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

Abstract. Blue carbon ecosystems, such as saltmarshes, play a crucial role in sequestering atmospheric carbon dioxide by storing it as buried organic carbon, also known as blue carbon, for centuries to millennia. This has generated significant interest in restoring these ecosystems for climate change mitigation benefits. While international methodologies exist for generating blue carbon credits through coastal wetland restoration, their application in Aotearoa New Zealand is limited by a lack of data on saltmarsh carbon stocks and accumulation rates. Additionally, to improve carbon mitigation estimates, research is needed to better understand the sources, composition and preservation of organic carbon in saltmarshes and the factors influencing its long-term storage. This study quantifies these metrics at five saltmarsh sites in Aotearoa New Zealand using 45 sediment cores analysed for elemental composition, stable isotopes, X-ray fluorescence (XRF), lipid biomarkers and Ramped-Pyrolysis Oxidation-Accelerator Mass Spectrometry (RPO-MS) in combination with Pyrolysis-Gas Chromatography-Mass Spectrometry (Py-GC-MS). Results show high variability in soil organic matter properties, carbon stocks (40.7 ± 9.1 to 112 ± 100.3 Mg C ha-1), and accumulation rates (0.56 ± 0.23 to 2.5 ± 0.44 Mg C ha-1 yr-1). An initial assessment indicates increased carbon accumulation following restoration at two sites. Stable isotope and lipid biomarker results show substantial contributions from saltmarsh vegetation to the organic carbon pool. Preliminary analysis suggests long-term preservation of plant-derived organic carbon in the oldest basal soil samples. The findings highlight the importance of accounting for spatial variability within saltmarsh ecosystems in carbon assessments and underscore the need for further research to determine environmental factors influencing long-term carbon storage and preservation.