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Alegret, Ortiz & Kaminski (eds.), 2012. Ninth International Workshop on Agglutinated Foraminifera, Abstract Volume
Agglutinated benthic foraminifera during ocean acidification: what holds them
together?
Ellen THOMAS 1,2
1
Department of Geology and Geophysics, Yale University, New Haven CT, USA
2
Department of Earth and Environmental Sciences, Wesleyan University, Middletown CT, USA.
e-mail: ellen.thomas@yale.edu
Deep-sea benthic foraminifera suffered rapid, severe extinction at the beginning of the
Paleocene-Eocene Thermal Maximum (PETM), a period of extreme global warming. Such
extinctions are rare, because the habitat of deep-sea benthic foraminifera is very large and
their motile propagules enables them to re-migrate quickly from refugia after local-regional
disappearance (e.g., Thomas, 2007). The cause(s) of the PETM extinction thus must have
been global. Warming, de-oxygenation, ocean acidification, changes in export productivity or
some combination of these have all been implicated (Thomas, 1998; Alegret et al., 2010;
Speijer et al., 2012; Winguth et al., 2012). Acidification, however, might have been a more
important driver of the extinction than thought until recently.
First, ostracods (metazoans which share deep-sea environments with benthic
foraminifera) did not suffer major extinction during the PETM (Webb et al., 2009). Active
metazoans are more sensitive than protists to oxygen depletion, but marine ectothermic
metazoans with an extensive extracellular fluid volume may be less sensitive to ocean
acidification than protists (Melzner et al., 2009). This different sensitivity to acidification thus
might explain the different response of foraminifera and ostracods to the PETM.
Second, at the rates of acidification estimated for the PETM, the effects of
acidification are modeled as less severe at neritic than at bathyal-abyssal depths (Hoenisch et
al., 2012), and the benthic extinction was much less severe at neritic than at bathyal-abyssal
depths (Alegret and Ortiz, 2006; Speijer et al., 2012).
Third, with PETM ocean acidification, the lysocline and calcium carbonate
compensation depth (CCD) in the oceans moved upwards. At depths below the lysocline (i.e.,
in a large part of the world’s oceans) calcite saturation in pore waters may have risen as the
result of carbonate sediment dissolution. Under such conditions, infaunal species calcifying
from pore waters will experience less undersaturation than epifaunal species calcifying from
bottom waters, and thus be under less stress (e.g., Brown et al., 2011). This difference in
saturation of bottom and pore waters thus could explain the increase in relative abundance of
infaunal taxa during the PETM observed at many sites. An increase in relative abundance of
infaunal taxa during ocean acidification thus may be a proxy for acidification, and not for
decreased bottom water oxygenation and/or increased food supply, in contrast to accepted use
(e.g., Thomas, 2007).
We can test the hypothesis that acidification was an important cause of deep-sea
extinction by studying extinction of agglutinated foraminifera during the PETM: obligatory
calcite-cemented or calcite-particle using agglutinated foraminifera would be expected to
suffer extinction at rates similar to those of calcareous foraminifera, whereas organicagglutinated
taxa would be expected to have preferentially survived. It has indeed been
argued that the Deep-Water Agglutinated Foraminiferal assemblages (DWAF) commonly
seen in flysch-settings, at least some of which occurred below the carbonate compensation
depth and did not use calcite in test construction, suffered less severe extinction at the PETM
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