Distribution & Dynamics of Colored Dissolved Organic Materials (CDOM) in the Open Ocean

Distribution & Dynamics of Colored Dissolved Organic

Materials (CDOM)in the Open Ocean

Dave Siegel

Institute for Computational Earth System Science &

Department of Geography

University of California, Santa Barbara

Thanx ...•Collaborators

Norm Nelson, Stéphane Maritorena, Craig Carlson, Chuck McClain, Mike Behrenfeld, Dennis Hansell, Debbie Steinberg, Samar Khatiwala, Pat Hatcher, …

•StudentsJon Klamberg, Chantal Swan, Stu Goldberg, DeDe Toole, Tiho

Kostadinov, Toby Westberry, Eric Brody, Sara Garver, …

•Technical StaffManuela Lorenzi-Kayser, Margaret O’Brien, Erik Fields, Dave

Menzies, Liz Caporelli, Nathalie Guilocheau, Ellie Wallner, David Court, Natasha MacDonald, …

•SupportNASA Ocean Biology & Biogeochemistry Program & NSF OCE

Talk Outline

• What is CDOM?

• Why do we care? (well … its only me really)

Contributions to open ocean optical properties

• What is the global CDOM distribution?

Ocean color remote sensing & hydrographic observations

• What regulates open ocean CDOM cycling?

• What is known about CDOM in the deep sea?

What is CDOM?

• Colored Dissolved Organic Matter

• Also called gelbstoff, gilvin, yellow

matter, chromophoric DOM, …

• Colored portion of the oceanic DOM pool

– Probably a small fraction of the total DOM

– Don’t really know its composition (like DOM)

What is CDOM?

• CDOM is defined operationally by filtering

Using a 0.2 m filter

Absorption relative to “pure water” standard

• Typically, spectrum decreases

exponentially

ag() = ag(o) exp(-S(-o))

S varies from 0.015 to 0.024 nm-1

Light Absorption Spectral Shapes

CD

OM

phytoplankton

water

detritus

Wavelength (nm)

Why should we care about CDOM?

• Dominates light availability for < 450 nm

Huge role in marine photo-processes

• CDOM is part of the ocean carbon budget

Does CDOM = DOC?? -> NO!!!

• Precursor for photochemical rxn’s

Emission of trace gas (DMS, COS, CO, CO2)

Bioavailability of trace metals (Fe, Mn, Cu, etc.)

• A natural tracer of water mass exchange

How Important is CDOM?

• Assess the light absorption budget:

at() = aw() + aph() + ag() + ad()

total water phyto- CDOM detrital plankton particulate

• Examine the relative importance of each

component using a global dataset

Optical Importance of CDOM

• Global Data set (N=1044 - pre-NOMAD)

• Use Chlorophyll as an ecosystem index

– High Chl => more eutrophic ocean

– Low Chl => more oligotrophic

• Evaluate for 440 nm (peak of the chl-a abs)

• Data set is a little “green” (mean Chl ~ 1.5

mg m-3)

aw(440)/at(440)aph(440)/at(440)

ag(440)/at(440)

adet(440)/at(440)

Mean = 41%

Mean = 9%

Mean = 40%

Relative Spectral Contributions

aw()/at()

aph()/at()

ag()/at()

adet()/at()

Global Data Set

• CDOM and Chl equally dominate the at(440)

budget

• CDOM is more important for < 440 nm

• Phytoplankton dominates from 440 to 490 nm

• Water dominates for > 500 nm &

• Detritus is small part of at() budget

• CDOM importance will increase if data set were

less “green” & better represents the global ocean

Sargasso Sea

(open ocean; summer-time)

Nelson & Siegel [2002]

CDOM

Total Particles

How Large is Ocean CDOM??

6th Floor Ellison Hall – UC Santa Barbara

UC Santa Barbara Drinking Water

250 300 350 400 450 500

10-2

10-1

100

101

wavelength (nm)

ag(

) (m-1)

ICESS UCSB Drinking Fountain

Wavelength (nm)

a g(

) (m

-1)

UCSB Drinking

Water

SargassoSea

Where does ocean CDOM come from?

• Lots of places!!

– Pelagic ecosystem processes

– Inputs from terrestrial biosphere

– Benthic environments

• Allochthonous or Autochthonous

Where does ocean CDOM come from?

Historically, only terrestrial discharge sources were considered

First optical oceanographers worked on the Baltic Sea

There, gelbstoff is obvious component of water clarity & related to land-ocean exchange

Results in CDOM = f(Salinity)

Observations from the Baltic Sea

After Jerlov [1953]

??

Example From Delaware Bay

Vodacek & others, L&O, (1997)

linear mixing lineoceanvalue

Does Ocean CDOM = 0??

Where does ocean CDOM come from?

• Simple mixing analyses suggest near zero CDOM at oceanic salinities

• What are the oceanic CDOM sources?

– Is it simply mixing of terrestrial waters?

– Or are autochthonous sources important?

– How can we diagnose these sources?

– Need to know the time/space CDOM distribution

So, What is the Global CDOM Distribution??

• Field observations are too scarce (&

uncertain)

• If CDOM dominates optics of the sea, it

should be part of the ocean color signal

• Fortunately, there are excellent ocean

color sensors in orbit

• Relationship between LwN() & ocean optical

properties is known

• Component spectral shapes are constant – only their magnitudes vary

• Solve least-squares problem for 3 components– Water properties are known

– Nonlinear processes are ignored

– Detrital absorption is small

The GSM Ocean Color Model

• Problems– Only first order understanding– Parameterizations are imperfect

Garver & Siegel, JGR [1997]

The GSM Ocean Color Model

GSM ModelLwN()

Products(Chl, CDM & bbp)

Parameters(aph

*(), S, etc.)

Compiled a global LwN() & validation data set

Used it to “tune” the parameters in the modelMaritorena et al. [2002] AO (… the GSM01 model)

Optimizing the GSM Model

UCSB OceanColor ModelLwN() Products

“Tuned”Parameters

Optimization

Validation Data

• Algorithm alone…

• Matchup with NOMAD data (IOCCG IOP report; Lee et al. 2006)

• Model-data fits are pretty good – though not excellent

• GSM01 is optimized for all 3 retrievals

Does this all work??

• Independent global match-up data set of SeaWiFS & CDM observations

• Regression is pretty good (r2 = 71%)

Does this all work??

Siegel et al. [2005] JGR

Comparison of Patterns

A16N

A20

A22

r2 = 0.65; N = 111slope = 1.16

Siegel et al. [2005] JGR

Global CDOM Distribution

Siegel et al. [2005] JGR

Global CDOM Distribution

% non-water absorption(440) due to CDM

Seasonal CDOM Cycle

• Seasonal changes at most latitudes

• Lower in summer

• Reduced in tropics

• Higher towards poles

• Hemispheric asymmetry

%CDM

CDM

Role of Rivers

Large River Outflows…

Maximum annual change due to global rivers is 0.005 m-1

River inputs are just not important on a global scale

Global CDOM & DOC

• CDOM DOC

• Completely different

Tropics vs. high latitudes

Subtropical gyres

• Different processes driving CDOM & DOC

CDM

DOC

Siegel et al. [2002] JGR

Summary of Satellite CDOM

• Large latitudinal trends (low in tropics)

• Large seasonal trends (low in summer)

• Ocean circulation structures are apparent

– CDOM follows basin-scale upwelling patterns

• Rivers are small, proximate sources

• CDOM is not related to DOC simply

These are global surface CDOM values …

what are the roles of vertical

processes??

Seasonal Cycles of CDOM at BATS

BATS - Sargasso Sea (after Nelson et

al. 1998)

Seasonal cycle

CDOM DOC

CDOM POC

CDOM Chl

Temperature

DOC

CDOM

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.14

0.15

0.16

0.17

Jan Feb Mar Aprl May Jun Jul Aug Sept Oct Nov Dec

0

20

40

60

80

100

120

140

160

180

Seasonal Cycle of CDOM at BATS

Dep

th (

m)

Month

Deep Mixing

Summer Stratification

Fall Mixing

Net Production of CDOM

Summer – Spring CDOMBATS data

Sargasso Sea(Nelson et al. 1998)

Production max at 40-60 m

Similar to the bacterial production

Seasonal CDOM Cycle at BATS

Links mixing, photolysis & production

• Low summer ML CDOM due to bleaching

• Shallow summer max of CDOM production

• Mixing homogenizes the system

• Again, not related to DOC [CDOM] << [DOM]

Photobleaching of CDOM

Wavelength (nm)

300 350 400 450 500 550 600-3

-2

-1

0

1

2

3

235 139 23 0 hrs

0

2

4

6

8

10

12

14

16

Wavelength (nm)

300 400 500 600

0.0

0.2

0.4

0.6

0.8

A

B

Light exposure S (NLF) 0 hr 0.0174 23 hr 0.0201 139 hr 0.0225 235 hr 0.0249

time

Laboratory incubations of filtered Delaware Bay water

Blough & Del Vecchio [2002]

Microbial Production of CDOM

CDOM

Bacteria Microbes producelong-lived CDOM

Experiments from BATS 60m water by Nelson & Carlson

After Nelson et al. [2005]

Zooplankton & CDOM

8 hour excretion experiments from Sargasso Sea Steinberg et al. [2005] - MEPS

0

0.5

1

1.5

2

2.5

3

3.5

4

250 300 350 400 450 500 550 600 650 700

Wavelength (nm)

Absorbance (1/m)

FSW control

FSW + tow water control

Candacia ethiopica (copepod)

Example spectra for controls vs. plankton

The Open Ocean CDOM Cycle

• Losses due to photobleaching

– Mixed layer climate & UV dose

• There is a net biological source

– Likely microbial activity on organic carbon

– Zooplankton too, BUT this material seems

to be labile (and back to microbes…)

– Likely biological sinks too…

The Open Ocean CDOM Cycle

Siegel et al. [2002] JGR

The Hydrography of CDOM

Nelson, Carlson & Siegel - Support by NSF & NASA

The Global CDOM Project

CLIVAR - Repeat Hydrography Survey

Full hydrographic suite T,S,O2,Nuts,CFC’s,CO2,...

CDOM measured using WPI Ultrapath

CDOM reported as ag(325)

Nelson et al., DSR-I [2007] & more in review…

www.icess.ucsb.edu/GlobalCDOM

Just North of Hawaii

Surface CDOM & SeaWiFS

A16N

A20

A22

r2 = 0.65; N = 111slope = 1.16

Siegel et al. [2005] JGR

Atlantic A22 Temperature

STMW

GS

DeepCaribbean

AAIW

NAD

WNelson et al. [2007] - DSR-I

Atlantic A22 CDOM

STMW

DeepCaribbean

AAIW

NAD

WG

S

Grand Banks Orinoco

CDOM in Subtropical Mode Water

BATS observations

Nov 11, 2001

Low CDOM in 18o

water mass

18o water ventilates

north of BATS bringing low CDOM surface waters to south

18o water low CDOM

PotentialTemperature

CDOMag(325)

Atlantic A22 CDOM

STMW

DeepCaribbean

AAIW

NAD

W

GS

Grand Banks Orinoco

How do DOC distributions look??

A22 DOC Distribution

CDOM DOC

Data from Dennis Hansell [UM] & Craig Carlson [UCSB]

How to diagnose the spatial

patterns in CDOM??

Mankind to the rescue!!!

CFC’s as Transient Tracers

Mixing with ocean imprints ventilated waters w/ CFC levels

Provides a ventilation “age” for water mass

Atmospheric CFC’s are now dropping

Good for O3 hole

Bad for tracer work…

A22 CFC-12 Concentrations

STMW

GS

DeepCaribbean

AAIW

High CFC’s – recently ventilated waters

Low CFC’s – old water

NAD

W

Atlantic A22 CFC-12 Age

Age calculations by Bill Smethie & Samar Khatiwala [LDEO]

STMW

DeepCaribbean

AAIW

NAD

W

Layer Abbreviation n range (kg m-3)

Surface SURF 18.00 – 25.00

Upper Thermocline UTCL 25.00 – 26.40

Subtropical Mode Water STMW 26.40 – 26.60

Lower Thermocline LTCL 26.425 - 27.00

Upper Antarctic Intermediate Water

uAAIW 27.00 – 27.50

Lower Antarctic Intermediate Water

lAAIW 27.50 – 27.80

Labrador Sea Water LSW 27.80 – 27.975

Iceland-ScotlandOverflow Water

ISOW 27.975 – 28.05

Denmark StraitOverflow Water

DSOW 28.05 – 28.14

Antarctic Bottom Water AABW 28.14 – 29.00

Definitions follow Joyce et al. [2001]

Regressions between age and CDOM P < 0.025 P < 0.025

P < 0.025

P < 0.025

T ~ 10y

T ~ 50y

T > 200 y

a*cdom(325)

a*cdom = CDOM / DOC

Upper layers bleaching & production signals

a*cdom increases w/

depth & age

CDOM is largely recalcitrant

Nelson et al. [2007] DSR-I

Atlantic CDOM • CDOM follows hydrographic & transient

tracer patterns

• Ventilation & advection appear to be the dominant processes (CDOM is set at surface)

• Evidence of a remineralization source (CDOM vs. age relationship decreases w/ depth)

• CDOM DOC (it is also recalcitrant)

Nelson et al. [2008]

Pacific P16 Temperature

Along 150oW done in 4 legs over 2 campaigns (2005/06)Swan et al. [ in prep]

AA

Front

AAIW

Pacific P16 Salinity

AA

Front

AAIW NPIW

AABW

Pacific P16 CDOM

•SH Subtropical CDOM low extends thru upper 1000 m •High CDOM in NH thermocline

Pacific P16 CDOM

AA

Front

AAIW NPIW

• Low CDOM in SH subtropical gyre• Very high in NH thermocline• Some subducting water mass signature are seen in CDOM

Pacific P16 CFC-12

AAIW

AABWVery Old Water

CFC concentrations at or below detection levels-> can’t calculate ages using CFC’s - too old

Pacific P16 AOU

Apparent Oxygen Utilization = O2sat – O2

Measure of past remineralization of organic carbon

Pacific P16 CDOM

•SH Subtropical CDOM low extends thru upper 1000 m •High CDOM in NH thermocline

Pacific AOU vs. CDOM

Satellite

Water-leaving

Z > 700 m

AOU (mol/kg)

a g(3

25)

(m-1)

Swan et al. [in prep]

Atlantic A22 CDOM

STMW

DeepCaribbean

AAIW

NAD

W

GS

Grand Banks Orinoco

Atlantic A22 AOU

STMW

DeepCaribbean

AAIW

NAD

W

GS

North Atlantic AOU vs. CDOM

Z > 700 m

• Ratio of time scales Tphys/Tbio

• Large Tphys/Tbio Slow ventilation & Fast biology Biogeochemical control Pacific

• Small Tphys/Tbio

Fast ventilation & Slow biology Ventilation control North Atlantic

Deep Open CDOM Cycling

Open Ocean CDOM

• CDOM distributions are consistent with hydrographic & transient tracer patterns

• Ventilation & net BGC production are the two dominant processes

• Their relative importance determines the observed patterns for each ocean

• CDOM DOC

Talk Outline

• What is CDOM?

• Why do we care?

Contributions to biogeochemistry & optical properties

• What is the global sea surface CDOM distribution?

Ocean color remote sensing

• What regulates open ocean CDOM cycling?

• What is known about CDOM in the deep sea?

Thank You!!

Regressions between age and CDOM

Layer CDOM vs. Age(m-1yr-1)

R2 n t-test

SURF 0.005 0.031 80 N.S.

UTCL -0.002 0.019 48 N.S.

STMW 0.0100 0.028 20 P < 0.025

LTCL 0.0030 0.197 113 P < 0.025

uAAIW 0.0006 0.064 144 P < 0.025

lAAIW 8 E-5 0.003 83 N.S.

LSW -0.0002 0.010 131 N.S.

ISOW 0.0002 0.016 58 N.S.

DSOW -0.0005 0.042 138 P < 0.025

AABW 0.002 0.107 37 P < 0.025

Table 1: Water mass layer definitions in terms of neutral density intervals (based on Joyce et al. 2001) and estimated CFC-12 saturation at formation (this study)

Layer Abbreviation n range (kg m-3) CFC-12 Saturation

Surface SURF 18.00 – 25.00 100%

Upper Thermocline UTCL 25.00 – 26.40 100%

Subtropical Mode Water STMW 26.40 – 26.60 94%

Lower Thermocline LTCL 26.425 - 27.00 90%

Upper Antarctic Intermediate Water

uAAIW 27.00 – 27.50 70%

Lower Antarctic Intermediate Water

lAAIW 27.50 – 27.80 70%

Labrador Sea Water

LSW 27.80 – 27.975 70%

Iceland-ScotlandOverflow Water

ISOW 27.975 – 28.05 43%

Denmark StraitOverflow Water

DSOW 28.05 – 28.14 75%

Antarctic Bottom Water AABW 28.14 – 29.00 70%

• CDOM cycle and water mass

ventilations

– Subducted waters should be “imprinted”

with a net photobleaching signal

– Obducted waters should see the effects

of higher micobial production on CDOM

• Would CDOM be a useful age tracer?

CDOM As A Geochemical Tracer?

CDOM As A Geochemical Tracer?

BATS data

Temp/TOC/CDOM

CDOM low from

1997-1999

Related to water

mass renewal?

Temperature

TOC

CDOMlow CDOM

High TOC

Modeling the CDOM Cycle

• Are these simple concepts useful for the quantitative

modeling of the CDOM cycle?

• Cycle links mixing, photolysis & production

• Model Components …

– CDOM Photolysis = f(solar radiation)

– CDOM Production = f(microbial production)

– PWP mixed layer model seasonal cycle

Modeling the CDOM Cycle

__________

= a(z) [CDOM] - b _______ [CDOM] exp(-cz)

a(z) = production rate by microbial activity

b = photobleaching rate

c = extinction coefficient

• Parameter values chosen from BATS data

∂[CDOM]

∂t Sol(t)

Solo

Zooplankton & CDOM

Debbie Steinberg, Norm Nelson & Craig Carlson (unpubl. data)

0

0.5

1

1.5

2

2.5

250 300 350 400 450 500 550 600 650 700

Wavelength (nm)

Absorbance (1/m)

Pleuromamma xiphias

Clausocalanus sp.

Candacia ethiopica

Sapphirina sp.

Copepods

Trichodesmium & CDOM

Debbie Steinberg, Norm Nelson & Craig Carlson (unpubl. data)

0

0.5

1

1.5

2

250 300 350 400 450 500 550 600 650 700

Wavelength (nm)

Absorbance (1/m)

Tricho 1

Tricho 2

Trichodesmium (cyanobacteria)

How to Quantify Ocean Color?

• Ocean color is quantified by the normalized water-leaving radiance, LwN()

• LwN() = Fo() Lw() / Ed()

where Lw() is the nadir upwelling radiance

Ed() is the downwelling irradiance

Fo() is the ET irradiance spectrum

Ocean Color Remote Sensing

• Simple in concept

green water is productive, blue water is not,

brown is turbid, yellow is CDOM-rich, etc.

• The bio-optical link can be quantified

Water-leaving radiance = LwN() can be modeled as a f(in-water constituents)

Step is called a bio-optical algorithm

But, Ocean Color Remote Sensing is

Difficult…• The ocean is a dark target

• Atmosphere path radiance is >90% of satellite signal

• Correcting for the atmospheric signal is still difficult

• Satellites are delicate optical instruments launched from rockets (on-board calibration)

• Theory …

Ocean Color = LwN() = ________________

• Optical properties can vary independently

• Three components of ocean color

– CDOM, phytoplankton & particulate scattering

Modeling Ocean Color

backscattering

absorption

What is Ocean Color?

• Light backscattered from the ocean

• Quantified by water-leaving radiance

LwN()

ocean

atmosphere

GSM01 Chlorophyll

SeaWiFS chlorophyll distribution

Where does ocean CDOM come from?

• Lots of places!!

• Washed to sea or formed in situ

– Allochthonous or Autochthonous

– Land vs. Ocean

• Not all CDOM is equal

– Different reactivities by region