| The Early Aptian is referred to as a time of major perturbations in the ocean-atmosphere system, documented by the widespread deposition of organic carbon-rich shales in the world oceans, accompanying biological turnover and pronounced excursions in the carbon-isotope record. Even though these changes have been described in great detail from different pelagic and hemipelagic settings, few studies have been carried out focusing on shallow water environments and their responses to climatic and oceanographic disturbances during this time interval.
Mixed carbonate-siliciclastic coastal deposits of Early Aptian age from the Portuguese Algarve basin have been chosen as the near-shore archives to study from a sedimentological, geochemical and palynostratigraphic perspective. Carbon-isotope stratigraphies have been established for two sections based on bulk organic matter (OM) of terrestrial origin. The dominance of continent-derived plant material in the bulk OM fraction has been confirmed by optical studies, Rock-Eval pyrolysis and by comparison with the  signature of different types of fossilized land-plant particles. Both sections exhibit a very similar carbon-isotope pattern displaying a pronounced isotopic minimum (-27.8  ) followed by a prominent shift towards higher values (up to -19.4 ). This terrestrial carbon-isotope record is compared with org data of marine origin, obtained from a stratigraphically well-constrained hemipelagic succession (Vocontian basin, SE France), representing the OAE 1a black shale. This distinct organic carbon-rich horizon corresponds to a stable interval in the record (chemostratigraphic segment C5) within the step-like, negative-positive carbon-isotope shift occurring above polarity chron M0 in the NC6 nannofossil Zone and the lowermost Leupoldina cabri planktic foraminiferal Zone.
Remarkable similarities between the terrestrial and marine carbon-isotope records, in combination with the palynostratigraphic results, facilitate a detailed chemostratigraphic correlation from the basinal to the coastal realm and enable comparison of the sedimentary evolution in the different environments at a high-resolution. In contrast to the hemipelagic succession, none of the studied coastal sections comprise an organic-rich layer. The basinal black shale horizon corresponds to a marked interval of siliciclastic shedding in the near-shore environment, documented by the occurrence of a ca. 10 m thick sequence of bedded silt- and sandstone and a pronounced drop in CaCO3 content. These siliciclastics are overlaid by a 6 m thick package of peritidal carbonates, forming prominent banks in the clay- and marlstone-dominated sedimentary system. The deposition of the coastal succession took place during a transgressive depositional sequence following a major discontinuity of Early Bedoulian age. The studied near-shore succession shows pronounced changes in the sedimentological pattern during deposition of OAE 1a. Whether these changes are controlled by short-term sea-level fluctuations and/or reflect variations in weathering conditions due to climatic disturbances during the Early Aptian has to be solved. |