady-state as well (or at least one which does not see mass amounts of biomass sent into the oceans and rivers and destroying them as they rot and change the water chemistry). In short I do not think there is much of an argument there for this aspect as something meaningful/measurable.
A good thought exercise though: If I cut down the entire continent of trees and replanted them, dumping all the cut biomass into the ocean - what would the net effect be?
Unlikely but theoretically possible. As the bits of tree work their way down to the sea, they are going to oxidise and release CO2. This would appear to our eyes as "rotting" and breaking down. Leaves aren't going to make it, although they might get buried in river sediments. Trunks might. Once they reach the open sea, they are going to have to become water logged and sink to the bottom of the sea or ocean. They are going to then have to either fall into an area of anoxic bottom waters (restricted water flow => very little oxygen if any), or be quickly buried by sediment that cuts off the oxygen.
Speaking generally, when I look at ocean sediments, I have yet to see fossil wood. I do see fossil wood in places like river deltaic sediments though (eg. the Carboniferous sandstones of West Yorkshire).
One way to lock some of the carbon in plant biomass into the soil for a very long time is, paradoxically, by burning it. While most of the carbon in burning vegetation does literally go up in smoke, turning back into CO2, a small fraction — around 1% to 5% or so — of it turns into ash and charcoal, collectively known as black carbon or pyrogenic carbon.
Once formed, this black carbon can stay in the soil for a very long time (half-life measured in thousands of years) since, being essentially inorganic elemental carbon, it is not easily broken down by microorganisms. Some of it can also get transported by air and/or rivers into lakes and seas, where it can become locked up in sediments for even longer times.
For instance, to quote Forbes, Raison & Skjemstad, "Formation, transformation and transport of black carbon (charcoal) in terrestrial and aquatic ecosystems", Science of the Total Environment 370 (2006), pp. 190–206 (PDF):
"BC [= black carbon] can comprise up to 40% of the OC [= organic carbon] in terrestrial soils and between 12% and 31% of the OC in deep ocean sediments, and has radiocarbon ages in soils in excess of thousands of years. Hence, BC appears to have a significant half-life, in the order of thousands of years. This relative inertness means that the projected <3% of the carbon converted to BC during forest, savanna and grassland fires, must be considered a significant component of the global carbon cycle with a very slow turnover."
In recent years, there's been increasing interest in the deliberate conversion of biomass into black carbon, often known as biochar in this context. Such artificial charring can achieve much higher conversion ratios than natural burning, on the order of 50% or so, while simultanenously allowing the rest of the biomass to be converted e.g. into biogas and/or directly into energy. The resulting biochar can then e.g. be mixed into farm soil (where it can apparently improve water retention and pH and otherwise improve the soil quality), or it could potentially be dumped into the ocean for very long-term storage.
All this makes biochar production a very attractive proposition. It's almost an environmental engineer's dream come true — a power plant / biogas generator with a negative net CO2 emission rate, effectively burning the hydrogen in hydrocarbon biofuel for energy while locking down the carbon into an inert form that — as icing on the cake — can then be sold as soil improvement material. Of course, as usual with emerging technologies, it's not entirely free of practical problems, but it does show promise.
Ps. Of course, there are also other mechanisms by which carbon in biomass can become locked down for long periods. For example, in peat bogs, the dead moss and other vegetation does not decay normally due to the low pH and lack of oxygen, but rather accumulates as peat. This can also sequester the carbon in it for thousands of years — assuming, of course, that no pesky humans come along to dig it up and burn it.
An important role of vegetation in carbon sequestration is related to what happens belowground. Plant roots comprise a signficant portion of vegetative biomass and remain in the soil even when a plant dies or is harvested. Plants also exude organic carbon through their roots into the subsurface, in part serving a symbiotic purpose with microorganisms. The top three meters of soil/sediment has been estimated to contain about 2344 Pg of organic carbon, more than the atmosphere and aboveground vegetation combined (Jobbagy and Jackson, Ecological Applications 10(2): 423-236, 2000). Deeper subsoils, although less rich than surface soils, also contain a significant pool of organic carbon. Most of this carbon is plant-derived, either directly (through leaf litter and root decomposition, root exudates, etc.) or indirectly (through subsurface organisms such as bacteria or fungi that depend on plant-derived carbon). Some of this carbon cycles back into the atmosphere quickly through microbial degradation, but part of it is protected from degradation by various mechanisms or moves to deeper regions such that it becomes sequestered for long time periods. So yes, vegetation can contribute to long-term carbon sequestration, although primarily by mechanisms other than the river transport and ocean deposition you proposed.
Plant matter decomposes, partly it is digested aerobically (releasing CO2 again) but partly it forms humic acids. The latter are a large part of topsoil. In this form, carbon can be sequestered in the ground for a long time.
