Synchronous negative carbon isotope shifts in marine and terrestrial biomarkers at the onset of the early Aptian oceanic anoxic event 1a: Evidence for the release of ¹³C‐depleted carbon into the atmosphere

Abstract

A common feature of records of the early Aptian oceanic anoxic event (OAE) 1a is the sharp negative δ13C excursion displayed in both carbonate and organic matter at the onset of this event. A synchronous negative δ13C excursion has also been noted for terrestrial organic matter. This negative excursion has been attributed to either an injection of 13C‐depleted light carbon into the atmosphere or, in case of marine sediments, recycling of 13C‐depleted CO2. However, most studies were done on separate cores, and no information on the relative timing of the negative spikes in terrestrial versus marine records has been obtained. Here we examine early Aptian core sections from two geographically distal sites (Italy and the mid‐Pacific) to elucidate the causes and relative timing of this negative “spike.” At both sites, increased organic carbon (Corg) and decreased bulk carbonate contents characterize the interval recording OAE 1a (variously referred to as the “Selli event”). The organic material within the “Selli level” is immature and of autochthonous origin. Measured δ13C values of marine and terrestrial biomarkers largely covary with those of bulk organic carbon, with lowest values recorded at the base of the organic‐rich section. By contrast, sediments enveloping the “Selli level” exhibit very low Corg contents, and their extractable Corg is predominantly of allochthonous origin. Hydrous pyrolysis techniques used to obtain an autochthonous, pre‐Selli δ13C value for algal‐derived pristane from corresponding sample material yielded a negative δ13C shift of up to 4‰. A negative δ13C shift of similar magnitude was also measured for the terrigenous n‐alkanes. The results are collectively best explained by means of a massive, syndepositional, rapid input of 13C‐depleted carbon into the atmosphere and surface oceans, likely delivered either via methane produced from the dissociation of sedimentary clathrates or perhaps by widespread thermal metamorphism of Corg‐rich sedimentary rocks.

The Late Hauterivian Faraoni ‘Oceanic Anoxic Event’: an update

“…Such negative CIEs are known for T-OAE [Jenkyns, 1988;Hesselbo et al, 2000;Schouten et al, 2000;Kemp et al, 2005;Sabatino et al, 2009;Hesselbo and Pienkowski, 2011;Hermoso et al, 2012], OAE 1a [Menegatti et al, 1998;Jenkyns and Wilson, 1999;Ando et al, 2002;Herrle et al, 2004;van Breugel et al, 2007], OAE 1b Luciani et al, 2004] as well as for OAE 2 [Kuroda et al, 2007;Elrick et al, 2009], although the CIE is not always recorded in the latter. [Haq et al, 1988;Haq, 2014] illustrating the stratigraphic position and nomenclature of OAEs discussed in the text.…”

Environmental conditions in the South Atlantic (Angola Basin) during the Early Cretaceous

“…5‰ shift in compound specific d 13 C values at other locations (e.g. van Breugel et al, 2007;Méhay et al, 2009). As such, it is unlikely that the Aptian organicrich shales at Site 364 are a local manifestation of OAE 1a, although higher resolution studies are needed to verify this conclusion.…”

Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)

“…The early Aptian OAE 1a or Selli event (~120 Ma) has been much studied in terms of carbon-, calcium-, osmiumand strontium-isotope stratigraphy: Blättler et al (2011); Bottini et al (2012); Erba et al (2010); ; ; Jones & Jenkyns (2001); Keller et al (2011); Méhay et al (2009) ;Menegatti et al (1998); Tejada et al (2009);van Breugel et al (2007); Wissler et al (2004). This event is conventionally interpreted as triggered by the massive outgassing of the Ontong Java, Manihiki and Hikurangi submarine large igneous province (LIP; Larson & Erba, 1999;Méhay et al, 2009;Tarduno et al, 1991Tejada et al, 2009, which introduced large quantities of isotopically light carbon into the atmosphere-ocean system, producing a stepwise decline in  13 C and lowering the pH of the oceans.…”