  Exposed scarps images and ice-penetrating radar measurements in the North
Polar Layered Deposits (NPLD) of Mars show alternating layers that provide an
archive of past climate oscillations, that are thought to be linked to orbital
variations, akin to Milankovitch cycles on Earth. We use the Laboratoire de
Meteorologie Dynamique (LMD) Martian Global Climate Model (GCM) to study
paleoclimate states to enable a better interpretation of the NPLD physical and
chemical stratigraphy. When a tropical ice reservoir is present, water vapor
transport from the tropics to the poles at low obliquity is modulated by the
intensity of summer. At times of low and relatively constant obliquity, the
flux still varies due to other orbital elements, promoting polar layer
formation. Ice migrates from the tropics towards the poles in two stages.
First, when surface ice is present in the tropics, and second, when the
equatorial deposit is exhausted, from ice that was previously deposited in
mid-high latitudes. The polar accumulation rate is significantly higher when
tropical ice is available, forming thicker layers per orbital cycle. However,
the majority of the NPLD is sourced from ice that temporary resided in the
mid-high latitudes and the layers become thinner as the source location moves
poleward. The migration stages imprint different D/H ratios in different
sections in the PLDs. The NPLD is isotopically depleted compared to the SPLD in
all simulations. Thus we predict the D/H ratio of the atmosphere in contact
with NPLD upper layers is biased relative to the average global ice reservoirs.
