Post on 20-Dec-2015
Chapter 10The Marine Environment
Continental shelf: the submerged edge of a continent.
Shelf break: the point where there is a change in slope on the shelf from~0° to 2°-4°.
Neritic environment: that part of the ocean that overlies the continental shelves.
Oceanic environment: that part of the ocean that overlies the ocean basin.
Vertical structure of the ocean
Ekman Transport
Surface currents are deflected by the Coriolis effect. In the northern hemisphere, the causes a clockwise rotation.
If the ocean current is regarded as layered, then each deeper layer moves more slowly than the overlying layer.
Layers that move slower will be acted on more strongly than those that moved faster. Therefore, the lowest layers are rotated 90°and more to the surface layer.
Upwelling: where Ekman transport causes surface waters to diverge or move away from the coast and deeper (often cold and nutrient-rich) water to be brought to the surface.
Downwelling: where Ekman transport causes surface waters to converge or impinge on the coast, displacing surface waters to converge or impinge on the coast, displacing surface water downward thickening the surface layer.
Thermohaline circulation and specific water masses
These intermediate and deep ocean water masses are characterized by their temperature and salinity. When two water masses interact, the temperature and salinity of the mixed waters are linearly related to the relative proportions of the contributing water masses.
Side bar… Thermohaline circulation and climate change…
As might be expected, the different water masses often do not have a unique temperature and salinity, but rather a range of characteristic temperatures and salinities.
Example 10-1 Water samples were collected in the North Atlantic in the depth range 850m to 1500m. The sample collected at 850m had T & S values typical of MIW, whereas the sample collected at 1500m had T & S characteristic of NADW. The temperature and salinity values for the 5 samples are given in the table below.
When plotted, the data lie on a straight line, with the NADW & MIW as end members. The other samples represent simple mixtures of MIW & NADW. The numbers in parentheses indicate the fraction of MIW in each sample.
Chemical composition of the oceans
Constancy of composition: the ratio among all dissolved elements is constantNormalized to Cl-
Salinty: the total amount in grams of solid material dissolved in 1kg of seawater when all the carbonate has been converted to oxide, all the iodine and bromine have been replaced by chlorine, and all organic matter has been completely oxidized.
(S ‰ = 1.80655 Cl- o/oo)
Residence time varies by element
Table 10-2. Elemental oceanic residence times (in years) base d on river in put*
Log (y) Log (y) Log (y) Log (y)
Li 6.3 S 6.9 Cu 4.0 Sb 4.0
B 7.0 Cl 7.9 Zn 4.0 I 6.0
C 4.9 K 6.7 As 5.0 Cs 5.8
N 6.3 Ca 5.9 Se 4.0 Ba 4.5
O 4.5 Sc 4.6 Br 8.0 La 6.3
F 5.7 Ti 4.0 Rb 6.4 Au 5.0
Na 7.7 V 5.0 Sr 6.6 Hg 5.0
Mg 7.0 Cr 3.0 Zr 5.0 Pb 2.6
Al 2.0 Mn 4.0 Mo 5.0 Ra 6.6
Si 3.8 Fe 2.0 Ag 5.0 Th 2.0
P 4.0 Co 4.5 Cd 4.7 U 6.4
*Data source, Holland (1979)
Residence times = seawater conc. / input rate of element
Short residence time elements are highly reactive, and not recycled
Inputs of elements : atmosphere, continental weathering, seawater-seafloor interactions
Removal of elements : atmosphere, precipitation, adsorption, reactions with seafloor
Atmosphere supplies and removes gasesContinents supply major elements
Direct precipitation is relatively unimportantBiological removal (Si, Ca)
Adsorption onto primarily oxyhydroxides. pH = 8.0, oxyhydroxides havenegative charge….so cations are the adsorbed
Water cycling through seafloor basalts add some and remove other elements.
Biological controls on seawater composition
Redfield ratio, C:N:P= 106:16:1
Expanded redfield ratio include trace elements
O2 minima, nutrient maxima
Only a fraction of the biogenic elements created in the upper ocean is buried in ocean sediments
f = [D]
[R]
[S]
[R]-
1
1 + 20
D: deep water conc.S: surface water conc.R: River water conc.
Assuming upwelling is 20x river input:
g = 20[S]/[R]
20 [D]/[R] + 11 -
g is the fraction of biogenic element removed
= 1600 y / fg
is the residence time
Example 10-2: The following average concentrations, in mol L-1, were determined for P in the Pacific ocean: surface water = 0.2 and deep water = 2.5. For average river water, P = 0.7 mol L-1. Calculate f, g,and for P.
f = 1 / 20(([2.5] – [0.2])/[0.7])) + 1 = 0.015
g = 1 – ((20[0.2]/[0.7])/(20[2.5]/[0.7] + 1)) = 0.92
t = 1600 y / (0.015)(0.92) = 115,942 y
This tells us that 92% of the phosphorus is removed from the surface waters as biogenic particles, and that over 98% of the phosphorus contained in the organic particles is returned to the water column by decomposition and dissolution. The calculated residence time of P in the ocean is relatively long because it is continually recycled through the oceanic system.
Seawater – sediment interactions
At seawater pH clay particles have neg. charge
At high ionic strength, monovalent cations preferentially exchangefor divalent cations (Na+ swaps out for Ca2+)
Redox conditions, determined primarily by organic matter availability
Biolimiting elements: elements that are almost totally depleted in the surface waters.
Biointermediate elements: elements that are only partly depleted in the surface waters.
Biounlimited elements: elements that show no measurable depletion in the surface waters.
Seawater-basalt interactions
Mid ocean ridges, range of temperaturesMajor basaltic minerals olivine (Fe,Mg)2SiO4
pyroxene Ca(Mg,Fe)Si2O6 , (Mg,Fe)SiO3
Ca-plagioclase CaAl2Si2O8
obsidian
High Temp reactions:Reactions remove Mg2+ and SO4
2-, Reactions add Ca2+ , H4SiO4, and K+
Low Temp reactions:Reactions remove Mg2+ and K+, Reactions add Ca2+ , H4SiO4
Table 10-4. Concentration changes for seawater species duringhigh temperature seawater - basalt interactions*
Concentrations ( mmol L-1)
Seawater Hot springs Δ ( mmol L-1)
Mg2+ 54 0 -54
Ca2 + 10 36 26
K+ 10 26 16
SO24 28 0 -18
H4SiO4 (aq) ~0 20 ~20
ΔCa2 + - Δ SO24 54
*From Berner and Berner (1996)
K+
SO24
H4SiO4 (aq)
ΔCa2 + - Δ SO24
*From Berner and Berner (1996)
Seawater basalt interaction
Table 10-11. Chemical composition of oceanic Fe-Mn deposits
Element abundance (µg g-1)
Element Hydrogenous crust Oxic nodule Sub-oxic nodule Hydrother mal crust
Mn 222,000 316,500 480,000 550,000
Fe 190,000 44,500 4,900 2,000
Co 1,300 280 35 39
Ni 5,500 10,100 4,400 180
Cu 1,480 4,400 2,000 50
Zn 750 2,500 2,000 2,020
Mn:Fe 1.2 7.1 98 275
Mineralogyof Mn phase δ-MnO2
todorokiteδ-MnO2
todorokitebirnessitetodorokite
Growth rate( mm/106 y) 1 -0 2 10 - 50 100 - 200 1000 - 2000
Seawater chemistry
Ionic strength (I) of seawater related to salinity
I = (19.92 x S‰) / {1000 - (1.005 x S‰}
Alkalinity of seawater should include contributions of other species capable of accepting electrons in addition to the carbonate species
Ex: Peng et al. 1987
TA = [HCO3-] + 2[CO3
2-] + [H2BO3-] + [H3SiO4
-] + [H2PO4-] + 2[HPO4
2-]+ 3[PO4
-] + [OH-]
However, the majority of seawater alkalinity still comes from CO32-
and HCO3- .
CA (carbonate alkalinity) = HCO3- + 2CO3
2-
The Redfield ratio is C:N:P = 106:16:1.
Alkalinity cont.
PhotosynthesisDIC decreasesAlk stays the same
RespirationDIC increasesAlk stays the same
CaCO3 dissolutionDIC increasesAlk goes up
CaCO3 precipDIC goes downAlk goes down
youngerolder
CH2O + O2 CO2 + H2O
CO2 + H2O H2CO3
H2CO3 + H2O H+ + HCO3-
HCO3- + H2O H+ + CO3
2-
CaCO3(calcite) Ca2+ + CO3
2-
Calcite, aragonite solubility and the CCD
Saturation horizion = depth at which ocean becomes undersaturated withrespect to calcite and aragonite
Lysocline = depth where waters are increasingly undersaturated withrespect to calcite and aragonite
CCD = depth below which, calcite and aragonite is so undersaturatedthat calcareous sediments won’t accumulate on the seafloor.
Carbonate Compensation Depth (CCD)
Calcite, aragonite solubility and the CCD
Ca2+ can be calculated using salinityCa2+ = 0.01028(S/35)
Surface waters are strongly oversaturated with respect to calciteAragonite more soluble than calcite by a factor of 50
Saturation state for both is essentially a function of CO32- conc.
Less CO32- in Pacific (see Example 10-3) = shallower CCD in Pacific
Buffering capacity of seawater includes contributions from the carbonatesystem and boric acid
Maximum buffering capacity range for the boric acid and carbonate systems are not within the normal pHs found in seawater.
Although seawater is well-buffered on long timescales by reactions with calcite,pH can vary on the short-term (remember photosynthesis versus respiration)
Trace metals in seawater
Sources: hydrothermal (Mn, Fe, Ba, Li, Rb) rivers atmosphere
Sinks: removed by particles (either lithogenic, or biogenic)
Open ocean, particles are biogenicCoastal ocean, particles both biogenic and lithogenic
Metals removal by biology: Uptake, Redfield : C:N:P:Fe:Zn:Mn:Ni:Cd:Cu:Co:Pb
180:23:1:0.005:0.002:0.001:0.0005:0.0004:0.0002:0.00004
Metals enriched in shell material (Ba, Sr, Cu, Ag, Zn, Pb, Ti, Cr, Mn, Fe, Ni)
EF = Me conc. in biogenic material / Me conc. in sewater Ligand complexes with secreted organic material
Enrichment factor (EF) = Metal concentration in biogenic materialMetal concentration in seawater
Metals removal by adsorption and precipitation
Sinking particles composed of conglomerates of:organic matter, oxyhydroxides, clay minerals, shell
Particles have net negative charge
Metals are scavenged by particles
Conservative vs. non conservative distributions
Figure 10-18. Schematic plot of concentration versus salinity. Species A shows conservative behavior. Species B and C show nonconservative behavior. Internal processes are adding species B to the seawater. Internal processes are removing species from the seawater.
uptake
production
Mixing line
Vertical profiles
Biolimiting trace metals will be depleted in surface waters
If a trace element is associated with organic matter, there will be a mid water column maxima
If a trace element is associated with CaCO3 shell material there will be a deep water maxima (CCD)
If a trace element is associated with opaline shell material, its concentration will be correlated to Si
Methane Hydrates
Global distribution of Methane Hydrates
It is estimated that there is a total 1.2x1017 m3 of methane gas (expanded to atmospheric conditions) or equivalently 74,400 Gt of CH4 in ocean hydrates, which is three orders of magnitude larger than worldwide conventional natural gas reserves. Of this, it is estimated 4.4x1016 m3 of methane expanded to STP exists on the continental margins.
Marginal Marine Environment
Water and salt balances
Water balance : Ti + P = To +ETidal inputs plus precipitation = tidal outputs plus evaporation
Salt balance: TiSi = ToSo
Tidal inputs times ocean salinity = tidal outputs times basin salinity
Combine:
Ti = [So(E-P)] / (So – Si)
To = [Si(E-P) / (So-Si)]
SillSill
Brine
Principal types of estuaries based on physiographic characteristics.
Ganges-Brahmaputra River Delta
Mississippi River Delta
Water column chemistry in marginal or estuarine waters
Mixing of fresh and salt waters impact many of the parametersthat govern mobility of elements (e.g. adsorption, redox, aqueous complexes, pH etc)
Examples: Fe and Al precip out in seawater due to higher pHIncreased ionic strength leads to flocculation of colloids (turbidity max)Redox sensitive phosphate, iron oxyhydroxide reactions
Use salinity vs. species mixing curves as a first indicator of reactivity
AdsorptionMetals will associate with dissolved organic matter and suspended particulatematter. Increased dissolved organic matter will decrease the amount of metalsscavenged by suspended particulate matter
In general: an increase in salinity will release adsorbed metals into solutionbut enhance adsorption of organic species.
RedoxThere are a number of redoxclines in coastal marine / esturarine settings
Low redox – Fe, Mn, Co soluble Cd, Cu, Zn insoluble (ppt as sulfides, or sulfide complexes stuck to silicate particles)
High redox – Fe, Mn, Co exist as insoluble metal oxyhydroxides
Biomarkers in sediments
Retain original source-specificStructure
Combined with radioisotopes
Enrichment above 1800’s levels
PAH fingerprints