Geoscience Reference
In-Depth Information
1977 ; Aller 1994 ) and thereby reduce their effect on
sediment pH. The depth to which each electron
acceptor, particularly O
2
, penetrates sediments is
critical in driving production of H
+
. This pene-
tration depth in turn depends on physical charac-
teristics such as sediment porosity, sediment
resuspension, and the supply and nature (fresh or
degraded) of organic matter which control the rate of
remineralization and thereby the rate with which
each electron acceptor is successively depleted
(Westrich and Berner 1984). In sediments deplete of
large infauna, pH is laterally homogeneous (Zhu
et al.
2006b ), rel ecting the fact that each of the processes
outlined above dominates a discrete depth layer.
However, studies of burrow structures of macrofauna
have revealed that the chemical environment of bur-
rows and burrow walls differs substantially from that
of the surrounding sediments (Aller and Yingst 1978).
Recent technological innovations in the determina-
tion of O
2
, pH, and
p
CO
2
with optodes have high-
lighted the vertical, lateral, and temporal variability
of these parameters (see below and review by
Stockdale
et al.
2009). Having introduced the main
sources of H
+
in sediments, it is informative to con-
sider relevant H
+
sinks which buffer pH changes.
Marine sediments have a high pH buffering
capacity due to mineral dissolution-precipitation,
adsorption-desorption reactions between solutes
and minerals and acid-base reactions in porewa-
ters. Field observations and modelling studies have
shown that dissolution of calcium carbonate min-
eral (hereafter CaCO
3
, including calcite, Mg-calcite,
and aragonite) is a major sink for H
+
in marine sedi-
ments (Wenzhöfer
et al.
2001 ; Jourabchi
et al.
2005 ;
Morse
et al.
2006). However, as with the supply of
CO
2
to sediments, there are also strong biological
controls on pH buffering processes. Thereby, sea-
sonal patterns in abundance of benthic foraminifera
in coastal waters exert a strong inl uence on Ca
2+
concentration and calcite saturation state in pore-
waters (Green
et al.
1993 ). Macrofauna bioturbation
plays a critical role in driving dissolution of CaCO
3
shells by maintaining the exchange of solutes, pre-
venting the build-up of total alkalinity and thereby
sustaining redox processes that lead to H
+
forma-
tion ( Aller 1978 , 1982 ; Green
et al.
1993 ). Further
examples of biological control of the pH buffering
capacity of sediments can be found in the literature.
For example, model as well as experimental data
suggest that seagrasses introduce O
2
directly into
the sediments, increasing the mineralization of
organic matter and thereby dissolution of CaCO
3
( Burdige
et al.
2008). On the other hand, Burdige
et al.
(2008) showed that seagrass foliage decreased
the bottom water l ow, and thereby the advective
uptake of O
2
by sediments. The net balance between
these opposing effects depends on seagrass density,
with carbonate dissolution from direct seagrass O
2
input becoming the dominant process at densities
above 0.5 m
2
of leaf area per m
2
of seal oor (Burdige
et al.
2008). Photosynthetic activity by microphyto-
benthic communities may also introduce O
2
directly
into sediments following diel patterns (de Beer
et al.
2005 ; Burdige
et al.
2008). The result of carbonate
mineral dissolution in marine sediments is an
increase in total alkalinity (see Chapter 1) which in
turn determines the concentrations of bicarbonate
(HCO
3
-
) and carbonate (CO
3
2-
) in porewaters. It has
been suggested that the increased bicarbonate due
to the dissolution of carbonate minerals may re-pre-
cipitate as a different metastable mineral (e.g. arag-
onite dissolution followed by calcite precipitation;
Hu and Burdige 2007). In addition to mineral disso-
lution-precipitation reactions, microbial redox
processes (including those described above) affect
the concentration of acid-base pairs in porewaters
( Soetaert
et al.
2007). However, the effect of micro-
bial processes on sediment pH is partly cancelled
out during re-oxidation reactions (e.g. Mn reduc-
tion increases pH, but Mn
2+
re-oxidation decreases
pH). Many of the microbially mediated redox reac-
tions in sediments tend towards an equilibrium pH
(>5.2) which is far lower than the typical seawater
pH of 8.2 (free scale) (Soetaert
et al.
2007 ). These
equilibria contribute to pH buffering in marine sed-
iments. Given the scale of the predicted changes in
pH of the overlying water under ocean acidii ca-
tion, sediments are likely to buffer part of these
changes through the dissolution of carbonate min-
erals. The dissolution of different phases of these
minerals will depend on their respective mineral
stability (see Chapter 7). For example, Morse
et al.
(2006) showed that high-Mg calcite is likely to
respond earlier than aragonite in a progressive dis-
solution process. As some infaunal animals main-
tain their burrow pH at or near that of overlying
