Evidence of biomass burning in ice cores

At www.physorg.com/print210529446.html there is a press release on a paper from the December 2nd issue of Science and it seems that Antarctic ice cores have found some striking evidence of biomass burning in the southern hemisphere over the last 650 years – with a series of peaks and troughs. They go back to 1350AD and therefore embrace the period of the Black Death epidemic (see Mike Baillie, New Light on the Black Death). The press release does not describe the details, as uch, but claims landscape fires as well as cooking and communal fires, contributed to the signature. The amount of change between one century and another was apparently quite large – on the order of 60 to 70 per cent. What might be to blame?