Late Quaternary Quantitative Planktonic Foraminiferal Biostratigraphy in Turbidite Sequences of the Central California Continental Margin

Abstract

We defined a latest Pleistocene and Holocene biostratigraphy in seven cores from Monterey and Delgada Fans. We tested every muddy sample for its turbiditic or hemipelagic character using grain-size analysis of the silt fraction. Previous results indicated that muds were turbiditic if mean silt size was coarser than 6.4 phi and if modes occurred in medium and coarse silt sizes (Brunner and Ledbetter 1987). We rejected all identified turbiditic samples from the biostratigraphic data set. Q-mode cluster analysis clustered samples into four groups distinguished by four assemblages of planktonic foraminifers: 1) a subarctic assemblage dominated by Globigerinita uvula (Egger), 2) a transitional assemblage dominated by Globigerina quinqueloba Natland, 3) an upwelling assemblage dominated by Globigerina bulloides d'Orbigny, and 4) a dissolution assemblage dominated by Neogloboquadrina pachyderma (Ehrenberg). We defined three assemblage zones based on the above foraminiferal assemblages, the coiling ratio between sinistral and dextral morphs of Nq. pachyderma, and frequency of subtropical taxa. After exclusion of samples with mud turbidite components, the sediment sequences exhibit a pattern of assemblage succession that is coherent between the two fans. The zonation draws attention to the similarity between middle Holocene and full glacial Pleistocene faunas, and highlights the peculiar nature of deglacial assemblages relative to full glacial and interglacial assemblages. We tentatively correlate a brief cooling within the deglaciation to the Younger Dryas event of the North Atlantic basin and northern Europe.