The role of shelf nutrients on glacial‐interglacial CO₂: A negative feedback

Abstract

[1] In the past 800 thousand years and before industrialization, the largest variations in atmospheric CO 2 concentration (pCO 2 ) occurred in connection with the glacial cycles that characterized Earth's climate over this period. One curious feature of at least the last four glacial-interglacial cycles is that atmospheric pCO 2 reached about the same upper limit of 280 ppm during peak interglacial periods and about the same lower limit of 180 ppm during peak glacial periods. Here, we show using a numerical model of earth system that enhanced shelf sediment weathering during glacial sea level lowstands tends to raise pCO 2 even after carbonate compensation and thus stabilize pCO 2 from further reduction. This is because not all nutrients from weathering will be utilized by biology but more importantly because the spatial distributions of carbon and phosphorus from weathering become decoupled in such a way that carbon is preferentially stored in the upper ocean and phosphorus in the deep ocean. In addition, the C:P ratios in continental margin sediments are generally much higher than the Redfield ratio due to preferential remineralization of phosphorus in shelf sediment diagenesis. When these factors are accounted for in our model, the input of organic matter, which corresponds to the observed negative shift in ocean d 13 C during glacial periods, raises pCO 2 by approximately 14 ppm. The same mechanisms operating in the opposite directions during interglacial highstand tend to lower pCO 2 and stabilize it from further increase. The impact of sea level-driven continental shelf exposure and submersion of CO 2 is therefore a negative feedback that may have contributed to limiting the variation of Pleistocene pCO 2 to the observed 100 ppm range.Citation: Ushie, H., and K. Matsumoto (2012), The role of shelf nutrients on glacial-interglacial CO 2 : A negative feedback, Global Biogeochem. Cycles, 26, GB2039,

Different mechanisms of silicic acid leakage and their biogeochemical consequences

“…The interior ocean ventilation of MESMO 2 is well calibrated against measured and derived tracers such as natural and bomb radiocarbon, anthropogenic carbon, and chlorofluorocarbon. The model is successfully used in a number of process studies of the global carbon and climate [Chikamoto et al, 2008;Lee et al, 2011;Matsumoto and McNeil, 2013;Matsumoto et al, 2010;Matsumoto and Yokoyama, 2013;Sun and Matsumoto, 2010;Ushie and Matsumoto, 2012] as well as in community-wide model intercomparison projects [Archer et al, 2009;Cao et al, 2009;Eby et al, 2013;Joos et al, 2012;Weaver et al, 2012;Zickfeld et al, 2013].…”

Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric <i>p</i>CO<sub>2</sub> and seawater composition over the last 130 000 years: a model study

“…Applying very high molar C : P ratios for POM in shelf sediments (ca. 200), it was previously calculated that shelf weathering resulted in a net increase rather than decrease in atmospheric pCO 2 (Ushie and Matsumoto, 2012). However, most of the phosphorus in shelf sediments and riverine particles is not organic but bound in other reactive, inorganic phases such as carbonate-fluoro-apatite (Berner and Rao, 1994) which release DP when exposed to weathering (Ruttenberg, 1992;Ruttenberg and Berner, 1993).…”

“…Phosphate cycling in marine sediments is affected by oxygen conditions in ambient bottom waters and sediments (Krom and Berner, 1981;Van Cappellen and Ingall, 1994;364 K. Wallmann et al: Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization Neritic carbonate burial Wallmann, 2003). Phosphate is released from sediments under suboxic and anoxic conditions due to the reduction of iron and manganese oxides and the preferential degradation of P-bearing organic matter (POP).…”

MESMO 2: a mechanistic marine silica cycle and coupling to a simple terrestrial scheme

“…The model is also well calibrated on centennial time scale with respect to the abundance of natural 14 C ( 14 C) in the deep Pacific and Indian. MESMO 1 has been used successfully in a number of carbon and climate process studies (Lee et al, 2011;Matsumoto and McNeil, 2013;Matsumoto and Yokoyama, 2013;Sun and Matsumoto, 2010;Ushie and Matsumoto, 2012) as well as in model intercomparison projects Eby et al, 2012;Weaver et al, 2012;Zickfeld et al, 2012).…”