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