Temporally variable diagenetic overgrowth on deep-sea nannofossil carbonates across Palaeogene hyperthermals and implications for isotopic analyses

Calcareous nannofossil assemblages of deep-sea sediments were subjected to intensive diagenetic alterations during early Palaeogene hyperthermal events. These alterations may have significantly modified bulk isotopic and trace metal signals and nannofossil preservation, thus biassing palaeoceanographic and palaeoecological interpretation. We present a detailed characterisation of the temporal variation in degree of diagenetic overgrowth on nannoliths during the PETM and Early Eocene Thermal Maximum (ETM2) using scanning electron microscopy (SEM), and explore in detail the consequences of these changes in overgrowth for interpretation of nannofossil assemblages and geochemical records covering the ETM2 at ODP Site 1265 where the event is well recognised. Results show that the nannofossil genera Discoaster and Zygrhablithus are particularly receptive to significant amounts of diagenetic calcite overgrowth, which was confirmed by Sr/Ca variations within single discoasters. Overgrowths show a strong correlation with changes in sediment carbonate content across the hyperthermals, with notably less overgrowth in low carbonate intervals. This secondary calcite affects stable isotope, notably oxygen isotopes, and assemblage composition modifying the dissolution susceptibility of taxa. In particular, the size fraction with a high contribution of overgrown discoasters has heavier

Earth system feedback statistically extracted from the Indian Ocean deep-sea sediments recording Eocene hyperthermals

Abstract Multiple transient global warming events occurred during the early Palaeogene. Although these events, called hyperthermals, have been reported from around the globe, geologic records for the Indian Ocean are limited. In addition, the recovery processes from relatively modest hyperthermals are less constrained than those from the severest and well-studied hothouse called the Palaeocene–Eocene Thermal Maximum. In this study, we constructed a new and high-resolution geochemical dataset of deep-sea sediments clearly recording multiple Eocene hyperthermals in the Indian Ocean. We then statistically analysed the high-dimensional data matrix and extracted independent components corresponding to the biogeochemical responses to the hyperthermals. The productivity feedback commonly controls and efficiently sequesters the excess carbon in the recovery phases of the hyperthermals via an enhanced biological pump, regardless of the magnitude of the events. Meanwhile, this negative feedback is independent of nannoplankton assemblage changes generally recognised in relatively large environmental perturbations