Revisiting diagenesis on the Ontong Java Plateau: Evidence for authigenic crust precipitation in Globorotalia tumida

Abstract

The calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analyzed to generate trace element and isotope paleoproxy records. These sediments constitute a distinct physio-chemical environment from the conditions in which the tests formed. Storage in sediments can modify the trace element and isotopic content of foraminiferal calcite through diagenetic alteration, which has the potential to confound their paleoceanographic interpretation. A previous study of Globorotalia tumida from the Ontong Java Plateau, western equatorial Pacific, found that preferential dissolution of higher-Mg chamber calcite and the preservation of a low-Mg crust on the tests significantly reduced whole-test Mg/Ca and Sr/Ca. Here we revisit specimens with a combination of synchrotron X-ray computed tomography (sXCT) and electron probe microanalyses to reevaluate the nature of their diagenetic alteration. The dissolution of higher-Mg calcite with depth was directly observed in the sXCT data, confirming the inference of the previous study. The sXCT data further reveal a thickening of the chemically and structurally distinct calcite crust with depth. We propose that these crusts have a diagenetic origin, driven by the simultaneous dissolution of high-Mg chamber calcite and precipitation of low-Mg crust from the resulting modified pore water solution. While the breadth of the study is limited by the nature of the techniques, the observation of both dissolution and reprecipitation of foraminiferal calcite serves to demonstrate the action of two simultaneous diagenetic alteration processes, with significant impacts on the resulting paleoproxy signals.

Magnesian calcite solid solution thermodynamics inferred from authigenic deep-sea carbonate

“…We use our measured equilibrium K d Mg values, along with literature values obtained from recrystallized abiogenic calcite (Katz, 1973;Kozdon et al, 2013;Branson et al, 2015) and from calcite growth experiments extrapolated to near-equilibrium (Mavromatis et al, 2013), to fit a subregular solution thermodynamic model for the Tdependent solid activity coefficients for magnesite in magnesian calcite (Bénézeth et al, 2011) down to 2°C (Figure 2a). A plot of the calculated W 21 interaction parameter values as a function of T is given in Figure 2b for…”

“…Magnesium partitioning behavior determined from magnesian calcite synthesis (Katz, 1973;Berner, 1975;Mucci and Morse, 1983) and solubility (Chave et al, 1962;Mucci and Morse, 1984;Walter, 1984; Busenberg and Plummer, 1989) experiments have frequently been assumed to reflect equilibrium partitioning behavior (Lea et al, 1999;Kozdon et al, 2013;Branson et al, 2015), although several studies have underscored the influence of kinetics (e.g., (Berner, 1975;De Choudens-Sánchez and González, 2009). These experiments typically give Mg distribution coefficients (K d Mg )-the concentration ratio of Mg/Ca in the solid normalized by that of the fluid-in the range of K d Mg ≈ 0.01-0.05 at 25°C.…”

“…These experiments typically give Mg distribution coefficients (K d Mg )-the concentration ratio of Mg/Ca in the solid normalized by that of the fluid-in the range of K d Mg ≈ 0.01-0.05 at 25°C. In contrast, studies of marine carbonate diagenesis suggest that low-Mg calcite persists at the expense of higher Mg calcite in core-top foraminifer tests (Hover et al, 2001), and that recrystallized calcite has a very low Mg content corresponding to K d Mg < 0.001 (Baker et al, 1982;Higgins and Schrag, 2012;Kozdon et al, 2013;Branson et al, 2015). This large discrepancy in K d Mg has been explained in terms of a reaction with a separate, lower Mg/Ca fluid in the marine carbonate sediment during diagenesis that is isolated from the bulk pore fluid, although this explanation conflicts with measured Sr distribution data (Kozdon et al, 2013;Branson et al, 2015).…”

Relationships Between Temperature, pH, and Crusting on Mg/Ca Ratios in Laboratory‐Grown Neogloboquadrina Foraminifera

“…Encrusted fossil foraminifera are characterized by a thicker shell, a distinct euhedral texture, low δ 18 O, and a relatively low-Mg/Ca ratio (Arikawa, 1983;Bolton & Marr, 2013;Kozdon, Ushikubo, et al, 2009;Sadekov et al, 2005). Analyses of core top and down core neogloboquadrids have been unable to resolve whether the low-Mg/Ca ratio of crust calcite is the result of (1) precipitation in deeper, and hence colder, more CO 2 -rich waters than ontogenetic calcite; (2) differing Mg/Ca:temperature sensitivities between the two types of calcite (Bolton & Marr, 2013;de Nooijer et al, 2014;Hathorne et al, 2003;Steinhardt et al, 2015); (3) diagenesis (Branson et al, 2015); or (4) a combination of factors. Recent evidence argues against a temperature trigger for the formation of crust calcite (Jonkers et al, 2016), due to the presence of a low-Mg/Ca ratio crust in shells grown in a deep mixed layer of uniform temperature.…”

Selective Preservation of Coccolith Calcite in Ontong‐Java Plateau Sediments

“…Although foraminiferal Mg/Ca content does not scale with habitation depth (Figure , Rebotim et al, ), Mg content does appear to scale with calcification depth (Anand et al, ; Bender et al, ) and therefore could be related to calcification rate (Lammers & Mitnick, ). However, authigenic calcites precipitated in equilibrium with seawater appear to contain more Mg than foraminiferal tests (Branson et al, ; Lammers & Mitnick, ), and it is therefore unlikely that Mg incorporation alone leads to less stable calcite. Alternatively, concentration of minor and trace elements has been linked to the primary organic membrane and the formation of amorphous calcium carbonate (Branson et al, ; de Nooijer et al, ; Politi et al, ; Wang et al, ).…”