A chemoautotrophy maximum is present in many anoxic basins at the sulfidic layer's upper boundary, but the factors controlling this feature are poorly understood. In 13 of 31 cruises to the Cariaco Basin, particulate organic carbon (POC) was enriched in 13C (δ13CPOC as high as −16‰) within the oxic/sulfidic transition compared to photic zone values (−23 to −26‰). During “heavy” cruises, fluxes of O2 and [NO3− + NO2−] to the oxic/sulfidic interface were significantly lower than during “light” cruises. Cruises with isotopically heavy POC were more common between 2013 and 2015 when suspended particles below the photic zone tended to be nitrogen rich compared to later cruises. Within the chemoautotrophic layer, nitrogen-rich particles (molar ratio C/N< 10) were more likely to be 13C-enriched than nitrogen-poor particles, implying that these inventories were dominated by living cells and fresh detritus rather than laterally transported or extensively decomposed detritus. During heavy cruises, 13C enrichments persisted to 1,300 m, providing the first evidence of downward transport of chemoautotrophically produced POC. Dissolved inorganic carbon assimilation during heavy cruises (n = 3) was faster and occurred deeper than during light cruises (n = 2). Metagenomics data from the chemoautotrophic layer during two cruises support prevalence of microorganisms carrying RuBisCO form II genes, which encode a carbon fixation enzyme that discriminates less against heavy isotopes than most other carbon fixation enzymes, and metatranscriptomics data indicate that higher expression of form II RuBisCO genes during the heavy cruises at depths where essential reactants coexist are responsible for the isotopically heavier POC.