Late Holocene variations in Pacific surface circulation and biogeochemistry inferred from proteinaceous deep-sea corals
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Abstract. 15N and 13C data obtained from samples of proteinaceous deep-sea corals collected from the North Pacific Subtropical Gyre (Hawai'ian archipelago) and the central equatorial Pacific (Line Islands) document multi-decadal to century scale variability in the isotopic composition of surface-produced particulate organic matter exported to the deep sea. Comparison of the 13C data, where Line Island samples are 0.6 more positive than the Hawai'ian samples, support the contention that the North Pacific Subtropical Gyre is more efficient than the tropical upwelling system at trapping and/or recycling nutrients within the mixed layer. 15N values from the Line Island samples are also more positive than those from the central gyre, and within the Hawai'ian samples there is a gradient with more positive 15N values in samples from the main Hawai'ian Islands versus French Frigate Shoals in the Northwestern Hawai'ian Islands. The gradient in the Hawai'ian samples likely reflects the relative importance of algal acquisition of metabolic N via dissolved seawater nitrate uptake versus nitrogen fixation. The Hawai'ian sample set also exhibits a strong decrease in 15N values from the mid-Holocene to present. We hypothesize that this decrease is most likely the result of decreasing tradewinds, and possibly a commensurate decrease in entrainment of more positive 15N-NO3 subthermocline water masses.
