Geol 330_634 Problem Set #2 Autumn 2016 Due: 10:30 a.m., Tuesday, September 20 1. Why is seawater salty? (5) 2. Weathering reactions are the main source for most ions in river water. True/False a) Silicate minerals weather to make clay minerals (2) b) Calcite weathering produces two HCO3- for each CO2 consumed (2) c) In both seawater and river water Ca2+ is the main cation and Cl- is the main anion (2). d) The composition of rivers differs between continents because of the variable proportions of carbonate and silicate rocks present on the different continents. (2) 3. Weathering reactions in the Sarmiento and Gruber Box Model. In the Global Carbon Cycle shown in Figure 1 of Sarmiento and Gruber, 0.2 PcC/yr are consumed from the atmosphere during weathering while 0.2 PgC/yr come from the rocks weathered. These fluxes should suggest to you the kind of rocks that are being weathered in this Global Carbon Cycle model. What are they? Write the reaction. (5)

4. What are the two main ways that the composition of river water is different from seawater? (5) 5. Can you make seawater by evaporation of river water? Explain your answer. (5) 6. Why is the Urey Reaction (CaCO3 + SiO2 = CaSiO3 + CO2) required to make the global carbon cycle balance? (5) 7. The Redfield equation can be written with either nitrate (NO3) or ammonia (NH3) as the nitrogen source. Write the reactions and explain why the ΔO2 / ΔCO2 ratios are different for these two versions. (5) 8. There are major areas of low oxygen at mid-depths in the world’s oceans. These are called oxygen minimum zones. You may assume that when seawater was at the sea surface at high latitude with 0° C and S = 35 it initially had an oxygen content of 350 µM. If all this oxygen was consumed in the oxygen minimum zone, what concentration of NO3 would you predict the water to have. (5)

9. Composition of River Water Britta Voss (http://www.whoi.edu/hpb/Site.do?id=11933) has recently completed a comprehensive study of the dissolved inorganic composition of the Fraser River, Canada (Voss et al., 2014, GetCA, 124, 283-308). Britta took this course when she was a student at UW in Winter 2008. Here is a summary of the discharge-weighted average concentration of major ions (mmol L-1). The global average river concentrations (from Lecture 4) are included for comparison. Ion Cl- Na+ Mg2+ SO42- K + Ca2+ HCO3- SiO2 a)



Fraser Concentration 29 (mmol L-1) 110 123 69 19 365 851 81

Global Average 220 (mmol L-1) 274 171 117 59 375 958 218

Assume that there are no evaporate rocks in the Fraser River drainage basin. Assume all the Cl- comes from marine aerosols. Calculate the seawater aerosol contributions to all the major ions in the Fraser River. (5) b) Calculate the aerosol corrected concentrations for all ions and % of total that come from weathering. (5) c) Calculate the % of how much of the HCO3- comes from silicate versus carbonate rocks. There are different ways you could do this calculation. Do the one you think best and state your assumption(s). (5) d) How does the % HCO3- from weathering silicate rocks for the Fraser River compare with that from the global average rivers? (5)





10. 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 6). The main removal is by hydrothermal processes-- the concentration of Mg2+ in 350°C end member hydrothermal solutions is zero (you can see the table in the major ions lecture (Lecture Notes 3 and 6) for more details). Here we'll see how this balance between sources and sinks as a control on the composition of seawater works. a.) Draw a schematic diagram for a one-box model calculation for Mg2+ in seawater which shows the source (rivers) and sink to a hypothetical mid-ocean ridge. (5) 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. (5) c.) Assume the ocean Mg cycle is at steady state, rivers are the only input and hydrothermal removal of Mg is the only sink. How long will it take for all the water in the ocean to pass through the mid-ocean ridge vent system? Is this an upper or lower limit? (5) 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? (5)