Geoscience Reference
In-Depth Information
few episodes that appear most relevant to us in rela-
tion to the ongoing acidii cation event caused by
humans. Our list is not comprehensive, and there
may be other events in earth's history that deserve
more attention in the context of ocean acidii cation
(see, e.g., Kump
et al.
2009 and Chapter 4 ).
accompanied this event that affected marine life
(see Caldeira 2007 for more details).
2.5.3 Palaeocene-Eocene Thermal Maximum
The PETM (~55 Myr) appears to be the closest ana-
logue for the future that has so far been identii ed in
the geological record. The onset of the PETM was
marked by a global increase in surface temperatures
by 5 to 9°C within a few thousand years (e.g. Kennett
and Stott 1991 ; Thomas and Shackleton 1996 ; Zachos
et al.
2003 ; Sluijs
et al.
2006). At nearly the same time,
a substantial carbon release occurred, as evidenced
by a large drop in the
13
C/
12
C ratio of suri cial car-
bon reservoirs. The carbon release led to ocean acid-
ii cation and widespread dissolution of deep-sea
carbonates (e.g. Zachos
et al.
2005 ; Zeebe
et al.
2009 ;
Ridgwell and Schmidt 2010). Different sources for
the carbon input have been suggested, which has
led to speculations concerning the mechanism.
Some, such as volcanic intrusion, imply that the car-
bon drove the warming. Others, such as the destabi-
lization of oceanic methane hydrates, imply that the
carbon release is a feedback that can exacerbate
warming (Dickens
et al.
1995 ; Dickens 2000 ; Pagani
et al.
2006). Note that regarding impacts on ocean
acidii cation, it is of minor importance whether the
carbon source was in the form of CO
2
or methane,
as methane would have been oxidized rapidly to
CO
2
in the water column and/or the atmosphere.
Remarkably, even the lower estimates for the car-
bon release during the onset of the PETM (~1 Pg C
yr
-1
) and over the past 50 years from anthropogenic
sources appear to be of similar order of magnitude.
The PETM exhibits several characteristics that are
essential for a meaningful comparison with the
anthropogenic perturbation: (1) it was a transient
event with a rapid onset (not a long-term steady
state); (2) it was associated with a large and rapid car-
bon input. Also, in contrast to aberrations that
occurred in the more distant past, the PETM is
relatively well studied because a number of well-
preserved terrestrial and marine palaeorecords for
this time interval are available (on the marine side
accessible through ocean drilling). The PETM may
therefore serve as a case study for ocean acidii cation
caused by CO
2
released by human activities. However,
it is important to keep in mind that the climatic and
2.5.1
Aptian Oceanic Anoxic Event
One possible example of ocean acidii cation is the
Aptian Oceanic Anoxic Event (OAE1a; ~120 Myr),
an interval characterized by the widespread deposi-
tion of organic-rich sediments. It has been suggested
that a marine calcii cation crisis occurred during
OAE1a (e.g. Erba and Tremolada 2004). However,
rather than being of a transient nature (e.g. showing
a decay pattern after an initial perturbation), the
event was long-lasting, with a total duration of ~1
Myr. The timescale of its onset has been estimated at
~20 to 44 kyr (Li
et al.
2008 ; Mehay
et al.
2009 ), and
was most likely slower than the onset of the PETM,
for example. Also, the substantial decline in nanoco-
nid abundance (calcareous nanoplankton, proposed
as an indicator of the calcii cation crisis) had already
started ~1 Myr prior to the onset of the event (Erba
and Tremolada 2004 ; Mehay
et al.
2009 ). As pointed
out in Section 2.4.5, the ocean carbonate saturation
state is generally well buffered on timescales >10 000
yr, which makes it improbable that effects on calcii -
cation would have lasted over millions of years. This
view is supported by the fact that other heavily cal-
cii ed taxa peaked in abundance precisely during
the interval of minimum nanoconid abundance
(Erba and Tremolada 2004). Some species such as
the coccolithophore
Watznaueria barnesiae
show little
change in abundance during the onset of the event
( Mehay
et al.
2009 ).
2.5.2
End-Permian and K/T boundary
An excellent summary of the end-Permian mass
extinction (~252 Myr) is provided by Knoll and
Fischer in Chapter 4 and shall not be repeated here
(see also Knoll 2003). The Cretaceous-Tertiary
boundary (~65 Myr) represents another possible
ocean acidii cation event, probably involving
sulphur compounds which acidii ed the surface
ocean. Undoubtedly, other dramatic perturbations
