Lecture Notes (PDF) - Aqueous and Environmental Geochemistry

Marine Chemistry I
Environmental Geochemistry
DM Sherman, University of Bristol
The Hydrologic Cycle
1

Average Rivers vs. Ocean
Concentration
in Average
River
(mmol/kg)
Seawater
Concentration
(mmol/kg)
(M/Cl)sw
(M/Cl)riv
Cl - 0.16
54.6
Na + 0.224 48.0
Mg +2 0.138 5.4
Ca +2 0.334 1.0
SO
-2
4 0.068 2.9
K + 0.033 1.0
HCO 3 0.852 0.211
Br - --
0.087
H 4 SiO 4 0.173 0.10
1
0.62
0.11
0.12
0.009
0.09
0.0007
0.002
Why is seawater different from river
water?
•You cannot get seawater by evaporating river water
(instead, you get an alkaline NaHCO 3 brine).
•Ca 2+ , HCO 3
-
and H 4 SiO 4 are taken up by marine
organisms and lost as sediments.
•Mg 2+ and SO 4
-2
are taken up by basalt-seawater
interactions at Mid-Ocean Ridges.
•Cations such as K+ may be taken up in clays via
“reverse weathering” and ion exchange.
2

Sillen’s (1961) Equilibrium Model
•Major ion composition of seawater determined by
chemical equilibrium.
•Cations (Ca, Si, Al, Na, K and Mg) supplied by
weathering igneous rocks
•Anions (Cl, HCO 3- ,) suppled by volcanic emission.
•Formation of clay minerals from solution and by
reverse weathering:
(K, Ca, Mg..) + kaolinite + Si(OH) 4 + HCO 3
-
=
(K,Ca, Mg)-clays + CO 2
Arguments against Equilibrium Model
•Reverse weathering reactions are extremely slow.
•No evidence has been found for such reactions in
marine sediments.
•Distribution of clays suggests detrital (eolian and
riverine) origin; not formed authigenic.
3

Mackenzie and Garrels (1966):
Steady State Model
Input
Oceans
Output
•Rivers
•Hydrothermal
Vents
•Atmosphere
V = 1.4 x 10 21 L
•Sedimentation
•Evaporation
•Hydrothermal
alteration
If we assume that the oceans are at steady state, then the
concentrations of elements are determined by the balance of
input and output fluxes.
Example: Mg Cycle
Net flux = -1.5
(not steady state)
Atmosphere/Evaporites
0.5 x10 12 mol/y
River Input
8.0 x10 12 mol/y
Oceans
Mg = 69 x 10 18 moles
Ion Exchange/
Reverse Weathering
1.2 x10 12 mol/y
Hydrothermal Alteration
7.8 x10 12 mol/y
4

Residence Times of Chemical Species
Water in the Oceans:
Volume of Oceans (V ocean ) = 1.37 x 10 21 L
Global Riverine Input (F Riv ) = 3.6 x 10 16 L/y
t H oceans = V oceans
F Riv
= 1.37 x1021 L
3.6 x10 16 L / yr = 3.81x 104 years
!
Residence Times of Chemical Species
Sodium in the Oceans:
Concentration in Oceans (C ocean ) = 0.47 mol/L
Avg. Conc. in Rivers = 2.2 x 10 -4 mol/L
oceans = C Na
oceans
=
Na F River
t Na
C River
V oceans
(0.47mol / L)(1.37 x10 21 L)
(2.2x10 "4 mol / L)(3.6 x10 16 L / yr ) = 8.1 x 107 years
!
5

Residence Times of Chemical Species
Highly reactive elements have short residence
times.
Element
log(t R /Yr)*
Element
log(t R /Yr)
Na 7.9
K 6.7
Mg 7.0
Ca 5.9
*Based on riverine input.
Fe
Co
Cu,Zn
Pb
2.0
4.5
4.0
2.6
Distribution of Marine Sediments
6

Silica in Seawater
Dissolved Si as (H 4 SiO 4 )
is taken up by
radiolarians in surface
waters and deposited as
siliceous ooze in bottom
sediments.
Silica(am) and quartz are undersaturated in seawater but
the dissolution of SiO 2 tests is slow.
Clay Mineral Input to Oceans
Clay minerals are input into
oceans as part of the
suspended load of rivers.
aeolian dust, and volcanic
ash.
Reverse weathering:
take up Mg and K
Ion Exchange: take up
Na and release Ca
7

Clay Minerals
Reverse Weathering Examples
2K + + 3Al 2 Si 2 O 5 (OH) 4
= 2 KAl 3 Si 3 O 10 (OH) 2 + 3H 2 O + 2H+
10Mg 2+ + 2Al 2 Si 2 O 5 (OH) 4 + 2Si(OH) 4 + 10H 2 O
= 2 Mg 5 Al 2 Si 3 O 10 (OH) 8 + 20H +
18Mg 2+ + 2Na + + Al 2 Si 2 O 5 (OH) 4 + 20Si(OH) 4
= 6 Na 0.33 Mg 3 Al 0.33 Si 3.67 O 10 (OH) 2 + 38H + + 18H 2 O
8

Reverse Weathering
Ion Exchange with Riverine Clays
2Na + + Ca-Clay = Na 2 -Clay + Ca 2+
Average Equivalent
Fraction
River
Clays
Marine
Clays
Net Removal
from Ocean
(10 12 g/year)
Percentage
of River
Input
Na + 0.04 0.42
45
30
K + 0.01 0.06
9
13
Ca +2 0.6 0.16 -40
-8
Mg +2 0.25 0.32
4
3
These reaction are kinetically fast and should reach
equilibrium.
9

Hydrothermal Input/Output
Distribution of Hydrothermal Vents
Hydrothermal Input/Output
10

Hydrothermal Fluxes
River Flux
(10 10 mol/yr)
F - 16
-1.1
Rb + 0.04
0.24
Mg +2 530
-800
Ca +2 1200 350
SO
-2
4 400
-400
K + 190
190
H 4 SiO 4 600
300
(Edmund et al., 1979)
Hydrothermal
Flux
(10 10 mol/yr)
Ferromanganese Deposits in the Ocean
Vast deposits of Fe-Mn oxides (todorokite, birnessite..)
occur as ferromanganese nodules and crusts on the
seafloor. These contain up to 1-2 wt % Co and Ni.
11

Distribution of Ferromanganese
Crusts/Nodules
Authigenic Manganese(III,IV) (Hydr)oxides
Birnessite
(Na,K) 4 Mn 14 O 27 xH 2 O
Todorokite
(Ca,Na,K) 4 Mn 5 O 12 xH 2 O
12

Authigenic Iron(III) (Hydr)oxides
Goethite (α-FeOOH)
Lepidocrocite (γ-FeOOH)
Akaganeite (β-FeOOH)
and Schwertmannite
(Fe 8
O 8
(OH) 6
SO 4
)
Sorption Edges to FeOOH
(Approximate..)
13

Element Behavior in the Oceans
Conservative: these elements (e.g., Na + , Cl - ) are
unreactive; their concentrations are determined by
physical processes such as mixing, dilution and
evaporation. They show constant concentrations relative
to salinity.
Nutrient: these elements (e.g.,P, N, Si) are taken-up
by organisms. Nutrients are depleted in surface water
but enriched in deep water by decomposition.
Scavenged: these elements (e.g., Fe, Mn, Co, REE)
are strongly sorbed onto “particulate matter”. They
occur at very low concentrations but may be elevated
in surface water by dissolution of dust input.
Photosynthesis and Nutrient Uptake
Before, we simplified the fixation of carbon by
the reaction
CO 2 + H 2 O → CH 2 O + O 2
Based on the average composition of marine
plankton, a better model would be:
106 CO 2 + 16 NO 3
-
+ HPO 4
-
+ 122 H 2 O + 18 H+
→ C 106 H 263 O 110 N 16 P + 138 O 2
C 106: N 16: P is known as the Redfield Ratio
14

Biolimiting Micronutrients
H
1
Li
3
Na
11
K
19
Rb
37
Cs
55
Fr
87
Be
4
Mg
12
Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
20 21 22 23 24 25 26 27 28 29 30
Sr
38
Ba
56
Ra
88
Y
39
La
57
Ac
89
Zr
40
Hf
73
Nb Mo Tc
41 42 43
Ta
73
B
5
C
6
W Re Os Ir Pt Au Hg Tl Pb
74 75 76 77 78 79 80 81 82
U
92
Ru
44
Rh
45
Pd
46
O
8
F
9
Al Si P S Cl
13 14 15 16 17
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
58 59 60 61 62 63 64 65 66 67 68 69 70 71
Th
90
Pa
91
Ag
47
Cd
48
Ga
31
In
49
Ge
32
Sn
50
N
7
As
33
Sb
51
Bi
83
Se
34
Te
52
Po
84
Br
35
I
53
At
85
He
2
Ne
10
Ar
18
Kr
36
Xe
54
Rn
86
Vertical Profiles and Element Behavior
Biolimiting
(P, N, Si)
Intermediate
(ΣCO 2 )
Conservative
(Cl, Na, Mg)
15

Summary
•Seawater composition results from
river input + hydrothermal input
- biological uptake +/- basalt interactions
+/- ion exchange +/- reverse weathering
•Whether chemical species are in steady state or
chemical equilibrium is unclear.
•Some elements (e.g., Mg) may not even be in steady
state.
16