Ba distribution in surface sediments of the Southern Ocean

We present excess Ba (Baxs) data (i.e., total Ba corrected for lithogenic Ba) for surface sediments from a north-south transect between the Polar Front Zone and the northern Weddell Gyre in the Atlantic sector and between the Polar Front Zone and the Antarctic continent in the Indian sector. Focus is on two different processes that affect excess Ba accumulation in the sediments: sediment redistribution and excess Ba dissolution. The effect of these processes needs to be corrected for in order to convert accumulation rate into vertical rain rate, the flux component that can be linked to export production. In the Southern Ocean a major process affecting Ba accumulation rate is sediment focusing, which is corrected for using excess 230Th. This correction, however, may not always be straightforward because of boundary scavenging effects. A further major process affecting excess Ba accumulation is barite dissolution during exposure at the sediment-water column interface. Export production estimates derived from excess 230Th and barite dissolution corrected Baxs accumulation rates (i.e., excess Ba vertical rain rates) are of the same magnitude but generally larger than export production estimates based on water column proxies (234Th-deficit in the upper water column; particulate excess Ba enrichment in the mesopelagic water column). We believe export production values based on excess Ba vertical rain rate might be overestimated due to inaccurate assessment of the Baxs preservation rate. Barite dissolution has, in general, been taken into account by relating it to exposure time before burial depending on the rate of sediment accumulation. However, the observed decrease of excess Ba content with increasing water column depth (or increasing hydrostatic pressure) illustrates the dependence of barite preservation on degree of saturation in the deep water column in accordance with available thermodynamic data. Therefore correction for barite dissolution would not be appropriate by considering only exposure time of the barite to some uniformly undersaturated deep water but requires also that regional differences in degree of undersatuation be taken into account.

我们发布了大西洋扇区极锋带与威德尔环流北部之间、以及印度洋扇区极锋带与南极大陆之间的南北断面表层沉积物的过剩钡（excess Ba，缩写为Baxs，即经陆源钡校正后的总钡含量）数据。本研究聚焦于两类影响沉积物中过剩钡累积的关键过程：沉积物再分配与过剩钡溶解作用。为将累积速率转换为可与输出生产力挂钩的通量组分——垂直沉降速率，需校正这两类过程的影响。南大洋中影响钡累积速率的核心过程为沉积物聚集作用，可通过过剩²³⁰钍（²³⁰Th）进行校正，但由于边界清除效应的存在，该校正并非总能简便实施。另一类影响过剩钡累积的重要过程为沉积物-水界面暴露过程中的重晶石溶解作用。基于过剩²³⁰钍与经重晶石溶解校正的Baxs累积速率（即过剩钡垂直沉降速率）得到的输出生产力估算值，量级与基于水柱代用指标（上层水柱的²³⁴钍亏损、中层水柱的颗粒态过剩钡富集）得到的输出生产力估算值相近，但整体更高。我们认为，由于对Baxs的保存速率评估不够准确，基于过剩钡垂直沉降速率得到的输出生产力数值可能被高估。通常而言，重晶石溶解作用的校正会将其与埋藏前的暴露时长挂钩，而暴露时长取决于沉积物的累积速率。然而，观测到的过剩钡含量随水柱深度（或静水压力）增加而降低的现象，结合现有热力学数据表明，重晶石的保存程度取决于深水柱中的饱和状态。因此，仅通过考虑重晶石在某一均匀不饱和深水中的暴露时长来校正重晶石溶解作用并不恰当，还需纳入饱和亏损程度的区域差异因素。