correctly reproduced the transfer of the tracer at the air-sea
interface, as well as its penetration below the sea surface into
the ocean.
Oceanic Paleocirculation
In the meantime, from the systematic
14 C measurements in
the world ocean layers, the idea quickly emerged to recover
past changes in ocean circulation from
14 C analyses of
marine biocarbonates. The
14
C concentration of large volume samples such as corals and mollusks shells were first
measured by radioactive decay measurements (Stuiver et al.
1986; Druffel 1989). This could be achieved for small volume samples with the new AMS
14 C technique. The
planktonic and poorly abundant benthic foraminifera contemporaneously deposited in deep-sea sediment cores from
different water-depths could henceforth be easily dated
allowing the paleo-ventilation of the paleocean layers to be
recovered to the limit of the
14 C dating (theoretically 10
times the half-life). Meantime the changes of the Earth’s
orbital parameters, and thus changes of the insolation, the
atmospheric and oceanic circulation changes modulate the
Earth’s climate. Considerable efforts, compiled in (Zhao
et al. 2018), have been made to measure the
14
C differences
in the deep to surface ocean water masses in the past from
benthic-planktonic foraminifera. Benthic corals have provided additional estimates of the deep Δ
14 C as a function of
calendar ages as they can be dated by both the U-Th and
14 C
methods (Adkins et al. 1998; Goldstein et al. 2001; Robinson et al. 2005; Burke and Robinson 2012; Chen et al.
2015). Opposite results in the estimates of the
paleo-ventilation of the ocean during the deglaciation
emerged when using either the
14 C dating of paired
benthic-planktonic foraminifera or paired
14
C/U-Th dating
of benthic corals (Adkins et al. 1998; Goldstein et al. 2001;
Robinson et al. 2005; Burke and Robinson 2012; Chen et al.
2015) or the paired atmospheric-marine
14 C dating from
ash-layers (Sikes et al. 2000; Ikehara et al. 2011; Siani et al.
2013; Ezat et al. 2017). Values of Δ
14 C are obtained from
the absolute (calendar) age and the
14 C age (see Box 1).
Thus the conflicting results are very likely related to the
estimated Δ
14 C from paired benthic-planktonic foraminifera
due to (i) an incorrect estimate of the sea surface reservoir
age subtracted to marine
14 C ages (in order to be referenced
to calendar ages), (ii) the use of different atmospheric
14 C
calibration records, notably the much smoother variations of
14 C in IntCal13 than those estimated in previous records
(Reimer et al. 2013), (iii) the use of
14 C values of foraminifera picked in deep-sea sediment cores with a low
sedimentation rate (see below: the bioturbation effects), and
(iv) few planktonic foraminifera spend their entire life at the
sea surface, and their
14 C content represents that of the upper
ten to hundreds of meters of the water column in which they
lived. More constraints on the changes of the sea surface
reservoir ages allowing robust calculation of Δ
14 C from
benthic foraminifera and more
14 C analyses from deep sea
corals will be very useful for model simulations of the
oceanic circulation during the last glacial maximum and the
deglaciation using three-dimensional models (Tagliabue
et al. 2009). In the study by Zhao et al., (2018), the box
model resolution, which should at least take into account the
changing geometry of water masses in the last 25 kyr
(Michel et al. 1995), is a limitation to estimate regional
changes in paleoventilation.
Mineralization of Organic Matter in Soil
The contribution of soils and their role as sinks and sources
in the global carbon cycle remain misunderstood until now.
The stock of soil organic matter is defined as a balance
between the input of organic matter through vegetation and
the loss through microbial decomposition. The balance can
be disrupted by changes in agricultural practices and climate
variations. For instance, a temperature increase may clearly
increase the activity of soil microorganisms and the subsequent soil organic matter mineralization. No consensus has
however been reached on the relative importance of the
various climatic factors that affect soil organic matter
dynamics, such as temperature, aridity, land use. To better
evaluate the effect of these disturbances on the global carbon
cycle, it is essential not only to characterize soil carbon
stocks but also soil carbon dynamics. To do so,
14 C is a
powerful tool as it can be considered as a clock that registers
the carbon residence time in the soil organic mixture
(Scharpenseel and Shiffmann 1971; Balesdent and Guillet
1982).
Conceptual views of soil organic carbon dynamics have
greatly evolved with time. Carbon sequestration was considered to be related to the chemical structure of the components (lignin having a longer mean residence time than
sugar), to the accessibility of organic matter in aggregates
(the higher the pore, the more labile the organic matter), to
the affinity between organic matter and mineral surface (the
stronger the bond, the more refractory the components) (Six
et al. 2002). The conceptual view is still developing (Kleber
et al. 2007) and
14 C brings powerful elements.
14 C measurement is thus done on bulk organic matter, on density
fractions, on granulometric fractions, on molecular fractions
and on molecules according to the process or to the turnover
to be characterized.
Isotopic methods, such as dating by carbon-14, natural
(percentage of plants in C3 and in C4) and artificial
carbon-13 labeling are very powerful tools in so far as they
make it possible to estimate the residence time of natural
64
M. Paterne et al.
interface, as well as its penetration below the sea surface into
the ocean.
Oceanic Paleocirculation
In the meantime, from the systematic
14 C measurements in
the world ocean layers, the idea quickly emerged to recover
past changes in ocean circulation from
14 C analyses of
marine biocarbonates. The
14
C concentration of large volume samples such as corals and mollusks shells were first
measured by radioactive decay measurements (Stuiver et al.
1986; Druffel 1989). This could be achieved for small volume samples with the new AMS
14 C technique. The
planktonic and poorly abundant benthic foraminifera contemporaneously deposited in deep-sea sediment cores from
different water-depths could henceforth be easily dated
allowing the paleo-ventilation of the paleocean layers to be
recovered to the limit of the
14 C dating (theoretically 10
times the half-life). Meantime the changes of the Earth’s
orbital parameters, and thus changes of the insolation, the
atmospheric and oceanic circulation changes modulate the
Earth’s climate. Considerable efforts, compiled in (Zhao
et al. 2018), have been made to measure the
14
C differences
in the deep to surface ocean water masses in the past from
benthic-planktonic foraminifera. Benthic corals have provided additional estimates of the deep Δ
14 C as a function of
calendar ages as they can be dated by both the U-Th and
14 C
methods (Adkins et al. 1998; Goldstein et al. 2001; Robinson et al. 2005; Burke and Robinson 2012; Chen et al.
2015). Opposite results in the estimates of the
paleo-ventilation of the ocean during the deglaciation
emerged when using either the
14 C dating of paired
benthic-planktonic foraminifera or paired
14
C/U-Th dating
of benthic corals (Adkins et al. 1998; Goldstein et al. 2001;
Robinson et al. 2005; Burke and Robinson 2012; Chen et al.
2015) or the paired atmospheric-marine
14 C dating from
ash-layers (Sikes et al. 2000; Ikehara et al. 2011; Siani et al.
2013; Ezat et al. 2017). Values of Δ
14 C are obtained from
the absolute (calendar) age and the
14 C age (see Box 1).
Thus the conflicting results are very likely related to the
estimated Δ
14 C from paired benthic-planktonic foraminifera
due to (i) an incorrect estimate of the sea surface reservoir
age subtracted to marine
14 C ages (in order to be referenced
to calendar ages), (ii) the use of different atmospheric
14 C
calibration records, notably the much smoother variations of
14 C in IntCal13 than those estimated in previous records
(Reimer et al. 2013), (iii) the use of
14 C values of foraminifera picked in deep-sea sediment cores with a low
sedimentation rate (see below: the bioturbation effects), and
(iv) few planktonic foraminifera spend their entire life at the
sea surface, and their
14 C content represents that of the upper
ten to hundreds of meters of the water column in which they
lived. More constraints on the changes of the sea surface
reservoir ages allowing robust calculation of Δ
14 C from
benthic foraminifera and more
14 C analyses from deep sea
corals will be very useful for model simulations of the
oceanic circulation during the last glacial maximum and the
deglaciation using three-dimensional models (Tagliabue
et al. 2009). In the study by Zhao et al., (2018), the box
model resolution, which should at least take into account the
changing geometry of water masses in the last 25 kyr
(Michel et al. 1995), is a limitation to estimate regional
changes in paleoventilation.
Mineralization of Organic Matter in Soil
The contribution of soils and their role as sinks and sources
in the global carbon cycle remain misunderstood until now.
The stock of soil organic matter is defined as a balance
between the input of organic matter through vegetation and
the loss through microbial decomposition. The balance can
be disrupted by changes in agricultural practices and climate
variations. For instance, a temperature increase may clearly
increase the activity of soil microorganisms and the subsequent soil organic matter mineralization. No consensus has
however been reached on the relative importance of the
various climatic factors that affect soil organic matter
dynamics, such as temperature, aridity, land use. To better
evaluate the effect of these disturbances on the global carbon
cycle, it is essential not only to characterize soil carbon
stocks but also soil carbon dynamics. To do so,
14 C is a
powerful tool as it can be considered as a clock that registers
the carbon residence time in the soil organic mixture
(Scharpenseel and Shiffmann 1971; Balesdent and Guillet
1982).
Conceptual views of soil organic carbon dynamics have
greatly evolved with time. Carbon sequestration was considered to be related to the chemical structure of the components (lignin having a longer mean residence time than
sugar), to the accessibility of organic matter in aggregates
(the higher the pore, the more labile the organic matter), to
the affinity between organic matter and mineral surface (the
stronger the bond, the more refractory the components) (Six
et al. 2002). The conceptual view is still developing (Kleber
et al. 2007) and
14 C brings powerful elements.
14 C measurement is thus done on bulk organic matter, on density
fractions, on granulometric fractions, on molecular fractions
and on molecules according to the process or to the turnover
to be characterized.
Isotopic methods, such as dating by carbon-14, natural
(percentage of plants in C3 and in C4) and artificial
carbon-13 labeling are very powerful tools in so far as they
make it possible to estimate the residence time of natural
64
M. Paterne et al.