Climate data operator (CDO) code for the analyses:
#subset set ice variable from base file
cdo selname,ICEFRAC,time decadal_mean_annual_seaice.nc ice.nc
cdo seltimestep
#calculate annual mean values
cdo timmean -seltimestep,496/506 ice.nc 17kamean.nc
cdo timmean -seltimestep,596/606 ice.nc 16kamean.nc
cdo timmean -seltimestep,646/656 ice.nc 15.5kamean.nc
cdo timmean -seltimestep,696/706 ice.nc 15kamean.nc
cdo timmean -seltimestep,746/756 ice.nc 14.5kamean.nc
#Autumn
Cdo selname,ICEFRAC,time trace.01-36.22000BP.cam2.ICEFRAC.22000BP_decavgSON_400BCE.nc Autumn.nc
cdo timmean -seltimestep,496/506 Autumn.nc 17kaAutumn.nc
cdo timmean -seltimestep,596/606 Autumn.nc 16kaAutumn.nc
cdo timmean -seltimestep,646/656 Autumn.nc 15.5kaAutumn.nc
cdo timmean -seltimestep,696/706 Autumn.nc 15kaAutumn.nc 
cdo timmean -seltimestep,746/756 Autumn.nc 14.5kaAutumn.nc 
#Spring
Cdo selname,ICEFRAC,time trace.01-36.22000BP.cam2.ICEFRAC.22000BP_decavgMAM_400BCE.nc Spring.nc
cdo timmean -seltimestep,496/506 Spring.nc 17kaSpring.nc
cdo timmean -seltimestep,596/606 Spring.nc 16kaSpring.nc
cdo timmean -seltimestep,646/656 Spring.nc 15.5kaSpring.nc
cdo timmean -seltimestep,696/706 Spring.nc 15kaSpring.nc 
cdo timmean -seltimestep,746/756 Spring.nc 14.5kaSpring.nc 
#Summer
Cdo selname,ICEFRAC,time trace.01-36.22000BP.cam2.ICEFRAC.22000BP_decavgJJA_400BCE.nc Summer.nc
cdo timmean -seltimestep,496/506 Summer.nc 17kaSummer.nc
cdo timmean -seltimestep,596/606 Summer.nc 16kaSummer.nc
cdo timmean -seltimestep,646/656 Summer.nc 15.5kaSummer.nc
cdo timmean -seltimestep,696/706 Summer.nc 15kaSummer.nc 
cdo timmean -seltimestep,746/756 Summer.nc 14.5kaSummer.nc 
#Winter
Cdo selname,ICEFRAC,time trace.01-36.22000BP.cam2.ICEFRAC.22000BP_decavgDJF_400BCE.nc Winter.nc
cdo timmean -seltimestep,496/506 Winter.nc 17kaWinter.nc
cdo timmean -seltimestep,596/606 Winter.nc 16kaWinter.nc
cdo timmean -seltimestep,646/656 Winter.nc 15.5kaWinter.nc
cdo timmean -seltimestep,696/706 Winter.nc 15kaWinter.nc 
cdo timmean -seltimestep,746/756 Winter.nc 14.5kaWinter.nc