Natural organobromine in marine sediments : new evidence of biogeochemical Br cycling

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 24 (2010): GB4017, doi:10.1029/2010GB003794.Organobromine (Brorg) compounds, commonly recognized as persistent, toxic anthropogenic pollutants, are also produced naturally in terrestrial and marine systems. Several enzymatic and abiotic bromination mechanisms have been identified, as well as an array of natural Brorg molecules associated with various marine organisms. The fate of the carbon-bromine functionality in the marine environment, however, remains largely unexplored. Oceanographic studies have noted an association between bromine (Br) and organic carbon (Corg) in marine sediments. Even so, there has been no direct chemical evidence that Br in the sediments exists in a stable form apart from inorganic bromide (Brinorg), which is widely presumed conservative in marine systems. To investigate the scope of natural Brorg production and its fate in the environment, we probed Br distribution and speciation in estuarine and marine sediments using in situ X-ray spectroscopy and spectromicroscopy. We show that Brorg is ubiquitous throughout diverse sedimentary environments, occurring in correlation with Corg and metals such as Fe, Ca, and Zn. Analysis of sinking particulate carbon from the seawater column links the Brorg observed in sediments to biologically produced Brorg compounds that persist through humification of natural organic matter (NOM). Br speciation varies with sediment depth, revealing biogeochemical cycling of Br between organic and inorganic forms as part of the burial and degradation of NOM. These findings illuminate the chemistry behind the association of Br with Corg in marine sediments and cast doubt on the paradigmatic classification of Br as a conservative element in seawater systems.This investigation was funded by the U.S. Department of Energy, Office of Basic Energy Sciences (DOE‐BES) Chemical and Geosciences Programs, the National Science Foundation (NSF) Chemical Sciences Program, and an NSF Graduate Research Fellowship (ACL). Use of the ALS was supported by the DOE‐BES Materials Sciences Division under contract DE‐AC03‐ 76SF00098. Use of the SSRL, a national user facility operated by Stanford University, was supported by the DOE‐BES. Use of the NSLS was supported by the DOE‐BES under contract DE‐AC02‐98CH10886. Portions of this work were performed at beamline X26A at the NSLS. Beamline X26A is supported by the DOE‐Geosciences (DE‐FG02‐92ER14244 to the University of Chicago–CARS) and DOE Office of Biological and Environmental Research, Environmental Remediation Sciences Division (DE‐FC09‐96‐SR18546 to the University of Georgia)