25th International Meeting on Organic Geochemistry IMOG 2011

P-471
Integrating biomarker and microfossil records across the
Cretaceous-Paleogene mass extinction event
Julio Sepulveda 1 , Laia Alegret 2 , Emily Wooton 1 , Changqun Cao 3 , Roger E. Summons 1
1 Massachusetts Institute of Technology, Cambridge, United States of America, 2 Universidad de Zaragoza,
Zaragoza, Spain, 3 Chinese Academy of Sciences, Nanjing, China (corresponding author:juliosep@mit.edu)
Currently, two hypothesized post-extinction models
(―Strangelove‖ and ―Living Ocean‖ [1,2]) for the status
of marine primary productivity and the global carbon
cycle following the Cretaceous-Paleogene (K-Pg)
mass extinction exist. However, these models have
been recently challenged by geochemical and
micropaleontological studies suggesting an early
resurgence in marine primary production and food
supply reaching the seafloor after the K-Pg [3,4].
Fundamentally, these results conflict with the
mechanisms proposed to elucidate carbon isotopic
anomalies preserved in deep-sea sediments. Here,
we present a comprehensive investigation of the postimpact
ecosystem recovery based on the study of
lipid biomarkers and bulk stable N- and C-isotopes in
combination with the assemblage of benthic
foraminifera. Eight remarkably expanded K-Pg
sections (clay layers) representative of different
bathymetric zones along a latitudinal gradient were
studied (Stevns Klint in Denmark, Loya and Bidart in
France, Agost and Caravaca in Spain, and El Kef and
Aïn Settara in Tunisia).
The Tunisian outcrops presented the lowest degree of
thermal maturity and appear as the most suitable for
organic geochemical work. Sections from Spain,
France and Denmark presented intermediate and
fluctuating levels of maturity, likely invoking mixtures
of organic matter of differing ages and thermal
histories through differential preservation of in-situ
organic matter. Only minor changes in eukaryotic
sources relative to bacteria were observed between
pre- and post-boundary sediments, as evidenced by
the relative contribution of steranes over hopanes
[S/S+H], except for a conspicuous and short-lived
algal decrease in the Danish section. Similarly, the
relative contribution of C28 and C29 steranes [C28/C29]
indicated insignificant changes in the ecological
dominance of their algal precursors throughout the
boundary layers. Peaks of dinosteranes were only
found in the clay layers of Tunisia, whereas they
remained unchanged or were absent in other
sections. A marked latitudinal trend in decreasing
prevalence of algal dominance (low [S/S+H]) and a
reduction in the C28/C29 ratio were observed towards
higher latitudes. A high contribution of hopanoid-
producing cyanobacteria (2-methylhopanoid index
>10%) is only observed in the aftermath of midlatitude
and upper-bathyal sections (Agost, Caravaca
and Bidart). The � 15 N record exhibits generally more
negative values before and immediately above the
boundary clay layer, whereas a gradual enrichment is
observed up section. The benthic foraminiferal
assemblage displays a reduction of infaunal species
relative to epifaunal groups following the boundary,
suggesting a reduction of food supply to the seafloor.
These results allow us to provide the following
insights regarding the post-impact environment:
a) Major changes in planktonic microfossils across the
K-Pg are not reflected in the biomarker record.
b) The lack of major variations in the relative
contribution of main phytoplanktonic groups and the
persistence of algal biomarkers throughout the
boundary suggest that primary production was only
slightly or briefly affected by the impact event.
c) A decreasing algal contribution towards high
latitudes, associated with major changes in algal
ecology, suggest a northward increase in
bacterioplankton dominance and possible mixotrophy.
d) The co-occurrence of epifaunal, shallow infaunal
and deep infaunal species, and the lack of significant
extinctions among benthic foraminifera, suggests a
relatively continuous nutrient supply to the seafloor in
the aftermath of the K-Pg, as also suggested by the
biomarker record.
References:
[1] Hsü et al., 1982. Science 216, 249–56; [2] D‘Hondt
et al., 1998. Science 282, 276–79; [3] Sepulveda et
al., 2009. Science 326, 129-132; [4] Alegret and
Thomas, 2009. Mar. Micropal. 73, 105-116.
596