Characterization of the Negative Carbon Isotope Shift in Segment C2 of the Organyà Basin, El Pui Section, Spain; a Potentially Useful Chemostratigraphic Marker Prior to OM-rich Interval Associated with Oceanic Anoxic Event 1a (OAE 1a)

Abstract

Abstract

Lower Cretaceous deposits are widely recognized to show intermittent prominent accumulation of organic-rich sediments at the origin of hydrocarbon source rocks. These intervals are often identified by the presence of negative and positive spikes. The original standard pattern registered by the δ¹³C_org and δ¹³C_carb curves in Lower Aptian sediments of the Alpine Tethys include distinct isotope segments coined (C1 to C8)[1]. The same pattern is further observed worldwide and thus provides a calibrated chemostratigraphic tool useful for correlation and geochronology. The longest interval C2 preceding OM-rich segments C3-C6 (OAE 1a) is punctuated by a negative spike of 0.5 to 2.06 ‰ found in many expanded sections of the Tethyan realm: e.g. Cismon in Italy, Djebel Serdj in Tunisia, Bab Basin in Oman, and El Pui in Spain [1,4,5,3,2].

We use TIC, TOC, δ¹³C_org, microfacies, paleontological and biomarker (n-alkanes) as proxies to provide further insight into the character of the depositional environment associated with this negative inflection and assess the effects of local factors on the isotope curve associated with the C2 negative shift at El Pui. The results show unvarying allochemical contents with biota mainly dominated by calcareous nannofossils associated with a TIC average of 72.8 CaCO₃ %, scarce minute planktonic foraminifera and an abundance of roveacrinid fragments. N-alkanes distribution prior to the negative spike suggests that the source of the organic matter is mainly from protists and bacteria (≤ nC19), with some contribution from macrophytes (nC20 - nC25) and terrestrial plants (>nC25). During and after the negative spike, the n-alkanes are consistently derived from protists and bacteria with minor contribution from macrophytes, and none from terrestrial plants. The terrestrial/aquatic ratios TARs (nC27+nC29+nC31)/(nC15+nC17+nC19) averaging 0.112 before and 0 during and after the negative shift also imply that local factors did not affect the δ¹³C_org curve during segment C2. Therefore, the negative shift in segment C2 reflects a change in the global ocean carbon reservoir, that can be a useful chemostratigraphic marker before OAE1a.

[1]Menegatti et al 1998 Paleoceanography 13 530–545;[2]Sanchez-Hernandez et al 2014 Cretaceous Research 51,1 – 21;[3]Sanchez-Hernandez & Maurrasse 2015 Palaeo 3; 2014 Chemical Geology 372 12–31;[4] Heldt et al 2008 Palaeo 3 261, 246–260;[5]Yamamoto et al 2013 Geochemistry Geophysics Geosystems 14.4 1104-1130.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016