Mr. Chumpville basically has two points. One is that the 13C/12C measurement is a very small number. Well, he cites Peedee Belemnite and he also cites mass spectrometry. That's because, due to the importance of determining isotope ratios for a wide variety of geochemical pursuits, the scientists who do this are quite specialized and the instruments they use are very, very accurate. They need to be. In fact, using mass spectrometry, it's possible to use 14C to date materials back to about 50,000 years old -- when theoretically there's only about 3 atoms of 14C left.
So, yes, it's a small number. Isotope geochemistry specializes in making very precise measurements of very small numbers.
But... do we rely on one measurement of the Suess effect, by one lab, and for one material? No, of course not. Many different labs have done it, on many different materials, and their results show the same thing. The Suess effect, starting when industrialization got going, and increasing as more and more CO2 went into the atmosphere.
Should you take my word for it? No. Look at the graphs. And I've got a few. They are below.
Also, Mr. Chumpville would like to have measurements verifying that fossil fuels are depleted in 13C. OK, let's start with the little graph below, showing where fossil fuels should fall (I assume that the range is an amalgamation of measurements that have been made).
And then I offer this abstract. Is 114 petroleum samples sufficient?
"D/H and 13C/12C ratios were measured for 114 petroleum samples and for several samples of related organic matter. δD of crude oil ranges from −85 to −181‰, except for one distillate (−250‰) from the Kenai gas field; δ13C of crude oil ranges from −23.3 to −32.5‰,"
So I feel pretty confident in the 13C/12C ratio for fossil fuels shown in the first plot.
So, Mr. Chumpville, take a look at the graphs below. The Suess effect is real, and well-measured, and independently verified, and it is powerful supporting evidence for the addition of large amounts of fossil fuel CO2 to the atmosphere, which is the main/simplest/best explanation for the increasing trend of atmospheric CO2 concentrations.