Estimation of Marine Versus Terrigenous Organic Carbon in Sediments Off Southwestern Taiwan Using the Bromine to Total Organic Carbon Ratio as a Proxy

Abstract We measured bromine to total organic carbon (Br:TOC) ratios as an organic source indicator in terrestrial and marine settings of Taiwan, a tectonically active margin that differs from previously studied, passive margins. Terrestrial lake sediments, soils, and river bed and suspended sediments showed Br:TOC ratios ranging from 0.02 to 2.8 mg‐Br/g‐TOC. In ocean margin sediments, Br:TOC ratios ranged from terrigenous values up to 7.6 mg‐Br/g‐TOC. A two‐component Br:TOC mixing model for ocean margin sediments indicates roughly equal proportions of terrigenous and marine organic matter (FT and FM, respectively). Terrestrial δ13C values varied from −31.5 to −24.7‰, a range which is twice the difference between the isotopic terrigenous and marine end‐members used to calculate these proportions. FT and FM based on δ13C were similar to those using Br:TOC if averaged across all marine sediments but often differed at the level of individual samples. Depth profiles of amounts of these two forms of organic matter (OCterr and OCmar) using Br:TOC ratios show that OCmar burial is relatively constant while OCterr and hence TOC vary more strongly; this pattern suggests either episodic terrigenous carbon inputs or winnowing before burial. Variations in TOC:TN and δ13C are consistent with this control of TOC by terrigenous carbon. This study validates Br:TOC as a source indicator for terrigenous versus marine‐derived organic matter in sediments derived from mountainous areas in Oceania. As a single parameter source indicator it should have less error than δ13C due to less uncertainty of the terrestrial end‐member.