Understanding the Distribution and Behavior of Si Isotopes in the Ocean

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22 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

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Understanding the Distribution and
Behavior of Si Isotopes in the Ocean


Christina L. De La Rocha

Alfred Wegener Institute

Diatoms

diatom frustule

made from opal

aka

amorphous, hydrated silica

SiO
2
.
n
H
2
O

Fragilariopsis kerguelensis

10
m
m

photo from G. Cortese

Diatoms Matter

They carry out:


>75% of the new production in coastal, high nutrient
environments



~ half of all marine primary production



~ 20% of all primary production occurring on Earth
each year



and much of the production in the Southern Ocean

The Silica Cycle

in Tmol y
-
1

6


weathering

silica production

240

dissolution

river input

120

dissolution

91

upwelling

115

sedimentation

29

dissolution

23

net burial

6
-
7

weathering

0.4

hydrothermal

0.5

numbers from

Tr
éguer et al., 1995

DeMaster 2002

Elderfield and Schultz, 1996

eolian deposition

0.5

Questions


Can dissolved Si (DSi) utilization by diatoms be
reconstructed from Si isotopes?



What is the distribution and behavior of Si isotopes in
the modern ocean and how has it been in the past?



Do sediments faithfully record an annually integrated
nutrient utilization signal from surface waters?

Notation

Silicon has 3 stable isotopes:



28
Si






29
Si



30
Si


92.23%




4.67 %




3.10%


Silicon isotope ratio variations are expressed in permil:




where R
SAM

and R
STD

are the
30
Si/
28
Si ratio in a sample
and standard, respectively.

3
30
10
x
R
R
R
Si
STD
STD
SAM



Fractionation of Si Isotopes

During Opal Biomineralization


30
Si
opal

-


30
Si
Si(OH)4

Marine Diatoms

Skeletonema costatum


1.0
±

0.4 ‰

Thalassiosira weissflogii


1.3
±

0.4 ‰


1.5
±

0.2 ‰

Thalassiosira
sp.


0.9
±

0.3 ‰

data from De La Rocha et al., 1998;

Milligan et al., 2004

Rayleigh Distillation of Isotopes

F
(fraction of DSi remaining)
0.0
0.2
0.4
0.6
0.8
1.0

30
Si
(‰)
0
2
4
6
8
Si(OH)
4
BSi
accumulating BSi

30
Si vs Depth


30
Si (‰)
0
1
2
3
Depth (m)
0
1000
2000
3000
4000
5000

30
Si (‰)
0
1
2
3
Depth (m)
0
1000
2000
3000
4000
5000
data from De La Rocha et al., 2000; Varela et al., 2004;

Cardinal et al., 2005

blues = Pacific

central and coastal N Pac

Antarctic (Pac Sector)

Pacific subantarctic


pink/purple = Atlantic

BATS and coastal N Atl

Atlantic subantarctic

Fractionation of Si Isotopes


Field Samples
-

Monterey Bay

ln[DSi]
3.0
3.1
3.2
3.3
3.4
3.5

30
Si (

)
1.0
1.2
1.4
1.6
1.8
2.0
y = -1.09x + 5.0
r
2
= 0.89
from De La Rocha et al., 2000


30
Si (

)
1.0
1.2
1.4
1.6
1.8
Depth (m)
0
20
40
60
80
100
DSi (
m
M)
20
24
28
32
36
Depth (m)
0
20
40
60
80
100
Fractionation of Si Isotopes:
Southern Ocean

from Varela et al., 2004

Silicic acid (
m
M)
0
20
40
60
Latitude (
o
S)
55
60
65
70

30
Si (‰)
1.5
2.0
2.5
3.0

30
Si in Southern Ocean Sediments

data from

De La Rocha et al., 1998

Brzezinski et al., 2002


30
Si (‰)
0.4
0.8
1.2
1.6
Age (kyr)
0
20
40
60
80
100
120

15
N
bulk
(‰)
1
2
3
4
5
1 2 3-5
Regional Variability

but why?

Depth (mbsl)
0.0
0.5
1.0
1.5
2.0
2.5
3.0

30
Si (

)
0.5
1.0
1.5
Depth (mbsl)
0
2
4
6
8
10
Depth (mbsl)
0.0
0.5
1.0
1.5
2.0
RC 13-269
Atlantic Sector
E 50-11
Indian Sector
E 11-94
Indian Sector
data from De La Rocha et al., 1998

Potential Influences on Signal

(Other Than Silicic Acid Utilization)


secular variation in whole ocean

30
Si



regional/temporal variability in

30
Si upwelled
to euphotic zone



sediments that do not integrate equally over
entire growing season (e.g. bias towards resting
spores, more robustly silicified diatoms, etc)


Si Isotope Budget

Inputs

T mol y
-
1


30
Si (‰)

river

5.6

+0.3 to +3.4

hydrothermal

0.5

0.3

low
-
T basalt
weathering

0.4

0.3

eolian

0.5

0.3

total

7.0

?



Outputs

biogenic opal

6.5 to 7.4


3.7 to +2.0

The Silica Cycle

in Tmol y
-
1

6


weathering

silica production

240

dissolution

river input

120

dissolution

91

upwelling

115

sedimentation

29

dissolution

23

net burial

6
-
7

weathering

0.4

hydrothermal

0.5

numbers from

Tr
éguer et al., 1995

DeMaster 2002

Elderfield and Schultz, 1996

eolian deposition

0.5

Si Isotope Budget

Inputs

T mol y
-
1


30
Si (‰)

river

5.6

+0.3 to +3.4

hydrothermal

0.5

0.3

low
-
T basalt
weathering

0.4

0.3

eolian

0.5

0.3

total

7.0

?



Outputs

biogenic opal

6.5 to 7.4


3.7 to +2.0

Inputs

T mol y
-
1


30
Si (‰)

river

5.6

+1.4

hydrothermal

0.5

0.3

low
-
T basalt
weathering

0.4

0.3

eolian

0.5

0.3

total

7.0

+1.2



Outputs

biogenic opal

6.5 to 7.4

+1.2

Examination With Simple 2
-
box Model

Surface
Deep
Input
Downwelling
Production
Dissolution
Dissolution
Sedimentation
Upwelling
Surface
Deep
F R
in in
W C R
ex surf surf
BSi R
prod BSi
R
BSi
BSi/2
prod
Diss R
deep BSi
F R
o BSi
W C R
ex deep deep
A
B
from De La Rocha and Bickle, 2005

Impact of Changing Riverine Si Flux

[Si(OH)
4
] (
m
M)
0
30
60
90
120
150
180

30
Si ( ‰)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Time (kyr)
0
20
40
60
80
100
Production (Tmol Si y
-1
)
0
100
200
300
400
500
Time (kyr)
0
20
40
60
80
100
A
B
C
D
E
F
2.5x modern 0.5x modern

Regional Variability

but why?

Depth (mbsl)
0.0
0.5
1.0
1.5
2.0
2.5
3.0

30
Si (

)
0.5
1.0
1.5
Depth (mbsl)
0
2
4
6
8
10
Depth (mbsl)
0.0
0.5
1.0
1.5
2.0
RC 13-269
Atlantic Sector
E 50-11
Indian Sector
E 11-94
Indian Sector
data from De La Rocha et al., 1998


30
Si vs Depth


30
Si (‰)
0
1
2
3
Depth (m)
0
1000
2000
3000
4000
5000
data from De La Rocha et al., 2000; Varela et al., 2004;

Cardinal et al., 2005

blues = Pacific

central and coastal N Pac

Antarctic (Pac Sector)

Pacific subantarctic


pink/purple = Atlantic

BATS and coastal N Atl

Atlantic subantarctic

Modeled Mixed Layer

30
Si


Wischmeyer et al., 2003

Rayleigh Distillation of Isotopes

F
(fraction of DSi remaining)
0.0
0.2
0.4
0.6
0.8
1.0

30
Si
(‰)
0
2
4
6
8
Si(OH)
4
BSi
accumulating BSi
Modeled Mixed Layer

30
Si


Wischmeyer et al., 2003


30
Si vs DSi

DSi (
m
M)
0
25
50
75
100
125
150
175

30
Si (‰)
0
1
2
3
blues = Pacific

central and coastal N Pac

Antarctic (Pac Sector)

Pacific subantarctic


pink/purple = Atlantic

BATS and coastal N Atl

Atlantic subantarctic

data from De La Rocha et al., 2000; Varela et al., 2004;

Cardinal et al., 2005













SO Surface Waters
-

Model vs Reality

DSi (
m
M)
0
25
50
75
100
125
150
model
-

Wischmeyer et al., 2003





data
-

Varela et al., 2004; Cardinal et al., 2005

Outlook

Suggested areas of attack (samples + modeling)


mapping of

30
Si at high spatial resolution


especially upper water column


relative to trace elements (Fe), N isotopes



comparing data with models of Si isotopic
composition of ocean, sediments


do we understand all the processes controlling

30
Si?




investigating links between species composition
and isotopic composition of sediments


improving methods for isolating diatoms from sediments