Records of the early Aptian OAE 1a (Southern Iberian Paleomargin, Western Tethys): multiproxy evidence from expanded successions

The early Aptian OAE 1a is one of the most prominent hyperthermal events in Earth’s history. This event is associated with global environmental and biotic perturbations, including global rise in temperatures, ocean oxygen depletion, and widespread deposition of organic-rich marine sediments.

The onset of OAE 1a coincided with a major disturbance of the global carbon cycle, recorded with a marked negative spike in the C-isotope record. A complex response of the environmental and biotic system occurred, ultimately triggered by volcanism of the Ontong-Java Plateau.

In this study, we present two expanded records of OAE 1a from the Southern Iberian Palaeomargin (SIP, Western Tethys): the Carbonero and Cau sections, which comprise high-resolution C-isotope records that have served as the basis for a precise stratigraphic correlation.

The Carbonero section is composed of black shales, radiolarites and marls, deposited on a fault-bounded, highly subsiding sector of the pelagic basin of the SIP. The Cau section (also studied in a core), consists of an alternance of hemipelagic marls and marly limestones deposited in the distal platform settings of the SIP. Previous studies have provided with multiproxy evidence, including stratigraphy and sedimentology, biostratigraphy (ammonites, planktonic foraminifera, calcareous nannofossils and radiolaria), C-O and Re-Os isotope stratigraphy, elemental composition and biomarker distributions. The time model has been based on a combination of biostratigraphy and cyclostratigraphy. Atmospheric CO₂ concentrations records across OAE 1a have been derived from bulk and compound-specific C-isotope data from the Cau section. The high-resolution C-isotope stratigraphy from the Cau core has been used to refine the previously defined C-isotope segments of the Aptian, and to correlate the succession with other records worldwide.

The onset of the OAE 1a has been studied at ultrahigh-resolution scale (0.2–0.5 kyr spacing) revealing a succession of sharp C-isotope negative spikes, interpreted as a record of pulses of volcanism and methane emissions. The largest spike was rapid (<10 kyr) and marks the base of OAE 1a, which occurs within a longer-term falling C-isotope trend. The C-isotope profile across OAE 1a records the negative (C3/Ap3), positive (C4/Ap4), steady (C5/Ap5), and positive (C6/Ap6) segments that were defined from Cismon (Italy) and subsequently recognized worldwide.

Our results illustrate a complex evolution of environmental and biotic changes throughout the OAE 1a event, with an estimated duration of ca. 1.47 Ma. Moreover, the unprecedented resolution of the study of the onset of the event reveals rapid environmental and biotic changes at the scale of the thousands of years, providing valuable insights on the mechanisms acting at shorter time intervals and enabling comparisons with more recent hyperthermal events.