Ocean nitrogen isotopic change in the early Eocene

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Author(s): Kast, Emma; Stolper, Daniel A.; Higgins, John A.; Ren, Haojia Abby; Wang, Xingchen Tony; Sigman, Daniel Mikhail
Author Affiliation(s): Primary:
Princeton University, Princeton, NJ, United States
California Institute of Technology, United States
Academia Sinica, Taiwan
Volume Title: AGU 2017 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2017; American Geophysical Union 2017 fall meeting, New Orleans, LA, Dec. 11-15, 2017. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The long term variability of the marine nitrogen (N) cycle is an open question. The Cenozoic provides a well-studied framework for investigating the marine N cycle over long time scales and across large climate transitions. However, only sparse bulk Cenozoic sediment δ15N data exist, the utility of which for reconstructing environmental conditions is unclear. We present a record of foraminifera-bound organic matter δ15N from the Paleocene to late Eocene. At three distant sites, foraminifera-bound δ15N decreases dramatically between 56 Ma and 50 Ma: from 14 ppm to 2 ppm in the northwest Pacific (ODP site 1209), from 12 ppm to 4 ppm in the southeast Atlantic (ODP site 1263), and from 9 ppm to 4 ppm in the northwest Atlantic (IODP site U1409). This foraminifera-bound δ15N change is on par, if not greater, than the largest changes that have been observed in bulk sediment δ15N over the last 600 million years. The shared change among the sites implies a change in mean δ15N of oceanic fixed N, which is thought to be sensitive to the ratio of water column to sedimentary denitrification, with a higher δ15N reflecting a greater proportion of denitrification occurring in the water column. Today, water column denitrification occurs in the shallow subsurface, in regions where these waters are suboxic. Thus, the δ15N decrease may reflect a slowing of water column denitrification, which can be generated by a decline in shallow subsurface suboxia. A key factor in the extent of shallow subsurface suboxia is the amount of "preformed oxygen," the initial concentration of dissolved O2 in the water that flows from the surface into the shallow subsurface: a decline in suboxia would require a rise in preformed oxygen from 56 to 50 Ma. The δ15N decline occurs before the onset of cooling in the Eocene, eliminating global temperature change as the driver of increased preformed oxygen. Instead we favor explanations that involve tectonically driven changes in continental configuration and shallow and mid-depth ocean bathymetry. Indeed, the δ15N decline appears coincident with the initiation of bathymetric effects from the collision of India with Asia. This category of explanation is consistent with the overlap of the δ15N decline with the previously identified increase in marine barite δ34S at 51 Ma.
Year of Publication: 2017
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Atlantic Ocean; Cenozoic; Eocene; Expedition 342; Geochemical cycle; IODP Site U1409; Integrated Ocean Drilling Program; Isotope ratios; Isotopes; Leg 198; Leg 208; Lower Eocene; N-15/N-14; Nitrogen; Nitrogen cycle; North Atlantic; North Pacific; Northwest Pacific; ODP Site 1209; ODP Site 1263; Ocean Drilling Program; Pacific Ocean; Paleogene; Shatsky Rise; South Atlantic; Stable isotopes; Tertiary; Walvis Ridge; West Pacific
Coordinates: N323900 N324000 E1583100 E1583000
S283200 S283200 E0024700 E0024700
N411744 N411745 W0491359 W0491400
Record ID: 2018081959
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