Physical oceanography, nutrients, and δ¹⁸O measured on water bottle samples during POLARSTERN cruise ARK-XXII/2

Extremely low summer sea-ice coverage in the Arctic Ocean in 2007 allowed extensive sampling and a wide quasi-synoptic hydrographic and d18O dataset could be collected in the Eurasian Basin and the Makarov Basin up to the Alpha Ridge and the East Siberian continental margin. With the aim of determining the origin of freshwater in the halocline, fractions of river water and sea-ice meltwater in the upper 150 m were quantified by a combination of salinity and d18O in the Eurasian Basin. Two methods, applying the preformed phosphate concentration (PO*) and the nitrate-to-phosphate ratio (N/P), were compared to further differentiate the marine fraction into Atlantic and Pacific-derived contributions. While PO*-based assessments systematically underestimate the contribution of Pacific-derived waters, N/P-based calculations overestimate Pacific-derived waters within the Transpolar Drift due to denitrification in bottom sediments at the Laptev Sea continental margin.Within the Eurasian Basin a west to east oriented front between net melting and production of sea-ice is observed. Outside the Atlantic regime dominated by net sea-ice melting, a pronounced layer influenced by brines released during sea-ice formation is present at about 30 to 50 m water depth with a maximum over the Lomonosov Ridge. The geographically distinct definition of this maximum demonstrates the rapid release and transport of signals from the shelf regions in discrete pulses within the Transpolar Drift.The ratio of sea-ice derived brine influence and river water is roughly constant within each layer of the Arctic Ocean halocline. The correlation between brine influence and river water reveals two clusters that can be assigned to the two main mechanisms of sea-ice formation within the Arctic Ocean. Over the open ocean or in polynyas at the continental slope where relatively small amounts of river water are found, sea-ice formation results in a linear correlation between brine influence and river water at salinities of about 32 to 34. In coastal polynyas in the shallow regions of the Laptev Sea and southern Kara Sea, sea-ice formation transports river water into the shelf's bottom layer due to the close proximity to the river mouths. This process therefore results in waters that form a second linear correlation between brine influence and river water at salinities of about 30 to 32. Our study indicates which layers of the Arctic Ocean halocline are primarily influenced by sea-ice formation in coastal polynyas and which layers are primarily influenced by sea-ice formation over the open ocean. Accordingly we use the ratio of sea-ice derived brine influence and river water to link the maximum in brine influence within the Transpolar Drift with a pulse of shelf waters from the Laptev Sea that was likely released in summer 2005.

2007年北冰洋夏季海冰覆盖度极低，使得研究人员得以开展大规模采样，并在欧亚海盆（Eurasian Basin）、马卡罗夫海盆（Makarov Basin）直至阿尔法海岭（Alpha Ridge）与东西伯利亚大陆边缘，采集得到一套大范围准同步的水文与δ¹⁸O（氧同位素18）数据集。本研究旨在确定盐跃层（halocline）中淡水的来源，通过结合盐度与δ¹⁸O数据，对欧亚海盆上部150米水体中的河水与海冰融水占比进行了定量分析。研究采用两种方法——基于预形成磷酸盐浓度（preformed phosphate concentration, PO*）与硝磷比（nitrate-to-phosphate ratio, N/P），将海洋组分进一步区分为大西洋源与太平洋源贡献；基于PO*的评估会系统性低估太平洋源水体的占比，而基于N/P的计算则会因拉普捷夫海（Laptev Sea）大陆边缘海底沉积物的反硝化作用，高估穿极漂流（Transpolar Drift）内的太平洋源水体占比。在欧亚海盆内，观测到一条东西走向的海冰净融化与净生成锋面；在以海冰净融化为主的大西洋水团影响区域之外，约30至50米水深处存在一层受海冰形成过程中释放卤水显著影响的水体，该水体在罗蒙诺索夫海岭（Lomonosov Ridge）上方达到浓度峰值，这一峰值的空间分布特征表明，陆架区域的信号以离散脉冲形式快速释放并通过穿极漂流输送。北冰洋盐跃层各层内，海冰来源卤水影响强度与河水占比的比值大致恒定；卤水影响强度与河水占比的相关性可分为两个聚类，分别对应北冰洋内两种主要的海冰形成机制：在开阔大洋或大陆坡冰间湖（polynyas，此处河水占比较低）中，当盐度约为32至34时，海冰形成过程会导致卤水影响强度与河水占比之间呈现线性相关；而在拉普捷夫海与喀拉海（Kara Sea）南部浅水区的沿岸冰间湖内，由于紧邻河口区域，海冰形成过程会将河水输送至陆架底层水体中，因此当盐度约为30至32时，二者间会形成第二种线性相关关系。本研究明确了北冰洋盐跃层的哪些层段主要受沿岸冰间湖的海冰形成过程影响，哪些层段主要受开阔大洋的海冰形成过程影响；据此，我们利用海冰来源卤水影响强度与河水占比的比值，将穿极漂流内的卤水影响峰值与2005年夏季释放的拉普捷夫海陆架水体脉冲关联起来。