Introduction to Biological Oceanography

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Introduction to Biological Oceanography
Department of Oceanography, Dalhousie
University Spring, 2004 Marlon R. Lewis Lecture
Biogeochemical Cycles
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Readings (Required) Falkowski, P.G., R.T.
Barber, V. Smetacek. 1998. Biogeochemical
controls and feedbacks on ocean primary
production. Science 281 -200-206
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The Suns fusion reactions provide the energy
necessary for the physical, chemical and
biological processes on Earth. Our sun should
have begun rather small and dim and grown in
diameter through time. The amount of sunlight
reaching the Earth should thus have increased by
some 15 to 30 since the earth formed some 4.5
billion years ago.

If nothing else was different than today, this
would mean the surface of the earth world have
changed in temperature tremendously, and no
liquid water could have been present on the Earth
prior to 2 billion years ago. However, we see
instead by looking at the geological record,
that there has been liquid water on the earth
since it its crust solidified, and in general the
Earth's surface seems to have remained within a
surprisingly narrow range. Why is that?
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The answer has everything to do with the presence
of carbon dioxide in the atmosphere. Here is
whats happened over the last 40 years
What causes annual fluctuations? Seasonal
cycle of photosynthesis and the asymmetry in land
mass area between the northern and southern
hemispheres. What causes the long-term
trends? You and Me.
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More recent correlations
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What we do know We know that atmospheric CO2
is increasing. We know that anthropogenic
emissions of CO2 are increasing. We know the
radiative properties of CO2 quite well. And we
know the radiative properties of other
greenhouse gasses (e.g. methane) well. All
else equal, this should translate into warmer
Earth. But.all else is not equal, and a better
understanding of global bio- geochemical cycles,
particularly carbon, is needed to assist in
accurate prediction of future habitability of the
Earth.
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Where is the carbon today?
Reservoirs Sub-Reservoir Amount (1015 g C)
Atmosphere 720
Biota Land Oceans 827 2
Oceans (dissolved) 38,000
Sediments Organic Matter 15,000,000
Carbonate Rocks 20,000,000
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Two kinds of biogeochemical cycles maintain the
Earth's atmospheric levels of CO2 fast and
slow. The fast cycle operates on time
scales of hundreds to thousands of years.
The second operates on hundred of thousands to
millions of years. Both are essential, but
are often confused.
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First, the fast cycle The critical chemical
reactions are Photosynthesis and
Respiration CO2 H20 e-
CH2O O2 Carbonation
CO2 H2O H2CO3 H HCO3- Calcium
Carbonate dissolution and precipitation
Ca2 2HCO3- CaCO3 H2O CO2
Carbonate equilibrium in seawater
H2CO3 H CHO3- H CO32-
Photosynthesis and respiration are the clear
controllers of the seasonal cycle of CO2. Note
also that any carbon not immediately respired
results in the accumulation of O2 in the
atmosphere. We have O2 in the atmosphere because
of the C buried as organic matter in sediments
and rocks.
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A negative feed back loop keeps O2 levels from
getting too high If O2 levels get too high,
land biomass will burn and photosynthesis will go
down, and O2 will go down. Also the more carbon
is buried, the more nutrients are buried, putting
another brake on the system. CO2 in the
atmosphere is in equilibrium with the ocean. The
ocean has a vast amount of carbon in it in the
form of carbonate (CO32-), and bicarbonate
(HCO3-). Over hundreds to thousands of years,
adding more CO2 to the atmosphere is just sucked
up by the ocean, lowering the pH and thus
producing more bicarbonate to neutralize it from
carbonate thus driving the equilibrium equation
back towards the acid side. Lowering atmospheric
CO2 has the opposite effect, and results in the
precipitation of CaCO3. Because the ratio of
ocean C to atmospheric C is about 50 to 1,
doubling or tripling atmospheric CO2 does little
to the oceans or the net atmospheric CO2 on the
long run. The only reason we are having an effect
on the atmosphere is because the RATE of the
input exceeds that of the removal by the oceans!
Over thousands of year our contribution to the
atmosphere via fossil fuel burning would be
nil. And I kinda liked the greenhouse effect.
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Why is the atmosphere at 250-350 ppm instead of
other amounts? This must be a function of the
amount of carbonate in the oceans. That is
controlled by the long term cycle of carbon.
Thus, the burial of organic carbon and carbonate
carbon (ocean biology) are the controllers of O2
in the atmosphere and the carbonate pool in the
oceans, respectively. The latter controls the
CO2 in the atmosphere. Because of plate
tectonics nearly all of this buried carbon is
returned via subduction and metamorphism over
about 200 million years. In total about 0.2 x
1015 g of C is buried each year and just about
that is returned by outgassing.
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In the above diagram, Corg is organic carbon,
primarily the breakdown products of carbohydrates
produced by photosynthesis. THUS, THE ATMOSPHERIC
CO2 IS JUST WHAT REMAINS BETWEEN THE OUTGASSING
CO2 FROM IGNEOUS AND METAMORPHIC SOURCES AND
CONSUMPTION OF CO2 BY PHOTOSYNTHESIS AND
WEATHERING. The most important lesson of all
this, is that, the composition of the Earth's
atmosphere is constantly maintained by life.
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OK, in the long run, no worries (and I was hoping
for a Costa del Newf). But what about
short-time scale (100s of years) variability?
The buried Corg is being removed to fuel our
houses, cars etc., and advancing the geochemical
cycle. Exchange of atmospheric CO2 with the
oceans proceeds at a much faster rate. The sea
takes up CO2 in its surface layer, and slower
processes then exchange some of this CO2 with
deeper waters and ocean sediments. Much of the
carbon residing in the shallow oceans is in the
form of dissolved CO2. The capacity of ocean
water to store dissolved CO2 is diminished as the
water temperature increases. This constitutes a
positive feedback mechanism whereby an increase
in global temperature results in more atmospheric
CO2, which results in an increase in
global temperature, etc.
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Much of the exchange of carbon between ocean and
atmosphere is (in the short term see above),
purely physical/chemical. This is called the
solubility pump. It is quite active in areas
where deep water is formed, for example in the
North Atlantic. But what about the short term
biological impacts? Here, the nutrient cycles,
and in particular vertical exchange of nutrients
between surface and deep ocean, play a role. It
is complicated.
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Hypothesis A large body of evidence leads to
the conclusion that light limits the growth of
phytoplankton. The distribution of phytoplankton
should reflect the distribution of light.
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Welllooks like light kills phytoplankton.
Hypothesis rejected
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Hypothesis There is also evidence leads to the
conclusion that higher temperatures enhance the
growth of phytoplankton. The distribution of
phytoplankton should reflect the distribution of
surface temperature.
Maximum Growth Rate (d-1)
Temperature (oC)
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SST
Looks like phytoplankton have a low boiling
point. Hypothesis rejected.
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Well, its not light, not temperature, what could
it be?
Mar.
Aug.
Perhaps something to do with the fluid
dynamical environment?
Mixed Layer Depths
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But how might this translate into biological
production?
Annual average surface nitrate concentration.
Vigorous fluid mixing introduces a net flux of
nitrate (read nutrients) into the surface,
well-lit layer.
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Solubility Pump
Biological Pump
CO2
NO3-
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Explain this one!
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How about this?
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OK, how about these?
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Conclusions The chemistry of ocean,
atmosphere, and land, is largely related to
biological oceanographic processes, on both short
and long time-scales. The chemistry of carbon,
which concerns us quite a bit due to its
increases and radiative properties, is intimately
tied up with cycles of major (nitrate, phosphate,
silicate) and minor (e.g. iron) nutrients. In
turn, the supply of these nutrients, which
control the biological processes, is controlled
by the physical oceanographywhich in turn is
related to the air-sea heat exchangewhich is
related to atmospheric radiation.which is
related to biological productionGaia lives!