shell or bone material is far more resistant until carried to
undersaturated conditions usually at considerable depth in
the ocean (Dittert et al., 1999; Berger and Wefer, 2008) or
within seabed sediments.
Interestingly, as organic matter undergoes diagenesis in
sinking particles, reactions occur which add new material
to the debris. The reactions lead to precipitation of new
minerals, like barite. It appears that the amount of biogenic
barite created is related to the quantity of reactive organic
matter settling to depth (Paytan and Griffith, 2007). The
diagenetic products accumulate at the seabed and can provide a record of biological organic matter production.
Diagenesis (recycling, consumption) continues at the
seafloor, which is actually a highly reactive interface
where biological activity and chemical reactions alter
what arrives. Benthic fauna feed on reactive organic compounds and biotic activity combined with dissolution reactions destroy skeletal materials (Ragueneau et al., 2000;
Zeebe and Wolf-Gladrow, 2001; Sarmiento and Gruber,
2006).
Tracers of past biological production
Historically, tracking paleoproduction was attempted first
by measuring contents of organic carbon and carbonate in
deep-sea sediments. In the equatorial Pacific, for example,
there are distinct cycles of carbonate percent in
Pleistocene-age sediments (Berger, 1992). Percent organic
carbon content was also observed to vary regionally and in
time, in ways that related to patterns of biological production in the ocean. However, the analysis of the carbonate
cycles was confounded by argument about whether they
were due to changes in production or preservation
(Berger, 1992). The cycles could alternatively reflect variation in biological production in the upper ocean or
changes in the properties of deep ocean waters (changes
in degree of corrosiveness to carbonate) related to climate
shifts.
The problem with contents is that, for each biogenic
material, it depends on the supply and preservation of all
the components going in the sediments, so interpreting
content data is complicated. Because of this, measuring
accumulation rates (weight of material accumulated per
cm
2 of seabed per 1,000 years) offers a better avenue to
reconstruction paleoproduction.
Accumulation rates of biogenic components have been
used extensively to infer changes in productivity in times
past. However, these rates are still subject to the problem
of varying preservation (what accumulates is a function
of supply minus diagenetic loss). Also, the simple use of
accumulation rates for inference of supply changes
assumes that delivery of materials from the surface ocean
to the seabed is completely vertical, with no lateral transport or redeposition.
Examination of accumulation rates requires correction
for changes in preservation and for possibly variable water
column or seabed transportation. These issues remain
problematic. Methods have been developed to quantify
changes in carbonate preservation (Mekik et al., 2002),
and methods based on thorium-230 concentrations in
deep-sea sediments have been derived for measuring lateral sediment transport (Francois et al., 2004). Application
of the method has produced some notable disagreements
over the Pleistocene cycles of carbonate and organic carbon accumulation in the eastern equatorial Pacific
(Loubere and Richaud, 2006).
Tracers not dependent on accumulation rate calculations
offer escape from the complications described above. Relative abundances (percentages) of benthic (bottom dwelling)
foraminifera (shell-producing microorganism) species in
deep-sea sediments have been found to reflect flux of labile
organic carbon reaching the seafloor. This is because the
carbon flux is food to the benthic community whose ecology is structured depending on how large that flux
is. Composition of benthic fossil assemblages has been
empirically calibrated to both average annual organic carbon production and seasonality of that production for the
open ocean (Loubere and Fariduddin, 1999a, b).
Tracers of paleoproductivity are also available in biogenic components which are thought to be highly resistant
to alteration in the water column and seabed or whose characteristics are set in the upper ocean. These include barite
(biogenic barite produced as explained above and a tracer
of reactive organic carbon flux from the upper ocean),
alkenones (lipids) whose concentration has been taken to
reflect productivity of the Prymnesiophyceae algae
(including coccolithophorids and Phaeocystis) (e.g., Sachs
and Anderson, 2005), elemental ratios in pelagic sediments
(Anderson and Winckler, 2005), and elemental or isotope
ratios in planktonic material (e.g., Stoll et al., 2007; see
Fischer and Wefer, 1999, pp. 315–468).
Has oceanic productivity varied in the past?
In principle, we would expect that global and regional
marine productivity would vary in response to changes
in nutrient concentrations in seawater and to the structure
of the upper ocean water column (degree of stability influences availability of sunlight). In short to intermediate
geological timescales, nutrient concentrations will depend
on the whole ocean exchange between deep and surface
waters and on regional patterns of upwelling and vertical
mixing (Sarmiento and Gruber, 2006). Changes in ocean
circulation should be linked to global climate change.
On the longer geological timescale, ocean circulation will
change in response to shifting continents, and nutrient
concentrations might change in the global ocean due to
variations in the ultimate supply, the erosion of continents
(the key element phosphorus is an example; Filippelli,
2002).
On the grandest scale, strong decreases in marine biological productivity have been inferred for major extinction events in global history (e.g., Coxall et al., 2006).
The exact causes are unclear as yet, but impacts on both
planktonic community composition and changes in ocean
circulation and nutrient supply likely all played a role.
PALEOPRODUCTIVITY
651
undersaturated conditions usually at considerable depth in
the ocean (Dittert et al., 1999; Berger and Wefer, 2008) or
within seabed sediments.
Interestingly, as organic matter undergoes diagenesis in
sinking particles, reactions occur which add new material
to the debris. The reactions lead to precipitation of new
minerals, like barite. It appears that the amount of biogenic
barite created is related to the quantity of reactive organic
matter settling to depth (Paytan and Griffith, 2007). The
diagenetic products accumulate at the seabed and can provide a record of biological organic matter production.
Diagenesis (recycling, consumption) continues at the
seafloor, which is actually a highly reactive interface
where biological activity and chemical reactions alter
what arrives. Benthic fauna feed on reactive organic compounds and biotic activity combined with dissolution reactions destroy skeletal materials (Ragueneau et al., 2000;
Zeebe and Wolf-Gladrow, 2001; Sarmiento and Gruber,
2006).
Tracers of past biological production
Historically, tracking paleoproduction was attempted first
by measuring contents of organic carbon and carbonate in
deep-sea sediments. In the equatorial Pacific, for example,
there are distinct cycles of carbonate percent in
Pleistocene-age sediments (Berger, 1992). Percent organic
carbon content was also observed to vary regionally and in
time, in ways that related to patterns of biological production in the ocean. However, the analysis of the carbonate
cycles was confounded by argument about whether they
were due to changes in production or preservation
(Berger, 1992). The cycles could alternatively reflect variation in biological production in the upper ocean or
changes in the properties of deep ocean waters (changes
in degree of corrosiveness to carbonate) related to climate
shifts.
The problem with contents is that, for each biogenic
material, it depends on the supply and preservation of all
the components going in the sediments, so interpreting
content data is complicated. Because of this, measuring
accumulation rates (weight of material accumulated per
cm
2 of seabed per 1,000 years) offers a better avenue to
reconstruction paleoproduction.
Accumulation rates of biogenic components have been
used extensively to infer changes in productivity in times
past. However, these rates are still subject to the problem
of varying preservation (what accumulates is a function
of supply minus diagenetic loss). Also, the simple use of
accumulation rates for inference of supply changes
assumes that delivery of materials from the surface ocean
to the seabed is completely vertical, with no lateral transport or redeposition.
Examination of accumulation rates requires correction
for changes in preservation and for possibly variable water
column or seabed transportation. These issues remain
problematic. Methods have been developed to quantify
changes in carbonate preservation (Mekik et al., 2002),
and methods based on thorium-230 concentrations in
deep-sea sediments have been derived for measuring lateral sediment transport (Francois et al., 2004). Application
of the method has produced some notable disagreements
over the Pleistocene cycles of carbonate and organic carbon accumulation in the eastern equatorial Pacific
(Loubere and Richaud, 2006).
Tracers not dependent on accumulation rate calculations
offer escape from the complications described above. Relative abundances (percentages) of benthic (bottom dwelling)
foraminifera (shell-producing microorganism) species in
deep-sea sediments have been found to reflect flux of labile
organic carbon reaching the seafloor. This is because the
carbon flux is food to the benthic community whose ecology is structured depending on how large that flux
is. Composition of benthic fossil assemblages has been
empirically calibrated to both average annual organic carbon production and seasonality of that production for the
open ocean (Loubere and Fariduddin, 1999a, b).
Tracers of paleoproductivity are also available in biogenic components which are thought to be highly resistant
to alteration in the water column and seabed or whose characteristics are set in the upper ocean. These include barite
(biogenic barite produced as explained above and a tracer
of reactive organic carbon flux from the upper ocean),
alkenones (lipids) whose concentration has been taken to
reflect productivity of the Prymnesiophyceae algae
(including coccolithophorids and Phaeocystis) (e.g., Sachs
and Anderson, 2005), elemental ratios in pelagic sediments
(Anderson and Winckler, 2005), and elemental or isotope
ratios in planktonic material (e.g., Stoll et al., 2007; see
Fischer and Wefer, 1999, pp. 315–468).
Has oceanic productivity varied in the past?
In principle, we would expect that global and regional
marine productivity would vary in response to changes
in nutrient concentrations in seawater and to the structure
of the upper ocean water column (degree of stability influences availability of sunlight). In short to intermediate
geological timescales, nutrient concentrations will depend
on the whole ocean exchange between deep and surface
waters and on regional patterns of upwelling and vertical
mixing (Sarmiento and Gruber, 2006). Changes in ocean
circulation should be linked to global climate change.
On the longer geological timescale, ocean circulation will
change in response to shifting continents, and nutrient
concentrations might change in the global ocean due to
variations in the ultimate supply, the erosion of continents
(the key element phosphorus is an example; Filippelli,
2002).
On the grandest scale, strong decreases in marine biological productivity have been inferred for major extinction events in global history (e.g., Coxall et al., 2006).
The exact causes are unclear as yet, but impacts on both
planktonic community composition and changes in ocean
circulation and nutrient supply likely all played a role.
PALEOPRODUCTIVITY
651