Long-term peat organic matter stability influenced by botanical composition and oceanic bromide inputs

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Abstract

Peatlands serve as essential terrestrial reservoirs of carbon. The stability of organic matter (OM) in peat depends on its organic composition, with oxidized carbon (O-alkyl) more labile than aromatic and aliphatic components. Peat tends to be enriched in halogens, which bond to OM and are retained over unknown timescales. Although halogens are generally assumed to render organic molecules resistant to degradation, their role in peat OM dynamics is poorly understood. Here we assess controlling factors for OM recalcitrance in four long (>400-cm) Patagonian peat cores, finding that 1) the recalcitrance of peat OM depends on botanical composition; and 2) bromination of aromatic compounds in peat OM confers stability on millennial scales. Through analysis of plant macrofossils combined with infrared spectroscopy of solid-phase peat material, we find that the proportion of Sphagnum moss is directly related to the relative abundance of carbohydrates and anticorrelated with aromatic content in the cores. Using X-ray absorption spectroscopy, we show that all Br in the peat is bonded to aromatic carbon, and this organobromine persists over multiple millennia, without evident debromination. Aromatic organobromine becomes enriched alongside aromatic content in peat OM over the first several millennia of the record. However, in the two cores with the longest chronological records, organobromine declines from 7,500 to 14,000 years before present, with no corresponding decrease in aromatic content. Organobromine concentrations correlate with sea level through this period, suggesting that lower bromide deposition in these rain-fed ecosystems yielded less organobromine during the early Holocene. This association broaches the potential for using organobromine in peat as a geochemical marker for post-glacial sea level rise. It also suggests that increasing sea levels will stimulate (bio)geochemical organobromine production in coastal peatlands, enhancing long-term preservation of aromatic residues in peat OM. Taken together, these results illustrate the influence of plant community composition as well as oceanic bromide inputs on the long-term stability of peat OM.

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Keywords

Spectroscopy, Environment and Air / water / soil quality

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