High resolution palaeoceanography and palaeoclimatology from mid and high latitude Late Cretaceous laminated sediments
Late Cretaceous laminated diatomaceous sediments from the Marca Shale (California) and CESAR 6 core (Arctic Ocean) have been analysed using scanning electron microscopy techniques and found to contain marine varves. The varve interpretation is based on the pervasive occurrence of laminae types that can be related to different seasons and placed into robust annual flux cycles. In the Marca Shale this consists of intermittent near-monospecific laminae of Azpeitiopsis morenoensis deposited in the late summer, succeeded by a ‘fall dump’ mixed assemblage laminae, dominated by fragmented Hemiaulus polymorphus, Stephanopyxis spp. and Stellarima spp. Intermittent near-monospecific Chaetoceros-type resting spore laminae, deposited in spring follow and are in turn overlain by terrigenous laminae, reflecting enhanced fluvial runoff during the summer. A mixed floral diatomaceous and terrigenous laminae couplet constitute the annual flux cycle in the majority of varves. Summer terrigenous flux may indicate that the ~43°N palaeolatitude estimate is incorrect, and deposition actually occurred at lower latitudes or alternatively, that the Marca basin was influenced by intense monsoonal summer storms. The CESAR 6 annual flux cycle consists of alternating diatom resting spore and vegetative cell laminae (often dominated by Hemiaulus), interpreted to represent spring bloom flux and ‘fall dump’/intermittent summer blooms flux, respectively. Near-monospecific Rhizosolenia laminae frequently occur above vegetative laminae, relating to subsequent ‘fall dump’ flux. Common silicoflagellate/setae-rich laminae, often containing diatom hash, occur beneath resting spore laminae, interpreted as the early spring flux of silicoflagellate blooms and grazed diatoms. Seasonally occurring detrital ‘blebs’ and rare detrital laminae are also observed, which may represent ice rafted debris, consistent with Campanian model simulations. Diatom taxa adapted to exploit stratified conditions, sedimented over the summer or in a ‘fall dump’, are important components of both sequences. Spring bloom flux was important in the CESAR 6, but was only a subsidiary component in the Marca Shale. The abundance of Hemiaulus spp. at both localities may be evidence for diatom blooms powered by N2-fixing cyanobacteria. Diatom biostratigraphy indicates a late Late Campanian age for the CESAR 6, although an earliest Maastrichtian age cannot be discounted. Time-series analysis of laminae thickness data from both sequences and bioturbation index data from the Marca Shale, reveal both sites contain strong quasi-biennial signals (mean periodicities of 27.4 and 30.4 months, respectively), inferred to relate to the Quasi-Biennial Oscillation (QBO). The CESAR 6 contains strong sub-decadal/quasi-decadal peaks interpreted to relate to Northern Hemisphere Annular Mode (NAM) variability. Together with the QBO signals, this suggests NAM variability was robust during the Late Cretaceous. Similar frequency peaks occur in the Marca Shale which, supported by several sedimentological lines of evidence, indicates the presence of an El Niño Southern Oscillation (ENSO)-like oscillation. Theory dictates that a weakening of the ‘Bjerknes’ feedback loop in the equatorial Pacific will lead to a reduction or shutdown of ENSO variability, resulting in a permanent El Niño climate. The Late Cretaceous was witness to a severe weakening of the ‘Bjerknes’ feedback, yet the results of this study demonstrate that there was robust ENSO variability during this greenhouse period, adding to the evidence against permanent ‘El Niño’ climate states. A 10.66 year harmonic peak in the CESAR 6 is taken as evidence for modulation of the NAM by solar variability. Peaks at 10.0, 10.3 and 10.8 years in the Marca Shale are interpreted to relate to the solar modulation of strong ENSO events. Both sites also contain multi-decadal peaks, the frequency of which may also have been modulated by solar variability. Quasi-bidecadal peaks are inferred to relate to the NAM or an oscillation analogous to the Pacific Decadal Oscillation (PDO), possibly influenced by the Hale cycle. A 44 year peak in the Marca Shale is also inferred to relate to the PDO. The CESAR 6 contains a harmonic peak at 78.74 years, interpreted to relate to modulation of the NAM over the Gleissberg solar cycle or alternatively, to modulation of the PDO.