The oceanic biological carbon pump (BCP) is thought to be driven by the vertical export of particulate organic carbon (POC) derived from fresh photosynthetically-fixed carbon in the surface waters to depth. Yet the chemical and isotopic composition of deep ocean POC is often inconsistent with this one-dimensional perspective of the BCP. Here, we investigate the sources and temporal dynamics of POC in the deep-sea using the concentration and carbon isotopic composition (Δ14C and δ13C) of POC intercepted by time-series sediment traps deployed in the Sargasso Sea near Bermuda over a 43-month time period. Seasonal variability in POC collected at 3200 m between 2012 and 2015 reveals a recurring springtime increase in Δ14C and δ13C, indicating rapid transfer of POC produced in the surface ocean to depth. In contrast, summer and autumn periods are characterized by lower Δ14C and δ13C values, reflecting higher contributions of millennial to centennial aged carbon sources. Isotopic mass balance calculations indicate that, on average, 63 ± 14 % of sinking POC originates from freshly produced surface-derived OC and 24 ± 15 % derives from mineral-free suspended pre-aged POC (partly surface-derived), while mineral-associated OC (8 ± 6 %), and DOC (5 ± 4 %) further contribute to the aged carbon signature. The contribution of surface-derived OC is highest during spring, while the fraction of aged mineral-bound OC, likely linked to Gulf Stream-driven lateral sediment transport from the Northwest Atlantic continental margin, is highest in autumn. Our findings highlight the complexity of the BCP in the open ocean, where pre-aged OC contributes significantly to deep-sea carbon sequestration, challenging traditional one-dimensional vertical perspectives of carbon export.
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