Sulfide flux from cold seeps using an ISUS chemical sensor

Josh Plant

Ken Johnson

Luke Coletti

Steve Fitzwater

Carole Sakamoto

Motivation

• Demonstrate optical measurements of HS- at depth over extended periods

• Chemical flux at cold seeps difficult to measure!

• Provide direct in situ measurements of chemical flux with enhanced temporal resolution

ISUSIn Situ Ultraviolet Spectrometer

Unique UV absorption spectrum allows direct measurement

Deconvolution of overlapping peaks enables determination of ion concentrations

Sample absorbance spectrum is sum of all absorbing species

HS- NO3- & Br- are the primary UV absorbers in seawater!

Change in spectrum is due to change in ion concentrations

Wavelength, nm200 230 260 290

Ab

sorb

ance

0

1

2

3

840 µM Br-

50 µM HS-

Sample spectrum

30 µM NO3-

HS- Standards in Seawater (mM)

Wavelength (nm)

200 220 240 260 280 300 320 340

Ab

sorb

ance

0

1

2

3

265 285

0.2

0.8

0 0.05

0.20

0.481.9

6.7

0.10

HS- is a strong UV absorber between 230 – 300 nm

Cold seep fluids high in HS-

Spectral deconvolution uses multiple wavelengths

Shift fit window to measure higher concentrations

RecircPump

ISUSHS-

FlushPump

pH

CTD

Sediment

RecircPump

ISUSHS-

CTD

pH

FlushPump

HS-

Tube of

100 m mesh

DeploymentRecirculationMeasureWaitRecirculationMeasureFlush

HS- benthic flux measurementsChamber concentration proportional to difference between

seawater and seep fluid

Cfluid

CChamber

Cseawater

Minutes0 25 50 75 100

Cch

amb

er ,

mM

0

1

2

3

4

0.5 m/day1.0 m/day

4.0 m/day

Cchamber = Cfluid ( 1 – e-kt)

K = Fluid velocity / chamber height

Flux = fluid velocity * Cfluid

H

Clams Barren Zone

Bacterial Mat

Most active seeps found in Monterey Bay

Very focused flow

Support chemosynthetic communities

960 meter depth

Extravert Cliff Seeps

2m x 1m

July 2006 Daily Velocity Calculations

Chamber flushed dailySampled hourly

Cfluid 2 – 4 mM HS-

Velocity 5 – 20 m day-1

Time , days0 2 4 6 8 10 12 14V

elo

cit

y,

m d

ay-1

0

10

20

Cfl

uid

, m

M

0

2

4

Hours0 2 4 6

HS

- mM

0

1

2

3Cfluid = 2.6 mM

3.0 m day-1

Time, days

2 4 6 8 10 12 14

Te

mp

, C

4

6

HS- ,

mM

0

2

4

July 2006 Flux Chamber data

Seep fills chamber in less than an hour!

Fluid Velocity an underestimate in high flow cases

Temperature and HS- correlated

959

960

961

HS

- , m

M

0

1

2

3

4

T1

T2 Tide

Time, days2 4 6 8 10

De

pth

, m

959

960

961

Te

mp

, C

4

5

6

7

September 2006 Flux Chamber dataFlush every 30 minutes

2 samples per cycle, Time 1 and Time 2

Chamber concentration related to tides

September 2006 Velocity Calculations

Strong tidal signal - highest flow at low tide!

Velocity similar to July data 5 to 30 m day-1

30 to 180 Liters day-1 through 8cm diameter chamber

Time, days2 4 6 8 10

Vel

oci

ty, m

day

-1

0

10

20

30

Dep

th, m

959

960

961Tide

Velocity

Temperature & HS- highly correlated

Relationship similar on July & Sept deployments

3 degrees above ambient

Warm sulfide rich source – a warm seep!!

HS-, mM0 1 2 3 4 5

Tem

per

atu

re, C

4

5

6

7

JULY

SEPT

Conclusions

ISUS effective for high resolutionHS- measurements

Fluid flow 100x faster than previous measurements in area

Enabled resolution of velocity response to tides

System adaptable

AcknowledgementsDavid and Lucile Packard Foundation

R / V Point Lobos crew and Ventana pilots

Shannnon Boedecker & Annette Gough