Clues to Ocean History: a Brief Overview of Proxies
23
pheric CO 2 is on average 9 %0 lower than that in
surface ocean CO 2 and the extent offractionation
depends on temperature (e.g. Broecker and MaierReimer 1992; Charles etai. 1993; Lynch-Stieglitz
et ai. 1995). As a result, high latitude surface water have a higher 13CfI 2 C ratio than low latitude
surface water. Generally, this thermodynamic imprint is not very strong due to very slow isotopic
equilibration (Broecker and Peng 1982), however,
if there is sufficient time for gas exchange, a significant signal is produced (Charles and Fairbanks
1990). The concept of "preformed 813C mc " (at zero
nutrients) was introduced (e.g. Broecker and
Maier-Reimer 1992; Lynch-Stieglitz and Fairbanks
1994; Oppo and Fairbanks 1989) to investigate the
decoupling between 8 13 C mc and nutrients and use
8 13 C mc as a conservative tracer for water masses.
It is generally assumed that the 8 13 C minimum recorded in planktic and benthic foraminifers at the
deglaciation is such a water mass effect. (preformed 8 13 C mc )'
Another source of variation is the so called
"Mackensen effect". Mackensen et ai. (1993) hypothesized that growth and reproduction of
benthic foraminifers coincides with the seasonal
flux of phytodetritus. Because 8 13 C of freshly accumulated phytodetritus is 3 to 4 %0 lower than
surface sediment organic carbon they argued
that 8 13 C of epibenthic shells, formed during
remineralization ofthis fluffy layer (locally depleting 8 13 CL C02 )' is depleted compared to 8 13 CL c02
found throughout the rest of the year.
It has long been known that the sand content in
calcareous deep-sea sediments decreases with
increasing dissolution (Johnson et ai. 1977; Berger
et ai. 1982). The reason for this is that the sand
fraction consists primarily of planktic foraminifers
and the tests become weak and the individual chambers collapse under dissolution conditions. This
results in a lower sand content and increased silt
and clay content. Analysis of other dissolution indices confirms the clear relationship between dissolution intensity and grain size (RebbeIn et ai. 1990;
Yasudaet ai. 1993). In addition to the8 13 C values,
Bickert and Wefer (1996) also used the sand fraction in reconstructing deep-water circulation for the
Late Quaternary in the South Atlantic. Using this,
they were able to determine changes in the position of the lysocline, separating a lower and upper
Southern Component Water body during the glacial period. During the LGM, this border lay at a
depth of3 800 m near the equator, becoming shallower toward the south.
Qualitative statements about the bottom-water
flow direction and speed can be made based on the
diatom distribution and clay-mineral composition.
Burckle and Stanton (1975) used diatoms to determine the spread of Antarctic Bottom Water
(AABW) from the Antarctic to the Argentine
Basin. Another parameter for reconstructing the
flow of the AABW is the clay mineral distribution.
Using knowledge of the source and formation conditions of recent clay mineral facies, Diekmann et
ai. (1996) were able to obtain additional important
information relating to the spread of Antarctic
Bottom and Deep Water (Fig. 14).
Strictly speaking, clay minerals and redeposited
diatoms are not "proxies" in the sense we use the
concept, since there is no generally valid equation
relating abundance of redeposited materials to the
properties of the transporting agent. Nevertheless,
we mention these techniques here to remind ourselves that traditional sedimentologic methods have
much to offer regarding reconstruction (qualitatively) of the oceanic environment.
Reconstruction of Productivity from
Organic Matter
Reconstructions of productivity patterns in the
ocean are of great interest because of important
links to current patterns, mixing, wind, carbon cycle, and biogeography. As Arrhenius (1952) pointed
out, productivity patterns are related to wind patterns. Specifically, upwelling is closely linked to
trade winds. Productivity is defined as flux of carbon; dimensions are carbon per area per time. Biologists measure the flux from the reservoir of dissolved carbon into organically fixed carbon, in the
photic zone. They distinguish "recycled production"
and "new production", depending on whether the
nitrogen used in producing organic matter is derived
from ammonia and urea (recycled) or nitrate (new)
(Eppley and Petersen 1979). Ocean chemists are
interested in the flux out of the photic zone, termed
"export production" (Berger et ai. 1989). This is the
23
pheric CO 2 is on average 9 %0 lower than that in
surface ocean CO 2 and the extent offractionation
depends on temperature (e.g. Broecker and MaierReimer 1992; Charles etai. 1993; Lynch-Stieglitz
et ai. 1995). As a result, high latitude surface water have a higher 13CfI 2 C ratio than low latitude
surface water. Generally, this thermodynamic imprint is not very strong due to very slow isotopic
equilibration (Broecker and Peng 1982), however,
if there is sufficient time for gas exchange, a significant signal is produced (Charles and Fairbanks
1990). The concept of "preformed 813C mc " (at zero
nutrients) was introduced (e.g. Broecker and
Maier-Reimer 1992; Lynch-Stieglitz and Fairbanks
1994; Oppo and Fairbanks 1989) to investigate the
decoupling between 8 13 C mc and nutrients and use
8 13 C mc as a conservative tracer for water masses.
It is generally assumed that the 8 13 C minimum recorded in planktic and benthic foraminifers at the
deglaciation is such a water mass effect. (preformed 8 13 C mc )'
Another source of variation is the so called
"Mackensen effect". Mackensen et ai. (1993) hypothesized that growth and reproduction of
benthic foraminifers coincides with the seasonal
flux of phytodetritus. Because 8 13 C of freshly accumulated phytodetritus is 3 to 4 %0 lower than
surface sediment organic carbon they argued
that 8 13 C of epibenthic shells, formed during
remineralization ofthis fluffy layer (locally depleting 8 13 CL C02 )' is depleted compared to 8 13 CL c02
found throughout the rest of the year.
It has long been known that the sand content in
calcareous deep-sea sediments decreases with
increasing dissolution (Johnson et ai. 1977; Berger
et ai. 1982). The reason for this is that the sand
fraction consists primarily of planktic foraminifers
and the tests become weak and the individual chambers collapse under dissolution conditions. This
results in a lower sand content and increased silt
and clay content. Analysis of other dissolution indices confirms the clear relationship between dissolution intensity and grain size (RebbeIn et ai. 1990;
Yasudaet ai. 1993). In addition to the8 13 C values,
Bickert and Wefer (1996) also used the sand fraction in reconstructing deep-water circulation for the
Late Quaternary in the South Atlantic. Using this,
they were able to determine changes in the position of the lysocline, separating a lower and upper
Southern Component Water body during the glacial period. During the LGM, this border lay at a
depth of3 800 m near the equator, becoming shallower toward the south.
Qualitative statements about the bottom-water
flow direction and speed can be made based on the
diatom distribution and clay-mineral composition.
Burckle and Stanton (1975) used diatoms to determine the spread of Antarctic Bottom Water
(AABW) from the Antarctic to the Argentine
Basin. Another parameter for reconstructing the
flow of the AABW is the clay mineral distribution.
Using knowledge of the source and formation conditions of recent clay mineral facies, Diekmann et
ai. (1996) were able to obtain additional important
information relating to the spread of Antarctic
Bottom and Deep Water (Fig. 14).
Strictly speaking, clay minerals and redeposited
diatoms are not "proxies" in the sense we use the
concept, since there is no generally valid equation
relating abundance of redeposited materials to the
properties of the transporting agent. Nevertheless,
we mention these techniques here to remind ourselves that traditional sedimentologic methods have
much to offer regarding reconstruction (qualitatively) of the oceanic environment.
Reconstruction of Productivity from
Organic Matter
Reconstructions of productivity patterns in the
ocean are of great interest because of important
links to current patterns, mixing, wind, carbon cycle, and biogeography. As Arrhenius (1952) pointed
out, productivity patterns are related to wind patterns. Specifically, upwelling is closely linked to
trade winds. Productivity is defined as flux of carbon; dimensions are carbon per area per time. Biologists measure the flux from the reservoir of dissolved carbon into organically fixed carbon, in the
photic zone. They distinguish "recycled production"
and "new production", depending on whether the
nitrogen used in producing organic matter is derived
from ammonia and urea (recycled) or nitrate (new)
(Eppley and Petersen 1979). Ocean chemists are
interested in the flux out of the photic zone, termed
"export production" (Berger et ai. 1989). This is the