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