Variability of extreme Cretaceous-Paleogene climates - Evidence from Blake Nose (ODP Leg 171B)

The importance of drilling Cretaceous and Paleogene marine deposits is discussed for understanding climate variability during extreme warm climates. We present evidence from ODP Leg 171 drilling at Blake Nose for climate variability during the Late Paleocene Thermal Maximum (LPTM) and late Albian-early Cenomanian time periods. Borehole logs and continuous core measurements, such as magnetic susceptibility and color variations, reveal that most of the sediment variability is forced by the Earth's orbital variations Sediment variability before the LPTM is dominated by the precessional cycle, which possibly transgressed into the obliquity cycle prior to the carbon anomaly, or alternatively sedimentation rates increased, The climate was changing before the LPTM which suggests that some unknown climatic threshold was surpassed, ultimately paving the way to the climatic conditions of the LPTM. The downhole gamma ray log shows a large excursion coinciding with the carbon isotope anomaly. The gamma ray excursion at the LPTM is not unique and may be part of a longer climate cycle of about 2 m.y. The sedimentary record of the late Albian-early Cenomanian period is characterized by cycles of black shales alternating with limestones (time equivalent of Oceanic Anoxic Event 1d). The resistivity log reveals the nature of cyclicity variations during this time period. We assume that precessional cycling is evident in the lower part of the record. Bundling of the high frequency cycles into an unknown low frequency cycle (100 k.y.?) occurred in the upper part of the sequence. We conclude that climate during warm periods was not stable and longer term orbital cycles in the order of two million years may have triggered the most extreme intervals such as the LPTM and the termination period of OAE 1d: although they could also reflect long term sedimentary cycles unrelated to orbital dynamics. The abrupt termination of OAE 1d suggests intensification of intermediate/deep water ventilation in the Cenomanian, possibly generated by the opening of an unknown gateway.