Guide to I Chi Spreadsheets
This folder contains an electronic models for an example presented in this book.
The model is constructed in Excel workbooks with spreadsheet calculations to illustrate
isotope examples of Chapter 6. To orient yourself to what is supposed to happen in these
workbooks, you may want to find the corresponding section in the book, then read the
text about that example before actually opening a workbook.
When you open the workbook, you will find spreadsheets that generally have
master variables in the upper left sections. These master variables are highlighted in bold
and are there for you to manipulate. Type in a value and watch what happens in the
isotope graphs. Change those master values to get a feeling for linkages between mixing
and fractionation, the big themes of the book. One of the simplest and most informative
things to do in each spreadsheet is to set fractionations equal to zero, so that only mixing
controls dynamics. Change the gain terms, then the loss terms and observe the effects on
mixing. After understanding these mixing effects, then turn on fractionation again by
typing in a non-zero  fractionation value, and watch what happens. Try it! With gains
and losses involved in mixing fixed at constant values, you can evaluate fractionation.
And don’t forget to try various combinations of the individual mixing and fractionation
values you have been using – the combined effects are most complex, and most
interesting. In sum, try manipulating the master variables in the spreadsheets of this
Appendix – you may be surprised how much isotopes change, depending on how you set
up mixing and fractionation in these examples.
In some examples, you may find that changing master variables leads to odd and
nonsense results. Two particular problems deserve mention. First, when loss terms are
too great, pools or reservoirs start to have negative values and many odd features result in
the I Chi graphs. If this is happening, make the loss terms smaller, so that amounts in
pools always stay positive. Second, changing master variables can give data off the
charts, beyond the normal scales. To reset the scale of an x-axis or y-axis in a graph, use
your cursor to click on the axis. A box labeled “format axis” should appear. Click again
on the tab labeled “scale”, and reset the “maximum” value to a higher number, then click
on the “ok” button. The scale will reset, and now your data should appear on the chart.
Back-up versions of the workbooks are provided, so that if you change variables
and overwrite the original material, you have a spare copy to use for reference. Generally,
if you want to save your work, you can make your own copy of the workbook and save
your work therein.
Beyond simply typing in different values and saving your new scenarios, also you
can access the mathematics in these worksheets. You can look more closely at the math
underlying the spreadsheets by clicking on individual cells, examining the formulas used
at each location. Those formulas appear in the toolbar line just above the worksheet, and
if you use your cursor to click on the formula there, color highlighting appears so that
you can see spreadsheet values used in the equation.
The examples have similar math, the “I Chi” equations introduced in section 4.3.
Calculations are done sequentially in horizontal rows in the spreadsheet, with the results
in the final columns copied into the first columns of the next row. Each row represents
one time step with typically several processes happening across the columns in that time
step/row. This is the step-by-step philosophy of I Chi. There are detailed examples in
Section 4.4 about how this math is set up and actualized in the spreadsheets – consult that
section 4.4 to get a feel for how the calculations are set up in a sequential manner in rows
and columns.
Finally, you can write your own examples, and introduce your own I Chi
equations, amending the workbook models. The equations are actually simple algebra,
summarized in Section 4.3. Writing your own I Chi is not nearly as hard as it may look at
first glance, but then again, thinking with math is its own challenge.