Understanding the Distribution and Behavior of Si Isotopes in

the Ocean

Christina L. De La RochaAlfred Wegener Institute

Diatoms

diatom frustulemade from opal

akaamorphous, hydrated silica

SiO2.nH2O

Fragilariopsis kerguelensis

10 m

photo from G. Cortese

Diatoms MatterThey 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 Cyclein Tmol y-1

6

weathering

silica production 240

dissolution

river input

120

dissolution91

upwelling

115

sedimentation29

dissolution23

net burial 6-7

weathering

0.4

hydrothermal

0.5 numbers fromTréguer et al., 1995DeMaster 2002Elderfield 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:

28Si 29Si 30Si 92.23% 4.67 % 3.10%

Silicon isotope ratio variations are expressed in permil:

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

330 10xR

RRSi

STD

STDSAM

Fractionation of Si Isotopes During Opal Biomineralization

30Siopal - 30SiSi(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)

0.00.20.40.60.81.0

30S

i (‰

)

0

2

4

6

8

Si(OH)4

BSi

accumulating BSi

30Si vs Depth30Si (‰)

0 1 2 3

Dep

th (

m)

0

1000

2000

3000

4000

5000

30Si (‰)

0 1 2 3

Dep

th (

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 IsotopesField Samples- Monterey Bay

ln[DSi]

3.0 3.1 3.2 3.3 3.4 3.5

30S

i (‰

)1.0

1.2

1.4

1.6

1.8

2.0y = -1.09x + 5.0

r2 = 0.89

from De La Rocha et al., 2000

30Si (‰)

1.0 1.2 1.4 1.6 1.8

Dep

th (

m)

0

20

40

60

80

100

DSi (M)

20 24 28 32 36

Dep

th (

m)

0

20

40

60

80

100

Fractionation of Si Isotopes: Southern Ocean

from Varela et al., 2004

Silic

ic a

cid

(M

)

0

20

40

60

Latitude (oS)

55606570

30 Si

(‰)

1.5

2.0

2.5

3.0

30Si in Southern Ocean Sediments

data from De La Rocha et al., 1998Brzezinski et al., 2002

30 S

i (‰

)

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 Variabilitybut why?

Depth (mbsl)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

30 S

i (‰

)

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 13-269Atlantic Sector

E 50-11Indian Sector

E 11-94Indian Sector

data from De La Rocha et al., 1998

Potential Influences on Signal (Other Than Silicic Acid Utilization)

• secular variation in whole ocean 30Si

• regional/temporal variability in 30Si 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 30Si (‰)

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 Cyclein Tmol y-1

6

weathering

silica production 240

dissolution

river input

120

dissolution91

upwelling

115

sedimentation29

dissolution23

net burial 6-7

weathering

0.4

hydrothermal

0.5 numbers fromTréguer et al., 1995DeMaster 2002Elderfield and Schultz, 1996

eolian deposition

0.5

Si Isotope Budget

Inputs T mol y-1 30Si (‰)

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 30Si (‰)

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

S u rfa c e

D e e p

In p u t

D o w n w e llin g

P ro d u c tio n

D isso lu tio n

D isso lu tio n

S e d im e n ta tio n

U p w e llin g

S u rfa c e

D e e p

F Rin in

W C Re x s u r f s u rf

B S i Rp ro d B S i

R B S iB S i /2 p ro d

D iss Rd e ep B S i

F Ro B S i

W C Re x d e e p d e e p

A B

from De La Rocha and Bickle, 2005

Impact of Changing Riverine Si Flux

[Si(

OH

) 4]

( M

)

0

30

60

90

120

150

180

30 S

i (

‰)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Time (kyr)

0 20 40 60 80 100

Pro

duct

ion

(Tm

ol S

i 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 Variabilitybut why?

Depth (mbsl)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

30 S

i (‰

)

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 13-269Atlantic Sector

E 50-11Indian Sector

E 11-94Indian Sector

data from De La Rocha et al., 1998

30Si vs Depth30Si (‰)

0 1 2 3

Dep

th (

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 30Si

Wischmeyer et al., 2003

Rayleigh Distillation of Isotopes

F(fraction of DSi remaining)

0.00.20.40.60.81.0

30S

i (‰

)

0

2

4

6

8

Si(OH)4

BSi

accumulating BSi

Modeled Mixed Layer 30Si

Wischmeyer et al., 2003

30Si vs DSi

DSi (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)

0 25 50 75 100 125 150

model- Wischmeyer et al., 2003 data- Varela et al., 2004; Cardinal et al., 2005

OutlookSuggested areas of attack (samples + modeling)• mapping of 30Si 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 30Si?

• investigating links between species composition and isotopic composition of sediments

• improving methods for isolating diatoms from sediments