²¹⁰Po and ²¹⁰Pb distributions during a phytoplankton bloom in the North Atlantic: Implications for POC export

Highlights

• Changes in the distributions of ²¹⁰Po and ²¹⁰Pb in the North Atlantic Ocean (46°N, 20°W) over the course of a spring phytoplankton bloom.

• ²¹⁰Po and ²¹⁰Pb distributions reflect course of the bloom, with ²¹⁰Po deficits relative to ²¹⁰Pb increasing over time.

• Particle uptake of ²¹⁰Po in early bloom: deficits relative to ²¹⁰Pb as particles aggregate and sink as nutrients are depleted.

• ²¹⁰Po deficit (150 m) is converted to the sinking flux of particulate organic carbon (POC) using the POC/²¹⁰Po.

• Po-derived POC fluxes agree (factor of 2) with those determined independently using floating sediment traps and ²³⁴Th/²³⁸U.

Abstract

During the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS), water column sampling for particulate and dissolved ²¹⁰Po and ²¹⁰Pb was performed four times (26 April and 4, 20, 30 May 1989) during a month-long Lagrangian time-series occupation of the NABE site, as well as one-time samplings at stations during transit to and from the site. There are few prior studies documenting short-term changes in ²¹⁰Po and ²¹⁰Pb profiles over the course of a phytoplankton bloom, and we interpret the profiles in terms of the classical “steady-state” (SS) approach used in most studies, as well as by using a non-steady state approach suggested by the temporal evolution of the profiles. Changes in ²¹⁰Po profiles during a bloom are expectable as this radionuclide is scavenged and exported. During NABE, ²¹⁰Pb profiles also displayed non-steady state, with significant increases in upper water column inventory occurring midway through the experiment. Export of ²¹⁰Po from the upper 150 m using the classic “steady-state” model shows increases from 0.5 ± 8.5 dpm m⁻² d⁻¹ to 68.2 ± 4.2 dpm m⁻² d⁻¹ over the ~one-month occupation. Application of a non-steady state model, including changes in both ²¹⁰Pb and ²¹⁰Po profiles, gives higher ²¹⁰Po export fluxes. Detailed depth profiles of particulate organic carbon (>0.8 μm) and particulate ²¹⁰Po (>0.4 μm) are available from the 20 and 30 May samplings and show maxima in POC/Po at ~37 m. Applying the POC/²¹⁰Po ratios at 150 m to the “steady state” ²¹⁰Po fluxes yields POC export from the upper 150 m of 8.2 ± 1.5 mmol C m^{− 2} d⁻¹ on 20 May and 6.0 ± 1.6 mmol C m⁻² d⁻¹ on 30 May. The non-steady state model applied to the interval 20 to 30 May yields POC export of 24.3 mmol C m⁻² d⁻¹. The non-steady state (NSS) ²¹⁰Po-derived POC fluxes are comparable to, but somewhat less than, those estimated previously from ²³⁴Th/²³⁸U disequilibrium for the same time interval (37.3 and 45.0 mmol m⁻² d⁻¹, depending on the POC/Th ratio used). In comparison, POC fluxes measured with a floating sediment trap deployed at 150 m from 20 to 30 May were 11.6 mmol m⁻² d⁻¹. These results suggest that non-steady state Po-derived POC fluxes during the NABE agree well with those derived from ²³⁴Th/²³⁸U disequilibrium and agree with sediment trap fluxes within a factor of ~2. However, unlike the ²³⁴Th-POC flux proxy, non-steady stage changes in profiles of ²¹⁰Pb, the precursor of ²¹⁰Po, must be considered.