Introduction

The first applications of Terrestrial Cosmogenic Nuclide (TCN) geochronology appeared about 20 years ago (Kurz, 1986; Nishiizumi et al., 1986; Phillips et al., 1986). The method has rapidly developed since those early days, truely revolutionizing geomorphology and related fields in the process. TCN dating is no longer a specialized tool used by a small group of experienced users, but has found an ever growing base of users who are not necessarily familiar with all the details of the method. Today we are facing a paradoxal situation. On the one hand, a better understanding of cosmogenic nuclide production systematics has improved the accuracy of TCN dating. But on the other hand, many users of the method may be less familiar with its intricacies than was the case in the pioneering days. An important example of this situation is that of the production rate scaling factors. In a landmark paper, Lal (1991) presented a method to calculate cosmogenic nuclide production rates as a function of latitude and elevation. Lal's scaling factors are elegant and easy to use, but overestimate the importance of muons and are only valid for standard atmosphere. Later authors introduced several improvements, incorporating atmospheric effects and improved muon production systematics. The scaling factors of Stone (2000), Dunai (2000), and Desilets et al. (2003, 2006) more accurately represent the scaling of cosmogenic nuclide production rates with latitude and elevation, but the increased sophistication of these methods is an obstacle to their widespread use.

CosmoCalc is an add-in to MS-Excel developed with the intention to alleviate this problem. The CosmoCalc interface was designed to be as user friendly and easy-to-use as possible. Default parameters are set so that beginning users only have to make a minimal number of decisions. At the same time, all the default parameters can be changed so that CosmoCalc is highly customizable and also experienced users should find it useful. The program as well as a spreadsheet with test data can be downloaded from the CosmoCalc website (http://cosmocalc.googlepages.com), which also provides detailed installation instructions. The add-in requires MS-Excel 2000 or higher. Because of small differences between the MS-Windows and Apple OS-X implementations of Excel, two versions of CosmoCalc are provided. The functionality of both programs is the same, but the Macintosh version is significantly slower than the PC-version.

After installing the CosmoCalc add-in, a toolbar menu appears (Figure 1) that guides the user through the data reduction and closely follows the outline of this paper:

• Production rate scaling factors (Section 2)
  • Lal (1991)
  • Stone (2000)
  • Dunai (2000)
  • Desilets and Zreda (2003)
  • Desilets et al. (2006)
• Shielding corrections (Section 3)
  • Topographic shielding
  • Self shielding
  • Snow shielding
• Banana plots (Section 4)
  • $^{26}$Al-$^{10}$Be
  • $^{21}$Ne-$^{10}$Be
• Age/erosion rate calculations (Section 5)
  • Single nuclide: exposure age and erosion rate calculations for $^{26}$Al, $^{10}$Be, $^{21}$Ne, $^{3}$He, $^{36}$Cl and $^{14}$C
  • Two nuclides: simultaneous calculation of (burial or exposure) age and erosion rate.
• Converters (Section 6)
  • Convert elevation to atmospheric pressure or -depth and back, under standard and Antarctic atmosphere.
  • Convert geomagnetic latitude to -inclination or cutoff rigidity and back.
• Settings: customizing CosmoCalc (Section 7)
  • Specify TCN production rate calibration sites
  • Specify the relative importance of various production pathways

The following sections will provide more details about these calculations. Thus, the present paper serves as an abridged review of TCN calculations, with an emphasis on the numerical methods that are needed to solve the equations. More details about the physics of TCN production are given in the review article of Gosse and Philips (2001). CosmoCalc is not the first computational tool for TCN calculations. Useful alternatives are CHLOE, an Excel spreadsheet for cosmogenic $^{36}$Cl calculations (Phillips and Plummer, 1996) and the CRONUS-Earth web-calculator (Balco and Stone, 2007; http://hess.ess.washington.edu/math). CosmoCalc was developed independently from these other tools, except for its topographic shielding correction function, which was translated into VBA from the Matlab code of Balco and Stone (2007). The reader is strongly encouraged to try these other programs. CosmoCalc is optimized for $^{26}$Al, $^{10}$Be, $^{21}$Ne and $^{3}$He dating. Because geomagnetic field fluctuations and thermal neutron reactions are ignored, results for $^{36}$Cl and $^{14}$C may be inaccurate. CosmoCalc can be used as an exploratory tool for these nuclides, but for more accurate results, CHLOE or the spreadsheet of Lifton et al. (2005) are recommended.

Figure 1: CosmoCalc's main menu guides the user through the data reduction and follows the outline of this paper.