Ocean 520 Problem Set #1 Fall 2009 Due: 9:30 a.m., Friday, Oct. 16 1) Profiles of trace metals: clues to controls Two representative profiles of the infamous oceanographic element My (Mysterium) in the Atlantic and Pacific are shown below. What factors control the distribution of My in seawater? What might explain the difference between ocean basins? What are two plausible sources of My?2) Box Model Problem: What controls the Mg2+ composition of seawater? You might think it is well known, but there is some debate about what controls the magnesium concentration in seawater. The main input is rivers (see Power Point Lecture Notes 9). The main removal is by hydrothermal processes-- the concentration of Mg2+ in 350°C endmember hydrothermal solutions is zero (you can see the table in the major ions lecture (Lecture Notes 2) for more details). Here weʹll see how this balance works. a.) Draw a schematic diagram for a one-box model calculation for Mg2+ in seawater. Include a hypothetical mid-ocean ridge. b.) Calculate the residence time of water in the ocean, once relative to river input, and again relative to hydrothermal circulation. From notes use: Mass of ocean = 1.38 x 1021 kg; River discharge = 3.5 x 1016 kg y-1; Hydrothermal circulation ≈ 1 x 1014 kg y-1. c.) Calculate the residence time of Mg2+ in seawater relative to these two processes, individually. You will note that at present, using the numbers given above, the ocean hydrothermal sink is slightly larger than the river source. The fluxes are close however supporting the hypothesis that there are the main two fluxes in the overall mass balance. Either the ocean Mg concentration is not at steady state or else there is uncertainty in the fluxes. Most likely the hydrothermal circulation flux is the most suspect of being incorrect. d.) How would the Mg2+ concentration in seawater change and what would the steady state concentration be if the rate of hydrothermal circulation was to double?3) Lizard aquarium – air / water compositions You have been asked to create a new line of fish aquaria for pet lizards that includes an abiotic ocean atmosphere system sealed in a glass box with a volume of 2 m3 with 1 m3 of distilled water to be incubated at 20°C. You start out with a system that has 1 m3 distilled water that has had all atmosphere evacuated. There is an inlet through which you can add solid salts and flow meter through which you can add accurate volumes of gases. You want the model ocean to have the chemistry of the real ocean with Molar concentrations (moles liter-1) concentrations of rounded to the nearest 10 mmolal. Na+ 470 mM Mg2+ 50 mM Ca2+ 10 mM K+ 10 mM Cl- 540 mM SO42- 30 mM The density of pure water at this temperature is 0.993 g cm-3. Note that the H+ and OH- will balance when you are finished so the pH will be neutral. The model atmosphere should be one atmosphere with dry air mole fractions of : XN2 = 0.80 XO2 = 0.20 But the final air will include H2O as well as N2 and O2. The partial pressure of H2O in equilibrium with seawater at 20°C can be found in Fig 3.10 in Emerson and Hedges. Assume there are 22.414 liters gas per mole. You have the following salts in your lab and a balance: NaCl, KOH, MgSO4, CaCl2, Mg(OH)2, HCl (assume HCl is a solid) And the pure gases N2 and O2 a) What is the recipe and how much of each chemical (including water) (in grams) is required b) The pH ends up being about neutral (pH = 7). Explain why. c) Each gas (in volume) should be added to achieve the correct concentrations (or mol fractions)? Remember that some gas dissolves into the water at equilibrium. d) Present the solution concentrations in units of mol kg-1. e) What will the ionic strength of the solution