Application of calcite Mg partitioning functions to the reconstruction of paleocean Mg/Ca

Neogene ice volume and ocean temperatures: Insights from infaunal foraminiferal Mg/Ca paleothermometry

“…We adjust the 690 Mg/Ca record by −9.1% to account for the different cleaning technique relative to the core top samples [ Yu et al ., ]. Echinoderm Mg/Ca ratios have been used to estimate Mg/Ca sw at 53 Ma as 1.7 mol/mol [ Dickson , ], although subsequent reinterpretation of these data using a power law dependency has revised this to 1.3 mol/mol [ Hasiuk and Lohmann , ]. Our calculations assume that seawater Mg/Ca was invariant over the duration of the Paleocene‐Eocene data set; we compensate for this simplification by using a range of Paleocene‐Eocene Mg/Ca sw of 1.0, 1.3, and 2.5 mol/mol to calculate the constants in equations and (Tables and ).…”

“…Benthic foraminiferal Mg/Ca paleothermometry has shown great promise in deconvolving the temperature and δ 18 O sw signal from foraminiferal δ 18 O records, although it is not without its complications [ Lear et al ., , ; Billups and Schrag , , ; Martin et al ., ; Shevenell et al ., ; Cramer et al ., ; Bohaty et al ., ]. Chiefly, these are as follows: (1) variations in Mg/Ca temperature sensitivity between foraminiferal genera necessitating genus‐ or species‐specific temperature calibrations [ Lear et al ., ], (2) the influence of carbonate saturation state at low saturation [ Elderfield et al ., ], (3) uncertainties in seawater Mg/Ca reconstructions [ Coggon et al ., ], and (4) the relationship between seawater Mg/Ca and foraminiferal calcite Mg/Ca [ Hasiuk and Lohmann , ; Ries , ; Cramer et al ., ; Evans and Müller , ]. The last two complications do not compromise the use of Mg/Ca paleothermometry on short (<1 Myr) timescales, making the proxy ideally suited to examining relative temperature changes over geologically short time intervals [ Lear et al ., ; Elderfield et al ., ; Mawbey and Lear , ].…”

Mg isotope fractionation during calcite precipitation: An experimental study

“…The oceanic Mg/Ca ratio has varied from $1 to 5.2 mol/mol over geological times (Sandberg, 1983;Wilkinson and Algeo, 1989;Hardie, 1996;Horita et al, 1996Horita et al, , 2002Stanley and Hardie, 1998;Zimmermann et al, 2000;Lowenstein et al, 2001Lowenstein et al, , 2003Dickson, 2002;Coggon et al, 2010;Hasiuk and Lohmann, 2010;Broecker and Yu, 2011). These variations are recorded in fluid inclusions in marine halite Kovalevich et al, 1998;Zimmermann, 2000;Lowenstein et al, 2001Lowenstein et al, , 2003, and by the mineralogy of evaporitic minerals (Hardie, 1996;Holland et al, 1996;Horita et al, 2002;Ries, 2010) and of marine carbonates ("aragonite" sea vs "calcite" sea) (Wilkinson, 1979;Sandberg, 1983;Mackenzie et al, 1986;Stanley and Hardie, 1998;Dickson, 2002;Ries et al, 2008).…”

Micro‐ to nanostructure and geochemistry of extant crinoidal echinoderm skeletons

“…Dickson (2002Dickson ( , 2004) used a single fractionation coefficient (DcMg = 0.03757), derived from modern tropical echinoids, to reconstruct palaeo-oceanic Mg/Ca from the Mg/Ca ratio of fossil crinoid and echinoid ossicles and concluded that fossil echinoderms 'provide an excellent seawater archive'. More recently, Hasiuk & Lohmann (2010) argued that a power-law partition function is better applied to (palaeo) oceanic Mg/Ca reconstruction and that fossil echinoderms 'serve as ideal records of Phanerozoic Mg/Ca'. More recently, Hasiuk & Lohmann (2010) argued that a power-law partition function is better applied to (palaeo) oceanic Mg/Ca reconstruction and that fossil echinoderms 'serve as ideal records of Phanerozoic Mg/Ca'.…”