ACR MLG LOSS OF CABIN PRESSURE. EMERGENCY DESCENT.

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WE DEPARTED MLB AT XA19 LCL. PASSING 26000' DURING CLB WE HEARD A VIBRATION AS IF SOMETHING WAS FLAPPING AGAINST THE SIDE OF THE ACFT. SHORTLY AFTER THAT THE CABIN RATE OF CLB INDICATOR WAS PEGGED AT 6000 FPM RATE OF CLB. WE WERE UNABLE TO CONTROL THE PRESSURE; SO WE PUT ON THE O2 MASKS--RAN THE DEPRESSURIZATION CHKLIST; DECLARED AN EMER; AND REQUESTED VECTORS FOR JACKSONVILLE. WE REQUESTED THE EMER EQUIP TO STANDBY FOR OUR ARR. WE MADE AN UNEVENTFUL LNDG AT JACKSONVILLE ARPT AT XB20 AND REQUESTED THAT THE MEDICAL TEAM MEET US AT THE GATE. ONE WOMAN WAS HOSPITALIZED THAT HAD A PREVIOUS HEART CONDITION. THE CAUSE OF THE PRESSURE PROB APPEARS TO BE THE BOTTOM OF THE FORWARD CARGO DOOR SEAL THAT CAME OFF. JACKSONVILLE CENTER; APCH CTL AND TWR ALL DID AN EXCELLENT JOB OF HANDLING THIS PROB. SUPPLEMENTAL INFO FROM ACN 99815: HEARD A RUMPLING NOISE. THINKING IT WAS A POSSIBLE NOSE GEAR DOOR HANGING CYCLED GEAR HANDLE FROM 'OFF' TO 'UP'; THEN BACK TO THE 'OFF' POS WITH NO NOTICEABLE CHANGE OF NOISE. UNABLE TO CONTROL CABIN. ACCOMPLISHED EMER DSCNT/RAPID DECOMPRESSION CHKLIST; DECLARED AN EMER AND LANDED UNEVENTFULLY. TO PREVENT A RECURRENCE; THE GND AGENT COULD VISUALLY INSPECT ALL EXTERIOR DOORS (CARGO; SVC) FOR SECURITY PRIOR TO PUSHING BACK THE AIRPLANE.