EXPLORING THE HALOGEN GEOCHEMISTRY OF MARINE SEDIMENTS

The chemical and isotopic compositions of marine sediments are proxies for paleo- temperature, redox, and salinity. However, potential diagenetic, biochemical, and kinetic effects add uncertainty when reconstructing paleoenvironmental information. In this study, we measured the halogen concentration (F, Cl, Br I) and chlorine isotope composition of marine cherts and carbonates throughout the Precambrian and Phanerozoic to investigate broad inter-element and isotopic relationships to address several long-standing questions. 1. Has ocean salinity significantly changed through time (Knauth, 2005)? 2. Are the halogen contents of marine carbonates proportional to the seawater they are derived from (Kitano et al. 1975), and 3. Do they provide information about redox conditions (Lu et al., 2010)?

We present several observations. 1. Precambrian and Phanerozoic cherts contain variable halogen concentrations but invariant halogen ratios (i.e., I/Cl, Br/Cl), suggestive of minimal changes in seawater salinity resulting from halite sequestration. 2. The halogen contents of marine carbonates do not reflect simple partitioning from seawater and are influenced by several variables including ambient seawater chemistry (i.e., Ca/Mg), dominant carbonate polymorph, and biochemistry of biomineralizing organisms. F contents largely reflect carbonate mineralogy and are far more abundant in aragonite. Normalizing halogen concentrations to chlorine is seemingly effective in controlling for mineralogical differences. In contrast, the I and Br contents of carbonates are directly related. We find the highest I/Cl and Br/Cl ratios measured in the carbonate skeletons of biomineralizing organisms such as red algae with high abundances of haloperoxidases enzymes which function to oxidize halides. Given the understanding of iodine (Lu et al. 2010), and bromine (Midgley et al. 2021) partitioning into carbonates as halates, we suggest that the I and Br contents of carbonates do not solely reflect changes in redox conditions. Overall, we show that halogen geochemistry in marine sediments has potential for novel insights into paleoenvironmental reconstructions.