Now that we've seen it three times, I guess we understand it. ;-)
Byrd station is not the GISP or Vostok CO2 data. The small figure below kinda shows how the Vostok record splices into the modern era. Essentially when CO2 rose at the end of the last glacial period, it went up to ~280 ppm, and that's where it stayed until the slight rise began in the mid-1700s/early 1800s.