Publication Date:
2021-09-29
Description:
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 species Neogloboquadrina dutertrei and Neogloboquadrina incompta over 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.
Description:
Plain Language Summary:
The 15N/14N ratio of sediments provides information on the past marine nitrogen (N) cycle through the production of N‐bearing organic matter in the surface ocean and its burial in the sediments. Previous measurements of the sedimentary 15N/14N ratio in the eastern equatorial Pacific (EEP) indicate lower values during the last ice age compared to the Holocene (the current warm period). This has been interpreted to reflect an ice age reduction in the oceanic N loss process known as “denitrification” that occurs between 200 and 500 m depth in this region of the ocean. However, the 15N/14N ratio measured on the whole sediment can be biased by biological and chemical processes in the sediments and by foreign N inputs. To avoid these complications, we measured the 15N/14N ratio of organic N embedded in the calcite shell of unicellular zooplankton (foraminifera) in two sediment cores from the EEP. We found similar foraminifera‐bound 15N/14N ratios during the last ice and the Holocene. This may argue against the long‐held interpretation of a reduction in denitrification during the last ice age. However, the oceanographic setting of these equatorial cores leaves open alternative interpretations, calling for further work at other eastern tropical Pacific sites.
Description:
Key Points:
Foraminifera‐bound δ15N was similar during the last ice age and the Holocene in the eastern equatorial Pacific, unlike bulk sedimentary δ15N.
Bulk sediment δ15N is likely biased to lower ice age values by foreign N inputs and weaker sedimentary diagenesis.
The foraminifera‐bound δ15N data may reflect that water column denitrification was not reduced during the last glacial period.
Description:
Swiss National Science Foundation
Description:
US National Science Foundation
Description:
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung
http://dx.doi.org/10.13039/501100001711
Description:
National Science Foundation
http://dx.doi.org/10.13039/100000001
Keywords:
551.9
;
Pacific Ocean
;
nitrogen isotopes
;
denitrification
;
suboxia
;
Last Glacial Maximum
;
Holocene
Type:
map
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