Each deuterium measurement is based on collecting and melting a fixed
thickness of ice (my expectation is a ~5cm length, but you'd have to
study the references to know for sure). As a result each measurement
is intrinsically an average over the amount of time represented by the
ice in that piece. Since time is more compressed as one goes down,
this effect becomes greater with depth.
Hence a 5cm sample collected early in the core might record average
temperature for a single year (or less), but a 5cm sample collected
from deep ice would average across several years.
Variations in the amount of averaging intrinsic to the sampling method
probably accounts for your observation that the standard deviation is
larger in recent ice than it is in old ice.
-Robert Rohde
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