Pore-water chemistry of abyssal Arabian Sea surface sediments

Benthic fluxes and pore-water compositions of silicic acid, nitrate and phosphate were investigated for surface sediments of the abyssal Arabian Sea during four cruises (1995-1998). Five sites located in the northern (NAST), western (WAST), central (CAST), eastern (EAST), and southern (SAST) Arabian Sea were revisited during intermonsoonal periods after the NE- and SW-Monsoon. At these sites, benthic fluxes of remineralized nutrients from the sediment to the bottom water of 36-106, 102-350 and 4-16 mmol/m**2/yr were measured for nitrate, silicic acid and phosphate, respectively. The benthic fluxes and pore-water compositions showed a distinct regional pattern. Highest fluxes were observed in the western and northern region of the Arabian Sea, whereas decreasing fluxes were derived towards the southeast. At WAST, the general temporal pattern of primary production, related to the NE- and SW-Monsoon, is reflected by benthic fluxes. In contrast, at sites NAST, SAST, CAST, and EAST a temporal pattern of fluxes in response to the monsoon is not obvious. Our results reveal a clear coupling between the general regional pattern of production in surface waters and the response of the benthic environment, as indicated by the flux of remineralized nutrients, though a spatially differing degree of decoupling during transport and remineralization of particulate organic matter and biogenic opal was observed. This has to be taken into account regarding budget calculations and paleoceanographic topics.本研究于1995-1998年的四次科考航次中，对阿拉伯海深海区（abyssal Arabian Sea）表层沉积物中的底栖通量（benthic fluxes）及硅酸（silicic acid）、硝酸根（nitrate）、磷酸根（phosphate）的孔隙水（pore-water）组成展开了调查。选取阿拉伯海北部（NAST）、西部（WAST）、中部（CAST）、东部（EAST）与南部（SAST）的5个站位，在东北季风（NE-Monsoon）及西南季风（SW-Monsoon）结束后的季风间歇期（intermonsoonal periods）对其进行了重访考察。上述站位中，沉积物向底层水（bottom water）输出的再矿化营养盐（remineralized nutrients）底栖通量分别为：硝酸根36~106 mmol/(m²·yr)、硅酸102~350 mmol/(m²·yr)、磷酸根4~16 mmol/(m²·yr)。底栖通量与孔隙水组成呈现出显著的区域分布格局。阿拉伯海西部与北部区域的通量最高，通量值向东南方向逐渐降低。在WAST站位，与东北、西南季风相关的表层初级生产（primary production）的整体时间变化模式可通过底栖通量得到反映。与之相反，NAST、SAST、CAST及EAST站位的通量对季风响应的时间变化模式并不显著。本研究结果表明，尽管在颗粒有机质（particulate organic matter）与生物成因蛋白石（biogenic opal）的搬运及再矿化过程中，不同空间尺度的解耦程度存在差异，但表层水体初级生产的整体区域模式与底栖环境的响应（以再矿化营养盐通量为表征）之间存在显著的耦合关系。在进行物质预算计算及古海洋学（paleoceanographic）相关研究时，需充分考虑这一现象。