UNIVERSITY OF EAST ANGLIA School of Environmental Sciences Main Series Undergraduate Examination 2012-2013 CHEMICAL OCEANOGRAPHY ENV-2A45 Time allowed: 2 hours. Answer THREE questions, at least ONE question from EACH section. Answer EACH question in a SEPARATE answer book. All questions have equal weighting. Provided: ENV Data Book Notes are not permitted in this examination. Do not turn over until you are told to do so by the Invigilator. ENV-2A45 Module Contact: Dr Alex Baker, ENV Copyright of the University of East Anglia Version 1
2 SECTION A 1. a) The unusual dissolved oxygen distribution of the Black Sea gives rise to changes in the redox speciation of a number of elements that are not commonly observed in other regions of the world s oceans. Give at least three examples of elements whose depth profiles are influenced in this way. You should illustrate your answer with sketch diagrams of the depth profiles of the species concerned, identify the processes that control their speciation and provide chemical equations where appropriate. [75%] b) Briefly describe the geographic and hydrological conditions responsible for the depth distribution of dissolved oxygen in the Black Sea. [25%] 2. a) Geo-engineering options being considered for mitigating climate change on Earth are divided into two broad categories. What are these two categories? Discuss briefly the primary advantages and disadvantages of each category. [20%] b) Discuss in detail four ocean geo-engineering options that have been proposed by scientists to mitigate climate change. Include in your discussion known advantages and disadvantages of each option and how effective they may be, citing the published literature where relevant. [80%] 3. a) Describe the processes influencing the spatial and temporal distribution of dissolved oxygen in the global ocean. Illustrate your answer with diagrams and chemical equations, and include definitions for the terms oxygen saturation, apparent oxygen utilisation, oxygen utilisation rate, water mass age, thermocline, oxygen minimum zone, hypoxic and suboxic. [70%] b) Describe how the manipulation of oxygen concentration during wastewater treatment can i) enhance the removal of nitrogen and phosphorus and ii) produce energy. [30%] 4. a) Describe the factors that regulate primary production in the ocean, and how these vary with latitude. [50%] b) Describe using diagrams the difference between new and recycled production. [30%] c) Briefly describe the influence of phytoplankton community structure on the sinking flux of organic carbon. [20%]
3 SECTION B 5. Analysis of aerosol samples from three marine locations (the North Sea, Barbados and the Falkland Islands) yielded the following mean concentrations for aluminium and copper: Al (ng m -3 ) Cu (ng m -3 ) North Sea 150 8.7 Barbados 520 0.7 Falkland Islands 27 0.08 The abundances in upper crustal rock of Al and Cu are 80,000 µg/g and 28 µg/g respectively. a) Calculate enrichment factors for Cu with respect to upper crustal rock for all three sites. Discuss the potential sources of atmospheric copper at each site, based on the evidence provided by these enrichment factors and the Al and Cu concentration data. [40%] b) The phosphorus (P) content of Saharan dust is 880 µg/g. Assuming that all Al at Barbados is derived from Saharan dust, use the information above to calculate the average aerosol P content at Barbados, and the daily deposition flux of P to the ocean around the island. The dry deposition velocity for Saharan dust is 1.2 cm/s. [30%] c) Describe the potential impact of dust deposition on nitrogen fixing organisms, such as Trichodesmium, in the oligotrophic North Atlantic gyre. [30%] PLEASE TURN OVER
4 6. a) Draw a rough sketch of a Bjerrum plot for the ocean carbonate system (CO 2(aq), HCO 3 - and CO 3 2- ), showing relative mole fraction versus ph. [15%] b) What is the approximate present-day average global ocean ph, and at this ph, what are the approximate relative distributions, in percent, of the three carbonate species shown on your plot? [10%] c) The oceans absorb large amounts of CO 2 from the atmosphere, becoming more acidic in the process. Consider a hypothetical scenario where the average ph of the global ocean were to change from a value of 7.5 to 7.3. Using the information and data given below, calculate the relative distribution of the three carbonate species in percent at each ph. Now calculate the change in the total global ocean DIC content for this ph change. DIC is Dissolved Inorganic Carbon and is equivalent to the sum of CO 2(aq), HCO 3 - and CO 3 2-. [75%] Useful information: The ocean carbonate chemistry system can be represented by: CO 2(gas) CO 2(aq) (K 0 ). That is, [CO 2(aq) ] = K 0 [CO 2(gas) ] CO 2(aq) + H 2 O H + - + HCO 3 (K 1 ) - HCO 3 H + 2- + CO 3 (K 2 ) For 1 unit of [CO 2(aq) ], the ratio of the carbonate species can be represented by: [CO 2(aq) ] : [HCO 3 - ] : [CO 3 2- ] 1 : K 1 / [H + ] : K 1 K 2 / [H + ] 2 Typical values (at 20 C) of K 0 = 10-1.6 mol kg -1 atm -1, K 1 = 10-5.9 mol kg -1, K 2 = 10-9.0 mol kg -1. ph = -log 10 [H + ] Take the atmospheric CO 2 concentration as 390 ppm (equivalent to 390 μatm). 7. Define Ω calcite and describe the biological and physical factors which cause its variability with depth in the North Atlantic. Why does the depth where Ω=1 differ between the North Atlantic and the North Pacific? [40%] b) Describe how the Ion Activity Product (IAP) of calcite in seawater is determined. State the analytical measurements required and show how these measurements are used to derive an equation for IAP. [30%] c) Calculate Ω calcite for North Atlantic seawater with the following properties: ph = 8.0, temperature = 25ºC, carbonate alkalinity = 2.0 x 10-3 equivalents kg -1, [Ca 2+ ] = 0.01 mol kg -1. Assume the thermodynamic second dissociation constant of carbonic acid is 10-10 mol 2 kg -2, and the activity coefficients of Ca 2+, CO 3 2- and HCO 3 - are 0.20, 0.02 and 0.50 respectively. [30%]
8. The table below gives surface water nutrient data measured in the North Atlantic Ocean and the Mediterranean Sea. An additional water sample collected from 3000 m in the North Pacific has a nitrate concentration of 35 mmol m -3 and a water mass age of 2100 years. North Atlantic Ocean Mediterranean Sea Nitrate (mmol m -3 ) 15 22 Phosphate (mmol m -3 ) 1.07 1.25 5 a) Assuming a Redfield molar C:N:P:O 2 ratio of 106:16:1:-138, calculate the average respiration rate of the North Pacific deep water in µmol O 2 m -3 d -1. [50%] b) Define the term N* and describe how it can be used to indicate the occurrence of N 2 fixation and denitrification. Using the data above, calculate N* to determine which nutrient limits production at the North Atlantic and Mediterranean sampling sites. [50%] END OF PAPER