The naturally-occurring radionuclides protactinium-231 (²³¹Pa) and thorium-230 (²³⁰Th) are produced at approximately uniform rates in the ocean and thought to be removed from the water column through a reversible exchange with settling particles. Recent measurements along the U.S. GEOTRACES North Atlantic transect (GA03) revealed two features which are at odds with current understanding about ²³¹Pa and ²³⁰Th cycling in the ocean: (i) a sharp decrease in dissolved ²³¹Pa (²³¹Pa${}_d$) and ²³⁰Th (²³⁰Th${}_d$) activities with depth below 2000–4000 m and (ii) very high particulate ²³¹Pa (²³¹Pa${}_p$) and ²³⁰Th (²³⁰Th${}_p$) activities near the bottom, at a number of stations between the New England continental shelf and Bermuda. Concomitant measurements of light attenuation from beam transmissometry showed that both features occur in benthic nepheloid layers (BNLs), which suggests that these features may stem, at least partly, from the presence of resuspended sediment in the deep water column.

Here we explore the behaviour of ²³¹Pa and ²³⁰Th in BNLs by using (i) radionuclide, optical, and hydrographic data from the western segment of GA03 (west of Bermuda) and (ii) a simplified model of particle and radionuclide cycling that includes a lateral particle source. First, the BNLs observed at GA03 stations are characterized from measurements of the beam attenuation coefficient converted to particle concentrations. At all stations, particle concentrations below the clear water minimum were the highest in the bottom mixed layer, whose thickness ranged from 95 to 320 m, and decreased generally with height above the bottom. The thickness of strong BNLs varied from 482 to 1358 m and the vertical integral of particle concentration in excess to that at the clear water minimum varied from $1\times 10^4$ to $2\times 10^6$ mg m⁻², among different stations. Second, the particle-radionuclide model is fitted to data from stations GT11-04 (New England continental rise) and GT11-08 (Hatteras abyssal plain), where samples for radionuclide analyses were collected in the BNL. The model can reproduce simultaneously the increase of particle concentration with depth, the low ²³¹Pa${}_d$ and ²³⁰Th${}_d$ in the BNLs, and the high ²³¹Pa${}_p$ and ²³⁰Th${}_p$ near the bottom. According to the model, at heights less than about 300 m above the seafloor, the dissolved phase was set primarily by a balance between adsorption and desorption, with vertical turbulent mixing playing a secondary role, whilst the particulate phase behaved largely as a non-reactive constituent supplied laterally and transported vertically by particle settling and turbulent mixing. Sensitivity tests with the model suggest that lateral particle sources near continental slopes and similar reliefs can produce significant biases both in the ²³⁰Th normalization method and in the interpretation of sediment ²³¹Pa/²³⁰Th records. Our findings yield insights into the influence of sediment resuspension and transport on ²³¹Pa and ²³⁰Th in the deep ocean and highlight the need for considering these processes in paleoceanographic applications.

天然存在的放射性核素 protactinium-231 ( ²³¹ Pa) 和钍-230 ( ²³⁰ Th) 在海洋中以大致均匀的速率产生，并被认为通过与沉降颗粒的可逆交换从水柱中去除。最近沿美国 GEOTRACES 北大西洋横断面 (GA03) 进行的测量揭示了两个与目前对海洋中²³¹ Pa 和²³⁰ Th 循环的理解不一致的特征：(i) 溶解的²³¹ Pa ( ²³¹ Pa${}_d$) 和²³⁰ Th ( ²³⁰ Th${}_d$) 深度低于 2000–4000 m 的活动和 (ii) 非常高的颗粒²³¹ Pa ( ²³¹ Pa${}_{磷}$) 和²³⁰ Th ( ²³⁰ Th${}_{磷}$) 活动在底部附近，在新英格兰大陆架和百慕大之间的一些站点。光束透射法对光衰减的伴随测量表明，这两个特征都出现在底栖类星体层 (BNL) 中，这表明这些特征可能至少部分源于深水柱中重新悬浮沉积物的存在。

在这里，我们探讨了²³¹ Pa 和²³⁰通过使用 (i) GA03 西段（百慕大西部）的放射性核素、光学和水文数据和 (ii) 包括横向粒子源的粒子和放射性核素循环的简化模型，BNL 中的 Th。首先，在 GA03 站观测到的 BNL 是通过测量转换为粒子浓度的波束衰减系数来表征的。在所有站点，低于清水最小值的颗粒浓度在底部混合层中最高，其厚度范围为 95 至 320 m，并且随着底部上方的高度普遍降低。强 BNL 的厚度从 482 到 1358 m 不等，并且超过清水最小值处的粒子浓度的垂直积分从$1\times 10^4$ 到 $2\times 10^6$mg m ^-2，在不同站点之间。其次，将粒子-放射性核素模型拟合到来自 GT11-04（新英格兰大陆隆起）和 GT11-08（哈特拉斯深海平原）站的数据，其中放射性核素分析的样本是在 BNL 中收集的。该模型可以同时再现粒子浓度随深度增加的情况，低²³¹ Pa${}_d$和²³⁰的Th${}_d$ 在 BNL 和高²³¹ Pa${}_{磷}$和²³⁰的Th${}_{磷}$ 靠近底部。根据该模型，在低于海底约 300 m 的高度处，溶解相主要由吸附和解吸之间的平衡决定，垂直湍流混合起次要作用，而颗粒相在很大程度上表现为非反应性成分通过颗粒沉降和湍流混合横向供应并垂直输送。该模型的敏感性测试表明，靠近大陆斜坡和类似地形的横向粒子源在²³⁰ Th 归一化方法和沉积物²³¹ Pa/ ²³⁰ Th 记录的解释中都会产生显着的偏差。我们的研究结果深入了解沉积物再悬浮和运输对²³¹深海中的Pa 和²³⁰ Th，强调了在古海洋学应用中考虑这些过程的必要性。