Compositional and isotopic characteristics of organic matter for the early Aptian Oceanic Anoxic Event at Shatsky Rise, ODP Leg 198

Abstract

The occurrence of intervals of enhanced sequestration of organic matter (OM) during the Mid-Cretaceous—collectively termed ‘Oceanic Anoxic Events’ (OAE)—reflect abrupt changes in global carbon cycling. These episodes raise questions about the causes for such perturbations, and their linkages to fluctuations in climate, marine biota, and ocean geochemistry preserved in the sedimentary record. Detailed assessment of the early Aptian event (OAE1a) in the Pacific has been precluded by the limited recovery of this stratigraphic interval, in contrast to the many records from the Tethyan realm. The recovery of pelagic sequences of OM-rich sediments at Site 1207 on Shatsky Rise (ODP Leg 198) corresponding to OAE1a has enabled recognition of high-resolution temporal variations in the elemental and isotopic compositions of this event in the Pacific. However, the continuous 45 cm OM-rich interval recovered is likely only the initiation of OAE1a rather than a complete section based on natural gamma and uranium logs, which suggest that the full thickness of this interval is ∼ 1.2 m. Exceedingly high organic carbon (C org > 20%) and total sulfur (TS > 5%) contents—among the highest ever recorded for pelagic Cretaceous sequences—attest to the enhanced sequestration of OM during OAE1a. δ 13C org values are lowest (− 27.7‰) at the base of the recovered OM-rich interval, followed by a 2.4‰ positive shift. This isotopic profile resembles part of the δ 13C carb record at Resolution Guyot and is comparable with segments C3 and C4 of the Cismon δ 13C org record. Low δ 15N values (< − 2‰) provide evidence for nitrogen fixation, perhaps coupled with elevated cyanobacterial populations, that suggest that the marine nitrogen cycle in the Cretaceous was different from the modern. Overall, the temporal fluctuations in elemental and isotopic data through OAE1a at Shatsky Rise describe differences in production and preservation of OM that likely reflect variations in ocean conditions associated with both local and global environmental changes.