anchor.diff:  !   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                                        )
  +            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]
  
positive.code:   !       -> 'pf' means use Perez & Fraga (1987) formulation for Kf (recommended by Dickson et al., 2007)
    !               **** BUT Valid for  9 < T < 33°C and 10 < S < 40.
    !       -> 'dg' means use Dickson & Riley (1979) formulation for Kf (recommended by Dickson & Goyet, 1994)
  
    !     OUTPUT variables:
    !     =================
  
negative.code: 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, optGAS                          )
  
    !   Purpose:
    !     Computes air-sea CO2 flux & surface ocean carbonate system vars (pH, CO2*, HCO3- and CO32-, OmegaA, OmegaC, R)
  