Isotope tracers (Delta δ¹³C) in Weddell Sea water during POLARSTERN cruise ANT-XII/3

Stable oxygen isotopic composition of sea water and stable carbon isotopes of dissolved inorganic carbon (DIC) on the continental shelf in the southern Weddell Sea are presented. Using the stations sampled during the summer 1995 two sections can be constructed, one closely parallel to the ice shelf edge and the other perpendicular to the upper continental slope. Generally, delta18O values clearly separate between different shelf water masses depending on the content of meteoric meltwater added during melting of glacial ice. Extrapolation of the mixing line between the cores of High Salinity Shelf Water (HSSW) and supercooled Ice Shelf Water (ISW) reveals delta18O values of the glacial ice of -27 per mil, whereas extrapolation of the mixing line between the delta18O values of the most-saline HSSW and lowest temperature ISW results in delta18O values of -34 per mil for glacial ice. These values point to an origin of meltwater from below the ice shelf, where ice is less depleted in 18O, since deep beneath the ice shelf close to the grounding line, values may reach -40 per mil. If values between -34 and -27 per mil are used as delta18O end member values for glacial ice, the amount of meltwater from the ice shelf that adds to the formation of ISW off the Filchner-Ronne Ice Shelf ranges from 0.2 to 0.8%, in agreement with previous studies based on delta18O and 4He. Carbon isotopic fractionation due to gas exchange between the atmosphere and the ocean at cold temperatures results in Delta delta13CDIC values of 0.20 +/- 0.17 per mil for Weddell Sea Deep Water, the water mass that ventilates the global abyssal ocean, typically defined as Antarctic Bottom Water (AABW). This confirms the low end of the range estimated previously (0.2-0.4 per mil), and thus corroborates the dominance of biology in shaping the deep and bottom water delta13C signal. It has been hypothesized that different modes of glacial/interglacial Antarctic bottom water formation may be separated by different stable isotopic compositions of deep-sea foraminiferal calcite. Here I show that differences between Delta delta13C and delta18O values of HSSW and ISW, both of which contribute to bottom water formation today, are too small to be resolved in deep and bottom water masses. Therefore, glacial/interglacial changes in relative proportions of these water masses in Antarctic deep and bottom water cannot be separated by stable isotopes of fossil benthic foraminiferal calcite.

本数据集收录了威德尔海南部大陆架海域海水的稳定氧同位素组成，以及溶解无机碳（dissolved inorganic carbon, DIC）的稳定碳同位素数据。基于1995年夏季采集的站位样品，可构建两条断面：一条与冰架边缘近乎平行，另一条垂直于上大陆坡。总体而言，δ¹⁸O值可依据冰川冰消融过程中掺入的大气降水融水含量，清晰区分不同的陆架水团。对高盐陆架水（High Salinity Shelf Water, HSSW）核心与过冷水冰架水（supercooled Ice Shelf Water, ISW）之间的混合线进行外推，可得到冰川冰的δ¹⁸O值为-27‰；而对盐度最高的HSSW与温度最低的ISW的δ¹⁸O值之间的混合线进行外推，则得到冰川冰的δ¹⁸O值为-34‰。上述数值表明，融水来源于冰架下方区域——此处的冰的¹⁸O亏损程度更低，因为在冰架下方靠近接地线的位置，δ¹⁸O值可低至-40‰。若将-34‰至-27‰区间的数值作为冰川冰的δ¹⁸O端元值，则菲尔希纳-龙尼冰架附近形成冰架水（ISW）过程中掺入的冰架融水占比为0.2%~0.8%，这与此前基于δ¹⁸O和氦-4（⁴He）开展的研究结果一致。低温条件下大气与海洋之间的气体交换引发的碳同位素分馏，使得威德尔海深层水（该水团为全球深海大洋提供通风作用，通常被定义为南极底层水（Antarctic Bottom Water, AABW））的Δδ¹³C_DIC值为0.20±0.17‰。这一结果验证了此前估算的区间下限（0.2‰~0.4‰），从而证实了生物作用在塑造深层及底层水的δ¹³C信号中占据主导地位。已有假说提出，不同冰期-间冰期模式下的南极底层水形成过程，可通过深海有孔虫方解石的稳定同位素组成加以区分。本研究表明，现今均参与底层水形成的HSSW与ISW的Δδ¹³C和δ¹⁸O值差异过小，无法在深层及底层水团中被分辨。因此，无法通过化石底栖有孔虫方解石的稳定同位素，区分南极深层及底层水中上述两类水团的相对比例变化。