Final answer:
The low proportions of 13C and 14C within atmospheric CO₂ indeed suggest the source is fossil carbon, which is true, as fossil fuels have low levels of these isotopes due to the decay of 14C over millions of years.
The statement that low proportions of 13C and 14C within atmospheric CO₂ indicate the source is fossil carbon is true.
Step-by-step explanation:
Fossil fuels, when burned, release carbon dioxide (CO₂) into the atmosphere. Because fossil fuels have been buried for millions of years, the 14C they once contained has decayed away, meaning that the CO₂ released is richer in 12C and has much lower proportions of 13C and 14C. The increasing accumulation of 12 CO₂, largely from the combustion of these fossil fuels, leads to a decrease in the 14C:12C ratio historically present in the atmosphere. This change in isotope ratios is a clear indicator of the anthropogenic or man-made contribution to atmospheric CO₂ levels, aligning with the well-documented overlap of the Keeling Curve and the rise in carbon emissions from fossil fuel use.