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C323: Deconvolving the Cainozoic δ18O signal of foraminiferal calcite using Mg/Ca and clumped isotopes (Lead Supervisor: David Hodell, Earth Sciences)

Supervisor: David Hodell (Earth Sciences)

Importance of the area of research:

One of the shortcomings of oxygen isotope palaeothermometry is the d18O of calcite is not only dependent upon temperature but also the d18O of seawater. Mg/Ca of foraminifera is also a palaeotemperature proxy and when combined with d18O can be used to separate the temperature and d18Oseawater signal (Lear et al., 2000).  However, this method is not foolproof because foraminiferal Mg/Ca can be affected by dissolution (carbonate ion effect), Fe-Mn oxide coatings, and changes in the Mg/Ca of seawater on long time scales.  More recently, it has been shown that the formation of 13C-18O bonds (i.e., “clumped isotopes”) in carbonate minerals is a function of temperature, and is independent of the δ18O of the ambient water (Eiler 2011).  At Cambridge, we have developed a new triple approach to the problem by measuring d18Ocalcite, Mg/Ca and clumped isotopes simultaneously on the same specimens of foraminifera. This method provides redundancy with two independent estimates of both temperature and the d18Oseawater.  

Project summary:

This project will apply the triple approach (d18Ocalcite, Mg/Ca and clumped isotopes) to deconvolve the temperature and d18Oseawater signal from the Cenozoic d18O record.  Variations in the d18Oseawater are related to salinity that changes with continental ice volume. There is also considerable uncertainty in the magnitude of secular changes in the Mg/Ca of seawater during the Cainozoic (Broecker and Yu, 2011). The triple approach also has the potential to assess variation in the Mg/Ca of seawater through time if the Mg/Ca-temperature sensitivity has remained constant.  Sample preservation will be evaluated using scanning electron microscopy and strontium-calcium ratios.

What the student will do:

The student will process sediment samples, pick foraminfera, evaluate diagenetic alteration, pretreat and clean samples, and simultaneously measure the d18Ocalcite, Mg/Ca and clumped isotopes of foraminiferal calcite.  He/she will test if the Mg/Ca versus D47 follows the exponential curve predicted from modern temperature calibrations, which has been verified using core-top samples. Outliers will indicate a problem with either Mg/Ca or D47, or both.  Additional tests may be conducted to evaluate outliers.  For example, reductive cleaning may be necessary for some samples. Sr/Ca and SEM will be used to carefully screen samples for diagenesis and dissolution. There is also the opportunity for an ancillary project using clumped isotopes to study differential recrystallization rates of bulk sediment and foraminfera. The redundancy inherent in the triple approach constitutes a rigorous test of individual methods and has widespread applicability for interpreting the temperature and d18Oseawater signals during the Cainozoic.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Broecker, W., and J. Yu. What do we know about the evolution of Mg to Ca ratios in seawater?, Paleoceanography, 26, PA3203, doi:10.1029/2011PA002120 (2011).

Eiler, J.M., Paleoclimate reconstruction using carbonate clumped isotope thermometry. Quaternary Science Reviews, 30, 3575–3588 (2011).

Lear, C.H., Elderfield, H., and Wilson P.A., Cenozoic deep-sea temperatures and global Ice volumes from Mg/Ca in benthic foraminiferal calcite.  Science 287, 269-272 (2000).

Follow this link to find out about applying for this project

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