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Abstract Bulk sediment δ15N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ15N, then a deglacial δ15N maximum, followed by a gradual decline to a late Holocene δ15N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ15N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ15N) measurements for the two speciesNeogloboquadrina dutertreiandNeogloboquadrina incomptaover the last 35 ka in two sediment cores from the eastern equatorial Pacific (EEP), both of which have the typical LGM‐to‐Holocene increase in bulk sediment δ15N. FB‐δ15N contrasts with bulk sediment δ15N by not indicating a lower δ15N during the LGM. Instead, the FB‐δ15N records are dominated by a deglacial δ15N maximum, with comparable LGM and Holocene values. The lower LGM δ15N of the bulk sediment records may be an artifact, possibly related to greater exogenous N inputs and/or weaker sedimentary diagenesis during the LGM. The new data raise the possibility that the previously inferred glacial reduction in ETP water column denitrification was incorrect. A review of reconstructed ice age conditions and geochemical box model output provides mechanistic support for this possibility. However, equatorial ocean circulation and nitrate‐rich surface water overlying both core sites allow for other possible interpretations, calling for replication at non‐equatorial ETP sites.
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Ongoing Increase in Eastern Tropical North Pacific Denitrification as Interpreted Through the Santa Barbara Basin Sedimentary δ 15 N Record
Abstract Decades of observations show that the world's oceans have been losing oxygen, with far‐reaching consequences for ecosystems and biogeochemical cycling. To reconstruct oxygenation beyond the limited scope of instrumental records, proxy records are needed, such as sedimentary δ15N. We combine two δ15N records from the Santa Barbara Basin (SBB), a 24‐year‐long, biweekly sediment trap time series, and a 114‐year, high‐resolution sediment core together spanning the years 1892–2017. These records allow for the examination of δ15N variability on seasonal to centennial timescales. Seasonal variability in SBB δ15N is consistent in timing with the poleward advection of a high δ15N signal from the Eastern Tropical North Pacific in the summer and fall. Strong El Niño events result in variable δ15N signatures, reflective of local rainfall, and neither the Pacific Decadal Oscillation nor North Pacific Gyre Oscillation impose strong controls on bulk sedimentary δ15N. Seasonal and interannual variability in sediment trap δ13Corgis consistent with local productivity as a driver; however, this signal is not retained in the sediment core. The time series from the sediment trap and core show that bulk sedimentary δ15N in SBB has now exceeded that measured for the past 2,000 years. We hypothesize that the change in δ15N reflects the increasing influence of denitrified waters from the Eastern Tropical North Pacific and ongoing deoxygenation of the Eastern Pacific. When juxtaposed with other regional δ15N records our results further suggest that SBB is uniquely situated to record long‐term change in the Eastern Tropical North Pacific.
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- PAR ID:
- 10459546
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 34
- Issue:
- 9
- ISSN:
- 2572-4517
- Page Range / eLocation ID:
- p. 1554-1567
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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