Ice Age‐Holocene Similarity of Foraminifera‐Bound Nitrogen Isotope Ratios in the Eastern Equatorial Pacific
Anja S Studer, Figen Mekik, Haojia Ren, Mathis P. Hain, Sergey Oleynik, Alfredo Martínez‐García, Gerald H. Haug, Daniel M. Sigman
Abstract
Abstract Bulk sediment δ 15 N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ 15 N, then a deglacial δ 15 N maximum, followed by a gradual decline to a late Holocene δ 15 N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ 15 N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ 15 N) 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 δ 15 N. FB‐δ 15 N contrasts with bulk sediment δ 15 N by not indicating a lower δ 15 N during the LGM. Instead, the FB‐δ 15 N records are dominated by a deglacial δ 15 N maximum, with comparable LGM and Holocene values. The lower LGM δ 15 N 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.