Phosphate cycling in Bering Sea sediments

Phosphate cycling in Bering Sea sediments

Emily S Davenport, David H Shull and Allan H DevolImage: C. Ashjian

Background

Questions

Results

Bering Sea Domains

Importance of Benthos

Sediment surface

Dissolved phosphate

Oxic sediments

(Oxygen present) Organic phosphorus

Dissolved phosphate

Fe-bound phosphate

Anoxic sediments

(Minimal oxygen present)

Dissolved phosphate

Fe-bound phosphate

Organic phosphorus

Authigenic apatite

Adapted from Sundby et al. 1992

Organic phosphorus

40-50%

Fe-bound phosphate

40-50%

The role of the benthos in phosphorus geochemistry

Overlying water column

Oxic zone

Anoxic zone

Overlying water column

Oxic zone

Anoxic zone

HPO42-

HPO42-

HPO42-

HPO4+

Fe(OH)3

HPO4+

Fe(OH)3

HPO4+

Fe(OH)3

Fe2+

Fe2+

Fe2+

1. Does phosphate flux co-vary with oxygen flux?

2. Does phosphate flux co-vary with amorphous iron-hydroxide concentrations?

3. Does phosphate flux co-vary with burrow numbers?

Questions

Image: S. Dahle, NOAA

Alaska

St. Lawrence Island

Off shelf- Continental slope

and Aleutian Basin

Outer shelf

Middle shelf

Northern Bering shelf Alaska

St. Lawrence Island

200m

50m

100m

Phosphate adsorption

Image: D. Shull

Shake for 24 hours0µM, 10µM, 20µM, 30µM,

40µM or 50µM

20mL sediment

Centrifuge

Filter HPO42-

Outer-shelf domain

*All linear regressions are statistically significant at the α = 0.05 level

-200

20406080100

120

0 2 4 6 8 10-10

-5

0

5

10

15

20

25

0 5 10 15 20 -20

0

20

40

60

80

0 2 4 6

y=61.1x-393R2=0.824

y=3.10x-37.9R2=0.608

Station 31 Station 106Station 79

C (µmole L-1)

Final concentration of dissolved phosphate

C*

(µm

ole

kg-1)

Mas

s of

pho

spha

te a

dsor

bed/

deso

rbed

Freundlich isotherm: C*=KCK= linear adsorption coefficient

Middle-shelf domain

0

10

203040

5060

0 2 4 6 8 10 -20

0

20

40

60

80

0 1 2 3 40

20

40

60

80

100

0 2 4 6 8 10 12 14

Station 21 Station 27 Station 59

C (µmole L-1)

Final concentration of dissolved phosphate

C*

(µm

ole

kg-1)

Mas

s of

pho

spha

te a

dsor

bed/

deso

rbed

Middle-shelf domain

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35-5

0

5

10

15

20

25

0 5 10 15 20 25 300510152025303540

0 15 30 45

Cinitial (µmole L-1)

ΔC

(C

final-C

initi

al; µ

mol

e L-1

)

y=0.983x-6.14R2=0.986

y=0.994x-2.98R2=0.999

y=1.01x-9.35R2=0.976

Station 21 Station 27 Station 59

*All linear regressions are statistically significant at the α = 0.05 level

Phosphate fluxes:Whole-core incubations

Images: D. Shull, A. Devol

M.I.M.S. SmartChem

Amorphous iron-hydroxides

Shake, 4 hours in the

dark

Centrifuge, dilute to

50mls and analyze for

iron on flame AAS

Ammonium oxalate, oxalic acid mixture

Chao and Zhou, 198325mL

Outer shelf vs. middle shelf:

Phosphate flux

Outer shelf: Phosphate flux co-varies with sediment oxygen consumption

-20

0

20

40

60

80

100

120

-4000-3500-3000-2500-2000-1500-1000-5000

Sediment oxygen consumption (µmole m-2 d-1)

Pho

spha

te f

lux

(µm

ole

m-2 d

-1)

y=-0.260x+9.25

R2=0.604

p=0.00819

0

1

2

3

4

5

6

7

8

0 50 100 150 200

Top 0.5-cm sediment amorphous iron-hydroxides (µmole g-1)

Top

0.5

-cm

por

e-w

ater

dis

solv

ed p

hosp

hate

(µ

mol

e L-1

)

y=-0.0279x+7.93R2=0.352F[1,8]=5.43p=0.0421

Outer shelf

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

Burrow numbers

Pho

spha

te f

lux

(µm

ole

m-2 d

-1)

y= 2.07x-17.8

R2=0.751

p= 0.00117

Middle shelf: Phosphate flux co-varies with burrow numbers

Dissolved phosphate (µmole L-1)

0

5

10

15

20

25

0 20 40 60 80 100

Stn 21

Stn 27

Stn 59

Dep

th (

cm)

A special thanks to:Dr. Suzanne Strom

Funding from NSF

The captain and crew of the USCGC Healy

Bonnie Chang and Heather Whitney for assistance with sample processing

Erin Macri and Calvin Mordy for assistance with sample analysis

My family and friends for all their encouragement and support

QUESTIONS?