Biogeochemical cycles - The Hydrosphere The hydrosphere contains all the water on earth. Ocean...
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Biogeochemical cycles - The Hydrosphere The hydrosphere contains all the water on earth. Ocean represent 97% of it so we will focus mainly on oceans, but
Biogeochemical cycles - The Hydrosphere The hydrosphere
contains all the water on earth. Ocean represent 97% of it so we
will focus mainly on oceans, but rivers will also be discussed
Slide 2
Physical structure of the ocean (very brief ! ) Thermocline
Surface Mixed layer Deep ocean Wind driven horizontal circulation
Horizontal and vertical movements Thermohaline circulation Euphotic
layer vertical Vertical structure Horizontal structure
Slide 3
Ocean chemistry Motivation : global carbon cycle, ecosystem
functioning Key questions : What controls the mean composition (and
pH) of seawater ?
Slide 4
Observed Mean Ocean Concentrations large range Can we explain
todays mean ocean composition ? Major elements Trace elements..
Including nutrients (C,N,P,Si,Fe)
Slide 5
River Water Sea Water Mainly Ca 2+ /HCO 3 - Mainly Na + /Cl -
Both the composition and key ratios are different ppm = mg kg -1
Linking Ocean and river water composition ?
Slide 6
Weathering (chemical) of limestone is considered a congruent
reaction (all solid dissolves) CaCO 3 (s) + CO 2 (g) + H 2 O Ca 2+
+ 2 HCO 3 - 1 2 Weathering of alumino-silicate minerals to clay
minerals are examples of incongruent reactions (solid partially
dissolves) silicate minerals + CO 2 (g) + H 2 O clay minerals + HCO
3 - + 2 H 4 SiO 4 + cations 1 1 2 KAlSi 3 O 8 (s) + CO 2 (g) +
11/2. H 2 O 1/2. Al 2 Si 2 O 5 (OH) 4 (s) + K + + HCO 3 - + 2.H 4
SiO 4 Weathering of rocks (orthoclase feldspar) (kaolinite) What
Controls the chemical composition of Rivers? Minerals weather at
different rates e.g.
Slide 7
Variability in water vapor composition Europe, North America
and Asia are more calcareous continents. Most of variability due to
Ca 2+ and HCO 3 - which come from weathering of carbonate rock SO 4
2- and Cl - come from aerosols and weathering of evaporite rocks
(e.g. Salt or NaCl). Na +, K +, Mg 2+, SiO 2 come from weathering
of silicate rocks
Slide 8
Can the ocean composition results from the evaporation of river
water ? Examples: Mono Lake, CA Soap Lake, WA When river water
evaporates, it makes a Na, HCO 3 -, CO 3-- brine and the resulting
pH is very basic. pH Calculated from the charge balance H + + Na +
+ K + + 2Ca 2+ + 2Mg 2+ = Cl - + 2 SO 4 2- + HCO 3 - + 2CO 3
2-
Slide 9
Can the ocean composition results from a chemical
thermodynamical equilibrium ? Sillen hypothesis (1959, 1961) :
Sources of dissolved ions = Weathering reactions and rivers Sinks =
Reverse weathering reactions : formation of mineral (solid) in the
ocean from dissolved compounds (reaching saturation) and
sedimentation. Organizational framework: Gibbs Phase Rule f = c + 2
pf = degrees of freedom which means the number of intensive
properties, which are independent of other intensive variables, and
required to define the system. (e.g. T, P, concentrations Na +, Cl
-, Ca 2+, SO 4 2- ) c = components (ingredients composing the
system, e.g., HCl, NaOH, MgO ) p = phases at thermo equilibrium
(domains of uniform composition, e.g. gas, liquid, pure solids)
http://serc.carleton.edu/research_education/equilibria/phaserule.html
Slide 10
Sillen ocean model : Nine component model (C = 9) Acids: HCl, H
2 O, CO 2 Bases: KOH, CaO, SiO 2, NaOH, MgO, Al(OH) 3 The ocean
chemistry is viewed as a giant acid-base titration. Acids from the
volcanoes and bases from the rocks weathering. If these phases are
at equilibrium, then the system is determined by 2 independent
intensive variables, taken as T and [Cl-]. The sea water
composition is then fixed and it could only change if temperature
or [Cl - ] changed. Kaolinite, illite, chlorite, montmorillonite
and phillipsite are types of clay minerals
Slide 11
Specific reverse weathering type reactions proposed to remove
excess ions. In this approach newly formed clays would have to
equal 7% of sedimentary mass to account for the river input and
explain the observed sea water composition. The measured
distribution of clay mineral in sediment does not support these
reactions. chlorite in deep-sea sediments illite in deep-sea
sediments Instead most clays are detrital, reflecting continental
sources (so not locally found) rather than a inverse weathering
source (which would produce clays in the ocean itself) detrital =
particles of rock derived from pre-existing rock by weathering and
erosion
Slide 12
So, a thermodynamical equilibrium approach doesn t work : The
phases suggested do not appear to be at equilibrium The composition
of seawater has changed in the past (evidence from paleo-climatic
studies) But there is some evidence that inverse weathering
reactions do occur especially in near shore sediments Reverse
weathering not totally eliminated but cannot be a control for an
equilibrium global ocean. Current models for seawater composition
emphasize the balance between inputs and removals. The balance
sheet has become more important that solubility relationships for
explaining ocean chemistry. The difference has many important
ramifications. We would expect a thermodynamic ocean to have a
constant composition of the ocean and its sediments over geological
time. According to the kinetic view we would expect changes in
paleo ocean chemistry as inputs and removals varied in the geologic
past. Kinetic Model of Seawater
Slide 13
What is the origin of seawater s composition? Sources Rivers??
Mid-Ocean Ridges?? Other?? Aerosols Sinks Sediments?? Mid-Ocean
Ridges?? Other?? Aerosols Kinetic Model of Seawater - A Mass
Balance Approach Q = input rate (e.g. moles y -1 ) S = output rate
(e.g. moles y -1 ) [M] = total dissolved mass in the box (moles) =
mass / input or removal flux = M / Q = M / S Sites of Hydrothermal
Vents on Mid-Ocean Ridges
Slide 14
Kinetic model of seawater mass balance model Main input and
removal fluxes for major ions in seawater (from McDuff and Morel,
1980)
Slide 15
Group Ia Cl short term cycle = aerosols and rivers main sink
over geological time = evaporites = controlled by tectonics,
geometry of marginal seas residence time is so long (~100 My) that
changes in concentration are hard to see. Group Ib Mg, SO 4,
probably K input from rivers main sink through ocean crust /
hydrothermal activity The dominant control of mass balance is
hydrothermal activity, thus tectonics. Classification of elements
according to cycling processes Group II (e.g. Ca, Na, the remaining
cations with long residence times) Consider the charge balance for
seawater: 2[Ca 2+ ] + [Na + ] + 2[Mg 2+ ] + [K + ] = [HCO 3 - ] +
[Cl - ] + 2[SO 4 2- ] or rearranged: 2[Ca 2+ ] + [Na + ] - [HCO 3 -
] = [Cl - ] + 2[SO 4 2- ] - 2[Mg 2+ ] - [K + ] This side is
controlled by tectonics Therefore this sum is also controlled by
tectonics The controls on the relative proportions of elements on
the left hand side are complicated but include: a) Ca/Na ion
exchange in estuaries b) Ca/HCO 3 regulation by calcium carbonate
equilibrium
Slide 16
Group III (e.g. nutrients (Si, P, C, N) and trace metals) The
main balance is input from rivers and removal as biological debris
to sediments
Slide 17
Dissolved O 2 is important ! New production Recycling
Slide 18
Overview of nutriment cycling and time scales
Slide 19
Regional variability of ocean chemical composition: : Salinity
Why does surface salinity vary? S = 30 to 37 In oceanography, it
has been traditional to express salinity not as percent, but as per
mille (parts per thousand) (), which is approximately grams of salt
per kilogram of solution
Slide 20
Evaporation and Precipitation Effects on Surface Salinity
Slide 21
Regional variability : Ocean currents control on marine biology
Basin, planetary scale ~ 10,000 km Large-scale atmospheric wind
patterns Upwelling Downwelling Upwelling
Slide 22
Regional variability : Nutrient limitation on marine biology.
The example of iron Fe is necessary nutrient for marine
phytoplankton Major source is atmospheric dust deposition Southern
Ocean, far from continental dust sources, is depleted in Fe
Illustration : SOIREE (Boyd et al. 2001) Purposeful iron addition
experiment Increased marine productivity after iron addition It is
not yet clear its long-term impact on the oceanic carbon uptake
Some organisms can utilize N from dissolved N 2 gas (N 2 fixers)
Increased Fe input to the ocean can stimulate N 2 fixation
(Falkowski 1997)
Slide 23
Antarctic ice core data Increased dust deposition during cold
and arid ice age climates (Vostok ice core data) Iron hypothesis
(Martin, 1990) .. The Iron example illustrates complex geochemical
couplings between different elements, geospheres and climate
evolution
Slide 24
Summary Salinity of seawater is determined by the major
elements. Early ideas were that the major composition was
controlled by equilibrium chemistry. Modern view is of a kinetic
ocean controlled by sources and sinks. River water is main source
composition from weathering reactions. Evaporation of river water
does not make seawater. Reverse weathering was proposed but the
evidence is weak. Sediments are a major sink. Hydrothermal
reactions are a major sink. Biological control on nutriment
concentration and sedimentation.