Evidence for changes in subsurface circulation in the late Eocene equatorial Pacific from radiolarian-bound nitrogen isotope values

摘要

Microfossil-bound organic matter represents an important archive of surface ocean environmental information. Sedimentary nitrogen (N) isotope reconstructions of surface nitrate consumption and nitrogen source changes are made using fossil diatom (autotrophs) and planktic foraminiferal (heterotrophs)-bound organic matter with success. However, because diatoms and planktic foraminifera are poorly preserved and sedimentary organic matter content is near zero during the late Eocene, our ability to examine nutrient dynamics across this important climate transition is limited. Here we present new data exploring the use of N isotope records from radiolarian tests. A comparison of surface ocean nitrate and core top bulk and radiolarian N isotope values (as delta N-15) from the equatorial Pacific indicates that radiolarian-N records delta N-15 variability with fidelity but that a significant offset exists between bulk sedimentary and diatom delta N-15 values and those measured from radiolarians (similar to 7.1 +/- 1.1 parts per thousand). A downcore profile of radiolarian delta N-15 values is compared to siliceous microfossil assemblage changes across the Eocene-Oligocene boundary. Average of radiolarian-bound delta N-15 values is 0.5 +/- 2.0 parts per thousand, which, when corrected using the offset derived from the modern surface samples, suggests that the mean nitrogen isotopic composition of the early Cenozoic eastern Pacific was not significantly different from today. The overall trend, of decreasing delta N-15 values with decreasing export productivity, is consistent with either a regional decline in pelagic denitrification or a large-scale change in nutrient sources to the eastern equatorial Pacific (EEP), both linked to the cooling climate and changing intermediate water circulation. Decreasing/low delta N-15 values cooccur with high radiolarian species turnover at similar to 35.5 and 34Ma, suggestive of a significant ecological change in the EEP, consistent with cooling and water mass distribution changes. The preliminary results suggest that radiolarian-bound organic nitrogen represents another promising archive and underscores the fact that the different microfossil fractions must be separated to ensure robust results.