Regional palaeoenvironmental influence on organic matter sequestration and characteristics of carbon isotope segment C5 in a hemipelagic sequence, Organyà Basin, northeast Spain

Abstract

AbstractThe Cretaceous marine sedimentary record is punctuated by episodes of unusually high rates of organic carbon burial termed oceanic anoxic events. The El Pujal Section archives the response to perturbations of the global carbon cycle associated with Oceanic Anoxic Event 1a in a hemipelagic setting of the Organyà Basin. Here, the aim is to understand the lithological and geochemical responses to changing palaeoenviromental conditions associated with carbon isotope segment C5 of Oceanic Anoxic Event 1a. A multi‐proxy approach is applied to characterize how regional factors influenced the accumulation of organic‐rich sediments during this period of severe oceanic oxygen deficiency. Source specific biomarkers demonstrate that the bulk organic matter derives from in‐situ production, with lesser inputs from allochthonous sources. High‐resolution stable carbon isotope data (δ13Corg) were used to establish the refined details of segment C5. Pulses of fluvial fluxes associated with climate fluctuations supplied variable quantities of terrestrial organic matter with more negative isotopic values, which left their intrinsic signature superimposed on the global δ13Corg signal of segment C5 as minor negative spikes. Primary production sustained by fluvial inputs of biolimiting elements provided abundant labile organic matter conducive to oxygen‐deprived conditions as attested by concomitant peaks in total organic carbon, relatively lower benthic faunal counts, lower bioturbation index, higher concentration of pyrite and enrichments in redox sensitive trace elements. However, organic matter preservation was not chiefly controlled by redox conditions as physical encapsulation by clay minerals probably played a significant role. In addition, coincident inputs of terrestrial organic matter with more inert properties and prone to be bound to clay minerals supplemented the accumulation of organic matter, especially during intervals of enhanced terrigenous fluxes. The combined results underscore how despite sharing common global forcing factors, regional palaeoenvironmental conditions are more important in determining the sedimentary expression of Oceanic Anoxic Event 1a.