From: “Science”, 23 September 2016, RESEARCH SECTION, Pgs. 1427 – 1430
by D.A.Stolper, M.L.Bender, G.B.Dreyfus, Y.Yan, J.A.Higgins
This article defined the research and methodology used to determine the contribution of O2 to atmospheric pressures over the last 800,000 years. The medium used to determine this were Pleistocene ice cores from Greenland. Comparisons were made with the contribution of CO2.
The finding was that although the contribution of CO2 to atmospheric pressure (PCO2) remained steady, the contribution of O2 to atmospheric pressure (PO2) declined by approximately 2%. This indicates that the O2 sinks were larger than the O2 sources.
The differences were attributable to the contribution of eroded and buried pyrite that tended to consume O2 during its oxidation process.
The article considered factors that required adjustments and corrections for such things as air bubbles, particular geographical domes that the core samples were taken from and many other variables. Included in these adjustments was a rather lengthy discussion of the cooling of ocean temperatures during the periods under consideration.
This is an interesting fact. Cooling ocean temperatures should be, and probably are, considerations factored into global temperature models.
The following is from Wikipedia.
To investigate the possibility of climatic cooling, scientists drill into the Greenland ice caps to obtain core samples. The oxygen isotopes from the ice caps suggested that the Medieval Warm Period had caused a relatively milder climate in Greenland, lasting from roughly 800 to 1200. However, from 1300 or so the climate began to cool. By 1420, the “Little Ice Age” had reached intense levels in Greenland. Excavations of midden or garbage heaps from the Viking farms in both Greenland and Iceland show the shift from the bones of cows and pigs to those of sheep and goats. As the winters lengthened, and the springs and summers shortened, there must have been less and less time for Greenlanders to grow hay. A study of North Atlantic seasonal temperature variability showed a significant decrease in maximum summer temperatures beginning in the late 13th century to early 14th century—as much as 6-8 °C lower than modern summer temperatures. The study also found that the lowest winter temperatures of the last 2,000 years occurred in the late 14th century and early 15th century. By the mid-14th century deposits from a chieftain’s farm showed a large number of cattle and caribou remains, whereas, a poorer farm only several kilometers away had no trace of domestic animal remains, only seal. Bone samples from Greenland Norse cemeteries confirm that the typical Greenlander diet had increased by this time from 20% sea animals to 80%.