Homework #4 – CO2 Concentrations in the Phanerozoic (last 543 million years)

Homework #4 – CO2 Concentrations in the Phanerozoic (last 543 million years)The goal of this exercise is to graph the estimates of past CO2 concentrations in the atmosphere over tectonic time scales (i.e. the last 500 million years). The Excel file that accompanies this homework assignment is a compilation of hundreds of CO2 estimates from many different publications using a variety of different proxy methods (Royer, 2006). Later you are going to compare the CO2 data to estimates of temperature over this time interval.Royer, D. L., 2006. CO2-forced climate thresholds during the Phanerozoic. Geochimica et Cosmochimica Acta 70: 5665–5675.Step 1: Create a graph of CO2 concentration vs. time.Take a moment to look at the data contained within the Excel file. The estimates are grouped by proxy method (e.g. from Paleosols (ancient soils), or from Phytoplankton, or from Stoma indices). Within each of these methods you will see a number of different publications, each with a few analyses. Column B and C are the ages of each of the samples analyzed in Ma (millions of years ago) and CO2 values (ppm is parts per million, respectively. For reference pre-industrial CO2 was 280 ppm and we are currently around 400 ppm. You will see that the headings get repeated throughout and there are blank spaces, both of which confuse Excel, so I have reformatted the data on the second page of the work book. Look on the bottom left and you will see a “For Plot” tab. I have eliminated the blank spaces and extra headings and also placed the values in chronological order (most recent on top; oldest on the bottom). Use this page to make the graph. Highlight all of the values and insert an X-Y scatter graph. This is important you cannot use a line graph for this data. I recommend using data points that are not connected by lines. Here are some key points for success:• Make sure the graph has a title and that your axes are labeled with the appropriate units.• Reverse the orientation of the time axis so that it runs in the appropriate direction.• Make sure that there is a citation on the graph (see above).• Once your graph is printed use a pencil to draw a line of best fit that passes as close as possible to as many data points as possible. This will help you ignore the outliers in the data and focus where most of the data lie. Ask if you are not sure about how to do this.Step 2: Compare the CO2 values to estimates of temperatureThere are many different ways to estimate past temperatures. We are going to look at three different ways of doing this and I would like you to compare these estimates to the CO2 values. There are six different questions for you to answer.A) Icehouse vs. Greenhouse conditionsIf there are glaciers, then the world is cold. If there are no glaciers, then it is warm. There have been three icehouse periods over the last 500 Ma: a brief event around 440 Ma, 330-260 Ma, and 32-0 Ma.Questions:1) How do each of these three icehouse events compare with CO2 levels at that time?2) Does this argue for or against the notion that CO2 concentration is one of the factors that control global temperature?B) Global sea levelGlaciers are created when water evaporates out of the ocean and then is stored on land as snow andice. If glaciers melt then sea level will rise. The amount of space that the ocean water occupies is alsodetermined by its temperature, if the water warms it will expand, and the surface of the ocean will riseup. Therefore, global sea level is another estimate of Earth’s temperature. There are other geologicalfactors at play (if the sea floor is pushed up then the water surface will rise with it), but it is a reasonablystraight forward proxy for temperature. The graph below has two estimates of global sea levelsspanning the last 500 million years.https://upload.wikimedia.org/wikipedia/commons/4/44/Phanerozoic_Sea_Level.pngQuestions:3) How do each of these sea level curves compare with CO2 levels over time? The red and the blueline are sort of the same but they are also different. Do you think that there is a differencebetween the two lines? Note that time is running in the opposite direction in this graph, sotake care.4) Does this argue for or against the notion that CO2 concentration is one of the factors thatcontrol global temperature?C) Chemical proxies for temperatureThe last point of comparison is to look at oxygen isotope data for this time interval. It is a tricky businessto estimate temperature from isotope values from so long ago because quite a bit can changed aboutboth oceans and organisms over such a long time. People who study these super ancient isotoperecords feel that some corrections are needed to account of all of these changes. I am mostly (but not100%) convinced that they are justified. If you want I can give you the paper where they explain the basis for the corrections, but it involves some serious chemistry.In the graph below, the uncorrected values are shown with the blue line. The corrected values are shown with either the red or the dashed black line (two different ways of doing the corrections). They are roughly the same, so let’s use the dashed black line as it is a little easier to see.From Royer et al., 2004. CO2 as a primary driver of Phanerozoic climate. GSA Today v. 14 n. 3 pp. 4-10.Questions:5) How well do the uncorrected d18O values correspond with CO2 levels? What about the corrected dashed black line?6) Does this argue for or against the notion that CO2 concentration is one of the factors that control global temperature?

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