Abstract:
Oceanic Anoxic Events (OAEs) in the Early Aptian (OAE 1a; approximately 120 Ma) and proximal to the Aptian/Albian boundary (OAE1b; approximately 112 Ma) represent two brief time intervals of large, abrupt, and widespread climate changes in the Mid-Cretaceous greenhouse world. Climate changes during these OAEs are pertinent to understanding modern anthropogenic warming climate. However, the lack of a high-resolution timescale for these OAEs has impeded our understanding of their brevity and causal mechanisms. As part of an ongoing collaborative research aimed at refining the chronology of these two OAE intervals by integrating biostratigraphy, chemostratigraphy, and cyclostratigraphy, we carried out a high-resolution rock magnetic study of the highly expanded OAE1a and 1b intervals in a hemipelagic succession, Deep Sea Drilling Project Site 398 located at Vigo Seamount in the North Atlantic. The purpose of this study is to examine whether changes in magnetic properties of the hemipelagic sequence record orbital variations and to establish an orbital timescale for the OAE1a and 1b intervals. The OAE intervals are composed of siltstones, sandstones, and dark-gray to gray mudstones. Unoriented samples were collected, on average, every approximately 25-30 cm for magnetic measurements including low-field magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM). Carbonate contents of selected samples were measured. The lack of strong correlation between carbonate contents and MS suggests that MS is controlled by the amount of terrigenous influx and changes in lithological composition of the terrigenous component. ARM data were used as a measure of variations in terrigenous influx. Spectral analyses of ARM depth series reveal dominant cycles with wavelength ratios resembling those of modern orbital periodicities, implying that orbital variations may have modulated depositional processes. An obliquity-based orbital timescale is obtained for the OAE1a and 1b intervals. The sedimentation rates derived from the orbital timescale are comparable with those estimated independently from biostratigraphy, adding confidence to our newly resolved orbital timescale.