Chapter 14 Ocean Particle Fluxes Jim Murray(5/7/01) Univ. WashingtonThe flux of particulate material to the deep sea is dominated by large rapidly settlingparticles, especially:zooplankton fecal pelletsmarine snow aggregates14-1: Settling RateEstimated from Stokes Law of Settling.This equation was derived assuming smooth, spherical particlessinking rate = Us = α B r2 (cm sec-1)where:r is a linear dimension, the equivalent spherical radius of the particleα is a shape factor; ρp is the density of particles and ρw is the density of seawater.B is a parameter that depends on the nature of the fluid and the particulate material but noton size.B = 2 g (ρp - ρw) / 9η (cm-1 sec-1)For water (at 20°C) the density of seawater is ρw = 1.025 g cm-3 and viscosity η = 0.01 gcm-1 sec-1.Thus: B = 2.18 x 104 (ρp - ρw)There is always substantial uncertainty in assigning the equivalent spherical radius becausenatural particles are rarely spherical.14-2: Particle Density:The density of euphaussid fecal pellets was determined by Komar et al (1981) to be 1.23 gcm-3. This is much larger than typical seawater density of about 1.025 g cm-3. Pellets sinklike little rocks. It was originally thought that fecal pellets were the main flux material butnow it is felt that fecal pellets are relatively rare compared to marine snow (Pilskaln et al)Most recent work has suggested that the large marine snow particles are primarilyresponsible for the vertical transport of biogenic material (See Alldredge and Gotschalk,1988, Limnol. Oceanogr. 33, 339). These particles have a very small density differencefrom seawater, typically 1 x 10-2 to 1 x 10-5 g cm-3. The density difference decreases withincreasing particle size (Fig 14-1)Fig 14-1 ∆Density (ρp - ρw) versusparticle size for marine snowparticles (Aldredge and Gotschalk,1988).The most recent idea is that organic matter does not sink unless it is associated with mineralmatter that can result in a large enough density difference to create sinking particles. This isthe "ballast" hypothesis.14-3: Techniques for determining particle flux1. Sediment traps:Styles: Bottom moored in deep sea Drifting near sea surfaceDesign: Cylinders (e.g. PITS) Cone3 (e.g. IRS)There are lots of pros and cons about sediment traps in the community but they have playeda valuable role.2. Radiotracer techniques: especially using the deficiency of 234Th relative to itsradioactive parent 238U. This will be discussed later when we get to radiochemistry.3. Marine snow camera approach (Fig 14-2) (Asper et al )Fig 14-2: Sinking speeds ofaggregates in the water column of theGulf of Mexico (Asper et al ). Therange of size is 1-3 mm, ∆ρ ≈ 10-4 gcm-3 and the range of sinking speedsare 10 - 100 m d-1.14-4 Fluxes in the upper oceanThe Vertex Project (Martin et al (1987)Fig 14-3 Station locations in the NEPacific where the Vertex Projectdeployed surface teathered, driftingsediment traps. Deployments werefor days to weeks so the fluxesrepresent a shapshot in time.Fig 14-4 The flux ofparticulate organic carbon(in mol C m-2 y-1) along theVertex Transect. The dotsare the data points. The fluxdecreases with depth andwith distance from land.14-5 Flux Gradients and Respiration RatesThe composite profile of the upper ocean vertex data from locations Vertex 2, 4, 5, II, IIIand NPEC is shown in Fig. 14-5. The decrease in flux was interpreted as respiration andmodeled as:dΦorgC / dz = Rwhere respiration equales the gradient in the flux. This probably represents an upper limitas the model does not account for particle break up.Fig 14-5 Composite Vertex ParticleFluxes. The solid line represents thenormalized power function:F = F100 (z/100)bIn this case the equation is:F = 1.53 (z/100)-0.858with units of mol C m-2 y-1.They measured the C:H:N content of thesediment trap material and calculated O2utilization from the differences betweensequential levels according to:C + O2 → CO2 C : O2 = 1 : 1NH3 + 1.5 O2 → NO3- + H2O + H+N : O2 = 1 : 1.52H2 + O2 → 2 H2OH : O2 = 1 : 0.2The vertical pattern in respiration areshown in Fig 14-6. The line labeledas OOC is from Vertex, J82 is fromJenkins and Goldman (1985) JMR43, 465 and F & C 78 is fromFiadeiro and Craig (1978) JMR, 36,323. The integrated respiration ratefrom 100m to 1000m was:∫ R dz = 1.5 mol O2 m-2 y-1Fig 14-614-6 Deep Ocean FluxesThere have been numerous studies of deep ocean fluxes using bottom moored sedimenttraps. Here are some data from a set of Honjo et al (1982; Deep-Sea Research, 29, 609)traps. The locations are shown in Fig 14-7 and the mass flux in Fig 14-8. The organiccarbon flux is in Fig 14-9 and the CaCO3 flux is in Fig 14-10.Fig 14-7 Fig 14-8Fig 14-9 Fig 14-1014-7 Seasonal patterns in Deep Ocean FluxesDeuser (1986; Deep-Sea Res., 33, 225-246) maintained a time series of bottom mooredtraps in the Sargasso Sea near Bermuda at 3200m. The total flux collected from 1978 to1984 is shown in Fig 14-11 and the composite seasonal fluxes for total flux, organic carbonflux and CaCO3 flux (all in mg m-2 d-1) are shown in Fig 14-12. The spring bloom standsout clearly.Fig 14-11 Total Fluxes at3200m at Bermuda.Fig 14-12 CaCO3 and organic C are about 60% and 5% of the total,