Tech Briefs Magazine - October 2023 - Comsol-28A

RESEARCH SPOTLIGHT
grains and the seawater that
surrounds them. The water's
alkalinity, density, and chemical
composition were set to match
typical deep-sea conditions.
The team modeled various
solids and simulated their
interactions with seawater
and seafloor sediment. Sulpis
also added seashell models,
based on scans of actual
specimens, to his sediment-
water interface. For example,
the H. inflatus pteropod shell
highlighted in Figure 3 is based
on a CT scan of a specimen
from the Cariaco Basin off the
coast of Venezuela. Such 3D
images enabled the simulation
to capture how the irregular
shape of a shell could affect its
dissolution.
FIGURE 3 Simulations of the shells of four species, showing changes after being
submerged for one minute. The H. Inflatus pteropod shell is made from aragonite, while
the others are calcite. Models in blue show CaCO3
saturation levels in the surrounding
seawater (top) and models in the Heat Camera color table show the rate at which the shells
dissolve (bottom).
as they occur in an environment
where we cannot physically
go. " Physically removing fragile
specimens from deep-sea
sediment is also challenging.
" It is really hard to recover sea
butterfly shells with a sediment
trap, " says Sulpis. " By the time
you bring them up from the
deep, they will likely already
have dissolved. So, there is a
lack of good data about calcium
carbonate reactions at deep sea
pressures and temperatures. "
Previous attempts to
mathematically model the
behavior of calcium carbonate
in seawater provide limited
value for Sulpis' research.
" Most models have treated
all CaCO3
as calcite, rather
than creating separate models
28a COMSOL NEWS
of aragonite. Also, existing
diagenetic 'continuum'
models do not capture what
is happening at the scale of a
single grain or a single pore in
a seashell, " he explains.
Another issue is that
older models have rendered
CaCO3
grains as smooth,
uniform objects, which is not
accurate. " These grains are
complex and heterogenous
micrometer-scale shapes with
insides and outsides. " Sulpis
does acknowledge that some
simplifications are necessary,
but says, " We wanted to
replicate the actual shapes
as closely as possible, at the
smallest scale possible. Before
deciding to simplify some
structures, we wanted our
simulation to confirm that
these simplifications would
not compromise results. "
» SIMULATED DEEP
DIVE INTO THE
OCEAN-SEDIMENT
BOUNDARY ZONE
For a deeper understanding
of how calcite and aragonite
interact at the seafloor,
Sulpis developed a novel 3D
model using the COMSOL
Multiphysics®
software. This
model makes it possible
to move virtually among
the boundaries between
oceanic organisms and their
environment. It enables
researchers to simulate the
dissolution reactions occurring
among aragonite and calcite
The simulation indicates
that the inner shape of a shell
may not have a significant
impact on how it reacts with
seawater. " If you look at the
top row, it tells us that inside
these shells, the trapped
water can become completely
saturated with CaCO3
. This
prevents further dissolution
from occurring along the
complex inner surface,
and so the shells dissolve
from the outside in, " Sulpis
says. These results suggest
that some simplification
of a shell's modeled shape
will not necessarily affect
simulation results, at least
when the shell is completely
surrounded by seawater.
Now, what happens when a
pteropod shell and seawater
are added to calcite-rich
sediment? Figure 4 presents
the simulated effects of this
interaction. A dissolving sea
butterfly shell is shown to
prevent the dissolution of
calcite grains mixed into the
sediment. These grains were
rendered as spheres - a
simplification that Sulpis
made based on the results of
simulations such as those in
Figure 3.

Tech Briefs Magazine - October 2023

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