ocaml-rdf should include an implementation of the Turtle serialization that allows reading and writing Turtle files.
The existing zoggy/ocaml-rdf library contains an implementation of Turtle (https://framagit.org/zoggy/ocaml-rdf/-/blob/master/src/rdf_ttl.ml). Maybe this can be directly re-used. zoggy/ocaml-rdf is licensed as LGPL so copying the code is permitted.
Why not just use zoggy/ocaml-rdf?
It does a bit too much (SPARQL, queries over HTTP, crypto stuff). Things that make it hard to port the library to Mirage or browser (via js_of_ocaml).