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.

中文翻译：

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从 12°N 到 70°S 印度洋和南大洋的表面无机碘形态

海洋碘形态已成为初级生产力、沉积输入和海洋氧化的潜在示踪剂。碘化物与海面臭氧的反应也被确定为对流层臭氧的最大沉积汇和大气中碘的主要来源。将这些过程准确纳入大气模型需要更好地了解海气界面处的碘化物浓度。海面碘化物的观测相对稀少，在印度洋盆地尤其缺乏。在这里，我们检查了在印度洋和南大洋印度段的 3 次航行中进行的 127 个新海面（≤10 m 深度）碘化物和碘酸盐观测。观测跨越纬度从~12°N 到~70°S，包括三个不同的水文状况：南印度亚热带环流、南大洋和北印度洋，包括南孟加拉湾。海面碘化物的浓度和空间分布遵循与其他海洋盆地相同的总体趋势，碘化物浓度随着纬度的增加（和海面温度的降低）而降低。然而，这种关系的梯度在印度洋亚热带水域比在大西洋或太平洋更陡峭，这表明它可能无法通过广泛使用的基于海面温度的参数化来准确表示。流域之间的梯度差异可能源于浮游植物群落组成和/或碘化物生产率的差异。热带北印度洋的碘浓度比其他地方更高，而且变化更大。在孟加拉湾遇到了两个极高的碘化物浓度（1241 和 949 nM），被认为与低氧条件下的沉积输入有关。排除这些异常值，海面碘化物浓度范围为 20 至 250 nM，中值为 61 nM。使用最先进的碘循环模型研究了对印度洋海面碘浓度的控制。发现多种相互作用因素驱动碘化物分布。通过垂直混合和混合层深度浅滩稀释是关键控制，两者也调节生物地球化学碘化物形成和损失过程的影响。排除这些异常值，海面碘化物浓度范围为 20 至 250 nM，中值为 61 nM。使用最先进的碘循环模型研究了对印度洋海面碘浓度的控制。发现多种相互作用因素驱动碘化物分布。通过垂直混合和混合层深度浅滩稀释是关键控制，两者也调节生物地球化学碘化物形成和损失过程的影响。排除这些异常值，海面碘化物浓度范围为 20 至 250 nM，中值为 61 nM。使用最先进的碘循环模型研究了对印度洋海面碘浓度的控制。发现多种相互作用因素驱动碘化物分布。通过垂直混合和混合层深度浅滩稀释是关键控制，两者也调节生物地球化学碘化物形成和损失过程的影响。