Does Carbonate Ion Influence Foraminiferal Mg/Ca? Ed Boyle Department of Earth, Atm., and Planetary Sciences Massachusetts Institute of Technology Cambridge MA Jonathan Erez Hebrew University Jerusalem, Israel

Summary A dataset of Mg/Ca in benthic foraminifera (Cibicidoides) shows a strong correlation between Mg/Ca and carbonate ion concentration. ? How can this be? Isn’t foraminiferal Mg/Ca supposed to be a temperature tracer? It turns out that throughout the ocean, temperature and [CO 3 = ] are highly correlated. A dataset that shows a strong correlation with one of these properties will show a strong correlation with the other property. Is Mg/Ca controlled by temperature or [CO 3 = ] (or both) ?

Preface We didn’t set out to investigate foraminiferal Mg- [CO 3 = ] relationships; we were trying to test the hypothesized (and observed in laboratory studies) relationship between foraminiferal [SO 4 = ] and [CO 3 = ]. BUT: laboratory study and microchemical analysis show that the first stage of calcification involves laying down a layer of high Mg calcite, which is also high in [SO 4 = ]. SO: in order to test the response of foraminiferal [SO 4 = ] to [CO 3 = ], we have to make a correction for foraminiferal Mg. We aimed to test these ideas on a core top database of benthic foraminiferal Mg/Ca and S/Ca compared to water column [CO 3 = ]. And as all good geochemists would, we plotted everything versus everything else (easy to do when you only have three properties).

Core Sites

Wait a minute - isn’t Mg/Ca a paleotemperature indicator?

GEOSECS Global [CO 3 = ] - T data [CO 3 = ] and T are correlated on a global scale…

[CO 3 = ] and T are correlated on a global scale - WHY?? Accidental - Because of CO 2 depletion in shallow waters and CO 2 enrichment of deep waters, shallow waters have higher [CO 3 = ] and deeper water have lower [CO 3 = ]; “coincidentally”, shallow waters are warm and deeper waters are cold. [CO 3 = ] of a surface water sample in equilibrium with the atmosphere is partly determined by temperature because CO 2 is more soluble in cold waters than surface waters (hence making colder surface waters more acidic and lower in [CO 3 = ] than warm waters). Biogeochemical processes and ocean circulation patterns that lead to higher dissolved inorganic carbon in older waters also lead to coincidentally colder temperatures in some of the older waters. Carbon system apparent thermodynamic constants are functions of temperature.

Equilibration of surface waters with atmospheric CO 2 as temperature changes: 25°C surface water in equilibrium with pre- anthropogenic atmospheric CO 2 : p CO 2 = 280 TCO 2 = 1918 Alk = 2290 CO 3 = = 274 cool to 15°C, re-equilibrate with atmosphere: p CO 2 = 280 TCO 2 = 2000 Alk = 2290 CO 3 = = 211 This factor results in a strong correlation between carbonate ion and temperature in surface waters. It also will similarly affect experimental results where foraminifera are cultured at different temperatures in equilibrium with the atmosphere.

Given that [CO 3 = ] and T are correlated, core top calibrations don’t distinguish between one factor or the other as the true controlling property (unless the core tops are chosen to maximize the separation of variables).

Let’s not just pick on Mg/Ca temperature estimations…. Any property used to estimate paleo- temperatures that is based on correlation rather than a well defined mechanism could just as well be controlled by [CO 3 = ] as by temperature: e.g. foraminiferal paleoecological transfer functions; alkenones, etc.

And there is some precedent here: For many years, it was believed that the distribution of reef-building corals was controlled by temperature, with a lower bound T of 18°C (Bermuda winter water). More recently, Buddemaier et al. have made a convincing case that carbonate ion is the true controlling factor for coral reef growth.

Is there any evidence against [CO 3 = ] control of foraminiferal Mg? … a little - but it probably isn’t conclusive: Lea, Mashiotta, & Spero (1999) planktonic culture experiments Russell et al. (in press) planktonic culture experiments Glacial/Interglacial planktonic Mg/Ca (tropical planktonic Mg/Ca goes down, but surface [CO 3 = ] went up)

Do Carbonate Ion and/or Temperature Influence Benthic Foraminiferal Mg/Ca? Based on core top correlations alone, it is difficult to tell the difference between T control and [CO 3 = ] control. A little bit of planktonic culture data does not support [CO 3 = ] control, but it is not conclusive (few data; benthic vs. planktonic). Even if it proves that T is the dominant control on Mg/Ca, it would be important to know if [CO 3 = ] is a partial influence.