316
Riihlemann et al.
al. 1993), may be significantly affected by biological activity. Primary producers living in surface waters convert CO 2 into organic matter. Part of this
organic matter sinks into deeper waters where it is
oxidized, releasing initially fixed carbon back to water as CO 2 (Broecker andPeng 1993). This mechanism, known as the 'biological pump' (Volk and
Hoffert 1985), lowers the CO 2 partial pressure in
surface waters and atmosphere. CaC0 3 production in the photic zone, on the other hand, counteracts the effect of organic matter production by the
release of CO 2 , However, the solubility ofCaC0 3
again decreases the CO 2 partial pressure in deep
waters. Opal production or dissolution does not affect the partial pressure of CO 2 in any direction.
Consequently, the efficiency of the biological pump
is determined by the magnitude of primary production, by the fraction ofthis production escaping recycling within the mixed layer and being exported
to deeper waters, and by the ratio of organic carbon to carbonate carbon in sinking biogenic particles (Berger and Keir 1984). For this reason, investigations of biogenic export production rates are
a central theme in the reconstruction of the oceanic carbon budget. The influence and response of
marine biota to climate change is difficult to determine on time scales of years or even decades.
Sediment cores from the deep-sea floor, however,
provide the opportunity to study the relationships
between biological productivity and climate dynamics over hundreds of thousands of years during
periods of climatic extremes.
Here we focus on the use of organic carbon and
carbonate as paleoproductivity indicators by applying the results of recent studies. We first describe
the modem geographic pattern of marine primary
production and carbonate production, and the glacial-to-Holocene change of paleoproductivities, in
the different ecological domains of the world's
oceans. We then review the factors influencing
organic carbon and carbonate preservation during
transit flux from the surface waters to the sea floor
and into the sediment. The applicability and the problems related to the two paleoproductivity proxies are
exemplified by comparing a sediment core from the
high productivity region off Angola with one from
the low productivity ocean area located north of
Brazil. We will point out to the principle differences
of organic carbon accumulation at each study
site and compare three equations for estimating
paleoproductivity from organic carbon (MUller and
Suess 1979; Stein 1986; Sarntheinet al. 1992). We
will then evaluate, how paleoproductivity can be
estimated from carbonate accumulation rates as
proposed by Brummer and van Eijden (1992).
Finally, the potential causes for countercyclic
paleoproductivity variations which have been reconstructed from the two sediment cores, are discussed.
Primary Production and Proxies of
Paleoproductivity
Productivity: Basic Terms
The quantity of 'primary production', that is the
photosynthetic fixation of carbon per unit area and
per unittime, is primarily limited by the availability
of nutrients (nitrate, phosphate, silicate, and trace
elements such as iron) and by light intensity. 'New
production' is that portion of primary production
which results from allochthonous nutrient inputs
such as from deep waters (upwelling, deep vertical mixing), river discharge or atmospheric deposition, and includes the fixation of molecular nitrogen
(Eppley 1980; Fig. 1). 'Regenerated production'
results from the recycling of nutrients within the
surface waters. 'Export production' denotes that
portion of primary production which is transported
from the euphotic zone into deeper water where
most of the organic matter is decomposed and
transformed into dissolved nutrients. Because a
steady state of nutrient flux has to be maintained,
export production should be equivalent to new production on time scales longer than a year (Williams
et al. 1989).
Primary Production in the Tropical Atlantic
The overall pattern of primary production in the
tropical-subtropical Atlantic is characterized by a
pronounced asymmetry: a highly productive eastern boundary coastal ocean and divergence zone,
and the oligotrophic gyres and western ocean (Fig.
2). This dichotomy is caused by differences in nutrient availability. Generally, nutrients are depleted
Riihlemann et al.
al. 1993), may be significantly affected by biological activity. Primary producers living in surface waters convert CO 2 into organic matter. Part of this
organic matter sinks into deeper waters where it is
oxidized, releasing initially fixed carbon back to water as CO 2 (Broecker andPeng 1993). This mechanism, known as the 'biological pump' (Volk and
Hoffert 1985), lowers the CO 2 partial pressure in
surface waters and atmosphere. CaC0 3 production in the photic zone, on the other hand, counteracts the effect of organic matter production by the
release of CO 2 , However, the solubility ofCaC0 3
again decreases the CO 2 partial pressure in deep
waters. Opal production or dissolution does not affect the partial pressure of CO 2 in any direction.
Consequently, the efficiency of the biological pump
is determined by the magnitude of primary production, by the fraction ofthis production escaping recycling within the mixed layer and being exported
to deeper waters, and by the ratio of organic carbon to carbonate carbon in sinking biogenic particles (Berger and Keir 1984). For this reason, investigations of biogenic export production rates are
a central theme in the reconstruction of the oceanic carbon budget. The influence and response of
marine biota to climate change is difficult to determine on time scales of years or even decades.
Sediment cores from the deep-sea floor, however,
provide the opportunity to study the relationships
between biological productivity and climate dynamics over hundreds of thousands of years during
periods of climatic extremes.
Here we focus on the use of organic carbon and
carbonate as paleoproductivity indicators by applying the results of recent studies. We first describe
the modem geographic pattern of marine primary
production and carbonate production, and the glacial-to-Holocene change of paleoproductivities, in
the different ecological domains of the world's
oceans. We then review the factors influencing
organic carbon and carbonate preservation during
transit flux from the surface waters to the sea floor
and into the sediment. The applicability and the problems related to the two paleoproductivity proxies are
exemplified by comparing a sediment core from the
high productivity region off Angola with one from
the low productivity ocean area located north of
Brazil. We will point out to the principle differences
of organic carbon accumulation at each study
site and compare three equations for estimating
paleoproductivity from organic carbon (MUller and
Suess 1979; Stein 1986; Sarntheinet al. 1992). We
will then evaluate, how paleoproductivity can be
estimated from carbonate accumulation rates as
proposed by Brummer and van Eijden (1992).
Finally, the potential causes for countercyclic
paleoproductivity variations which have been reconstructed from the two sediment cores, are discussed.
Primary Production and Proxies of
Paleoproductivity
Productivity: Basic Terms
The quantity of 'primary production', that is the
photosynthetic fixation of carbon per unit area and
per unittime, is primarily limited by the availability
of nutrients (nitrate, phosphate, silicate, and trace
elements such as iron) and by light intensity. 'New
production' is that portion of primary production
which results from allochthonous nutrient inputs
such as from deep waters (upwelling, deep vertical mixing), river discharge or atmospheric deposition, and includes the fixation of molecular nitrogen
(Eppley 1980; Fig. 1). 'Regenerated production'
results from the recycling of nutrients within the
surface waters. 'Export production' denotes that
portion of primary production which is transported
from the euphotic zone into deeper water where
most of the organic matter is decomposed and
transformed into dissolved nutrients. Because a
steady state of nutrient flux has to be maintained,
export production should be equivalent to new production on time scales longer than a year (Williams
et al. 1989).
Primary Production in the Tropical Atlantic
The overall pattern of primary production in the
tropical-subtropical Atlantic is characterized by a
pronounced asymmetry: a highly productive eastern boundary coastal ocean and divergence zone,
and the oligotrophic gyres and western ocean (Fig.
2). This dichotomy is caused by differences in nutrient availability. Generally, nutrients are depleted