by ggpab » Nov 24, 2006 2:17 pm
Hi,
True - iron and dirt are often argued to be linked to changed in speleothem colour. More specifically, there is more organic particulate matter in darker calcite. But nothing is ever so simple though.
You have probably heard that there can be annual banding in the calcite at a microscopic and sometimes if they grow real fast, at the "visible to the naked eye" resolution. The banding in speleothem calcite is in part due to seasonal and inter-annual (decadal, century, etc) changes in the hydrology, climate above the cave, etc etc. In addition to the particulate matter, some aspects of the changes in colour over time (the banding) is due to a real soup of complex organic acids leached out of the soil and organic matter above the cave as the water percolates down and this varies of course over the seasons in temperate environments.
The colour of speleothem calcite is therefore a really great subject, as you would expect the colour to be different depending on the hydrology (fast or slow dripping water), and climate (how much evaporation/evapotranspiration and so concentration of the organic molecules in the drip water) and of course, the type and density of vegetation which produces the organic matter in the first place. If you have significant change in the landscape/ecosystem, then these changes should be reflected in the drip water chemistry, but also the colour of the calcite formed by that drip! If you are looking at a broken stal and you can see a big change in colour or really noticeable rings, then you are probably looking at evidence of big changes.
You mention that different colour speleothems can be right beside each other in the same cave/chamber. I have seen this too. I am curious though if you have noticed which colours have water actively dripping on them, and if there is a distinction between colour / drip rate (fast-slow).
I am also curious if anyone out there knows of documented colour changes in the speleothem calcite before and after a specific landuse change - maybe when a parking lot is put on top of the cave, or the area has been clear cut logged.
Cheers
Trish Beddows.