Presentation on theme: "Outline : Carbon cycling and organic matter biogeochemistry"— Presentation transcript:
1 Outline : Carbon cycling and organic matter biogeochemistry Global carbon cycle - pools, sources, sinks and fluxespools of organic carbon - POC, DOC - vertical & horizontal segregation, vertical fluxesOcean productivityBiological carbon pumpPreservation of organic carbonVertical flux of POM – sediment trapsDissolved organic carbon (DOC)Concentrations & distributionCharacterization of DOC pool - molecular size and reactivitySources and fates of POM & DOMAge and long-term sinks for DOM
2 Operational pools of carbon in seawater POM - 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 carbonPIC – Particulate inorganic carbon (CaCO3)DIC - dissolved inorganic carbon (all forms)Organic nutrient poolsPON & 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)
4 Organic carbon = Reduced carbon Includes all carbon other than CO2, HCO3-, H2CO3, CO, CO32-, and carbonate mineralsIncludes 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.
5 Net export from surface 8-15 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)Atmospheric CO2 784Terrestrial biota 600River DIC 0.5Exchange90Soil & detritus 1500Net export from surface 8-15Ocean DIC 38,000Marine biota 1-2Detrital POC 30DOC 7000.2Sedimentary reservoirs are huge!Organic sediments10,000,000Fossil fuels 3577Limestone & dolomite50,000,000
6 Most organic carbon in the sea is dissolved or colloidal. Biomass pools are very small Operationally-dissolved Dissolved and Colloidal materials are operationally Dissolved
7 Based on Table 9.1 in Millero, 2006 Sources of organic matter to the open oceans% of totalPrimary productionPhytoplanktonMacrophytesRiversGroundwaterAtmospheric inputRivers are a small source of organic matter to open ocean!Based on Table 9.1 in Millero, 2006
8 Ocean Net Primary Production in different trophic regimes Trophic zone Mixed layer Chl a (μg L-1)Net Primary Production(1015 gC y-1)% of Ocean NPPOligotrophic <22.7(<100 gC m-2 y-1)Mesotrophic56.5( gC m-2 y-1)Eutrophic >18.7( gC m-2 y-1)Macrophytes2.1Total ocean production = 48.5Total terrestrial production = 56.4Total global production = 104.9
9 Global primary productivity pattern as deduced from satellite imagery Behrenfeld et al Nature 444:Considerations:Depth distribution i.e. euphotic depthSeasonal variations, esp. in polar regionsInterannual variationsOceanic/oligotrophic areas– dominated by picoplankton < 2 μmUpwelling, coastal & temperate areas have larger phytoplankton (> 2 μm) as major primary producers
10 Behrenfeld et al 2006. Nature 444: Temporal changes in global average Chlorophyll anomaly and Net Primary Productivity (NPP) anomaly.was a strong El Nino year which reduced NPP. Rapid recovery ensued, with slow decline thereafter.Behrenfeld et al Nature 444:
11 The Biological Carbon Pump Exporting carbon below the pycnoclineCO2 (g)CO2 (aq) + H2O <=> H2CO3 <=> H+ + HCO3- <=> H+ + CO32-AirSeaEuphotic zonePhotosynthesiscalcificationPycnoclinesinkingUpwelling of high DIC, high pCO2 waterPOMCaCO3Some DOMDIC & alkalinityCaCO3-rich sediment above CCDNo preservation of CaCO3 below CCDCO2Deep SearespirationspreadingAlkalinityPOMCCDRidge crestCaCO3 dissolutionCarbon burial & preservation as POM and CaCO3Non-carbonate sedimentcarbonates
12 Export Productionper yearFalkowski et al., Science 298:
13 POMflux(z) = POMflux (100)(z/100)-0.858 Flux of organic matter decreases exponentially with depth :POMflux(z) = POMflux (100)(z/100)-0.858Where 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!Very little organic matter (POM) reaches the deep ocean – and what does reach the bottom is lower qualityData for the figure of Bishop et al came from Martin et al. 1987Vertical flux of POM is via dead phytoplankton, fecal pellets, molt shells, fragments, mucous feeding nets etc.
14 DOC export from surface ocean represents 8-18% of the total organic carbon export. Modeled DOC downward fluxDOC/POC downward flux ratioHansell et al., 2010
15 Sediment traps - particle interceptors Poison or preservativeBaffle to reduce hydrodynamic effectsParticle flux Base of euphotic zone m500 mCapture flux decreases exponentially with depth1000 m3000 m
16 Many different designs of sediment traps have been used Time series traps - rotating cylinders within trap collect for certain period of time1-1.5 metersLarge surface area trap for oceanic sampling
17 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)
18 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?
19 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:
20 Joaquim Goes and his team deploy simple sediment traps in the Southern Ocean
21 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)
22 Significance of Organic Carbon Burial Much of the present global carbon burial (preservation) is in marine environmentsLittle 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 BurialBurial 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
23 Percent of primary production accumulated in the sediments The greater the overall sedimentation rate of particles, the greater the fraction of surface primary production delivered to sedimentsSediment accretion rate (cm per 1000 y)0.111010010000.0010.01Percent of primary production accumulated in the sedimentsCoastal areas – maximum of ~10%y = x1.25See Fig in Pilson for actual data graph>5000 m depth> m depth>2000 m depth incl. Black Sea
24 Most burial nearshore on continental margins Burial will be a small fraction of the carbon delivered to the sediments. Most will be respired to CO2 and diffuse back to water column.Libes, Chapter 25
25 Reasons for high carbon burial on the continental margins: high productivity - > high POM flux to benthoshigh 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 descentremineralization slower under anoxia - still a debatable issue.
26 Coastal waters can have much higher DOC Dissolved organic carbon - the largest pool of organic matter in seawaterMeasured by converting DOC into CO2 via:Wet-chemical oxidationHigh temperature catalytic combustionUV-oxidationSealed tube combustionDOC concentrations are µM in surface waters of the open ocean, and µ M at depth.Coastal waters can have much higher DOC
27 Surface ocean (30 m) DOC concentrations Dots are measured values, background color field is modeledHansell et al., 2010
28 Deep ocean (3000 m) DOC concentrations decrease along ocean conveyor (meridional overturning circulation)Dots are measured values, background color field is modeledNADW starts with about 46 µM DOCThe 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 et al., 2010
29 DOC Concentration (µM) BCDDOC Concentration (µM)1000200030004000Depth (m)10203040506070Labile DOC; Small pool; τ = hours to daysSemi labile DOC; larger pool (25-30 µM) in sfc;τ = weeks to monthsOpen 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.Ultra-refractory DOC; τ = >6000 yRefractory DOC; τ = ~1000 yearsAfter Benner, 2002
30 The average 14C age of deep DOC is 6000 years|!
31 DOC is generally conservative with salinity in estuaries DOC (µM)Salinity3640075Freshwater end-memberImplies 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 observedSeawater end-member ~ µM
32 DOC concentration decreases away from shore Much of the DOC delivery to ocean is lost on the shelf, close to shore
33 Constituents of DOMHigh molecular weight >5000 Da (includes colloids)proteinspolysaccharides (mucus, structural polymers)nucleic acidssome humic substancesMedium Molecular weight Dahumic substances (refractory)oligopeptides, oligonucleotideslipidspigmentsLow molecular weight < 500 Damonomers (sugars, amino acids, fatty acids)osmolytes (DMSP, betaines, polyols)toxins, pheromones and other specialty chemicalsSee Chapter 22 in Libes for structures of organic compoundsModerate labilityMixed lability – some very refractoryHigh lability
35 Examples of some polysaccharides that might be part of a semi-labile, high molecular weight pool of DOM.Pectin contains O-methoxy groupsChitin is an amino sugar, i.e. it contains N
36 Depolymerization - Polymer hydrolysis Conversion of high molecular weight DOM or POM into low molecular weight DOMCarried out primarily by bacteria but really aconsortium of microbes.Proteins -> free amino acids & peptides by proteasesPolysaccharides to monosaccharides by glucosidases, chitinases, cellulasesPeptides to amino acids by peptidasesRNA or DNA to nucleotides by nucleases
37 Origin of labile DOM in seawater Exudates - Amino acids, sugars, some high molecular weight labile polysaccharides - rapidly consumedDeath or lysis of cells - rapid uptake by bacteriaSloppy feeding - leaking of phytoplankton cell contentsDigestion - 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.
38 Marine Snow. Agglomerated organic matter - amorphous aggregates Enriched with bacteria and protozoanspossible low oxygen conditionselevated nutrientsStill understudied.Some species of phytoplankton release mucilage i.e. Phaeocycstis sp.TEP - Transparent ExoPolymer. Is a form of marine snowMarine 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 foam.
39 Sea foam generated from Phaeocystis bloom in Dutch Wadden Sea Phaeocystis globosa colony –cells embedded in mucous form spherical colonySea foam generated from Phaeocystis bloom in Dutch Wadden Sea
40 Blowing sea foam at Nags Head, North Carolina during Hurricane Sandy, October 2012 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)
41 What isn’t there may be most important! Biogeochemists rule # 1What 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
42 kloss Glycine (2 nM) production loss 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.Glycine(2 nM)pseudo-steadystate conc.productionlossklossPool sizeProduction = loss under steady stateHypothetical example of amino acid turnoverk = 50 d-12 nM x 50 d-1 = 100 nM d-1The flux of carbon through a particular compound is a function of: turnover (Conc. X Kloss ) and carbon content per molecule.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.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 times per day!
43 Carbon utilization efficiency affects trophic transfer and CO2/O2 dynamics In terms of carbonCarbon Assimilation EfficiencyMicrobial Growth Efficiency (MGE)Biomass Production (BP)==BP + RespirationFrom the literature: MGE varies from 0.05 to 0.30 in different ocean waters (up to 0.52 in estuaries)Microbial Carbon Demand = Microbial C ProductionMicrobial Growth EfficiencyThese terms are often referred to as bacterial growth efficiency (BGE) and bacterial carbon demand (BCD) (until discovery of ocean Archaea complicated things)
44 Oligotrophic (from some recent studies) eutrophicOligotrophic (from some recent studies)Microbial Growth Efficiency = MGE = [Microb. Prod/(Microb. Prod + Respiration)]See also del Giorgio et al L&O 56:1-16
45 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 largeTurnover of humic substances and other refractory material may be very long (years)DOC in the deep sea is very refractory (14C-ages of 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)
46 How is this material ultimately removed from the ocean? 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?Photochemical oxidation may be the key (Mopper and Kieber et al. 1991).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).Hansell et al. (2009) also suggest particle adsorption (scavenging) in the deep see may remove some refractory carbon
47 Photochemical Blast Zone - some DOM oxidized Photooxidation as a major sink for refractory DOM in the seaPhotochemical Blast Zone - some DOM oxidizedNADW formation. Labile DOM is utilized in relatively short time - leaving old refractory carbon to make another circuitUpwelling of refractory, old DOMDeep water transit (= 1000 y)Little alteration of old, refractory carbonThis is a highly conceptualized diagram! Its not this simple!
48 Relative C:N ratiosAmino 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. 5-7C:N of POM increase with depth (more labile N-containing compounds are removed in upper water column)
49 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, Chemical composition and reactivity of marine dissolved organic matter.C:NDOM has much higher C:N and C:P than plankton (Redfield)RedfieldC:P
50 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 < 4Fulvic acids - soluble at all pH’sHumic acids + fulvic acids = humic substancesSignificant 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
51 Soil humic acid showing amorphous structure and many functional groups Ligand bound FeAdsorbed Al-Silicate clayNo two humic molecules will be the same
52 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 preservationCan be labile compounds – just not bioavailable when stuck to sediment
53 Organic carbon (weight percent) SA = Surface area of sediment particlesOC/SA = Organic carbon per unit surface areaRelatively constant amount of organic carbon per surface areaKeil and Hedges, Nature 370:549
55 Organic matter desorbed from sediment particles is rapidly degraded Age of the sediment layer from which Organic Matter was desorbed.This material persisted for at least 460 years but when desorbed, it degraded in days. Therefore it is labile stuff – protected by adsorptionHedges & Keil, 1995
56 More than monolayer equivalent Less than monolayer equivalentPercent 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)
58 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 preservationCan be labile compounds – just not bioavailable when stuck to sediment
61 Reactivity of DOM vs. molecular size (after Amon and Benner, 1996) Counterintuitive? Big molecules more reactive than small?Applies to Bulk DOC – not to individual compoundsMany small molecules have VERY high reactivity e.g. amino acids, DMSP
62 Latitudinal variation of DOC in the deep ocean 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.
63 Larger pool of semi-labile DOC in surface water; τ = weeks Small pool of very labile (easily degradable) DOC in surface waters; τ = hours to 1 dayLarger pool of semi-labile DOC in surface water; τ = weeksRefractory DOC; τ = yearsUltra-refractory DOC; τ = >6000 yOpen ocean surface DOC concentration is about 70 µM. Its about 44 µM in the deep Sargasso and about 34 µM in the deep Pacific.
66 Hydrolyzing/fermenting cell Under anoxic conditions it takes a consortium of organisms to degrade complex organic matterVery high Mol Wt DOCFermentative cellHigh Mol Wt DOCLow Mol Wt DOCHydrolyzing/fermenting cellRespiring cellLow Mol Wt DOC
67 Different organic fractions degrade at different rates
68 Ocean productivity by province % of Ocean Prod.81%18%1%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 x 1015 gC/y. The distribution percentages, however will be similar to those shown here.
72 Global Carbon Cycle Problem Global CO2 release is known, but net increase in atmosphere is less than predictedWhere 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!
74 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-.
75 Oceanic/oligotrophic areas– dominated by picoplankton < 2 μm Upwelling, coastal & temperate areas have larger phytoplankton (> 2 μm) as major primary producersConsiderations:Depth distribution i.e. euphotic depthSeasonal & interannual variations-seasonal variations
76 Deep DOC ~5900 years oldDeep DOC ~4100 years old
77 Fig. 2. Observed values of the total Corg rparticle surface area loading of sediment in riverine, deltaic, nondeltaic continentalmargin, and deep-sea environments sediments Mayer, 1994a,b;Keil et al., 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., 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..
78 Polysaccharides & Proteins Revised Molecular Size-Reactivity Continuum Model for Marine DOC (after Amon and Benner, 1996)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.HighLabile Monomers- Amino acids- DMSP- sugarsLog ReactivityLowconcentrationLabilePolysaccharides & ProteinsRefractory humicsubstancesquantityLow10000 MW1000 MW500 MW0 MWHighLowLog Molecular SizeAll scales are somewhat arbitrary, and should probably viewed as a log-type scale