Figure 3: Distribution of D. altispira Mg/Ca values from LA-ICP-MS profiles of the 1551-1554 m sample. (A) A summary of all Mg/Ca values, where open circles denote individual measurements, and filled circles denote mean Mg/Ca values for each specimen. The horizontal black line is the mean of all depth profiles from the sample, and the gray bar the ±2 SE sample uncertainty. (B) The evolution of the sample 2 SE with increasing specimens. Only profiles that passed data screening are included (n=72). Data is provided in Supplementary Table S5.
2.7 Mg/Ca paleo-sea surface temperature calculations
The influence of calcification temperature (T) on the Mg/Ca ratio of foraminiferal calcite can be explained by an exponential curve of general form Mg/Ca = BexpAT where the pre-exponential constant (B) and exponential constant (A) are species specific (Anand et al. , 2003; Lear et al. , 2002; Nürnberg et al. , 1996; Rosenthal et al. , 1997). To convert raw Mg/Ca ratios to absolute temperatures, several secondary controls on Mg/Ca must be considered, and accounted for (Gray et al. , 2018; Hollis et al. , 2019; Holland et al., 2020).
In this study we use Mg/Ca values from D. altispira , a near surface dweller present from the Oligocene to the Pliocene. Since this is not an extant species, we consider two approaches to calculating SST: (i) using the multi-species calibration equation from Anand et al. (2003) and (ii) using the Globigerinoides ruber Mg/Ca-SST equation and pH correction from Evans et al. 2016.
In scenario (i), we apply a compilation of nine modern planktic foraminifera (Anand et al., 2003). This calibration is commonly applied to extinct planktic foraminiferal species such as D. altispira and applies a power law relationship, where H is a constant that describes the sensitivity of Mg/CaCALCITE to seawater Mg/Ca (Mg/CaSW) (Hasiuk and Lohmann,2010; Cramer et al., 2011; Evans and Müller , 2012) (Equation 1).
\begin{equation} \frac{\mathbf{Equation\ 1:\ \ \ Mg}}{\mathbf{\text{Ca}}}\mathbf{=}\frac{\mathbf{B}}{\frac{\mathbf{\text{Mg}}}{\mathbf{\text{Ca}}}_{\mathbf{\text{SW}}}^{\mathbf{t=0}^{\mathbf{H}}}}\mathbf{\text{\ x\ }}\frac{\mathbf{\text{Mg}}}{\mathbf{\text{Ca}}}_{\mathbf{\text{SW}}}^{\mathbf{t=t}^{\mathbf{H}}}\mathbf{\text{ex}}\mathbf{p}^{\mathbf{\text{AT}}}\nonumber \\ \end{equation}
Fluxes of Mg2+ and Ca2+ into and out of the oceans leads to secular variation in Mg/Casw. This variability must be accounted for when determining absolute sea surface temperatures on Cenozoic timescales (Hollis et al. , 2019). Reconstructions of Mg/Casw based on large benthic foraminifera (Evans et al. , 2018), calcite veins (Coggon et al. , 2010), fluid inclusions (Horita et al. , 2002), and echinoderms (Dickson , 2002) have constrained this variability through the Cenozoic (Supplementary Figure S4). The Eocene-Oligocene demonstrates relatively stable values of 2.0-2.5 mol/mol (Coggon et al. , 2010; Evans et al. , 2018). However, only one data point exists from the Miocene, through which Mg/Casw more than doubles from ~2.2 mol/mol in the late Oligocene (Coggon et al. , 2010) to the well constrained value of 5.2 mol/mol in the modern ocean (Broecker et al. , 1982;Dickson , 2002; Horita et al. , 2002; Kısakürek et al. , 2008). Therefore, the method of Lear et al. (2015) is followed by fitting the fourth-order polynomial curve fit through the compiled Mg/Casw proxy records (Supplementary Figure S4). We use a ±0.5 mol/mol uncertainty window in the following temperature calculations, this error envelope incorporating the majority of the spread in the proxy data.
The power law function negates the assumption that the temperature sensitivity remains constant, independent of changing Mg/CaSW through the Cenozoic era. We apply a power law constant of H=0.41, similar to the value applied for T. trilobus , a symbiont-bearing, mixed layer dweller (Delaney et al., 1985;Evans and Müller , 2012). Adapting Equation 1 to include our H value, a modern-day Mg/Casw value of 5.2 mol/mol, and the calibration constants of Anand et al., (2003) derives Equation (2).
\begin{equation} \mathbf{Equation\ 2:\ }\nonumber \\ \end{equation}\begin{equation} \frac{\mathbf{\text{Mg}}}{\mathbf{\text{Ca}}}\mathbf{=}\frac{\mathbf{0.38\pm 0.02}}{\mathbf{5.2}^{\mathbf{0.41}}}\mathbf{\text{\ x\ }}{\mathbf{Mg/Ca}_{\mathbf{\text{sw}}}}^{\mathbf{0.41}}\mathbf{\text{\ ex}}\mathbf{p}^{\mathbf{(0.090\pm 0.003\ x\ SST)}}\nonumber \\ \end{equation}
This first calibration approach assumes that foraminiferal Mg/Ca is not influenced by changes in the carbonate system. However, studies have shown that planktic foraminiferal Mg/Ca is influenced by changes in the carbonate system, the ratio increasing with decreased pH and/or Δ[CO32-] (Evans et al. , 2016; Gray and Evans , 2019; Gray et al. , 2018;Russell et al. , 2004; Yu and Elderfield , 2008). However, the ultimate driver of this effect is not certain and some species are insensitive to changes in the carbonate system. Further, it has been shown that for Orbulina universa dissolved inorganic carbon (DIC) plays a role in test Mg/Ca variability (Holland et al. , 2020). We follow recent results which interpret pH, as opposed to Δ[CO32-] or DIC, as the parameter which controls the carbonate system’s influence on Mg/Ca (Evans et al. , 2016; Gray et al. , 2018). Furthermore, unlike with either DIC or Δ[CO32-], it is possible to reconstruct pH through the Neogene using boron isotopes in foraminifera (Foster and Rae , 2015; Greenop et al. , 2014; Henehan et al. , 2013; Sosdian et al. , 2018). For these reasons we use the recent Neogene boron isotope compilation of Sosdian et al.(2018), which provides well constrained estimates of pH across this time interval (Supplementary Figure S5; Supplementary Table S9). Linear interpolation between these pH values allows us to estimate a mean pH value, and associated uncertainty envelope, for each Sunbird-1 sample, where the uncertainty envelope is maximum and minimum pH at the 17% and 83% confidence interval (~± 0.06 pH units).
Therefore, in addition to scenario (i) we also consider the approach from Evans et al . (2016) which corrects for pH changes using the interpolated Neogene pH record of Sosdian et al. (2018) (Supplementary Figure S5). Measured planktic foraminiferal Mg/Ca values are corrected for this influence of pH using the equation of Evans et al. (2016) (Equation 3).
\begin{equation} \frac{\mathbf{Equation\ 3:\ \ Mg}}{\mathbf{\text{Ca}}_{\mathbf{\text{CORRECTED}}}\mathbf{\ =\ }\frac{\frac{\mathbf{\text{Mg}}}{\mathbf{\text{Ca}}_{\mathbf{\text{MEASURED}}}}}{\frac{\mathbf{0.66}}{\mathbf{1+exp(6.9}\left(\mathbf{pH-8.0}\right)\mathbf{)}}\mathbf{+0.76}}}\nonumber \\ \end{equation}
The preferred equation of Evans et al. (2016) is used to account for the influence of changing Mg/Casw when estimating SST. These authors determined that the best fit to culture-derived calibration lines is when both the pre-exponential (B) and exponential (A) coefficients vary quadratically with Mg/Casw(Equation 4 and 5).
Equation 4: B = (0.019 x Mg/Casw2) – (0.16 x Mg/Casw) + 0.804
Equation 5: A = (-0.0029 x Mg/Casw2) + (0.032 x Mg/Casw)
We substitute these equations into the general exponential calibration, Mg/Ca = BexpAT, to account for changing Mg/Casw. Although the Evans et al. (2016) equation is specific to G. ruber , this species inhabits a shallow water depth of 0-50m (Schiebel and Hemleben , 2017) similar to the inferred mixed-layer habitat depth D. altispira (Aze et al. , 2011). Furthermore, as with G. ruber , D. altispirawas a tropical/subtropical species, with symbionts (Aze et al. , 2011).
Salinity can exert a secondary effect on foraminiferal Mg/Ca, sensitivity measurements from culture and core-top studies show this to be ~3-5% per practical salinity unit (psu) (Gray et al. , 2018; Hollis et al. , 2019; Hönisch et al. , 2013;Kısakürek et al. , 2008). In the absence of a robust, independent salinity proxy (although we do note the promise of Na/Ca (Bertlich et al. , 2018; Geerken et al. , 2018)) and the relatively minor effect of salinity on foraminiferal Mg/Ca, this potential secondary control is not empirically accounted for. Sunbird-1 was located in a coastal setting and likely experienced a highly variable hydrological cycle due to changes in the position of the ITCZ making it susceptible to changes in salinity. Therefore, an error of ± 0.5⁰C is incorporated into the final sea surface temperature estimates, equivalent to an assumed salinity variability of ~± 1 PSU.
Mg/Ca-derived sea surface temperature estimates calculated using both approaches (i) and (ii) yield extremely similar trends (Supplementary Figure S6). Across the time interval of the Sunbird-1 dataset (~13.5 Ma – 9.5 Ma) pH changes by a small amount and thus the choice of approach has little influence on the Sunbird-1 absolute SST record. In our discussion below, we adopt approach (i); the multi-species calibration equation from Anand et al. (2003) without a pH correction. This approach avoids any potential species-specific effects from applying the Evans et al. (2016) calibration specific to G. ruber to the extinct D. altispira used in this study. Furthermore, D. altispira has been considered to be symbiont bearing, so may demonstrate a muted response to changes in pH and insensitivity to pH changes, similar toTrilobatus trilobus (Gray and Evans , 2019).
The uncertainties (± 2SE) associated with the conversion from Mg/Ca to absolute SST estimates incorporate the uncertainty on the Mg/Casw record, and the potential uncertainty due to varying salinity. Additionally, scenario (i) incorporates the uncertainty in the calibration of Anand et al. (2003) (Equation 2), and scenario (ii) using the approach of Evans et al. ( 2016) incorporates the uncertainty in the pH correction. These combined are termed the calibration uncertainty and are considerably greater than the independent analytical uncertainty, which only incorporates the intra- and inter- specimen variability (±2 SE). Absolute sea surface temperature estimates, and associated uncertainties, calculated using approach (i) and (ii) are available in Table 1 and Supplementary Table S9 respectively