Early diagenesis in marine sediments

Transcription

1 Early diagenesis in marine sediments

2 Why study this part of the ocean? Particle flux to the sea floor ocean surface sediments early diagenesis layer Biogeochemical reactions

3 Why study this part of the ocean? Particle flux to the sea floor Solute exchange : effects on ocean chemistry early diagenesis layer Biogeochemical reactions

4 Why study this part of the ocean? Particle flux to the sea floor early diagenesis layer Biogeochemical reactions Accumulating sediments: composition differs from particle inputs

5 Marine Sediments 1) The flux of particles to the sea floor 2) Preservation rates of biogenic components of the flux 3) Consequences of early diagenesis 4) Specifics: For each of : Organic matter, CaCO 3, Biogenic SiO 2 : a) mechanism for decomposition / dissolution b) how do we know?

6 The composition of the particulate rain to the sea floor -- Examples Flux: mg/m2/day Composition: wt. % Eastern Equatorial Atlantic Eastern Equatorial Atlantic Flux: mg/m 2 /day Equatorial Pacific Percent Equatorial Pacific Total CaCO 3 North Atlantic Org Matter Opal Lithogenic Total CaCO 3 Org Matter North Atlantic Opal Lithogenic - By weight Figure by MIT OCW.

7 Composition of the particulate rain to the sea floor -- the Southern Ocean - Figure by MIT OCW.

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9 A specific example: Balance of fluxes in the central equatorial Pacific Berelson et al., 1997, DSR II Flux (mmol m -2 d -1 ) C org CaCO 3 Opal. Burial Remineral. Rain Burial + Remineral. Budgets of the biogenic constituents in the EqPac region, corrected and adjusted as describeded in the text. Figure by MIT OCW. Burial : measured accumulation rates Remineralization : in situ benthic flux chamber determinations Rain : sediment traps

10 Benthic Fluxes >> Burial Rates: Does it matter? Goal : To learn about changes in rain rates to the sea floor over time from measurements of sediment accumulation rates *Assume steady state. For a given component of particle flux: Rain rate to sea floor = Accumulation rate (preserved) + Diagenetic reaction rate (lost to reaction) In symbols R = A + D Case 1: D << R : R ~A R = 1 1 D A R ===> Case 2: D/R constant : R α A Case 3: D~R, D/R variable: small (D/R) --> large R

11 In picture form: 1 R = 1 D A R

12 Some consequences of early diagenesis A. Low and variable preservation rates of biogenic components and the interpretation of the sedimentary record B. Early diagenesis and atmospheric oxygen (long time scales) C. In the contemporary ocean 1. Deep-water oxygen consumption ~ 5% of O2 consumption below 1m occurs in sediments 2. Denitrification >~ 5% of dentrification in the modern ocean occurs in sediments

13 Continental margin sediments: O2 --> near the sediment-water interface! Continental Margin Continental Slope Sea-level Vertical Exaggeration x O 2 penetration depth (mm) Depth (km) Shelf Break Continental Shelf Continental Rise Sea-level Vertical Exaggeration x Abyssal Plain Water Column Depth (m) Distance From Shore (km) The depth where O 2 in continental margin sediments Northwest Atlantic (Lohse et al., 1998) Northeast Atlantic (Martin and Sayles, 24) Northeast Pacific (Reimers et al., 1994) Figure by MIT OCW.

14 How sedimentary processes differ from water column processes Particles! Surface sediments ~ 4-7% particles by weight Time : Particles fall through the water column: τ 35m 5 m day 7 days swi Mixed layer ~ reaction layer τ ~ 8cm.1 cm y ~ 8yr 2cm.1cm y ~ 2yr (pelagic) (coastal) Reactions that are too slow to occur in the water column can happen in surface sediments

15 How sedimentary processes differ from water column processes In sediments, reactants are supplied from above: Particles from Water column solutes diffuse from water column Particles from Sediment surface React with solutes diffusing from above particle mixing First order approximation*: sediments have a layered structure

16 Mechanism for organic matter oxidation Familiar Processes: Organic matter e - acceptor products reduced e - acceptor Mechanism: more complex than overall rxn written! Order : decreasing G Evidence : Benthic fluxes and pore water profiles of reactants and / or products

17 Pore water profiles :O 2 all done by in situ microelectrode profiling 5 O2 (µmol/l) O 2 (µmol/l) O2 (µmol/l) Depth (cm) Ceara Rise 32 m (Hales et al., 1996) Depth (cm) m JSL II Dive 2949-a JSL II Diev c JSL II Dive d Fit Range of x* Depth (cm) 1 2 BBay August 23 dep_22 dep_12 dep_13 Total Corg ox. Rate (µmol/cm2/y)

18 Interpretation of pore water profiles : 1. Qualitative interpretation Assume: ** steady state ** + ~ constant porosity & diffusivity, negligible advection. Concentration-Solute I C A.4 C B Concentration of Solute II C B C A 1. 2 x A A 2 x A A Depth (cm) x B PRODUCTION between XA and XB: A straight line shows what the profile would be with NO net production between A and B. At each depth, the concentration is GREATER than that on a straight line between A &B B Depth (cm) x B B CONSUMPTION between A and B. The profile between A and B would be a straight line if no reaction were occurring. Conc. is smaller at each depth than the no-reaction case ==> the solute is being consumed between A and B

19 CONCENTRATION CHARACTERISTIC REACTION DEPTH O 2 - NO 3 Mn 2+ Ft 2+ ZONE 1 ZONE 1 ZONE 1 ZONE 1 ZONE 1 ZONE 1 ZONE 1 d 2 [MO - 3 ] 1. dz 2 < d 2 [NO - 3 ] 2. dz 2 = 3. d 2 [Mn 2+ ] > dz 2 d 2 4. [Mn 2+ ] dz 2 d 2 [NO - 3 ] 5. dz 2 6. d 2 [Fe 2+ ] dz 2 d 2 [Fe 2+ ] 7. dz 2 < > > < 1 Diffusion Schematic representation of trend in pore water profiles. Depth and concentration axes in arbitrary units. The zones, characteristic curvature OF the gradients and reaction numbers are discussed in the text. Figure by MIT OCW.

20 Interpretation of profile shapes : quantitative Steady-state mass balance in a sediment layer: Rate of reaction within the layer = net flux out of the layer oxic 1 NO 3 - μmol/l Diffusive flux : R = F out F in F = φd sed dc dx 1 denitrification 2 Flux at pt. 1 (x=) : gives total, net NO3 Production in sediment column Depth (cm) CVP - Site M NO3 data Fit: A&S stoich. Flux at pt. 2 : gives rate of NO3 consump. By denitrification Sum of absolute values of Flux at 1 + Flux at 2: Gives rate of NO3 production by oxic Decomposition of organic matter 6

21 Example: pore water data 45m water depth, NW Atlantic O 2 (µmol/1) NO - 3 (µmol/1) Depth (cm) m Dive JSL II 2941 Dive JSL II 2944 Fit Range of X * NH4 Depth (cm) Depth where O O 2 wcs 46m m Data and best fit Depth (cm) 5 1 NO 3 O N. W. Atlantic continental Margin 15 A Figure by MIT OCW.

22 Depth (cm) Mn 2+ (µmol/l) 25 Example : pore water data Coastal site C org ox. Rate ~ 85 µmol C / cm 2 /y Depth (cm) Fe 2+ (µmol/l) 4 8 Hingham Bay 21 January June October Depth (cm) H 2 S (µmol/l) SO 4 (mmol/l) Buzzards Bay Aug 4 Hingham Bay Oct 2 dsbw

23 Which electron acceptors are used the most in sediments for organic matter oxidation? Electron Acceptors in Pelagic Sediment (1) Site - Region C org ox. rate (µmol/cm 2 /y) O 2 NO 3 - Mn(IV) FE(III) SO 2-4 MANOP H MANOP C E. Eq. Atlantic E. Eq. Pacific Central Eq. Pac -3 o N, 6-16 O W Electron Acceptors in Continental Margin Sediments % of organic C oxidation by different election acceptors Location Water depths Total Corg ox (µmol/cm 2 /y) O 2 NO 3 - Mn Fe SO 4 N.E. Atlantic (1) N.W. Atlantic (2) N.E. Pac: O2<5 µμ (3) N.E. Pac: O2 = (3) (1) Lohse et al., 1998; (2) Martin and Sayles, 24; (3) Reimers et al., 1992 Figure by MIT OCW.

24 Which electron acceptors are used the most in sediments for organic matter oxidation?

25 ORGANIC CARBON BURIAL IN MARINE SEDIMENTS Organic Carbon Burial Rates (and percentages)in Different Ocean Regimes Sediment type Deltaic Shelf Slope Pelagic Total Data from Gershanovich et at. (1974) All sediment types () 23 (1) 195 (88) 5 (2) 223 Σ = 223 Data from Berner(1989) Terrigenous deltaic-shelf sediments Biogenous sediments (high-productivity - zones) Shallow-water carbonates Pelagic sediments (low-productivity zones) Anoxic basins (e.g. Black sea) 14 (82) 6 (5) 1 (1) 7 (6) 3 (2) 5 (4) Σ = 126 Recalculation of data from Berner (1989) a Deltaic sediments Shelves and upper slopes Biogenous sediments (high-productivity zones) Shallow-water carbonates Pelagic sediments (low-productivity zones) Anoxic basins (e.g. Black Sea) 7 (44) 68 (42) 6 (4) 1 (.5) 7 (4) 3 (2) 5 (3) Σ = 16 Units are 1 12 g C yr -1 (parenthetical units = % of total burial) a Deltaic-shelf sediments were reapportioned assuming that 33% of the sediment discharge from rivers is deposited either along nondelatic shelves or upper slopes, and assuming that those deposits have total loadings of 1.5% organic carbon rather then.7% as in delatic regions. Estimates for all other regions remain the same. Figure by MIT OCW.

26 The distribution of organic matter in marine sediments : What determines the observed pattern?.local productivity?.variable preservation? Image removed due to copyright restrictions.

27 So: And: And: Organic carbon preservation A correspondence between regions of high 1 productivity and high % C org in sediments, These regions of high %C org are ALSO regions of low bottom water O 2 in many cases, It has been shown that some naturally occurring organic molecules REQUIRE O 2 for decomposition Does sedimentary %C org (C org accumulation rate, really) depend on: productivity? preservation? (bw O 2 ) both?

28 Oxygen Exposure Time Hartnett et al. (1998) Nature 391, Studied 2 areas: - 1) squares: Washington margin: higher productivity, less intense O 2 min 2) Circles: Mexico margin: lower productivity, intense O 2 min. Water Column Depth (m) Depth in Sediments (cm) Oxygen Concentration (µ mol 1-1 ) Organic Carbon (wt%) Figure by MIT OCW.

29 Oxygen Exposure Time Hartnett et al. (1998) Nature 391, They defined oxygen exposure time : And examined its effect on C org burial efficiency (= burial rate / rain rate)

30 Oxygen Exposure Time Hartnett et al. (1998) Nature 391, ( Linear scale ) Oraganic carbon burial efficlency (%) OC burtal efficiency (%) Oxygen exposer time (yr) Oxygen Exposure Time (yr) (Log Scale) Organic carbon burial efficiency as a function of oxygen exposure time Promising idea, Limited data Note: Provides feedback between atmospheric O 2 + Corg burial rate Figure by MIT OCW.