LAUSR

Significance The Bahama Banks produce huge volumes of carbonate mud. These sediments represent a major sink in the modern carbon cycle, and likely an even larger sink in the ancient carbon cycle. Yet, it is still debated where and how carbonate mud is formed. We use geochemical fingerprinting to show that mud cannot be derived from the breakdown and mixing of any combination of known carbonate grains or skeletal sources. Rather, mud represents a distinct seawater precipitate. We use ancient sedimentary and modern oceanographic evidence to show that high seawater alkalinity is required to produce carbonate mud. Finally, we illustrate how the size and strength of the shallow carbonate factory plays a first-order control in setting global pCO2 and climate.