Stable Carbon Isotopes in Benthic Foraminifera: Proxies for Deep and
Bottom Water Circulation and New Production
A. Mackensen 1 * and T. Bickerf
1 Alfred-Wegener-lnstitutfur Polar- und Meeresforschung, Postfach 12 0161,
D-27515 Bremerhaven, Germany
2 Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 0440,
D-28334 Bremen, Germany
*corresponding author (e-mail):amackensen@awi-bremerhaven.de
Abstract: The /i13C values recorded in benthic foraminiferal calcareous tests reflect the /ilJC values
of the dissolved inorganic carbon (/i13C~C02) of ambient deep and bottom water masses and as such
are widely used as a nutrient proxy to reconstruct deep ocean paleocirculation. This paper summarizes
new results on the thermodynamiciy caused decoupling of nutrients and the /i13C'C02 signal in the
Weddell Sea and its potential influence on paleoceanographic reconstruction. In addition, new data
are presented on high latitude benthic foraminiferal (Cibicidoides spp.) /i13C and bottom water
/i13C~C02 values, which prove a significant deviation from the ideal one-to-one relationship used in
paleoceanography. Foraminiferal/i13C values by 0.2 - 0.6 %0 lower than the ambient bottom water
/i13C'C02 values coincide with areas of highly seasonal productivity, the development of plankton
blooms, and subsequent rapid sedimentation of phytodetritus. On the one hand the phytodetritus
triggers foraminiferal calcification and reproduction, but at the same time it produces a lJC depleted
microenvironment at the sediment/water interface. Benthic /il3C values of Cibicidoides spp. by
0.2 - 0.4 %0 higher than bottom water values are found close to the Antarctic and the Arctic Eurasian
continental margins. Therefore, we here speculate that during processes of bottom water formation
in polynyas and brine formation on the continental shelves, water masses with enriched /i13C EC02
values spill down the continental slopes and influence the benthic foraminiferal /i13C record.
Compensation for an artificial lowering ofthe Cibicidoides /iBC signal in areas of seasonally high
primary production in the South Atlantic Ocean, allows for a paleoceanographic interpretation of
very high glacial to interglacial amplitudes of up to 1.6 %0 in benthic /i13C records in agreement with
reconstructions based on other proxies. In particular, low Antarctic Cd values and regionally low
/i 13 C values in glacial benthic foraminifera now can be consistently interpreted as indicating relatively
low nutrient concentrations and thus relatively young Southern Ocean deep and bottom water
masses compared to glacial Pacific deep water.
Introduction
In most oceans, a linear correlation is observed to reconstruct deep ocean paleocirculation ( e.g.
between /il3C~C02 values and nutrient contents of Duplessy et al. 1984; Curry et al. 1988; Oppo et al.
deep and bottom water masses because the distri- 1990; Raymo et al. 1990; Boyle 1992; Sarnthein et
bution of both are controlled by the interaction of aI. 1994). Although different methods and tools have
biologic uptake at the sea surface and decomposi- been applied to reconstruct glacial nutrient distribution in deeper water masses with the general cir- tion and circulation patterns of the ocean, quantitaculation of the ocean (Kroopnick 1980; Kroopnick tive estimates regarding exchange fluxes between
1985). This is important, because the /il3C signal of the main basins or between surface, intermediate
the water masses is recorded in benthic foraminifera and deep water masses have not been well
and as such is extensively used as a nutrient proxy constrained (cf. Zahn et a1. 1994). Moreover,
From FISCHER G, WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 229-254
Bottom Water Circulation and New Production
A. Mackensen 1 * and T. Bickerf
1 Alfred-Wegener-lnstitutfur Polar- und Meeresforschung, Postfach 12 0161,
D-27515 Bremerhaven, Germany
2 Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 0440,
D-28334 Bremen, Germany
*corresponding author (e-mail):amackensen@awi-bremerhaven.de
Abstract: The /i13C values recorded in benthic foraminiferal calcareous tests reflect the /ilJC values
of the dissolved inorganic carbon (/i13C~C02) of ambient deep and bottom water masses and as such
are widely used as a nutrient proxy to reconstruct deep ocean paleocirculation. This paper summarizes
new results on the thermodynamiciy caused decoupling of nutrients and the /i13C'C02 signal in the
Weddell Sea and its potential influence on paleoceanographic reconstruction. In addition, new data
are presented on high latitude benthic foraminiferal (Cibicidoides spp.) /i13C and bottom water
/i13C~C02 values, which prove a significant deviation from the ideal one-to-one relationship used in
paleoceanography. Foraminiferal/i13C values by 0.2 - 0.6 %0 lower than the ambient bottom water
/i13C'C02 values coincide with areas of highly seasonal productivity, the development of plankton
blooms, and subsequent rapid sedimentation of phytodetritus. On the one hand the phytodetritus
triggers foraminiferal calcification and reproduction, but at the same time it produces a lJC depleted
microenvironment at the sediment/water interface. Benthic /il3C values of Cibicidoides spp. by
0.2 - 0.4 %0 higher than bottom water values are found close to the Antarctic and the Arctic Eurasian
continental margins. Therefore, we here speculate that during processes of bottom water formation
in polynyas and brine formation on the continental shelves, water masses with enriched /i13C EC02
values spill down the continental slopes and influence the benthic foraminiferal /i13C record.
Compensation for an artificial lowering ofthe Cibicidoides /iBC signal in areas of seasonally high
primary production in the South Atlantic Ocean, allows for a paleoceanographic interpretation of
very high glacial to interglacial amplitudes of up to 1.6 %0 in benthic /i13C records in agreement with
reconstructions based on other proxies. In particular, low Antarctic Cd values and regionally low
/i 13 C values in glacial benthic foraminifera now can be consistently interpreted as indicating relatively
low nutrient concentrations and thus relatively young Southern Ocean deep and bottom water
masses compared to glacial Pacific deep water.
Introduction
In most oceans, a linear correlation is observed to reconstruct deep ocean paleocirculation ( e.g.
between /il3C~C02 values and nutrient contents of Duplessy et al. 1984; Curry et al. 1988; Oppo et al.
deep and bottom water masses because the distri- 1990; Raymo et al. 1990; Boyle 1992; Sarnthein et
bution of both are controlled by the interaction of aI. 1994). Although different methods and tools have
biologic uptake at the sea surface and decomposi- been applied to reconstruct glacial nutrient distribution in deeper water masses with the general cir- tion and circulation patterns of the ocean, quantitaculation of the ocean (Kroopnick 1980; Kroopnick tive estimates regarding exchange fluxes between
1985). This is important, because the /il3C signal of the main basins or between surface, intermediate
the water masses is recorded in benthic foraminifera and deep water masses have not been well
and as such is extensively used as a nutrient proxy constrained (cf. Zahn et a1. 1994). Moreover,
From FISCHER G, WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 229-254