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The history of atmospheric oxygen ( P O 2 ) and the processes that act to regulate it remain enigmatic because of difficulties in quantitative reconstructions using indirect proxies. Here, we extend the ice-core record of P O 2 using 1.5-million-year-old (Ma) discontinuous ice samples drilled from Allan Hills Blue Ice Area, East Antarctica. No statistically significant difference exists in P O 2 between samples at 1.5 Ma and 810 thousand years (ka), suggesting that the Late-Pleistocene imbalance in O 2 sources and sinks began around the time of the transition from 40- to 100-ka glacial cycles in the Mid-Pleistocene between ~1.2 Ma and 700 ka. The absence of a coeval secular increase in atmospheric CO 2 over the past ~1 Ma requires negative feedback mechanisms such as P co 2 -dependent silicate weathering. Fast processes must also act to suppress the immediate P co 2 increase because of the imbalance in O 2 sinks over sources beginning in the Mid-Pleistocene. 

Carbonate mud represents one of the most important geochemical archives for reconstructing ancient climatic, environmental, and evolutionary change from the rock record. Mud also represents a major sink in the global carbon cycle. Yet, there remains no consensus about how and where carbonate mud is formed. Here, we present stable isotope and trace-element data from carbonate constituents in the Bahamas, including ooids, corals, foraminifera, and algae. We use geochemical fingerprinting to demonstrate that carbonate mud cannot be sourced from the abrasion and mixture of any combination of these macroscopic grains. Instead, an inverse Bayesian mixing model requires the presence of an additional aragonite source. We posit that this source represents a direct seawater precipitate. We use geological and geochemical data to show that “whitings” are unlikely to be the dominant source of this precipitate and, instead, present a model for mud precipitation on the bank margins that can explain the geographical distribution, clumped-isotope thermometry, and stable isotope signature of carbonate mud. Next, we address the enigma of why mud and ooids are so abundant in the Bahamas, yet so rare in the rest of the world: Mediterranean outflow feeds the Bahamas with the most alkaline waters in the modern ocean (>99.7th-percentile). Such high alkalinity appears to be a prerequisite for the nonskeletal carbonate factory because, when Mediterranean outflow was reduced in the Miocene, Bahamian carbonate export ceased for 3-million-years. Finally, we show how shutting off and turning on the shallow carbonate factory can send ripples through the global climate system.