Pelagic Sargassum is considered an ecological plague that is causing adverse economic impacts to the tourist and fishing industries in the Caribbean. However, its proliferation might be playing an important role to reduce global warming, as it removes a high content of CO from the atmosphere and transforms it into calcium carbonate, in its calcite phase, producing sediment after it dies. We quantified the amount of calcite in Sargassum samples collected from the Mexican Caribbean coast in 2019. Samples were divided into three parts: vesicles, thallus, and leaves. In each part, the amount of carbon, oxygen, and calcium was determined by means of X-ray energy dispersion to confirm the existence of a calcite crystalline phase. Imaging methodologies and IR spectroscopy complemented the structural studies. The thermogravimetric analysis determined that approximately 5% of the CO captured by the Sargassum was converted into calcite. Thus, by extrapolation, the Atlantic Sargasso Belt retained approximately 19.3 million tons of CO from 2011 to 2019.