DOI: 10.25921/89rg-cp89
title: NOAA/WDS Paleoclimatology - Hawaiian Gold Coral Bulk and Compound Specific Nitrogen Isotope Data for the Last Millennium

description: The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean. Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters. It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes. It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend. Here we analyse records of bulk and amino-acid-specific 15N/14N isotopic ratios (d15N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, d15N decreases between 1850 and the present. The total shift in d15N of 2 per mil over this period is comparable to the total change in global mean sedimentary d15N across the Pleistocene–Holocene transition, but it is happening an order of magnitude faster. We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 percent increase in nitrogen fixation over this time period. A comparison with independent records suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.
description: Colonies of Hawaiian gold coral (K. haumeaae) were collected from approx. 450 m water depth from 3 sites in the Hawaiian archipelago (Makapuu, Cross Seamount, and French Frigate Shoals) with the HURL/NOAA Pisces V submersible between 1997 and 2004. Air-dried colonies were sectioned near the skeletal base, polished and photographed under a binocular microscope. A computerized Merchantek micromill was used to mill samples, parallel to growth banding, at increments of 0.1 mm, along radial transects from the outer edge to the center of each section. Each sample represents from 1 to 20 (average 5) years of growth, depending on growth rate. Radiocarbon dating of sample aliquots was used for age modeling. Bulk nitrogen isotope ratios were measured using a Carlo Erba 1108 elemental analyzer interfaced to a ThermoFinningan Delta Plus XP isotope ratio mass spectrometer (IRMS). Values are reported in per mille units relative to atmospheric N2. Amino acid specific nitrogen isotope atios were measured on 15-20 mg sample composites (combining 3-10 separate samples). Composites were hydrolysed in 6 N HCl (100 ml, 20 h), spiked with a norleucine internal standard followed by formation of trifluoroacetyl/isopropyl ester (TFA) derivatives. Derivatives were analyzed on a Thermo Trace Ultra GC, fitted with a SGE BPX5 capillary column (60m x 0.32mm ID, 1 micrometer), in line with oxidation and reduction furnaces, and linked to a ThermoFinnigan Delta Plus XP IRMS. Samples were analysed in quadruplicate. Analytical accuracy was monitored by analysis of the norleucine internal standard and co-derivatized AA external standards for which authentic d15N values of each AA were determined offline. Reproducibility for individual AA values was typically better than 1 per mille.