The Apectodinium acme and terrestrial discharge during the Paleocene-Eocene thermal maximum: new palynological, geochemical and calcareous nanoplankton observations at Tawanui, New Zealand

Abstract

Manifestations of profound perturbations in biogeochemical systems during the Paleocene–Eocene thermal maximum (PETM) include a prominent global negative δ13C and a pronounced increase in the relative abundance of dinoflagellate cysts (dinocysts) assigned to the genus Apectodinium. While motile representatives of Apectodinium were most likely thermophilic and heterotrophic, the underlying causes of this dinoflagellate response are not well understood. Here we provide new insight by examining the palynology, chemistry and calcareous nannoplankton across the PETM in a continental slope section at Tawanui, New Zealand. Across the PETM, marked changes in the relative abundance of Apectodinium vary antithetically with significant changes in the δ13C of carbonate and organic matter. In general, the high relative abundance of Apectodinium relates to enhanced concentrations of dinocysts, signifying a ‘bloom’ of Apectodinium in surface waters during the PETM. Changes in Apectodinium and δ13C records correspond to variations in many other parameters, including a smaller negative shift in bulk carbonate δ13C than expected, increased terrestrial palynomorphs, elevated TOC and C/N ratios, lower carbonate contents, higher SiO2 and Al2O3 contents, and lower Si/Al ratios. All of these variations can be explained by an increase in delivery of terrigenous material to the continental margin. A peak in the relative abundance of Glaphyrocysta dinocysts at the onset of the PETM may indicate greater down slope transport of neritic material. Changes in calcareous nannoplankton abundances suggest increased nutrient availability in surface waters during the PETM. The combined results show that Apectodinium-dominated assemblages, global perturbations in carbon isotopes and enhanced terrigenous delivery closely correspond in time at Tawanui. A sudden and massive carbon injection to the ocean–atmosphere system may have enhanced weathering and increased terrigenous inputs to continental margins during the PETM. We further suggest that these inputs caused the Apectodinium acme by elevating primary productivity in marginal seas.