Lecture 1: Determinants of Seawater Composition

Fall 2004 12.742 - Marine Chemistry Lecture 1 - Determinants of Seawater Composition Prof. Scott Doney We start off discussing sources and sinks for major ions. Today we focus on box models, residence times, and distributions. • Main points – Box models/residence times – Groupings of chemical elements (major vs. minor) – Spatial gradients (vertical / horizontal) – Resources: · Y. Nozaki et. al. “A fresh look at element distributions in the North Pacific”, EOS, May 1997. http://www.ajn.org/eos_elec/97025e.html · MBARI periodic table of the elements. http://www.mbari.org/chemsensor/pteo. htm • Box Model Figure 1. input internal cycling loss – Assume for the moment, a well mixed box Figure 2. Concentration = Ci Volume = V0 Inventory = Ci V0 1 d(Ci V0 ) = input − loss dt – Assume loss rates are proportional to the inventory - negative feedback d(Ci V0 ) = Ii − ki Ci V0 dt where ki has units of Units: 1 time : ki = 1 τi 1 mol 1 mol mol = · 3 · m3 · m3 · = m3 s s s m Can solve for a “steady state solution”: d(Ci V0 ) = 0 = Ii − ki Ci∗ V0 dt Ii 1 C ∗ V0 Ci∗ = τi = = i ki ki V0 Ii residence time bigger input → bigger Ci∗ bigger τi (smaller ki ) → bigger Ci∗ – Time evolving system: Ci0 is Ci at time zero. as t → ∞, Ci → Ci∗ Ci = (Ci0 − Ci∗ )e−ki t + Ci∗ Exponentially approach Ci∗ Ci * Ci time Figure 3. At t = τ have approached 63% of the way from Ci0 to Ci∗ , and at t = 3τ have approached 95%. · τi and ki functions of constituent (not reservoir) 2 vs. Ci − Ci∗ = e−ki t Ci0 − Ci∗ residence time >> residence time << time−scale of peturbation in input time−scale of peturbation in input Figure 4. · sets up lag in ocean response - characteristic time-scales · very different spatial distributions depending upon sources and sinks, time-scales/residence times • Sources and sinks sea salt wet/dry aerosol deposition gas exchange rain rivers biology scavenging sedimentation sediment resuspension pore water sediments mid−ocean ridge/ hydrothermal basalts sediment burial Figure 5. For most elements, rivers are the primary sources, while sedimentation/burial are the primary sinks. Estimating residence times: τ= amount in ocean amount in ocean = input Vriver × Criver 3 Complications and issues: – River inventories highly variable – Cyclic salts – Anthropogenic activities (positive bias), these bias pre-industrial estimates – Other sources τ= – Estimate using outputs instead amount in ocean ! inputs τ is O(102 ) − O(108 ) years τ ↔ chemical reactivity • Major ions (those with high quantities), with residence times of 0.5 − 200 × 106 years Na, Mg, Ca, K, Sr, Cl, SO24− , Br, F scale of ocean) τ & 1000 years (overturning circulation time- • Trace elements with long residence times Rb, Li, Mo, Cs, U 0.3 − 3 × 106 years Low quantities because there are low quantities in source rocks Solubilized by stable oxyanions • Trace elements with short residence times τ < 1000 yrs, 50 − 1000 yrs No longer “well-mixed” Fe, Ag, Hg, Th, Pa, Al, Pb, REE (Rare Earth Elements) input loss Figure 6. 4 Elements can also be categorized based on source and internal cycling pathways • Aeolian (dust) deposited and water-column scavenged elements dust input Al scavenging depth Figure 7. Atlantic versus Pacific - large differences in dust deposition • Biologically recycled elements Intermediate residence times/rapid uptake and recycling in the water column C, N, P (along with many trace elements) taken up by organism organisms dissolved OM colloidal OM remineralization suspended – Rapid biological uptake sinking particles – Almost complete remineralization – Only ∼ 1% sediment burial burial Figure 8. • Two components of biologically produced material – “Soft organic matter” - cells, carcasses, mucus, etc. C, N, P (O2 ) Simple respiration and photosynthesis: O2 + CH2 O ! CO2 + H2 O Elemental stoichiometry for marine plankton (Redfield ratios): 106 CO2 + 16 N O3− + HP O42− + 122 H2 O + 18 H + → C106 H263 O110 N16 P1 + 138 O2 Shallow remineralization 5 – “Hard organic matter” - shells, tests CaCO3 - forams, coccoliths, pteropods, corals Si - diatoms, radiolarians SrSO4 - Acantharia Deep remineralization “Nutrient-like” elements associated with both “soft” and “hard” biological material soft 1 km particle flux hard 5 km depth Figure 9. Concentration profile of “Nutrient-like” elements rivers removal two box model mixing surface remineralization depth paticle flux deep burial Figure 10. 6 frivers ≈ fburial net upward mixing flux ≈ particle flux fluxes into surface box = fluxes out of surface box VR [CR ] + Vmix [Cdeep ] = Vmix [Cshallow ] + Fsink fluxes into deep box = fluxes out of deep box Vmix [Cshallow ] + γFsink = Vmix [Cdeep ] Vmix ≈ 30 VR γ = fraction of sinking particle flux that remineralizes in deep box (1 − γ) = fraction that is buried Atlantic Pacific Figure 11. Deep waters form in North Atlantic and Southern Ocean. Oldest deep waters are in the North Pacific 7 Ocean Conveyor Belt Circulation: Elevated concentrations of “nutrient-like” elements are found in the deep North Pacific because of continued organic matter remineralization along the deep-water circulation path Depth North Atlantic Figure 12. North Pacific An example is N O3− , O2 , silicate from Broecker and Peng (1982). Start building more complex box models. 8