The climatology of the deep particle flux in the oligotrophic western North Atlantic gyre, 1978–2022

Abstract

The oceanic particle flux controls, in part, ocean biogeochemical cycles and long-term carbon sequestration. The Oceanic Flux Program (OFP), the longest running time series of its kind, has continuously measured the deep particle flux in the oligotrophic Sargasso Sea southeast of Bermuda since 1978. This paper describes the deep flux climatology at the OFP site over the 1978–2022 time period. Mass flux at 500 m, 1500 m and 3200 m depths has averaged 27.2, 34.8 and 36.8 mg m⁻² d^-1, respectively. Carbonates comprise ∼ 60 % of the flux, with lesser amounts of organic matter, opal and lithogenics. Flux magnitude and composition vary seasonally with large interannual variability, particularly in the winter/spring flux maximum. Flux frequency distributions are strongly skewed, especially at 500 m depth where flux magnitude and compositional variability are highest. Flux seasonality, skewness and compositional heterogeneity decrease markedly with depth. A significant component of the deep flux is sourced from large particle production in the deep water column (e.g. suspended material repackaging) rather than directly from the overlying export flux. Lithogenic flux increases five-fold between 500 m and 3200 m depths, underscoring the importance of deep lateral advection and lithogenic particle removal via particle cycling processes. Multidecadal averages in deep carbon fluxes are compared with concurrent monthly data on overlying net primary production (PP) and surface export flux (EF, measured by drifting traps at 150 m depth) at the nearby Bermuda Atlantic Time Series (BATS) site. Carbon fluxes are temporally coherent throughout the water column (within the sample resolution) and lag primary production by ∼ one month. Approximately 0.6 %, 0.5 % and 0.4 % of PP reaches the 500, 1500 and 3200 m depth horizons, respectively, with the highest depth penetration occurring during the Jan-Mar period of peak primary production. Annually, 7.6 % and 6.2 % of the EF reaches the1500 and 3200 m depth horizons, respectively, with the highest transfer efficiency (9.1 % and 7.4 %, respectively at 1500 and 3200 m depths) during the post-bloom (Apr-Jun) period. The OFP flux climatology summarized here provides an important baseline for assessing future consequences of a changing climate on ocean functioning in the oligotrophic North Atlantic gyre.