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Organic and black carbon 13C and 14C through the Santa Monica Basin sediment oxic‐anoxic transition

Black carbon (BC) is a significant percentage of sedimentary organic carbon (SOC) at abyssal ocean sites, but its presence in shelf sediments is not well studied. Approximately 1600AD, Santa Monica Basin bottom waters shifted from oxic to very low oxygen (dysoxic) deposition conditions. Under oxic deposition conditions BC was 11 ± 4% of SOC, whereas after the overlying water became dysoxic (and sediments became anoxic), BC was 5.2 ± 1.2% of SOC. This shift may reflect the preferential remineralization of non‐black SOC under oxic conditions. There is an offset between BC and SOC 14C ages which changes with oxidation conditions, suggesting that BC storage is related to oxygen exposure and confirming a previously published report of the vulnerability of BC to sedimentary oxidation [ Middelburg, 1999 ]. Terrestrial carbon is 17 ± 5% of total SOC in this core's anoxic region, and 31 ± 11% of this terrestrial carbon is BC.