  There remain many open questions relating to the depletion of elements into
dust, e.g., exactly how are C and O incorporated into dust in dense clouds and,
in particular, what drives the disappearance of oxygen in the denser
interstellar medium? This work is, in part, an attempt to explain the
apparently anomalous incorporation of O atoms into dust in dense clouds. We
re-visit the question of the depletion of the elements incorporated into the
carbonaceous component of interstellar dust, i.e., C, H, O, N and S, in the
light of recent analyses of the organics in comets, meteorites and
interplanetary dust particles. We find that oxygen could be combined with ~ 10
to 20 % of the carbon in the dust in dense regions in the form of a difficult
to observe, organic carbonate, which could explain the unaccounted for 170-255
ppm oxygen depletion. We conclude that, while C, O and N atoms are depleted
into an amorphous a-C:H:O:N phase, we posit that a significant fraction of C
and O atoms could be sequestered into an organic carbonate, which provides a
viable solution to the oxygen depletion problem. Further, the thermal or
photolytic decomposition of this carbonate may have a bearing on the formation
of CO_2 in the ISM.
