Dinoflagellate cysts across the Cretaceous–Paleogene (K/Pg) boundary in the North Pacific; biostratigraphy, diversity, and paleoenvironmental reconstructions




McLachlan, Sandy Melvin Stuart

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The central objective of this study is to understand phytoplankton community response following the global mass extinction event at the Cretaceous–Paleogene (K/Pg) boundary. The objective is approached through analysis of dinoflagellate cyst assemblages across the boundary interval in the North Pacific. Dinoflagellate cysts are powerful tools for deep time paleoenvironmental reconstructions and this group of microfossils has been vastly underutilized in this region of the world. On this premise, comprehensive marine palynological surveys were undertaken for the Oyster Bay Formation of eastern Vancouver Island, British Columbia, Canada and Shatsky Rise in the northwest Pacific. The Oyster Bay Formation work resulted in the discovery of the first K/Pg boundary succession west of the Rocky Mountains based on biostratigraphic controls and refined taxonomy for the genus Cannosphaeropsis found within these strata. Three new taxa are described: Cannosphaeropsis franciscana subsp. vacuoseptata subsp. nov., Cannosphaeropsis franciscana subsp. vesiculata subsp. nov. and Phelodinium fensomei sp. nov. The cyst assemblages reveal endemic associations and signals of transition between offshore coastal to estuarine settings in keeping with global eustatic trends. Oyster Bay Formation results and interpretations are compared to analyses of core samples from Deep Sea Drilling Project Site 577 at Shatsky Rise. Contrast is seen between a diverse, highly productive coastal to estuarine environment in the Oyster Bay Formation as represented by organic-walled taxa and an oligotrophic bathyal environment at Shatsky Rise during the same interval as represented by a small number of calcareous taxa. These two regions form the basis for comparison between differing assemblages in order to ascertain the extent to which phytoplankton communities were affected by changes in sea-surface and water mass conditions in conjunction with the K/Pg event. The findings reveal measurable impacts of climate and paleoenvironmental change reflected by shifts in assemblage composition and cyst morphology. A lack of extinction among many forms is consistent with studies from around the globe as presented in an exhaustive review of the literature. The dinoflagellates were marginally impacted with the most specialized taxa presenting a record of sea-surface temperature fluctuation, nutrient supply and opportunistic niche exploitation.



paleontology, micropaleontology, palynology, fossils, microfossils, microscopy, biostratigraphy, dinoflagellates, dinoflagellate cysts, phytoplankton, biostratigraphy, paleoecology, Cretaceous, Paleogene, K/Pg boundary, extinction, climate change, North Pacific