8. Given an arithmetic sequence $\left\{a_{n}\right\}$ with the sum of the first $n$ terms as $S_{n}$,
$$
\begin{array}{l}
\left(a_{6}-1\right)^{3}+2013\left(a_{6}-1\right)=1, \\
\left(a_{2008}-1\right)^{3}+2013\left(a_{2008}-1\right)=-1 .
\end{array}
$$

Then which of the following conclusions is correct? ( ).
(A) $S_{2013}=2013, a_{2008}a_{6}$
(C) $S_{2013}=-2013, a_{2008} \leqslant a_{6}$
(D) $S_{2013}=-2013, a_{2008} \geqslant a_{6}$
8. A.

Construct the function $f(x)=x^{3}+2013 x$, which is an odd and increasing function.
From $f\left(a_{6}-1\right)=1, f\left(a_{2008}-1\right)=-1$, we know $\left(a_{6}-1\right)+\left(a_{2008}-1\right)=0$,

and $a_{6}-1>a_{2008}-1$.
Therefore, $a_{6}+a_{2008}=2$, and $a_{6}>a_{2008}$.
Thus, $S_{2013}=2013$.