LAB SIMULATED WHALE FALLS PRODUCE FRAMBOIDS RESEMBLING THOSE FROM EUXINIC BASINS

The distribution of pyrite framboid diameters from a sample population is commonly used as an indicator of past water chemistry. Framboids generally form in anoxic environments enriched in sulfide (euxinic conditions), often as the result of sulfide produced by heterotrophic sulfate reducing bacteria. Euxinic environments are commonly found in subsurface pore-waters in oxygenated ocean basins. In addition to forming in sediment pore waters, framboids can also form in the water column in closed, stagnant basins or under other ocean anoxic conditions. Framboids that form in euxinic water columns are on average smaller (diameter < 5 µm) and less variable in size because they rapidly fall out of suspension after nucleation, halting framboid growth. Conversely, framboids that form in sediment overlain by oxic water are larger (diameter > 8 µm) because they remain in geochemical conditions that allow for continuing growth. Here, we provide an alternative scenario for the generation of small framboids populations based on their appearance on submerged bone surfaces during laboratory experiments. Twelve de-fleshed domestic goat rib sections were placed in marine water and sediments from the coast of Massachusetts and were incubated at 10⁰C for eight months. Two weeks after emplacement, bone surfaces were densely colonized by dark-colored microbial mats thought to be inhabited by sulfate-reducing bacteria. After four weeks, putative sulfide-oxidizers (light colored mats) colonized the bone surface and only the water adjacent to bone and sediment was anoxic. Framboids were found on bone surfaces as early as four weeks after emplacement. Average framboid diameter is 6.1 µm with a standard deviation of 1.45 µm (n=20), indicating they are closer is size and distribution to framboids formed in euxinic water column. Our results suggest that large pulses of organic carbon can promote localized euxinic conditions that extend above the sediment/water interface. Our observations suggest that the bacterially-mediated euxinic conditions are transient due to nutrient availability and changing microbial communities. We propose that the smaller framboids that form in the microbial mat result from shorter exposure to conditions that favor framboid growth than those that precipitate in sediment pore waters.