A capless retractable sealed marking instrument has a sealed chamber incorporated in a marker casing. The sealed chamber includes a chamber, a wiper ring, and an annular rim and a seal portion with a cross-slit aperture defining surfaces or flaps which are moved out of contact by a linearly-advanced marker nib to open the seal for extension of the marker nib. The configuration of the disk provides positive sealing force between the lateral sealing surfaces of the seal flaps. Retraction of the marker nib draws the flaps back into the sealed chamber configuration to protect the marker nib and cartridge from atmospheric degradation.