Spatial sedimentary distribution, seasonality and the characteristics of organic matter on Fernando de Noronha insular shelf

Abstract The present study was conducted in the Fernando de Noronha archipelago (4°S/32°W). The objective is the evaluation of the spatial distribution and seasonal variations in the sediments and sedimentary organic matter in the northern insular shelf of Fernando de Noronha ("Mar de Dentro"). Nineteen surface sediment samples were collected between December 2013, July 2014 and November 2014. The studied methods included analysis of the grain size, coarse fraction, morphoscopy, total organic matter content, calcium carbonate, organic carbon, total nitrogen, sedimentary phosphorus (organic, inorganic and total), elemental ratios (C/N, C/P) and stable isotopic ratios (δ13C-δ15N). The results allowed to infer that there is no seasonal variation in sediment distribution. Whereas, the shelf sediments present a calcareous sandy sedimentary cover (CaCO3≈ 88.3%), predominantly of well-sorted fine sands, with low organic matter content (TOM<2.87%, TOC<4.29%, TP<0.44µmol/g) and are mainly of marine origin (δ13C ≈ -22.17%PDB). As conclusions was observed a spatial variation, where the southwestern part is dominated by gravelly and northeast part is composed for fine sand, conditioned by the bottom morphology, sediments sources and, wave action. The geochemistry showed some stations with relative enrichment in organic matter content (TOC>3.0%; TN>0.4%) of mixed origin (δ13C= -24.5 to -23.0%PDB), which were related to anthropogenic impacts and the biotic and abiotic local processes.

摘要 本研究于费尔南多·迪诺罗尼亚群岛（Fernando de Noronha archipelago，4°S/32°W）开展，旨在评估该群岛北部岛架"内海（Mar de Dentro）"区域内沉积物与沉积有机质的空间分布特征及季节变化规律。研究于2013年12月、2014年7月至2014年11月期间共采集19个表层沉积物样品，采用的分析方法包括粒度、粗组分、形貌观测、总有机质（Total Organic Matter, TOM）含量、碳酸钙、有机碳、总氮（Total Nitrogen, TN）、沉积磷（有机磷、无机磷及总磷）、元素比值（C/N、C/P）以及稳定同位素比值（δ¹³C-δ¹⁵N）分析。研究结果推断，沉积物分布不存在显著季节变化。该岛架沉积物整体为钙质砂质覆盖层（碳酸钙含量约88.3%），以分选良好的细砂为主，有机质含量较低（TOM<2.87%、总有机碳（Total Organic Carbon, TOC）<4.29%、总磷（Total Phosphorus, TP）<0.44µmol/g），且主要来源于海洋（δ¹³C≈-22.17‰ PDB）。研究结论显示，沉积物分布存在空间差异：西南部以砾质沉积物为主，东北部则以细砂为主，该分布格局受底床地貌、沉积物来源及波浪作用共同调控。地球化学分析表明，部分站位存在有机质相对富集现象（TOC>3.0%、TN>0.4%），其来源具有混合性（δ¹³C=-24.5~-23.0‰ PDB），该现象与人为活动影响及本地生物、非生物过程密切相关。