Updated estimates of sedimentary potassium sequestration and phosphorus release on the Amazon shelf

The Fate of Sedimentary Reactive Iron at the Land‐Ocean Interface: A Case Study From the Amazon Shelf

“…Spiegel et al (2021) reported pore water profiles that are indicative of coincident removal of dissolved silica and potassium (K), which is consistent with the formation of authigenic silicate minerals via reverse weathering. Spiegel et al (2021) also reported elevated sedimentary K/Al across the entire Amazon shelf (redrawn in Figure 4b) and quantified sedimentary K uptake by multiplying excess K relative to Amazon River suspended material by the annual discharge of particulate material. Their findings are consistent with early studies of reverse weathering and sedimentary K uptake in the study area (e.g., Michalopoulos & Aller, 1995Michalopoulos et al, 2000).…”

“…Geochemical evidence for the formation of such clay minerals within Amazon shelf sediments has previously been presented based on the same sample set as analyzed within this study. Spiegel et al (2021) reported pore water profiles that are indicative of coincident removal of dissolved silica and potassium (K), which is consistent with the formation of authigenic silicate minerals via reverse weathering. Spiegel et al (2021) also reported elevated sedimentary K/Al across the entire Amazon shelf (redrawn in Figure 4b) and quantified sedimentary K uptake by multiplying excess K relative to Amazon River suspended material by the annual discharge of particulate material.…”

“…During this process, terrigenous Fe oxide minerals react with biogenic silica and cations dissolved in seawater to form authigenic clay minerals such as glauconite or potassium‐rich smectite (Baldermann et al., 2015; Mackenzie & Garrels, 1966; Michalopoulos & Aller, 1995; Michalopoulos et al., 2000; Scholz, Severmann, McManus, Noffke, et al., 2014). The incorporation of Fe into authigenic silicates has been demonstrated to be particularly prevalent in tropical shelf environments such as the Amazon shelf (Michalopoulos & Aller, 1995; Michalopoulos et al., 2000; Spiegel et al., 2021) where reactive Fe oxide minerals originating from intense tropical weathering in the hinterland are particularly abundant (Michalopoulos & Aller, 2004). Conversion of reactive Fe oxide minerals to authigenic clay minerals may explain low proportions of reactive Fe in continental margin sediments compared to river suspended particles, as these clay minerals are not considered part of the reactive Fe or Fe HR pools.…”

“…Even though clay minerals are generally not considered part of the highly reactive Fe pool, it is currently unknown if authigenic clay minerals are dissolved by the standardized sequential extraction scheme, which is widely applied to determine Fe HR in marine sediments and to evaluate early diagenetic Fe cycling in modern marine sediments (e.g., Baldermann et al., 2015; Henkel et al., 2016; Lenstra et al., 2019; Scholz, Schmidt, et al., 2019) and paleo‐redox conditions in the geological record (e.g., Poulton & Canfield, 2011; Poulton et al., 2010; Raiswell et al., 2018). In this study, we evaluate the impact of authigenic clay formation (i.e., reverse weathering) on sedimentary Fe speciation across the Amazon shelf, which is a well‐known type locality for reverse weathering (Michalopoulos and Aller, 1995, 2004; Spiegel et al., 2021). We also test the hypothesis that reverse weathering may reduce the proportion of reactive Fe in continental margin sediments relative to river suspended particles.…”

Trace metal stoichiometry of dissolved organic matter in the Amazon plume

“…Fluctuations in concentrations of DIP and pH in the inner estuary could therefore be largely explained by changes in phytoplankton productivity. In the outer estuary (15 < S < 35), we observed the highest chlorophyll a concentration (27.5 μg liter −1 ), and DIP was variable although consistently <0.2 μM, suggesting that a dynamic DIP cycle likely influenced by a reported large sedimentary flux of DIP at the time ( 37 ) and high phytoplankton biomass in the plume ( Fig. 2C ).…”