Ocean acidification: the other carbon problem
Ocean acidification (OA) is one of the largest issues
resulting from anthropogenic pumping of CO₂into the atmosphere. CO₂in the
atmosphere dissolves into oceans, leading to a decrease in pH (called the
bicarbonate buffer system):
CO₂+H₂O ↔ H₂CO₃ ↔ HCO₃¯+H⁺ ↔ CO₃¯+H⁺
Atmospheric CO₂doubling will likely lower pH of the entire ocean > 0.1 unit.
The normal variation of pH in open seawater (7.6𔃆.2),
so .1 units is very significant.
Effects on marine organisms include physiological responses (regulating
acid-base imbalance) as well a dissolution of calcium carbonate support
structures like shells, tests, or exoskeletons as saturation horizons rise. It
is possible deep sea organisms may be particularly vulnerable. For example, the
internal control of pH is critical for proper physiological functioning, so
many organisms have evolved elaborate methods to regulate internal pH. However,
the pH in most of the deep sea is stable over thousands of years, so deep sea
organisms have not needed methods to rapidly adapt to or regulate changes in
pH. Ocean acidification may be too fast for these deep sea organisms to adapt
to (Seibel and Walsh, 2002).
Changes in carbonate saturation horizons are one of the biggest worries when
people talk about effects of OA on organisms. The solubility of calcium
carbonate (CaCO₃) increases with decreasing temperature and increasing
pressure. In the north Pacific, the rate of rise for aragonite saturation
horizon is around 1 m y¯¹. At atmospheric CO₂
= 780 ppm (near the end of this century), the subarctic North Pacific and
Southern Ocean will be undersaturated with respect to aragonite (Fabry et al.,
2008; Feely et al., 2006; Orr et al., 2005.) Major planktonic calcium carbonate
producers like coccolithophores, foraminifera, and euthecosomatous pteropods
(organisms responsible for nearly all the export flux of calcium carbonate to
the deep sea) may be at risk as well. In lab studies, foraminifera and
pteropods for example showed possible reduced calcification with decreasing pH
(Feely et al., 2004; Orr et al., 2005).
So
to be blunt, this is bad.
But what is going to happen? Is there another way, besides controlled
laboratory experiments, to gauge where the ocean is headed? Other than just
sitting and waiting around to see what happens, I mean.
We could look to the past.
The Paleocene-Eocene Thermal Maximum
About 55 million years ago, sea surface temperatures rose rapidly, about 5-10°C
in only a thousand years. This rapid warming was likely due to increases in
greenhouse forcing, just like today. Rather than anthropogenic however, this
rapid influx of carbon to the atmosphere may have come from methane hydrates at
the bottom of the ocean. For those of you who are isotopically inclined, δ¹³C
records from deep sea sediment cores show a rapid initial decrease (around
20,000 years) followed by a gradual recovery (~130,000 years) back to similar δ¹³C
values found before the excursion. The magnitude of the drop in δ¹³C values (-3‰)
suggest the carbon source was very depleted in 13C, pointing to methane
hydrates as the likely source. This carbon isotope excursion was the signature
of the Paleocene-Eocene Thermal Maximum (PETM) (Zachos et al., 2005).
Part of influx of CO2 to the atmosphere dissolved into the oceans, lowering pH.
This resulted in a rise in the lysocline and calcite compensation depth (CCD)
which promoted the dissolution of seafloor carbonate. The CCD shoaled over 2 km
within just a few thousand years, but recovery was gradual, around 60,000
years. Ultimately this CO2 would be sequestered through chemical weathering of
silicate rocks (Zachos et al., 2005).
All marine communities experienced major changes, including migrations to
higher latitudes, evolutionary radiations, and extinctions. There were also
distinct responses between planktonic and benthic organisms. Planktonic
organisms weren't particularly affected, but did experience radiation and
diversification. However for benthic organisms, the PETM marked the largest
extinction event in the last 90 million years. 30 to 50% of benthic species
became extinct. It's important to keep in mind that gauging PETM effects on
marine ecosystems relies entirely on microfossils, as no macroinvertibrate
fossils have been described (Rodriguez-Tovar et al., 2011; McInerney and Wing,
2011).
Surprisingly, the driving force affecting these organisms may have been
temperature rather than ocean acidification. Temperature affects bottom water
oxygenation and increases metabolic rates, meaning organisms need more food to
maintain base metabolism (McInerney and Wing, 2011).
Applicability?
Can we expect to see similar responses from marine organisms today? How similar
was the PETM to today's global warming/ ocean acidification problem? It turns
out we are likely in for much worse. for one, the rate of carbon input was much
different. In the PETM, carbon was input to the atmosphere over an 8,000 year
period. Contrast that with our pumping CO2 into the atmosphere over a mere 300
years. That's less than the mixing time of the ocean. The longer CO2 input rate
during the PETM meant less severe acidification and carbonate dissolution in
the surface ocean.
Something else to consider: there was no ice during the PETM. No glaciers,
snow. Nothing. No ice means no ice/albedo feedback (warming leads to melting of
ice, revealing darker surfaces which absorb more heat, leading to more warming)
which means an absence of greater warming at polar latitudes. In contrast, the
most drastic warming today is at the poles (McInerney and Wing, 2011; Ridgwell
and Schmidt, 2010).
Check out this video:
We are heading for something unprecedented. The PETM at best
provides a framework for the mimimum damage today's anthropogenically induced
problems will cause deep sea marine life.
Helpful link:
