GEOL212 Due 11/19/18 Homework XI

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1 GEOL212 Due 11/19/18 Homework XI General instructions: Although you are allowed to discuss homework questions with your classmates, your work must be uniquely your own. Thus, please answer all questions in your own intelligible words. (When in doubt, use complete sentences.) If calculations are involved, show all work (so that I will have some basis for giving partial credit.) Be sure to use appropriate units and significant figures, where appropriate. Stable isotope notation: A major task of geochemists (regardless of where their samples come from) is finding ways to present data on concentrations of elements and their isotopes that make it easy to make comparisons. Previously we discussed one method chondrite normalization in which the concentration of an element in a given sample is expressed in comparison to its concentration in CI carbonaceous chondrites. A second geochemical convention involves expressing proportions of stable isotopes of a particular element. Fractionation: Recall that a given element may have several stable (i.e. non-radioactive) isotopes. Although globally, there are typical ratios of these isotopes, natural processes tend to sort or fractionate them in distinct ways. For example: Carbon has two natural stable isotopes, 12 C and 13 C. (Forget 14 C for now. It is radioactive, not stable.) On Earth, much carbon is tied up in the biosphere. It gets there by way of photosynthesis, however photosynthesizing organisms have a strong preference for 12 C. Thus, photosynthesis fractionates carbon, yielding organic carbon (i.e. carbon that has passed through the biosphere) that is depleted in 13 C. 1.) You are familiar with percent notation (%) which expresses parts per hundred. Per mil notation ( ) expresses parts per thousand in a similar way. Now, after analyzing a specimen with a mass-spectrometer, you determine that the concentration of 13 C is g for every 1 g of 12 C. Express this in parts per mil ( ). (Watch significant figures!) Rather than using absolute concentrations, it is customary to plot ratios of stable isotope concentrations with respect to a reference with known, unchanging relative concentrations. These tend to be arbitrary and based on convenience. For decades, the standard reference for carbon was PDB - Pee Dee Belemnite (a fossil critter with a robust CaCO3 skeleton recovered from the Pee Dee Formation of North America.) PDB has g of 13 C is for every 1 g of 12 C. Now we need a convenient way to express the ratio of 13 C/ 12 C we find in a sample with respect to this standard. If a sample has a 13 C/ 12 C ratio of Rs and the reference (like PDB)

2 has a 13 C/ 12 C ratio of Rr, the sample ratio could be expressed as: δ 13 C = (Rs /Rr -1) * 1000 This is called delta notation and the value δ 13 C would be pronounced "Del Carbon thirteen." Formulated differently: Three things to note: The ratio is expressed as parts per mil ( ) If the sample ratio is identical to the reference standard, then δ 13 C = 0 δ 13 C could be positive or negative. A sample with a positive value has more 13 C than the standard, a negative sample has less. 2.) Calculate δ 13 C for the sample discussed in Q 1 using the PDB standard. If your answer isn't negative, you have made a mistake. What does the fact that the result is negative mean about the amount of 13 C? Now for some applications:

$As the figure above indicates, these pathways fractionate carbon differently, yielding different values of δ 13 C. Hint: Pay attention to the negative signs on x-axis values! 3.$

3 From There are two distinct photosynthetic chemical pathways used by different plants. Most employ the C3 pathway, but some, including grasses, use the C4 pathway. As the figure above indicates, these pathways fractionate carbon differently, yielding different values of δ 13 C. Hint: Pay attention to the negative signs on x-axis values! 3.) Based on the figure above, which plants exclude 13 C more vigorously during photosynthesis, C3 or C4 plants? (Hint: Pay attention the sign of the x axis.) From 4.) The figure above tracks the concentration of 13 C in Pakistan over the last fifteen million years (oldest values on the left). During which interval is a major shift in in δ 13 C values taking place?

$Several isotope systems are useful to geochemists, including: 16 O, 17 O, and 18 O 32 S, 33 S, 34 S, 36 S 84 Sr, 86 Sr, 87 Sr, 88 Sr In contrast to carbon, which is fractionated by biological$

4 Given the information discussed in question 3, what change might have occurred in the dominant plant types during that interval? It s not just carbon. Several isotope systems are useful to geochemists, including: 16 O, 17 O, and 18 O 32 S, 33 S, 34 S, 36 S 84 Sr, 86 Sr, 87 Sr, 88 Sr In contrast to carbon, which is fractionated by biological effects, oxygen is fractionated by changes in physical state. When sea water evaporates (see below), water molecules containing 16 O, the lightest isotope, are more likely to enter the vapor phase. Conversely, in the vapor phase, molecules with heavier isotopes (especially 18 O, which is more common) are more likely to condense. 5.) Normally, because water circulates constantly between reservoirs of the hydrologic cycle, this is not important, but how would the δ 18 O of sea water change over time if evaporating water were prevented from flowing back into the ocean?

6.) The upper line of the graph below plots δ18o of ocean sediments over the last 500,000 years. (We care only about the y-axis scales on the left. Ignore the lower curve.

5 6.) The upper line of the graph below plots δ18o of ocean sediments over the last 500,000 years. (We care only about the y-axis scales on the left. Ignore the lower curve. In this graph, the oldest values are on the right and lowest values of δ18o are above!) From Yasuhara et al., Describe the general pattern of change in marine 18O concentrations over time. (Be sure to read the axes properly!) To what major global environmental changes might these variations be related? (Feel free to use references.) How would these produce the observed changes in δ18o?

6 The commonly used standard for δ 18 O is Standard Mean Ocean Water (SMOW). In SMOW, 16 O atoms make up 99.76% and 18 O make up %. (The remaining % would be the rare isotope 17 O.) An analyzed sample shows % of 16 O and % of 18 O. Calculate the sample s δ 18 O.

7 7.) Isotopes of strontium fractionate for yet again different reasons. Hydrothermal processes in the deep ocean (associated with sea-floor spreading) tend to dissolve 86 Sr, adding it to ocean water, whereas the erosion of continental rocks results in enriched 87 Sr. Thus, the ratio of 87 Sr / 86 Sr reflects the relative contribution of continental erosion and sea-floor spreading. Periods of rapid sea-floor spreading have more 86 Sr and therefore lower values of 87 Sr / 86 Sr. Periods of rapid continental erosion are the opposite. (Because Sr isotopes have similar concentrations, researchers typically just calculate the ratio directly and don t use δ notation.) The graph above shows this ratio in marine sediments for the last 135 my. During which geologic period would you conclude that sea-floor spreading was most active, the Neogene, Paleogene, or Cretaceous?

8 8.) By odd coincidence, the ratio of 87 Sr / 86 Sr has increased in an almost linear fashion over the last 50 my. This allows it to be used as a tool for geochronology. For example, suppose you know nothing about a specimen of marine sediments except that it has a 87 Sr / 86 Sr of and is definitely younger than 50 my. Roughly how old is it?

9 Fractionation lines: Suppose we have a pristine sample substance that is so pure that if we were to plot it in a graph with δ 18 O on one axis and δ 17 O on the other, all samples would plot in essentially the same place. Then we allow it to be subjected to some process that fractionates these stable isotopes. In nature, different parts of the sample will become fractionated to different degrees depending on how intensely they are exposed to the fractionating process. Nevertheless, the general trend of the fractionation will be the same. Same direction but different rates. When different samples of the substance are analyzed and plotted, their values will fall on a fractionation line. This is because the fractionating process will have been working on both isotopes at the same rate. Generally, when we see data points plotting along such lines, we can infer that the samples have a common origin but have been subjected to different degrees of fractionation. From Horn et al., ) In the graph above, values for several classes of samples (identified by different symbols) are plotted according to their δ 57 Fe and δ 56 Fe values. Which classes of samples show evidence of a common origin?

GY 112 Lecture Notes Stable Isotope Stratigraphy

GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Stable Isotope Stratigraphy Lecture Goals: A) Stable isotopes of use to geology (fractionation) B) Delta values and isotopic standards C) Delta