Ocean 520 Name:_________________ Chemical Oceanography 2 December 2009 Fall 2009 Points are in parentheses (show all your work) (use back if necessary) MID-TERM #2 1) Box Models of Nitrogen Cycling Nitrogen fixation has become the latest "hot topic". Lets assess its importance. Construct a two-box ocean model for nitrogen as nitrate. Include river inflow, atmospheric deposition of NO3, nitrogen fixation, upwelling, downwelling, denitrification in the deep box and a biological flux of organic N (B). The removal rate to the sediments (S) is expressed as S = f x B. Assume for today that denitrification is a sink process that removes nitrate by converting it to N2 gas that escapes back to the atmosphere. Nitrogen fixation is a source process whereby organisms ultimately convert atm N2 to nitrate. From the literature we find some information that we may (or may not) need: Global ocean denitrification = 175 TgN y-1 f = 0.01 River Input = 76 TgN y-1 Area = 3.61 x 108 km2 Atmospheric Deposition = 30 TgN y-1 Sediment Burial = 25 TgN y-1. Upwelling rate V = 300 cm y-1 a) Draw the Broecker style two-box model for this problem and label the fluxes (including transport terms (Vm) and biological fluxes (B) and the fluxes listed above.(5 pts). b) Write the mass balance equations for nitrate the surface and deep boxes. You can assume that the N buried in the sediments (fB) equals the river flux (VrCr). Derive an expression for the B flux from the surface box to deep box. Use the mass balance for nitrate in the deep box explain what controls the nitrate concentration. (10 pts)2) Ocean Carbonate System Imagine you have a sample of surface seawater in the lab. Indicate what will happen to the following properties when you perform the operations listed on the left by inserting an up arrow (↑), down arrow (↓), or horizontal line (if it remains unchanged). No explanation is necessary. (21 pts) Operation [CO2] + [H2CO3] [HCO3-] [CO32-] pH DIC Alk δ13C Bubble with N2 gas Respire OM to DIC Dissolve CaCO3(s) 3) Radioactive Decay A shortened version of the 238U decay chain is shown below. The activities of the two isotopes of U and Th in the deep sea are also given. Isotope dpm 100kg-1 238U 240 234Th 240 234U 280 230Th 0.15 a) U is a conservative element in seawater but the activities of 238U and 234U are different. Which isotope has the higher molar concentration (you don’t need to calculate the concentration) and explain why? (10 pts) b) Are 238U and 234Th in secular equilibrium in the deep ocean? Explain what conditions are required for secular equilibrium and what this means.(10 pts) c) Explain in a few short sentences and/or equations why the activities of 234Th and 230Th in the deep ocean are so different.(5 pts)4) CaCO3 versus organic carbon We know from ice core records that atmospheric CO2 varied from high values of about 280 ppm during interglacials to 180 ppm during glacial periods. The regular variation of PCO2 over many glacial cycles has been one of the major unexplained puzzels in marine geochemistry. During the last glacial period organic matter degradation took place deeper in the ocean causing the deep water to have a lower pH and be more corrosive to CaCO3. If everything else was the same as now, except that a lot more CaCO3 dissolution took place in ocean sediments over a period of about 10 kyr, a) How would the present day plot of DIC vs Alk (from Emerson and Hedges Plate 2) change? Draw the qualitative new trend and explain. (10 pts) b) How would the new Alk vs DIC, which after time would be mixed into the surface ocean, affect the PCO2 of the atmosphere. Support your answer with carbonate system justification. Is this in the right direction to explain the glacial/interglacial changes? (10 pts)5. Biological stoichoimetry Nitrate and oxygen have been measured as a time series in the subacrctic North Pacific at Station P (Whitney et al., 2007, Progress in Oceanography, 75, 179-199). The rates of change in integrated concentrations between 100m to 600m between 1994 and 2003 (see lines in the Figure) were +0.26 mol NO3 m-2 y-1 and -2.4 mol O2 m-2 y-1. a) What is the magnitude of the relative change in O2 to NO3 ? Is this consistent with the Redfield equation for the stoichiometry of biological respiration? Explain what the data suggest.(10 pts) b) Explain how these trends could be explained by changes in ocean new production or rate of thermocline ventilation. (10 pts) c) These changes could also result in a natural ocean acidification. What would you predict to be the rate of addition of CO2 to these waters from this change in respiration (5 pts). O2