466
10.4 Organic Matter in the Oceans
10.4.1 Influx of Terrestrial Organie Matter
The organic matter of the oceans comes from two
sources: (1) riverine and eolian influx of terrestrial,
plant-derived dissolved (DOC) and particulate organic matter (POC) and (2) primary marine production of organic matter in the oceans. In addition, dissolved inorganic carbon (DIC), mostly in the form of
hydrogencarbonate HC0 3 , is transported by the
rivers to the sea. We first discuss the riverine and
eolian influx of organic carbon (OC) or total organic
carbon (TOC).
The river input of both DOC and POC from the
continents significantly contributes to the total carbon budget of the oceans. However, the flux rates
from the present-day continents and morphoc1imatic
regions markedly vary (Fig. 10.5a)
- Regions with high relief and forests, particularly
tropical rain forests (for example Oceania) contribute
larger amounts of organic matter per unit area to the
oceans than do areas of low relief and dry c1imate
(e.g. major parts of Africa).
- The present-day specific TOC yield oftropical wet
regions is about 8 tlkm 2 /a and makes up -55% of the
global terrestrial TOC flux into the ocean. The second and third position in terrestrial TOC yield take
the zones of temperate wet c1imate (-4 tlkm 2 /a) and
the widely extended regions of tundra and taiga vegetation (-3 tlkm 2 /a). Each of these zones delivers
about 17% of the global TOC yie1d to the ocean. Dry
regions inc1uding deserts contribute only minor
amounts to the global terrestrial TOC flux. About
30% of the land surface have internal drainage systems which do not reach the ocean.
- The total carbon (TOC) flux from the present-day
continents to the oceans reaches about 400 x 10 12
gC/a. This is about 1 % of the terrestrial productivity.
The mean terrestrial TOC yield of all coritinents (a
type of specific "terrestrial new productivity", see
below) is 2.7 glm 2 /a = 2.7 t/km 2 /a. Of these, about
55% enter the oceans as dissolved organic compounds (DOC).
- Plant-derived TOC, i.e. DOC and POC, reaches
almost the same order of magnitude as the mean
"new productivity" of major parts of the ocean (Table 10.2). It therefore potentially constitutes an important portion of the organic matter preserved in
marine sediments (Sect. 10.5). It appears, however,
that its contribution to the organic matter preserved
in modem sediments is limited (Hedges et al. 1997).
- Furthermore, particulate organic carbon supplied
by winds is a major proportion of the organic matter
accumulated in the organic carbon-poor sediments of
the central open-ocean regions (Zafiriou et al. 1985;
Prahl and Muehlhausen 1989; Stein 1991). Whether
Chapter 10 Sedimentation Rates and Organic Matter
the contribution of wind-borne POC is as great as
assumed by some authors (23% of the total amount
of carbon buried in sediments; e.g. Romarikevitch
1984) is an open question.
Dissolved inorganic carbon (DIC), derived to a large extent
from the atmosphere (-70%) is not further considered here,
but it is an important factor in the global carbon budget.
Data for carbon fluxes are still subjected to significant
errors and evaluated by some authors as "best guess". The
TOC yields of the continents to the ocean and that of the
surface layer of the ocean to the deeper sea and sediment
are mostly found by some sort of extrapolation (see e.g.
Schlünz 1998). The global values of terrestrial TOC yield
determined by different authors therefore range from 335 to
500 x 10 12 gC/a (e.g. Degens et al. 1991; Hedges et al.
1997; Ludwig et al. 1998). In addition, riverine carbon
transport data only partially inc1ude the effects of peak
floods which deli ver large quantities of organic matter.
They therefore often underestimate especially partic1e flux.
Better data are available for selected smaller regions,
such as the drainage areas of specific rivers or parts of
ocean basins. The results from these regions confirm the
general mies listed above. The river Amazon, for example,
delivers about 10% of the global terrestrial OC yield, and
on the Amazon shelf 6-10% of the global riverine OC discharge are deposited (Schlünz 1998). In the Laptec Sea of
the Arctic ocean, terrestrial organic carbon also dominates
in the sediments (Stein and Fahl 1999). This is caused by
large rivers which drain tundra and taiga regions delivering
relatively high amounts of TOC (Fig. 1O.5a). Marine primary production is very low in this basin.
The sources of organic matter present in sediments can
be identified by different techniques (Sect. 14.1). As long
as the sediments are immature, i.e., not deeply buried and
substantially heated, marine organic matter (kerogen type
II) derived from algae, phytoplankton, zooplankton and
bacteria is characterized by a high "hydrocarbon index"
(Fig. 1O.5c). These organisms contain a high proportion of
hydrogen-rich lipid material, whereas terrestrial organic
matter (kerogen type III), dominated by cellulose and
lignin, renders low hydrocarbon index values. In addition,
C-isotopes and specific biomarkers can aid in the identification of carbon sources (e.g. Killops and Killops 1993;
Westhausen et al. 1993; Peters and Moldowan 1993). Detailed work on the composition of organic matter in many
Mesozoic and Cenozoic sediment cores taken during scientific ocean drilling has shown that a great portion of the
kerogen (insoluble organic compounds, Sect. 14.1) must
have come from neighboring continents (e.g., Stein et al.
1986,1989; Stein and Fahl 1999).
In this and the following sections, only the quantitative
importance of land-derived organic matter in marine sediments is considered.
10.4.2 Primary Produetion and Heterotrophie
Organisms
Primary Produetion
In the modem oceans the main producers of primary
organic matter are diatoms, coccolithophorids, and
dinoflagellates. These phytoplankton can grow only
10.4 Organic Matter in the Oceans
10.4.1 Influx of Terrestrial Organie Matter
The organic matter of the oceans comes from two
sources: (1) riverine and eolian influx of terrestrial,
plant-derived dissolved (DOC) and particulate organic matter (POC) and (2) primary marine production of organic matter in the oceans. In addition, dissolved inorganic carbon (DIC), mostly in the form of
hydrogencarbonate HC0 3 , is transported by the
rivers to the sea. We first discuss the riverine and
eolian influx of organic carbon (OC) or total organic
carbon (TOC).
The river input of both DOC and POC from the
continents significantly contributes to the total carbon budget of the oceans. However, the flux rates
from the present-day continents and morphoc1imatic
regions markedly vary (Fig. 10.5a)
- Regions with high relief and forests, particularly
tropical rain forests (for example Oceania) contribute
larger amounts of organic matter per unit area to the
oceans than do areas of low relief and dry c1imate
(e.g. major parts of Africa).
- The present-day specific TOC yield oftropical wet
regions is about 8 tlkm 2 /a and makes up -55% of the
global terrestrial TOC flux into the ocean. The second and third position in terrestrial TOC yield take
the zones of temperate wet c1imate (-4 tlkm 2 /a) and
the widely extended regions of tundra and taiga vegetation (-3 tlkm 2 /a). Each of these zones delivers
about 17% of the global TOC yie1d to the ocean. Dry
regions inc1uding deserts contribute only minor
amounts to the global terrestrial TOC flux. About
30% of the land surface have internal drainage systems which do not reach the ocean.
- The total carbon (TOC) flux from the present-day
continents to the oceans reaches about 400 x 10 12
gC/a. This is about 1 % of the terrestrial productivity.
The mean terrestrial TOC yield of all coritinents (a
type of specific "terrestrial new productivity", see
below) is 2.7 glm 2 /a = 2.7 t/km 2 /a. Of these, about
55% enter the oceans as dissolved organic compounds (DOC).
- Plant-derived TOC, i.e. DOC and POC, reaches
almost the same order of magnitude as the mean
"new productivity" of major parts of the ocean (Table 10.2). It therefore potentially constitutes an important portion of the organic matter preserved in
marine sediments (Sect. 10.5). It appears, however,
that its contribution to the organic matter preserved
in modem sediments is limited (Hedges et al. 1997).
- Furthermore, particulate organic carbon supplied
by winds is a major proportion of the organic matter
accumulated in the organic carbon-poor sediments of
the central open-ocean regions (Zafiriou et al. 1985;
Prahl and Muehlhausen 1989; Stein 1991). Whether
Chapter 10 Sedimentation Rates and Organic Matter
the contribution of wind-borne POC is as great as
assumed by some authors (23% of the total amount
of carbon buried in sediments; e.g. Romarikevitch
1984) is an open question.
Dissolved inorganic carbon (DIC), derived to a large extent
from the atmosphere (-70%) is not further considered here,
but it is an important factor in the global carbon budget.
Data for carbon fluxes are still subjected to significant
errors and evaluated by some authors as "best guess". The
TOC yields of the continents to the ocean and that of the
surface layer of the ocean to the deeper sea and sediment
are mostly found by some sort of extrapolation (see e.g.
Schlünz 1998). The global values of terrestrial TOC yield
determined by different authors therefore range from 335 to
500 x 10 12 gC/a (e.g. Degens et al. 1991; Hedges et al.
1997; Ludwig et al. 1998). In addition, riverine carbon
transport data only partially inc1ude the effects of peak
floods which deli ver large quantities of organic matter.
They therefore often underestimate especially partic1e flux.
Better data are available for selected smaller regions,
such as the drainage areas of specific rivers or parts of
ocean basins. The results from these regions confirm the
general mies listed above. The river Amazon, for example,
delivers about 10% of the global terrestrial OC yield, and
on the Amazon shelf 6-10% of the global riverine OC discharge are deposited (Schlünz 1998). In the Laptec Sea of
the Arctic ocean, terrestrial organic carbon also dominates
in the sediments (Stein and Fahl 1999). This is caused by
large rivers which drain tundra and taiga regions delivering
relatively high amounts of TOC (Fig. 1O.5a). Marine primary production is very low in this basin.
The sources of organic matter present in sediments can
be identified by different techniques (Sect. 14.1). As long
as the sediments are immature, i.e., not deeply buried and
substantially heated, marine organic matter (kerogen type
II) derived from algae, phytoplankton, zooplankton and
bacteria is characterized by a high "hydrocarbon index"
(Fig. 1O.5c). These organisms contain a high proportion of
hydrogen-rich lipid material, whereas terrestrial organic
matter (kerogen type III), dominated by cellulose and
lignin, renders low hydrocarbon index values. In addition,
C-isotopes and specific biomarkers can aid in the identification of carbon sources (e.g. Killops and Killops 1993;
Westhausen et al. 1993; Peters and Moldowan 1993). Detailed work on the composition of organic matter in many
Mesozoic and Cenozoic sediment cores taken during scientific ocean drilling has shown that a great portion of the
kerogen (insoluble organic compounds, Sect. 14.1) must
have come from neighboring continents (e.g., Stein et al.
1986,1989; Stein and Fahl 1999).
In this and the following sections, only the quantitative
importance of land-derived organic matter in marine sediments is considered.
10.4.2 Primary Produetion and Heterotrophie
Organisms
Primary Produetion
In the modem oceans the main producers of primary
organic matter are diatoms, coccolithophorids, and
dinoflagellates. These phytoplankton can grow only