B737 FLT CREW REPORTS CABIN ALTITUDE WARNING PASSING 10000 FEET MSL AND QUICKLY LEVELS OFF; BUT NOT BEFORE PAX OXYGEN MASKS DEPLOY. CAPTAIN DISCOVERS BLEED SWITCHES OFF DURING RETURN TO DEPARTURE ARPT.

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TAXIED OUT FROM RAMP AREA WITH 1 ENG RUNNING. DURING TAXI OUT WE WERE GIVEN A RWY CHANGE WHICH REQUIRES REDOING THE TAXI CHKLIST. WE ACCOMPLISHED THESE ITEMS AND SHORTLY THEREAFTER I CALL FOR THE DELAYED ENG START WHICH THE FO ACCOMPLISHES AND THEN WE DO THE BEFORE TKOF CHK WHILE HOLDING SHORT OF RWY. WE ARE THEN CLRED ON TO HOLD THEN ALMOST IMMEDIATELY CLRED FOR TKOF ON THAT RWY. I XFER CTL OF THE ACFT TO FO AND HE COMMENCES THE TKOF. UNEVENTFUL TKOF ROLL AND LIFT-OFF. AFTER INITIATING L TURN TO 260 DEGS AT 400 FT AGL; AND PASSING APPROX 2500 FT AGL; THE AFTER TKOF CHKLIST IS INITIATED. DURING THIS PROCESS WE ARE SWITCHED TO DEP FREQ AND CHK IN. I THEN OBSERVE TFC AT L 10 O'CLOCK POS AND INFORM DEP OF THIS. SHORTLY THEREAFTER WE GET A TCAS TA SHOWING TFC AT R 2 O'CLOCK POS FOR A COUPLE OF MI ON DOWNWIND. DEP CTL CONFIRMS THIS. AS WE ARE PASSING 8000 FT MSL; I START LOOKING FOR THE TCAS ADVISORY TFC AND MONITORING OUR ALT AS I AM CONCERNED ABOUT THE TFC ON DOWNWIND AND THE TCAS ADVISORY; I MAKE THE 9000 FT FOR 10000 FT CALL AND CONFIRM THAT THE FO IS LEVELING OFF. WHILE THIS IS OCCURRING; DEP CTL STATES THAT TFC IS NO FACTOR AND CONTINUE CLB TO FL230. I INQUIRE ABOUT DELETING THE 250 KT SPD RESTR AND DEP GIVES US NORMAL SPD AND TELLS US TO TURN L ON A HDG TO INTERCEPT THE DEP. I WAIT FOR THE ACFT TO ROLL OUT ON THE ASSIGNED HDG AND ENGAGE LNAV FOR THE FO. ALMOST IMMEDIATELY AFTER LNAV IS ENGAGED; THE INTERMITTENT CABIN WARNING HORN STARTS TO GO OFF. AT THIS TIME WE NOTICE THE CABIN ALT CLBING ABOVE 10000 FT. WE PUT ON OUR OXYGEN MASKS; PERFORM THE RECALL ITEMS AND ESTABLISH COCKPIT COMS. VERY SHORTLY AFTER THIS I TELL THE FO TO LEVEL OFF AND I TALK TO ATC; STATING WE NEED TO DSND BACK DOWN TO 10000 FT. ATC ADVISES HIS MVA IS 11000 FT AND I REQUEST A TURN AND DSCNT. ATC CLRS US N AND DOWN TO 11000 FT. DURING THIS WHOLE PROCESS; THE LEAD FLT ATTENDANT IS ATTEMPTING TO CALL ME AND INFORM ME THE PAX OXYGEN MASKS HAVE DEPLOYED. ALSO DURING THIS PROCESS; I TOLD THE FO TO TAKE ALL COMS WITH ATC WHILE I ACCOMPLISH THE QRH PROC FOR CABIN ALT WARNING HORN. AS WE ARE COORDINATING FOR RETURN I INITIATE THE DSCNT AND APCH CHKLIST. IT IS AT THIS TIME I NOTICE THAT BOTH ENG BLEED SWITCHES ARE OFF. I THEN TURN ON THE L ENG BLEED SWITCH; PAUSED FOR A FEW MOMENTS AND THEN TURN ON THE R ENG BLEED SWITCH. AS WE COMPLETE THE APCH CHK; AND WITH THE PRESSURIZATION PANEL OPERATING IN MANUAL WITH THE VALVE CLOSED PER QRH CABIN ALT WARNING HORN PROC; I MANUALLY OPEN THE OUTFLOW VALVE TO REDUCE DIFFERENTIAL PRESSURE TO ZERO PRIOR TO LNDG. LNDG AND ROLLOUT WERE UNEVENTFUL. OBSERVATIONS: DUE TO THE HIGHER ELEVATION OF THE ARPT; AND OUR FAIRLY LIGHT WT/HIGHER RATE OF CLB COUPLED WITH FREQ CHANGE; TCAS TA DISTR AND LOOKING FOR TFC; INTERMEDIATE LEVELOFF; AND SUBSEQUENT INSTRUCTIONS/CLRNC FROM DEP CTL; I HAD NOT YET FINISHED THE AFTER TKOF CHK PRIOR TO ACFT CLBING ABOVE 10000 FT MSL. THUS HAD NOT YET CONFIRMED THE PRESSURIZATION/ENG BLEED CONFIGN PRIOR TO CABIN ALT WARNING HORN GOING OFF.