Publication: Organic geochemistry, stable isotopes, and facies analysis of the Early Aptian
OAE—New records from Spain (Western Tethys)
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Publication Date
2012
Authors
Quijano López, María Luisa
Castro Jiménez, Jose Manuel
Pancost, Richard D.
Gea Guillén, Ginés Alfonso
Najarro, María
Aguado Merlo, Roque
Rosales, Idoia
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Elsevier
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Abstract
The Early Aptian Oceanic Anoxic Event (OAE1a) is a time interval characterized by increased organic carbon
accumulation in marine sediments, notable sedimentary and biotic changes, and abrupt carbon-isotope
excursions indicative of significant major palaeoenvironmental changes linked to a perturbation in the global
carbon cycle. Here we present the study of four sections recording the OAE1a (Early Aptian) in Spain, which
are located in two broad basins respectively located in the South and the North of Iberia: the Southern Iberian
Palaeomargin (Carbonero, La Frontera and Cau sections) and the Basque–Cantabrian Basin (Puentenansa
section), which represent depositional settings ranging from shallow marine (distal ramp -Cau- and drowned
platform -Puentenansa-) to pelagic environments (Carbonero, La Frontera). Biomarker compositions,
C-isotope profiles, biostratigraphic data and facies analysis from the four sections are correlated and integrated.
The C-isotope curves all present a clear negative excursion followed by a positive shift. The integration of
the C-isotope curves with the biostratigraphic data has been used to correlate the studied sections and to tentatively
identify the eight segments formerly proposed from the Alpine domain, and subsequently identified
in sections worldwide. Four main groups of compounds are present in all sections: n-alkanes, isoprenoids,
hopanes and steranes. n-Alkanes and acyclic isoprenoids (pristane and phytane) are dominant in most samples.
The hopanes are represented by a range of C27 to C35 components,with the specific isomers varying amongst the
sections due to differences in thermal maturity. Steranes occur as a range of C27, C28 and C29 isomers, whereas
diasteranes only occur in the most thermally mature section (Carbonero). Other compounds of interest include
gammacerane and dinosterane. Differences in thermal maturity appear to be the first order control on different
biomarker assemblages amongst the studied sections. The Carbonero section is thermally mature, whereas the
nearby La Frontera and Cau sections are immature. Puentenansa has intermediate values. Organic matter is derived
from a range of terrestrial, marine and bacterial sources. The dominance of the C29 sterane isomers in all
sections suggests a strong contribution from higher plants. The presence of gammacerane indicates water column
stratification, and high C29/C30 hopane ratios suggest anoxia at the water/sediment interface, respectively.
Sedimentologic analysis also suggests anoxic conditions during sedimentation, but evidence for strong and persistent
water column anoxia is equivocal. The correlation of the sections reveals that sedimentation of
organic-rich facies started earlier in pelagic and later in the shallow marine settings, which can be related to
an expansion of the favorable conditions for organic matter accumulation and preservation from deep marine
waters to shallower platform environments during the development of OAE1a.