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Supporting Online Material
Figs. S1 to S9
1 September 2010; accepted 4 March 2011
Published online 24 March 2011;
Latitudinal Gradients in Greenhouse
Seawater d18O: Evidence from
Eocene Sirenian Tooth Enamel
Mark T. Clementz1,2 and Jacob O. Sewall3*
The Eocene greenhouse climate state has been linked to a more vigorous hydrologic cycle at
mid- and high latitudes; similar information on precipitation levels at low latitudes is, however,
limited. Oxygen isotopic fluxes track moisture fluxes and, thus, the d18O values of ocean surface
waters can provide insight into hydrologic cycle changes. The offset between tropical d18O values
from sampled Eocene sirenian tooth enamel and modern surface waters is greater than the
expected 1.0 per mil increase due to increased continental ice volume. This increased offset could
result from suppression of surface-water d18O values by a tropical, annual moisture balance
substantially wetter than that of today. Results from an atmospheric general circulation model
support this interpretation and suggest that Eocene low latitudes were extremely wet.
The Eocene epoch represents an extremely warm interval in Earth’s climate history. Greenhouse gas concentrations were up to
five times as high as present-day levels (1, 2), and
annual global temperatures during the Early Eocene
Climatic Optimum (EECO) at ~50 million years
ago (Ma) were as much as 12°C higher than modern values (3–5). From this temperature peak, conditions gradually declined with only a single
prolonged interruption in this trend by a respite
to warmer conditions during the Middle Eocene
Climate Optimum (MECO) (4, 6). The rate of global temperature deterioration increased markedly
at the end of the Eocene as the volume of continental ice on Antarctica rapidly increased. This
event defines the Eocene-Oligocene boundary
and heralded a major shift in Earth’s climate state
from greenhouse to icehouse conditions.
The warmer temperatures of the Eocene may
have promoted a more vigorous hydrologic cycle,
where high global temperatures would have led to
an increase in the concentration of water vapor in
Earth’s atmosphere through intensified subtropical
evaporation (7–9). Atmospheric circulation pat-
1Department of Geology and Geophysics, University of Wyoming,
Laramie, WY 82071, USA. 2Program in Ecology, University of
Wyoming, Laramie, WY 82071, USA. 3Department of Physical
Sciences, Kutztown University, Kutztown, PA 19530, USA.
*To whom correspondence should be addressed. E-mail:
terns and a reduced latitudinal temperature gradient [e.g., (10)] would have limited rainout along
a longitudinal trajectory and facilitated the meridional transport of water and, thence, latent heat to the
poles, thus producing a warmer and more humid
early Paleogene climate state. Current understanding
of atmospheric water vapor content in the Eocene
has primarily focused on specific time intervals
(e.g., Eocene hyperthermal events) and regions
(mid- and high latitudes) (9, 11–13), and evidence
that these wetter, more humid conditions extended to lower latitudes or throughout the entire
Eocene is so far lacking. In addition to characterizing the tropical greenhouse climate, knowledge
of the Eocene hydrologic cycle is critical to accurately calculating paleotemperatures from planktonic foraminifera, as changes in evaporation and
precipitation will influence the oxygen isotopic
composition of seawater and, thence, that of foraminiferal tests.
One means of acquiring more information on
the Paleogene hydrologic cycle comes from the
examination of latitudinal changes in the oxygen
isotopic composition of ocean surface waters
(d18Osw; <100 m depth). A stronger and more
immediate connection between marine surface
waters and the atmosphere heightens the sensi-
tivity of this oceanic layer to changes in regional
hydrology (i.e., precipitation and evaporation)
(14, 15). Today, meridional gradients in d18Osw are
a product of latitudinal differences in evaporation
and precipitation and the isotopic fractionation
of water vapor by Rayleigh distillation during
transport in the atmosphere (16 ). The net result
is that whereas ocean bottom waters (>1000 m
depth) maintain salinities and d18O values that
are more or less uniform globally, d18Osw values
(Fig. 1A) and surface-water salinities (Fig. 1B) can
vary significantly with latitude (17). Enhanced
evaporation at lower latitudes leads to higher sa-
linities and d18Osw values (as much as 2.0‰ greater
than bottom waters). In contrast, reduced evapora-
tion and higher precipitation levels at high lati-
tudes result in lower salinities and d18Osw values
(more than −2.0‰ lower than those for bottom
waters). Thus, one might expect that intensifi-
cation of the hydrologic cycle during the Eocene
would affect the magnitude of this meridional gra-
dient in d18Osw values and should be recorded in
marine isotope records.