CO2 cycle revisited

A previous post had a look at the CO2 cycle and questioned the cause.
This post adds a couple more years of data but still does not solve the dip cause.

Latest Data Showing the approx dip measurement points:

Depth of dip replotted.

Note the innaccuracy of determining the depth gives a different plot to the last investigation.

The cause?

Ice covering phyto plankton - as more is exposed to sunlight photosynthesis overcomes respiration and co2 is absorbed from the sea wich then absorbs it from the air.

http://www.ldeo.columbia.edu/~marra/PhytoRespGRLv5.pdf

Change at equinox considering respiration/photosynthesis in phytoplankton and plants
Daylight time is greater than dark time prior to 22nd September and less than dark after this date? Trouble is the dip is around the end of August and the 1st week in September.
Why should the dip be earlier?

Growth/decay of land plants
Growth - ok CO2 is converted to biomass
Decay - much too slow over the winter months to return the CO2

Water temperature.
Warm water takes up less CO2 than cold so is wrong direction for dip.


Most likely
 equinox and length of day/night affecting the respiration/photosynthesis balance?
Depth of dip is greatest at Barrow and gets smaller the further south you go. Eventually the phase of the dip changes as you would expect for the southern hemisphere.


Still no further forward!