Introduction to Biological Oceanography

Department of Oceanography, Dalhousie University

Spring, 2004

Marlon R. Lewis

Lecture: Biogeochemical Cycles

Readings (Required):

Falkowski, P.G., R.T. Barber, V. Smetacek. 1998. Biogeochemical controls and feedbacks on ocean primary production. Science 281: -200-206

The Sun’s 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?

The answer has everything to do with the presence of carbon dioxide in the atmosphere. Here is what’s 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.

More recent correlations:

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.

Reservoirs Sub-Reservoir Amount (10^15 g C)

Atmosphere

720

Biota Land

Oceans

827

2

Oceans (dissolved) 38,000

Sediments Organic Matter 15,000,000

Carbonate Rocks 20,000,000

Where is the carbon today?

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.

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+ + CO3

2-

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.

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 (CO3

2-), 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….

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 10^15 g of C is buried each year and just about that is returned by outgassing.

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.

OK, in the long run, no worries (and I was hoping for a “Costa del Newf”). But what about short-time scale (100’s 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 inmore atmospheric CO2, which results in an increase in globaltemperature, etc.

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.

The distribution of the marine phytoplankton is not uniform over the global ocean. This leads to questions: A.     What limits the growth and accumulation of biomass in the world’s oceans?B.     What are the consequences for fluxes and distributions of biogeochemical compounds?C.     How do these processes translate into higher trophic levels, and fluxes to the sea bottom?

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.

0

1

2

3

4

5

6

7

0 500 1000 1500 2000 2500

Irradiance(mol quanta m-2 s-1)

Pho

tosy

nthe

sis

mgC

(m

g C

hl)-1

h-1

“High Light” Cells

“Low Light” Cells

Well…looks like light kills phytoplankton. Hypothesis rejected

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.

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40

Temperature (oC)

Max

imum

Gro

wth

Rat

e (d

- 1)

Looks like phytoplankton have a low boiling point. Hypothesis rejected.

SST

Well, its not light, not temperature, what could it be?

Perhaps something to do with the fluid dynamical environment?

Mixed Layer Depths

Mar.

Aug.

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.

CO2

Biological Pump

Solubility Pump

NO3-

Explain this one!

How about this?

OK, how about these?

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 oceanography…which in turn is related to the air-sea heat exchange…which is related to atmospheric radiation….which is related to biological production…Gaia lives!