DOI: 10.25921/hxs7-5s18
title: NOAA/WDS Paleoclimatology - Late Holocene North Pacific Subtropical Gyre deep-sea proteinaceous coral d13C and d15N

description: δ15N and δ13C data obtained from samples of proteinaceous deep-sea corals collected from the North Pacific Subtropical Gyre (Hawaiian Archipelago) and the central equatorial Pacific (Line Islands) document multidecadal to century-scale variability in the isotopic composition of surface-produced particulate organic matter exported to the deep sea. Comparison of the δ13C data, where Line Islands samples are 0.6‰ more positive than the Hawaiian samples, supports 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. δ15N values from the Line Islands samples are also more positive than those from the central gyre, and within the Hawaiian samples there is a gradient with more positive δ15N values in samples from the main Hawaiian Islands versus the French Frigate Shoals in the Northwestern Hawaiian Islands. The gradient in the Hawaiian samples likely reflects the relative importance of algal acquisition of metabolic N via dissolved seawater nitrate uptake versus nitrogen fixation. The Hawaiian sample set also exhibits a strong decrease in δ15N values from the mid-Holocene to present. We hypothesize that this decrease is most likely the result of decreasing trade winds, and possibly a commensurate decrease in entrainment of more positive δ15N-NO3 subthermocline water masses.
description: Hawaiian gold corals (Kulamanamana haumeaae; Sinninger et al. 2013, doi:10.1371/journa.pone.0052607), both living and fossil were collected using the Hawaii Undersea Research Laboratory’s DSRV Pisces IV and V. K. haumeaae is a cosmopolitan colonial zoanthid that forms tree-like structures with heights of several meters and basal attachment diameters up to a few 10s of cm. K. haumeaae have radial growth rates of <100µm-yr-1 and can attain ‘life spans’ of centuries to millennia (Roark et al. 2006, doi:10.3354/meps327001; Roark et al. 2009, doi:10.1073/pnas.0810875106). In Hawai’ian waters they are most abundant at ~450m, near the top of the regional low oxygen zone, and are directly attached to exposed, hard substrates. Radiocarbon (14C) analyses of polyps and a finely sectioned radial skeletal disk of a live harvested branch both documents incorporation of recently exported particulate organic carbon (Roark et al., 2009), while stable isotope analysis shows that polyps are ≤ 2 trophic levels relative to exported POC/PON (Roark et al. 2009, doi:10.1073/pnas.0810875106). Resu lts presented here include samples collected from the Hawai’ian archipelago in 2004 and 2007 (Big Island, Cross Seamount, Oahu in the Main Hawai'ian Islands; MHI, and French/East French Frigate Shoals and Brooks Bank in the North West Hawai'ian Islands; NWHI). Upon recovery, the external polyps and tissue were removed using a seawater hose from all live-collected individuals. After removing the tissue layer, the skeletons (live collected and sub-fossil) were washed with seawater followed by fresh water and allowed to air dry on deck. Stumps were cut into ~0.7-cm thick cross-section disks taken close to the basal attachment. In a small number of cases additional disks were also taken higher on the branch (or stump). Sampling was done by microdrilling center (inner) and outer samples from radial transects using a 1.8 mm spherical carbide bur. Depending on the size of the cross-section disk sub-samples between the inner and outer samples were also taken. Milled samples were decarbonated using 1N HCl at 90C, rinsed three-times with milliQ water and dried overnight on a heating block. Sub-samples were transferred to tin boats and the mass determined. Carbon and nitrogen stable isotope analyses were made via continuous-flow IRMS using a Carlo-Erba elemental analyzer connected to an Optima IRMS in the UCSC Light Isotope Facility. Results are reported in conventional per mil notation relative to V-PDB (δ13C) and air (δ15N). Reproducibility of the isotope results is ± 0.12‰ and ±0.06 (1-sd) based on reproducibility of standards (acetanilide, n=20) and replicate analyses (n=29) of coral samples for δ13C and δ15N, respectively. C:N ratios determined during analysis were ±0.15 (1-sd). Radiocarbon sub-samples were first converted to CO2 via sealed-tube combustion, and the resulting purified CO2 reduced to graphite in the presence of an iron catalyst. Individual graphite-catalyst mixtures were pressed into aluminum target holders and analyzed via accelerator mass spectrometry. Results include a background subtraction based on similarly prepared 14C-free coal and wood, sample specific δ13C corrections, and are reported as Fmodern, D14C, and conventional radiocarbon years as per Stuiver and Polach (1977, doi:10.1017/S0033822200003672), with one-sigma uncertainties. We note that in the published version of these data that IntCal 09 (Reimer et al. 2009, doi:10.1017/S0033822200034202) was used as the 14C-calendar calibration curve. Since that time IntCal 13 (Reimer et al. 2013, doi:10.2458/azu_js_rc.55.16947) has been released. We strongly recommend that the users of these data use the appropriate, most recent and up-to-date IntCal calibration data product to convert radiocarbon years to calibrated ages. Being of marine origin, the data require a site specific correction for the marine reservoir effect. Until further empirical observations are available, we recommend for Hawai’ian samples, and based on a ~40 year average of reef-building hermatypic coral data, a DELTA_R of -28±4 to be used (Druffel et al. 2001, doi:10.1017/S0033822200031593).