Found some O2 data for Alert Alaska. Plotting this on the same graph as Barrow give the plot below.
This shows that O2 is the inverse of CO2 with the peak of O2 occurring (as near as can be determined from the sparse data) at the minimum of CO2. They appear to be in synchronism.
To me this shows that the O2 and CO2 are linked to the same process.
From http://www.elcamino.edu/faculty/tnoyes/Readings/10AR.pdf this plot shows that there is a peak phytoplankton growth in summer at the poles which is significantly higher than spring or autumn growths.
Phytoplankton in sunlight photosynthesises using CO2 and creating O2. In the dark O2 is used in phytoplankton respiration and CO2 is released.
Decomposition requires warm temperatures for the bacteria to work.At the end of summer temperatures are falling rapidly so decomposition will not release CO2 rapidly and certainly would not continue into December. This suggests that spring growth and autumn decomposition would not cause the CO2 dip.
The CO2 dip is caused by the action of sunlight on phytoplankton.
The Full O2 record with CO2