Ocean acidification and metal speciation

EFFECTS OF CHANGES IN CARBONATE CHEMISTRY
ON NUTRIENT AND METAL SPECIATION
Hein De Baar,
Baar Loes Gerringa
and Charles-Edouard Thuroczy
Biological Elements in Seawater
C N
Al Si P
Ge
Ca
Mn Fe Co Ni Cu Zn
Ag Cd
Ba La
L Ce
La
C Nd
S Eu
Sm
E Gd Tb Dy
D Ho
H Er
E Tm
T Yb Lu
L
Element in biochemical function (soft tissue)
Element in skeletal part (hard shell, frustule)
Apparent coupling with ocean biological cycle
Metals Abundance & Biological Evolution
Mn
Fe
Co
Ni
Cu
Zn
9550
900000
2250
49300
522
1260
Ag
Cd ?
0.49
1.61
H
Hg
Pb
0.34
315
numbers of atoms versus 1 million Si atoms
Biological Evolution used abundant metals: essential
L
Low
abundant
b d t metals
t l no bi
bio-functions:
f
ti
ttoxic
i
Case studies for Fe, Cu, Zn, Cd
De Baar & La Roche (2003)
Contents
• Small changes in CO2 chemistry are significant
– effects on underwater sound
– effects on biology
• Small changes in nutrient chemistry
– Silicate
• sidestep: Aluminium
– Phosphate
Ph
h t
•
•
•
•
Speciation of trace metals Zn, Cu, Cd
S
Speciation
i ti off iiron F
Fe
Future work
Summary
What is Speciation
• Different physical-chemical
physical chemical states of a
chemical element in seawater
– truly
t l di
dissolved,
l d colloids,
ll id llarger particles
ti l
– oxidation state, Fe(II) versus Fe(III)
– weak acid (H2CO3) or weak base (NH4OH)
– inorganic complexation (e.g. NaHCO30)
– organic complexation (e.g. Fe(III)Lorganic)
DIC and Alkalinity at the Zero Meridian
50 0S station
Steven van Heuven, unpubl. data, 2008
Zero Meridian Station 107
50
00 16.13’
16 13’ S
S, 01026.71’
26 1’ W
2200.0
2300.0
7.8
2400.0
7.9
8.0
20.0
8.1
0
0
0
500
500
500
1000
1000
1500
1500
1000
pH
Depth (m)
Depth (m)
Depth (m)
2100.0
2000
2000
DIC
2500
Alk
30.0
CO2
1500
2000
2500
2500
3000
3000
3000
3500
3500
3500
deep waters already are
natural laboratory for low pH
Steven van Heuven, unpubl. data, 2008
Today and future scenario at 49 m depth
Station 107 (50 0S,
S 01 0W)
year
y
DIC
ALK p
pCO2
pH
p
2008
2144 2287
360
8.07
2100 ? 2249 2287
800
7.76
2300 ? 2325 2287
1500
7.5
Future:
DIC and ALK above units micromol per kg seawater
these were converted to micromol per Liter for Mineql+ Version 4.6 (2007) simulations
CO2 System Speciation
with
ith M
Major
j IIons iin S
Seawater
t
CO2 System Speciation
at pCO2=360 ppm and pH=8.07
CO3(2-)
CaHCO3+
H2CO3 (aq)
HCO3HCO3
MgHCO3+
NaHCO3 (aq)
CaCO3 (aq)
Mineql+
Version 4.6
(2007)
MgCO3
g
((aq)
q)
NaCO3-
Unanticipated Consequences of Ocean Acidification:
A Noisier Ocean at Lower pH
Keith Hester, Edward Peltzer, William Kirkwood and Peter Brewer
Geophysical Research Letters (2008)
Decrease in ocean sound absorption for frequencies lower than 10 kHz.
kHz
This effect is due to known pH-dependent chemical relaxations in the
B(OH)3/B(OH)4- and HCO3-/CO32- systems.
systems
The pH change of 0.3 units anticipated by mid-century, results in a
decrease of sound absorption by almost 40%.
40%
Simplified scheme is a coupled exchange reaction with ion-pairing:
In fact the species CaCO3(aq) and CaHCO3+ as well as MgCO3(aq) and MgHCO3+ are involved
Also Presentation by D. Browning at Wednesday morning
Will marine mammals adapt ?
Underwater
U
d
t sound
d is
i already
l
d
increasing due to human activity
(ships, sonar, etc.)
59
oSouth,
February 2008
The additional decrease of sound
absorption is an extra concern
Simplified CO2 System in Seawater
CO2 System in Seaw ater
at pCO2=360 ppm and pH=8.07
CO2
HCO3
HCO3CO32CO2 System Speciation
at pCO2=360 ppm and pH=8.07
CO3(2-)
CaHCO3+
H2CO3 (aq)
HCO3
HCO3MgHCO3+
NaHCO3 (aq)
CaCO3 (aq)
MgCO3 (aq)
NaCO3-
Future changes CO2 system:
changes of minor players can make a difference
2500
[Micromol] = [10-6 M]
2000
CO2
(aqueous)
CO32-
1500
1000
HCO3500
0
8.07
1
360
7.76
2
800
7.5
3
1500
pH
pCO2
Major
M
j player
l
HCO3- remains
i
quite
it stable
t bl
Minor players CO2 and CO32- change more
this is our concern for photosynthesis and calcification, respectively
Equilibrium Thermodynamics in Mineql+4.6(2007)
The thermodynamic equilibrium simulation only mimics
the existing or predicted situation
The rate or kinetics of any chemical reaction is not
assessed
Conversion between chemical species:
H2O + CO2 = HCO3- + H+
This is a slow reaction
Therefore chemical speciation is important
(acceleration by enzyme Carbonic Anhydrase)
When the rates of conversion reactions are fast, then
chemical speciation in seawater may be less important
Silicate Speciation
[10-6 M] or Percent
15
Si(OH)4 [10-6M]
[10 6M]
H3SiO4- [10-6M]
H3SiO4- [%]
10
5
pH
8.07
7.76
7.5
7.50
3
Si(OH)4 [10-66 M]
12 0
12.0
12 1
12.1
12 2
12.2
At tenfold expanded scale:
H3SiO4- [10-6 M]
0.151
0.074
0.041
Total Si [10-6 M]
12.2
12.2
12.2
Percent H3SiO4-
1.2 %
0.6 %
0.3 %
%
%
%
0
8.07
1
7.76
2
pH
[1 0-6 M] or P
Percent
1.5
%
1
%
0.5
%
0
8.07
1
7.76
2
pH
7.50
3
Si(OH)4 [10-6M]
H3SiO4- [10-6M]
H3SiO4- [%]
Silicate speciation: does it matter ??
((who cares ?))
Milligan et al (2004)
Si uptake and dissolution by diatoms
conditions pH = 8.5 and pH = 7.8
Si uptake rate not affected by pH
Si dissolution rate is 6-7 fold higher at low pH
at low pH more Si efflux from cell: lower Si/C ratio
effect on Si stable isotope fractionation yet to be assessed
Del Amo & Brzezinski (1999)
4 diatoms at pH 8.0 and pH 9.2
Si(OH)4 is by far the preferred Si species for diatom uptake
one diatom P. tricornutum behaves different from other 3 diatoms
Recommendation:
assess conceivable effect on fractionation of stable isotopes of Si
use more realistic pH value ranges in diatom studies
With gratitude to Damien Cardinal for responding to my questions
Biological Elements in Seawater
C N
Al Si P
Ge
Ca
Mn Fe Co Ni Cu Zn
Ag Cd
Ba La
L Ce
La
C Nd
S Eu
Sm
E Gd Tb Dy
D Ho
H Er
E Tm
T Yb Lu
L
Element in biochemical function (soft tissue)
Element in skeletal part (hard shell, frustule)
Apparent coupling with ocean biological cycle
Titanium ultraclean sampling system
with Kevlar hydrowire
for trace elements in seawater
(De Baar et al., 2008, Marine Chemistry)
Ultra Clean Sub-sampling
s de cclean
ea co
container
a e
inside
Al in high latitude Arctic Ocean
Steady increase of dissolved Al with increasing depth
Looks like silicate
Rob Middag, unpublished data, Arctic Geotraces, Polarstern 2007
Aluminium: major shifts
70
60
53
Percentage [%]
P
50
Al(OH)2+
Al(OH)3 (aq)
Al(OH)4( )
69
65
Aluminium Speciation
p
at pH = 8.07 and pCO2 = 360 ppm
44
Al(OH)2+
Al(OH)3 (aq)
Al(OH)4-
40
27
30
15
20
10
3
16
8
0
1
8.07
2
7.76
3
7.5
pH
pH = 8.07
pH = 7.76
pH = 7.5
Al(OH)2+
Al(OH)3 (aq)
Al(OH)4-
Major shifts Al speciation: does it matter ?
Gehlen et al (2002); Al incorporation in growing diatoms
Thalassiosira nordenskjioldii
Al : Si = 1.3
1 3 : 1000
Al : Si = 3.8 : 1000
uptake ratio varies with Al/Si ratio in
seawater
t medium
di
Dissolution of opal diatom frustules
decreases significantly with higher Al:Si
ratio of the biogenic opal (Van Bennekom)
Speciation changes of Al in seawater likely
affect the uptake ratio Al:Si by diatoms
The 6-7 fold higher opal dissolution at
l
lower
pH
H (Milli
(Milligan etal
t l 2004) may well
ll b
be
due to shifts in Al speciation affecting the
Al:Si ratio of opal hence the dissolution
Phosphate speciation
Phosphate Speciation with Major Ions
at pH = 8.07
MgHPO4 (aq)
NaHPO4H2PO4HPO4-2
CaPO4-
Phosphate Speciation at pH = 8.07
Simplified in 3 Categories
H2PO4HPO42PO43
PO43-
Phosphate speciation with ocean acidification
100
Percenttage [%]
80
H2PO4HPO42HPO42
PO43-
60
40
20
0
1
8.07
2
7.76
3
7.5
pH
pH
8.07
7.76
7.5
H2PO4-
22%
2.2
44%
4.4
78%
7.8
HPO42-
96.2 %
94.8 %
91.7 %
PO43-
1.6 %
0.8 %
0.4 %
At tenfold expanded scale:
10
Percentage [%
%]
8
H2PO4HPO42PO43-
6
4
2
0
1
8.07
2
7.76
pH
3
7.5
•
•
•
•
•
•
major shifts of two minor species
(just like the CO2 system)
does it matter ?
probably not ?
onl 1 stable isotope 31P
only
who cares ?
Acidification effect on third major nutrient
Nitrogen
• Nitrate is strong ion
– only one species NO3- and this will not be affected
• Ammonia is weak ion
– speciation obviously will be affected
Zinc, Copper and Cadmium
Mn
Fe
Co
Ni
Cu
Zn
9550
900000
2250
49300
522
1260
Ag
Cd
0.49
1.61
H
Hg
Pb
0.34
315
numbers of atoms versus 1 million Si atoms
Zinc Inorganic Speciation
Zn is key
y element in enzyme
y
Carbonic Anhydrase
y
Zinc Inorganic Speciation at pH = 8.07
Total Zn = 0.4 nM
Zn(2+)
ZnHCO3+
Zn Inorganic Speciation
ZnCO3 (aq)
100
Zn(2+)
90
Zn(SO4)2-2
ZnSO4 (aq)
Percentage [%
%]
80
ZnHCO3+
70
60
ZnCO3 (aq)
50
Zn(SO4)2-2
40
30
ZnSO4 (aq)
20
10
0
1
2
3
pH
decrease of ZnCO3(aq) species
Zn Inorganic Speciation
15
P
Percentag
ge [%]
Zn(2+)
ZnHCO3+
10
10
ZnCO3 ((aq)
q)
6
Zn(SO4)2-2
5
3
0
1
8.07
2
7.76
ZnSO4 (aq)
3
7.5
pH
ZnCO3(aq) would decrease from 10 % to 6% to 3 % with ocean acidification
Zinc Speciation with Natural Organic Ligand
Total
T
t lZ
Zn = 0.4
0 4 nM
M
Total Ligand = 2.2 nM
K(ZnL) = 10.8
Zinc-binding Ligand after
Ellwood 2004; Lohan et al., 2005;
Bruland 1989
Percentage [%]
Assumption:
L binding strength does not
change
h
with
ith pH
H
100
Log scalle
This Ligand also binds Cd
and Cu at same time
Sum (Zn+Cd+Cu)=1.2 nM
Zinc Organic Speciation
96.6
97
97.2
10
1.6
1
81 07
8.07
1.6
7.76
7 276
1.6
7.5
7 35
pH
Very strong organic complexation stabilizes everything
No impact of ocean acidification on Zn speciation
No tipping point manuscript to Nature/Science
ZnL
Zn(2+)
( )
Copper (Cu) Inorganic Speciation
90
Cu Inorganic Speciation
at pH = 8.07 at pCO2 =360 ppm
80
70
Cu(2+)
Cu(2+)
Cu(OH)2 (aq)
CuCl+
Pe
ercentage [%]
CuOH+
60
CuOH+
Cu(OH)2 (aq)
50
CuCl+
40
CuCO3 (aq)
Cu(CO3)2-2
30
CuCO3 (aq)
20
Cu(CO3)2-2
10
Cu Inorganic Speciation mostly carbonates
and also OH species
Very promising for major changes ??
0
1
8.07
2
7.76
3
7.5
pH
pH
8.07
7.76
7.5
Free Cu(2+)
6%
11 %
17 %
CuCO3 (aq)
81 %
80 %
72 %
Cu(CO3)22-
7%
4%
2%
Copper speciation with natural organic ligand
Total Cu = 0.6 nM
Total Ligand = 2.2 nM
K(CuL) = 13.8
100
This Ligand also binds Cd and
Cu at same time
Sum (Zn+Cd+Cu)=1.2
(Zn+Cd+Cu)=1 2 nM
Assumption:
L binding strength does not
change with pH
Cu binding Ligand after
Gerringa et al.,
al 1995;
Laglera & Van Den Berg, 2003;
Moffett, 2007
Percenttage [%]
80
60
CuL
40
20
0
8.071
7.76
2
3
7.5
pH
H
Full 100% organic complexation at every pH value
No impact of ocean acidification on Cu speciation
No tipping point manuscript to Nature/Science
Cadmium Speciation
without and with Organic Ligand
80
Cd Inorganic Speciation
64%
64%
64%
Per centage [%]
40
Free Cd(2+)
CdCl+
CdCl3CdCl2 (aq)
30
20
Percentage [[%]
50
60
Free Cd(2+)
( )
CdCl+
CdCl3CdCl2 (aq)
CdL
40
20
10
0
0
1
8.07
2
7.76
3
7.5
pH
3 % free Cd2+ ion
For the rest only Chlorine species
No pH dependence at all
8.07
1
7.76
2
37.5
pH
Total Cd = 0.2 nM
Total Ligand = 2.2 nM
K(CdL) = 10.8 after Ellwood, 2004
This Ligand also binds Zn and Cu at same time
Sum (Zn+Cd+Cu)=1.2 nM
64 % Cd
Cd-L
L organic complex
1 % free Cd2+ ion
For the rest only Chlorine species
No pH dependence at all
Gold is for the mistress
Silver for the maid
Copper for the craftsman
cunning at his trade.
“Good”
Good said De Baaron
sitting in this hall:
“But iron – cold iron –
is master of them all.
all ”
free after Rudyard Kipling (1910)
Surface Ocean: Iron Speciation crucial for biological uptake by
phytoplankton in the euphotic zone
photoreduction
colloids
0
[FeCO 3 ]
[FeOH+ ]
solids
Fe2O3
Fe(OH)3
FeOOH
+
2+
3+
[Fe ]
[[Fe(II)L]?
( ) ]
[Fe ]
?
?
[Fe(OH) 2 ]
[Fe(OH)2+]
cell
photoreduction
? ?
sidero
siderophores
[Fe(III)L]
organic complexes
Gerringa et al, 2000
Deep Ocean: Competition between Stabilization by Organic Complexation
versus Removal via Colloids controls Residence Time of Fe in the Oceans
removal
stabilization
stable
dissolved
[Fe3+]-organic
complex
dissolved
[Fe3+]
fine
colloids
Feparticles
Feparticles
> 0.2
micron
sedimentation
seafloor
40
000m de
eep oce
ean
upper ocean
Case 1: inorganic speciation, no light (dark ocean)
photoreduction
colloids
0
[FeCO 3 ]
[FeOH+ ]
solid
FeOOH
goethite
+
2+
3+
[Fe ]
[[Fe(II)L]?
( ) ]
[Fe ]
?
?
[Fe(OH) 2 ]
[Fe(OH)2+]
cell
photoreduction
? ?
sidero
siderophores
[Fe(III)L]
organic complexes
Gerringa et al, 2000
Case 1: inorganic speciation, no light (dark ocean)
Surface sample at 49 m depth:
< 0.2 micron
[Fe] = 0.096 nM = 96 pM
Assume no Ligand
Fe'
GOETHITE
1.E-10
9.E-11
88 pM
82 pM
8.E-11
72 pM
Dissolved Inorganic Iron
Fe(OH)2+
Concentration
7.E-11
6.E-11
5.E-11
4.E-11
24 pM
3.E-11
Fe(OH)3 (aq)
Fe(OH)4(
)
14 pM
2.E-11
1.E-11
8 pM
0.E+00
1
8.07
2
7.76
3
7.5
pH
Sum off all
S
ll inorganic
i
i species:
i
Fe’ = Fe(OH)2+ + Fe(OH)3 + Fe(OH)4-
90 % of the dissolved Fe would precipitate
Case 2: include organic ligand, delete solid FeOOH,
no light (dark ocean)
photoreduction
colloids
0
[FeCO 3 ]
[FeOH+ ]
solid
FeOOH
goethite
+
2+
3+
[Fe ]
[[Fe(II)L]?
( ) ]
[Fe ]
?
?
[Fe(OH) 2 ]
[Fe(OH)2+]
cell
photoreduction
? ?
sidero
siderophores
[Fe(III)L]
organic complexes
Gerringa et al, 2000
Case 2: include organic ligand, delete solid FeOOH,
no light (dark ocean)
Prediction of speciation model:
Surface sample at 49 m depth:
<0
0.2
2 micron
[Fe] = 0.096 nM = 96 pM
Ligand = 0.668 nM = 668 pM
100
FeL [pM]
80
96
pM
96
pM
96
pM
60
FeL
40
20
0
8.07
1
7.76
2
7.5
3
pH
All the dissolved Fe is bound by the ligand, thus stable in seawater
Case 2: Measure the organic
complexation in real sample
• deep water sample (2500m depth)
– measurement onboard: Fe=0.95
Fe 0.95 nM, L=1.64
L 1.64 nM
• adjusted to 3 different pH values 7.75, 8.05, 8.25 in laboratory
• measure ligand concentration L and its stability constant K
pH
L [nM]
error (L)
logK
error logK
Onboard
8.05
7.75
8 05
8.05
8.25
1.638
0.110
22.42
0.18
1.686
1 734
1.734
1.694
0.067
0 081
0.081
0.065
22.52
22 82
22.82
22.84
0.11
0 15
0.15
0.10
Ligand concentration L does not change significantly
Stability constant logK does not change significantly
This supports modeling assumption that K is independent of pH
Case 3: include organic ligand, dissolve FeOOH as
much as possible, no light (dark ocean)
photoreduction
colloids
0
[FeCO 3 ]
[FeOH+ ]
solid
FeOOH
goethite
+
2+
3+
[Fe ]
[[Fe(II)L]?
( ) ]
[Fe ]
?
?
[Fe(OH) 2 ]
[Fe(OH)2+]
cell
photoreduction
? ?
sidero
siderophores
[Fe(III)L]
organic complexes
Gerringa et al, 2000
Case 3: include organic ligand, dissolve FeOOH as
much as possible, no light (dark ocean)
800
Surface sample:
674
Original Fe = 96 pM
Fe' or FeL or Total D
Diss Fe [pM]
700
Ligand = 0.668 nM =
668 pM
688
522
712 Total Dissolved Fe = Fe’ + FeL
698
600
514
Fe'
500
FeL
400
Total
Dissolved Fe
300
200
100
8
14
24
7.76 2
7.5 3
Original Fe = 96 pM
0
8.07
1
pH
The Excess Ligand of Todays Ocean Can Accomodate more dissolved Fe
By Ocean Acidification the dissolved Fe in Seawater can increase even more
Dissolution of solid phase Fe and Mn is much
more a function of low oxygen conditions
than of pH
Strong Fe and Mn maxima exist in subsurface
oxygen minimum
i i
zones off Equatorial
E
t i l East
E t
Pacific, Northwest Indian Ocean
(L di
(Landing
ett al.
l 1987
1987, Saager
S
ett al.
l 1989)
... however this meeting happens to be about pH not oxygen
How about photoreduction to Fe(II) in open ocean ?
How about the fine colloids in open ocean ?
photoreduction
colloids
0
[FeCO 3 ]
[FeOH+ ]
solids
Fe2O3
Fe(OH)3
FeOOH
+
2+
3+
[Fe ]
[[Fe(II)L]?
( ) ]
[Fe ]
?
?
[Fe(OH) 2 ]
[Fe(OH)2+]
cell
photoreduction
? ?
sidero
siderophores
[Fe(III)L]
organic complexes
Gerringa et al, 2000
Photochemistry in upper euphotic zone
of the open ocean
Photoreduction of Fe(III) to Fe(II)
only in upper euphotic zone
beyond chemical thermodynamics prediction
rapid re-oxidation but slower in cold polar waters
small
ll portion
ti
off th
the d
dominant
i
tF
Fe(III)L
(III)L species
i di
dissociates
i t
into free Fe(II) and free L
the free Fe(II) likely is more available for uptake by algae
Photochemical breakdown of organic ligand L
yes but only of small labile portion of total L pool
ligand L is very stable and uniform throughout world ocean
this supports the notion of long term chemical stability
new ligand
li
db
being
i
fformed
d iin surface
f
waters
t
from
f
biota
bi t
surface waters tend to have large excess of ligand versus Fe
Future work in open ocean waters
• response of Fe
Fe-colloids
colloids size class to acidification
–
–
–
–
adjust 0.2 micron filtered seawater to different pH values
wait some time
ultrafiltration and then measure Fe and L in size fractions
physical-chemical modeling ?
• Acidification effects on other metals
– Mn manganese
– lanthanides series (dominated by CO32- species)
• but who cares about the lanthanides ?
Summary
• shifts in Silicate speciation may affect fractionation of Si stable
isotopes
• major shifts
f in Aluminium speciation may affect
ff
its uptake in
diatom frustules, hence dissolution of biogenic Silicon
• major shifts of two minor species of Phosphate
Phosphate, any effects on
biota unknown
• Zn, Cu and Cd are strongly stabilized by organic complexation
• excess organic ligand in modern ocean may accomodate more
dissolved Fe than is currently present
• if so
so, then in the future more acidic ocean somewhat more Fe
may dissolve
• caveats:
– in fact the open ocean ligand does not carry as
much Fe as it could

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