| Abstract: | Oceanic anoxic events (OEAs) have become a major source of discussion over the past decade mainly with the success of deep-time drilling from the ODP. During the drilling of Blake Nose (ODP Leg 171B), mid-Cretaceous sediments were recovered, and OAE 1b (Lower Albian) was identified with exceptional foraminifera preservation. Subsequent studies on this time interval revealed that during the black shale interval (~143 mbsf), oxygen-isotope ratios between benthic and planktic species diverged leading Erbacher et al. (2000, Nature 409:325-27) to attribute this to increased water column thermal stratification. The effect on carbon-isotope ratios was minor. Erbacher et al. thus concluded that the formation of the OAE 1b black shale could have involved similar processes as those leading to the formation of Quaternary sapropels.
Upon a closer inspection of colour photographs of Core 1049C-12X, we noted that ~2 m prior to the black shale interval studied by Erbacher et al. there was a major change in sediment colour cycles from alternating red and white to white and then grey. To determine whether this colour change had any paleoceanographic significance we requested samples every 10cm through all 6 cores (~8.5m). Although most foraminiferal studies use 10cc, we requested 1cc in order to obtain as high a resolution as we could while still obtaining enough foraminifera for isotopic analysis. All samples were processed using normal procedures and split into three fractions (<63µm, 63-150µm, >150µm).
Stable-isotope analyses were first conducted on the bulk 63-150µm-fraction, predominantly consisting of juvenile and/or small planktonic foraminifera, and this revealed a large (2-2.5 ), rapid negative shift in  coinciding exactly with the interval of colour change from white to grey (~145.3m). Subsequently, the interval was re-sampled every 1 cm and monospecific assemblages of planktonic (Hedbergella trocoidea) and benthic (Gyroidinoides infracretaceous) foraminifera were picked and analysed. All records have shown a similar isotopic excursion across the white-grey interval. Given the published sedimentation rate for this core (Ogg & Röhl 1999, Eos 80:F491-92), the excursion would have occurred during 5kyr or less.
Although the of H. trocoidea and G. infracretaceous changed simultaneously, the magnitude of the initial shift was different: 1.5 and 1.9 respectively. The bulk 63-150µm-fraction shifted slightly later (by one sample) in the record, and with a greater magnitude (>2). A similar pattern was discovered in the  record; the bulk 63-150µm-fraction shifted later and by ~1, H. trocoidea shifted by only 0.4 and G. infracretaceous by 0.7. What is most striking about the record is that H. trocoidea returned to pre-excursion values whereas G. infracretaceous remained at post-excursion values: the difference between planktic and benthic species prior to the excursion was 1, after the excursion it was only ~0.16. These isotopic patterns would suggest that diagenesis and bioturbation had only a minor effect on the shape of the isotopic signals and that a major perturbation of the global carbon cycle must have occurred prior to the sensu stricto black shale of OAE1b in the Blake Nose ODP core. At present, a massive dissociation of continental margin methane gas hydrates is the only explanation for the magnitude and abruptness of the carbon isotopic shift, although such an explanation would require further corroborating evidence. |