644
Land-Ocean Systems in the Siberian Arctic: Qynamics and History
minerals, especially illite) may become a significant proportion of the total nitrogen (Stevenson
and Cheng, 1972; Miiller, 1977), causing (too) low CIN ratios. At Core PS2471, illite contents
vary between 40 and 65% (c. Miiller, unpubl. data 1997). A total organic carbon versus total
nitrogen may help to estimate the amount of inorganic nitrogen (Figure 7; Ruttenberg and Goni,
1997). The positive total nitrogen intercept at TOC=O recorded in the turbidite facies of Core
PS2471 may indicate a background of inorganic nitrogen (i.e., 0.02% absolute). In the
"pelagic" facies of all three cores, on the other hand, inorganic nitrogen seems to be a minor
proportion of the total nitrogen pool, as based on the total organic carbon/total nitrogen
diagrams. This is surprising because the illite contents of the "pelagic" intervals in Core
PS2471 are higher than those determined in the turbidites (45-65 % and 40-45 %, respectively;
C. Miiller, unpubl. data 1997). Thus, the terrigenous proportion may be underestimated in the
CIN ratios, and the CIN data should only be interpreted in combination with other organicsource indicators (see below).
In order to get some more detailed qualitative informations about the marine and terrigenous
proportions of the total organic carbon fraction in more detail, organic geochemical bulk
parameters (Le., hydrogen indices and CIN ratios; see above) and specific biomarkers were
presented for Core PS2471 sediments (Figure 8). The intervals of maximum TOC contents of
the turbidites (Unit C) correlate well with minimum hydrogen index values « 100 mg HC / g
TOC), high CIN ratios (> 10), and low amounts of short-chain n-alkanes and alkenones,
indicating the dominance of terrigenous organic matter. On the other hand, elevated hydrogen
index values (100-250 mg HC / g TOC), low CIN ratios « 10) as well as high amounts of
short-chain n-alkanes, alkenones, and fatty acids suggest relatively increased marine organic
matter proportions in the intervals between the turbidites and in the upper silty clay sequence of
Unit B. In the uppermost, near-surface sediments (Unit A), the proportion of terrigenous
material increases as suggested by low hydrogen index values and low amounts of short-chain
n-alkanes, short-chain fatty acids, and alkenones.
The relative abundances of photosynthetic pigments (tetrapyrroles) determined as
phaeopigment absorbance at a wave length of about 410 nm, are low in the non-turbidite
intervals, but display maximum values in the turbidites (Figure 8). Because of the coincidence
of the pigment maxima with low hydrogen index values, high CIN ratios, minima of shortchain n-alkanes and fatty acids, and minima in alkenones (Figure 8), as well as maxima in
lignin phenols (1. Lobbes, unpubl. data, 1997), the pigment data are not interpreted as a signal
for diagenetic products of marine-derived chlorophyll-a (e.g., Rosell-Me1e, 1994). Possibly the
maxima in the turbidites are caused by chlorophyll-b, which has been synthezised by terrestrial
plants like Spermato-, Pterido-, or Bryophyta (Stryer, 1987).
The carbon preference index (CPI) as a measure for the maturity of the terrigenous organic
matter, varies from 1.5 to 3 in the sedimentary record of Core PS2471, showing neither a
general trend with depth nor any difference between turbidite and non-turbidite intervals.
Discussion
Modem processes controlling organic carbon deposition in the Laptev Sea continental margin
area
As shown in studies of surface sediments (Stein and Niirnberg, 1995; Stein, 1996; Fahl and
Stein, 1997a), the organic-carbon enrichments in the Laptev Sea and the adjacent continental
margin and deep sea (Figure 3) are mainly controlled by increased supply of terrigenous
organic matter. Especially the TOC maxima in the coastal zones off the rivers Lena and Olenek
are directly related to riverine material. This is supported by low hydrogen index values and
high CIN ratios (Stein, 1996) as well as high concentrations of long-chain n-alkanes (Fahl and
Land-Ocean Systems in the Siberian Arctic: Qynamics and History
minerals, especially illite) may become a significant proportion of the total nitrogen (Stevenson
and Cheng, 1972; Miiller, 1977), causing (too) low CIN ratios. At Core PS2471, illite contents
vary between 40 and 65% (c. Miiller, unpubl. data 1997). A total organic carbon versus total
nitrogen may help to estimate the amount of inorganic nitrogen (Figure 7; Ruttenberg and Goni,
1997). The positive total nitrogen intercept at TOC=O recorded in the turbidite facies of Core
PS2471 may indicate a background of inorganic nitrogen (i.e., 0.02% absolute). In the
"pelagic" facies of all three cores, on the other hand, inorganic nitrogen seems to be a minor
proportion of the total nitrogen pool, as based on the total organic carbon/total nitrogen
diagrams. This is surprising because the illite contents of the "pelagic" intervals in Core
PS2471 are higher than those determined in the turbidites (45-65 % and 40-45 %, respectively;
C. Miiller, unpubl. data 1997). Thus, the terrigenous proportion may be underestimated in the
CIN ratios, and the CIN data should only be interpreted in combination with other organicsource indicators (see below).
In order to get some more detailed qualitative informations about the marine and terrigenous
proportions of the total organic carbon fraction in more detail, organic geochemical bulk
parameters (Le., hydrogen indices and CIN ratios; see above) and specific biomarkers were
presented for Core PS2471 sediments (Figure 8). The intervals of maximum TOC contents of
the turbidites (Unit C) correlate well with minimum hydrogen index values « 100 mg HC / g
TOC), high CIN ratios (> 10), and low amounts of short-chain n-alkanes and alkenones,
indicating the dominance of terrigenous organic matter. On the other hand, elevated hydrogen
index values (100-250 mg HC / g TOC), low CIN ratios « 10) as well as high amounts of
short-chain n-alkanes, alkenones, and fatty acids suggest relatively increased marine organic
matter proportions in the intervals between the turbidites and in the upper silty clay sequence of
Unit B. In the uppermost, near-surface sediments (Unit A), the proportion of terrigenous
material increases as suggested by low hydrogen index values and low amounts of short-chain
n-alkanes, short-chain fatty acids, and alkenones.
The relative abundances of photosynthetic pigments (tetrapyrroles) determined as
phaeopigment absorbance at a wave length of about 410 nm, are low in the non-turbidite
intervals, but display maximum values in the turbidites (Figure 8). Because of the coincidence
of the pigment maxima with low hydrogen index values, high CIN ratios, minima of shortchain n-alkanes and fatty acids, and minima in alkenones (Figure 8), as well as maxima in
lignin phenols (1. Lobbes, unpubl. data, 1997), the pigment data are not interpreted as a signal
for diagenetic products of marine-derived chlorophyll-a (e.g., Rosell-Me1e, 1994). Possibly the
maxima in the turbidites are caused by chlorophyll-b, which has been synthezised by terrestrial
plants like Spermato-, Pterido-, or Bryophyta (Stryer, 1987).
The carbon preference index (CPI) as a measure for the maturity of the terrigenous organic
matter, varies from 1.5 to 3 in the sedimentary record of Core PS2471, showing neither a
general trend with depth nor any difference between turbidite and non-turbidite intervals.
Discussion
Modem processes controlling organic carbon deposition in the Laptev Sea continental margin
area
As shown in studies of surface sediments (Stein and Niirnberg, 1995; Stein, 1996; Fahl and
Stein, 1997a), the organic-carbon enrichments in the Laptev Sea and the adjacent continental
margin and deep sea (Figure 3) are mainly controlled by increased supply of terrigenous
organic matter. Especially the TOC maxima in the coastal zones off the rivers Lena and Olenek
are directly related to riverine material. This is supported by low hydrogen index values and
high CIN ratios (Stein, 1996) as well as high concentrations of long-chain n-alkanes (Fahl and