3.3. Decay-induced changes of pH.
The analysis revealed a complex pattern of pH gradients in the
experimental tubes. Inside the sediments with the buried carcasses, the
pH varied from acidic (pH 6.05 in the kaolinite) to alkaline (pH 11 in
the montmorillonite) (Table 4). This is very different from the original
(control) values of the artificial sea water (pH 7.8) which was used in
experiments with the kaolinite, silica, clinochlore and chamosite, and
fresh water (pH 7.0) used in the montmorillonite experiment. The pH of
the liquid above the sediment with nauplii became more alkaline than
inside the sediments (except montmorillonite). The difference between
the pH in the middle of the sediment and the supernatant was more
pronounced in the experimental tubes with nauplii than in the controls
without nauplii (Table 4). Besides, in three experimental sediments
(kaolinite, silica and clinochlore), the pH had a local maximum in the
water-sediment interface (Table 4, shown in bold). This means that the
pH in the topmost layer of the sediment was higher than in the middle of
the sediment and in the water column above the sediment (silica,
clinochlore), or, in the case of the kaolinite, there was a thin layer
of water immediately above the sediment where the pH was lower (7.04)
than in the topmost sediment (7.13), although elsewhere in the water
column the pH was high (7.76; Table 4).
This complex pattern of pH gradients becomes even more intricate when we
consider the light-colored spots around the carcasses. These spots
probably imply lower local pH because the discoloration of the otherwise
evenly darkened kaolinite or dark-green clinochlore can be explained by
the acidic dissolution of dark-colored compounds (e.g., hydrotroilite in
the kaolinite sediment (Naimark et al., 2016a). We could not directly
measure the pH within the spots but microenvironmental pH minima have
been shown to form locally around decaying biomass in marine sediments
(Zhu et al., 2006).
The results are compatible with the idea that decaying organic matter
induces chemical changes in the surrounding medium that potentially can
affect the sediment and the set of ions released by sediment particles
(see below).