Marine Chemistry Flashcards
Properties of water
polar substance - relatively high
strong dissolver of polar substances
highest surface tension of all liquids- except Hg- Important as capillary action in plants
Highest heat capacity of all liquids (except NH3)- prevents rapid fluctuations in temperature
latent heat of fusion- highest of all solids and liquids- heat transfer at poles- keeps polar ice caps cold
latent heat of evaportion- highest- heat transfer and evaportaiton
thermal expansion-15% greater than Hg- sea levels rise
Transparency- high- photosynthesis
Latent heat of fusion
amount of thermal energy required to change the state of a liquid to solid or gas to liquid
Photosynthesis happens where
top 30cm of ocean
water + salt means
higher density lower freezing point higher boiling point increases electrical conductivity increases viscosity
mass of sea water that is water
97.5%
salinity
• The quantity of dissolved inorganic salts in the
water
• Measured in ppt (‰) - g of inorganic dissolved
ions in 1 kg seawater – generally 34-37 ppt
• Measured by titration, or by conductivity
Measuring salinity using chlorinity
• Chlorinity is a measure of the amount of chlorine in seawater, which is directly proportional to the salinity • Measured via titration • An accurate method independent of temperature & pressure
Factors that influence salinity
evaporation, freezing -> ↑ salinity
rainfall, runoff, snowfall -> ↓ salinity
Why measure salinity?
• Salinity affects:
• the density of water (and hence oceanic circulation)
• the rate and equilibrium state of chemical reactions
• Dissolved inorganic ions in seawater can alter
the solubility of substances in the ocean
chemical equilibrium
Chemical equilibrium is where there is no net
change over time in the chemical activities or
concentrations of the reactants and products
aA + bB ⇔ cC + dD
Ionic strength (I)
The influence of ions in a solvent on solubility is measured by ionic strength (I). I = 0.5 Σ mi zi 2 m = molality: concentration in mol.kg-1 z = ion charge/valence
Why calculate I?
• Property of H2O is high polarity & readily dissolves polar solids • As I ↑, amount of charged particles in H2O ↑ and solubility of polar substances and proteins increases (to a point)
Chemical equilibrium at I=0
• For solvents with low ion concentrations (e.g.
pure water, river water), dissolved substances
behave “ideally” :
aA + bB ⇔ cC + dD
• We can ignore the influence of dissolved ions
and determine the equilibrium point as follows:
Keq = [C]c[D]d / [A]a[B]b
Chemical equilibrium at I>0
• When I>0 reactants & products behave in an
“unideal” manner and K based on
concentrations is wrong
• Equilibrium constant now written as:
• Keq = {C}c{D}d / {A}a{B}b
• {} = ion activity, total concentrations must be
corrected by ionic strength effects to work out
the equilibrium point in salt water
Ionic Activity
• A measure of how ions in a non-ideal reaction
interact
Ionic strength
a measure of the concentration
of dissolved ions in a solution
Ionic activity
describes how ions behave
Implications of increasing salinity
Increasing salinity in freshwater systems may increase phosphate solubility (Nielsen et al 2003). Solubility of complex metals may increase along an estuary (Gerringa et al 2001). Dissolved nutrients and metals may be toxic to biota
Summary seawater properties
• Water is a polar substance with a unique
combination of properties which make water
essential to life on Earth
• Seawater has consistent ionic composition
• Salinity alters the equilibrium state of reactions,
esp. the solubility of polar substances &
proteins
Euphotic zone:
enough light to
support
photosynthesis
Disphotic
measurable
levels of light,
insufficient for
photosynthesis
Aphotic
no
measurable
light
Light availability & primary production
SeaWiFS
Sea-viewing Wide Field-of-view
Sensor
Light attenuation
• With no scattering,
light is attenuated
exponentially through the water column:
• IZ = I0 exp(-Kd dz)
• Iz [Watts m-2] is the light at a depth z, dz [m]
below I0
• I0 [Watts m-2] light at top of the water column
• Kd [m-1] is the attenuation coefficient
Factors that influence Kd
→ Kd = Kw + KTSS . TSS + KChl . Chl
TSS: concentration of Total Suspended Solids [kg.m-3]
kTSS - TSS-specific att. coef. ~ 30 m2 kg-1
Chl: concentration of Chlorophyll (Chl a) [mg.m-3]
kChl - Chl-specific att. coef. ~ 0.02 m2 mg (Chl a)-1
Kw: attenuation coefficient of pure H2O ≈ 0.04 m-1
Measuring light attenuation & Kd
- Secchi disc
* Light meter
Summary light transmission
• Light attenuates exponentially with depth
• All photosynthesis occurs within photic zone
(depth to which 1% available light penetrates)
• Attenuation quantified by Kd
• High K = rapid attenuation
Biogeochemical cycles
The transport and transformation of elements or
compounds as a result of biological, chemical and
geological processes
Element pools (reservoirs) and fluxes
(movements)
• Understanding these allows us to create a budget
Closed systems:
new nutrients are not added to the
system and must be recycled
Positive flux
source (elements flow out)
Negative flux
sink (more in than out)
The ocean is a CO2…
– CO2 highly soluble in seawater
– 60 times more CO2 in the ocean
than the atmosphere
• Coal deposits are a CO2 source
DIC
• DIC (dissolved inorganic carbon) – CO2, HCO3 (≈ 90%), CO3 2- (≈ 9%) • DIC : organic carbon ≈ 40 : 1
Research into C cycle involves huge
international collaborations:
– JGOFS: Joint Global Ocean Flux Study
– IPCC: Inter-governmental Panel for
Climate Change
Global Carbon Cycle
Predominantly a gaseous cycle • CO2 is the main vector linking atmosphere, oceans and terrestrial habitats • Carbon is the “currency” in which energy is stored in food webs – Predominantly enters food webs from the bottom up (used in photosynthesis)
Physical/solubility pump
• The movement of C between the
deep and shallow open ocean
• Relies on 2 main processes:
– Enhanced solubility of CO2 in cold
water
– Upwelling of deep water at the
equator
Solubility pump
• CO2 more soluble in cold than warm water (i.e. more
soluble at the poles than the tropics)
• At the poles CO2 is ‘pumped’ from atmosphere to
ocean
• Cold, dense water sinks taking CO2 to the deep ocean
and flows at depths to equatorial latitudes (thermo- haline circulation)
• Deep water up-wells in equatorial latitudes, as it warms
CO2 solubility drops and outgases to the atmosphere
Biological pump
A biological carbon linkage between
ocean surface and deep ocean
Form of organic carbon
dead animals “remineralized” by bacteria to inorganic carbon
Continental shelf pump
• A mechanism of transfer of CO2 from atmosphere to ocean in shallow coastal waters • Proposed by Tsunogai et al (1999) • Relies on same principles that drive the biological and solubility pumps
• Shallow waters tend to be cooler than the open ocean (due to
evaporation etc.)
• CO2 is more soluble in cool, dense water (remember the solubility
pump)
• Dissolved CO2 is incorporated into food webs
• Organisms die & sink to seafloor, organic C carried offshore
(remember the biological pump)
• Estimated global capacity ~ 1 Gt C.yr-
Coal deposits
C sink
• Human activities
liberate C from this
sink
Potential human impact on biological pump positive feedback
Positive feedback
• temperature of ocean surface leading to ocean mixing
• ocean mixing means nutrients brought up from deep
water
• strength of biological pump removing CO2 from
atmosphere
• A positive feedback as CO2 also rate of this process
Potential human impact on biological pump negative feedback
Warmer planet will have more droughts.
• More dust with trace nutrients gets from land to ocean
• trace nutrients strength of biological pump removing
more CO2 from atmosphere.
• CO2 results in CO2 uptake into biological pump
• A negative feedback
Solubility pump is dependent on 2 factors:
CO2 is more soluble in cold water
– Thermo-haline circulation
Global warming is predicted to
– Increase sea surface temperatures
– Slow the thermo-haline circulation
deep ocean storage
CO2 lakes
Acidity
pH = -log10 [H+]
Liquid water dissociates by the reaction:
H2O ↔ H+ + OH
Alkalinity
Alkalinity (A) of a solution is a measure of the
negative ions present that can neutralise H+
Seawater has a strong
buffering capacity
Carbonate buffering system
Carbonate buffering system plays essential role in maintaining ocean pH: If too acidic: CO3 2- + H+ → HCO3 - If too basic: HCO3- → CO3 2- + H+
average pH of ocean
This maintains ocean pH at an average of 8.1
gets it from organisms
2 reactions when CO2 dissolves in seawater
CO2 reacts with H2O to form carbonic acid (H2CO3)
CO2+ H2O → H2CO3
Carbonic acid then dissociates releasing H+ and
bicarbonate ions
H2CO3 ↔ H+ + HCO3
-
So if we ↑ CO2, the initial reaction is ↓ pH (i.e. more H+
ions)
Predictions for 2100
IPCC A2 ‘differentiated world or delayed development’ scenario puts us at 850 ppm of atmospheric CO2 in 2100 (presently 390, up from 280 in 1750). This will lead to a 50% decrease in CO3 2- , reducing oceanic pH by 0.35 Ocean pH will change from 8.20 to 7.85
animals most as risk form decrease in pH
those that rely upon the
synthesis of calcium carbonate (CaCO3)
Calcification by pteropods
Fish rely on CaCO3 otoliths (earstones)
to navigate towards reef habitats
Ocean acidification may result in the production of
deformed otoliths
Fish with deformed otoliths struggle to find suitable reef
habitats
snails unable to make thick shells to fend off prey so have to run
Summary - Acidification
The ocean is an important sink for anthropogenic CO2 • Dissolution of CO2 in ocean lowers ocean pH (i.e. acidifies the ocean) CO2 \+ H2O ↔ H2CO3 H2CO3 ↔ H+ + HCO3 - H+ + CO3 2- ↔ HCO3 -
Summary - Calcification
Ocean acidification reduces calcification
H+ + CO3
2- ↔ HCO3
-
Ca2+(aq) + CO3
2-
(aq) ↔ CaCO3(s)
Impacts upon survival, physiology and behaviour
of marine life and predator-prey interactions etc.
What is nitrogen used in
N used in amino acids, proteins, nucleic acids
(DNA and RNA)
• Incorporated into chlorophyll used in
photosynthesis
• A limiting nutrient, particularly in temperate
marine ecosystems
N compounds
N2 nitrogen gas NO2 - nitrite NO3 - nitrate NH3 ammonia NH4 \+ ammonium
Inorganic N
• The majority takes forms not accessible to organisms
• Major sink is the atmosphere – N2(g) cannot be used
by vast majority of life
For N to be useful
must be converted to forms available to
organisms (nitrite NO2-, nitrate NO3-, ammonia NH3,
ammonium NH4+)
nitrogen bottleneck
Process of converting N2(g) to a useful form known as ‘nitrogen fixation’ Fixation may be: Biotic: performed by bacteria Abiotic: lightning strikes Anthropogenic: Haber process
Nitrifying bacteria
convert N2(g) to
ammonia NH3 (nitrogen fixing) and then to nitrates and
nitrites- approx. 90% N fixed by bacteria
Nitrosomonas: ammonium NH3 → nitrite NO2-
Nitrobacter: nitrite NO2- → nitrate NO3-
These forms of N can be used by plants in various
functions (amino acids, component of DNA etc.)
Denitrifying bacteria
Decomposing organic material: decomposing bacteria
also produce ammonia (NH3)
NH3 converted by bacteria back into NO2- and NO3-
(denitrifying bacteria)
NO3
- → N2(g) Pseudomonas
Nitrification
conversion of atmospheric N to ammonia to
nitrite and then to nitrate by microbes
N2(g) → NH3 → NO2
- → NO3
Denitrification
conversion of nitrate to atmospheric N by
bacteria
NO3
- → N2(g)
Fixation may be: 3 types
Biotic: performed by bacteria
Abiotic: lightning strikes
Anthropogenic: Haber process
Abiotic N fixation
Lightning responsible for
8% global N fixation, although estimates very ‘rubbery’ (Nesbitt et al 2000) N2(g) + O2(g) → 2NO (nitric oxide) 2NO + O2 → 2NO2 (nitrogen dioxide) 2NO2 + H2O → HNO2 + HNO3 (nitrous acid + nitric acid) HNO2 ↔ H+ + NO2 - HNO3 ↔ H+ + NO3 -
Haber-Bosch fixation
Anthropogenic N fixation
Carried out at high pressure and temperatures
• Fertilizer produced using the catalysed reaction:
N2(g) + 3H2(g) ==> 2NH3(g)
• NH3(g) converted to a crystalline form
N fixation for agriculture 3 facts
• Haber-Bosch has facilitated agricultural intensification • 40% of world’s population is alive because of it • An additional 3 billion people by 2050 will be sustained by it
Eutrophication:
accelerated production of organic matter (esp. algae) in a water body as a result of increasing nutrient inputs Results in harmful algal blooms (HABs) Algae consume dissolved oxygen leading to anoxic conditions fish kills Attributed to anoxic conditions and harmful algae as a result of nutrient enrichment
Excess N in estuaries
- Degradation of seagrass and algal beds
- Formation of nuisance algal mats
- Coral reef destruction
- Increased oxygen demand and hypoxia
- Increased nitrous oxide (greenhouse gas)
Eutrophication and coral reefs
• Nutrient enrichment may lead to ‘coral-algal’ phase shifts on tropical reefs • Macroalgae are fastgrowing species which typically benefit from increases in nutrients • Algae are able to grow rapidly by acquiring excess nutrients and may outcompete corals
P cycle differs to the N and C cycles in many
ways:
- one of the slowest biogeochemical cycles
- Phosphorus not abundant in the atmosphere
- bacteria do not play a major role
P in organisms 4 points
P often the ‘ultimately limiting nutrient’ in marine and
freshwater ecosystems
An essential nutrient for plants and animals (PO4
3- and
HPO4
2-
)
Used in photosynthesis (ATP and ADP)
A component of fats, cell membranes, bones and teeth
P cycling
So slow that except across long time-scales P does not
seem cycle:
Land → Ocean → Seafloor
Substantial losses of P to sediments
Phosphorus liberated from rock
as inorganic phosphate (PO4 3- ) through erosion Washed into oceans and available to organisms in this form
P cycling
• Cycle is slow overall
but available phosphorus is used
by marine organisms rapidly
• Phosphates from decomposing organisms in the ocean
taken up within minutes by plankton
The P cycle is slow due to
the geological fluxes
Sources of oceanic P- at
Atmosphere:
• Accounts for < 1% P inputs
• More important offshore away from rivers
• No precise estimates of exact quantities
Sources of oceanic P- vol
Volcanic processes:
• Eruptions release P4O10 and PO4
• Localized importance only
Sinks of oceanic P
Organic matter in sediments:
Organic matter in sediments:
• No stable gaseous form of P, thus major sink is the
seafloor
• Reliant upon upwelling and geological uplift to return
to surface waters
Sinks of oceanic P
Adsorption to iron particles:
• P has complex chemical interactions with iron and
colloidal material in estuaries
• P absorbs to small iron particles in the water column
making it un-available as a nutrient
• As ionic strength increases, P starts to re-dissolve
making it available to organisms
Sinks of oceanic P
Hydrothermal vents:
• Fluid coming from vents
contains lots of iron
• Binds P and takes it to the
seafloor
Redfield ratio
• C:N:P is 106:16:1 for optimal phytoplankton growth • Low N/P ratios = nitrogen limitation • High N/P ratios = phosphorus limitation
Anthropogenic P inputs
Fish farms introduce much P to underlying sediments May have strong impacts upon infaunal organisms living in sediments E.g. 1 g fish tissue produced = 11 mg phosphorus (Xu et al 2007).
Summary phosphorus cycle
Very slow (~ 50 000 yrs in sediments)
• Sediment a major sink
• Geological processes responsible for fluxes
