Introduction
Foraminifera, being extensively distributed and highly abundant in most
marine environments, are essential proxies for reconstructing the chemical
and physical properties of past oceans. Several trace-element-to-calcium (Ca)
ratios analysed on foraminiferal tests have been developed as proxies in the
last decades. Perhaps one of the most conventional approaches is the
reconstruction of seawater temperatures using Mg / Ca (e.g. Nürnberg
et al., 1996; Elderfield et al., 2006). Other established trace-elemental
proxies also include Ba / Ca to trace salinity changes due to continental
run-off (Lea and Boyle, 1989; Hönisch et al., 2011) and Cd / Ca to
reconstruct water masses (Marchitto and Broecker, 2006). Whilst there is a
wealth of research applying the geochemistry of foraminiferal calcite for
palaeo-oceanographic reconstruction and copious sedimentary redox proxies have
been developed (e.g. Gooday et al., 2009), utilising the trace-elemental
composition of foraminiferal shells (tests) to reconstruct oxygen conditions
is still in its infancy. One redox-sensitive element that has recently gained
more interest is manganese (Mn), both as a trace element in biogenic
foraminiferal calcite (Mn / Ca) and in bulk sediment samples (Reichart et
al., 2003; Glock et al., 2012, Groeneveld and Filipsson, 2013; Lenz et al.,
2014; Koho et al., 2015). Here we aim to explore the potential of Mn / Ca
by analysing both benthic foraminiferal tests and comparing them to Mn / Al
of bulk sediment samples from an upwelling record to determine if changes in
oxygen conditions during different primary productivity regimes are
detectable by these methods.
At the sediment–water interface, the concentration of dissolved
redox-sensitive elements such as Mn varies significantly between oxic and hypoxic
(hypoxia defined as < 1.42 mL L-1 O2 following Levin et al.,
2009) settings. In sea water, redox-sensitive Mn is mainly present as
Mn2+, which under oxic conditions precipitates as Mn oxyhydroxide
(Burdige, 1993; Glasby, 2006). The Mn flux across the sediment–water
interface is driven by reductive dissolution of reactive Mn oxyhydroxide
(Froelich et al., 1979).
Under oxic conditions, dissolved O2 is present in the pore waters, and
thus benthic foraminiferal tests are expected to incorporate less Mn into
their test. In contrast, under low-oxygen conditions, Mn oxyhydroxide is
reduced and the Mn2+ concentration increases (Tribovillard et al.,
2006), becoming available to be incorporated in the foraminiferal tests.
Especially under hypoxic conditions, Mn concentrations will be concentrated
in the pore water because the Mn cannot escape into the overlying oxic bottom
water, meaning that benthic foraminiferal Mn / Ca will be highest. Hence
foraminiferal Mn / Ca has potential to be used as a proxy of hypoxic
conditions. Accordingly, we expect bulk Mn to be depleted in the sediment and
exhibit the opposite trend. On the other hand, under anoxic conditions the Mn
is able to diffuse upwards and into the overlying water column, or, when pore
waters become supersaturated with respect to Mn, it is precipitated as
MnCO3 (rhodochrosite) (Froelich et al., 1979; Pedersen and Price, 1982;
Tribovillard et al., 2006). Therefore, under low-oxygen conditions,
foraminiferal Mn / Ca is expected to be higher during hypoxic conditions
than during anoxic conditions, but still higher than under oxic conditions.
Whilst benthic foraminiferal Mn / Ca has been conventionally used as an
indicator of contamination by Mn oxyhydroxide or Mn carbonate (Boyle, 1983;
Barker et al., 2003), new studies are pioneering Mn / Ca as a potential
proxy of related changes in bottom/pore-water oxygen and redox conditions
(Ní Fhlaithearta et al., 2010; Glock et al., 2012; Groeneveld and
Filipsson, 2013; Koho et al., 2015). Mn / Ca signatures of the ambient
bottom water are recorded by benthic foraminifera. For example, culture
experiments have confirmed that the species Ammonia tepida
incorporates Mn into the test proportional to the concentration in the
ambient water masses (Munsel et al., 2010). Thus, during benthic
foraminiferal calcification under hypoxic conditions, more Mn will be
assimilated into their calcite tests, whereas under anoxic conditions, or oxic
conditions in particular, Mn is expected to decrease, albeit to different
amounts (Pena et al., 2005). Hence Mn / Ca used in this study could
provide a reliable means of reconstructing the former seafloor oxygen
settings at the time of deposition as opposed to sediment bulk measurements,
which can continue to oxidise and be mobilised post-deposition.
Traditionally, trace element / Ca is analysed on solution-based samples
containing larger numbers of foraminifera specimens to give a representative
result (Groeneveld and Filipsson, 2013). When a sufficient amount of
specimens are not available for solution-based techniques, or if diagenesis
has affected the tests, a micro-analytical technique upon single specimens
such as secondary ion mass spectrometry (SIMS) is a valuable tool. From an
analytical perspective SIMS has enhanced our ability to determine how trace
elements are distributed within foraminiferal tests at high spatial
resolution and precision on individual foraminifera (Allison and Austin,
2003; Bice et al., 2005; Kunioka et al., 2006; Glock et al., 2012). Recently,
SIMS-determined Mn / Ca of benthic foraminifera has been found to be
representative of Mn / Ca in the top centimetre of the pore water, confirming
that the foraminiferal calcite composition relates to the level of oxygen
depletion (Glock et al., 2012).
Where a sufficient amount of foraminiferal specimens are available, we
additionally used flow-through inductively coupled plasma optical emission
spectroscopy (FT-ICP-OES; Haley and Klinkhammer, 2002). Flow-through analysis
is a means of determining elemental composition from samples of foraminiferal
tests which permits complete monitoring of the effects of cleaning and
dissolution (Haley and Klinkhammer, 2002). However, due to the small size of
Eubuliminella exilis, a larger number (up to 50 specimens in this
case) of foraminiferal tests from the same core sample is required to give a
representative average signal. Therefore we utilise both SIMS and FT-ICP-OES
to explore the potential of Mn / Ca for interpreting down-core oxygen
studies.
To explore how Mn / Ca works as a potential proxy for bottom/pore-water
oxygen conditions, we study a site from the low-latitude NE Atlantic
upwelling system. Upwelling systems are an ideal environment to test this
proxy, as they are renowned for high export rates of labile organic matter
from surface waters, which provokes severe oxygen depletion in the underlying
intermediate waters and at the seafloor (Böning et al., 2004). We study
core GeoB7926-2 from the upwelling region off coastal NW Africa (Fig. 1) and
compare foraminiferal and sedimentary Mn with published diatom and benthic
foraminiferal species composition (Romero et al., 2008; Filipsson et al.,
2011; Kim et al., 2012; McKay et al., 2014). In general, coastal upwelling
systems are the most productive of the world ocean, resulting in vulnerability
to oxygen minima within the water column and underlying seafloor (Helly and
Levin, 2004; Bakun et al., 2010). At present, the benthic environment of this
particular upwelling system is not especially susceptible to low-oxygen
conditions and is well ventilated with bottom-water oxygen of ca. 5 mL
l-1(Goretski and Koltermann, 2004). However, based on benthic
foraminiferal faunal studies, there is evidence of previous periods of oxygen
depletion at the sea floor during the Younger Dryas (YD, 13.5–11.5 ka) and
Heinrich Event 1 (H1, 18–15.5 ka) in particular. This is inferred from the
predominance of the low-oxygen-tolerant benthic foraminiferal species
Eubuliminella exilis (synonymised taxa: Bulimina exilis)
(Filipsson et al., 2011; McKay et al., 2014). Therefore, we selected samples
allocated to late Marine Isotope Stage 3 (MIS3, 35–27 ka), the Last Glacial
Maximum (LGM), Heinrich Event 1 (H1), Bølling Allerød (BA) and the
Younger Dryas (YD) to reconstruct past bottom-water oxygen. These climatic
intervals were chosen in order to test if Mn / Ca can confirm the low-oxygen
conditions during different productivity regimes as reported by
previous studies from this particular sediment core (Filipsson et al., 2011;
McKay et al., 2014). We focus on utilising SIMS and compare this method with
FT-ICP-OES where a sufficient number of E. exilis specimens were
available. We also present Mn / Al sediment bulk measurements from the
same sediment for further comparison.
Locality of gravity core GeoB7926-2 (black star) in the low-latitude
NE Atlantic upwelling area. Arrows indicate the major oceanic currents in the
study area. Inset: location of the study area off coastal NW Africa. Modified
after Romero et al. (2008).
The low-oxygen-tolerant benthic foraminiferal species Eubuliminella exilis is an infaunal species and therefore has the ability to migrate
within the sediment and experience variable pore-water conditions. This
migration could affect the Mn / Ca incorporated within the test; however
as E. exilis is so low oxygen tolerant, it possibly migrates less
than other infaunal species which are not as tolerant. Therefore E. exilis likely incorporates more Mn and is therefore the right recorder,
suitable for reconstructing oxygen levels. Furthermore, abundant populations
of this taxon are reported from a range of settings, including coastal
upwelling sites, sapropels, oxygen minimum zones and other environments, for
example, in the Bering Sea and the Mississippi River mouth (Caralp, 1989;
Jorissen, 1999; Jannink et al., 1998; Rasmussen et al., 2002; Khusid et al.,
2006). Thus E. exilis also has potential to be a useful proxy for
other marine environments susceptible to strong oxygen deficiency and high
fluxes of organic export. Eubuliminella exilis has been found to
correlate with diatom accumulation rate and clearly dominates the
foraminiferal fauna during very high diatom input (Caralp, 1984; Filipsson et
al., 2011; McKay et al., 2014, 2015). Therefore E. exilis, being present (albeit in considerably varying abundance) throughout
the GeoB7926-2 record, is an ideal candidate to record oxygen changes in the
environment in which they lived and also provide an opportunity to test if
export productivity is causing low-oxygen conditions as opposed to the dominance
of this species merely being a fresh phytodetritus diet signal (Caralp,
1989).
We hypothesise that higher foraminiferal Mn / Ca will occur during times
of high diatom accumulation rate and lower bottom-water oxygen concentrations
and accordingly higher E. exilis abundance, with the opposite effect
during times of low surface productivity.
Method
Gravity core GeoB7926-2 from the NE Atlantic upwelling system
(20∘13′ N, 18∘27′ E, 2500 m water depth) was recovered
during R/V Meteor cruise M53/1 (Meggers et al., 2003). The
age model for the core was published by Kim et al. (2012), and the timing and
duration of the climatic intervals were adopted from the δ18O of
the North Greenland Ice Core Project (NGRIP Members, 2004) and Sánchez Goñi and Harrison (2010). We
selected well-preserved foraminiferal specimens based on the criteria of
high- and low-surface-productivity regimes as demonstrated from diatom abundance
(Romero et al., 2008). Details of sample preparation for benthic
foraminiferal faunal analysis have previously been published (Filipsson et
al., 2011; McKay et al., 2014).
SIMS analysis
From the > 150 µm size fraction, a total of 48 specimens of the
benthic foraminifera species Eubuliminella exilis were hand-picked
under a binocular microscope for SIMS analysis (Table 1). We acknowledge that
the presence of Mn-rich authigenic coatings (e.g. Mn (oxyhydr)oxides and
organic matter) can be problematic for trace-elemental analysis of
foraminifera (Boyle, 1983; Pena et al., 2005; Klinkhammer et al., 2009).
Therefore, we employed a rigorous pre-treatment cleaning technique to remove
possible organic contamination following the method of Glock et al. (2012)
and avoided potential diagenetic coatings during the SIMS analysis by
measuring within the massive centre of the test walls (Fig. 2).
Sample list: climatic intervals, export productivity according to
diatom export, average foraminiferal Mn / Ca for different samples
determined by SIMS and FT-ICP-OES.
Sampleno.
Depth(cm)
Age(ka)
Climaticinterval
Exportproductivity
SIMS: averageMn / Ca(µmolmol-1)
SD(µmolmol-1)
SIMS:averageMn / Ca (µmolmol-1)per sampledepth
FT-ICP-OES:average Mn / Ca(µmolmol-1)
YD A
170
12.2
YD
High
138
62
198
220
YD B
170
12.2
YD
High
191
116
YD C
170
12.2
YD
High
321
251
YD D
170
12.2
YD
High
141
56
YD E
270
12.7
YD
High
178
82
164
160
YD F
270
12.7
YD
High
96
53
YD G
270
12.7
YD
High
117
48
YD H
270
12.7
YD
High
164
120
YD I
270
12.7
YD
High
183
51
YD J
270
12.7
YD
High
111
27
YD K
270
12.7
YD
High
261
275
YD L
270
12.7
YD
High
204
78
BA A
365
13.5
BA
Moderate–high
155
37
Only 1specimenavailable
280
BA B
430
15.1
BA
Moderate–high
242
42
130
110
BA C
430
15.1
BA
Moderate–high
63
51
BA D
430
15.1
BA
Moderate–high
217
51
BA E
430
15.1
BA
Moderate–high
49
18
BA F
430
15.1
BA
Moderate–high
179
88
BA G
430
15.1
BA
Moderate–high
175
87
BA H
430
15.1
BA
Moderate–high
67
149
BA I
430
15.1
BA
Moderate–high
47
27
H1 A
500
16.7
H1
High
57
17
117
140
H1 B
500
16.7
H1
High
154
36
H1 C
500
16.7
H1
High
186
81
H1 D
500
16.7
H1
High
191
142
H1 E
500
16.7
H1
High
76
57
H1 F
500
16.7
H1
High
92
37
H1 G
500
16.7
H1
High
62
19
H1 H
500
16.7
H1
High
116
36
LGM A
723
20.7
LGM
Low
27
14
74
LGM B
723
20.7
LGM
Low
146
28
LGM C
723
20.7
LGM
Low
115
71
LGM D
723
20.7
LGM
Low
7
4
Insufficient no.
LGM E
773
22.7
LGM
Low
7
3
65
of specimens
LGM F
773
22.7
LGM
Low
10
3
LGM G
773
22.7
LGM
Low
70
49
LGM H
773
22.7
LGM
Low
175
91
MIS3 A
928
29.9
MIS3
Low
38
13
43
MIS3 B
928
29.9
MIS3
Low
48
38
MIS3 C
928
29.9
MIS3
Low
45
24
Insufficient no.
MIS3 D
1058
34.0
MIS3
Low
67
23
61
of specimens
MIS3 E
1058
34.0
MIS3
Low
55
62
For the SIMS analysis, foraminifera from individual sample depths were rinsed
over a 63 µm sieve with Milli-Q water. After this rinsing step, the
foraminifera were transferred into vials and sonicated for 20 s.
Subsequently, the foraminifera were rinsed with methanol and sonicated again
for 1 min. Any residual methanol was then removed with Milli-Q water. An
oxidative cleaning step was performed to remove organic matter which
consisted of mixing the following reagents: 100 µL 30 %
H2O2 to 10 mL of 0.1 M NaOH solution. Three hundred and fifty microlitres of this
reagent was added to each individual vial, and the vials were put into a water
bath at 92 ∘C for 20 min. Afterwards another 20 s sonic bath was
undertaken; the foraminifera samples were rinsed again with Milli-Q water in
the 63 µm sieve to remove any residues. For the final step, the
specimens were transferred back into their respective vials and
250 µL of 0.001 M HNO3 was added to each vial. The vials were
put into a sonic bath for 20 s and finally rinsed one last time with Milli-Q
water. After the cleaning procedure, the specimens were checked under a
binocular microscope to ensure sufficient cleaning and that the tests
remained intact.
SEM image (scale bar: 100 µm) and cross-section image
during SIMS analysis of a single Eubuliminella exilis specimen. The
white circles highlight the selected spots for SIMS analyses, measuring
5 µm in diameter. Note that the black areas of the SIMS measurement
targets visible in this image are actually the 5 µm spots plus the
10 µm pre-sputters. Inset is a close-up of the SIMS targets: the
red square is the approximate pre-sputter area
(15 × 15 µm, i.e. 5 µm spot +10 µm
raster), the yellow area is the field of view admitted to the mass
spectrometer (controlled by magnification and field aperture) and the blue
ellipse is the nominal 5 µm spot.
Foraminifera specimens were embedded in low-viscosity epoxy resin at JAMSTEC,
Japan. The foraminifera were then ground to expose a cross section across the
test wall using 16 µm silicon carbide paper at the Department of
Geosciences, University of Edinburgh, UK. Resin pieces were mounted into
low-viscosity epoxy resin disks (Struers) at the NORDSIM laboratory, Laboratory
for Isotope Geology at the Swedish Museum of Natural History, Stockholm,
Sweden. The mounts were polished using a Struers Rotopol-2 at 150 rpm for
1 min, first with 3 µm diamond suspension and again with
1 µm diamond suspension. Between each grinding and polishing step,
mounts were cleaned with ethanol. Each cross-sectioned foraminifera test was
examined under high-power reflected light microscopy to evaluate the quality
of the carbonate and to assist in assessing the progress of polishing until
the cross sections were clear. Subsequently, the mounts were cleaned in
high-purity ethanol and coated in a 20-nm thick, high-purity Au coat.
The reference material used for the SIMS was a polished piece of Oka calcite
crystal supplied from GEOMAR, Kiel University, Germany (E. Hathorne, pers.
comm). This standard was obtained from a matrix-matched specimen for which
Mn / Ca has been reported by solution ICP-MS (Glock et al., 2012). During
calibration, the Oka was analysed n=16 times, yielding a high sensitivity
with 1 standard deviation repeatability of 1.2 % for Mn concentration.
The Mn / Ca analyses of the test cross sections were performed using a
Cameca IMS 1280 ion microprobe at the NORDSIM laboratory at the Swedish
Museum of Natural History, Stockholm, Sweden. Analysis used a
16O2- ion beam accelerated at 23 kV impact energy (-13 kV
primary beam, +10 kV secondary beam). It is vital to only analyse Mn which
is located internally within the original test wall to attain the most
representative Mn / Ca for developing it as a redox proxy. Therefore, a
50 µm aperture in the primary column was used to shape a slightly
elliptical 5 µm spot on the sample surface, which, together with
careful placement, reduced the effects of sample contamination from the test
wall outer surface. Prior to each analysis, the analytical location was
pre-sputtered for 2 min with the ion beam rastered over a
10 × 10 µm raster to remove the Au coat and any remaining
surface contamination. During the initial pre-sputtering, the 44Ca
distribution was monitored using the ion imaging system of the instrument and
maximised to ensure high-precision beam targeting on the fine foraminiferal
test walls. The mass spectrometer was operated at a mass resolution of M/
ΔM ∼ 6000 to resolve the 55Mn peak from nearby molecular
interferences. A 400 µm contrast aperture was employed for maximum
transmission together with a 60 µm entrance slit, a
2001 µm field aperture restricting the field of view on the sample
to an area of ca. 12 × 12 µm at the transfer magnification
of ca. 160×, and a 45 eV wide energy window – all of which combined to
yield adequately flat-topped peaks on the species of interest. Each analysis
comprised of 16 cycles of 44Ca (1 s integration cycle-1) and 55Mn
(2 s). Each analysis lasted approximately 9 min. Secondary ions were
measured using a low-noise (< 0.01 cps) ion-counting electron multiplier.
Multiple analysis points were undertaken upon each individual test of
E. exilis (ca. 6–10 targets per individual specimen) starting from
the aperture and taking measurements alternating between the outer wall and
internal walls. For the best targets, programming was performed manually to
ensure that widest chamber walls and “t” junctions were targeted since
they provide a wider test wall for the analyses (Fig. 2). Furthermore, at
such high spatial resolution and precision, it is easy to visually observe
and avoid encrusting prior to selecting analysis targets via the connected
screen and avoid measuring secondary calcite or authigenic clays which would
otherwise affect measurements. With cautious positioning of the primary beam
on the test walls and observations of the element distributions during
measurements, such detrital material and potential contaminants were avoided
and annulled. Therefore only the elements actually incorporated into the
calcitic tests were measured. As an additional prerequisite to this, analyses
with Ca values > 500 kcps were classified as being reliable. Mn / Ca
was first normalised to those determined in the Oka standard and subsequently
converted to the true value in the Oka based on the Glock et al. (2012) value
for Mn / Ca of 4920 µmol mol-1.
The advantage of SIMS is that it is non-destructive, and, as the foraminiferal
cross sections are preserved within the mounts, they can be stored for
further analyses. Mounts are archived at the NORDSIM laboratory.
FT-ICP-OES analysis
For FT-ICP-OES, 20–50 specimens per sample depth of E. exilis from
the GeoB7926-2 record were selected from samples corresponding to H1, BA and
the YD for comparisons with the SIMS data. These three climatic intervals
encompassed the only samples where a sufficient number of pristine E. exilis individuals were present. The tests were gently crushed in a 0.5 mL
vial, and fragments were transferred into a PTFA filter with 0.45 µm
mesh.
For analysis, the filters were connected to a flow-through automated
cleaning device (Klinkhammer et al., 2004; Haarman et al., 2011). Automatic
cleaning prevents the loss of material which occurs with traditional
cleaning, allowing the analysis of very small samples (∼ 20 µg). The
flow-through was then connected to an ICP-OES (Agilent Technologies, 700
Series with autosampler (ASX-520 Cetac) and micro-nebulizer). Time-resolved
analysis (TRA) was used to analyse the samples at MARUM, University of
Bremen, Germany. After an initial rinse (5 min) with buffered Seralpur water
to remove clays, the samples were slowly dissolved using an acid ramp formed
by mixing of Seralpur with 0.3 M QD HNO3 (no additional oxidative
and/or reductive cleaning was performed). Starting with pure Seralpur the
acid contribution was stepwise increased every minute to 100 % acid after
30 min. The flow speed of the solution was 250 µL L-1.
Mn / Ca of the foraminiferal calcite was determined by identifying the
TRA interval which showed a consistent linear relationship between Mn and Ca
counts. Potential diagenetic phases like Mn(oxy)hydroxides are avoided this
way, as they would have a different slope, i.e. a significant Mn signal along
with the absence of a Ca signal. Mn / Ca was then calibrated using the
characteristic slope of this linear relationship of a known in-house standard
solution analysed on the same acid ramp. The average standard error on the
determination of the slope for Mn / Ca was 0.75 %. We analysed an
international limestone standard (ECRM752-1) for Mg / Ca to validate the
results following the same FT protocol. The average Mg / Ca of the
ECRM752-1 (n=4) was 3.76 mmol mol-1, which compares well with
the average published value of 3.75 mmol mol-1 (Greaves et al., 2008).
Mn bulk measurements
For geochemical bulk analyses, samples were dried and ground in an agate
mortar and homogenised. Total dissolution of samples was undertaken using HF
and HNO3 following the standard procedures of Gallego-Torres et
al., (2007). Mn and Al content for samples corresponding to 35–10 ka was
determined by atomic absorption spectrometry, using Re and Rh as internal
standards at the Analytical Facilities (Centro de Instrumentación Cientifica) at the University of Granada, Spain.
Redox-sensitive Mn was normalised to Al content in order to correct for
detrital variations (van der Weijden, 2002). Mn / Al data corresponding
to 25–10 ka have previously been published (Gallego-Torres et al., 2014), and
here we extend the Mn / Al record to 35 ka.
Conclusions
Our study contributes to the development of Mn / Ca in benthic
foraminiferal calcite as a proxy for reconstructing past oxygen conditions.
The results based on data from the low-latitude NE Atlantic upwelling system
indicate that shifts in oxygen levels occurred during different productivity
regimes between 35 and 11.5 ka, and thus foraminiferal Mn / Ca can assist
our understanding of the past environment in the region studied. The
foraminiferal Mn / Ca results are supported by benthic foraminiferal
faunal data.
The highest foraminiferal Mn / Ca and greatest Mn variability within
individual tests were obtained during the YD and indicate Mn enrichment which
coincides with very high primary productivity and the dominance of
low-oxygen-tolerant benthic foraminifera Eubuliminella exilis. The results
confirm our hypothesis that Mn / Ca in E. exilis can be applied
as a proxy for oxygen deficiency, in this case instigated by the increase in
diatom input. Therefore, whilst the benthic faunal abundance data are
indicative of such a scenario, foraminiferal Mn / Ca allows a more
comprehensive interpretation. Our initial down-core data set raises the
implication of calibrations. Once developed, Mn / Ca determined by the
SIMS method in particular may have the potential to be applied to other study
regions and foraminiferal species for reconstructing former bottom-water
oxygen conditions.
Furthermore, we conclude that SIMS-determined Mn / Ca in individual
tests is comparable with bulk foraminiferal Mn / Ca measured by
FT-ICP-OES. However, due to the processing time required to program and
target delicately thin foraminiferal test walls, SIMS may not be practical
for Mn / Ca studies where large numbers of samples must be processed.
Nevertheless, we emphasise that SIMS has great potential to provide reliable
Mn / Ca results from just a few individual foraminifera. Therefore, SIMS
is a robust alternative method to FT-ICP-OES; idea for employment on samples
that lack a sufficient abundance of individual benthic foraminiferal
specimens for solution-based bulk analyses. Furthermore, SIMS is also
non-destructive, and thus foraminiferal test cross sections can even be
remeasured.
In contrast, foraminiferal Mn / Ca data do not continuously exhibit a
consistent trend with Mn / Al determined from bulk sediment measurements.
The reason for this discrepancy is that Mn related to redox fronts within the
sediment provides a diagenetic signal, and thus it continues to react and
shift after deposition, whereas foraminiferal tests record the Mn
concentration at the time of calcification.