Abstract
AbstractBulk 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.
Highlights
The eastern tropical Pacific (ETP) is important in climate and biogeochemical cycles
In the δ15Nbulk records measured in ME-24 and ME-27 by Dubois et al (2011), δ15Nbulk is lowest (4–5‰) during the last ice age, increases to 6–7‰ during the deglaciation, and slightly increases (ME-24) or decreases (ME-27) during the Holocene (Figure 2)
At ME-24, foraminifera shell-bound δ15N (FB-δ15N) declines abruptly during the second half of the BA and is at a minimum during the YD, peaks again during the early Holocene, and declines into the middle Holocene. Another difference between the two core sites is that at ME-27, FB-δ15N of N. dutertrei and N. incompta are very similar, while at ME-24, FB-δ15N of N. dutertrei is on average 0.6‰ higher than FB-δ15N of N. incompta
Summary
The eastern tropical Pacific (ETP) is important in climate and biogeochemical cycles It hosts three upwelling systems: the equatorial upwelling, the coastal upwelling off California in the eastern tropical North Pacific (ETNP), and the Peru-Chile coastal upwelling in the eastern tropical South Pacific (ETSP) (Figure 1). In these regions, cold, nutrient-rich waters upwell to the surface, fueling productivity. Within the ODZs, denitrification reduces nitrate to N2, providing the oxidant for the respiration of organic matter This water column denitrification is a major driver of fixed N loss from the ocean (Deutsch et al, 2001), and it can produce nitrous oxide, a potent greenhouse gas, as a side-product (Pierotti & Rasmussen, 1980). Reconstruction of the history of denitrification may provide insight into past changes in ocean circulation and the strength of the global ocean's biological carbon pump (Altabet et al, 1995, 2002; Ganeshram et al, 1995; Martinez et al, 2006; Robinson et al, 2007)
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