Data_Sheet_1_Surface Inorganic Iodine Speciation in the Indian and Southern Oceans From 12°N to 70°S.pdf

Marine iodine speciation has emerged as a potential tracer of primary productivity, sedimentary inputs, and ocean oxygenation. The reaction of iodide with ozone at the sea surface has also been identified as the largest deposition sink for tropospheric ozone and the dominant source of iodine to the atmosphere. Accurate incorporation of these processes into atmospheric models requires improved understanding of iodide concentrations at the air-sea interface. Observations of sea surface iodide are relatively sparse and are particularly lacking in the Indian Ocean basin. Here we examine 127 new sea surface (≤10 m depth) iodide and iodate observations made during three cruises in the Indian Ocean and the Indian sector of the Southern Ocean. The observations span latitudes from ∼12°N to ∼70°S, and include three distinct hydrographic regimes: the South Indian subtropical gyre, the Southern Ocean and the northern Indian Ocean including the southern Bay of Bengal. Concentrations and spatial distribution of sea surface iodide follow the same general trends as in other ocean basins, with iodide concentrations tending to decrease with increasing latitude (and decreasing sea surface temperature). However, the gradient of this relationship was steeper in subtropical waters of the Indian Ocean than in the Atlantic or Pacific, suggesting that it might not be accurately represented by widely used parameterizations based on sea surface temperature. This difference in gradients between basins may arise from differences in phytoplankton community composition and/or iodide production rates. Iodide concentrations in the tropical northern Indian Ocean were higher and more variable than elsewhere. Two extremely high iodide concentrations (1241 and 949 nM) were encountered in the Bay of Bengal and are thought to be associated with sedimentary inputs under low oxygen conditions. Excluding these outliers, sea surface iodide concentrations ranged from 20 to 250 nM, with a median of 61 nM. Controls on sea surface iodide concentrations in the Indian Ocean were investigated using a state-of-the-art iodine cycling model. Multiple interacting factors were found to drive the iodide distribution. Dilution via vertical mixing and mixed layer depth shoaling are key controls, and both also modulate the impact of biogeochemical iodide formation and loss processes.

海洋碘形态（marine iodine speciation）已成为表征初级生产力、沉积输入与海洋氧化还原状态的潜在示踪剂。海表碘离子（iodide）与臭氧的反应，同时被证实是对流层臭氧（tropospheric ozone）最大的沉积汇，也是大气中碘的主要天然来源。若要将这些过程精准纳入大气模型，需深化对海-气界面（air-sea interface）碘离子浓度的认知。当前海表碘离子的观测数据相对匮乏，印度洋海域尤为不足。本研究针对印度洋及南大洋印度洋扇区三次科考航次中获取的127组全新海表（水深≤10 m）碘离子与碘酸根（iodate）观测数据展开分析。此次观测覆盖纬度范围约为北纬12°至南纬70°，涵盖三类典型水文区域（hydrographic regimes）：南印度洋副热带环流（subtropical gyre）区、南大洋海域，以及包括孟加拉湾南部在内的北印度洋海域。海表碘离子的浓度与空间分布，与其他大洋盆地呈现一致的整体趋势：碘离子浓度随纬度升高（及海表温度降低）呈下降态势。不过，印度洋副热带海域的该浓度-纬度梯度，较大西洋与太平洋更为陡峭，这表明当前广泛采用的基于海表温度的参数化方案（parameterizations），或无法精准还原这一关系。不同大洋盆地间的梯度差异，可能源于浮游植物（phytoplankton）群落组成的不同，或是碘离子生成速率的差异。北印度洋热带海域的碘离子浓度，较其他区域更高且波动更为显著。孟加拉湾海域曾观测到两组极高的碘离子浓度（1241 纳摩尔每升（nM）与949 nM），推测其与低氧环境下的沉积输入过程相关。剔除这些异常值后，海表碘离子浓度范围为20~250 纳摩尔每升（nM），中位数为61 nM。本研究借助当前最先进的碘循环模型，探究了影响印度洋海表碘离子浓度的调控因素。研究发现，多种相互作用的因子共同驱动碘离子的空间分布：垂直混合稀释与混合层（mixed layer）深度变浅是核心调控因素，二者同时还可调节生物地球化学过程中碘离子的生成与损耗效应。