anchor.diff:  SUBROUTINE flxco2(co2flux, co2ex, dpco2,                                                    &
                     ph, pco2, fco2, co2, hco3, co3, OmegaA, OmegaC, BetaD, rhoSW, p, tempis,  &
                     temp, sal, alk, dic, sil, phos, kw660, xco2, Patm, dz1, N,                &
  -                  optCON, optT, optP, optB, optK1K2, optKf                                  )
  +                  optCON, optT, optP, optB, optK1K2, optKf, optGAS                          )
   
     !   Purpose:
     !     Computes air-sea CO2 flux & surface ocean carbonate system vars (pH, CO2*, HCO3- and CO32-, OmegaA, OmegaC, R)
  
positive.code:   CHARACTER(3) :: opB
    CHARACTER(2) :: opKf
    CHARACTER(3) :: opK1K2
  
  ! Set defaults for optional arguments (in Fortran 90)
  ! Note:  Optional arguments with f2py (python) are set above with 
  
negative.code: !   Default is Lueker et al. 2000) for K1 & K2
      opK1K2 = 'l'
    ENDIF
    IF (PRESENT(optGAS)) THEN
      opGAS = optGAS
    ELSE
      opGAS = 'Pinsitu'
    ENDIF
  
    depth0 = fco2 * 0.0
    lat0   = depth0
  ! Compute derived variables from input (DIC, ALK, ...)
    CALL vars(ph, pco2, fco2, co2, hco3, co3, OmegaA, OmegaC, BetaD, rhoSW, p, tempis,  &
              temp, sal, alk, dic, sil, phos, Patm, depth0, lat0, N,                    &
              opCON, opT, opP, opB, opK1K2, opKf, opGAS                                 )
  
  ! Compute pCO2atm [uatm] from xCO2 [ppm], atmospheric pressure [atm], & vapor pressure of seawater
  ! pCO2atm = (Patm - pH20(i)) * xCO2,   where pH20 is the vapor pressure of seawater [atm]
  