GLOBAL IMPLICATIONS OF THE N-ALKANES DISTRIBUTION IN THE NEGATIVE CARBON ISOTOPE SPIKE OF SEGMENT C2, EL PUI SECTION, ORGANYà BASIN, CATALUNYA, SPAIN

Consistent patterns of coeval carbon isotope records of Lower Cretaceous deposits document shifts that signal overall simultaneous responses of temporal changes in the ocean carbon reservoir. Prominent negative and positive spikes recorded in the δ¹³C_org and δ¹³C_carb stratigraphy of Lower Aptian sediments in the Alpine Tethys are divided in isotope segments C1 to C8 [1], also found elsewhere, thus providing a calibrated chemostratigraphic curve useful for geochronology [2 – 3].

In the expanded El Pui section segment C2 precedes those assigned to anoxic event 1a (OAE 1a) (C3 – C6) [3] and is marked by a negative inflection of ~ 0.5 to 1.5 ‰, also present within that interval elsewhere (Tunisia, [4]; Cismon APTICORE, Italy, Santa Rosa Canyon, Mexico, [5]. In addition to previous proxies [2 -3], n-alkanes were investigated to further characterize the depositional environment associated with this negative spike. The results are unvarying throughout, with dominance of calcareous nannofossils associated with a TIC average ~ 72.3 CaCO₃ %, abundance of roveacrinid fragments, scarce minute planktonic foraminifera. The n-alkanes before the negative inflection shows that OM is mainly derived from protists and microbial sources, short-chain length homologues (≤ nC₁₉) dominate the highest values. During the negative spike, the n-alkanes are predominantly short-chain length homologues with very little to no contribution from shallow marine (nC₂₀ - nC₂₅), land-derived aquatic vegetation, and absolutely none from higher plants (>nC₂₅). The terrestrial/aquatic ratios TARs (nC₂₇+nC₂₉+nC₃₁)/(nC₁₅+nC₁₇+nC₁₉) averaging 0.36 before and 0 during the negative inflection imply that local factors did not enhance the 𝛿¹³C_org negative inflection during C2. Thus global factors such as clathrate destabilization [e.g. 6 - 7] and large igneous provinces (LIPs) may be invoked to cause similar negative shifts in the 𝛿¹³C_org curve [1 - 8].

[1] Menegatti et al (1998) Paleoceano 13 530–545; [2] Sanchez-Hernandez et al 2014 Cret Res 51, 1 – 21; [3] Sanchez-Hernandez & Maurrasse (2015) Palaeo 3 ; (2014) Chem Geo. 372 12–31; [4] Heldt et al (2008) Palaeo 3 261, 246–260; [5] Li et al (2008) EPSL 271, 88 – 100; [6] Renard et al (2005) Carnets Géol, Art (CG2005_A04); [7] Dickens et al (2001) Org Geoch 32, 1179 -1193; [8] Tejada et al (2009) Geol 37, 855–858.