Trace elements in marine biogenic carbonates: analysis and application to past ocean chemistry

Abstract

Trace elements in marine biogenic carbonates may be used as proxies for past ocean
chemistry provided that there is an established relationship between the trace element
proxy and a parameter of interest, this relationship is preserved within biogenic
carbonate, and the trace element can be determined sufficiently accurately. Successful
application of any trace element proxy requires both development of the analytical
methodology to ensure accurate data with the necessary sensitivity, and an
understanding of the relationship between proxy and seawater chemistry.
Herein I develop methods for the determination of Mg/Ca, Sr/Ca and Cd/Ca in
planktonic foraminiferal calcite, using inductively coupled plasma optical emission
spectrophotometry and isotope dilution thermal ionisation mass spectrometry, and
propose a potential reference material for Mg/Ca in foraminiferal calcite. The developed
techniques are applied to an investigation of the Mg/Ca temperature proxy over
Chatham Rise in the Southwest Pacific Ocean and a calibration study of the partition
coefficient, DCd, for cadmium incorporation into planktonic foraminifera.
Comparisons of planktonic foraminiferal Mg/Ca, shell weight and oxygen
isotope records from sites north and south of the Subtropical Front on Chatham Rise,
demonstrate the effects of hydrography, foraminiferal habitat and dissolution as controls
on Mg/Ca. Determinations of Cd/Ca in seven species of planktonic foraminifera
confirm that the dominant controls on Cd/Ca are foraminiferal habitat and hydrography,
with only a minor influence of post depositional dissolution. The major uncertainty in
determination of DCd from core top samples comes from uncertainty in estimation of the
depth distribution and seasons of calcification of planktonic foraminifera.

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