Outline : Carbon cycling and organic matter biogeochemistry

Global carbon cycle - pools, sources, sinks and fluxes

• pools of organic carbon - POC, DOC - vertical & horizontal segregation, vertical fluxes

• Ocean productivity

• Biological carbon pump

• Preservation of organic carbon

• Vertical flux of POM – sediment traps

Dissolved organic carbon (DOC)• Concentrations & distribution

• Characterization of DOC pool - molecular size and reactivity

• Sources and fates of POM & DOM

• Age and long-term sinks for DOM

Operational pools of carbon in seawaterPOM - particulate organic matter (includes not only carbon but

also H, O, N, P, S etc)

DOM - dissolved organic matter (about 50% C by weight)

POC - particulate organic carbon (refers only to the carbon)

DOC - dissolved organic carbon

PIC – Particulate inorganic carbon (CaCO3)

DIC - dissolved inorganic carbon (all forms)

Organic nutrient pools

PON & POP (the pools of N & P that are bound in organic particles larger than the operational cut-off)

DON & DOP - (the pools of N & P that are bound in organic matter that passes through the operational cut-off filter)

All pools are operational! (depend on selected criteria for filtration)

Organic particle size continuum

0.4-0.2 µm filtration

cut-off

Organic carbon = Reduced carbon

Includes all carbon other than CO2, HCO3-, H2CO3,

CO, CO32-, and carbonate minerals

Includes hydrocarbons CH4, CH3-CH3 etc & black carbon.

Nearly all reduced carbon is biogenic. However, some chemical/geochemical alteration of OM takes place, petroleum and natural gas formation being notable examples.

Because organic matter is mainly biogenic it typically contains not only reduced carbon but also some H, O, N, P and S etc.

Atmospheric CO2 784

Global Carbon reservoirs and exchanges (Figure based on Libes; data from Table 11.1 in Emerson & Hedges)

pools in 1015 gC (boxes) fluxes in 1015 gC y-1 (arrows)

Marine biota 1-2

Ocean DIC 38,000

Detrital POC 30

DOC 7000.2

Organic sediments10,000,000

Fossil fuels 3577

Limestone & dolomite50,000,000

Terrestrial biota 600

Soil & detritus 1500

River DIC 0.5

Sedimentary reservoirs are huge!

Exchange90

Net export from surface

8-15

Relative partitioning of organic carbon in the ocean

Dissolved ColloidalDetritus (POM)PhytoplanktonZooplanktonBacteria

•Most organic carbon in the sea is dissolved or colloidal.•Biomass pools are very small

Dissolved and Colloidal materials are operationally Dissolved

Operationally-dissolved

Sources of organic matter to the open oceans

Primary production

Phytoplankton 84.4

Macrophytes 6.2

Rivers 3.65

Groundwater 0.3

Atmospheric input 5.45

% of total

Rivers are a small source of organic matter to open ocean!Based on Table 9.1 in Millero, 2006

Trophic zoneMixed layer Chl a (μg L-1)

Net Primary Production

(1015 gC y-1)

Oligotrophic < 0.1 11.0Oligotrophic < 0.1 11.0

Mesotrophic 0.1 -1.0 27.4Mesotrophic 0.1 -1.0 27.4

Eutrophic > 1.0 9.1Eutrophic > 1.0 9.1

Macrophytes - 1.0Macrophytes - 1.0

Total ocean production = 48.5Total ocean production = 48.5

22.722.7

56.556.5

18.718.7

2.12.1

% of Ocean NPP

% of Ocean NPP

Total terrestrial production = 56.4Total terrestrial production = 56.4

Total global production = 104.9Total global production = 104.9Total global production = 104.9Total global production = 104.9

Ocean Net Primary Production in different trophic regimes

(<100 gC m-2 y-1)

(100-300 gC m-2 y-1)

(300-500 gC m-2 y-1)

Behrenfeld et al 2006. Nature 444:

Global primary productivity pattern as Global primary productivity pattern as deduced from satellite imagerydeduced from satellite imagery

Oceanic/oligotrophic areas– dominated by picoplankton < 2 μm

Upwelling, coastal & temperate areas have larger phytoplankton (> 2 μm) as major primary producers

Considerations:• Depth distribution i.e. euphotic depth

• Seasonal variations, esp. in polar regions

• Interannual variations

Behrenfeld et al 2006. Nature 444:

Temporal changes in global average Chlorophyll anomaly and Net Primary Productivity (NPP) anomaly.

Temporal changes in global average Chlorophyll anomaly and Net Primary Productivity (NPP) anomaly.

1997-98 was a strong El Nino year which reduced NPP. Rapid recovery ensued, with slow decline thereafter.

1997-98 was a strong El Nino year which reduced NPP. Rapid recovery ensued, with slow decline thereafter.

CO2 (g)

CO2 (aq) + H2O <=> H2CO3 <=> H+ + HCO3

- <=> H+ + CO32-

Air

Sea

Pycnocline

POM CaCO3

Some DOM

Non-carbonate sediment Carbon burial & preservation as POM and CaCO3

CCD

Deep Sea

Euphotic zone

DIC & alkalinity

respirationCO2

CaCO3 dissolution

The Biological Carbon Pump

Exporting carbon below the pycnocline

Ridge crest

Alkalinity

Upwelling of high DIC, high pCO2 water

No preservation of CaCO3 below

CCDspreading

carbonates

CaCO3-rich sediment above

CCD

Photosynthesis calcification

sinking

POM

Falkowski et al., 1998. Science 298:

Export Production

Export Production

per year

POM flux vs depth based on equation of Bishop (1989)

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

0 20 40 60 80 100

POM flux as % of flux at 100 m

De

pth

(m

)

Flux of organic matter decreases exponentially with depth :

POMflux(z) = POMflux (100)(z/100)-0.858

Where POMflux(100) is the downward flux at the base of the euphotic zone (100 m), and POMflux(z) is the flux of organic carbon at depth (z) measured with sediment traps.

At 5000 meters, the flux is only 3.5% of that at the base of the euphotic zone!

Vertical flux of POM is via dead phytoplankton, fecal pellets, molt shells, fragments, mucous feeding nets etc.

Data for the figure of Bishop et al came from Martin et al. 1987

Very little organic matter (POM) reaches the deep ocean – and what does reach the bottom is lower quality

Very little organic matter (POM) reaches the deep ocean – and what does reach the bottom is lower quality

Hansell et al., 2010

Modeled DOC downward flux

DOC/POC downward flux ratio

DOC export from surface ocean represents 8-18% of the total organic carbon export.

Sediment traps - particle interceptors

Base of euphotic zone 100-200 m

500 m

1000 m

3000 m

Capture flux decreases exponentially with depth

Particle flux

Poison or preservative

Baffle to reduce hydrodynamic effects

1-1.5 meters

Many different designs of sediment traps have been used

Time series traps - rotating cylinders within trap collect for certain period of time

Large surface area trap for oceanic sampling

Diagram of an automated time-series sediment trap used in the Arabian Sea. A baffle at top keeps out large objects that would clog the funnel. The circular tray holds collection vials. On a preprogrammed schedule (every 5 days to 1 month), the instrument seals one vial and rotates the next one into place. Scientists retrieve the samples up to a year later to analyze the collected sediment. (courtesy Oceanus magazine, WHOI) http://www.whoi.edu/instruments/gallery.do?mainid=19737&iid=10286

What results do you expect for POM captured in a sediment trap array deployed over a full oceanic depth profile?

• Quantity of POM?

• Quality of POM - C:N, specific biomolecules?, 14C-content?

Three sediment trap designs.

The original funnel design (moored trap) uses a large collection area to sample marine particulates that fall to great depths.

Surface waters produce enough sediment so that traps there don’t require funnels. Neutrally buoyant, drifting sediment traps catch falling material instead of letting it sweep past in the current. Drawings are not to scale.

Source: http://www.whoi.edu/instruments/gallery.do?mainid=19735&iid=10286

Joaquim Goes and his team deploy simple sediment traps in the Southern Ocean

WHOI scientists Ken Buesseler and Jim Valdes with one of the neutrally buoyant sediment traps they helped design. The central cylinder controls buoyancy and houses a satellite transmitter. The other tubes collect sediment as the trap drifts in currents at a predetermined depth, then snap shut before the trap returns to the surface. (Tom Kleindinst, WHOI)

http://www.whoi.edu/instruments/gallery.do?mainid=19750&iid=10286

Much of the present global carbon burial (preservation) is in marine environments

Little organic carbon preservation in terrestrial soils except for high latitude peats. Terrestrial burial of OM has been more significant in the geological past (i.e. Carboniferous coal deposits)Significance of Organic Carbon

Burial Burial and preservation of biogenic (reduced) carbon in sedimentary reservoirs removes atmospheric CO2 and allows excess O2 to remain in the atmosphere.

Burial of organic matter removes some nutrient elements and trace elements.

Carbon burial leads to petroleum, organic rich shales, & natural gas

Sediment accretion rate (cm per 1000 y)

0.1 1 10 100 10000.001

0.01

0.1

1

10

100

Per

cent

of

prim

ary

prod

ucti

on

accu

mul

ated

in th

e se

dim

ents

>5000 m depth

>2-5000 m depth

>2000 m depth incl. Black Sea

y = 0.028 x1.25

The greater the overall sedimentation rate of particles, the greater the fraction of surface primary production delivered to sediments

See Fig. 11.5 in Pilson for actual data graph

Coastal areas – maximum of ~10%

Most burial nearshore on continental margins

Most burial nearshore on continental margins

Libes, Chapter 25Libes, Chapter 25Burial will be a small fraction of the carbon delivered to the sediments. Most will be respired to CO2 and diffuse back to water column.

Reasons for high carbon burial on the continental margins:

high productivity - > high POM flux to

benthos

high particle flux leading to faster burial rate - OM preservation tied directly to

mineral surface area (see Keil et al. 94)

shallow depth - less organic matter

degradation on descent

remineralization slower under anoxia - still a debatable issue.

Dissolved organic carbon - the largest pool of organic matter in seawaterMeasured by converting DOC into CO2 via:

• Wet-chemical oxidation

• High temperature catalytic combustion

• UV-oxidation

• Sealed tube combustionDOC concentrations are 70-100 µM in surface waters of the open ocean, and 35-50 µ M at depth.

Vertical profile of DOC concentrations in the ocean

-3000

-2500

-2000

-1500

-1000

-500

0

0 20 40 60 80 100

DOC (micromolar)

De

pth

(m

)

Coastal waters can have much higher DOC

Surface ocean (30 m) DOC concentrations Dots are measured values, background color field is modeled

Hansell et al., 2010

Deep ocean (3000 m) DOC concentrations decrease along ocean conveyor (meridional overturning circulation)

Dots are measured values, background color field is modeled

Hansell et al., 2010

The semi-labile fraction of DOC degrades during the long transit from North Atlantic to the Pacific. What is left (~34 M) is ultra-refractory since it survived the ~1000 y trip through the deep ocean. This DOC is present as background DOC in surface waters and has an average age of ~6000 years.

NADW starts with about 46 µM DOC

A BC

D

DOC Concentration (µM)

1000

2000

3000

4000

0D

epth

(m

)

Ultra-refractory DOC; τ = >6000 y

Refractory DOC; τ = ~1000 years

Semi labile DOC; larger pool (25-30 µM) in sfc; τ = weeks to months

Labile DOC; Small pool; τ = hours to days

Open ocean surface DOC concentration is about 70 µM. It is about 44 µM in the deep Sargasso and about 34 µM in the deep Pacific.

0 10 20 30 40 50 60 70

After Benner, 2002

The average 14C age of deep DOC is 6000 years|!

DOC (µM)

Salinity 3600

400

75

DOC is generally conservative with salinity in estuaries

Freshwater end-member

Seawater end-member ~80-100 µM

Implies terrestrial DOC delivery to ocean – but most is lost on shelf (see next slide)

In fact, some modification of riverine DOC takes place in estuaries, but conservative pattern still observed

DOC concentration decreases away from shore

Much of the DOC delivery to ocean is lost on the shelf, close to shore

Constituents of DOMHigh molecular weight >5000 Da (includes colloids)

• proteins• polysaccharides (mucus, structural polymers)• nucleic acids• some humic substances

Medium Molecular weight 500-5000 Da• humic substances (refractory)• oligopeptides, oligonucleotides• lipids• pigments

Low molecular weight < 500 Da • monomers (sugars, amino acids, fatty acids)• osmolytes (DMSP, betaines, polyols)• toxins, pheromones and other specialty chemicals

Moderate lability

Mixed lability – some very refractory

High lability

See Chapter 22 in Libes for structures of organic compounds

See Chapter 22 in Libes for structures of organic compounds

Shift

Examples of some polysaccharides that might be part of a semi-labile, high molecular weight pool of DOM.

Examples of some polysaccharides that might be part of a semi-labile, high molecular weight pool of DOM.

Chitin is an amino sugar, i.e. it contains N

Chitin is an amino sugar, i.e. it contains N

Pectin contains O-methoxy groupsPectin contains O-methoxy groups

Depolymerization - Polymer hydrolysis

Conversion of high molecular weight DOM or POM into low molecular weight DOM

Carried out primarily by bacteria but really a consortium of microbes.

Proteins -> free amino acids & peptides by proteases Polysaccharides to monosaccharides by

glucosidases, chitinases, cellulases Peptides to amino acids by peptidases RNA or DNA to nucleotides by nucleases

Origin of labile DOM in seawater

Exudates - Amino acids, sugars, some high molecular weight labile polysaccharides - rapidly consumed Death or lysis of cells - rapid uptake by

bacteria

Sloppy feeding - leaking of phytoplankton

cell contents Digestion - Digestor theory. Jumars, Penry et al. Zooplankton maximize their organic matter assimilation by maximizing throughput not by being highly efficient. This results in considerable release of DOC from fecal pellets and zooplankton.

Marine Snow. Agglomerated organic matter - amorphous aggregates

• Enriched with bacteria and protozoans• possible low oxygen conditions • elevated nutrients• Still understudied.

Some species of phytoplankton release mucilage i.e. Phaeocycstis sp.

TEPTEP - Transparent ExoPolymer. Is a form of marine snow

Marine Snow or aggregates caused by surface phenomenon. Enrichment of OM at surfaces of bubbles, waves convergence zones. You can make snow in the lab by rotating filtered water samples in bottle. Snow, and DOC make, sea foamsea foam.

http://www.microscopy-uk.org.uk/mag/artapr01/foam.html

http://www.ifremer.fr/delec-en/projets/efflores%20phyto/phaeocystis/phaeocys.htm

Phaeocystis globosa colony –cells embedded in mucous form spherical colony

Sea foam generated from Phaeocystis bloom in Dutch Wadden Sea

Nags Head, N.C.High winds blow sea foam into the air as a person walks across Jeanette's Pier in Nags Head, N.C., Sunday, Oct. 28, 2012 as wind and rain from Hurricane Sandy move into the area. Governors from North Carolina, where steady rains were whipped by gusting winds Saturday night, to Connecticut declared states of emergency. Delaware ordered mandatory evacuations for coastal communities by 8 p.m. Sunday. (AP Photo/Gerry Broome)

Blowing sea foam at Nags Head, North Carolina during Hurricane Sandy, October 2012

Biogeochemists rule # 1

What isn’t there may be most important!

Substances with low concentrations may be especially important in biogeochemical fluxes - their concentrations are low because they are desirable molecules to microbes!This axiom isn’t always true, but it often is

Concentrations of most labile, low molecular weight organic compounds are low (typically in the 1-10 nM (10-9 – 10-8 Molar) range). Compare this to total DOC concentration in surface waters of about 75 µM C. But some LMW compounds have very fast turnover.

The flux of carbon through a particular compound is a function of: turnover (Conc. X Kloss ) and carbon content per molecule.

Thus, even substances with low concentrations can have high carbon fluxes if the turnover rate constant is large (fast turnover)

Glycine and DMSP dissolved pools may turn over 10-50 times per day!

Glycine (2 nM)

pseudo-steady state conc.

production loss

kloss

Pool size

Production = loss under steady state

Hypothetical example of amino acid turnover

k = 50 d-12 nM x 50 d-1 = 100 nM d-1

So for this example, 100 nM glycine d-1 x 2 mol C/mol glycine = 200 nM C d-1 flux through the glycine pool.

From the literature: MGE varies from 0.05 to 0.30 in different ocean waters (up to 0.52 in estuaries)

Microbial Growth Efficiency (MGE) =

Biomass Production (BP)

BP + Respiration=

Carbon Assimilation Efficiency

Carbon utilization efficiency affects trophic transfer and CO2/O2 dynamics

Microbial Carbon Demand = Microbial C Production

Microbial Growth Efficiency

These terms are often referred to as bacterial growth efficiency (BGE) and bacterial carbon demand (BCD) (until discovery of ocean Archaea complicated things)

These terms are often referred to as bacterial growth efficiency (BGE) and bacterial carbon demand (BCD) (until discovery of ocean Archaea complicated things)

In terms of carbon

Microbial Growth Efficiency = MGE = [Microb. Prod/(Microb. Prod + Respiration)]

Oligotrophic (from some recent studies)Oligotrophic (from some recent studies)

eutrophiceutrophic

See also del Giorgio et al 2011. L&O 56:1-16See also del Giorgio et al 2011. L&O 56:1-16

Turnover of higher molecular weight material is relatively slow

Polysaccharide material (relatively labile) may turnover on time scales of days, and because of relatively large pool sizes (micromolar C), the mass flux can be large

Turnover of humic substances and other refractory material may be very long (years)

DOC in the deep sea is very refractory (14C-ages of 4000-6000 years) - this explains its nearly uniform distribution (see Bauer, Williams and Druffel et al.)

Surface water DOC pool has average 14C age of ~1000 y - this DOC is composed of young (modern) carbon (14C age of +200 y) plus some of the old refractory material (14C age of ~6000 y)

If 14C-age of deep DOC is ~6000 years, then this material has survived several ocean mixing cycles.

How is this material ultimately removed from the ocean?

Photooxidation breaks down DOM into CO2 and smaller, often more labile molecules, thus returning it to biologically active pool of carbon (Kieber et al. Nature, 1989).

Photochemical oxidation may be the key (Mopper and Kieber et al. 1991).

Hansell et al. (2009) also suggest particle adsorption (scavenging) in the deep see may remove some refractory carbon

Photochemical Blast Zone - some DOM oxidized

Deep water transit (= 1000 y)

NADW formation. Labile DOM is utilized in relatively short time - leaving old refractory carbon to make another circuit

Upwelling of refractory, old DOM

Little alteration of old, refractory carbon

Photooxidation as a major sink for refractory DOM in the sea

This is a highly conceptualized diagram! Its not this simple!

Relative C:N ratios

• Amino acids (AA’s) < protein < lipids < carbohydrates.

• AA’s C:N 2-6 except for phenylalanine and tyrosine (C:N= 9)

POM concentration is generally high in the upper water column and euphotic zone. Very low at depth.

• C:N of POM in surface ocean is generally similar to Redfield, i.e. 5-7

• C:N of POM increase with depth (more labile N-containing compounds are removed in upper water column)

Molar ratios of C:N and C:P in marine plankton, DOM, and high molecular weight (HMW) DOM from the surface (<100 m) and deep (>1000 m) ocean. From Benner, 2002. Chemical composition and reactivity of marine dissolved organic matter.

Redfield

C:N

C:P

DOM has much higher C:N and C:P than plankton (Redfield)

Humic substances in the sea• Complex, amorphous organic matter Gelbstoffe (colored DOM or CDOM) (contain many functional groups incl. aromatics)

• Humic acids - insoluble at pH < 4

• Fulvic acids - soluble at all pH’s

Humic acids + fulvic acids = humic substances

Significant terrestrial input of humic substances to the sea via rivers, but most is destroyed on continental shelves before reaching open ocean, probably via photooxidation. Only a small fraction (~1%) of oceanic DOC is terrestrially-derived, but up to 10% of humic substances might be terrestrial (based on lignin biomarkers and 13C-content)

Autocthonous humic substances - marine origin. Lack lignin moieties. Result from condensation of marine DOM - possibly via photoreactions

Soil humic acid showing amorphous structure and many functional groups

Adsorbed Al-Silicate clay

Ligand bound Fe

No two humic molecules will be the same

Role of sediment adsorption of organic matter in the carbon cycle (after Hedges and Keil, 1999)

Adsorption of organic compounds to inorganic sediment surfaces may play a role in organic carbon preservation

Adsorption of organic compounds to inorganic sediment surfaces may play a role in organic carbon preservation

Can be labile compounds – just not bioavailable when stuck to sediment

Can be labile compounds – just not bioavailable when stuck to sediment

Keil and Hedges, 1994. Nature 370:549

Organic carbon (weight percent)

SA = Surface area of sediment particles

OC/SA = Organic carbon per unit surface area

Relatively constant amount of organic carbon per surface area

Hedges & Keil, 1995

Hedges & Keil, 1995

Age of the sediment layer from which Organic Matter was desorbed.

Organic matter desorbed from sediment particles is rapidly degraded

This material persisted for at least 460 years but when desorbed, it degraded in days. Therefore it is labile stuff – protected by adsorption

Monolayer equivalent

Less than monolayer equivalent

More than monolayer equivalent

Percent of global organic carbon burial that occurs in different depositional environments. The largest fractions are Delta (44%) and Shelf (45%) indicating that 90% of global carbon burial occurs on ocean margins. The shading indicates where organic content is More, Less or Equivalent to monolayer absorption based on surface area of sediment particles. (after Keil & Hedges)

finish

Role of sediment adsorption of organic matter in the carbon cycle (after Hedges and Keil, 1999)

Adsorption of organic compounds to inorganic sediment surfaces may play a role in organic carbon preservation

Adsorption of organic compounds to inorganic sediment surfaces may play a role in organic carbon preservation

Can be labile compounds – just not bioavailable when stuck to sediment

Can be labile compounds – just not bioavailable when stuck to sediment

Low Mol Wt DOC

High Mol Wt DOC

Photo – Jeff Cornwell

O2

From Davis and Benner, 2007

Counterintuitive? Big molecules more reactive than small?

Applies to Bulk DOC – not to individual compounds

Many small molecules have VERY high reactivity e.g. e.g. amino acids, DMSPamino acids, DMSP

Mol

ecul

ar s

ize

Reactivity of DOM vs. molecular size (after Amon and Benner, 1996)

Latitudinal variation of DOC in the deep ocean. The semi-labile fraction of DOC degrades during the long transit from North Atlantic to the Pacific. What is left (~34 M) is ultra-refractory since it survived the ~1000 y trip through the deep ocean. This DOC is present as background DOC in surface waters and has an average age of ~6000 years.

Latitudinal variation of DOC in the deep ocean. The semi-labile fraction of DOC degrades during the long transit from North Atlantic to the Pacific. What is left (~34 M) is ultra-refractory since it survived the ~1000 y trip through the deep ocean. This DOC is present as background DOC in surface waters and has an average age of ~6000 years.

Hansell.

Ultra-refractory DOC; τ = >6000 y

Refractory DOC; τ = 1000 years

Small pool of very labile (easily degradable) DOC in surface waters; τ = hours to 1 day

Larger pool of semi-labile DOC in surface water; τ = weeks

Open ocean surface DOC concentration is about 70 µM. Its about 44 µM in the deep Sargasso and about 34 µM in the deep Pacific.

Fermentativ

e cell

Respiring cell

Hydrolyzing/ferm

enting cell

Low Mol Wt DOC

Low Mol Wt DOC

High Mol Wt DOC

Very high Mol Wt DOC

Under anoxic conditions it takes a consortium of organisms to degrade complex organic matter

Different organic fractions degrade at different rates

Ocean productivity by provinceProvince Area

(106 km2) % of ocean

Mean productivity (gC m-2 y-1

Total Production

(1015 gC y-1)

Open ocean 326 90 50 16.3

Coastal zone 36 9.9 100 3.6

Upwelling areas

0.36 0.1 300 0.108

Total 20

These values for productivity are old and a low estimate! Other recent estimates of global ocean productivity (e.g. Martin et al. 1987) are closer to 40-50 x 1015 gC/y. The distribution percentages, however will be similar to those shown here.

81%

18%

1%

% of Ocean Prod.

Hopkinson & Vallino. Nature 433: 2005

Seasonal cycle of DOC at the BATS station in the Sargasso Sea - Carlson et al 1994

Spring build up of DOC

Winter mixing homogenizes upper 200m & mixes down some DOC

Global Carbon Cycle Problem

Global CO2 release is known, but net increase in atmosphere is less than predicted

Where does this carbon go? Some of the carbon can be accounted for by ocean uptake (see Quay et al.), but there is a missing sink of 0.7 GT. Terrestrial biomass (i.e. trees) might be missing sink.

0.7 GT of C is only 4% of net annual primary productivity on land and 3% of ocean carbon exchange with atmosphere, therefore it is hard to discern with accuracy. Ocean exchange in particular is difficult because of spatio-temporal shifts in carbon exchange.

The role of the oceans in Carbon exchange is being studied intensively!

G1

G2

G3

Carbon generally not considered limiting to primary productivity in the sea - plenty of bicarbonate or CO2 in seawater (DIC = ~2 mM). The ratio of C:N:P in surface seawater is 1000:16:1. Thus C not likely to be limiting to Primary Production.

However, the form of inorganic carbon available to phytoplankton does makes a difference. Phytoplankton take up predominantly the neutral species of DIC (CO2(aq) and H2CO3) so if pCO2 is low, phytoplankton can experience carbon limitation.

Some species may have “carbon concentrating mechanisms” to transport HCO3

-.

-seasonal variations

Oceanic/oligotrophic areas– dominated by picoplankton < 2 μm

Upwelling, coastal & temperate areas have larger phytoplankton (> 2 μm) as major primary producers

Considerations:• Depth distribution i.e. euphotic depth

• Seasonal & interannual variations

Deep DOC ~5900 years old Deep DOC ~4100 years old

Fig. 2. Observed values of the total Corg rparticle surface arealoading of sediment in riverine, deltaic, nondeltaic continentalmargin, and deep-sea environments sediments Mayer, 1994a,b;Keil et al., 1997.. Despite contributions of both terrestrial andmarine Corg , the particle surface area specific Corg load of deltaicmaterial is comparable to oligotrophic deep sea sites that areessentially entirely marine Corg , indicating major loss from deltaicsediments relative to all source material. Approximate terrestrialand marine percentages ";15% for deltaic, shelf; ";5% fordeep-sea.are based on typical bulk sediment isotopic rangese.g.,Showers and Angle, 1986; Emerson and Hedges, 1988; Bird et al.,1995; Keil et al., 1997.. The riverine and deep Pacific Corgloading values represent simple averages of reported data "SDindicated., deltaic and nondeltaic shelf values represent slopes ofCorg vs. particle surface area regressions"SE indicated.. Theasymptotic value of Corg rarea at depth in sediment is used at agiven site if a depth variation below the sediment–water interfaceis evidentMayer, 1994a,b..

Log Molecular SizeHigh Low

1000 MW 500 MW 0 MW10000 MW

Log Reactivity

High

Low

Revised Molecular Size-Reactivity Continuum Model for Marine DOC (after Amon

and Benner, 1996)

quantity

LabilePolysaccharides &

Proteins

Refractory humicsubstances

Labile Monomers- Amino acids- DMSP- sugars

This modification of the figure presented in Amon and Benner, 1996, attempts to illustrate that a large fraction of the total DOC (quantity is indicated by the distance between the two curves) is high molecular weight material (>10,000 MW). The material >1,000 MW, represented by polysaccharides, is relatively labile (high reactivity) when compared with the low molecular weight material (refractory humics) near and just below 1,000 MW. Together these pools make up the bulk of the DOC concentration. On the low end of the size spectrum, most compounds are labile (amino acids etc.), but their concentrations are very low (together making only 1% of DOC) but their reactivity is VERY high.

This modification of the figure presented in Amon and Benner, 1996, attempts to illustrate that a large fraction of the total DOC (quantity is indicated by the distance between the two curves) is high molecular weight material (>10,000 MW). The material >1,000 MW, represented by polysaccharides, is relatively labile (high reactivity) when compared with the low molecular weight material (refractory humics) near and just below 1,000 MW. Together these pools make up the bulk of the DOC concentration. On the low end of the size spectrum, most compounds are labile (amino acids etc.), but their concentrations are very low (together making only 1% of DOC) but their reactivity is VERY high.

Lowconcentration

All scales are somewhat arbitrary, and should probably viewed as a log-type scale