I'd be curious to know how stable these isotope ratios are over long periods of time. For example I could imagine as a river changes course and erodes different

I'd be curious to know how stable these isotope ratios are over long periods of time. For example I could imagine as a river changes course and erodes different sediments, that would change the ratios for plants growing downstream. Likewise an increase in rainfall in an area might dilute the amount of groundwater it consumes. I'd also be curious about patterns of human consumption. In the modern day, sea food is the primary source of strontium in people's diets, with dairy and meat products also being large contributors. Agricultural trade could easily lead to people eating animals raised in a different region which might have an outsized effect on a specific dietary mineral.