This update affects the default method (that is, the method selected with the 'std' flag) for relating sample elevation to average atmospheric pressure. The previous method relied on the standard atmosphere equation coupled with a global mean sea surface pressure grid as a means of taking account of geographic variation in air pressure. The new method uses a spatially variable 1000 mg temperature field derived from the NCEP reanalyses as well. This removes a systematic bias that tended to overestimate atmospheric pressure and underestimate production rates at high elevations in the tropics. The elevation/pressure relationship selected by the 'ant' flag is not affected. There is no change to any of the methods of scaling production rates as a function of atmospheric pressure.

The effect of this change varies depending on which pressure-elevation flag the user inputs. If the user submits atmospheric pressures (the 'pre' flag combined with pressure in hPa) or submits elevations and requests the Antarctic elevation/pressure relationship (the 'ant' flag with elevations in m), the new method will yield similar results to the old method. There will be a small change because the new air pressure scheme affects the mean atmospheric pressures assumed for the production rate calibration sites, which results in a small change in the nominal values for the reference production rates. As the calibration data are not in areas that are very sensitive to the difference between the two air pressure schemes, this effect is small, less than 1%. If the user submits site elevations with the 'std' flag, the results may change by several percent relative to the old method. This will be most pronounced at high elevation, low latitude sites. For exposure dating at a site which proves to be particularly sensitive to this upgrade, it is probably a good idea to search for local meteorological measurements to get a better idea of the actual mean atmospheric pressure.

As of this week, the standard to which Be-10 measurements at LLNL-CAMS are normalized will change. The new standards are called '07KNXXXX' where XXXX is a number indicating the isotope ratio of the standard. Be-10 results reported against these new standards are not compatible with the production rate calibration data set used in the calculators at present. Thus, new Be-10 results reported against these standards must be renormalized to the old standards before submission to the calculators. This can be accomplished by multiplying Be-10/Be-9 ratios reported relative to the new standard by a factor of 1.106. If you do not renormalize these new results to the old standards, exposure ages and erosion rates generated using the calculator will be incorrect.

For further information, contact Bob Finkel at LLNL or consult:

Nishiizumi K., Imamura M., Caffee M.W., Southon J.R., Finkel, R.C., McAnich, J., 2007. Absolute calibration of Be-10 AMS standards. Nuclear Instruments and Methods in Physics Research B., vol. 258, pp. 403-413.

This is an unsatisfactory, and temporary, fix. First, the new standardization implies a different half-life for Be-10 than the one used in the calculators at present. This has a negligible effect on exposure ages for relatively young samples, but is potentially important for long exposure periods. Second, other AMS facilities are likely to change over to the new standardization in the near future, so it is likely that we will eventually renormalize the production rate calibrations to the new standardization. Also, in an effort to avoid confusion to the extent possible, in future we will require the name of the AMS standard used as an input to the calculators, and carry out renormalization between standards internally. To summarize, it's now become rather important to keep track of the AMS standard used for your measurements.

Updated to Version 2. Version 2 includes multiple scaling schemes and minor changes to air pressure-elevation conversions. See the documentation for details.

Minor changes made to the constants file and the functions get_al_be_age and get_al_be_erosion. First, updated the reference production rates to correspond exactly with the calibration data spreadsheet. Second, changed the values for the Be-10 and Al-26 decay constants to make them consistent with the Nishiizumi Be-10 and Al-26 standards to which we believe all the calibration data are normalized. These changes should not result in significant changes to exposure age calculations in most cases.

The constants file and the get_ functions are now version 1.2.

With some browsers, you may be able to cut and paste directly from the browser window containing the output data into Excel. If this doesn't work, you can save the web page containing the output data as an HTML document, then open it in Excel.

We log the minimum amount of data necessary to get a good picture of the amount of use the calculators get. The main fate of this information is the CRONUS project's annual report to NSF, our funding agency, who would like to know how useful a service we're actually providing. For every exposure age and erosion rate calculation, we keep a record of the date and time, the IP address of the user (so we can get an idea of how many different users there are, how many are from .edu/.com addresses, etc.), and the location of each sample (so we can distinguish between different sample sites being submitted and the same site being submitted multiple times). You can view the log record here. The plots generated on the result pages are deleted after 1 week.