The Interaction Between Physics and Biogeochemistry in Nearshore Permeable Sediments

Transcription

1 The Interaction Between Physics and Biogeochemistry in Nearshore Permeable Sediments Frank Sansone Department of Oceanography, University of Hawaii

2 Acknowledgements Jon Fram -Postdoc, now at OSU Brian Glazer,Geno Pawlak-PIs / co-pis Kristen Fogaren, InChiehChen, Jenny Murphy -Grad Students Chris Colgrove, Mari Okahara- Undergrad Students Kilo Nalu field and tech support personnel National Science Foundation - Chemical Oceanography

3 Outline A very short history of the topic Physical forcing of water flow in permeable sediments Diagenetic porewater signals in nearshore sands, and their response to physical forcing Verification of modeling of porewater motion, using temperature as a tracer Calculation of porewater fluxes and sediment production/consumption rates

4 Composition of Marine Sediments Relative Proportion of Inner (< 60 m) Continental Shelf Deposits: Coral Shell Mud Rock & Gravel Sand Hayes, Degrees Latitude 60

5 Traditional Reasons Notto Study Sands Low organic matter content Assumed low reactivity Concentrations and fluxes can t be determined using conventional (i.e., fine-grained sediment) techniques They re just boring sands.

6 Some Compelling Reasons ToStudy Sands Efficient filtration of water-column POM (and DOC??) Rapid remineralization of retained POM (and DOC??) Very high exchange rates with overlying water column Large sediment-water dissolved fluxes Nutrients out of the sediment Dissolved oxygen into the sediment

7 Two Main Physical Forcings in Sands Bottom-current / obstacle interactions Wave pumping

8 Bottom-current / Obstacle Interactions Huettel& Rusch(2000)

9 Wave Pumping in a Rippled Sand Wave orbital currents P P P P P P P Sand Basement

10 When you work with a Physical Oceanographer. magical things (i.e.numerical models) can happen!

11 Advection Driven by Surface Pressure Gradients Advection Calculated via Darcy s Law (Shum 1992) Darcy s Law κ v = P φµ φ=porosity µ= viscosity

12 Calculation of Surface Pressure Gradients (Shum 1992) SWI pressure is from potential flow via 2 nd order Stokes wave expansion and conformal mapping over ripples P crest trough k 1 U 2 o a L 2 Steeper and narrower ripples have bigger crest-trough pressure differences U o =Max wave orbital U = f(h s, H, T) H s =sig. wave height H=water depth T=wave period a=ripple height L=ripple wavelength

13 Diffusion Calculated as Shear Dispersion (Webster 2003) Dominant process deeper than ~10-20 cm in sed D=f(D mol, φ, H s, H, T) φ=porosity H s =sig. wave height H=water depth T=wave period

14 So.How well does all this work??

15 Porewater Motion From Wave Pumping Color of water parcels = original depth White = overlying seawater Jon Fram et al. (in prep) H sig = 0.65 m, period = 13 s, depth = 13 m

16 Porewater Parcel Tracking Shows Seawater / Porewater Motion start 12 min

17 Porewater Motion Over Six Days A rough rule of thumb: Daily sediment ventilation depth ripple wave length Very fast!!!

18 Now, With Migrating Ripples.

19 Ripple migration rate = two ripples/day

20 Ripple Migration Reduces Depth of Exchange Two ripples per day No ripple migration

21 The Big Questions What is the rate of POM removal from the water column? What is the rate of seawater / porewater exchange? What are the chemical fluxes between the sediment and the overlying seawater? What are the production / consumption rates in the sediment?

22 The Big Questions What is the rate of POM removal from the water column? What is the rate of seawater / porewater exchange? What are the chemical fluxes between the sediment and the overlying seawater? What are the production / consumption rates in the sediment?

23 Our Current Field Site KiloNaluNearshore Ocean Observatory, Honolulu, Hawaii Two-way high-bandwidth data; 24-V power

24 Sand patch within a nearshore reef system 10-m water depth 300 m from shore

25 DIC Profiles Reflect Upper-Sediment Org Matter Oxidation -- Despite Rapid Flushing Fogaren et al., submitted

26 How Does Wave Height Affect Porewater Chem? Fogaren et al., submitted

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33 Nutrient Conc Response to Swells

34 But, what can we do to get: -Better vertical resolution of concs -Better temporal resolution of concs -Concentrations of a wide range of redox-sensitive species

35 InSituElectrochemical Analysis (EChem) 100-um Au wire sealed in glass or PEEK using marine epoxy, Hgelectroplated O 2, H 2 O 2, Fe 2+, Mn 2+, HS -, S 2 O 3 2-, S 4 O 6 2-, S x 2-, S 8(aq), Fe 3+ (aq),fes (aq) are all simultaneously detectable

36 EChem with Ripple-Monitoring Video Cameras Remotely operated and powered interactive experiments

37 Ripple-Monitoring Shum & Sundby 1996

38 Ripple-Monitoring Shum & Sundby 1996

39 One complication of remotely controlled experiments..

40 KiloNaluEChem Data April Jenny Murphy Ripple edge Ripple peak Ripple edge

41 Factors Driving O 2 / H 2 S Variability Variable seawater-sediment exchange Variable bottom currents and surface waves Sand ripple migration Diurnal O 2 production / consumption within sediment Variable organic matter delivery to sediment

42 How to Measure the Seawater-Porewater Exchange??

43 Estimating Benthic Fluxes in Fine- Grained Sediments 0 Depth Flux Conc But a different approach is required with permeable sediments.. Flux Conc Time

44 Porewater Transport Measurement using 10 cm a Natural Tracer: Temperature Miniature Thermistor Chain 1.5 cm 2 cm One sensor above the SWI 10 sensors over a 16-cm depth range 5 sensors at 90 to simultaneously measure at ~½ ripple-wavelength away Sensors are at the end of long thin hollow tines to minimize heat transport minitchain Built by Mike Head, PME

45 Sensors are in the Sed Sand

46 Temperature vs.depth vs.time The sub-daily temperature propagation can be used to calculate advective transport in the upper sediment

47 March -June 2011 MiniTchain can measure advective thermal signals to at least 20 cm (i.e., over depths where advection dominates over diffusion)

48 Tuning of SWI Pressure in Advection Model Model - forced with measured SWI temp Data November December

49 Modeled vs. Measured Temps for all Sensors Overall R 2 =0.94

50 Sub-daily Temp Variation Model does well down to ~16 cm Model: T T 1day o ( C) Data: T o o o o T1day ( C) Data: T T1day ( C) Data: T T1day ( C) Data: T T1day ( C)

51 Uses of Temp-validated Advection Model Ongoing Work Calculate porewater residence time in 2D Combine porewater advection data with conc profilesto calc vertical fluxeswithin the upper sediment and across the sediment-water interface Compute in situ production / consumption rates from the divergence / convergence of fluxes at depth

52 Porewater Residence Time

53 A Simple Proof of Concept Use the transport calculated by the mtc-validated model Assume a constant oxygen consumption rate for the sediments Tune that constant until profiles match the EChem results

54 Results with Assumed Uniform O 2 Consumption of 1umolcm -3 d -1 Depth, m O 2, um Chen and Glazer (in prep)

55 Advection vs. Wave Height More Work in Progress 2011

56 The Next Big Step: EChemArrays Cable to analyzer

57 Conclusions Physical forcing can cause rapid flushing of the upper layers of permeable sediments Numerical modeling of porewater advection appears very feasible Temperature is an effective tracer for porewater advection and, hence, model validation Temperature-validated porewater modeling shows promise for calculation of porewater fluxes and reaction rates Come see our results at Ocean Sciences next Feb!

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