Silicic acid leakage from the Southern Ocean: A possible explanation for glacial atmospheric pCO₂

Abstract

Using a simple box model, we investigate the effects of a reduced Si:N uptake ratio by Antarctic phytoplankton on the marine silica cycle and atmospheric pCO2. Recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added [Hutchins and Bruland, 1998; Takeda, 1998; Franck et al., 2000]. The Southern Ocean may have supported diatoms with reduced Si:N uptake ratios compared to today during the dustier glacial times [Petit et al., 1999]. A similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as Phaeocystis. Our model shows that reduced Si:N export ratios in the Southern Ocean create excess silicic acid, which may then be leaked out to lower latitudes. Any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes CaCO3 production and therefore diminishes the carbonate pump. In our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric CO2 from the interglacial 277 to 230–242 ppm, depending on where the excess silicic acid is consumed. By comparison, the atmospheric pCO2 sensitivity of general circulation models to carbonate pump forcing is ∼3.5–fold greater, which, combined with carbonate compensation, can account for peak glacial atmospheric pCO2. We discuss the importance of the initial rain ratio of CaCO3 to organic carbon on atmospheric pCO2 and relevant sedimentary records that support and constrain this “silicic acid leakage” scenario.

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

“…Water masses upwelling in the equatorial Atlantic and Pacific basins have distinct sources, and it is possible that circulation patterns in the SO produce greater dissolved Si concentrations in Atlantic or Pacific source waters under certain conditions. Southern source water upwelling in the equatorial Pacific originates for the most part in the Pacific sector of the SO, as discussed in section 2 [Toggweiler et al, 1991;Dugdale et al, 2002;Matsumoto et al, 2002]. Southern source water in the equatorial Atlantic, however, mostly originates from the Indian sector of the SO.…”

“…The following paragraph provides a brief description of CaCO 3 compensation, the impact of Si cycling on SO and equatorial ecosystems and the SALH. A more detailed description can be found in a number of previous studies Matsumoto et al, 2002;Bradtmiller et al, 2006;Kienast et al, 2006b].…”

“…Water upwelling in the modern tropical oceans has a distinct low-silicic acid, high-nitrate signature, which can be traced back to the water's source in the Southern Ocean Matsumoto et al, 2002], where the following processes shape the Si/N ratio of nutrients introduced into mode waters. Upper Circumpolar Deep Water upwells south of the Antarctic Polar Front (APF) with dissolved Si:N molar ratios between 2:1 to 3:1 .…”

“…[3] The Silicic Acid Leakage Hypothesis (SALH) Matsumoto et al, 2002] suggests that a reorganization of the ocean silicon cycle could have accounted for a large portion of glacial-interglacial atmospheric CO 2 variability. According to the SALH, during glacial periods a larger fraction (compared to interglacials) of the dissolved silicic acid that upwells in the Southern Ocean (SO) went unconsumed by diatoms.…”

Evidence for a major change in silicon cycling in the subarctic North Pacific at 2.73 Ma

“…Brzezinski et al, 2002]. This observation led to the development of the ''silica leakage hypothesis,'' which relates the global N and Si biogeochemical cycles to the enhanced drawdown of atmospheric carbon dioxide during glacial periods through the availability of Fe in the Southern Ocean Matsumoto et al, 2002]. It was shown that the uptake of Si and N by diatoms can be altered by the availability of Fe [Hutchins and Bruland, 1998].…”