[U05-P01] Precessionally Paced Foraminifera-bound Nitrogen Isotope Variations in the Solomon Sea Influenced by Eastern Equatorial Pacific Upwelling Intensity
★Invited Papers
キーワード：Equatorial Pacific, Foraminifera-bound nitrogen isotopes, Precessional
The surface water in the Western Equatorial Pacific (WEP), although is depleted with nitrate and other major nutrients, bears isotopic signature of partial nitrate assimilation in the nutrient-rich Eastern Equatorial Pacific (EEP). Driven by the easterlies, Equatorial Under Current upwells to the surface of the EEP, where biological uptake only partially consumes nitrate and leaves the remaining nitrate enriched in its nitrogen isotopic composition. Through multiple cycles of uptake and remineralization, the elevated nitrogen isotopic composition is recorded by organic matter and is spread out to regions away from the EEP, partly causing the WEP thermocline nitrate δ15N to be much more elevated than the global mean. We then expect the δ15N in the WEP to be sensitive to the expansion and shrinking of the EEP high nutrient region during past climatic cycles.
This study reports foraminifera-bound δ15N (FB-δ15N) of two planktonic species – Globigerinoides sacculifer and Globigerinoides ruber – from a sediment core in the Solomon Sea over the last 158 ka. The FB-δ15N values, similar between the two species, fall between 7‰ to 13‰. The core-top FB-δ15N (9.2‰) is approximately equal to the modern subsurface nitrate δ15N (9.4‰), the dominate nitrate source to the Solomon Sea surface. The FB-δ15N values are significantly elevated during periods with high local summer/fall insolation with a strong precessional pacing. While without a clear glacial/interglacial difference, at both terminations, the FB-δ15N hits lowest values (~8.3‰) before an abrupt increase.
The lack of clear glacial/interglacial change, yet a strong precessional pacing in the Solomon Sea FB-δ15N record, implies a low latitude control and an insignificant influence from oxygen minimum zones in the eastern tropical Pacific. Supported by modern hydrologic data and modeling exercises, we argue that the variations in our FB-δ15N record reflects changes in the EEP surface nutrient status, such that expansions of the high nitrate pool cause the observed high FB-δ15N values. Together with reconstruction in the EEP paleoproductivity, our record strongly indicates enhanced EEP upwelling during periods with local insolation maximum, consistent with previous modelling studies showing a strong precessional control on ENSO evolution.
This study reports foraminifera-bound δ15N (FB-δ15N) of two planktonic species – Globigerinoides sacculifer and Globigerinoides ruber – from a sediment core in the Solomon Sea over the last 158 ka. The FB-δ15N values, similar between the two species, fall between 7‰ to 13‰. The core-top FB-δ15N (9.2‰) is approximately equal to the modern subsurface nitrate δ15N (9.4‰), the dominate nitrate source to the Solomon Sea surface. The FB-δ15N values are significantly elevated during periods with high local summer/fall insolation with a strong precessional pacing. While without a clear glacial/interglacial difference, at both terminations, the FB-δ15N hits lowest values (~8.3‰) before an abrupt increase.
The lack of clear glacial/interglacial change, yet a strong precessional pacing in the Solomon Sea FB-δ15N record, implies a low latitude control and an insignificant influence from oxygen minimum zones in the eastern tropical Pacific. Supported by modern hydrologic data and modeling exercises, we argue that the variations in our FB-δ15N record reflects changes in the EEP surface nutrient status, such that expansions of the high nitrate pool cause the observed high FB-δ15N values. Together with reconstruction in the EEP paleoproductivity, our record strongly indicates enhanced EEP upwelling during periods with local insolation maximum, consistent with previous modelling studies showing a strong precessional control on ENSO evolution.