1. Methodology
 Samples were recovered from ODP Hole 757B located on the Ninety East
Ridge in the Indian Ocean during ODP Leg 121 (1988).
 Ostracods of the genus Krithe were identified under a microscope and
removed from samples (Fig. 1).
 Valves were weighed and transferred to 0.5 mL microcentrifuge tubes.
 Valves underwent chemical cleaning to remove clays, metal oxides and
organic matter (method modified from Boyle, 1981).
 Samples were then analysed for trace metals using inductively coupled
plasma mass spectrometry (ICP-MS) .
Earth's Greenhouse-Icehouse Transition Across the Eocene/ Oligocene Boundary
J.S. Crowe1 and H.K. Coxall2
1. School of Earth and Ocean Science, Cardiff University
2. Department of Geological Sciences, Stockholm University
Background
 The earliest indication of Eastern Antarctic glaciation at sea level occurs
at c. 45.5 Ma during the Eocene Epoch.
 By c. 36 Ma glaciation had significantly strengthened with the creation of
the Eastern Antarctic ice sheet.
 The Eastern Antarctic continent was entirely buried under ice by the
onset of the Oligocene at 33.9 Ma (Ehrmann and Mackensen, 1992)
 The Mg/Ca ratio found in shells of the deep-sea ostracod genus Krithe
has been used here to infer bottom water temperature (BWT).
 Ostracods are a type of bi-valved crustacean.
 Their shells are formed of calcite (CaCO3) and contain co-precipitated
magnesium.
 Concentration of magnesium with respect to calcium is known to be
temperature dependant (Burton and Walter, 1991).
Results Conclusions
 Average valve weight significantly decreases from 19.5 to 15.5 µg the
late Eocene to the early Oligocene (fig. 2).
 Magnesium calcium ratios found within ostracods from ODP Hole 757B
show a significant positive trend across the Eocene/ Oligocene (E/O)
boundary (Fig.3).
 These ratios correspond to a 1.5°C increase in bottom water temperature
between 30 and 36 million years (Fig.3).
 Increases in magnesium calcium ratios occur simultaneously with a
distinct rise δ18O ‰ (Fig. 2-3).
 Bottom water temperatures found here are outside the range of the
commonly used calibrations (Cronin et al., 1996., 2005., Dwyer et
al.,1995., 2002.)
References & Acknowledgments
Boyle, E.A., 1981. Earth Planet. Sci. Lett., 53, 11–35. Burton, E. A. & Walter, L.M., 1991. Geochim. Cosmochim. Acta. 55,777.
Cronin, T.M., et al., 1996. Geol. Soc. Spec. Publ.111,117-134 Cronin, T.M. et al., 2005. Marine Micropaleontology. 54, 249-261.
Dwyer, G.S., et al., 1995. Science. 270 (5240), 1347-1351. Dwyer, G.S, et al, 2002. Geophys. Monogr.Ser. 131, 205-225.
Ehrmann, W.U., et al., 1992. Palaeogeo, Palaeoclim, Palaeoecol. 93, 85 Elmore, A.C., et al., 2012. Geochem, Geiphys. Geosys. 13(9).
Many thanks: to: CUROP, C.H. Lear, E.M. Mawbey and A. Morte-Ródenas
Figure 1. A light microscope image of a selected ostracod.
Questions for Further Research
 Are bottom water temperatures calibrated using Dwyer et al., 1995
accurate over this temperature range?
 Does a ∆[CO3
2-]- corrected calibration (designed for low temperatures)
yield more accurate bottom water temperature estimates (Elmore et al.,
2012)?
 Do bottom water temperatures calculated using Mg/Ca or Sr/Ca ratios in
benthic foraminifera correlate with those found using ostracods?
 Why do Mg/Ca ratios increase when a cooling effect linked to glaciation
is expected?
 Is the increase in Mg/Ca ratio significant in regard to a reduction in valve
weight?
Figure 2. Valve weight per ostracod across the E/O boundary.
Figure 3. Mg/Ca ratios and calibrated BWT across the E/O boundary.
Figure 4. Oxygen isotope record across the E/O boundary.