Sedimentary organic bromine as indicator for marine organic carbon: a proxy for primary productivity in paleo-records

Abstract. The efficiency of the ocean as a carbon sink requires estimates for marine primary productivity and for the fraction of this primary productivity that is ultimately buried in the sediment. Partitioning sedimentary total organic carbon (TOC) into marine (OC_mar) and terrestrial (OC_ter) components is thereby essential for distinguishing the contribution of land-derived organic matter versus locally produced organic matter in a marine setting. Previous studies have proposed sedimentary organic-bound bromine (Br_org)-to-TOC (Br/OC) ratio as a proxy for OC_mar, as marine organic matter is enriched in bromine relative to terrestrial organic matter. Yet, the influence of other environmental and depositional conditions on Br_org and thereby its potential as a quantitative tool for unravelling terrestrial versus marine organic matter remain poorly constrained. To address this, we present a multi-regional dataset of Br_org in core-top sediments from the Baltic Sea and North Sea to the Atlantic Ocean, Black Sea, and Mediterranean Sea (n = 55). We investigate environmental controls on the Br/OC ratio and compare the performance of Br_org with traditional marine organic matter indicators such as the carbon isotope composition of bulk organic matter (δ¹³C_org) and the carbon-to-nitrogen (C/N) ratio in capturing OC_mar contributions. By better constraining the marine carbon flux, Br_org not only distinguishes OC_mar but also directly reflects its link to marine net primary productivity (NPP), as sedimentary OC_mar is primarily derived from NPP. Across all studied basins, Br_org shows a strong positive relationship with TOC (R² = 0.86). After accounting for preservation effects using bottom-water oxygen concentrations (oxic versus anoxic settings), we show that sedimentary Br_org deposited under oxic conditions exhibits the strongest relationship with NPP (R² = 0.65), compared to TOC (R² = 0.43), δ¹³C_org-derived OC_mar (R² = 0.38), and C/N-derived OC_mar (R² = 0.26). This suggests that Br_org not only traces OC_mar more effectively than traditional proxies but also outperforms them in reconstructing productivity signals. However, this relationship applies mainly to sediments deposited under oxic conditions (North Sea, Atlantic Ocean, and Mediterranean Sea). Under anoxic conditions (Baltic and Black Seas), reduced oxygen exposure may enhance the preservation of labile, Br-rich organic matter, causing these sediments to follow a different Br_org-NPP relationship than oxic settings. Finally, we apply this Br_org-NPP relationship to a North Sea sedimentary record by calibrating X-ray fluorescence core scanning Br data to Br_org using a multivariate log-ratio calibration. As such, this approach allows high-resolution Br_org records downcore to be generated and used to trace OC_mar and for quantitative past NPP (paleoproductivity) reconstructions.