--> Regional Paleoenvironmental Influence on OM Sequestration and Carbon Isotope Segment C5 in the Hemipelagic Sequence at the EL Pujal Section, Organyà Basin, Northeast Spain

AAPG ACE 2018

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Regional Paleoenvironmental Influence on OM Sequestration and Carbon Isotope Segment C5 in the Hemipelagic Sequence at the EL Pujal Section, Organyà Basin, Northeast Spain

Abstract

The El Pujal Section consists of interbedded dark gray to black hemipelagic limestones, argillaceous limestones and marls deposited during the Aptian in the Organyà Basin. Microscopically the matrix is comprised essentially of coccoliths throughout, with darker intervals showing lower degree of bioturbation index (3), higher proportion of pyrite, fewer benthic foraminifera and echinoid remains. High-resolution geochemical analyses of the upper 71.27 m section yield TIC values fluctuating between 54.92% and 81.46% (average of 69.86%), and total organic carbon (TOC) oscillating between 1.83% and 0.41% (average of 1.07%). Stable carbon isotope (δ13Corg) values vary between -25.09‰ and -23.44‰ with an average of -24.02‰, consistent with the pattern reported elsewhere for carbon isotope segment C5 [1]. However, δ13Corg values show a positive correlation with TIC (r = 0.50) while correlating negatively with TOC (r = -0.46). More negative δ13Corg values correspond with intervals of highest regional terrestrial influences, previously correlated with an increase of longer chain (>nC25) n-alkanes [2]. Similarly, fluctuation in major, terrestrially derived elements (Si, Al, Ti), bio-limiting elements (Fe, P), and redox sensitive trace elements (RSTEs) (V, Cr, Co, Ni, Cu, Mo, U) that show consistent enrichments in beds with higher TOC suggest transient episodes of dysoxia.

This pattern of transient episodes of dysoxia and enhanced OM sequestration is associated with punctuated pulses of riverine fluxes and wetter paleoclimatic conditions as attested by clay mineral assemblages characterized by smectite (11%), illite (63%), kaolinite (12%) and chlorite (14%) with higher proportions of kaolinite in OM rich layers (TOC r = 0.37) which further facilitated the preservation of OM. The concurrent negative correlation between δ13Corg and TOC, as well as terrestrially derived elements, RSTEs and kaolinite (r = -0.65) highlights the influence of higher regional inputs of terrestrial vegetation with lower δ13Corg values. Their isotopic signal is superimposed on the global δ13Corg signature associated with segment C5, correlative with enhanced preservation of OM in the sequence.

REFERENCES

[1] Menegatti et al., 1998. Paleoceanography 13 530–545;[2] Socorro & Maurrasse, 2016. AGU Fall Meeting Abstracts.