Single-specimen isotope temperatures
The average single-specimen δ18Occ of G. ruber reflects SSTs of
27.0 ± 2.2–28.4 ± 2.1 ∘C (based on
sediment trap calibrations from Fallet et al., 2010, and Wilke et al., 2009,
respectively), which is close to the satellite-derived annual mean SST of
27.6 ∘C (Fallet et al., 2010). When applying the equation of Kim
and O'Neil (1997) for conversion of δ18Occ into
temperature, SST is considerably higher (29.4 ± 1.3 ∘C). This
discrepancy may be caused by the fact that the calcite–water calibration of
Kim and O'Neil (1997) is based on inorganic precipitation experiments free
of vital effects and therefore may be offset compared to the
temperature–δ18Occ relationship of biogenic carbonates.
Nevertheless this temperature estimate based on Kim and O'Neil (1997) is in
good agreement with the average temperature of 28 ± 1.1 ∘C
during the investigated intervals. The inter-test variability of this species
can be explained by the high temperature variability at the sea surface, as
well as differences in symbiont activity. The shallow depth habitat of G. ruber in
the MC is in line with previous studies showing that this species is
confined to the photic zone (e.g. Deuser et al., 1981; Lončarić et
al., 2006; Peeters and Brummer, 2002) due to the light requirement of its
symbionts. Based on its relatively narrow preferred depth habitat, this
species is a suitable tracer for (sub)tropical surface-water (0–100 m, mixed layer) conditions (e.g. Deuser, 1987; Anand et al., 2003;
Field, 2004; Fallet et al., 2010). Birch et al. (2013) show that shell size
of specimens of G. ruber is not correlated with δ18Occ, confirming that
this species occupies a narrow calcification depth during its life. In
addition to its shallow living depth, G. ruber is known to occur in some areas
relatively equally throughout the year (e.g. Deuser, 1987; Mohtadi et al.,
2006; Tedesco et al., 2007), whereas in other areas, including the MC, it
occurs at highest densities during summer months (e.g. Tolderlund and Bé,
1971; Duplessy et al., 1981; Ganssen and Sarnthein, 1983; Deuser and Ross,
1989; Eguchi et al., 2003; Lončarić et al., 2006; Fallet et al.,
2010). This seasonal preference results in SSTs that are slightly biased
towards summer conditions when using fossil specimens of this species.
Based on an average δ18Occ-derived temperature of 24.3 ± 2 ∘C (Table 2), following the equation of Kim and
O'Neil (1997), calcification depths of N. dutertrei are in the range of 20–130 m (Fig. 6), with an average depth of 58 m. For eddy conditions, the average calcification
depth is approximately 80 m; for non-eddy conditions it is approximately 37 m. Average Mg / Ca-based temperature of 22.5 ± 4 ∘C is in
relatively good agreement with the average δ18Occ-derived
temperature (Table 2). The difference between Mg / Ca- and δ18O-based temperatures are smaller than the 1.2 ∘C
uncertainty associated with the Mg / Ca calibration (Anand et al., 2003).
Previous studies using N. dutertrei from Indian Ocean core-top samples and Mozambique
Channel sediment traps have reported similar depth ranges between 40 and 150 m
(Kiefer et al., 2006) and similar average depths of 80 m (Fallet et al.,
2011), respectively. Both of these studies used pooled specimens for their
stable isotope analysis and hence provided the population's average
calcification depth. Moreover, pooling of specimens from sediment core
samples (Kiefer et al., 2006) does not allow for resolving short-term
variability in calcification temperatures as do single specimens (e.g.
seasonality). The inferred calcification depth for N. dutertrei is in line with its
characterization as an intermediate deep-dwelling species, living
preferentially in the seasonal thermocline (e.g. Fairbanks et al., 1982;
Curry et al., 1983; Eguchi et al., 2003; Farmer et al., 2007), coinciding
with a deep chlorophyll maximum (Fairbanks et al., 1980; Ravelo et al.,
1990). Overall, the living depth of this species is confined to the upper 200 m
(Farmer et al., 2007; Kroon and Darling, 1995). Variability in Mg / Ca within
single-specimen shell walls of N. dutertrei from the Timor Sea suggested temperatures
between 12 and 23 ∘C, implying migration through the entire
thermocline (Eggins et al., 2003). However, most calcification seems limited
to a much smaller depth interval, and the extremes in Mg / Ca might reflect
upper and lower depth limits occupied by this species. Moreover, banding of
Mg / Ca in shell calcite has been viewed in terms of discrete calcification
events (Elderfield et al., 1996; Erez, 2003). Plankton tow studies
(Fairbanks et al., 1980) showed oxygen isotope equilibrium calcification for
N. dutertrei and P. obliquiloculata.
Apparent calcification depths of species are generally shallower during
non-eddy conditions. Apparent calcification depths for eddy (red) and
non-eddy conditions (blue) calculated from single-specimen δ18Occ using in situ temperature and
δ18Osw.
Calcification depth was determined by matching the measured foraminiferal
δ18Occ with the δ18Oeq, using the
equation of Kim and O'Neil (1997). We used δ18Osw from the
species calcification depth. The right panel is a close-up of
the top left part of the left panel (upper 200 m).
The δ18Occ-based calcification depths for P. obliquiloculata reported
here (48–125 m, with an average of 74 m; Fig. 6) are in close agreement with
those reported previously (e.g. between 60 and 80 m; Mohtadi et al., 2009).
Indeed, in plankton tows from the central equatorial Pacific the largest
abundance of adult P. obliquiloculata with a terminal cortex was found below 60 m (Watkins et
al., 1996). All specimens used in this study had the distinctive smooth
outer cortex that envelops the final whorl in the adult as well as an arched
aperture (Watkins et al., 1996). Non-corticated P. obliquiloculata (“juveniles”) are
confined mostly to the mixed layer (Watkins et al., 1996), indicating
migration to greater depths at the time of cortex formation during the
terminal stage of its life cycle (Erez and Honjo, 1981; Hemleben et al.,
1989; Ravelo and Fairbanks, 1992).
The average δ18Occ for G. scitula yields a calcification temperature
of 10.4 ± 3.9 ∘C, suggesting that this species calcifies
between 290 and 1100 m (Fig. 6), with an average depth of approximately 500 m. This
overlaps with the depth range indicated by the Mg / Ca temperatures of
14.4 ± 3.4 ∘C derived from the last few chambers added,
suggesting that these shells formed at a depth between about 250 and 350 m for non-eddy and eddy conditions, respectively. The δ18Occ-based estimates, however, do not consider possible vital
effects that were previously suggested for this species (e.g. Kahn and
Williams, 1981). If taken into account, this would lower the temperature and
depth habitat estimates by some 4 ∘C and 500 m, respectively.
Birch et al. (2013) support previous findings of a distinct positive
correlation between δ18O and size in G. scitula (e.g. Friedrich et al.,
2012), which is linked to a substantial ontogenetic vertical migration
through the water column. The largest individuals have been inferred to live
below the thermocline, consistent with the supposed absence of symbionts in
this species. This is in line with our observations, showing higher
inter-specimen variability in δ18Occ for G. scitula than in the
other species.
Habitat depth vs. calcification depth
Planktonic foraminifera collected by sediment traps might record δ18Occ signals comprising calcification at various depths and thus
document an apparent average calcification depth by integrating the entire
calcification history of the specimen. Given changes in seawater temperature
with water depth, even minor changes in the upper or lower range of the
depth at which planktonic species calcify can have a profound effect on the
average δ18Occ and reconstructed temperature. Since
evidence is accumulating that some species have a flexible calcification
range (e.g. due to seasonality or local hydrography; Lončarić et
al., 2006; Wilke et al., 2009), interpretation of down-core stable isotope
data in terms of thermal structure may be challenging. Therefore, it is
crucial to accurately quantify the impact of environmental factors on depth
preferences of planktonic foraminifera. Contrasting eddy and non-eddy
conditions, a short-term feature, allow us to disentangle seasonal and other
short-term local hydrography changes and their effect on foraminiferal
calcification depth.
While Mg / Ca-based temperatures of G. ruber and N. dutertrei record eddy-induced changes in upper
water column stratification (Steinhardt et al., 2014), δ18O-based temperatures are relatively similar for both species
(Fig. 6). Using the palaeo-temperature equation (Eq. 1; Kim and
O'Neil, 1997) and fitting δ18Ocalc with δ18Occ, we find that G. ruber calcifies on average at the sea surface
(down to 7 m during non-eddy conditions and down to 18 m under eddy
conditions, Fig. 6). N. dutertrei calcifies on average between 12 and 120 m during eddy
conditions (average calcification depth 81 m) and between 17 and 58 m under
non-eddy conditions (average 37 m). During eddy conditions, P. obliquiloculata calcifies
between 89 and 124 m (average 107 m), whereas it calcifies at shallower
depth, between 20 and 77 m (average calcification depth 60 m) during
non-eddy conditions. The largest changes in calcification depth in this study are
inferred from G. scitula. From a calcification range between 500 and 1100 m and an
average calcification around 716 m during eddy conditions it shifts to a
calcification range from 168 to 745 m and an average calcification depth of
343 m (Fig. 6).
Conversely, δ18O-based temperatures are significantly different
for P. obliquiloculata and G. scitula, while the Mg / Ca-based temperature of the last formed chambers of
P. obliquiloculata indicates similar calcification temperature (Table 1). Mg / Ca-inferred
calcification temperatures, representing the depth occupied at the later
stages of the foraminifer's life, were similar between eddy and non-eddy
conditions. Nonetheless, temperature mooring data show a steep temperature
gradient, coinciding with the habitat depth of G. scitula, and thereby revealing a wide
range of calcification depths for this species, changing significantly with
deepening of the thermocline during eddy passage.
Inferred higher variability in calcification temperature for G. ruber presented in
this study compared to observed satellite SST likely results from the
spatial resolution employed here. Inter-individual differences in depth
migration add to the variability in isotopes and element / Ca ratios when
measuring single specimens. Potential effects of ontogeny on stable isotope
composition are minimized by using narrow size fractions, as confirmed by
the lack of ontogenetic trends with shell size in our measurements. Russell
and Spero (2000) concluded that natural variability in oxygen isotopes is
species-specific. From measurements of single-specimen δ18Occ of G. ruber
shells from sediment traps in the eastern equatorial Pacific, they show that,
over a 1.5–3-day period, the standard deviation of δ18O
results in a temperature variability of ± 0.87 ∘C.
Such variability could explain between 12 and 38 % of the variability in
δ18O-based temperatures in our samples. Another cause of
natural variability might be differences in depth at which an individual
calcifies. In laboratory cultures, the addition rate of new chambers in G. sacculifer
ranges from 1.6 to 6.2 days (Bé and Spero, 1981), while chamber formation in G. hirsuta and
G. truncatulinoides takes about 5 to 6 h (Bé et al., 1979). Considering that our sample
duration ranges between 17 and 21 days, δ18O variability is
likely to be affected by other parameters (e.g. temperature). Therefore, the
observed variability in δ18O-based temperatures caused by
species-specific natural variability in δ18Occ (e.g.
Russell and Spero, 2000) during the time it takes to add new chambers, which
might be calcified under different conditions or water depth.
Reconciling δ18O and Mg / Ca-derived
calcification depths
Mg / Ca-derived temperatures indicate that calcification depths of N. dutertrei range
between 42 and 169 m (average depth: 81 m) under non-eddy conditions and between
13 and 196 m (average depth: 98 m) during eddy conditions (Steinhardt et
al., 2014). Thus, the shoaling in average calcification depth from 98 m
during eddy conditions to 81 m during non-eddy conditions indicated by the
whole-shell δ18Occ is less as than inferred from Mg / Ca,
derived from the calcification of the last chambers. A more pronounced trend
is present in Mg / Ca of P. obliquiloculata, shifting from depths of between 70 and 90 m (average 75 m) during
non-eddy conditions to depths of between 147 and 244 m (average 150 m) during
eddy conditions (Steinhardt et al., 2014). The Mg / Ca-derived shift is hence
larger than the shift inferred from δ18Occ (eddy:
107 m; non-eddy: 60 m). Mg / Ca-derived calcification temperatures for N. dutertrei and
P. obliquiloculata are hence cooler and indicative of deeper calcification of the final
chambers compared to that of the whole shell (based on δ18Occ). Calcification temperatures derived from Mg / Ca for G. scitula (Fig. 5) indicate an opposite trend, shifting between approximately 200 and 460 m
(average 330 m) during eddy conditions to shallower depths between
approximately 120 and 420 m (average 270 m) during non-eddy conditions
(Steinhardt et al., 2014). Although the δ18Occ suggests
calcification somewhat deeper than the Mg / Ca data, both Mg / Ca- and δ18O-derived calcification depth indicate a shoaling for this species
during non-eddy conditions. Furthermore, the average δ18O-derived calcification temperature of 10.4 ± 3.9 ∘C
is in good agreement with previously published results for this species
(Fallet et al., 2011; Birch et al., 2013). We refrain from correcting for a
vital effect, as this would lower δ18O-derived calcification
temperature to values unrealistically lower than the Mg / Ca-derived
calcification temperatures for the last chambers. The observed remaining
offset between single-specimen δ18O and single-chamber Mg / Ca in
G. scitula suggests either that (1) there is a vital effect resulting in more enriched
(i.e. positive) δ18O values than when this species would
precipitate its shell in isotopic equilibrium with seawater, that (2) a more
shallow calcification depth during formation of the final chamber, (3) that
crust carbonate adds significantly to the total shell mass, or that (4) the Mg / Ca
calibration for G. hirsuta (Anand et al., 2003) might be different from that of G. scitula.
Following the vital effect correction of Kahn and Williams (1981),
calcification temperature is 6.4 ± 3.9 ∘C, which
is equivalent to an average calcification depth for G. scitula between 600 and deeper
than 1100 m. This is in agreement with a suggested depth habitat within the
upper 1000 m for this species (Schiebel et al., 1995; Ortiz et al., 1996;
Itou et al., 2001). In our opinion the last two explanations are most
likely; however, irrespective of the underlying mechanism, it is clear that
the majority of the test carbonate precipitated at a depth greater or
comparable to that of the ontogenetic carbonate of the final chambers.
The range of uncertainties related to a species' average calcification depth
results from the relatively large natural inter-specimen variability in
Mg / Ca. Since we focus on relative differences within species between
hydrographic conditions, the uncertainty in calcification temperature
resulting from errors in the applied Mg / Ca–temperature calibration does not
affect the absolute temperature differences between the eddy and non-eddy
conditions. Instead, uncertainties in the calculated difference in
calcification depths between species will be caused by the inter-specimen
variability in Mg / Ca.
Cumulative calcification model
We used a conceptual oxygen isotope mass balance model (Wilke et al., 2006, 2009),
applying the temperature fractionation from inorganic calcite precipitation
of Kim and O'Neil (1997) to our measured δ18Occ. The model
equation describing foraminiferal migration as a function of depth used here
is known as the cumulative form of the Weibull function (Weibull, 1939). It
is a continuous probability function (Eq. 4), relating the shell mass (M) to
depth (z) using two constants (α and β) determining the shape
of this relationship:
M(z)=1-exp-1×z/βα.
Since shell size of planktonic foraminifera is thought to increase with
water depth (Hemleben and Bijma, 1994; Peeters and Brummer, 2002), shell mass
must also increase with depth. The isotopic composition of a single shell
thus represents the weighted sum of equilibrium calcite precipitated over a
depth range of the productive zone (i.e. where primary calcite formation
takes place).
Based on Eq. (5), the expected stable isotope composition of a specimen
for a discrete water depth interval can be calculated as follows:
δ18Omodel=∑inMi-Mi-1+δ18Oeq,iMi.
Given the δ18Oeq profile in the water column and the
measured δ18Occ of the planktonic foraminifera, it is
possible to model the mass development (growth pattern) by using the
determined Mg / Ca calcification depth of the last chambers, indicating the
base of the calcite production zone. The Mg / Ca-based temperature of the F-1
chamber was used to delimit 95 % of the calcite production. In Eq. (5), δ18Oeq,i denotes the interval-averaged δ18O of equilibrium calcite for the specified depth interval. For
convenience, shell mass at the sea surface was taken as zero and modelled
δ18Occ was done by adapting the variables “α” and
“β” in Eq. (5).
Increasing the value of “α” results in a growth curve with a narrow
calcification range. Higher values for “β” result in a deepening of
the growth curve, thereby determining the position of the base of the
productive zone. In contrast to Wilke's (Wilke et al., 2006, 2009) approach,
we have determined the calcification temperatures of the last three to four
chambers, which were used to constrain the base of the calcification range
and hence constrained values for “β”.
In this model, it is assumed that shell growth always follows the same
function, which is continuous and does not differ between species. Offsets
between δ18Occ and δ18Osw from
expected equilibrium (“the vital effect”) are assumed to be constant over
the temperature range in which the species calcifies. We have adapted
δ18Osw in metre steps as calculated from in situ salinity
measurements, which where interpolated for the upper 2000 m. We have
used expected δ18Oeq values of eddy and non-eddy
conditions
to compare depth distributions for all four species of planktonic
foraminifera.
Calcification depths inferred from the cumulative δ18O model
(Fig. 7) match previously published calcification depths and associated
temperatures for each of the species relatively well (e.g. Cléroux et
al., 2008, 2013; Wilke et al., 2009; Fallet et., 2010, 2011; Birch et al.,
2013). In three species, measured δ18Occ values reflect
shallower calcification depths than do single-chamber Mg / Ca-based
calcification depths, which is consistent with the general model of
migration to greater depth during growth. In the case of the deep-dwelling G. scitula, however,
δ18O-based calcification depth is below that of the final
chambers as derived from Mg / Ca temperatures. If a temperature
correction for δ18O-based calcification temperatures of G. scitula is not applied,
calcification depth based on δ18Occ can deviate up to
300 m from the Mg / Ca-based depths. This would suggest that the majority of the
previously formed calcite was precipitated deeper in the water column. The
model shows that species modulate their calcification pattern depending on
the hydrographical conditions they live in (e.g. eddy or non-eddy conditions).
For G. ruber, our results show that this species seems to be an exclusive surface
dweller, and hence an application of the cumulative calcification model only
confirms that the majority of the calcite is formed at the sea surface.
(a) Temperature profiles as well as δ18Oequilibrium (δ18Oeq) for the upper
1000 m for eddy (red) and non-eddy (blue) conditions. (b–e) δ18Ocumulative (δ18Omodel), mass development/growth pattern
and cumulative mass of the foraminifera (foram mass) are plotted for the
upper 500 m. Bulk δ18Oforam (triangles) Mg / Ca-derived
single-chamber calcification depths (crosses) are indicated in the relevant
plots for G. ruber (b), N. dutertrei (c), P. obliquiloculata (d) and G. scitula (e).
For the thermocline-dwelling species N. dutertrei, we find that this species calcifies
most of its calcite in a narrow depth range. Our model indicates that
calcification during eddy conditions is more intense in the deeper part of
the thermocline (α= 8.8, β= 85), whereas calcification
during non-eddy conditions is more equally distributed over the entire
thermocline (α= 1.9, β= 47). It is worth noting that N. dutertrei
appears to intensify its calcification efforts during eddy conditions deeper
in the thermocline, matching well with the deepening of the isopycnals and
hence a narrower range of optimal calcification conditions (Steinhardt et
al., 2014). This calcification response is also reflected in more enriched
δ13C values during eddy conditions. For P. obliquiloculata, modelled α and
β values are relatively high, particularly during eddy conditions
(α= 5.25, β= 133, compared to α= 3.1, β= 63 for non-eddy conditions). This indicates that most of the
calcification in P. obliquiloculata takes place at a water depth of around 125 m during eddy
conditions, and around 50 m during non-eddy conditions. The range at which
G. scitula calcifies is well below the seasonal thermocline, reflected by high values
for α and β (Fig. 7), and does not vary considerably during
eddy and non-eddy conditions.
In general, we conclude that temperature changes within the thermocline
induced by eddies mostly affect non-symbiotic species. Also, changes in
cumulative calcite addition with depth seem to be species-specific. We
modified the model by including Mg / Ca-based temperatures (following the
species-specific equations of Anand et al., 2003) of the F-1 chamber to
constrain the 95 % calcification level. This allows for expected
δ18Oeq to be predicted for different species and shell sizes (Spero et
al., 1997; Bijma et al., 1999; Itou et al., 2001; Peeters et al., 2002). The
extended version of the model does not distinguish between calcite deposited
during chamber formation (primary calcite) and calcite added as a result of
wall thickening due to gametogenic calcite or the precipitation of crust
(secondary calcite; Bé, 1980; Duplessy et al., 1981; Lohmann, 1995;
Jonkers et al., 2012). Secondary calcification might play an important role
of deeper-dwelling species such as G. scitula, which could explain the offset (about
1 ‰) between δ18Omodel and δ18Occ. This suggests that relatively more calcite is formed
deeper in the water column, or that secondary calcite is precipitated with a
fundamentally different calcification mechanism.
Carbon isotopes – testing the calcification model
The δ13C values found in planktonic foraminifera are primarily a
function of the carbon isotope composition of the dissolved inorganic carbon
(DIC) in seawater (e.g. Urey, 1947; Epstein and Mayeda, 1953; McCorkle et al.,
1990), which changes with water depth (e.g. Fairbanks et al., 1980; Curry
and Crowley, 1987). Therefore, we can use the cumulative mass balance model
output of the mass added per metre to calculated δ13Cexpect as the weighted sum of the δ13CDIC
(Wilke et al., 2006). Depth-resolved carbon isotope composition (δ13CDIC) available from locations closest to our study site
(locations between 37 and 43∘ E and 24.7∘ S; World Ocean
Database, 2009) was used to calculate the expected δ13C of each
species of foraminifera (δ13Cexpect). Since there is no
relation between size and stable carbon isotopes in our specimens, the
employed size fractions contained only mature (adult) specimens (Brummer et
al., 1986, 1987). Comparison of water column δ13CDIC data
(Fig. S1 in the Supplement) from several stations near the MC reveals that
absolute values and depth range over which values decrease are similar at the
different sites. To verify the depth-related calcification model, we compare
measured δ13Ccc with model-based δ13Cexpect values (Fig. 8).
Inter-species differences between expected δ13C
values, based on the cumulative mass balance model, and measured δ13C values of G. ruber, N. dutertrei, P. obliquiloculata and G. scitula. Dashed line indicates the 1:1 line of measured and expected δ13C. Red symbols represent values for eddy conditions; blue symbols
represent values for non-eddy conditions. Thick grey arrows indicate
intra-species trends between non-eddy and eddy conditions; thin arrows
indicate inter-specific trends. P. obliquiloculata does not calcify in
isotopic equilibrium with dissolved ∑CO2, but the deviation from
isotopic equilibrium is a linear function of temperature (Mulitza et al.,
1999), and hence there is no projected δ13Cexect; this is
indicated by the dotted lines. The open diamond indicates δ13Cexect for P. obliquiloculata non-eddy conditions.
Carbon isotope values become more negative from surface-dwelling G. ruber towards
deeper-dwelling P. obliquiloculata near the upper thermocline. Conversely, the δ13C of Globorotalia scitula increases with depth. Low temperatures and reduced food
availability have been suggested to result in relatively low metabolic rates
in deep-dwelling species, so that their δ13C likely approaches
δ13CDIC values (Birch et al., 2013). This suggests the
involvement of biological controls on the δ13C of the different
genera (Globigerinoides, Neogloboquadrina, Pulleniatina and Globorotalia). All δ13Cexpect values are higher than the measured
δ13Ccc.
Our cumulative mass balance shows that the majority of the carbonate of G. ruber is
formed in surface waters (Fig. 7). Equal δ13Cexpect values for eddy and non-eddy conditions are the result of similarly
enriched δ13CDIC in the mixed layer. The measured
differences in δ13Ccc (Fig. 8) are likely a consequence of
the deepening thermocline during passage of an eddy, carrying
nutrient-depleted waters (Kolasinski et al., 2012). Anticyclonic eddies are
characterized by accumulation of warm, nutrient-poor and
chlorophyll-depleted water in the centre, which implies that δ13CDIC is also more isotopically enriched. Still, local nutrient
enrichment potentially occurs at the outer edge as a result of high
turbulence along the isopycnal slope (e.g. Falkowski et al., 1991; Lévy,
2003). The strong response of the Mg / Ca and δ18O of N. dutertrei during
eddy conditions (deeper calcification) is also reflected by more depleted
δ13Ccc values. Remineralization of organic matter at greater
depth cause enrichment of δ13CDIC, resulting in the
incorporation of lighter carbon isotopes into the shell of N. dutertrei during eddy
conditions. Based on samples from a sediment trap in Cape Basin, Wilke et al. (2009) showed that the species N. dutertrei is an accurate recorder of the δ13CDIC. This is in agreement with previous findings (Mulitza et
al., 1999), showing that the carbon isotopic composition of N. dutertrei exhibits a
constant and temperature-independent offset from δ13CDIC
of ∼ 0.5 ‰ over a wide temperature range.
This difference is in line with the offset in our data set between δ13Cexpect and δ13Ccc of N. dutertrei (0.6 ‰).
The δ13C of the symbiont-barren G. scitula significantly deviates from
those of the shallower-dwelling species as a result of a decrease in δ13CDIC with water depth (Figs. S1
and S2). The more depleted δ13Ccc of G. scitula may also be a
consequence of a lower metabolism of this species (Vergnaud-Grazzini, 1976;
Kahn, 1977, 1979; Berger et al., 1978; Erez, 1978) compared to that of G. ruber and
N. dutertrei. At high metabolic activity, more isotopically lighter carbon is
incorporated, and since lower temperatures usually reduce metabolic rates,
species inhabiting deeper water depths may incorporate relatively heavier
carbon isotopes. Minor changes in δ13Ccc for G. scitula during
eddy compared to non-eddy conditions are in line with the minor response in
calcification depth for this species. Similar to previous conclusions, this
suggests that Mg / Ca-inferred temperature differences between N. dutertrei and G. scitula are good
indicators of eddies passing (Steinhardt et al., 2014). In addition, the
δ13Ccc differences between these species might very well
help to reconstruct eddy frequency in this area. The depth-integrated
difference between δ13C of N. dutertrei and G. scitula changes from 0.25 to 0.05 ‰.
In the comparison of δ13Cexpect and δ13Ccc for
P. obliquiloculata there is a discrepancy between eddy and non-eddy conditions (Fig. 8).
Similar to N. dutertrei, this species is mostly associated with the thermocline (Anand
et al., 2003; Cléroux et al., 2008; Sadekov et al., 2009). Our
cumulative calcification model showed a slightly deeper calcification depth
for N. dutertrei and a minor eddy response in the calcification range (Fig. 7). However,
δ13C values indicate a significant difference between eddy and
non-eddy conditions. Mulitza et al. (1999) showed that P. obliquiloculata does not calcify in
isotopic equilibrium with dissolved ∑CO2 and that the deviation from
isotopic equilibrium is a linear function of temperature (Fig. 8). While the
mean of the δ13C cannot be used to infer the actual
calcification depth, they argue that the spread and skewness of the
individual δ13C measurements should still be representative of
the range of calcification depths and habitat preferences within the
thermocline.
Also, changes in the carbonate ion concentration with depth potentially play
an important role in the observed differences between species and between
eddy and non-eddy conditions (Figs. S1 and S2). Since the
carbonate ion profile is expected to change in accordance with thermocline
deepening when an eddy passes we refrained from correcting for this. The
observed offsets between species, however, suggest that carbonate ion does
play a role there. The deeper living species show an increasing offset with
respect to the 1:1 line (Fig. 8). The exception is P. obliquiloculata, which responds to
temperature rather than δ13CDIC carbonate ion changes
(Mulitza et al., 1999).
Overall the relations observed here indicate that interpretation of the
foraminifera vertical distribution in the upper water column can be
unravelled by coupling various geochemical methods in order to retrieve
calcification temperature at different stages in a foraminifera's life
cycle. This in turn can be used to develop new proxies for the thermal and
nutrient structure of the upper part of the water column.