Characterization of the negative carbon isotope shift in segment C2,its global implications as a harbinger of OAEla

Lower Cretaceous C-isotope records show intermittent negative/positive spikes, and consistent patterns of coevalchemostratigraphic curves thus document shifts that signal simultaneous responses of temporal changes in the global carbon reservoir. The standard pattern registered by the δ13Corg and δ13Ccarb in Lower Aptian sediments includes distinct isotope segments C1 to C8 （Menegatti et al., 1998）. In the E1 Pui section, Organy Basin, Spain, C-isotope segment C2 is the longest interval preceding segments C3-C6 associated with oceanic anoxic event la （OAE la）, and reveals a distinct negative shift of-1.8‰ to -2.23‰ defining the C-isotope pattern within that interval. Total inorganic carbon （TIC）, total organic carbon （TOC）, δ13Corg, microfacies, n-alkanes show no difference before, during, or after the negative inflection. The biomarkers indicate that organic matter （OM） mainly originates from algal/microbial sources because short-chain length homologues （〈 nCtg） dominate, nC2o through nC25 indicate some contribution from aquatic vegetation, but little from higher plants （〉nC20, as also suggested by the terrestrial/aquatic ratio ofn-alkanes or （TAR） = [（nf27＋nf29＋nC31）/（nC15＋nCt7＋rlCt9）] （averages 0.085）. We suggest that conjoint pulses of contemporaneous LIPs （Ontong Java） and massive explosive volcanism in northeast Asia, the Songliao Basin （SB-V）, best conform to plausible causes of the negative intra-C2 carbon isotopic excursion （CIE） at that time. Because of its apparent common occurrence the intra-C2 inflection could be a useful marker harbinger to the more pronounced CIE C3, the hallmark of OAE 1 a.

Lower Cretaceous C-isotope records show intermittent negative/positive spikes,and consistent patterns of coeval chemostratigraphic curves thus document shifts that signal simultaneous responses of temporal changes in the global carbon reservoir.The standard pattern registered by theδ^(13)C_(org)andδ^(13)C_(carb)in Lower Aptian sediments includes distinct isotope segments C1 to C8(Menegatti et al.,1998).In the El Pui section,Organya Basin,Spain,C-isotope segment C2 is the longest interval preceding segments C3-C6 associated with oceanic anoxic event la(OAE la),and reveals a distinct negative shift of^1.8‰to^2.23‰defining the C-isotope pattern within that interval.Total inorganic carbon(TIC),total organic carbon(TOC),δ^(13)C_(org),microfacies,n-alkanes show no difference before,during,or after the negative inflection.The biomarkers indicate that organic matter(OM)mainly originates from algal/microbial sources because short-chain length homologues(≤nC_(19))dominate.nC_(20)through nC_(25)indicate some contribution from aquatic vegetation,but little from higher plants(>nC_(25)),as also suggested by the terrestrial/aquatic ratio of n-alkanes or(TAR)=[(nC_(27)+nC_(29)+nC_(31))/(nC_(15)+nC_(17)+nC_(19))](averages 0.085).We suggest that conjoint pulses of contemporaneous LIPs(Ontong Java)and massive explosive volcanism in northeast Asia,the Songliao Basin(SB-V),best conform to plausible causes of the negative intra-C2 carbon isotopic excursion(CIE)at that time.Because of its apparent common occurrence the intra-C2 inflection could be a useful marker harbinger to the more pronounced CIE C3,the hallmark of OAE1a.