Oxygen concentrations [O 2 ] in the oceans have been decreasing and oxygen minimum zones (OMZ) expanding since 1960 (Schmidtko et al., 2017). Climate models also show this decrease and predict that this trend will not only continue but accelerate in the future (Oschlies et al., 2008(Oschlies et al., , 2018. To better understand the longer-term oxygen cycle in the past, on time scales typically exceeding centuries to millennia, we can use proxy reconstructions (Moffitt et al., 2015). Most proxy reconstructions assess in-situ bottom water [O 2 ] and rely on sedimentary samples (e.g.,

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