Characterisation and quantification of organic carbon burial using a multiproxy approach in saltmarshes from Aotearoa New Zealand

Blue carbon ecosystems, such as saltmarshes, play a vital role in mitigating climate change by sequestering atmospheric carbon dioxide and storing it as buried organic carbon for centuries to millennia. While there are international methodologies for generating blue carbon credits through coastal wetland restoration, their application in Aotearoa New Zealand is limited due to insufficient data on saltmarsh carbon stocks, accumulation rates and the processes governing long-term carbon preservation. To quantify these metrics, we examined 45 sediment cores collected from five saltmarsh sites in Aotearoa New Zealand. The cores were analysed for elemental composition, stable isotopes, and lipid biomarkers. These data were collected using a range of techniques, including X-ray fluorescence (XRF), Ramped-Pyrolysis Oxidation-Accelerator Mass Spectrometry (RPO-AMS), and Pyrolysis-Gas Chromatography-Mass Spectrometry (Py-GC-MS). Results show high variability in soil organic matter properties, carbon stocks (41.3 +/- 9.4 to 92.3 +/- 66.2 Mg C ha-1; mean +/- SE), and accumulation rates (0.46 +/- 0.02 to 1.53 +/- 0.09 Mg C ha-1 yr-1; mean +/- SE). Stable isotope and lipid biomarker results indicate substantial contributions from saltmarsh vegetation to the organic carbon pool. Results suggest that plant-derived organic carbon is preserved in the oldest basal sediments. Our findings highlight that spatial variability must be considered when conducting carbon assessments in saltmarsh ecosystems. Further research is required to determine the environmental drivers that influence long-term carbon storage and to improve the accuracy of blue carbon assessments in Aotearoa New Zealand.