Dissolved aluminium in the Weddell-Scotia Confluence and effect of Al on the dissolution kinetics of biogenic silica

Microbial mediation of benthic biogenic silica dissolution

“…According to Van Cappellen et al (2002), each of these factors may theoretically result in dissolution rate variations of about one order of magnitude. Additionally, spatiotemporal variations of BSi reactivity are supposedly related to biomineralization conditions in surface waters (Van Bennekom et al, 1991;Van Beusekom et al, 1997). Nevertheless, a consistent BSi reactivity pattern should be expected within a given oceanographic setting.…”

“…Consequently, the array of proposed mechanisms extends from diffusive transport through a boundary layer, or a solid layer leached by preferential dissolution (transport limitation), to decreasing surface area due to dissolution, formation of coatings, adsorbtion of inhibitors, or precipitation of authigenic silicates (surface chemistry limitation) (Ragueneau et al, 2000;Greenwood et al, 2001). For instance, a conclusive empirical relation in field and in vitro data exists between asymptotic Si concentration and the detrital to opal ratio in surface sediments, suggesting a significant lowering of BSi solubility by aluminum released from lithogenic mineral phases (Van Bennekom et al, 1991;Van Cappellen and Qiu, 1997a;Dixit and Van Cappellen, 2003). However, in vitro BSi dissolution seems to be strongly accelerated by bacterial hydrolytic ectoenzymes, and to correlate with bacterial metabolic activity Azam, 1999, 2001;Bidle et al, 2002).…”

Possible mechanism of structural incorporation of Al into diatomite during the deposition process I. Via a condensation reaction of hydroxyl groups

“…For the former, Al uptake of diatoms during frustule biosynthesis does not exceed 0.8%, even when the diatoms are grown in high Al concentrations [24,33]. For the latter, previous studies have shown the Al incorporation into the framework of diatomite by mimicking the deposition process of the diatom shell at the sediment-water interface [26,22,34].…”

“…Because the existence of Al in the structure of diatomite is demonstrated to effectively lower the solubility of diatomaceous silica and is beneficial to the preservation of diatomite [13][14][15][16][17], many studies focus on the properties of structural Al in diatomite, such as its content and coordination number (tetrahedral or octahedral coordination) [18][19][20][21][22]. In addition, researchers have estimated the transport of Al from the oceans' surface to the sediment [23][24][25] by studying the structural Al in diatomite. This is an effective estimator because the Al/Si ratio can be as high as 0.16 [14,15,26], and the diatomaceous earth form is the most abundant form of silica on earth [27].…”

A comparison of the Si/Al and Si/time wet-alkaline digestion methods for measurement of biogenic silica in lake sediments

“…Within the water column, levels of diatom-bound Al are minimal with Al/Si ratios of 8.3x10 -3 to 7.0x10 -5 (Lewin 1961;Martin and Knauer 1973;Kamatani 1974;van Bennekom et al 1989;Schlüter and Rickert 1998;Beck et al 2002;Gehlen et al 2002). Although additional Al can become incorporated into the diatom frustule at the sediment-water interface (van Bennekom et al 1988(van Bennekom et al , 1991Dixit et al 2001;Dixit and van Cappellen 2002;van Cappellen et al 2002;Gehlen et al 2002;Rickert et al 2002;Koning et al 2007), the overall amount of Al originating from diatoms is likely to be negligible relative to contributions from non-BSi, particularly when sample BSi concentrations are low. Although van Bennekom et al (1989) reported fossilised diatom Al concentrations of up to 6 wt.% in sediments from the Zaire/Congo deep-sea fan, a wealth of other data indicates that fossilised diatom Al concentrations actually peak at c. 1.0 wt.%, with typical concentrations <0.3 wt.% and many concentrations c. 0.01 wt.% (e.g.…”