Terrestrial Organic Matter in Marine Sediments
553
biomass is enriched in hydrogen and consequently
produces higher HI values, compared to OM from
terrestrial biomass. This simple assumption is problematic if applied directly to TOC poor samples
« 0.5%; Espitalie et a!. 1984; Katz 1983), since
early diagenetic effects and retention of hydro carbons by the mineral matrix ("mineral matrix effect")
may lead to an overestimation of the terrestrial fraction. In order to identify and correct for mineral
matrix effects, Langford and Blanc-Valleron (1990)
suggest to determine the "real" HI from the regression of S2 vs. TOC. The relevance of mineral matrix effects, however, has to be evaluated for each
individual set of samples, taking results from TOC
poor pelagic sediments of the Norwegian-Greenland Sea (Wagner 1993) and the Equatorial Atlantic into account (Fig. la-c). Studies on matrix
adsorbtion suggests that it occurs in immature
marine sediments not uniformly but preferentially
in surface and glacial deposits. For comparison,
non-corrected and mineral matrix corrected HI are
displayed for Equatorial Atlantic sediments in van
Krevelen diagrams (Fig. Id-t). Results reveal that
mineral-matrix corrected HI of surface and interglacial samples in general plot into similar kerogen
fields as non-corrected samples, whereas glacial
samples reveal a stronger offset of corrected
values toward kerogen type II (mixed marine/
terrigenous). Three other aspects should be considered when HI values from recent, immature
marine deposits are discussed. First, hydrocarbons
are not exclusively generated by marine originated
OM, but may also be produced by recently deposited relatively fresh terrestrial plant matter or by
hydrocarbon-bearing reworked, fossil material (e.g.
from coal or black shale strata). Hence, interpretation of HI from near-continental, river-influenced
settings should be done carefully and obligatorily
in conjunction with other geochemical methods.
The second aspect concerns a typical pyrolytic
characteristic of immature OM in recent deposit~.
Liebezeit and Wiesner (1990) demonstrated that
labile biopolymers, which account for an important
fraction ofthe bulk organic pool in marine surface
sediments, generate hydrocarbons and non-hydrocarbons at very low temperatures. These products
are conventionally detected within the "S I " yield,
which in fossil source rocks comprise free hydrocarbons generated from the oil fraction (Espitalie
eta!. 1977; Tissotand Welte 1984). Hydrocarbons
detected in the SI temperature interval «300°C),
however, are not considered when HI are calculated (note HI=(S2* I OO)/TOC). For young immature sediments with labile OM it therefore appears
necessary to add the S 1 to the S2 yield before the
HI is calculated. Differences estimated between
conventional (S2) and modified (S I +S2) HI range
from 0-32% (average 9-14%) for Quaternary
pelagic sediments of the northern and equatorial
Atlantic (Wagner 1993; Wagner and Holemann
1995; Wagner 1998). Finally, alteration by bacterial sulfate reduction may strongly affect HI in surface sediments (Littke et a!. 1997b; Liickge 1997).
In conclusion, quantitative assessment of marine vs.
terrigenous OM based on HI alone is considered
unrealistic with regard to the almost impossible
definition of representative endmembers.
A second parameter obtained by Rock-Eval
Pyrolysis is TmID< which is not adequately discussed
in many paleoceanographic studies. TmID< delineates
the temperature of maximum hydrocarbon generation from the kerogen fraction (S2) which is a direct measure of the thermal maturity of the OM.
By definition, OM is considered thermally immature ifTmID< is below 435°C «0.5-0.6% Rm vitrinite
reflectance), mature if between 435-460°C (up to
1.3% Rm), and overmature when exceeding 460°C
(Tissot and Welte 1984). Hence, OM in young
marine sediments is expected to be immature revealing T mID< values below 400°C. Evidence for high
thermal maturity levels in Quaternary deposits,
however, is frequently reported from high latitude
settings such as the Baffin Bay and Labrador Sea
(e.g. Stein 1991), along the East Greenland margin (Stein et al. 1993), in the NorwegianGreenland Sea (Holemann and Henrich 1994;
Wagner and Henrich 1994, Wagner and Holemann
1995), the Fram Strait (Wagner 1993), and the
eastern Arctic Ocean (Stein et a!. 1994; Schubert
1995).
With regard to the widespread and often simultaneous occurrence of elevated T mID< in glacial sections of Nordic Sea sediments, a common cause
and depositional mechanism is indicated.
Sedimentological and organic-geochemical studies
performed in these areas have shown that glacial
553
biomass is enriched in hydrogen and consequently
produces higher HI values, compared to OM from
terrestrial biomass. This simple assumption is problematic if applied directly to TOC poor samples
« 0.5%; Espitalie et a!. 1984; Katz 1983), since
early diagenetic effects and retention of hydro carbons by the mineral matrix ("mineral matrix effect")
may lead to an overestimation of the terrestrial fraction. In order to identify and correct for mineral
matrix effects, Langford and Blanc-Valleron (1990)
suggest to determine the "real" HI from the regression of S2 vs. TOC. The relevance of mineral matrix effects, however, has to be evaluated for each
individual set of samples, taking results from TOC
poor pelagic sediments of the Norwegian-Greenland Sea (Wagner 1993) and the Equatorial Atlantic into account (Fig. la-c). Studies on matrix
adsorbtion suggests that it occurs in immature
marine sediments not uniformly but preferentially
in surface and glacial deposits. For comparison,
non-corrected and mineral matrix corrected HI are
displayed for Equatorial Atlantic sediments in van
Krevelen diagrams (Fig. Id-t). Results reveal that
mineral-matrix corrected HI of surface and interglacial samples in general plot into similar kerogen
fields as non-corrected samples, whereas glacial
samples reveal a stronger offset of corrected
values toward kerogen type II (mixed marine/
terrigenous). Three other aspects should be considered when HI values from recent, immature
marine deposits are discussed. First, hydrocarbons
are not exclusively generated by marine originated
OM, but may also be produced by recently deposited relatively fresh terrestrial plant matter or by
hydrocarbon-bearing reworked, fossil material (e.g.
from coal or black shale strata). Hence, interpretation of HI from near-continental, river-influenced
settings should be done carefully and obligatorily
in conjunction with other geochemical methods.
The second aspect concerns a typical pyrolytic
characteristic of immature OM in recent deposit~.
Liebezeit and Wiesner (1990) demonstrated that
labile biopolymers, which account for an important
fraction ofthe bulk organic pool in marine surface
sediments, generate hydrocarbons and non-hydrocarbons at very low temperatures. These products
are conventionally detected within the "S I " yield,
which in fossil source rocks comprise free hydrocarbons generated from the oil fraction (Espitalie
eta!. 1977; Tissotand Welte 1984). Hydrocarbons
detected in the SI temperature interval «300°C),
however, are not considered when HI are calculated (note HI=(S2* I OO)/TOC). For young immature sediments with labile OM it therefore appears
necessary to add the S 1 to the S2 yield before the
HI is calculated. Differences estimated between
conventional (S2) and modified (S I +S2) HI range
from 0-32% (average 9-14%) for Quaternary
pelagic sediments of the northern and equatorial
Atlantic (Wagner 1993; Wagner and Holemann
1995; Wagner 1998). Finally, alteration by bacterial sulfate reduction may strongly affect HI in surface sediments (Littke et a!. 1997b; Liickge 1997).
In conclusion, quantitative assessment of marine vs.
terrigenous OM based on HI alone is considered
unrealistic with regard to the almost impossible
definition of representative endmembers.
A second parameter obtained by Rock-Eval
Pyrolysis is TmID< which is not adequately discussed
in many paleoceanographic studies. TmID< delineates
the temperature of maximum hydrocarbon generation from the kerogen fraction (S2) which is a direct measure of the thermal maturity of the OM.
By definition, OM is considered thermally immature ifTmID< is below 435°C «0.5-0.6% Rm vitrinite
reflectance), mature if between 435-460°C (up to
1.3% Rm), and overmature when exceeding 460°C
(Tissot and Welte 1984). Hence, OM in young
marine sediments is expected to be immature revealing T mID< values below 400°C. Evidence for high
thermal maturity levels in Quaternary deposits,
however, is frequently reported from high latitude
settings such as the Baffin Bay and Labrador Sea
(e.g. Stein 1991), along the East Greenland margin (Stein et al. 1993), in the NorwegianGreenland Sea (Holemann and Henrich 1994;
Wagner and Henrich 1994, Wagner and Holemann
1995), the Fram Strait (Wagner 1993), and the
eastern Arctic Ocean (Stein et a!. 1994; Schubert
1995).
With regard to the widespread and often simultaneous occurrence of elevated T mID< in glacial sections of Nordic Sea sediments, a common cause
and depositional mechanism is indicated.
Sedimentological and organic-geochemical studies
performed in these areas have shown that glacial