Sedimentary organic bromine as an indicator of marine organic carbon and primary productivity in coastal and marginal seas

Partitioning sedimentary organic carbon (OC) into marine (OC_mar) and terrestrial (OC_ter) components is critical for understanding carbon cycling and sequestration in coastal and marginal seas. Organic-bound bromine (Br_org) has been proposed as a proxy for OC_mar due to the enrichment of bromine in marine organic matter. Although bromine has long been considered a conservative element in marine systems, it is now recognized as biogeochemically active. In the water column, hydrogen peroxide reacts with bromide to form reactive bromine species, resulting in phytoplankton and macroalgae to generate a wide range of brominated organic compounds. While volatile brominated compounds have been extensively studied due to their climatic relevance, non-volatile brominated organic compounds remain associated with organic matter and are transferred to the sediments. However, their environmental controls remain poorly constrained. Here, we present a dataset of Br_org from core-top sediments collected in the Baltic Sea, North Sea, Atlantic Ocean, Mediterranean Sea, and Black Sea. We assess relationships between Br_org and key environmental parameters such as sea surface salinity, net primary productivity (NPP), and bottom-water oxygen. NPP shows a stronger correlation with Br_org concentration (R² = 0.64) than with total organic carbon or carbon isotopic composition, indicating a higher sensitivity to variations in organic matter source, production, and reactivity. The relationship between NPP and Br_org differ between oxic and anoxic basins, highlighting the effect of preservation conditions. This proxy for marine organic carbon and productivity in oxic coastal settings can be used in downcore records to distinguish natural variability in older sediments from anthropogenic effects in more recent sediments.