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

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doi: 10.1002/2015PA002777
Author(s): Robinson, Rebecca S.; Moore, Theodore C.; Erhardt, Andrea M.; Scher, Howie D.
Author Affiliation(s): Primary:
University of Rhode Island, Graduate School of Oceanography, Kingston, RI, United States
University of Michigan at Ann Arbor, United States
University of Cambridge, United Kingdom
University of South Carolina, United States
Volume Title: Paleoceanography
Source: Paleoceanography, 30(7), p.912-922. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0883-8305 CODEN: POCGEP
Note: In English. 68 refs.; illus., incl. 3 tables, sketch maps
Summary: 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 δ15N) from the equatorial Pacific indicates that radiolarian-N records δ15N variability with fidelity but that a significant offset exists between bulk sedimentary and diatom δ15N values and those measured from radiolarians (∼7.1 ± 1.1 ppm). A downcore profile of radiolarian δ15N values is compared to siliceous microfossil assemblage changes across the Eocene-Oligocene boundary. Average of radiolarian-bound δ15N values is 0.5 ± 2.0 ppm, 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 δ15N 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 δ15N values cooccur with high radiolarian species turnover at ∼35.5 and 34 Ma, 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. Abstract Copyright (2015), . American Geophysical Union. All Rights Reserved.
Year of Publication: 2015
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Algae; Cenozoic; Chemical composition; Diatoms; East Pacific; Eocene; Equatorial Pacific; Expedition 320; Expeditions 320/321; IODP Site U1333; Integrated Ocean Drilling Program; Invertebrata; Isotope ratios; Isotopes; Living taxa; Lower Oligocene; Marine environment; Microfossils; Modern analogs; N-15/N-14; Nitrogen; North Pacific; Northeast Pacific; Ocean circulation; Oligocene; Pacific Ocean; Paleo-oceanography; Paleoecology; Paleoenvironment; Paleogene; Plantae; Productivity; Protista; Radiolaria; Stable isotopes; Tertiary; Upper Eocene
Coordinates: N103100 N103100 W1382510 W1382510
Record ID: 2015099144
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from John Wiley & Sons, Chichester, United Kingdom