MARMIC-01352; No of Pages 12
Marine Micropaleontology xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Marine Micropaleontology
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a r m i c r o
Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides
from Western Mediterranean plankton tow and core top samples
Ulrike Jannette van Raden a,⁎,1, Jeroen Groeneveld b,2, Markus Raitzsch b,3, Michal Kucera a
a
b
IFG Tübingen, Eberhard-Karls Universität Tübingen, Sigwartstrasse 10, 72076 Tübingen, Germany
MARUM, Universität Bremen, Leobener Strasse, 28359 Bremen, Germany
a r t i c l e
i n f o
Article history:
Received 22 April 2010
Received in revised form 23 November 2010
Accepted 25 November 2010
Available online xxxx
Keywords:
Mg/Ca paleothermometry
Planktonic foraminifera
Paleoceanography
Mediterranean
Recent
Habitat depth
Calcite saturation state
a b s t r a c t
Due to its strong gradient in salinity and small temperature gradient the Mediterranean provides an ideal
setting to study the impact of salinity on the incorporation of Mg into foraminiferal tests. We have
investigated tests of Globorotalia inflata and Globigerina bulloides in plankton tow and core top samples from
the Western Mediterranean using ICP-OES for bulk analyses and LA-ICP-MS for analyses of individual
chambers in single specimens. Mg/Ca observed in G. inflata are consistent with existing calibrations, whereas
G. bulloides had significantly higher Mg/Ca than predicted, particularly in core top samples from the easterly
stations. Scanning Electron Microscopy and Laser Ablation ICP-MS revealed secondary overgrowths on some
tests, which could explain the observed high core top Mg/Ca. We suggest that the Mediterranean intermediate
and deep water supersaturated with respect to calcite cause these overgrowths and therefore increased bulk
Mg/Ca. However, the different species are influenced by diagenesis to different degrees probably due to
different test morphologies. Our results provide new perspectives on reported anomalously high Mg/Ca in
sedimentary foraminifera and the applicability of the Mg/Ca paleothermometry in high salinity settings, by
showing that (1) part of the signal is generated by precipitation of inorganic calcite on the foraminifer test due
to increased calcite saturation state of the water and (2) species with high surface-to-volume shell surfaces
are potentially more affected by secondary Mg-rich calcite encrustation.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Mg/Ca in foraminiferal calcite is a widely applied proxy for
reconstructing past ocean temperatures. This proxy relies on the
observation that bulk Mg/Ca of foraminiferal tests are mainly
controlled by calcification temperature (Anand et al., 2003; Barker
et al., 2005; Dekens et al., 2002; Elderfield and Ganssen, 2000; Lea
et al., 1999; Mashiotta et al., 1999; Nürnberg, 1995, 2000; Nürnberg
et al., 1996, 2000; Rosenthal et al., 1997; Russell et al., 2004). An
advantage of this method is that the same biotic carrier can be used for
both Mg/Ca and oxygen isotope analyses, which assures a temporal
and spatial conformity of the used samples. Since the δ18O record of
planktonic foraminifera combines effects of sea surface temperature
and the isotopic composition of the ambient seawater (Rohling and
Cooke, 1999), Mg/Ca in the same biotic carrier can be used to subtract
⁎ Corresponding author. Tel.: + 41 44 632 07 06; fax: + 41 44 6321080.
E-mail address: [email protected] (U.J. van Raden).
1
Present address: Geological Institute, ETH Zürich, Sonneggstrasse 5, 8092 Zürich,
Switzerland.
2
Present address: Marum Excellence Cluster Alfred Wegener Institute (AWI) for
Polar and Marine Research, Columbussstrasse, 27568 Bremerhaven, Germany.
3
Present address: Lamont-Doherty Earth Observatory of Columbia University in the City
of New York, 61 Route 9W, Palisades NY 10964, USA.
the temperature effect on δ18O in order to gain information on past
sea water δ18O, which is directly related to variables like salinity and
continental ice volume (Elderfield and Ganssen, 2000; Groeneveld
et al., 2008; Lear et al., 2000; Rosenthal et al., 2000).
Secondary factors affecting shell Mg/Ca may include, (1) partial
dissolution of Mg-rich shell components in waters under-saturated
with respect to calcite (Brown and Elderfield, 1996; Dekens et al.,
2002; Regenberg et al., 2007); (2) salinity, with sensitivities
ranging from 4 ± 3% Mg/Ca per psu observed in culture studies
(Kisakürek et al., 2008; Lea et al., 1999; Nürnberg et al., 1996) up
to 15–59% per psu suggested by field studies on Mediterranean
core top samples (Ferguson et al., 2008); (3) pH (closely linked to
CO2−
3 ), showing a 6% decrease in Mg/Ca per rising pH unit (Lea
et al., 1999; Russell et al., 2004), (4) test size and weight, which
was observed for some foraminiferal species (Anand et al., 2003;
Elderfield et al., 2002; Chiessi et al. 2008, supplement). Presumably,
the shell size/weight effect relates to the different Mg/Ca ratios of
the outer layer (calcite crust) precipitated by many planktonic
species and the inner layer (ontogenetic primary calcite) of the
tests. Interestingly, some studies report lower Mg/Ca in the outer
than in the inner calcite layers of some planktonic species (Brown
and Elderfield, 1996; Elderfield and Ganssen, 2000; Hathorne et al.,
2003; Puechmaille, 1994), while other studies find higher Mg/Ca in
the outer than in the inner calcite layers (Allison and Austin, 2003;
0377-8398/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.marmicro.2010.11.002
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
2
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
Eggins et al., 2003, 2004; Gehlen et al., 2004; Nürnberg et al.,
1996). It was suggested that migration through the water column
and thus calcification within different water masses could be a
reason for this phenomenon (Eggins et al., 2003; Lohmann, 1995;
Lohmann and Schweitzer, 1990). Possible mechanisms which have
been suggested are the precipitation of two distinct calcite
compositions via different calcification pathways (Erez, 2003),
reservoir fractionation effects (Elderfield et al., 1996) and an active
regulation of the internal calcite saturation state under biological
control (Erez, 2003).
In this study, we aim to further constrain the potential impact of
varying water mass properties (temperature and salinity) on the
incorporation of Mg into foraminiferal tests. We analyzed Mg/Ca in
the planktonic foraminifera Globorotalia inflata and Globigerinoides
bulloides. These species were selected for this study since (1) both
species were abundant throughout all plankton tow and core top
samples and are commonly used in paleoceanographic reconstructions (Chiessi et al., 2008; Cléroux et al., 2008; Nürnberg and
Groeneveld, 2006), (2) both species are non-symbiont bearing, and
hence we can exclude a possible additional influence of symbiont
activity on the geochemical signal of the foraminiferal tests, and
(3) the species have different habitats which allows for the
comparison of Mg incorporation in surface dwelling species
(G. bulloides) vs. species that calcify throughout the upper water
column and the thermocline (G. inflata). We used plankton tow and
core top samples from multi cores of the Western Mediterranean
Sea, which is characterized by a higher salinity and calcite
saturation state than the open-ocean. A simultaneous analysis of
plankton and sedimentary material allows a direct comparison of
Mg incorporation at different water depths with measured water
mass properties and with the Mg/Ca signal preserved in the
sediment.
2. Regional setting
The Mediterranean Sea is a semi-enclosed basin and has, due to its
position and latitudinal dimension, a strong salinity gradient from
west (~36 psu near the Strait of Gibraltar) to east (~40 psu) (Boyer
et al., 2002; Lascaratos et al., 1999) accompanied with only a small sea
surface temperature (SST) gradient (~ 4 °C) (Stephens et al., 2002)
(Fig. 1). The increasing salinity towards the east reflects the dominant
circulation mode in the basin, which is controlled by the position of
the only connection with open ocean waters in the west through the
Strait of Gibraltar. Relatively low-saline Atlantic Water (AW) enters
the Mediterranean at the surface at the Strait of Gibraltar and thins
out towards the Strait of Sicily and into the Eastern Mediterranean Sea
becoming warmer and more saline due to evaporation. Below the
surface, in about 200–500 m depth, the warm and high saline
Levantine Intermediate Water (LIW) moves westward, mixes with
the Adriatic Deep Water (ADW) and flows through the Strait of Sicily
into the Western Mediterranean basin. In the Western Mediterranean
the Levantine Intermediate Water (LIW) mixes with Western
Mediterranean Deep Water (WMDW), forms the Mediterranean
Intermediate Water (MIW), and flows through the Strait of Gibraltar
into the Atlantic (Millot, 1999; Pinardi and Masetti, 2000; Rohling et
al., 2007) (Fig. 1).
The large salinity gradient in the Mediterranean in the absence of a
large temperature shift, therefore, provides an ideal setting for testing
the influence of salinity on Mg/Ca in planktonic foraminifera. We note
that residence times for Mg (13 Myr) and Ca (1 Myr) are long
compared to the mixing time of only 70 y in the Mediterranean Sea,
and that Mg and Ca concentrations in the basin can be expected to be
similar as in the open ocean and, hence, conservative (Broecker and
Peng, 1982). The Mg/Ca ratio of Mediterranean seawater is therefore
not expected to bias Mg/Ca in foraminiferal tests.
Fig. 1. a) Map with sampling locations (circles: core top locations, crosses: plankton tow locations) showing sea surface temperature (WOA01) in spring (April, May, June). b) Cross
section of the Western Mediterranean Sea with spring salinity (WOA01), sampling locations and water masses (AW: Atlantic Water, LIW: Levantine Intermediate Water, MIW:
Mediterranean Intermediate Water, WMDW: Western Mediterranean Deep Water). (Schlitzer, 2006).
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
3
intervals (Fig. 1; Table 1). The MSN with a mesh size of 100 μm and a
top opening of 50 × 50 cm was heaved with 0.5 m/s. The plankton
samples from individual depths were buffered with hexamethylendiamin to a pH of 8.5 and stored in a 4% formalin solution at 4 °C.
Planktonic foraminifera were then concentrated by wet sieving and all
foraminifera were quantitatively picked from the air-dried concentrates and counted on the basis of cytoplasm content and species
attribution, using the taxonomy of Hemleben et al. (1989).
At all five stations (POS-334 74, 75, 77, 79, and 81) sediment
samples of the upper few decimetres were recovered with a
multicorer (MUC) (Fig. 1; Table 1). The top 5 mm sections were
preserved in Rose Bengal and alcohol and stored at 4 °C until further
processing. For simplicity, these samples from the top of the
multicorer tubes will be referred to as “core top samples” from here
3. Materials and methods
The analyzed material was sampled in late March/early April 2006
(RV Poseidon, cruise POS 334) in the Western Mediterranean Sea. The
westernmost Station POS 334–74 is situated at the border between
the Alboran Sea and the Balearic Abyssal Plane at a water depth of
2033 m. Station POS 334–75 and POS 334–79 are located within the
Balearic Abyssal Plane at water depths of 2703 m and 2996 m,
respectively. Station POS 334–77 is at a shallower water depth of
1104 m in the proximity of the Balearic Islands. The easternmost
Station POS 334–81 has a water depth of 1226 m between Sardinia
and Sicily close to the Strait of Sicily (Table 1, Fig. 1).
For this study, plankton was sampled with a multinet (MSN) at
stations POS-74 and 81 down to 700 m water depth in different
Table 1
Sample locations, CTD data, mean spring and annual temperatures and salinities from World Ocean Atlas (WOA, 2001), and ICP-OES Mg/Ca data.
Ship
station
Sample
name
POS
MSN K
334–74 17 B5
MSN K
17 B4
MSN K
17 B3
MSN K
17 B2
MSN K
17 B1
MSN K
16 B4
MSN K
16 B3
MSN K
16 B2
MSN K
16 B1
MUC
651
POS
MUC
334–75 652
POS
MUC
334–77 653
POS
MUC
334–79 655
POS
MSN K
334–81 34 B5
MSN K
34 B4
MSN K
34 B3
MSN K
34 B2
MSN K
34 B1
MSN K
33 B4
MSN K
33 B3
MSN K
33 B2
MSN K
33 B1
MUC
657
WOA01 WOA01 WOA01 WOA01 Bulk Mg/Ca Bulk Mg/Ca G.inflata
CTD
Sampling CTD
containing
G.bulloides
annual annual G.inflata
spring
spring
mean mean
depth
S (psu) (mmol/mol) (mmol/mol) cytoplasm
S (psu) T (°C)
T (°C) S (psu) T (°C)
(m)
(tests/m3)
Location
Sampling
date
36°15`017 N;
01°59`532 W
26.03.2006 0–20
37°14`996 N;
00°29`997 W
38°25`004 N;
01°31`098 E
38°25`051 N;
05°24`165 E
38°45`166 N;
11°00`633 E
G.bulloides
containing
cytoplasm
(tests/m3)
15.5
36.53
17.55
36.86
18.31
36.87
2.45
3.92
11.2
20–40
15.2
36.57
16.62
36.94
17.19
36.93
2.57
3.54
8
16
40–60
15.0
36.63
15.09
37.15
15.42
37.13
2.22
3.47
8.4
20.4
60–80
14.7
36.75
14.40
37.38
14.63
37.40
3.17
3.96
14
27.2
80–100
14.6
36.82
14.20
37.53
14.39
37.55
2.9
3.46
11.6
21.6
100–200
13.7
37.95
13.61
37.98
13.66
37.98
2.72
3.31
6
1.8
200–300
13.3
38.39
13.34
38.30
13.23
38.33
2.39
3.61
4.6
1
300–500
13.3
38.49
13.25
38.32
13.20
38.39
1.91
3.8
3.2
1.3
500–700
13.3
38.52
13.10
38.30
13.11
38.38
1.75
3.89
3.1
0.4
2033
–
–
14.65a
37.54a
14.33b
37.82b
–
14.32
37.81
14.11
b
37.98
b
–
–
a
37.92
a
14.29
b
38.05
b
3.48
(3.7; 3.26)c
4.46
(4.25;4.66)c
6.1
–
a
2.44
(2.8; 2.07)c
2.13
(2.08;2.18)c
3.12
–
a
–
–
3.97
(3.34;4.59)c
3.54
–
–
23.2
12
27.03.2006 2703
–
28.03.2006 1104
–
–
14.30
30.03.2006 2996
–
–
14.61a
37.71a
14.61b
37.91b
01.04.2006 0–20
14.2
38.00
17.14
37.56
18.55
37.65
2.06
(1.89;2.22)c
2.05
20–40
13.8
38.03
16.08
37.60
17.50
37.67
2.07
2.76
40–60
13.4
38.08
14.66
37.71
15.38
37.74
2.78
3.57
60–80
13.3
38.13
14.14
37.85
14.48
37.88
2.9
3.52
7.6
d
21.2
15.2
14
10.8
12
22
7.2
12.4
80–100
13.3
38.22
14.07
37.94
14.32
37.97
2.7
(5.4)
100–200
13.8
38.53
13.98
38.25
14.07
38.29
2.39
3.04
3
0.5
200–300
14.0
38.67
14.04
38.59
14.06
38.61
2.28
3.56
0.1
0
300–500
13.9
38.70
13.96
38.65
13.96
38.66
2.22
3.14
1
0
500–700
13.7
38.65
14.61
38.61
13.70
38.63
1.55
3.25
1
0
1226
–
–
14.61a
37.97a
14.76b
38.23b
2.24
(1.65;2.82)c
7.52
–
–
a
Core top calcification temperature and salinity for G. bulloides: Average spring (April–June) T and S in 20–200 m water depth.
Core top calcification temperature and salinity for G. inflata: Average annual T and S in 20–500 m water depth.
Single measurements in () below the average w/o ().
d
G. bulloides only calcifies in the uppermost part of the water column. Hence, specimens from deeper tows are expected to be similar to the shallow ones. This is also indicated by
most samples except for this one. Its value is more than 2 standard deviations away from the average G. bulloides Mg/Ca ratios for this tow location. Hence, although no indications
are present that this sample was contaminated, it was excluded from further consideration.
b
c
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
4
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
on. For this study, the core top samples were cleaned from particles
b63 μm by wet-sieving, dried at 36 °C for 48 h and picked for
foraminiferal Mg/Ca analyses.
3.1. Analytical methods
Only adult specimens of G. bulloides and G. inflata (both from
plankton tow and the core top samples disregarding the cytoplasm
content) were prepared for geochemical analyses. We used the same
size fraction (150–250 μm in G. bulloides and 200–300 μm in G. inflata)
in all samples for analyses to exclude size dependant Mg/Ca
variations. All samples were cleaned according to the standard
cleaning protocol for foraminiferal Mg/Ca analysis of Barker et al.
(2003). This procedure was established for cleaning sediment
samples, which typically show high contamination of clay minerals
but low content of organic material. Since plankton tow samples are
not contaminated with clay but may still contain organic material, we
slightly modified the method for these samples in that the steps for
clay removal were minimized and an additional step for oxidizing
organic matter was included. 300–500 μg of uncleaned foraminiferal
material was used for analysis on the ICP-OES, which corresponds to
about 20 individuals of G. inflata and 30 individuals of G. bulloides.
Each sample was first gently crushed between two clean glass plates
in order to open all chambers and then transferred into an acidcleaned tube. The tests were cleaned from clay particles by alternating
ultrasonic treatment with washes in ultrapure water and methanol.
Then, the samples were treated two to three times with 250 μL of a hot
(97 °C) oxidizing 1% NaOH/H2O2 reagent for 10 min. Every 2.5 min,
the solution was carefully agitated to release any gaseous build-ups.
After 5 min, the samples were placed in an ultrasonic bath for a few
seconds to maintain the chemical reaction. The remaining oxidizing
solution was removed by three rinsing steps with ultrapure water.
After transfer into clean vials a weak acid leach was performed by
adding 0.001 QD M HNO3 and 30 s of ultrasonic treatment. After two
rinses with ultrapure water and the removal of any remaining
solution, dissolution of the foraminiferal material was achieved by
adding 500 μl QD 0.075 M HNO3 to the sample material. Then the
samples were centrifuged for 10 min (6000 rpm) to exclude any
remaining insoluble particles from the analyses. Finally, the samples
were diluted with ultrapure water (Seralpur) to a Ca concentration of
10–70 ppm.
Bulk trace element/Ca ratios were measured with an Inductively
Coupled Plasma Optical Emission Spectrometer (ICP-OES; Perkin Elmer
Optima 3300RL with autosampler and Ultrasonic Nebulizer U-5000 AT
(Cetac Technologies Inc.)) at the University of Bremen, Germany
(Table 1). Mn/Ca, Fe/Ca, and Al/Ca were analysed to monitor the
presence of any remaining clays or Mn-coatings. Obviously, no clay
contamination was present in the plankton tow samples. Ratios of
b0.10 mmol/mol in the core top samples indicate no significant amount
of remaining contaminants which could have had an influence on Mg/Ca
(Barker et al., 2003). Analytical precision based upon three replicates per
analysis was 0.19% for G. inflata and 0.08% for G. bulloides. An in-house
standard solution with Mg/Ca of 2.93 mmol mol−1 was run after every
five samples to monitor drift of the analyses (s = 0.28%). Whenever
there was enough material we did replicate measurements to detect the
natural internal variability within one sample. Reproducibility of the
results based on replicate samples was 0.07 mmol/mol or 2.4% (n = 11).
The international ECRM752-1 standard (Greaves et al., 2008) was
additionally analysed to validate the results.
For analyzing the internal shell structure and possible alterations,
we used LA-ICP-MS on shells from G. inflata. A Nd:YAG laser
(NewWave UP 193 nm), coupled to a Thermo-Finnigan Element 2
Sector Field ICP-MS, was employed to analyze selected shells for
potential diagenetic overgrowths or sedimentary contaminants. Time
resolved data provide the possibility to analyze trace element
variations throughout the test wall (ablation time ~ penetration
depth (Eggins et al., 2003)). From four core top and plankton tow
samples (MUC 651, 653, 657 and MSN K 33 B1, which span the entire
transect) the final three chambers (f, f-1, f-2) of four to six randomly
selected specimens of G. inflata per sample were investigated for
intra-shell trace element heterogeneities. We only used G. inflata since
shells of G. bulloides were too thin and fragile to obtain significant
laser ablation profiles of sufficient length of time. Before analysis,
uncrushed shells were cleaned following the protocol of Barker et al.
(2003), but with omission of the ultrasonication treatment. Spot sizes
were set at 35–50 μm, energy density at ~ 0.14 GW/cm2, and
repetition rate was 5 Hz. Element concentrations were determined
on the isotopes 25 Mg, 27Al, 43Ca and 55Mn. Aluminum and manganese
were monitored as indicators for clay fills and diagenetic overgrowths, respectively. Calcium was handled as internal standard at a
concentration of 40 wt.%. A silicate standard NIST 612 (Pearce et al.,
1997) and a carbonate standard JCt-1 (Inoue et al., 2004) for matrixmatched calibration were measured before and after five sample
analyses as external standards. Analytical precision of laser ablation
measurements was better than 97% with a standard error of 0.76% for
Mg/Ca.
We used scanning electron microscopy (SEM) to gain information
about possible diagenetic overprints on selective shells. Several test
fragments which were still large enough after cleaning were mounted
on a carrier for scanning electron microscope (SEM) analysis with a
magnification of up to 5000× (Zeiss Supra 40, Historical Geology and
Palaeontology Group at the University of Bremen) as an additional
tool to detect contamination, dissolution or high-Mg-calcite overgrowths that could lead to alteration of Mg/Ca ratios.
3.2. Hydrographic data and habitats of G. inflata and G. bulloides
In addition to plankton tow and core top samples, CTD (Conductivity–
Temperature–Depth probe) was deployed in the upper water column at
each station (Schulz et al., 2006) to gain information on water
conductivity (salinity) and temperature at each specific location and
water depth (Fig. 2; Table 1). To determine the calcification depth of
G. bulloides and G. jinflata all individuals of the plankton tow samples were
picked and counted with regard to their cytoplasm content (Fig. 2). While
the absence of cytoplasm in the inner test chambers indicates that the
individual was clearly dead when collected, the existence of cytoplasm in
the inner test chambers may indicate either living specimens or
specimens that died recently and are passively sinking trough the water
column. Nevertheless, the dominance of shells with cytoplasm in a specific
depth range is a reasonable way of deducing the preferred habitat of a
species. The combination of determined calcification habitat of both
species and CTD data allows for the estimation of the calcification
conditions for the analyzed foraminifera.
3.3. Calculating calcite saturation state of sea water
The calcite saturation state of seawater (Ωcc) indicates if calcite
tends to dissolve or precipitate and is given by:
Ωcc =
h
2+
Ca
i h
i
T CO2− = K
3
sp
ð1Þ
Where [Ca2+] and [CO2−
3 ] are the concentrations of calcium and
carbonate ions in sea water, respectively. Ksp is the solubility product
of calcium and carbonate. When Ωcc equals 1 equilibrium exists, while
saturation (N1) and dissolution (b1) occur otherwise (Zeebe and
Wolf-Gladrow, 2001). As a first approximation of ambient Ωcc we
calculated the saturation state of sea water in the Western
Mediterranean using the software package CO2sys (Pelletier et al.,
2005). The different values for carbonate system parameters were
taken from the World Ocean Atlas (WOA, 2001). Dissociation
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
5
Fig. 2. CTD derived temperature and salinity and abundance of cytoplasm bearing tests of G. inflata und G. bulloides of the plankton tow locations at stations (a) POS 334–74 and
(b) POS 334–81. Atlantic Water (AW), Levantine Intermediate Water (LIW) and Mediterranean Intermediate Water (MIW) are labeled.
constants are from Mehrbach et al. (1973); (modified by Dickson and
Millero (1987)) and used on a pH sea water scale.
4. Results
At the plankton tow stations POS 334–74 and 81 the abundance
maximum of both species was observed in the Atlantic Water. Between
16 and 27 cytoplasm bearing G. bulloides/m3 and ~7 to 15 G. inflata/m3
were sampled at station POS 334–74 in the upper 100 m of the water
column, whereas below this level only 0.5 to 2 G. bulloides/m3 and
~3.5 G. inflata/m3 were observed. At station POS 334–81 the same
decreasing abundance with water depth was found in G. bulloides,
although overall counts where lower (0–22 specimens/m3). G. inflata
showed abundances up to 24 specimens/m3 in the upper 100 m and less
than 3 specimens/m3 below 100 m at station POS 334–81. For precise
comparison of water properties and the foraminiferal Mg/Ca in the
plankton tow samples mean temperatures and salinities of each
sampling interval were derived from CTD data (Table 1, Fig. 2).
Previous studies from the Mediterranean (Barcena et al., 2004;
Pujol and Vergnaud Grazzini, 1995; Schiebel et al., 1997) conclude
that G. bulloides dominates in spring at an average depth of 20–200 m,
whereas G. inflata calcifies throughout the year between 20–500 m
water depth. Accordingly, mean spring (April, May, June) and mean
annual calcification temperatures and salinities of G. bulloides and
G. inflata, respectively, were derived from World Ocean Atlas 2001
data (WOA, 2001) for the respective habitat depths for the core top
samples (Table 1). Although the reproductive cycle of G. inflata has
never been studied systematically, Hemleben et al. (1989) assume a
monthly cycle. A short life cycle is consistent with the observation of
large temporal fluctuations in its standing stocks as reported for
example by Pujol and Vergnaud Grazzini (1995).
The bulk geochemical analyses reveal that G. bulloides has
inherently higher Mg/Ca than G. inflata, which is in accordance with
previous studies (Anand et al., 2003; Cléroux et al., 2008; Elderfield
and Ganssen, 2000). Mg/Ca range from 2.76 to 3.96 mmol/mol
(G. bulloides) and from 1.55 to 3.17 mmol/mol Mg/Ca (G. inflata) in
the plankton tow samples and from 3.34 to 7.52 mmol/mol
(G. bulloides) and from 1.65 to 3.12 mmol/mol Mg/Ca (G. inflata) in
the core top samples (Table 1).
In G. inflata of the plankton tow samples we observe medium Mg/Ca
(2–2.5 mmol/mol) in the top 50 m, high Mg/Ca (up to 3.2 mmol/mol)
between 50 and 100 m, and a clear decreasing trend down to water
depths of 700 m. No such trend is apparent in G. bulloides (Fig. 3). Mg/Ca
in G. inflata from core top samples seems to average the Mg/Ca in the
plankton tow samples (Fig. 3). Likewise, Mg/Ca in G. bulloides from the
core top samples at station POS 334–74 is within the range of Mg/Ca in
the plankton tow samples at this station (Fig. 3). In contrast, Mg/Ca
in G. bulloides from core top samples from the easternmost site
(POS 334–81) do not match Mg/Ca found in plankton tow samples
from the upper water column, but exhibit much higher ratios of
~7.5 mmol/mol (Fig. 3).
Laser ablation depth profiling in G. inflata shows that the internal
shell structure consists of different layers with distinct Mg/Ca ratios
(Fig. 4). In the plankton tow samples, the outer shell shows low Mg/Ca
of approximately 1 mmol/mol. The inner shell shows continuously
increasing Mg/Ca of up to 6 mmol/mol towards the inner shell surface.
It is striking that in contrast to the plankton tow samples, the outer
shell surfaces of the core top samples also exhibit high Mg/Ca peaks of
up to ~ 8 mmol/mol, which are accompanied by increased Mn/Ca
ratios (Fig. 4). In many cases of both plankton and core top samples,
the between-chamber variability further demonstrates that Mg/Ca in
the final chamber (f) is significantly higher than in the chambers f-1
and f-2. This difference in Mg/Ca between the final and the older
chambers may reach up to 35%.
5. Discussion
5.1. Mg/Ca in G. inflata
5.1.1. Plankton tow samples
G. inflata is a deep dwelling foraminifer which has been reported to
occur over a large depth range (Bé and Hutson, 1977; Wilke et al.,
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
6
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
Fig. 3. Bulk Mg/Ca in (a) G. inflata and (b) G. bulloides vs. water depth at both plankton tow stations POS 334–74 and POS 334–81. Gray and white areas mark depth ranges of
sampling intervals. Samples from the upper 700 m represent multi net samples, lower ones represent core top samples.
Fig. 4. Depth profiles for different chambers of G. inflata obtained with laser ablation ICP-MS for a) plankton tow samples and b) core top samples. Time resolved data show variations
of Mg/Ca and Mn/Ca over shell thickness. Shaded areas represent the within-spot reproducibility (~ 27% RSD) obtained from LA profiles through the carbonate standard JCt-1. Grey
vertical bars indicate Mg/Ca peaks, which are accompanied by increased Mn/Ca ratios (N0.01 mmol/mol). Although the signal is likely a mixed signal from different layers, an
increase of Mg/Ca towards the inner shell is evident for the existence of different calcite layers. In contrast to the plankton tow samples (a), the core top samples (b) are probably
affected by diagenetic Mn–Mg-rich overgrowths.
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
2006). Our plankton tow analyses show living G. inflata between 0 and
700 m at the westernmost station POS 334–74, and between 0 and
200 m at the easternmost station POS 334–81 (Fig. 2). Several studies
concluded that calcification takes place over a large range between
20–500 m water depth (Elderfield and Ganssen, 2000; Lončarić et al.,
2006); Chiessi et al. 2008, supplement). Cléroux et al. (2008)
concluded that G. inflata mainly calcifies at the bottom of the summer
thermocline and only descends to greater water depth when
temperatures get warmer than 16 °C. The decreasing Mg/Ca trend in
G. inflata with increasing depth in the plankton tow samples can thus
be readily explained by calcification continuing throughout the water
column, recording ever lower temperatures into their test calcite with
depth (Fig. 3). However, the observed decrease with depth in Mg/Ca
of 1.42 mmol/mol and 1.35 mmol/mol for the plankton tow samples
would imply a temperature decrease of 7.9 and 8.3 °C, respectively
(Chiessi et al. 2008, supplement) (Fig. 5, Table 2). This is significantly
more than the 2–2.5 °C change which has been measured by CTD
during sampling.
This apparent disagreement can probably be explained by
biomineralization changes during the life cycle of G. inflata. Although
care was taken that always the same size fraction was picked, the
shallower collected specimens were usually slightly smaller and
mostly uncrusted, whereas deeper collected specimens were slightly
bigger and contained larger amounts of calcite crust. This indicates
that the relative amount of calcite crust increases with depth. For
comparison, a difference in reconstructed temperatures of up to 6 °C
between encrusted and uncrusted specimens of G. inflata was found
for South Atlantic core top samples (Chiessi et al. 2008, supplement).
It was also suggested that the temperature dependency of calcite crust
is probably different from primary calcite (Erez, 2003). Therefore, the
temperature dependency of the shallow, less encrusted specimens
could be expected to be different from the dominantly encrusted deep
specimens. Since existing Mg/Ca paleotemperature calibrations for
G. inflata are mostly based on a specific size fraction and encrustation
stage (Chiessi et al. 2008, supplement), the estimated temperature
decrease for plankton tow samples based on existing calibrations for
sediment samples is most likely overestimated.
The differences in calcite composition of different calcite layers
within one shell are further illustrated by laser ablation analyses. We
observe a decreasing trend in Mg/Ca from the inner to the outer test,
which shows the difference between primary calcite in the inner part
of the test and calcite crust in the outer part (Fig. 4). Average Mg/Ca
for the calcite crust are b1 mmol/mol (Fig. 4). This is consistent with
plankton tow data, where the deepest samples, presumably containing the highest percentage of calcite crust, show Mg/Ca of about
1.5 mmol/mol. Also, Mg/Ca of the shallowest plankton tow samples
are 2.5–3 mmol/mol, similar to the inner calcite as shown by laser
ablation ICP-MS. These results are similar to laser ablation profiles for
G. inflata from a North Atlantic sediment trap study (Hathorne et al.,
2009).
A combination of bulk ICP-OES and LA-ICP-MS results suggests
that the low-Mg/Ca calcite crust, which forms on the outer part of the
tests, is the reason for the lower bulk Mg/Ca in the deeper samples
(below ~300 m) (Figs. 4, 6). Thus, we see that the overall Mg/Ca trend
seems to be in accordance with previous results. Taken together our
results highlight the importance of calcification changes during
ontogeny on the resulting bulk Mg/Ca signal in G. inflata.
7
Fig. 5. Bulk Mg/Ca in a) G. inflata and b) in G. bulloides vs. temperature in comparison to
existing paleotemperature calibrations (Table 2).
tow samples from 20 to 500 m depth at the same location (Fig. 3).
Although the plankton tow samples were taken during early spring, the
core top samples probably represent an annual average (Barcena et al.,
2004; Pujol and Vergnaud Grazzini, 1995). Nevertheless, as the annual
temperature range at intermediate water depth at the sampling
locations is small (b1.6 °C) (Locarnini et al., 2006) the core top samples
seem to reliably register water mass temperature at intermediate
depths as was also shown for various locations in the Atlantic (Lončarić
et al., 2006; Wilke et al., 2006; Chiessi et al. 2008, supplement). Three
additional core top samples (at stations POS 334–75, 77, and 79)
complete the transect through the Western Mediterranean. Although
the values show a relatively large spread they also record intermediate
water temperatures (Fig. 5). None of the values deviates substantially
from the expected Mg/Ca at the given SST and salinity range and we thus
conclude that G. inflata shells in the Western Mediterranean contain a
primary signal reflecting their calcification habitat.
5.2. Mg/Ca in G. bulloides
5.1.2. Core top samples
Based on earlier studies (Elderfield and Ganssen, 2000; Loncaric
et al., 2006) and our own results we conclude that G. inflata in the
Western Mediterranean calcifies throughout the year within 20–500 m
water depth. Considering this inferred habitat and calcification depth
range of G. inflata, Mg/Ca of bulk G. inflata from the core top samples are
comparable with the average Mg/Ca within the plankton tow samples.
G. inflata Mg/Ca in core top samples show values similar to the plankton
5.2.1. Plankton tow samples
G. bulloides is a planktonic foraminifer that mainly lives in the upper
part of the water column without experiencing migration to greater
water depths (Schiebel et al., 1997). Accordingly, calcification only
should take place down to a water depth of about 100 m (Ganssen and
Kroon, 2000; Schiebel et al., 1997). Our results, indeed, suggest that
G. bulloides calcifies very shallow (upper 200 m) as Mg/Ca do not change
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
8
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
Table 2
Mg/Ca-temperature calibrations for G. bulloides and G. inflata, source of the foraminifera, size fractions, and temperature range of the calibrations.
Species
Source
Size fraction
(μm)
Aa
Ba
r2
Temperature range
(°C)
Reference
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
Surface samples
Sediment trap
Sediment trap
Surface samples
Surface samples
Surface samples
Sediment trap
Culture study
Culture and surface samples
Culture study
n.a.
350–500
350–500
355–400
350–500
n.a.
212–355
n.a.
250–350
n.a.
0.49
0.56
0.299
0.71
0.831
0.56
1.2
0.53
0.474
0.528
0.10
0.058
0.09
0.06
0.066
0.1
0.057
0.1
0.107
0.102
n.a.
0.55
n.a.
0.72
0.78
n.a.
0.9
0.93
0.98
0.93
7.5–15
15–21
15–21
10.5–17.9
3–16
7.5–15
16–31
16–25
9–25
9–25
Elderfield and Ganssen (2000)
Anand et al. (2003)
Anand et al. (2003)
Cléroux et al. (2008)
Chiessi et al. (2008)
Elderfield and Ganssen (2000)
McConnell and Thunell (2005)
Lea et al. (1999)
Mashiotta et al. (1999)
Mashiotta et al. (1999)
a
inflata
inflata
inflata
inflata
inflata
bulloides
bulloides
bulloides
bulloides
bulloides
B is the temperature-sensitive component and A the y-axis intercept in the general exponential expression Mg/Ca = A * exp(B * temperature).
with depth (Fig. 3). Specimens found in deeper water in our plankton
tow samples thus mostly represent sinking specimens which probably
were not living and calcifying, even if some small amount of tests still
seemed to contain remains of cytoplasm (Fig. 2). If these specimens
were calcifying far below 100 m, we would expect a decreasing trend in
Mg/Ca caused by the temperature dependence of Mg uptake and colder
water temperatures (Fig. 3).
However, Mg/Ca of G. bulloides are significantly higher than in
other regions where calibrations have been developed and applied to
temperature reconstructions (Fig. 5, Table 2) (Anand et al., 2003;
Elderfield and Ganssen, 2000; Lea et al., 1999; Mashiotta et al., 1999).
Converted into temperature, G. bulloides Mg/Ca corresponds to values
up to 20 °C (Lea et al., 1999; Mashiotta et al., 1999; McConnell and
Thunell, 2005), whereas in situ temperatures measured via CTD
showed values of 15.5 °C. Since the uppermost part of the water
column where G. bulloides is calcifying is formed by lower saline
Atlantic inflow water rather than by the higher saline Eastern
Mediterranean water (Fig. 1), it seems unlikely that salinity was
responsible for these generally increased values.
5.2.2. Core top samples
In the core top samples of the westernmost station POS 334–74,
Mg/Ca in G. bulloides is about the average of the plankton tow samples
from the same location. At the easternmost station POS 334–81,
however, core top Mg/Ca are much higher than the plankton tow
samples with values up to 7.5 mmol/mol (Fig. 3). Since the plankton
tow samples do not show these extreme values there must be a
secondary factor influencing the sediment Mg/Ca. The core top
samples from station POS 334–77 have also highly increased Mg/Ca,
while core top samples from POS 334–75 and 79 show only slightly
higher values compared to the plankton tow samples (Fig. 5; Table 1).
These results are similar to the pattern in the core top results for
G. bulloides of Ferguson et al. (2008, supplement). The westernmost
samples are in agreement with the plankton tow samples, though in
general higher than expected.
5.3. Secondary influences on foraminiferal Mg/Ca
In G. inflata, the overall trend in Mg/Ca in both plankton tow and
core top samples is in accordance with previous results and is
consistent with vertical migration in the water column and ontogenetic calcification changes. In G. bulloides, however, the absolute Mg/Ca
values in all plankton tow samples are higher than expected from
existing calibrations, and some of the core top samples seem to contain
an additional imprint, leading to even higher Mg/Ca. These two
observations imply that other processes than known relationships
between Mg/Ca and SST have to be considered, as also suggested by
Wit et al. (2010). It also has to be explored why these factors seem to
have an influence on G. bulloides but not on G. inflata. Possible reasons
causing these unexpectedly high Mg/Ca are discussed in the following
sections.
Fig. 6. a) Bulk Mg/Ca in G. inflata vs. water salinity. ΔMg/Ca is the difference between
the analyzed Mg/Ca ratio and the value expected from the calibration of Chiessi et al.
2008, supplement. b) Bulk Mg/Ca in G. bulloides vs. water salinity. ΔMg/Ca is the
difference between the analyzed Mg/Ca ratio and the value expected from the
calibration of Mashiotta et al. (1999).
5.3.1. Salinity
Recently, several studies have investigated the influence of salinity
on Mg/Ca in planktonic foraminifera (Dueñas-Bohòrquez et al., 2009;
Ferguson et al., 2008; Hoogakker et al., 2009; Kisakürek et al., 2008;
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
Mathien-Blard and Bassinot, 2009). Although the mechanism is not
yet fully understood, it generally appears that higher salinities
increase the Mg content of foraminiferal tests. This was already
shown in the culturing studies by Nürnberg (1995) and Lea et al.
(1999) who suggested a 3–10% increase in Mg/Ca per 1 psu increase
in salinity. This slope was confirmed in a recent culturing experiment
by Kisakürek et al. (2008) on G. ruber, whereas Dueñas-Bohòrquez
et al. (2009) only found a dependency of ~2% on cultured G. sacculifer.
Ferguson et al. (2008) used a core top collection from the Eastern
Mediterranean to find apparent salinity dependencies for several
species varying from 19% to 59% increase in Mg/Ca per 1 psu increase
in salinity. Recently, however, it was shown that anomalously high
Mg/Ca found in Red Sea samples are probably caused by diagenesis
rather than calcification in high salinities (Hoogakker et al., 2009).
Although the Western Mediterranean is not characterized by
salinities as high as the Eastern Mediterranean or the Red Sea, salinity
is still higher than in the open-ocean with values up to 38.5 psu
(Fig. 1). Accordingly, we expect that some salinity effect on Mg/Ca
could have taken place. This influence could explain the difference in
Mg/Ca between the shallowest plankton tow samples (0–50 m) of
G. inflata in comparison with the slightly deeper ones (~100 m)
(Fig. 3). Inflowing Atlantic water (AW) into the Mediterranean with a
salinity of only 37 psu occupies the upper 50 m of the water column.
Below, saltier water (LIW, modified to MIW) originating in the Eastern
Mediterranean is dominant. As temperature is not changing significantly in the upper part of the water column, it can be argued that the
slightly higher Mg/Ca at 100 m for G. inflata than those at 0–50 m are
caused by calcification under different salinities. Since the AW mass is
reaching slightly deeper in the west and thinning out towards the east
(Figs. 1 and 2), we observe this increase in Mg/Ca slightly deeper at
station POS 334–74 than at station POS 334–81 (Fig. 3a). Assuming
that salinity is causing the observed differences in Mg/Ca of G. inflata
in the upper 100 m of the water column, we conclude that G. bulloides
only lives and calcifies in either one of the water masses since they do
not show this difference within the upper water column. However, it
is noteworthy, that in spring, salinity is only slightly increased in the
Western Mediterranean, where our plankton tow samples originate
and therefore, these samples in this setting are not expected to show a
strong salinity dependant change in Mg/Ca.
To assess the potential salinity effect, we corrected Mg/Ca for the
known temperature dependency. We calculated the expected Mg/Ca
ratio for the samples by applying the SST calibration of Chiessi et al.
2008, supplement for G. inflata and Mashiotta et al. (1999) for
G. bulloides and subtracted this from the analyzed values. The residual
ΔMg/Ca was then compared with salinity (Fig. 6). Since the apparent
relationship of ΔMg/Ca and salinity in G. bulloides core top samples is
only caused by two high values and shows a slope much higher than
any known salinity effect on planktonic Mg/Ca, it is rather unlikely
that the two extreme values really result from calcification under
increased salinity. Apart from these two extreme values, unlike the
observations by Ferguson et al. (2008), we do not observe any clear
correlation between salinity and plankton tow or core top sample
ΔMg/Ca. Consequently, beside a possible salinity effect on G. inflata at
the sea surface, the anomalous Mg/Ca values in plankton tow and in
particular in core top samples of G. bulloides cannot be explained by
changing salinities.
5.3.2. Diagenetic influence on foraminiferal Mg/Ca
Diagenetic alteration of foraminifer tests can influence or even
completely overprint the geochemical signature of the primary calcite
precipitated in the surface ocean (Hover et al., 2001; Walter and
Morse, 1984). In particular, Mg/Ca is easily altered by the influence of
inorganic carbonate phases on the foraminifer tests, which are often
not removed by standard cleaning techniques (Groeneveld et al.,
2008; Regenberg et al., 2007). We investigated several samples with
Scanning Electron Microscopy (SEM) to determine the possible
9
Fig. 7. SEM micrographs of G. bulloides of MUC 657 at station POS 334–81 showing
patchy diagenetical overgrowths of secondarily precipitated inorganic calcite.
presence of secondary carbonate phases (Fig. 7). Foraminifera used
for SEM analysis all underwent the normal cleaning process to remove
contaminants like clays and coccoliths, both from plankton tow and
core top samples. Tests from the plankton tow samples were found to
be free of any secondary phases. In the core top samples, however,
secondary overgrowths were found, but interestingly only on tests of
G. bulloides (Fig. 7). Similar overgrowths were also found on
foraminifera from the Caribbean, Red Sea, and Eastern Mediterranean
(Ferguson et al., 2008; Groeneveld et al., 2008; Regenberg et al.,
2007). SEM did not reveal any obvious crystal overgrowths on shells
of G. inflata.
Indications for diagenesis such as anomalous Mg/Ca ratios and
crystalline overgrowths visible via SEM analysis only occur in
G. bulloides in the more easterly core top samples of the transect
(stations POS 334–77, –79, and –81). A possible reason for this could
be the change in the calcite saturation state Ωcc of the water column.
Deep water coming from the Eastern Mediterranean is both warm and
more saline (Boyer et al., 2002; Stephens et al., 2002) and, hence, has a
very high saturation state (Ωcc). With an increase in salinity the
number of ions in seawater increases resulting in a higher ionic
strength. Also, the activity of ions in seawater with a higher salinity
decreases. As the activity for the different ions in the carbonate system
is not changing equally, a new equilibrium between them is formed in
which the concentration of CO2−
is higher than at lower salinity. As
3
the concentration of CO2−
mainly determines the saturation state of
3
seawater, an increase in salinity leads to an increase in saturation state
(Zeebe and Wolf-Gladrow, 2001).
It was recently shown during cultivating experiments that an
extremely high calcite saturation state causes inorganic calcite
precipitation on foraminifer tests (Raitzsch et al., 2010). The
consequence of these highly saturated conditions is that precipitation
of inorganic calcite can occur on any available surface. Therefore, the
surface of foraminiferal tests, as soon as they are formed and especially
after being deposited, become available as crystal nuclei for the growth
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
10
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
of inorganic calcite crystals. This would also provide an alternative
explanation for the high core top Mg/Ca data from Ferguson et al. (2008)
from the Eastern Mediterranean. Although a correlation between Mg/Ca
and salinity was found, it seems more likely that the high calcite
saturation state of the Eastern Mediterranean led to increased
precipitation of inorganic calcite on the foraminifer tests. On the other
hand, deep water in the westernmost part of the Mediterranean is a
mixture from the Eastern Mediterranean and the Gulf of Lyons deep
water, which has a substantially lower saturation state than pure
Eastern Mediterranean water (Ωcc = 5, Stommel, 1972). Therefore,
samples from the westernmost part of the transect are not affected by
diagenesis linked to saturation state of bottom waters.
In addition to this increase of Ωcc from west to east there is a decrease
in Ωcc from the surface towards the bottom, in line with an increase in
pressure with depth has been observed (Zeebe and Wolf-Gladrow,
2001). Since the solubility of calcium carbonate increases with pressure,
the concentration at which CO2−
is saturated increases with water
3
depth while the in situ CO2−
concentration decreases leading to a
3
decrease in saturation state.
These two trends (increasing Ωcc from west to east and decreasing
Ωcc with water depth) lead to overall supersaturated waters in the
Mediterranean, from the surface (Ωcc = 4–6) to the bottom (Ωcc = 2–3)
(Millero et al., 1979; Schneider et al., 2007). Especially the increased Ωcc
in the deeper waters is unusual as in the open ocean most bottom waters
are either undersaturated (dissolution occurs) or only slightly supersaturated (Ωcc just over 1). The core top samples of G. bulloides show
these two trends in their Mg/Ca, which reflect an increased potential for
diagenesis due to increased Ωcc towards the east. Since Ωcc is higher at
shallow water depths, the shallow core top samples at stations POS 334–
77 and –81 (~1000 m water depth) are more diagenetically altered and
thus show higher Mg/Ca than foraminifera at POS 334–79 at 2996 m
depth.
This observation is in accordance with the data presented by
Ferguson et al. (2008) where G. bulloides show the highest Mg/Ca in
core top T87 65B (Fig. 5), which is located in the eastern part of the
Western Mediterranean at a water depth of 904 m, thus at a location
and depth with highly elevated in Ωcc. All other core top samples of
G. bulloides reported by Ferguson et al. (2008) show lower Mg/Ca,
which is consistent with their greater sampling depth and, therefore,
less increased Ωcc. One core top sample from above 300 m water
depth in the Alboran Sea also shows low Mg/Ca, which is in this case
due to the fact that the sample location is mainly influenced by
inflowing Atlantic Water with a lower saturation state.
A possible reason why G. bulloides but not G. inflata would be
significantly affected by diagenesis might be due to the different ways
their shells are built. Tests of G. inflata form a thick calcite crust during
their life cycle, which is very homogeneous and massive (Sadekov
et al., 2009). Conversely, tests of G. bulloides are lightly built with
many pores and a relatively open crystal structure. This means that
the reactive area available for diagenesis is much larger for G. bulloides
than for G. inflata (Sadekov et al., 2009; Walter and Morse, 1984).
Consequently, G. bulloides can be expected to be much more
susceptible to diagenesis than G. inflata. A similar difference between
these two species has been shown in terms of dissolution susceptibility. Berger (1975) showed his dissolution index for planktonic
foraminifera that G. inflata is among the most resistant species while
G. bulloides is very susceptible to dissolution.
Laser Ablation analyses (Fig. 4) show that the tests of G. inflata
contain a thin coating of calcite with elevated Mg concentration on the
outer side of the tests which was not clearly visible by SEM. This
coating is not present in specimens from the plankton tow samples
even in the deepest layers and, hence, must have been formed after
the specimens descended onto the sediment surface. As Mn/Ca is also
increased, the coating most likely contains a Mn-carbonate phase with
slightly elevated Mg/Ca. As can be seen in Fig. 4, the coating is
volumetrically small compared to the bulk of the shell calcite and
given the range of bulk Mg/Ca for shells from core top samples of
G. inflata, the coating does not seem to have a significant influence on
the bulk Mg/Ca. However, as the total mass of a G. bulloides test is
much less than that of G. inflata, the same amount of diagenesis would
have a larger impact on shell chemistry in the former species.
Possible diagenetic overgrowths can explain the large difference in
Mg/Ca between plankton tow and core top samples in G. bulloides, but
they cannot explain why also the plankton tow samples, which are
obviously unaffected by diagenesis, show consistently higher Mg/Ca
in G. bulloides than expected from known calibrations (Fig. 5). A direct
effect of the calcite saturation state on Mg/Ca of G. bulloides can be
excluded, as it cannot be expected to lead to higher than normal
ratios. Several cultivating experiments on planktonic foraminifera,
including G. bulloides, have shown that less Mg rather than more Mg is
incorporated into the tests during calcification at higher saturation
states (Kisakürek et al., 2008; Lea et al., 1999; Russell et al., 2004). One
factor to be considered is the possibility that the Mediterranean
genotypes of G. bulloides are distinct from those used in previous
calibrations. Previously, it has been shown that different morphotypes
of Globigerinoides ruber, which are most likely representative of
different genotypes (Aurahs et al., 2009; Kuroyanagi et al., 2008), have
distinct Mg/Ca (Steinke et al., 2005). So far, only a sequence of
G. bulloides Type Ib has been identified in the Mediterranean by
deVargas et al. (1997). This genetic type is abundant in the northeastern Atlantic and together with the Indopacific Type Ia constitutes
the warm-water lineage within this morphospecies (Darling and
Wade, 2008; Kucera and Darling, 2002). The fact that this genetic type
is not restricted to the Mediterranean makes it unlikely that the
observed anomalously high Mg/Ca is due to genotype-specific vital
effects.
6. Conclusion
We have investigated Mg/Ca of tests of Globorotalia inflata and
Globigerina bulloides from the Western Mediterranean from a series of
plankton tow and core top samples. Our results show that:
1. The decreasing Mg/Ca trend with increasing water depth within
the plankton tow samples in G. inflata can be explained by
precipitation of low-Mg calcite crust in deeper and colder water
masses and the observed trend reflects the life cycle and habitat of
this deep dwelling species. The homogeneous Mg/Ca ratios in
G. bulloides throughout the upper 700 m of the water column at
both stations reflect the origin of the Mg/Ca signal in this species at
the surface within the same water mass. They also imply that
specimens with cytoplasm from lower levels in the water column
were no longer calcifying or that they represented sinking, dead
individuals.
2. Scanning electron microscopy analyses and Laser Ablation ICP-MS
of core top samples suggest secondary precipitation of inorganic
calcite on the foraminifer tests. This diagenetic overprint seen in
increased Mg/Ca is mainly present in foraminifera sampled at the
eastern locations with water depth less than 1500 m. This is in
accordance with two observed trends in the Mediterranean:
(1) increasing calcite saturation state from west to east and
(2) decreasing calcite saturation state with water depth. These
super-saturated waters can cause the precipitation of inorganic
calcite on the foraminifer test after their lifecycle has ended.
3. Diagenetic effects alter the test chemistry of G. bulloides more than
that of G. inflata due to the differences in test structure, i.e. thin,
porous test for G. bulloides, and encrusted, massive tests for
G. inflata. The core top samples of G. inflata might be slightly
influenced by diagenesis, as suggested by laser ablation ICP-MS
analyses. But the volumetric contribution to the bulk signal is small
(up to 5%, if the shell wall thickness is considered).
Please cite this article as: van Raden, U.J., et al., Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from
Western Mediterranean plankton tow and core top samples, Mar. Micropaleontol. (2010), doi:10.1016/j.marmicro.2010.11.002
U.J. van Raden et al. / Marine Micropaleontology xxx (2010) xxx–xxx
4. The reason for the consistently higher Mg/Ca in plankton tow
samples of G. bulloides of the Western Mediterranean cannot be
explained by a direct influence of salinity or calcite saturation state
on the incorporation of Mg into the test and this phenomenon
requires further investigation.
Acknowledgements
We thank Hartmut Schulz and the captain and the crew of RV
Poseidon POS 334 for the samples used in this study. Silvana Pape and
Martin Kölling are thanked for laboratory assistance; Stephan Steinke,
Ed Hathorne, and Prof. Christoph Hemleben for discussion; Petra
Witte for assistance with SEM analyses. JG would like to thank the
MARUM for a MARUM fellowship.
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