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Early diagenetic forward and reverse weathering reactions play a significant role in controlling alkalinity fluxes and silica, alkali metal and alkaline earth metal cycling in coastal systems. In Kongsfjorden, Svalbard, the inputs of autochthonous biogenic debris (diatomaceous silica) and allochthonous lithogenic material of varying reactivity (dominated by clays, especially illite and chlorite, and primary aluminosilicates, mostly plagioclase) drive complex balances of diagenetic silicate reactions in sediments. The rapid dissolution of reactive silica results in the release of dissolved silica (Si_d) into pore‐waters and sustains elevated benthic Si_dfluxes (−0.2 to −0.8 mmol m⁻² d⁻¹), which are on the upper end of values previously determined for Arctic environments. Increases with depth in pore‐water lithium (Li⁺), potassium, magnesium, and barium concentrations within the top centimeters provided evidence for forward weathering of clays quickly upon burial. Due to the prevalent occurrence of forward weathering, the benthic net Li⁺flux was associated with a light isotope signal. Decreases in pore‐water rubidium concentrations with depth at the near‐glacier station, elevated ratios of the authigenically altered silica to the biogenic silica pool at all sites, and small increases of pore‐water δ⁷Li values with depth showed that reverse weathering also takes place. Anoxic incubation of diatom frustule probes provided further evidence for the neoformation of cation‐rich clays. The superposition of reverse and forward weathering results in cryptic silica and cation cycling that muted net benthic fluxes. In deeper sediments, changes in pore‐water solute patterns indicated an interconnected occurrence of reverse and forward weathering, potentially driven by reactive silica‐limitation. 

Benthic organisms in coastal sediments affect elemental cycling and control benthic-pelagic coupling through particle reworking and ventilation of their burrows. Bioirrigation and associated porewater advection create intermittently oxic regions within sediments. The spatio-temporal patterns of such biogenic redox oscillations likely respond to seasonal factors, but quantitative information on the seasonality of bioirrigator behaviors and associated redox dynamics is scarce. We examined bioirrigation by the maldanid polychaete Clymenella torquata and its impacts on sediment oxygenation patterns in permeable sediments using high-resolution planar optode oxygen imaging. In sediment mesocosms with reconstructed summer-collected sediment, the durations of pumping and resting varied inversely with temperature. The average durations of pumping and resting increased from 4 min/4 min at 21 °C, to 6 min/6 min at 12 °C, to 15 min/14 min at 5 °C. In intact cores collected in summer, irrigation patterns (3.5 min/3.5 min) were similar to those observed at 21 °C during the temperature ramp. Pumping and resting durations in intact cores collected in winter at 6 °C averaged 9 min/26 min, significantly different from patterns at comparable temperatures in the temperature ramp. Pumping patterns strongly affected the temporal patterns of redox dynamics in surrounding sediments. In addition, temperature strongly affected burrow irrigation depth (exclusively within the top ∼10 cm at 21 °C, and down to ∼20 cm at 5–6 °C with an apparent transition at ∼15 °C), indicating that the zone with dynamic redox conditions migrates vertically on a seasonal basis. The differences in pumping patterns between in- and out-of-season experiments and the effect of temperature on irrigation depth underscore the importance of conducting experiments with bioturbators in-season and at field temperatures. The observed seasonal differences in bioirrigation patterns and associated spatio-temporal redox dynamics suggest that rates and pathways of redox-sensitive diagenetic processes and benthic chemical fluxes in permeable sediments likely show considerable seasonal variation. 

Permeable sediments, which represent more than 50% of the continental shelves, have been largely neglected as a potential source of Fe in current global estimates of benthic dissolved iron Fed fluxes. There are open questions regarding the effects of a range of factors on Fed fluxes from these deposits, including seasonal dynamics and the role of bioirrigation. To address these gaps, we performed laboratory-based sediment incubation experiments with muddy sands during summer (21 °C) and winter (7 °C). We used bioirrigation mimics to inject overlying water into the permeable sediment with patterns resembling the bioirrigation activity of the prolific bioturbating polychaete,Clymenella torquata. Newly developed in-line Fe accumulators were used to estimate Fe fluxes with a recirculating set-up. We found high Fed fluxes from sandy sediments, especially in benthic chambers with simulated bioirrigation. In the winter fluxes reached 200 µmol Fed m-2 d-1 at the onset of irrigation and then decreased over the course of a 13-day experiment while in the summer fluxes from irrigated sediments reached 100 µmol Fed m-2 d-1 and remained high throughout a 7-day experiment. Despite different geochemical expressions of Fe-S cycling and resulting porewater Fed concentrations in winter and summer, large Fed fluxes were sustained during both seasons. Solid-phase and porewater concentration profiles showed that maximum concentrations of key constituents, including total solid-phase reactive Fe, and porewater Fed and ammonium, were located closer to the sediment water interface (SWI) in irrigated cores than in non-irrigated cores due to the upward advective transport of dissolved porewater constituents. This upward transport also facilitated Fed fluxes out of the sediments, especially during times of active pumping. Our study demonstrates the potential for large Fed fluxes from sandy sediments in both summer and winter, despite relatively low standing stocks of labile organic matter and porewater Fed. The primary driver of these high fluxes was advective porewater transport, in our study induced by the activity of infaunal organisms. These results suggest that permeable sediments, which dominate shelf regions, must be explicitly considered in global estimates of benthic Fed fluxes, and cannot be simply extrapolated from estimates based on muddy sediments.