diff --git a/src/marbl_interior_tendency_mod.F90 b/src/marbl_interior_tendency_mod.F90
index 2675d99f..3e2df410 100644
--- a/src/marbl_interior_tendency_mod.F90
+++ b/src/marbl_interior_tendency_mod.F90
@@ -1072,8 +1072,14 @@ subroutine compute_carbonate_chemistry(domain, temperature, press_bar, &
 
       do k=1,dkm
         if (ph_prev_col(k)  /= c0) then
-          ph_lower_bound(k) = ph_prev_col(k) - del_ph
-          ph_upper_bound(k) = ph_prev_col(k) + del_ph
+          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
+          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
+          end if
         else
           ph_lower_bound(k) = phlo_3d_init
           ph_upper_bound(k) = phhi_3d_init
@@ -1097,8 +1103,14 @@ subroutine compute_carbonate_chemistry(domain, temperature, press_bar, &
       do k=1,dkm
         ph_prev_col(k) = pH(k)
         if (ph_prev_alt_co2_col(k) /= c0) then
-          ph_lower_bound(k) = ph_prev_alt_co2_col(k) - del_ph
-          ph_upper_bound(k) = ph_prev_alt_co2_col(k) + del_ph
+          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
+          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
+          end if
         else
           ph_lower_bound(k) = phlo_3d_init
           ph_upper_bound(k) = phhi_3d_init