Geoscience Reference
In-Depth Information
result is that, over long timescales, the CaCO
3
satu-
ration state of seawater globally is both stable and
close to that predicted by thermodynamic equilib-
rium (Ω ~ 1), despite tropical surface waters being
strongly supersaturated. For example, if CO
2
out-
gassing were to increase, so too would silicate
weathering (due to increased temperature). At the
same time, the rate of carbonate precipitation from
seawater would decrease due to carbonate compen-
sation (lowering Ω) and the system would arrive at
a new steady state with a higher atmospheric CO
2
,
but a similar Ω.
This thought experiment illustrates two impor-
tant concepts. First, it explains why, in times past,
CO
2
can have been far higher than today and yet
seawater Ω remained at levels adequate for calcii -
cation. Secondly, if we are concerned about ocean
acidii cation events in earth history, we need to look
for transient departures from long-term dynamic
equilibrium. Global deviations in Ω in seawater can-
not last long, in geological terms. Given enough
time (> 10 000 yr), carbonate compensation and sili-
cate weathering will work to balance CO
2
outgas-
sing (and inputs of acidity in general). In Chapter 2
Zeebe and Ridgwell provide a detailed discussion
of the mechanisms involved.
It is the rapidity of the increase in
p
CO
2
in present-
day oceans that is outstripping the buffering capac-
ity of the earth system and, potentially, the genetic
ability of populations to adapt. Thus, if we seek to
understand the lessons of the past for our future,
we need to identify brief intervals in the past when
CO
2
is inferred to have risen too rapidly for the
earth system to remain in equilibrium (e.g. Hoegh-
Guldberg
et al
. 2007 ; Knoll
et al
. 2007 ; and see Section
2.5.3). The geological record does indeed contain
several such events, and reveals that perturbations
to the marine carbonate system can have complex,
and in some cases devastating, effects on popula-
tions of calcifying organisms. In addition to these,
however, the rock record contains intervals in which
the patterns of biological calcii cation exhibit signal
features of stress like those of ocean acidii cation,
but sustained over timescales far longer than those
expected from our understanding of ocean acidii -
cation and the marine carbonate system (e.g. Knoll
et al
. 2007). These observations highlight an impor-
tant gap in our understanding and require an
additional class of hypotheses for processes respon-
sible for controlling Ω in surface seawater. The
mechanisms of interest are discussed in Section 4.3.3.
4.2.1 The Palaeocene-Eocene Thermal
Maximum
Because their calibration depends on assumptions
of equilibrium, models such as GEOCARB and
COPSE integrate over long intervals of time and
cannot be used to identify times of geologically
rapid
p
CO
2
increase in the geological record. We
need to i nd high-resolution geological records in
which geochemical data suggest rapid environmen-
tal change. Perhaps the best studied example is the
so-called Palaeocene-Eocene Thermal Maximum
(PETM), a brief interval of pronounced global
warming about 55 Myr ago (Kennett and Stott 1991;
Zachos
et al
. 1993 ).
Warming of 5 to 8°C, with larger increases at high
latitudes, has been inferred from a sharp excursion
of about -1.7‰ in the oxygen isotopic composition
of carbonate skeletons (Zachos
et al
. 2003 ). Other
geochemical proxies for sea-surface temperature
(Mg/Ca, the relative abundance of unsaturated
alkenones, and the structures of archaeal lipids) are
consistent with this estimate, as are biogeographical
changes among both corals and land plants
(reviewed by Scheibner and Speijer 2008). A -2.5 to
-3‰ shift in the C-isotopic composition of carbon-
ate skeletons coincides with the temperature excur-
sion, suggesting that increased atmospheric CO
2
supplied from an isotopically light source drove cli-
mate change. It has been hypothesized that cata-
strophic release of methane from shelf/slope
clathrate hydrates was involved in the PETM event
( Dickens
et al
. 1995), but the inability of clathrate
release to supply the quantity of carbon needed to
account for recorded C-isotopic change (Zachos
et al
. 2005) suggests that other mechanisms, includ-
ing thermogenic methane release associated with
end-Palaeocene l ood basalts, may have played a
role (Svensen
et al
. 2004 ; Higgins and Schrag 2006 ).
In any event, high-resolution stratigraphic and geo-
chemical data indicate that the PETM perturbation
was rapid and transient; the decrease in C-isotope
values occurred largely in two bursts, each less than
1000 yr in duration, and the system returned to its
