Quantifying hydrothermal ammonium mobilization from sediment and implications for the marine biosphere: A case study from the Guaymas Basin, Gulf of California

Nathan Rochelle-Bates^1*Annabel Long¹Graeme MacGilchrist¹Andreas Teske²Eva Stüeken¹

• ¹School of Earth and Environmental Sciences, Faculty of Science, University of St Andrews, St Andrews, United Kingdom
• ²Department of Marine Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States

Nitrogen is an essential and limiting nutrient for life on Earth. Understanding its environmental availability is therefore key to reconstructing the limits under which life has evolved and proliferated. Fluid-rock interactions in hydrothermal systems are capable of liberating ammonium (NH4 + ) from sedimentary organic material, and making it bioavailable for benthic and pelagic microbial communities. Hydrothermal systems in organic-rich sedimentary basins are therefore thought to have played a key role in supplying nitrogen to the early biosphere. However, quantifying the timescales of nitrogen remobilization and release remains difficult. To examine the magnitude and timeframe of nitrogen mobilization from sediment in hydrothermal systems, we examined sediments from active hydrothermal systems in the Guaymas Basin, a young oceanic spreading center in the Gulf of California. The basin floor is covered by organic-rich (1-2% total organic carbon) diatomaceous sediments, which are the result of high surface biological productivity. Intrusion of igneous bodies into these sediments drives hydrothermal systems that are rich in thermogenic alteration products like ammonium and methane. We investigated four shallow sediment cores that were taken at seepage sites in the Guaymas Basin's hydrothermally-active Southern Trough and found that ca. 54 % of the organically-bound nitrogen is remobilized by active seepage in the top 10 cm of the sediment package. Thus, hydrothermal seepage liberates more than half the accumulated sedimentary nitrogen within as little as 27-83 yr. Extrapolating these findings over the hydrothermally-active area of the basin yields an ammonium seepage flux of ca. 1.3-4.1 mol/s to the basin. In addition, high-temperature venting liberates ca. 156 -187 mol/s, as estimated from previous data. Assuming biological uptake of hydrothermally recycled ammonium in the watercolumn, sediment seepage and high-temperature venting could support up to 1.3 % and 58 % of export productivity, respectively. Our data also highlight the role of organic material in enhancing metal mobilization and accumulation in otherwise metal-starved hydrothermal seeps. We therefore conclude that hydrothermal activity in sedimentary basins can create a significant nutrient flux that could have enhanced microbial activity, in particular during time intervals when Earth's oceans are thought to have been nutrient-depleted.