The 4-to-2 mL/L oxygen transition narrowed in matched southern CalCOFI samples from 1984-2002 to 2003-2020

We tested whether two oxygen boundaries moved by the same mean distance off southern California. We used observed CalCOFI bottles from the 16 October 2023 database release. We found the shallowest downward crossings of 4 and 2 mL/L on the same cast. We matched 220 station-quarter groups at 56 stations, with 7,570 casts in 1984-2020. Mean boundary separation fell from 151.40 m in 1984-2002 to 120.76 m in 2003-2020. The decrease was 30.64 m, with a conditional 95% three-year block interval of 24.28 to 37.17 m. The 4 mL/L boundary became 10.88 m shallower and the 2 mL/L boundary became 41.52 m shallower. Gap, coverage, block and threshold checks retained a decrease. A shorter 1994-2020 comparison also gave a decrease. Thus the two boundaries did not have the same mean shift in these samples. This is a profile-shape diagnostic, not a direct habitat or hypoxia measure. Selected casts, finite bottle spacing, source methods, missing data and short dependent periods limit inference. The test cannot identify a physical cause. The package includes input rows, source hashes, code, all contrasts, saved resamples, a figure and numerical audits.

CalCOFI · oxygen transition · vertical structure · deoxygenation · matched sampling · block bootstrap