pH bounds for alt_co2 not set properly

[klindsay28]

pH bounds for alt_co2 are being set from ph_prev_col instead of ph_prev_alt_co2_col.
I think the following diff, against the development branch, corrects this:

diff --git a/src/marbl_interior_tendency_mod.F90 b/src/marbl_interior_tendency_mod.F90
index 698c80a..3a8a98e 100644
--- a/src/marbl_interior_tendency_mod.F90
+++ b/src/marbl_interior_tendency_mod.F90
@@ -1126,12 +1126,12 @@ subroutine compute_carbonate_chemistry(domain, temperature, press_bar, &
         ph_prev_col(k) = pH(k)
         if (ph_prev_alt_co2_col(k) /= c0) then
           if (domain%delta_z(k) > 1._r8 * unit_system%cm2len) then
-            ph_lower_bound(k) = ph_prev_col(k) - del_ph
-            ph_upper_bound(k) = ph_prev_col(k) + del_ph
+            ph_lower_bound(k) = ph_prev_alt_co2_col(k) - del_ph
+            ph_upper_bound(k) = ph_prev_alt_co2_col(k) + del_ph
           else
             ! Double initial bracket width for very thin layers
-            ph_lower_bound(k) = ph_prev_col(k) - 2._r8 * del_ph
-            ph_upper_bound(k) = ph_prev_col(k) + 2._r8 * del_ph
+            ph_lower_bound(k) = ph_prev_alt_co2_col(k) - 2._r8 * del_ph
+            ph_upper_bound(k) = ph_prev_alt_co2_col(k) + 2._r8 * del_ph
           end if
         else
           ph_lower_bound(k) = phlo_3d_init

[klindsay28]

It looks like this was introduced in #480.

[klindsay28]

Fixed by #488