Upwelling is the converse of
downwelling, and as the deep waters rise to the surface, they bring
with them carbon. These microbes (considered in terms
of their respiratory output) are very sensitive to the organic carbon
content of the soil as well as the temperature and water content,
respiring faster at higher carbon concentrations, higher temperatures
and in moister conditions (although if the soil is flooded with
water, conditions are worse since no oxygen can get into the soil --
the majority of the microbes need oxygen to respire, as can be seen
from the general equation for respiration given under the discussion
of plant respiration). In
defining this flow in our model, we need to split it into the carbon
added to the warm surface waters of the ocean and the carbon added to
the cold surface waters. This
runoff is eventually transported to the oceans by rivers. Learn how carbon moves through Earth's ecosystems and how human activities are altering the carbon cycle. Litter Fall (and below-ground
addition to the soil). Planktonic organisms make shells of
CaCO3,
and when these sink to the seafloor, they carry Ca2+
ions with them, thus reducing the alkalinity. This upwelling water also
brings with it nutrients such as nitrogen and phosphorus, making
these waters highly productive. Ultimately, we want to get an
expression for the concentration of CO2 gas contained in
the seawater at equilibrium; this is usually expressed as the partial
pressure of CO2, with units of
matm
(microatmospheres), or ppm (parts per million, by volume) rather than
a typical concentration, which would have units of moles per cubic
meter of water. This is the currently selected item. Our next
step is to combine the various equilibrium constants into a single
value as: keeping in mind that this value will
be a function of temperature (also salinity). Clearly, this biologic pump is an
important process. Adopting this pre-industrial case
as our steady state has another advantage in that we know the history
of CO2 emissions from human activities pretty well and we
know the present state of the carbon cycle pretty well and we even
know the rate of change of various parts of the carbon cycle. But how can this be the case? Land-Use Changes -- Forest Burning
and Soil Disruption. This increases the complexity of the
algebra, giving us a quadratic equation whose solution ends up
as: Regardless of whether we consider the
more digestible form (10) or the more precise form of expressing
HCO3-
and CO32-
(11), we are finally set, because if we look at the equation for the
partial pressure of CO2. Current estimates place the
total addition to the atmosphere from forest burning and soil
disruption at around 1.5 Gt C/yr; estimates divide this into 70% to
50% forest burning, with soil disruption making up the
remainder. These animals and plants eventually die, and upon decomposing, carbon is released back into the atmosphere. One option is that we could attach our model of the
carbon cycle to one of our models for the solar energy system. And yet if we don't make some attempt to describe this
process in the form of a global model, our understanding of the
dynamics of the global carbon cycle will languish in the early
stages. This transfer is often referred to as the biologic pump, and
it causes the concentration of CO2 gas, and also
SCO2
, the concentration of total dissolved inorganic carbon in the
surface waters to be less than that of the deeper waters. That reaction is: which is effectively the reverse
reaction for photosynthesis. by Zhang Nannan, Chinese Academy of Sciences. Then we can substitute
(4) and (6) into (5) to obtain another expression for the partial
pressure of carbon dioxide gas in seawater: The next thing we need to do is to
find expressions for the concentrations of carbonate and bicarbonate
in terms of the total amount of carbon dissolved in seawater, which
will change over time as the oceans release or absorb CO2
from the atmosphere. However,
some of the organic remains and the inorganic calcium carbonate
shells will sink down into the deep oceans, thus transferring carbon
from the shallow surface waters into the huge reservoir of the deep
oceans. This
will be provide us with a very nice way of assessing the significance
of our modeling results. The excess positive charge of
seawater that needs to be balanced by the different forms of
dissolved carbon is called the alkalinity of the seawater. 14C
dates on some of this soil humus give ages of several hundred to a
thousand years old. First, we need to define a term for the total
concentration of inorganic carbon in solution: This is an approximation since it
ignores CO2 gas and H2CO3, but both
of these are very minor components. The data used in our global carbon cycle model lead to a
residence time of about 26 years for the soil carbon
reservoir. But, we don't
want this to be a constant value; it will undoubtedly change as a
function of the size of the land biota reservoir. When deforestation occurs, most of the plant
matter is either left to decompose on the ground or it is burned, the
latter being the more common occurrence. By controlling the concentration of CO2 gas
dissolved in the surface waters, the planktonic organisms exert a
strong influence on the concentration of CO2 in the
atmosphere. An
interesting thing happens when the rate of photosynthesis increases;
the leaf interiors are filled with more CO2, so the
stomata close down to limit the intake of more CO2, and in
doing this, the plant also limits the amount of water that can
escape. Field experimental photos … As a general rule, the rates of most metabolic processes
increase with temperature, but there is usually an upper limit where
the high temperatures begin to destroy important enzymes, or
otherwise inhibit life functions. In addition, some
of the organic carbon is consumed by organisms living in the deep
waters and within the sedimentary material lining the sea floor. The answer is, yes, it would, except for the
fact that there are other flows of carbon out of the this cold water
reservoir, most importantly, downwelling. Let's see if we can summarize this
carbonate chemistry -- it is important to have a good grasp of this
if we are to understand how the global carbon cycle works. This sum, the concentration of
total dissolved carbon, is simply equal to the amount of carbon in
the ocean reservoir divided by the volume of the ocean. The
magnitude of this flow is small -- about 0.6 Gt C/yr -- relative to
the total amount transferred by sinking from the surface waters -- 10
Gt C/yr. The carbon cycle illustrates the central importance of carbon in the biosphere. Photosynthesis is
also limited by the availability of other nutrients, especially
nitrogen, which tends to be a limiting nutrient in many terrestrial
ecosystems. The truth of the matter is that we don't know enough
about the operation of our whole planet to solve this problem with
confidence -- the study of the global carbon cycle is in its early
stages. This decomposition thus
returns carbon, in the form of CO2, to seawater. The most straight-forward way to
represent the function of the marine biota in our model would be to
represent the marine biota in the form of two reservoirs, one for
each of the two surface water reservoirs. When sedimentary rocks deposited on
oceanic crust are subducted, they may melt or undergo metamorphism;
in either case, the carbon stored in calcium carbonate -- limestone
-- is liberated in the form of CO2, which ultimately is
released at the surface. Gross Primary Production differs from Net Primary
Production, which is the gross minus respiration. The
magnitude of this flow is quite small, and is adjusted here to a
value of 0.6 Gt C/yr in order to create a model in steady state. In doing this, we try
to represent the observation that the warm parts of the oceans give
up CO2 to the atmosphere while the colder parts absorb
CO2 from the atmosphere. Carbon is a constituent of all organic compounds, many of which are essential to life on Earth. The flows will furthermore be defined as standard
draining processes (first-order kinetic processes) as
follows: where Frow and Froc are
the runoff flows to the warm ocean and the cold ocean
respectively. In
our model of the carbon cycle, we will use an expression for soil
respiration that takes these observations into account: The temperature sensitivity part of
the equation is a linear function like that used in defining
photosynthesis. Temperature is another important
consideration in many life processes, and photosynthesis is no
exception. Or
more precisely, the respiration does depend upon temperature, but in
approximately the same way that photosynthesis does, so the ratio
between them stays the same. The flow of carbon in and
out of these marine biota reservoirs would be represented by three
flows -- a photosynthetic uptake flow, a respiration flow (which
combines plant respiration with rapid decomposition within the upper
100 m of water), and a sinking flow.
.
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