Variations in Terrestrial Organic Carbon Burial in Mid-Latitude Deglaciating Fjords

Abstract

Carbon burial in marine sediments plays an important role in regulating global climate dynamics across glacial timescales by reorganizing the distribution of carbon across the planet. Burial of organic carbon produced by terrestrial plants is a rapid, naturally-occurring form of atmospheric carbon dioxide removal that may sequester carbon for thousands to millions of years. However, the fate of terrestrial carbon in the ocean is poorly resolved in global climate models and there remains a missing sink in the modern carbon budget. Furthermore, there remains an open question as to the role of terrestrial carbon in natural carbon dioxide fluctuations across glacial cycles. Mid-latitude fjords play a globally significant role in land-to-ocean carbon transport and burial, storing more carbon per area than other coastal and marine environments. However, the effect of rapid deglaciation on carbon sequestration in fjord sediments is not well-quantified. Here, we applied a multi-proxy approach characterizing the evolution of carbon burial in mid-latitude fjords over space and time in four sites spanning a latitudinal and off-shore gradient in the Gulf of Alaska. We interpreted glacial history and depositional environment via high-resolution sediment core CT scans and radiocarbon-based age models. We determined relative organic carbon source contribution in modern surface and aged basal sediments by analyzing bulk sediment elemental and isotopic ratios and n-alkane abundances. We found that reduced glacial presence in the drainage basin was associated with an increase in the relative percentage of terrestrial plant carbon and an overall decrease in organic carbon preservation. Understanding the mechanisms and magnitude of this natural carbon dioxide removal in these and other nearshore systems may help resolve the unbalanced modern carbon budget, distinguish natural atmospheric carbon dioxide fluctuations across glacial time from anthropogenic inputs and inform future climate change response strategies.