Intense benthic nepheloid layers in the Northwest Atlantic Ocean lead to unexpected distributions of dissolved and particulate thorium-230 and protactinium-231

28 January 2022

Chen and colleagues (2021, see reference below) explore the behaviour of thorium-230 (²³⁰Th) and protactinium-231 (²³¹Pa) in the benthic nepheloid layers (BNLs) by using (i) hydrographic, optical, and radionuclide data from the western segment of GA03 (between Bermuda and the US coast) and (ii) a simplified model of particle and radionuclide cycling that accounts for lateral particle sources. This exhaustive exploration leads to the following results:

At all the deep stations, BNLs of thickness varying from 482 m to 1358 m were observed. The basal layer of these BNLs fits well with the bottom mixed layer (BML, with maximum suspended particle concentrations reaching 2200 µg/l) whilst these concentrations decrease with height above the BML.
Fitting the model to particulate matter, ²³⁰Th and ²³¹Pa data reveals that the second-order rate constant of adsorption and the first-order rate constant of desorption are lower for both tracers in the BNL than in other oceanic environments.

At two deep stations located on the continental rise (GT11-04) and on the abyssal plain (GT11-08), and at heights less than about 300 m above the seafloor, the dissolved phase is governed primarily by sorption reactions, whilst the particulate phase behaves more as a non-reactive tracer supplied by the lateral particle source.
The ²³⁰Th and ²³¹Pa activities of particles vary significantly with the origin and the strength of particle source, resulting in (i) an overestimate of ²³⁰Th-corrected mass flux and (ii) an increase in ²³¹Pa/²³⁰Th ratio of particles settling to the seafloor.

Regarding their paleoceanographic applications, these results highlight the potential biases introduced by sediment redistribution in regions near oceanic margins where benthic nepheloid layers may be present, complicating the application of ²³⁰Th normalization and the interpretation of sediment ²³¹Pa/²³⁰Th records, for deep-ocean cores raised from near continental rises and similar reliefs.

Figure: Upper left displays a regional map illustrating the bathymetry of the Northwest Atlantic basin and GEOTRACES GA03 stations considered in this study. Lower left panels show (a) concentrations of particulate matter, (b) dissolved ²³⁰Th and ²³¹Pa activities, and (c) dissolved ²³⁰Th and ²³¹Pa activities in the water column over depth at these stations; a change in gradient in dissolved activities concomitant with an increase in particulate activities near the bottom coincides with the presence of the benthic nepheloid layer (BNL). Upper right panel is a schematic of the processes considered in the model developed to explain the unexpected distributions of ²³⁰Th and ²³¹Pa in the BNL. Lower right panels illustrate the bias introduced by strong particle source and varying particle source origin to ²³⁰Th-corrected mass flux and particle ²³¹Pa/²³⁰Th.

Reference:

Chen, S.-Y. S., Marchal, O., Lerner, P. E., McCorkle, D. C., & Rutgers van der Loeff, M. M. (2021). On the cycling of ²³¹Pa and ²³⁰Th in benthic nepheloid layers. Deep Sea Research Part I: Oceanographic Research Papers, 177, 103627. doi: 10.1016/j.dsr.2021.103627

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