Terrestrial Organic Matter in Marine Sediments: Analytical Approaches
and Eolian-Marine Records in the Central Equatorial Atlantic
T. Wagner* and L.M. Dupont
Universitat Bremen, Fachbereich Geowissenschaften, Postfach 330440,
D-28334 Bremen, Germany
*corresponding author (e-mail):tw@mail.sedpal.uni-bremen.de
Abstract: Terrestrial organic matter (OM) in pelagic sediments is discussed with regard to depositional
processes and land-sea interactions in the modem and past glacial/interglacial Equatorial Atlantic.
Special emphasis is placed on a critical evaluation of different analytical approaches (CIN, Rock-Eval
Pyrolysis, stable carbon isotopes, palynology, organic petrology, and selected biomarkers) which
are currently used for the qualitative and quantitative assessment of terrigenous organic carbon. If
binary mixing equations are used to calculate terrestrial and marine proportions of organic carbon,
we consider the definition of endmember values to be most critical since these values may be biased
by a great number of independent controls. A combination of geochemical methods including optical
studies (organic petrology and palynology) is therefore suggested to evaluate each individual
proxy. Organic geochemical analyses performed on sediments from the modem and Late Quaternary
Equatorial Atlantic evidence fluctuations in eolian supply of terrigenous OM related to changes in
intensity of the trade winds. Quantification of this organic fraction leads to differing proportions
depending on the approach applied, i.e. the organic carbon isotopic composition or maceral analyses.
Modem distribution of terrigenous OM reveals a decrease in supply towards the basin contributing
less than a fifth ofthe total OM in pelagic areas. Organic geochemical data indicate that sedimentation
in the modem northeastern Brasil Basin is affected by lateral advection of reworked OM probably
from southern source areas. Glacial/interglacial deposits from the pelagic Equatorial Atlantic (ODP
Site 663), covering isotopic stages 12 and II, reveal that deposition ofterrigenous OM was higher
under past glacial conditions, in correspondence to generally enhanced dust fluxes. Proportions of
terrigenous OM, however, never exceed 50% ofthe total OM according to maceral analyses. Other
estimates, recently proposed by Verardo and Ruddiman (1996), are considered to be too high probably
for analytical reasons. Palynological records in the Equatorial Atlantic parallel dust records. Increased
portions of grass pollen suggest the admixture of C4-plant material under modem and past glacial
conditions. It is therefore assumed, as one possible interpetation, that C4-plant debris has an effect
on sedimentary /) 13 C"g and might explain differences between isotopic and microscopic quantitative
estimates. Using the difference between these two records, we calculate that maximum supply ofC4material remains below 20% ofthe total OM for the deep modem and past glacial/interglacial Equatorial
Atlantic.
Introduction
Organic matter (OM) in marine sediments may
either be of marine or terrestrial origin. The quantitative importance of the terrigenous organic fraction as part of the total organic carbon (TOC),
however, is still not completely understood. This
deficit is of relevance for productive pelagic environments where the composition ofTOC is commonly assumed to be dominantly marine (Emerson
et al. 1987; Samthein et al. 1988). Part of the terrestrial OM introduced into the ocean is digested in
the marine food chain (proteins, sugars, etc.) serving as a fertilizer. This type of OM is considered
marine because its terrestrial origin can no longer
be recognized. In contrast, organic components
From FISCHER G. WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg. pp 547-574
and Eolian-Marine Records in the Central Equatorial Atlantic
T. Wagner* and L.M. Dupont
Universitat Bremen, Fachbereich Geowissenschaften, Postfach 330440,
D-28334 Bremen, Germany
*corresponding author (e-mail):tw@mail.sedpal.uni-bremen.de
Abstract: Terrestrial organic matter (OM) in pelagic sediments is discussed with regard to depositional
processes and land-sea interactions in the modem and past glacial/interglacial Equatorial Atlantic.
Special emphasis is placed on a critical evaluation of different analytical approaches (CIN, Rock-Eval
Pyrolysis, stable carbon isotopes, palynology, organic petrology, and selected biomarkers) which
are currently used for the qualitative and quantitative assessment of terrigenous organic carbon. If
binary mixing equations are used to calculate terrestrial and marine proportions of organic carbon,
we consider the definition of endmember values to be most critical since these values may be biased
by a great number of independent controls. A combination of geochemical methods including optical
studies (organic petrology and palynology) is therefore suggested to evaluate each individual
proxy. Organic geochemical analyses performed on sediments from the modem and Late Quaternary
Equatorial Atlantic evidence fluctuations in eolian supply of terrigenous OM related to changes in
intensity of the trade winds. Quantification of this organic fraction leads to differing proportions
depending on the approach applied, i.e. the organic carbon isotopic composition or maceral analyses.
Modem distribution of terrigenous OM reveals a decrease in supply towards the basin contributing
less than a fifth ofthe total OM in pelagic areas. Organic geochemical data indicate that sedimentation
in the modem northeastern Brasil Basin is affected by lateral advection of reworked OM probably
from southern source areas. Glacial/interglacial deposits from the pelagic Equatorial Atlantic (ODP
Site 663), covering isotopic stages 12 and II, reveal that deposition ofterrigenous OM was higher
under past glacial conditions, in correspondence to generally enhanced dust fluxes. Proportions of
terrigenous OM, however, never exceed 50% ofthe total OM according to maceral analyses. Other
estimates, recently proposed by Verardo and Ruddiman (1996), are considered to be too high probably
for analytical reasons. Palynological records in the Equatorial Atlantic parallel dust records. Increased
portions of grass pollen suggest the admixture of C4-plant material under modem and past glacial
conditions. It is therefore assumed, as one possible interpetation, that C4-plant debris has an effect
on sedimentary /) 13 C"g and might explain differences between isotopic and microscopic quantitative
estimates. Using the difference between these two records, we calculate that maximum supply ofC4material remains below 20% ofthe total OM for the deep modem and past glacial/interglacial Equatorial
Atlantic.
Introduction
Organic matter (OM) in marine sediments may
either be of marine or terrestrial origin. The quantitative importance of the terrigenous organic fraction as part of the total organic carbon (TOC),
however, is still not completely understood. This
deficit is of relevance for productive pelagic environments where the composition ofTOC is commonly assumed to be dominantly marine (Emerson
et al. 1987; Samthein et al. 1988). Part of the terrestrial OM introduced into the ocean is digested in
the marine food chain (proteins, sugars, etc.) serving as a fertilizer. This type of OM is considered
marine because its terrestrial origin can no longer
be recognized. In contrast, organic components
From FISCHER G. WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg. pp 547-574