[M-IS18] Paleoclimatology and paleoceanography

[MIS18-P19] High-resolution osmium and carbon isotope stratigraphy at the Deep Sea Drilling Project Site 511, Falkland Plateau at the southern Atlantic Ocean

*Hironao Matsumoto¹, Junichiro Kuroda¹, Kotaro Shirai¹ (1.Atmosphere and Ocean Research Institute, the University of Tokyo)

Keywords:osmium, Aptian–Albian boundary

The early Cretaceous Aptian–Albian boundary is marked by several environmental perturbations, such as oceanic anoxic event (OAE) 1b, global warming, and the severe species turnover of planktonic foraminifera. Paleoenvironmental studies of the Aptian–Albian boundary have been carried out mainly using the Tethyan sedimentary sequence. However, due to the lack of a well-dated sedimentary sequence, few studies have focused on the environmental perturbations at the southern high latitude. Deep Sea Drilling Project (DSDP) Site 511 is a borehole core that was drilled on the Falkland Plateau, southern Atlantic Ocean, and is one of the few sedimentary sequences that continuously records the Aptian–Albian boundary. Although there is no sign of OAE1b, planktonic foraminiferal studies roughly determined the Aptian–Albian boundary. However, because of the lack of chemo-stratigraphy, there have been slight uncertainties in the previous stratigraphic correlations. In this research, we reconstructed high-resolution pale-marine osmium and carbon isotopic stratigraphy and tried to determine the exact position of the Aptian–Albian boundary. Furthermore, we attempted to reveal the cause of the environmental perturbations across the Aptian–Albian boundary. As a result, we found a sharp negative carbon isotopic shift at Core 55, section 4, which corresponds to the first occurrence of Microhedbergella renilaevis. Since this feature is consistent with the Tethyan Aptian–Albian boundary interval, we propose to place the Aptian–Albian boundary at the interval of carbon isotopic excursion. Also, the osmium isotopic ratio showed a clear radiogenic shift after the Aptian–Albian boundary, which is consistent with the osmium isotopic variations revealed in the Tethys and Atlantic regions. Since previous studies have proposed global warming at this interval at the low-latitude Atlantic regions, this radiogenic shift can be interpreted as the increase in continental weathering caused by global warming. Besides, since the Aptian–Albian boundary interval in this core contains the large marcasite crystal, the oceanic condition at this interval may have been oxygen-depleted and low-pH condition. Considering that the massive volcanic events forming the Kerguelen Plateau occurred at the high-latitude in the southern hemisphere, the intensive input of volcanic gas may have been the cause of these environmental perturbations.