DOI: 10.1594/pangaea.937786
title: Microchemistry of archaeological and present-day snapper (Chrysophrys auratus) otoliths from the Hauraki Gulf, New Zealand

description: We sampled the archaeological and modern-day snapper otoliths used for this analysis from sites in the Hauraki Gulf, on the east coast of the North Island of New Zealand. Long Bay (AL) samples were radiocarbon dated using Bayesian modelling to between 1430-1485 AD, while the Omaha (AO) samples were dated to between 1530-1640 AD (Campbell et al. 2004, 2019). For comparison we acquired modern samples from as close as possible to the archaeological middens. Kawau Island (MO, 2016 AD) served as the modern comparison to the Omaha midden and outside of the current Long Bay marine reserve (ML, 2020 AD) provided samples to compare with the Long Bay midden.Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis was conducted to measure Barium (138Ba) and Strontium (88Sr) compositions along an ablation path though the core to the proximal tip of each otolith. For each of the four sites, ten sagittal otoliths were transverse-sectioned and then mounted on a geological slide for laser ablation. Instrumentation was an Applied Spectroscopy RESOlution M-50 laser ablation system powered by a Coherent 193 nm ArF excimer laser and an Agilent 7900 quadrupole ICP-MS, located in the Centre for Trace Element Analysis in the Department of Chemistry, University of Otago (Dunedin, New Zealand).Slides with mounted otoliths were placed in an ablation cell in an atmosphere of pure helium to minimize any possibilities of experiencing re-condensation of ablated materials and elemental fractionations (Eggins et al. 1998). The video imaging system had suitable magnification to identify the core and was used for mapping transect pathways. Prior to obtaining measurements the 75 µm diameter transects were pre-ablated from core to the edge of the otolith to remove surface contaminants. The spot size employed for the transects was selected as a compromise between spatial sensitivity and detection power of the overall system (Taddese et al. 2019). The ablation with a laser firing frequency of 10 Hz and an on-sample fluence of 2.5 J/cm2 was operated along the pre-ablated transects with the sample stage moving at 10 µm/s, for determining elemental concentrations in correspondence to life cycle of the fish. The ICP-MS instrument was tuned to minimize oxide formation, double charge formation and mass fractionation. Signal intensities of Ba and Sr were maximized after carrying out gas tuning processes on software-controlled gas flows of He and N2 along with ICP-MS controlled Ar. Standards were run regularly with NIST610, NIST 612 and MACS3 used for instrument calibration, verification and matrix matched quality control respectively. Data reduction of the raw count data to molar ratios (element of interest/Ca) was conducted using Iolite 3.63 (School of Earth Sciences, University of Melbourne) which subtracts gas backgrounds and corrects for any drift in instrument response (Paton et al. 2011). Accuracy and precision of the analyses were assessed using NIST 612 and the MACS-3 otolith reference material (United States Geological Survey - USGS). For the glass control precision was excellent RSD &lt;3% and the accuracy was within ± 5% for all elements. For the otolith reference material precision was better than 5% with recoveries percentages of 97% and 96% for Sr and Ba, respectively.