ESE 148c Problem Set 1 Due April 12 in class or to Ashley by 5 PM 1 Isotopic Constraints on the Geologic Carbon Cycle a Typical average modern values of C for organic carbon and marine carbonates are 25 and 1 respectively It has also been estimated that 30 of the carbon buried in sediments is in organic form What is the average isotopic composition of carbon entering the surface carbon cycle through volcanic degassing b Marine sediments record an event about 55 Ma termed the Late Paleocene Thermal Maximum This event is characterized by rapid warming on a global scale but the trigger mechanism of such anomalous rapid warming remains unknown Carbonates from this same time period have anomalous 13C values as high as 2 Assuming no other changes in the carbon cycle ie Paleocene values for i and o are the same as modern what fraction of organic carbon burial do you predict for the Late Paleocene Now assume that Paleocene values for i and p the fractionation between a and o were the same as today and redo the calculation Which set of assumptions seems more reasonable c You should have just shown that the fraction of carbon buried as organic carbon is related to a change in global mean temperature Propose a mechanism to explain this hypothesized link 2 Carbonate Equilibrium Carbonate equilibrium is one of the most important chemical concepts in studying natural systems because essentially all natural waters and especially the oceans are buffered by the carbonate system You will find the following values for the carbonate equilibrium constants in seawater helpful Temperature C pK1 pK2 2 6 10 9 39 15 5 95 9 10 Concentrations are mol kg and Kw has units of mol kg atm pKH 1 42 a The global ocean has an average pH of 8 1 and an average temperature of 2 C The average bicarbonate HCO3 concentration is 2000 mol kg Considering all dissolved forms what mass of inorganic carbon is stored in the oceans the ocean has a mass of 1 4 x 1021 kg The pre industrial atmosphere had an average CO2 concentration of 280 ppm The mass of the atmosphere is 5 3 x 1018 kg What mass of carbon is stored in the atmosphere Which carbon reservoir is bigger b Consider the pre industrial atmosphere to be in equilibrium with the surface ocean which has an average temperature of 15 C and an average HCO3 concentration of 1600 mol kg What is the pH of the pre industrial surface ocean What is the pH of the modern surface ocean in equilibrium with today s atmosphere which has 380 ppm CO2 What about a potential future ocean in equilibrium with an atmosphere with 500 ppm CO2 c The concentration of carbonate ion CO32 is a critical environmental parameter for many marine organisms that build skeletons of calcium carbonate for example corals How much does the carbonate concentration decrease from the pre industrial 280ppm to the near future 500ppm atmospheric CO2 values 3 Studying the Hydrologic Cycle through Isotopes The small Mexican community of Popocatepetyl relies on a nearby lake for fishing A large manufacturing facility that is being planned has proposed to drill a deep well in the town to extract groundwater for industrial uses The locals are worried that this will ultimately divert groundwater from their lake and threaten that resource The critical question is the relative contributions of precipitation versus groundwater in supplying water to the lake The manufacturer has claimed that it will cost millions of pesos for a detailed hydrologic study to answer this question and shouldn t be necessary The townsfolk have asked you to give them some advice based on your knowledge of stable isotopes Answer the question quantitatively using the following data The water in the lake is completely replenished each year by precipitation and groundwater recharge The D of the lake water has a yearly average value of 70 Precipitation in the region is supplied by a nearly constant weather pattern that picks up water vapor over the Pacific Ocean mean annual temperature of 14 5 C and carries it eastward As these clouds pass over the town 25 of the vapor condenses into liquid water at a temperature of 10 C and falls on the lake as precipitation As the clouds continue to move over the nearby mountains 95 of the remaining vapor condenses out as snow at a temperature of 10 C This snow ultimately melts and provides the source of water to recharge regional groundwater Note that l v is temperature dependent At 15 C it is 1 08 at 10 C it is 1 095 and at 10 C it is 1 155 4 Oxygen in the Deep Ocean a The oxygen concentration of the deep ocean is initially set by equilibrium gas exchange occurring at high latitudes recall that this is where deep water is formed At equilibrium the high latitude surface ocean holds about 350 mol O2 kg seawater Oxygen is then consumed through deep heterotrophic consumption of organic matter raining from the surface this flux is roughly 0 6 mol C m2 yr Assume that deep heterotrophs utilize 90 of the organic matter raining into the deep ocean the remaining 10 gets buried in sediments What would you predict is the concentration of oxygen in the deep Atlantic Ocean In the deep Pacific Ocean b In the year 2020 the sudden collapse of the Greenland ice sheet introduces enough meltwater to the North Atlantic to completely stop thermohaline circulation Assuming that surface primary production continues as today how long would it take for the deep Atlantic to become anoxic You can assume that respiration continues at the same rate despite decreasing oxygen concentrations