The t-motivic mixed Carlitz zeta category and Carlitz-Thakur multi-zeta values

We construct the t-motivic mixed Carlitz zeta category over ${\text{F}}_q(t)$ and show that it contains all the (mixed) t-motives with Carlitz-Thakur multi-zeta values as periods constructed by Anderson and Thakur. Our construction is canonical and our category is Tannakian and neutral and every object is equipped with a weight filtration whose graded pieces are Carlitz motives over ${\text{F}}_q(t)$. For any finite separable extension L/\F_q(t) we show that existence of a similar category over $L$ is a consequence of a version of a conjecture of L. Taelman. Along the way we also prove the existence of the category of mixed $t$-motives and the category of mixed Carlitz motives over any $L$ (these two existence results are independent of any conjectures).