A320 Captain reported a fumes event during descent which was localized to the flight deck area.

Narrative

Descending through 14;000 ft.; on arrival to ZZZZ; both pilots began to notice an unusual odor in the flight deck. After brief discussion; it was agreed that the odor smelled like magic marker; or sharpie marker. Descending through 11;000 ft.; with the engines at idle in open descent; the odor became much stronger. No smoke was noticed in the flight deck. The flight was cleared directly to the IAF for the ZZZZ approach. The pilots briefly referenced Quick Reference Handbook S-7 (Fumes); and determined a diversion was necessary. ZZZZ was the closest; and safest; airport under the current situation; and the flight continued to its scheduled destination of ZZZZ. Time did not allow for any additional troubleshooting as the aircraft was already being configured for landing. The flight landed in ZZZZ approximately 11 minutes after the odor was first discussed. After landing; the flight deck windows were opened for fresh air ventilation; and oxygen was used intermittently by the Captain. After parking at the gate; a logbook entry was completed; an Aircraft type Cabin Odor Report was completed; and a detailed discussion commenced with ZZZZ Tech-Ops. It was discussed that the APU had been deferred as per MEL from a previous flight. A logbook history review also revealed this aircraft had a recent history of odor/fume events. The Inflight Crew members stated they didn't smell any odor in the cabin until the flight deck door was opened at the gate. After reviewing the 'Cockpit / Cabin Odor Recognition and Isolation' guide in the Supplemental Procedures section of FCOM Vol. 1; the event was identified as a Level 3. It was suggested that Engine #1 Bleed; or Pack #1; was the most likely source of the odor/fumes. Both pilots contacted Med-link as directed by the Chief Pilot on duty.