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

In the past 800 thousand years and before industrialization, the largest variations in atmospheric CO₂ concentration (pCO₂) 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 pCO₂ reached about the same upper limit of 280ppm during peak interglacial periods and about the same lower limit of 180ppm 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 pCO₂ even after carbonate compensation and thus stabilize pCO₂ 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 δ¹³C during glacial periods, raises pCO₂ by approximately 14ppm. The same mechanisms operating in the opposite directions during interglacial highstand tend to lower pCO₂ and stabilize it from further increase. The impact of sea level-driven continental shelf exposure and submersion of CO₂ is therefore a negative feedback that may have contributed to limiting the variation of Pleistocene pCO₂ to the observed 100ppm range.