25th International Meeting on Organic Geochemistry IMOG 2011

P-457
Molecular and isotopic evidence for differences in organic
matter delivery and preservation in two mid-Pleistocene lightdark
color cycles in sediments beneath the Benguela Current
Upwelling System
Philip Meyers 1 , Ioanna Bouloubassi 2 , Richard Pancost 3
1 Marine Geology and Geochemistry Program, Department of Geological Sciences, The University of
Michigan, Ann Arbor, United States of America, 2 L'OCEAN, UMR 7159 CNRS-IRD-UPMC-MNHN, IPSL,
Paris, France, 3 Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol, United
Kingdom (corresponding author:pameyers@umich.edu)
The light-dark color cycles that are distinctive features
of late Neogene-Quaternary sediment beneath the
Benguela Current Upwelling System imply repetitive
alternations in organic matter delivery and
preservation. We employed molecular and isotopic
proxies for paleoproductivity and for depositional
conditions to investigate the processes involved in
creating these cycles. We selected two sediment
sequences from ODP Site 1084 that correspond to
0.7 and 1.1 Mya, the earlier one representing a world
in which glacial-interglacial cycles were paced by
eccentricity and the older one a world dominated by
obliquity. TOC concentrations vary from ca. 5% in
lighter sediments to ca.18% in darker sediments in
both sequences. Large Rock-Eval HI values indicate
that marine organic matter dominates the TOC in all
these sediments. Bulk organic δ 13 C values in the
darker sediments at 0.7 Mya are -21.5 ‰ and -19.5 ‰
at 1.1 Mya, implying that marine productivity was
greater in the older sequence. However, alkenonebased
sea-surface temperatures were lower at 0.7
Mya (15°C) than at 1.1 Mya (21°C) and bulk δ 15 N
values are also different. Both parameters suggest
that water-mass properties had changed in the 400 ky
between the deposition of the two light-dark cycles.
Biomarker molecular compositions in both sequences
reflect a range of marine organic matter sources.
Algal biomarkers are abundant and diverse,
represented by 4-desmethyl and 4-methylsterols
(diatoms), dinosteral (dinoflagellates), and alkenones
(haptophytes). These biomarkers have δ 13 C values
ranging from ca. -21 to -24 ‰, consistent with a
marine origin. A difference in the algal communities
that produced the organic matter in the two light-dark
sequences is recorded by an alkenone/sterol
concentration ratio in the 0.7 Mya cycle that is twice
that in the 1.1 Myr cycle. Stenol/stanol ratios are
consistently >1, and they are somewhat larger in the
younger sequence than the older one, indicating
relatively better preservation of the marine organic
matter in the younger light-dark cycle. Distributions of
long-chain n-alkanes with CPI values that generally
are between 6 and 9 record inputs of land-plant
waxes, likely by eolian delivery from Namibia. n-
Alkane δ 13 C values that range from -24 to -28 ‰
suggest a mixture of C3 and C4 sources. The δ 13 C
values of the C33 n-alkane are consistently 2-3 ‰
larger than those of the C31 n-alkane, confirming an
important contribution of this vascular plant biomarker
from C4 grasses. All of the long-chain n-alkanes
exhibit smaller δ 13 C values in the younger cycle than
in the older one, suggesting that a drier continental
climate accompanied the colder sea surface
temperatures recorded by the alkenones in the 0.7
Mya cycle. Other terrestrial biomarkers (e.g.
triterpenoid acids and alcohols) are present but in
very low abundances. Our molecular and isotopic
comparison of these two cycles evidently captures
some of the consequences of the progressive cooling
of global climate and the associated changes in
ocean-continent interactions.
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