There is evidence that it is fossil fuel related. The concentrations of different isotopes of carbon are shifting. Fossil fuels don't have much Carbon 14 in them, since they haven't been exposed to the atmosphere for a long time.
AFAIK nobody is measuring and recording the isotope ratios of fossil fuels as they are extracted. Volcanic emissions, including hydrothermal vents in deep ocean, are also likely to be very low in C14. If anything coal would be most likely to contain C14, given a source of neutrons, since solid carbon is a good neutron moderator.
Oxygen isotope ratios in water are affected by temperature. What effect does temperature have on carbon isotope ratios in carbon dioxide?
In the recent past carbon dioxide concentration in the atmosphere has followed average temperature, with a delay of a few hundred years. Even if it can be demonstrated that human activities have had an effect on the isotope ratio, then that dosn't in itself show that these have made any difference to the concentration. There is much more carbon dioxide in the oceans than in atmosphere. More entering the atmosphere from other sources may simply equate to less entering the atmosphere from the oceans.
The fundermental problem is that we have no way of knowing what either concentration or isotope ratio of carbon dioxide would "naturally" be now. Moreover climate models predicated on atmospheric carbon dioxide concentration have failed to predicted anything. Thus "geo-engineering" involving attempts to manipulate the carbon dioxide concentration of the atmosphere is likely to be a complete waste of time. Even if something was done to actually reduce either carbon dioxide emitted by human activities or its concentration in the atmosphere. So far "green" methods have at best made no difference to human carbon dioxide emissions. About they only thing they appear to be good for is wasting money!