Part IIl. DIC and carbon isotopes, 25 points During photosynthesis, DIC is conve
ID: 294311 • Letter: P
Question
Part IIl. DIC and carbon isotopes, 25 points During photosynthesis, DIC is converted into organic carbon. Photosynthesis fractionates carbon isotopes by preferentially allowing 12CO2 to diffuse across cell membranes, while 1002 accumulates in the unused DIC pool. Use this information and the diagram below to answer the next few questions. Figure 4. DIC, 81BC, and alkalinity. DIC and alkalinity (umol kg") 1800 2000 2200 2400 100 (A) 200 300 400 DIC eTc 0.0 0.5 1.0 1.5 2.0 1. What causes the 81SC of DIC to be so positive in the surface mixed layer? (5 points) Below 100 m subsurface DIC begins to increase while 8'3C of DIC decreases. Explain these trends. (5 points) 2. What effect does increased photosynthesis and respiration have upon total alkalinity (Ar)? Is this surprising to you? Why or why not? (5 points) 3. Based on your understanding of carbon isotope cycling during photosynthesis and respiration, explain why deep waters in the North Atlantic have higher 83C values compared to deep waters in the North Pacific. (10 points 4.Explanation / Answer
Answers:
1. As given in the question itself, that, during photosynthesis 13CO2 accumulates in the unused DIC pool, delta 13 C of DIC is very much positive in the surface mixed layer where salinity is very much high. But, as we go deeper, the number of 13C decreases in DIC, and so the value of delta 13 C also decreases.
2. 100 m subsurface layer is known as temperature minimum layer or Tmin layer. Below it, DIC increases due to uptake of CO2 by gas exchange between sea and atmosphere. During photosynthesis, organisms preferentially take up the lighter isotope of carbon (12C) increasing surface ocean delta 13C. When the 13C-depleted organic matter is decomposed, this CO2 release decreases the delta 13C of seawater DIC at depth.
3. Photosynthesis means consumption of CO2while respiration means release of CO2. CO2 is acidic and so, when it is used up pH rises and when it is released and added to a solution pH falls, because ratio of carbonate to bicarbonate ions depends on pH. So, consumption of CO2 increases alkalinity while release of CO2 lowers alkalinity.
4. North Atlantic ocean has higher abundance in benthic foraminifera. The delta 18 O values are higher in deep water of North Atlantic than in North Pacific. The higher delta 13 C values in deep water may be attributed to the presence of carbonates in organic rich matters (i.e. foraminifera) which removes significant proportions of DIC.