The balance between silica production and silica dissolution in the sea: Insights from Monterey Bay, California, applied to the global data set

Abstract

Silicon isotope tracers were used to examine the relative magnitude of silica dissolution and silica production in the Monterey Bay, California, upwelling system. A diatom bloom dominated by Skeletonema costatum and Chaetoceros spp. was encountered under conditions of moderate upwelling. Profiles of silica production and dissolution rates were obtained at seven stations that sampled both inside and outside the bloom. Integrated silica production rates ranged from 5. ). The mean ratio of integrated silica dissolution to integrated silica production (# D : # P) between the surface and the 0.1% light depth was 0.075, omitting one station with an unusually high # D : # P of 0.61, indicating that, on average, 93% of silica production was supported by new silicic acid. The f-ratio for diatom nitrogen use estimated from silicic acid and nitrate depletion curves and the mean # D : # P ratio was found to be 0.83, indicating that silica was being regenerated at a rate that was only slightly slower than that for particulate organic nitrogen. These data provide direct evidence confirming earlier hypotheses that the silica pump is weak in Monterey Bay. Analysis of the global data set on # D : # P in the surface ocean leads to the hypothesis that low # D : # P (ϳ0.10 or less) are typical of diatom bloom events, with # D : # P rising to values in excess of 0.50 during nonbloom periods. This pattern is shown to be consistent with previous estimates that the annual mean # D : # P ratio in the upper 200 m of the global ocean is 0.5-0.6. A regional analysis reveals that the fraction of silica production supported by new silicic acid varies as a hyperbolic function of the level of gross silica production similar to the variation in the f-ratio for N use with primary productivity. These trends suggest that diatom blooms, especially those occurring in more productive waters, are the main vectors of silica export in the sea, with the majority of the silica produced during nonbloom periods being recycled in the euphotic zone.The relative rates at which silica and organic matter are recycled in the euphotic zone determine whether diatomdominated pelagic ecosystems are driven to silicon or nitrogen limitation. Dugdale et al. (1995) formalized this concept in what they termed the ''silicate pump.'' In their model, grazing and remineralization processes in the euphotic zone tend to recycle organic matter more rapidly than they do particulate silica, causing sinking particles to be enriched in Si over N compared to the relative rates at which particulate organic nitrogen (PON) and biogenic silica (bSiO 2 ) are produced. Thus, the biological pump removes bSiO 2 from the euphotic zone with greater efficiency than it does PON, with the ultimate result that the system is driven toward Si limitation. Here, we use the terminology ''silica pump'' rather than the term ''silicate pump'' because it is biogenic silica, not dissolved silicon, that is exported (i.e., pumped) from the surface ocean to depth. The silica pump seems especially 1 Cor...

Dissolved silicon isotopic compositions in the East China Sea: Water mass mixing vs. biological fractionation

“…This pattern, in accordance with the distributions of fucoxanthin (a typical diatom pigment; data not shown), indicates higher standing stock of diatoms in the winter water column with, however, lower biological consumption. The apparent discrepancy between a near complete depletion of Si(OH) 4 not matched by an equivalent increase in BSi observed in this study, as well as in previous ones (e.g., Brzezinski et al 2003;Leblanc et al 2003;Cao et al 2012) demonstrates that considerable BSi production had occurred but the majority of it had already been removed from the water column by additional processes such as particle settling and grazing, which lowered the BSi concentrations on the East China Sea shelf in summer (overall <0.1 mmol L 21 except in the nearbottom waters at station PN08; Table 3). In support of this, Chen et al (2012) estimated that increasing temperature largely enhances phytoplankton losses to microzooplankton herbivory in eutrophic waters.…”

Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography

“…Since Si(OH) 4 concentrations of surface and some subsurface samples at stations north of the Polar Front (PF) were too low (<10 µmol l −1 ) to directly apply the Si purification procedure required for Si-isotopic measurement (De La Rocha et al, 1996), a Si(OH) 4 preconcentration step was performed. It was achieved using a protocol adapted by Brzezinski et al (2003) and Reynolds et al (2006) from the MAGIC method (Karl and Tien, 1992). Briefly, the method consists of one or two steps (depending of the Si-recovery of the first step) of the co-precipitation of Si(OH) 4 with brucite (Mg(OH) 2 ), by increasing pH either with NaOH 1 µmol l −1 (20 ml l −1 seawater, Reynolds et al, 2006) or NH 4 OH 13.5 µmol l −1 (6 ml l −1 seawater Brzezinski et al, 2003).…”

“…It was achieved using a protocol adapted by Brzezinski et al (2003) and Reynolds et al (2006) from the MAGIC method (Karl and Tien, 1992). Briefly, the method consists of one or two steps (depending of the Si-recovery of the first step) of the co-precipitation of Si(OH) 4 with brucite (Mg(OH) 2 ), by increasing pH either with NaOH 1 µmol l −1 (20 ml l −1 seawater, Reynolds et al, 2006) or NH 4 OH 13.5 µmol l −1 (6 ml l −1 seawater Brzezinski et al, 2003). The precipitates were recovered by centrifugation and then redissolved with HCl.…”

“…After combustion of the silicomolybdate precipitate in covered Pt crucibles at 1000°C, the pure cristobalite phase was transferred to pre-cleaned polypropylene vials. Dissolution of cristobalite was done in a dilute HF/HCl mixture as described in Cardinal et al (2003). Isotopic measurements were carried out on a Nu Plasma MC-ICP-MS (ULB, Brussels) using Mg external doping in dry plasma mode following Abraham et al (2008).…”

“…The bSiO 2 dissolution:production (D:P ) ratio is assumed to increase linearly (from 0.0 to 1.0 over the expected range Brzezinski et al, 2003;Fig. 5d):…”