The review I linked has a remarkably readable discussion of this. In a nutshell, the filling of the site rendered everything that has been tested liable to contamination by the fill.
Apparently, the plaster on the walls (which contain lime, which I believe is thought to have come from fires) was one of the main pieces of evidence presented by Schmidt. But plaster can absorb carbon from the fill. This is plausible given both age (at least thousands of years) and the fact that plaster only occurs on the surface in direct contact with the contaminating fill.
AFAICT, there as been no enclosed space from which samples have been dated. It all comes from areas that were directly exposed to fill, or the fill itself.
The author of the review also notes something that should be obvious to those outside the field, but for some reason doesn't get discussed much inside:
> Thus, in questioning the claim about Gobekli Tepe’s date (of PPNB, possibly earlier) one might think that the one who does the questioning must have extraordinary and abundant as well as “almost beyond reasonable doubt” convincing evidence to counter what the archeological establishment has claimed about Gobekli Tepe. In effect, it seems it is no longer asked that the agency who makes the extraordinary claims about Gobekli Tepe provides the extraordinary evidence. But instead, the burden of proof has shifted to those who tend to counter the claims. Be that as it may, the paper will proceed as if the burden of proof is on the counterclaim. The paper will be setting the most stringent of all arguments and criteria in an attempt to support the counter arguments, although it doesn’t have to do so.
As Sagan famously said "extraordinary claims require extraordinary evidence." But the evidence on the dating of this site seems pretty weak.
Apparently, the plaster on the walls (which contain lime, which I believe is thought to have come from fires) was one of the main pieces of evidence presented by Schmidt. But plaster can absorb carbon from the fill. This is plausible given both age (at least thousands of years) and the fact that plaster only occurs on the surface in direct contact with the contaminating fill.
AFAICT, there as been no enclosed space from which samples have been dated. It all comes from areas that were directly exposed to fill, or the fill itself.
The author of the review also notes something that should be obvious to those outside the field, but for some reason doesn't get discussed much inside:
> Thus, in questioning the claim about Gobekli Tepe’s date (of PPNB, possibly earlier) one might think that the one who does the questioning must have extraordinary and abundant as well as “almost beyond reasonable doubt” convincing evidence to counter what the archeological establishment has claimed about Gobekli Tepe. In effect, it seems it is no longer asked that the agency who makes the extraordinary claims about Gobekli Tepe provides the extraordinary evidence. But instead, the burden of proof has shifted to those who tend to counter the claims. Be that as it may, the paper will proceed as if the burden of proof is on the counterclaim. The paper will be setting the most stringent of all arguments and criteria in an attempt to support the counter arguments, although it doesn’t have to do so.
As Sagan famously said "extraordinary claims require extraordinary evidence." But the evidence on the dating of this site seems pretty weak.