Ocean 520 - Chemical Oceanography Fall 2009 Problem Set #1 KEY 1. 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? (15 points) What factors control the distribution of My: (5 points) The profile of My is controlled by its sources and sinks. The surface enrichment indicates that there is an atmospheric source of My. It would also be possible to have a riverine source of My if these profiles were taken near shore. The concentration of My decreases with depth indicating that My is likely being scavenged by particles. Basin differences: (5 points) My has higher concentrations in the surface Atlantic because there is greater dust deposition in the Atlantic compared to the Pacific. At depth, My has lower concentrations in the Pacific than in the Atlantic, indicating that it is scavenged by particles along its flow path from the Atlantic to the Pacific. Concentrations are lower in the Pacific because this water is older and has had been exposed to continual particle scavenging along its flow path. My has a mid-depth maximum in the Atlantic, suggesting a possible hydrothermal vent or redox zone (oxygen minimum zone) source there. Two sources for My: (2.5 points for each of the two responses) • Surface enrichment from atmospheric deposition. • Riverine source of My • Mid-depth maximum from hydrothermal sources. • Mid-depth maximum from redox reactions in an oxygen minimum zone.2. Box Model Problem: What controls the Mg2+ concentration of seawater? You might think it is fundamental, but there is some debate about what controls the magnesium concentration in seawaters. The main input is rivers (see Power Point Lecture Notes 2 and 4). The main removal is by hydrothermal processes - the concentration of Mg+2 in 350°C end member hydrothermal solutions is zero (you can see the table in the major ions lecture for more details). Here we’ll see how this balance works. a) Draw a schematic diagram for this box model calculation. (2pts) Source: Rivers Inventory: Ocean Sink: H.V. Systems [Mg+2]R * 3.5 x 1016 kg water yr-1 [Mg+2]O * 1.38 x 1021 kg water [Mg+2]O * 1 x 1014 kg water yr-1 = mmol Mg+2 / yr = mmol Mg+2 = mmol Mg+2 / yr [Mg+2]R = 0.128 mmol/kg [Mg+2]O = 52.8 mmol/kg[Mg+2]HV = 0 mmol/kg [Mg+2]O[Mg+2]Rb) Calculate the residence time of water in the ocean, twice: once relative to river input, and again relative to hydrothermal circulation. (4pts) Mass of ocean = 1.38 x 1021 kg River discharge = 3.5 x 1016 kg yr-1 Hydrothermal circulation ~ 1 x 1014 kg yr-1 Residence time: assume steady state where sources = sinks. τ = inventory / sink or source rate River τR = 1.38 x 1021 kg = 3.94 x 104 yr = 39.4 thousand years 3.5 x 1016 kg yr-1 If the ocean was dry and the rivers were turned on it would take 39,000 years to fill it up. If the residence time of ocean circulation is 1000 years, a water molecule will circulate 39 times before it is lost to the atmosphere and transported to a continent and then to a river. Hydrothermal Vent Systems τHV = 1.38 x 1021 kg = 1.38 x 107 yr = 13.8 million years 1 x 1014 kg yr-1 A water molecule will circulate through the ocean 350 times before it makes a trip through a mid-ocean ridge. But those trips can be dramatic. In the case of Mg total removal occurs during that trip. c) Calculate the residence time of Mg+2 in seawater relative to these two processes, individually. (10pts) Ocean Inventory of Mg+2: [Mg+2]O * Mass Ocean = total amount of Mg+2 in ocean (52.8 mmol Mg+2/kg) * (1.38 x 1021 kg) = 7.28 x 1022 mmol Mg+2 River Source Flux of Mg+2: [Mg+2]R * River Flow Rate = amount of Mg+2 coming out of rivers / yr (0.128 mmol Mg+2/kg) * (3.5 x 1016 kg yr-1) = 4.48 x 1015 mmol Mg+2 /yr Hydrothermal Vent System Removal Flux of Mg+2: [Mg+2] O * Hydrothermal System Flow Rate = amount of Mg+2 removed by H.V. systems / yr Recall that the water going into a vent has the [Mg+2] concentration of seawater and that the water exiting the hydrothermal system has a [Mg+2] concentration of zero. Therefore the vent is removing all of the magnesium from the seawater. (52.8 mmol Mg+2/kg) * (1.38 x 1021 kg) = 5.28 x 1015 mmol Mg+2 /yr Note: As an example, let’s consider another species such as My. Let’s say that the water coming out of the hydrothermal vent end member has a concentration of [My] = 20 mmol My / kg. Now how would you determine the hydrothermal vent removal rate of My? Removal Rate: First we need to find the difference between the concentration of My in the ocean and in the H.V. end member. This value is the amount of My that has been removed from the seawater by the hydrothermal vent system. We then multiply this “missing” concentration by the rate of flow through the vent to get the removal rate of My. (H.V. flow rate) * ([My]O – [My]HV) = mmol My / yr When considering Mg the [Mg+2]HV = 0 so we only use [Mg+2]O in our calculation.Residence times of Mg+2 River τR = 7.28 x 1022 mmol Mg+2 = 1.62 x 107 yr = 16.2 million years 4.48 x 1015 mmol Mg+2 /yr Hydrothermal Vent System τHV = 7.28 x 1022 mmol Mg+2 = 1.37 x 107 yr = 13.7 million years 5.28 x 1015 mmol Mg+2 /yr These contrasting residence times indicate that the ocean is not in steady state with respect to magnesium because the rate of magnesium input to the ocean via rivers is slower than the rate of magnesium removal by hydrothermal vent systems. d) How would the concentration of Mg+2 in seawater change, and what would the steady state concentration be, if the rate of hydrothermal circulation were doubled? (4pts) If hydrothermal circulation is doubled then the sink term for Mg+2 must also be doubled since the rate of removal is equal to the concentration of magnesium in seawater times the rate of hydrothermal flow. Sink Term: [Mg+2] O * Hydrothermal System Flow Rate = amount of Mg+2 removed by H.V. systems / yr Since the source of magnesium to the ocean (rivers) has not been altered but the sink term has been doubled the concentration of magnesium in the ocean will decrease. As [Mg+2] O decreases the sink term will become smaller (see equation above) and at some point a new steady state