Supplemental Material: Northern-sourced water dominated the Atlantic Ocean during the Last Glacial Maximum

Southwest Atlantic hydrography; sample sites and neodymium isotope data; and estimates of benthic flux influence.<br>

Supplemental Material: Northern-sourced water dominated the Atlantic Ocean during the Last Glacial Maximum

Southwest Atlantic hydrography; sample sites and neodymium isotope data; and estimates of benthic flux influence.<br>

The Barents Sea Benthic Silica Cycle and its Sensitivity to Change: a Stable Isotopic and Reaction-Transport Model Study

&lt;p&gt;Biogeochemical cycling of silicon (Si) in the high latitudes has an important influence on the marine Si budget. The Barents Sea is divided aproximately equally into Arctic and Atlantic water (ArW and AW respectively) domains.&amp;#160; However, increases in the temperature and inflow of AW across the Barents Sea opening is driving an expansion of the AW realm. While the sensitivity of pelagic processes pertaining to primary production is receiving increasingly more attention, less is known of the effect on the benthic Si cycle. This knowledge gap could prove integral, as the flux of Si across the sediment-water interface (SWI) from Arctic shelf sediments could be up to 20% higher than that of riverine sources. This benthic flux is largely controlled by early diagenetic processes in sediment pore waters, including biogenic silica (bSi) dissolution and authigenic precipitation.&lt;/p&gt;&lt;p&gt;To improve our understanding of benthic Si dynamics in the Barents Sea and examine its sensitivity to future change, we analysed pore water and sediment samples from both the AW and ArW realms between 2017-2019 for dissolved silica (dSi) concentrations and stable silicon isotopic compositions. Moreover, to determine the composition and content of bSi, as well as Si sorbed onto metal oxides, we conducted a sequential digestion of surface sediment. Following this we coupled our analyses with reaction transport modelling to further improve our mechanistic understanding of the system and to quantitatively disentangle the relative importance of these diagenetic processes to pore water Si chemistry and benthic fluxes.&lt;/p&gt;&lt;p&gt;Our work suggests that both interannual and spatial variability of dSi are increased in the southern, AW region of the Barents Sea. Benthic flux estimates for the southern sites have been found to more than double (~30 to 100 mmol m&lt;sup&gt;-2 &lt;/sup&gt;yr&lt;sup&gt;-2&lt;/sup&gt;) between cruise years, compared to a more consistent flux in the north (~80 mmol m&lt;sup&gt;-2 &lt;/sup&gt;yr&lt;sup&gt;-2&lt;/sup&gt;). Therefore, future Atlantification of the northern region may enance the variability of dSi supply from the benthos to bottom waters, with potential consequences for diatom productivity in the region.&lt;/p&gt;

Seafloor sediment supply of nutrient silicon on the Greenland margin

&lt;p&gt;The biogeochemical cycling of nutrient silicon (Si) in the northern high latitudes has received increasing attention over recent years. This is in large part due to the discovery of silicon limitation of diatoms over seasonal timescales, the potential role of melting glaciers in supplying a significant amount of this nutrient to the coastal ocean, and the rapid environmental changes the polar ocean is experiencing as a result of global climate warming. However, our understanding of the nutrient Si in the polar ocean is severely restricted by the lack of knowledge of the benthic Si cycling and its controlling processes, which is due to the limited number of seafloor observations in the region. In this study, we address this knowledge gap through the acquisition of sediment pore water profiles and the execution of incubation experiments on sediment cores collected from the Greenland continental margin and the Labrador Sea.&lt;/p&gt;&lt;p&gt;Our results indicate a net (benthic) flux of dissolved silica (DSi) out of the sediment into the overlying seawater at the study sites. A new global compilation also reveals that benthic Si flux observed at our marginal sites are substantially higher than in the open ocean. This is likely because benthic flux in the open ocean is solely maintained by molecular diffusion along a concentration gradient, while there are additional processes: pore water advection and rapid dissolution of certain siliceous sponge groups, and other reactive silica phases, that contribute to the elevated benthic Si flux on the Greenland margin. This finding has important implications for existing evaluations of oceanic Si budgets, which have not accounted for any processes other than diffusion in the global estimation of benthic Si flux. Our results also suggest that strong benthic Si flux observed on the Greenland margin, combined with wind-driven coastal upwelling, could be a significant source of this nutrient to both the diverse (benthic) sponge communities and (planktonic) diatom productivity in the region. The magnitude of this benthic cycling could potentially rival the other continental inputs of Si in the northern high latitudes, as the first estimation of total benthic Si flux from the western Greenland shelf alone (0.04&amp;#8211;0.27 Tmol year&lt;sup&gt;-1&lt;/sup&gt;) is in the same order of magnitude as the total Si export from Greenland Ice Sheet (0.2 Tmol year&lt;sup&gt;-1&lt;/sup&gt;) and the pan-Arctic rivers (0.35 Tmol year&lt;sup&gt;-1&lt;/sup&gt;) respectively.&lt;/p&gt;

Bottom-water temperature controls on biogenic silica dissolution and recycling in surficial deep-sea sediments

This study calculated the dissolution rates of biogenic silica deposited on the seafloor and the silicic acid benthic flux for 22 Ocean Drilling Program sites. Simple models developed for two host sediment types – detrital and carbonate – were used to explain the variability of biogenic opal dissolution and recycling under present-day low (−0.3 to 2.14 °C) bottom-water temperatures. The kinetic constants describing silicic acid release and silica saturation concentration increased systematically with increasing bottom-water temperatures. When these temperature effects were incorporated into the diagenetic models, the prediction of dissolution rates and diffusive fluxes was more robust. This demonstrates that temperature acts as a primary control that decreases the relative degree of pore-water saturation with opal while increasing the silica concentration. The correlation between the dissolution rate and benthic flux with temperature was pronounced at sites where biogenic opal is accommodated in surficial sediments mostly comprised of biogenic carbonates. This is because the dissolution of carbonates provides the alkalinity necessary for both silica dissolution and clay formation; thus strongly reducing the retarding influence of clays on opal dissolution. Conversely, the silica exchange rates were modified by presence of aluminosilicates, which led to a higher burial efficiency for opal in detrital- than in carbonate-dominated benthic layers. Though model prediction of first-order silica early transformation suggests likely effects from surface temperatures (0–4 °C) on opal-CT precipitation over short geological times (