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Reconstructing southeastern Mediterranean bottom and surface water environments during Sapropel S1 using
laser-ICPMS elemental ratios on foraminiferal shells
Meryem Mojtahid, R. Hennekam, Lennart Jan de Nooijer, Gert-Jan Reichart, G.J. Jorissen, Gert de Lange
To cite this version:
Meryem Mojtahid, R. Hennekam, Lennart Jan de Nooijer, Gert-Jan Reichart, G.J. Jorissen, et al.. Reconstructing southeastern Mediterranean bottom and surface water environments during Sapropel S1 using laser-ICPMS elemental ratios on foraminiferal shells. Inconnu, Nov 2015, Baltimore, United States. 2015. �hal-02872429�
Study Context
We explore the potential of using Na/Ca, Ba/Ca, and Mn/Ca in benthic and planktic foraminiferal species as
proxies for reconstructing past changes in sea water salinity, productivity, and oxygenation. The eastern
Mediterranean is a perfect natural laboratory to perform such study as it is the theatre of one of the most
spectacular climatic phenomena that led to the formation of Sapropels. Two prerequisites have been
suggested for their formation: (a) freshwater flooding leading to stagnant bottom waters with reducing
conditions, and (b) high primary production. The present study focuses on the most recent Sapropel (S1; ~10 to
6 cal ka BP) from a sediment core PS009PC located in the southeastern Levantine Basin (Fig. 1 ). Core PS009PC
18 13
was earlier studied for its inorganic geochemical properties (Ti/Al, Ba/Al, V/Al), δ O
and δ C
G. ruberand
planktic foraminiferal assemblages [1; 2; 3]. This study presents a unique dataset in the Mediterranean and
during such a big environmental change of laser-ICPMS elemental ratios performed on six benthic
foraminiferal species and one planktic species.
G. ruber
Reconstructing southeastern Mediterranean bottom and surface water environments
during Sapropel S1 using laser-ICPMS elemental ratios on foraminiferal shells
1 2,3 3 2,3 1 2
M. MOJTAHID
, Hennekam, R. , De Nooijer, L. , Reichart, G.J. , Jorissen, F. , de Lange, G.
1 2 2
LPG-BIAF UMR-CNRS 6112, University of Angers, UFR Sciences, bd Lavoisier 49045, Angers Cedex 01, France (meryem.mojtahid@univ-angers.fr; frans.jorissen@univ-angers.fr); Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands. F.M.Hennekam@uu.nl; G.J.Reichart@uu.nl; G.J.deLange@uu.nl.; 3 Marine Geology and Chemical Oceanography, Royal Netherlands Institute for Sea Research,
Landsdiep 4, 1797 SZ 't Horntje Texel, The Netherlands. Lennart.de.Nooijer@nioz.nl.
Figure 1. Core location PS009PC (32°07.7'N, 34°24.4'E; 552 m water depth. SW: Mediterranean Surface Waters. The red arrows show modern surface water circulation. Photograph of the core with the location of S1 (Mojtahid et al., 2015).
Conclusions
- Foraminiferal Na/Ca and Ba/Ca present undeniably a good potential for reconstructing paleosalinity and
paleoproductivity variability during S1 whereas Mn/Ca is more likely to record primarily diagenetic processes
rather than the redox signal.
- Our study also shows that several effects certainly hamper the direct use of the equation Na/Ca = 0.22S-0.75 [4]
for quantitative estimates of past salinities → Further calibration studies are needed, for instance, using
deep-sea benthic formainifera under a larger range of salinity variation.
20 0 40 30 50 70 60 80 10 100 90 110 120 Egypt Israel Lebanon Syria Jordan Libya Palestine Cyprus Turkey 200 km Core PS009PC depth (cm) 200 m 1000 m 1500 m
PS009PC
SW Egypt Nile RiverMediterranean
Sea
130 140 150 160 170 180 190 Google earth Sapropel S1Quantitative estimates of Na/Ca = (0.22*Salinity)-0.75; [4]) (Fig. 2):
- Unrealistic values compared to the litterature (Figs. 2a-c; f-h).
- The difference of ~2.9 units between the max and min Na/CaG.ruber-surface salinities during
S1 (Fig. 2f) is coherent with the litterature (Fig. 2h) > the general trend of salinity variation is correctly described.
The possible factors leading to low salinity values using Wit et al. calibration :
1) Calcification rate and ontogenetic effect: In Wit et al. (2013), Na/Ca ratios measured on
A. tepida (<200 µm) correlated negatively with test size (Fig. 2e). This size effect is absent in
our samples (>200 µm) (Fig. 2e) > As the test grows larger, calcification rates slow down leading to lower E/Ca ratios in the larger tests? This pattern is not found in the different life stages of the measured specimens (Fig. 3; b).
2) Diagenesis and dissolution effects: Na+, due to the charge difference, is not directly
2+
substituting for Ca during calcite precipitation.
3) Inter-species effect (Fig. 2b): Faster growing individuals have partition coefficients closer to the abiotic seawater than slower growing individuals [5].
Past deep and surface salinity variations during S1:
Two specific periods (↔ S1a and S1b; [2]) with an interrruption at ~8.1 ka (Fig. 2a) :
salinities (
Period 1: A rise in bottom water
salinities during S1a → Hypothesis: a strong halocline at ~450 m separated stagnant old saline and anoxic waters (see the low abundances of benthic foraminifera (Fig. 2d) and the dominance of low oxygen indicative species (Fig. 2p)) from the well-ventilated upper ocean circulation ↔ the stop of LIW formation?
Period 2: Bottom water salinities raised
shortly during S1b before an abrupt drop around 7.6-7.0 ka, followed by a g r a d u a l i n c r e a s e ( F i g . 2 a ) → Hypothesis: weakening of the halocline due to decreasing freshwater
discharges (as recorded by Na/CaG. ruber;
Fig. 2f) → vertical mixing and freshening of deep waters → the LIW gradually formed again in the Aegean leading to the subsequent increase in bottom water salinities as recorded by
Na/Cabenthics (Fig. 2a).
3.7 4.2 4.7 5.2 5.7 20.2 21.2 22.2 23.2 24.2 25.2 26.2 27.2 28.2 29.2 Na/Ca mmol/mol All species Salinity (after ) Wit et al., 2013 5 37 14 C age (ka BP) 6 7 8 9 10 11 12 38 39 40 41 6 8 10 S1 cal ka BP Salinity Rohling (1994) LIW EMdIW 42 mmol/mol 0.0015 0.0025 0.0035 0.0045 0.0055 0.0065 0.0075 B. alata G. affinis G. altiformis G. orbicularis H. boueana U. peregina S1a S1b 5 6 7 8 9 10 11 12 Ba/Ca cal ka BP Ba/Al Corg (%) 0.4 0.8 1.2 1.6 0.001 0.002 0.003 0.004 V/Al 0.002 0.003 0.004 0.005 0.006 0.007 Hennekam et al. (2014) mmol/mol Ba/Ca 0.002 0.003 0.004 0.005 0.006 All species PFAR 0 3500 7000 10500 14000 Mojtahid et al. (2015) + PP - PP 2 (Ind./cm /Ka) Mn/Ca 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 All species mmol/mol 0 20 40 60 80 100 % Low oxygen indicative species Mn/Al 0.01 0.015 0.02 0.025 0.03 0.01 0.011 0.012 0.013 0.014 0.015 0.016 0.017 a1 a1 Mn/Al
Bottom water characteristics
Productivity ?
Ba/Ca mmol/mol 0.0010 0.0012 0.0014 0.0016 0.0018 0.0020 0.0022 0.0024 0.0026 0.0028 0.0030Salinity ?
Oxygenation ?
3.5 4.0 4.5 5.0 5.5 6.0 Na/Ca mmol/mol 6 7 8 9 10 11 12 S1a S1bcal ka BP 5 B. alata G. affinis G. altiformis G. orbicularis H. boueana U. peregina Size (µm) R² = 0.58 3 4 5 6 7 8 9 10 100 200 300 400 500 600 700 This studyWit et al. (2013): A. tepida
Na/Ca mmol/mol B. alata G. affinis G. altiformis G. orbicularis H. boueana U. peregina
Surface water characteristics
Mn/Ca 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 mmol/mol S1a S1b 5 6 7 8 9 10 11 12 cal ka BP
Figure 2. Bottom and surface water characteritics based on productivity, salinity and oxygenation proxies from this study (Ba/Ca, Na/Ca, and Mn/Ca respectively measured by LA-ICPMS on six benthic species (B. alata, G. affinis, G. altiformis, G. orbicularis, H. boueana, and U. peregrina and one planktic foraminiferal species G. ruber) and from the literature. PFAR: Planktic foraminiferal accumulation rates. LIW: Levantine Intermediate Waters. EMDW- Eastern Mediterranean Deep Waters. SSTUK’37 and SSTTEX86 data are from Castañeda et al. (2010) [8]. Corg, V/Al, Ba/Al, and Mn/Al were measured in the sediment. S1a and S1b: the two phases of Sapropel 1 as defined by Hennekam et al. (2014) [2] in the sediment from the same core.
5 6 7 8 9 10 11 12 38.73 37.73 36.73 35.73 39.73 40.73 41.73 42.73
Emeis et al. (2000) - Levantine Basin
Salinities from SST UK’37 18 δ O and G.ruber S1a S1b Age (cal ka BP) 18 Salinities δ OG.ruber using and SST TEX86 32 33 34 35 36 37 38 39 40 16 18 20 22 24 26 28 30 32 34 18 Salinities δ OG.ruber using and SST UK’37 18 δ OG. ruber ‰ -2.0 -1.0 0.0 1.0 2.0 3.0 Hennekam et al. (2014) Core PS009PC Na/Ca mmol/mol 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5 Salinity (W it et al., 2013) This study 28.8 29.3 29.8 30.3 30.8 31.3 31.8 32.3 32.8 33.3 G. ruber G. ruber 0.0010 0.0015 0.0020 0.0025 0.0030 Nile River discharge Strong Weak Ba/Ca Weldeab et al. (2014) mmol/mol G. ruber 5 6 7 8 9 10 11 12 Age (cal ka BP) S1a S1b
c
e
f
g
h
a
b
d
f
g
h
i
j
k
l
m
n
o
p
q
r
5 6 7 8 9 10 11 12 Age (cal ka BP) mmol/mol S1a S1b Mn/Ca 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13 G. rubers
Figure 3. a) Ba/Ca ratios measured in four chambers of G.
orbicularis during Sapropel S1. b) Ba/Ca ratios in the xix benthic foraminiferal species plotted against chamber stages from the oldest to the youngest stage.
b
1 2
3
4
100 µm G. orbicularis S1a S1b 5 6 7 8 9 10 11 12 cal ka BP No specimens 4.0 4.5 5.0 5.5 6.0 6.5 mmol/mol Na/Caa
Chamber stage B. alata G. affinis G. altiformis G. orbicularis H. boueana U. peregina R²=0.14 R²=0.33 Youngest stage Oldest stage mmol/mol Na/Ca R²=0.21 R²=0.02* * p<0.05 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 0 1 2 3 4 5-
Enhanced (Ba/Al) ratios indicate that more Ba
sedwas reaching the seafloor during
S1 (Fig. 2i). Enhanced BaSO fluxes are a function of both productivity and
4preservation.
- Ba/Ca
benthicsratios recorded during S1 (Fig. 2k) are well correlated to the PFAR (Fig. 2j)
often used as a paleoproductivity proxy.
- In surface waters, Ba is usually depleted through the precipitation of Barite [6].
However, runoff of tropical rivers (e.g., Nile) is highly enriched in dissolved Ba and
2+
may serve as an additional local source of Ba to the Levantine (Fig. 2n). This explains
the high Ba/Ca
G. rubervalues during S1 (Fig. 2m).
- Even though the described feature by Ba/Ca
benthicsratios is coherent with the literature
when considering all species together (Fig. 2k), some species-specific differences are
noted (Fig. 2l)
↔combination of several effects which are not yet well constrained (e.g.,
calcification rate and ontogenetic effects, diagenesis and dissolution effects,
microhabitat effect).
SO
4Productivity ?
Oxygenation ?
Several arguments favor the Mn/Ca recording primarily diagenetic processes; the redox signal being at the best drown in the
diagenetic signal: a) Our Mn/Ca values are far higher than values of primary shell material (<0.001 mmol/mol; [7]) (Figs, 2q; s),
although our study area is under the direct influence of Nile runoff which brought during S1 high C fluxes to the seafloor (Fig.
org2i). b) The absence of the usual anti-correlation between (Mn/Al) and (Mn/Ca)
sed benthics(Figs, 3o; q). c) The high Mn/Ca
G. ruberratios
(Fig. 3s) are also indicative of the presence of diagenetic Mn coatings as dissolved Mn in surface waters are nearly absent.
Alternatively, within S1, there was a significant positive correlation between Mn/Ca
benthics(Fig. 3q) and the percentages of low
oxygen indicative species (Fig. 3p). This might be the only time where we could discriminate between the redox and the
diagenesis signals using our (Mn/Ca)
benthics.
References
reproducibility of high-resolution paleoenvironmental records, Limnol. Oceanogr. Methods, 10, 991–1003; [2] Hennekam et al., 2014: Solar forcing of Nile discharge and sapropel S1 formation in the early- to mid-Holocene eastern Mediterranean, Paleoceanography, 2013PA002553; [3] Mojtahid et al., 2015: 13,000 years of southeastern Mediterranean climate variability inferred from an integrative planktic foraminiferal-based approach, Paleoceanography, 2014PA002705; [4] Wit et al., 2013: A novel salinity proxy based on Na incorporation into foraminiferal calcite, Biogeosciences, 10(10), 6375–6387; [5] A biomineralization model for the incorporation of trace elements into foraminiferal calcium carbonate, Earth Planet. Sci. Lett., 142(3–4), 409–423; [6] Lea, 1999: Trace elements in foraminiferal calcite, in Modern Foraminifera, pp. 259–277, B. K. Gupta, UK., 1999; [7] Lee et al., 2004: Ontogenetic trace element distribution in brachiopod shells: an indicator of original seawater chemistry, Chem. Geol., 209(1–2), 49–65. Castañeda et al., 2010: Millennial-scale sea surface temperature changes in the eastern Mediterranean (Nile River Delta region) over the last 27,000 years, Paleoceanography, 25(1), PA1208, doi:10.1029/2009PA001740.
Aknowledgments
This study makes part of a project MADHO (MediterraneAn Deltas in the Holocene) financed by the international program MISTRALS PaleoMEX. NWO is acknowledged for financial support to PASSAP cruise, PASS2- and PALM-projects. Special thanks to Wim Boer from the NIOZ for his technical support for LA-ICPMS analyses.[1] Hennekam and de Lange, 2012: X-ray fluorescence core scanning of wet marine sediments: methods to improve quality and
7 8 9 10 0 20 40 60 80 100 a1 N° of counted specimens N° of counted specimens 0 500 1000 1500 2000 a1 6 7 8 9 10 11 12 S1a S1bcal ka BP 5
