I have summarised the entire of the first year's content of an introduction to biogeochemistry module for oceanographers.
Summary Introduction to Biogeochemistry
1-2: Hydrothermal vents and introduction.
3: Why is the sea salty?
4: Nutrients and biogeochemistry
5: Nutrients and the ocean c...
Life requires energy and food. Supply of energy from sun continuous, but supply of chemical materials fixed and
finite (except extra-terrestrial input). So, for life to continue, Nature must recycle biologically important elements. The
study of the occurrence, distribution and cycling of material in the oceans is critical for Understanding Earth’s life
support system. Also, we know earth’s capacity for supporting life has changed over time, so need to know how
biological cycles have changed in the past.
The hydrological cycle
Water is required for life of any complexity to develop.
1. Planet must capture enough water (meteorites, comets)
2. Water must migrate from interior to planet surface
3. Water must not be lost to space (our gravitational pull is
large enough due to the Earth’s size)
4. Water must primarily be in liquid form so the right
temperature and right distance from the sun as well as
albedo are needed.
Important properties of water:
- Polar molecular, H-O covalent bond, inter-molecular Hydrogen bonds (Hydrogen bonds cause water to be
exceptionally attracted to each other. Therefore, water is very cohesive. This is the same for the polarity
causing the positive charge to be attracted to the negative version of the Oxygen).
- High boiling point with a heat capacity 4.18 J g-1 K-1 present as liquid, gas and a solid on Earth
- Very good solvent
- Density decreases on freezing causing ice sheets, ice caps etc.
Geographical variations in the hydrological cycle:
- Evaporation occurs mainly in equatorial oceanic regions
- Precipitation occurs over high land masses
- Seasonal systems such as Monsoons have dramatic effects on the hydrological cycle
- Some river systems dominate river runoff with the Amazon providing ~ 20% of freshwater input into the
oceans
- Rivers not only transport water into the oceans but they also input in weathering products: dissolved material
produced by chemical weather of rocks, and particulate material produced by physical weathering.
The major world rivers: Amazon largest by factor 5
(6300 km^3/yr cf 1250 for next; 17% of total discharge).
Congo, Orinoco, Yangtze, Brahmaputra (high sediment
yield as drains easily eroded foothills of Himalaya),
Mississippi, Yenisei, Lena (Arctic rivers low sediment
yield, mainly drain tundra), Mekong.
High elevation: lots of physical weathering so lots of
particulate material. Chemical weathering dominates
where flat, longer water-rock interaction time. Yenisei,
tundra dominated.
Weathering and global climate:
- Chemical weathering reactions result in drawdown of co2 which in turn controls climate.
Carbonic acid formation Chemical weathering of silicate rocks
2CO2 + 2H2O 2+ -
2H2CO3 + CaSiO3 + H2O Ca + 2HCO3 +
Chemical weathering of carbonate rocks
2+ -
H2CO3 + CaCO3 Ca + 2HCO3
,Biogeochemical processes in the ocean:
Once material is delivered to the oceans, most doesn’t just sit there – it’s subject to biogeochemical reactions.
An example is Iron distribution, delivered mainly by rivers but also by atmospheric dust.
Note very high levels of dFe in surface ocean around
55S- dust from S America
Concentrations very low at surface because Fe essential
nutrient for primary production. Increases at depth sue to
recycling decaying organic material.
Input from hydrothermal sources over mid-ocean ridge.
Close to Antarctic continent, supply of iron from
icebergs.
nmol/l?
n = nano, or 10-9
mol = mole. One mole = 6.02 × 1023 atoms (Avogadro’s number)
Mass of one mole of atoms = atomic mass (in grams)
Atomic mass of iron is 55.85
So, 1 nmol/l corresponds to 1 × 10-9 mol/l × 55.85 g/mol = 55.85 × 10-9 g of iron per litre of seawater (~10 mm
diameter rust particle in 1 litre of water)
Tectonic activity of the Earth:
The plates are not spreading apart at all the same rates, with fast and slow spreading ridges.
In bursts of volcanism, tectonic plates draw apart and lava wells up adding new seafloor to the separating plates.
Fissures open, seawater seeps in, is heated and rises in mineral rich springs that support oases of life.
Hydrothermal circulation: seawater being drawn down
at the newly formed seafloor that is then heated up and
rises as hot springs.
To explore these, we use seismic imaging where
density differences are picked out or directly explored
through ocean drilling which has been in operation to
try and pass through the gabbro.
Evidence of hydrothermal vents:
1970s seepage of warm fluids offshore Galapagos.
First high temperature (~350c) vents were observed in
the East Pacific Rise in 1981.
, Black smokers:
High temperature (>350 °C) fluids erupt at high flow rates through chimneys
Hi-T fluids are clear, but iron and other metals in the fluid precipitate as they mix with
seawater, forming ‘black smoke'.
Pure fluid is clear, black smoke on mixing with seawater
White smokers:
Lower temperature (<200 °C) fluids seep through small chimneys or cracks in the
seafloor
These fluids have lower metal concentrations, and the white smoke usually consists of
fine particles of anhydrite (calcium sulfate), barite (barium sulfate) or talc (magnesium
silicate)
Usually around 2-3km down.
TAG hydrothermal mound: over time the vents will block up and eventually the
minerals will coalesce and form mounds. Different types of deposit: black vs white
smokers, sulfide rubble, weathered sulfides, importance as a mineral resource.
Hydrothermal cycle removes some elements from seawater (sink) and adds other
elements (source). This has a significant impact on the chemistry of seawater and the
crust.
Hydrothermal plumes are hot vent fluids that are buoyant, so they rise through the
water column like smoke from a chimney. (Super plumes rise higher after eruptions).
These continually mix fluids with seawater and then will eventually cool and stop
rising.
These can have enormous amounts of production and fauna ->
In the Pacific Ocean tube worms convert hydrothermal fluids into energy.
Pacific ridges: large clams that also convert chemical compounds into
energy. Colorless shrimp with heat sensors that keep them safe whilst
collecting chemicals. Altogether unique oases of life reside here.
Biological interactions:
-Chemolithoautotrophic microbes are abundant around vents.
-Acquire energy from reduced compounds in fluids (rather than sunlight).
-Support ecosystem
around vent site.
-Low species diversity, dynamic environment.
-High biomass compared with deep ocean
Periodic table
, Groups: How many electrons in the outer shell.
Periods: How many shells.
What’s being delivered to the ocean is mirrored by what it is composed of.
Characterized by the minimal similarities.
How salty is the water?
You can evaporate the water and weigh salts but then you will lose important elements that will not then be measured,
resulting in fluctuating answers. -> Issue is loss of material, e.g., CO2 which varies with temperature
Titrate halides against AgNO3 – precipitate Ag halides
S = 1.80655 x Cl
Conductivity measurements relative to KCl solution – used since 1978 (known as salinity)
The practical salinity scale (PSS) has no units, i.e., it is a dimensionless number
Salinity
Salinity is nearly always calculated from conductivity measurements
S = 0.0080 – 0.1692R1/2 + 25.3851R + 14.094R3/2 – 7.0261R2 + 2.7081R5/2
where R is the conductivity
ratio at 15 °C and 1
atmosphere (standard
temperature and pressure)
Although salinity determined by
conductivity is temp and
pressure dependent, it may be
used as a proxy for the
concentration of total dissolved salts in seawater.
Salinity variation in the Atlantic
Most dramatic differences in the ocean surface are due to evaporation and
precipitation. Subtropical regions are the saltiest regions. At equatorial regions there
is higher precipitation and therefore we get slightly lower amounts of salinity. Determined by balance evaporation vs
precipitation; water below 1000m unaffected. At Equator, precipitation high so salinity slightly lower than in sub-
tropics, where evaporation is higher than precipitation.
(2) Increased evaporation = increased salinity.
(2) Reasonably close relationships between precipitation and salinity.
Constancy of composition of seawater
Concentrations of major dissolved ions (those which have concentrations of >1ppm by
weight) can vary from place to place in the oceans, but their RELATIVE proportions remain
virtually constant.
Major ions (mainly) show conservative behavior (due to relative proportions remaining
virtually constant).
Conservative behaviour (concentrations in seawater are not changed by biological and chemical
reactions that take place)
Ratio of sodium ions to salinity is constant throughout the oceans.
Some major ions show small variations e.g., calcium ions are utilized in surface waters and re-dissolve at depth.
The kink in the CA curve shows the utilization of calcium to calcium carbonate which is used by some plankton that
extract the calcium and use it for their shells hence the small decrease.
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