Dataset: Foraminiferal denitrification and deep bioirrigation influence benthic biogeochemical cycling in a seasonally hypoxic fjord

This dataset contains the data used in the manuscript "Foraminiferal denitrification and deep bioirrigation influence benthic biogeochemical cycling in a seasonally hypoxic fjord".Abstract of the paper: Benthic macro- and micro-biota often play significant roles in controlling the biogeochemical dynamics in sediments. Their activity can be influenced by oxygen availability and impacted by the rise in global hypoxia in coastal regions over the last decades. To understand how these organisms interact with coastal hypoxia and influence sediment biogeochemistry, we undertook a study of early diagenesis in Bedford Basin, a seasonally hypoxic fjord on the West Atlantic coast in Nova Scotia, Canada, using a combination of observations and reaction-transport modeling. We observed that the seafloor was a source of ammonium and sink of nitrate with average fluxes of 2.2±1.8 and -0.9±0.7 mmol m^-2 d^-1respectively. The diffusive oxygen uptake was 14±4.6 mmol m^-2 d^-1and the total organic carbon content in collected sediment cores was 5-7% with a C/N ratio of ~10. The pyrite content increased steadily from 0.5 wt.% Fe at surface to ~2 wt.% Fe at 20 cm depth. Hydrogen sulfide was negligible down to 25 cm depth most of the time. The sediment was inhabited by tube-forming polychaete <i>Spiochaetopterus sp. </i>that formed tubes up to ~30 cm in length. The living foraminiferal assemblage in the top 5 cm sediment was found to be dominated (&gt;85%) by nitrate-storing and denitrifying benthic foraminifera <i>Stainforthia fusiformis</i>. These observations were used to develop and constrain a biogeochemical reaction-transport model. The model results suggest that the observed decrease in porewater concentrations of ammonium and dissolved inorganic carbon below 5 cm depth, was due to deep bioirrigation by tubeworms. This bioirrigation was responsible for almost 50% of their benthic efflux. The model further revealed that the deep bioirrigation along with bioturbation and iron cycling prevented accumulation of free sulfide in the top 25 cm sediment despite oxygen penetration depths of ~1 mm. Modelled organic carbon and nitrogen deposition was 25.2 and 2.9 mmol m^-2 d^-1 with burial efficiencies of 23% and 17%, respectively. The model indicated a total denitrification rate of 1.3 mmol N m^-2 d^-1 that was largely (~70%) driven by benthic foraminifera. This study reports the first evidence of foraminiferal denitrification in western Atlantic coastal sediments, and suggests that eukaryote mediated denitrification is an important driver of sediment N-loss in seasonally hypoxic environments, a process that has been traditionally assumed to be carried out by prokaryotic microbes.

本数据集包含论文《有孔虫反硝化作用与深层生物灌溉对季节性缺氧峡湾底栖生物地球化学循环的影响》（Foraminiferal denitrification and deep bioirrigation influence benthic biogeochemical cycling in a seasonally hypoxic fjord）所使用的相关数据。
论文摘要：底栖大型与微型生物通常在调控沉积物生物地球化学动态过程中发挥关键作用。它们的活动既受氧气可获得性的制约，也受到近几十年来沿海区域全球缺氧现象加剧的影响。为厘清这类生物与沿海缺氧环境的相互作用机制及其对沉积物生物地球化学过程的影响，我们结合原位观测与反应-输运模型（reaction-transport modeling），对位于加拿大新斯科舍省大西洋西海岸的季节性缺氧峡湾——贝德福德盆地（Bedford Basin）的早期成岩作用（early diagenesis）开展了研究。
我们观测到，海底为铵盐（ammonium）的源与硝酸盐（nitrate）的汇，平均通量分别为2.2±1.8和-0.9±0.7 mmol m⁻² d⁻¹。扩散耗氧量（diffusive oxygen uptake）为14±4.6 mmol m⁻² d⁻¹；采集的沉积物岩芯总有机碳（total organic carbon）含量为5%~7%，碳氮比（C/N ratio）约为10。黄铁矿（pyrite）含量从表层的0.5 wt.% Fe逐步升高至20 cm深度处的约2 wt.% Fe。多数情况下，25 cm深度以浅均未检测到可观的硫化氢（hydrogen sulfide）。沉积物中栖息着可形成栖管的多毛类（polychaete）*Spiochaetopterus sp.*，其栖管长度可达约30 cm。沉积物表层5 cm以浅的活体有孔虫群落以储存硝酸盐并可进行反硝化作用的底栖有孔虫（benthic foraminifera）*Stainforthia fusiformis*占绝对优势（占比>85%）。
上述观测结果被用于构建并约束生物地球化学反应-输运模型。模型结果显示，5 cm深度以深孔隙水中铵盐与溶解无机碳浓度的下降，源于多毛类栖管动物的深层生物灌溉作用。该生物灌溉作用贡献了近50%的底栖物质通量。模型进一步揭示，尽管氧气渗透深度仅约1 mm，深层生物灌溉与生物扰动（bioturbation）及铁循环共同阻止了25 cm深度以浅沉积物中游离硫化物的积累。模拟得到的有机碳与氮沉降通量分别为25.2和2.9 mmol m⁻² d⁻¹，埋藏效率分别为23%与17%。模型估算总反硝化速率为1.3 mmol N m⁻² d⁻¹，其中约70%由底栖有孔虫介导。本研究首次报道了大西洋西海岸沿海沉积物中存在有孔虫介导的反硝化作用，并指出真核生物介导的反硝化作用是季节性缺氧环境中沉积物氮流失的重要驱动过程——这一过程此前被认为仅由原核微生物（prokaryotic microbes）完成。