USING MARBLE TOMBSTONES TO MEASURE WEATHERING RATES: NEW DATA AND NEW CONSTRAINTS

Vertical marble slab tombstones offer advantages for measuring historical weathering rates. There is ample material available, the date of exposure is easily determined, and identical or similar lithologies appear in diverse settings for comparison. The most readily measured property is surface recession rate, as defined by the difference in thickness between the top (relatively weathered) and base (unweathered) of the tombstone. This method has provided figures that are widely cited for weathering rates of carbonate rocks.

Previous use of this method makes several assumptions, all of which may be wrong in some circumstances: 1. Tombstones thicknesses are originally identical top and bottom. Measurements of recently emplaced tombstones show that there can be significant variation, which must be accounted for statistically. 2. The base is unweathered (to provide an estimate of the original thickness). Climate, especially mean annual precipitation, appears to be important, so that tombstones in arid and semiarid environments are more likely to have unweathered bases. The height of the tombstone and the design and placement of the base may also be factors. 3. Surface recession is due primarily to in-place dissolution of crystals. Loss of whole crystals also occurs. The measurement of thickness loss is therefore a minimum because calipers will not account for missing material below the crystals that protrude from the surface. 4. Weathering is even across the top. This assumption is not verified, but three measurements are usually sufficient to provide a mean. 5. There is measurable thickness loss. For this study, in Montana and southwestern Canada, marble tombstones are available for comparison over a wide area covering about 110+ years. Based on over 1000 measurements, the total mean surface recession rates were immeasurably small at a few rural sites in dry climates to a maximum of around 5 mm in wetter urban locations. 6. Weathering over the measured period of time is dominated by loss of thickness of the marble slab. In fact, expansion of some exposed surfaces is observed, accompanied by loosening of carbonate crystals, hydration of minerals, and growth of new minerals.

The new data reported here were collected and interpreted based on the refinement of constraints described above.