distribution & dynamics of colored dissolved organic materials (cdom) in the open ocean
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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
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