Complex and protracted environmental and ecological perturbations during OAE 1a - Evidence from an expanded pelagic section from south Spain (Western Tethys)

Abstract

The early Aptian Oceanic Anoxic Event (OAE) 1a represents a major perturbation in the global carbon cycle associated with significant environmental, biotic and sedimentary changes. The signature of this event is a global negative followed by a positive stable carbon isotope excursion (CIE), associated with the input of light-carbon into the climate system and subsequent widespread deposition of organic-rich sediments. This study uses biostratigraphy, C-isotope stratigraphy, biomarkers, and elemental geochemistry to stratigraphically and geochemically characterize OAE 1a at an expanded pelagic marine section from the western Tethys.The high-resolution δ 13 Corg analysis of the section highlights several successive negative CIEs during the onset of OAE 1a (isotope segment C3), pointing to rapid changes in global carbon cycling. The biomarker data indicate that the organic matter is thermally mature and is mainly of marine origin. The biomarker assemblage together with records of redox-sensitive trace elements suggest that sedimentation took place under generally well oxygenated waters punctuated by three short episodes of sea floor anoxia/dysoxia. The two first episodes of anoxia/dysoxia correlate with enhanced organic productivity during the main negative C-isotope excursion that represents the onset of the OAE 1a. They occurred at the final part of C-isotope negative excursions, likely as a response to C-release, increased temperatures and associated hydrological change and weathering, which resulted in increased fertilization-driven oxygen consumption. This was followed by the positive carbon isotope excursion, due to the burial of OM outpacing carbon input. The third episode of anoxia-dysoxia, which occurred later during the major positive C-isotope excursion, is not associated with fertilization, and perhaps instead reflects a general progressive depletion of oxygen during OAE 1a, linked to stagnation of marine waters. Interestingly, although the environmental parameters return to pre-OAE values at the end of the event, a biotic (especially microbial) perturbation persisted after OAE 1a. Our results suggest that instability in environmental conditions was the 2 main feature during OAE 1a in the western Tethys with notable changes extending from the onset to the post-OAE 1a interval.

High‐Resolution C‐Isotope, TOC and Biostratigraphic Records of OAE 1a (Aptian) From an Expanded Hemipelagic Cored Succession, Western Tethys: A New Stratigraphic Reference for Global Correlation and Paleoenvironmental Reconstruction

“…The negative excursion and minimum that define the Ap3 segment begin with the negative δ 13 C carb “spike” that are observed in several other sections (e.g., DSDP Site 463, Ando et al., 2008; Cismon core, Bottini et al., 2015; Menegatti et al., 1998; Figure 5), as well as in δ 13 C org records (Basque‐Cantabrian Basin, Millán et al., 2009; Carbonero section, Castro et al., 2019). Other differences between sections affecting the Ap3 segment are related to the position of the lowest values of δ 13 C carb , which may correspond to the base, middle part or uppermost part of the segment, and also to the definition of the Ap3/C3 segment in each study.…”

“…Interestingly, the third episode of TOC enrichment seems to be diachronous, taking place earlier in intrashelf basins (C5) and later in deep pelagic basins (C6), where C5 is represented by low TOC contents (Figure 7). This might be related to eutrophication from terrestrial inputs mostly affecting continental margins during C5, with the subsequent establishment of anoxia in deep marine basins during the C6 segment, as suggested by increases in OM deposition in deep marine environments during this interval (Figure 7), and proposed by Castro et al (2019) based on biomarker evidence. A shift from enhanced productivity to enhanced preservation across OAE 1a has also been proposed by previous studies on western Tethys sections (Bottini & Erba, 2018;Castro et al, 2019;Westermann et al, 2013).…”

“…These sedimentation rates lie within error of previous estimates from other sections. Differences are attributable to varying subsidence rates and sediment fluxes, but also to the use of different age models, different definitions of segment boundaries, and factors such as the presence of condensed intervals or discontinuities (Bottini et al., 2015; Castro et al., 2019; Huck et al., 2011; Li et al., 2008; Lorenzen et al., 2013; Malinverno et al., 2010; Scott, 2016).…”

“…The first episode of TOC enrichment coincides with the negative excursion at the onset of OAE 1a, considered to reflect a sharp input of light carbon into the atmosphere, either from volcanic and/or methane sources (e.g., Adloff et al., 2020; Méhay et al., 2009; Weissert, 2000). The consequent increase in temperatures could have resulted in an intensification of the hydrological cycle, increased terrestrial weathering and runoff, enhanced upwelling and ocean fertilization leading to a productivity‐driven episode of OM accumulation (e.g., Bottini et al., 2012; Castro et al., 2019). The second episode of TOC enrichment occurred in the opposite scenario, coeval to sharply rising δ 13 C (C4 segment).…”

“…(2019) based on biomarker evidence. A shift from enhanced productivity to enhanced preservation across OAE 1a has also been proposed by previous studies on western Tethys sections (Bottini & Erba, 2018; Castro et al., 2019; Westermann et al., 2013). A possible explanation for this evolution is a reduction in the oxygenation of deep marine waters, due to the oxidation of OM.…”