Underway measurements of dissolved gases along POLARSTERN cruise track ANT-XXVII/2

We present measurements of pCO2, O2 concentration, biological oxygen saturation (Delta O2/Ar) and N2 saturation (Delta N2) in Southern Ocean surface waters during austral summer, 2010-2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chla) concentrations in regions of frontal mixing and sea-ice melt. pCO2 and Delta O2 /Ar exhibited large spatial gradients (range 90 to 450 µatm and -10 to 60%, respectively) and co-varied strongly with Chla. However, the ratio of biological O2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time-scales of O2 and CO2 air-sea equilibration. We measured significant oceanic CO2 uptake, with a mean air-sea flux (~ -20 mmol m-2 d-1) that significantly exceeded regional climatological values. N2 was mostly supersaturated in surface waters (mean Delta N2 of +2.5 %), while physical processes resulted in both supersaturation and undersaturation of mixed layer O2 (mean Delta O2phys = 2.1 %). Box model calculations were able to reproduce much of the spatial variability of Delta N2 and Delta O2phys along the cruise track, demonstrating significant effects of air-sea exchange processes (e.g. atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment-corrected surface Delta O2 /Ar data, ranged from ~ -40 to > 300 mmol O2 m-2 d-1 and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and regions of sea-ice melt with shallow mixed layer depths, reflecting the importance of mixing in controlling surface water light and nutrient availability.

本研究报道了2010-2011年南半球夏季南大洋表层海水的二氧化碳分压（pCO₂）、氧气浓度、生物氧饱和度（ΔO₂/Ar）以及氮气饱和度（ΔN₂）观测数据。浮游植物生物量在不同水文学区差异显著，锋面混合区与海冰融化区域的叶绿素a（Chla）浓度较高。pCO₂与ΔO₂/Ar呈现显著空间梯度（范围分别为90~450 μatm与-10%~60%），且二者与Chla均呈显著共变关系。然而，生物氧累积量与溶解无机碳（DIC）消耗的比值显著低于光合化学计量比的理论预期，这反映了氧气与二氧化碳海-气平衡的时间尺度差异。本研究观测到显著的海洋CO₂吸收，平均海-气通量约为-20 mmol·m⁻²·d⁻¹，显著高于区域气候平均值。表层海水的N₂整体呈过饱和状态（平均ΔN₂为+2.5%），而物理过程则导致混合层O₂同时出现过饱和与欠饱和现象（平均ΔO₂_phys为2.1%）。箱式模型（Box model）计算结果能够复现航行轨迹上ΔN₂与ΔO₂_phys的大部分空间变异，证明了海-气交换过程（如大气压力变化与气泡注入）以及混合层夹卷对表层气体不平衡的显著影响。基于经夹卷校正的表层ΔO₂/Ar数据推导得到的净群落生产力（NCP）范围约为-40~>300 mmol O₂·m⁻²·d⁻¹，且与基于季节性混合层DIC亏损得到的独立NCP估算结果一致性较好。在水文学锋面区与混合层较浅的海冰融化区域观测到较高的NCP，这体现了混合作用在调控表层海水光照与养分可获得性方面的重要性。