This is probably a trivial question.

While reading the paper

R. Elkik, Singularites rationnelles et deformations, *Invent. Math.* 47 Ž1978., 139147.

I came across the following short exact sequence. Consider a flat morphism $f:X\rightarrow S=Spec(R)$ of k-schemes of finite type, $X$ normal + CM, and pick a regular parameter $t\in R$. If $X_t=X\times_{S} Spec R/tR$ is the fiber over $t$, then multiplication by $t$ induces a short exact sequence $$0 \rightarrow \omega_X \stackrel{t}{\rightarrow} \omega_X \rightarrow \omega_{X_t} \rightarrow 0$$

Is it straightforward to derive such a sequence?

Thanks in advance for any suggestion.