Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: pCO2 and paleo productivity

[摘要] A complete, well-preserved record of the Cenomanian/Turonian (C/T) OceanicAnoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southernNorth Atlantic Ocean (ODP site 1260). Across this interval, we determinedchanges in the stable carbon isotopic composition of sulfur-bound phytane (δ¹³C_phytane), a biomarker for photosynthetic algae. Theδ¹³C_phytane record shows a positive excursion at the onsetof the OAE-2 interval, with an unusually large amplitude (~7‰)compared to existing C/T proto-North Atlantic δ¹³C_phytanerecords (3–6‰). Overall, the amplitude of the excursion of δ¹³C_phytane decreases with latitude. Using reconstructed seasurface temperature (SST) gradients for the proto-North Atlantic, weinvestigated environmental factors influencing the latitudinal δ¹³C_phytane gradient. The observed gradient is best explained byhigh productivity at DSDP Site 367 and Tarfaya basin before OAE-2, whichchanged in overall high productivity throughout the proto-North Atlanticduring OAE-2. During OAE-2, productivity at site 1260 and 603B was thus morecomparable to the mid-latitude sites. Using these constraints as well as theSST and δ¹³C_phytane-records from Site 1260, wesubsequently reconstructed pCO₂ levels across the OAE-2 interval.Accordingly, pCO₂ decreased from ca. 1750 to 900 ppm during OAE-2,consistent with enhanced organic matter burial resulting in loweringpCO₂. Whereas the onset of OAE-2 coincided with increased pCO₂, inline with a volcanic trigger for this event, the observed cooling withinOAE-2 probably resulted from CO₂ sequestration in black shalesoutcompeting CO₂ input into the atmosphere. Together these results showthat the ice-free Cretaceous world was sensitive to changes in pCO₂related to perturbations of the global carbon cycle.

[学科分类] 地球化学与岩石