Characteristics and Seasonality of Particulate and Dissolved Organic Matter Discharged by the Lena River to the Arctic Coast

Abstract

Permafrost is considered a resting titan, exceptionally vulnerable to climate change. Warming temperatures intensify permafrost thawing, releasing previously stored organic matter (OM) into the biogeochemical cycle. Once released, OM enters lakes, streams, and rivers, eventually reaching larger water bodies. This study investigates particulate and dissolved organic carbon (POC and DOC) discharged by one of the largest Arctic rivers—the Lena River—into the Arctic Ocean. A key question drove this research: what is the composition and fate of OM transported by the Lena River across the continent, through its delta, to the Arctic coast?

This thesis was written as part of the German–British project CACOON and consists of three manuscripts. The first examines POC dynamics in the Lena River and its delta, showing that suspended matter and POC concentrations decrease by 70% from the main stem to the ocean. Dual-carbon isotope analysis reveals a dominant phytoplankton contribution to deltaic POC, along with permafrost-derived OM input. The second manuscript explores the seasonality of DOC and POC in the Lena Delta. It shows unexpectedly high winter DOC concentrations and older carbon signatures in cold months, offering new insights into Arctic carbon cycling. The third manuscript focuses on GDGT biomarkers used to trace environmental conditions. It highlights how temperature, salinity, and OM sources shape their distribution in the Lena Delta and Laptev Sea. Overall, this thesis provides a comprehensive study of OM in the Lena Delta, from its origins to transformations during transport to the Arctic nearshore, emphasizing seasonal changes and climate-driven impacts.