Oxygen and carbon stable isotope tracers of Weddell Sea water masses: new data and some paleoceanographic implications

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, δ¹⁸O 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 δ¹⁸O values of the glacial ice of ?27‰, whereas extrapolation of the mixing line between the δ¹⁸O values of the most-saline HSSW and lowest temperature ISW results in δ¹⁸O values of ?34‰ 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‰. If values between ?34 and ?27‰ are used as δ¹⁸O 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 δ¹⁸O and ⁴He. Carbon isotopic fractionation due to gas exchange between the atmosphere and the ocean at cold temperatures results in Δδ¹³C_DIC values of 0.20±0.17‰ 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‰), and thus corroborates the dominance of biology in shaping the deep and bottom water δ¹³C 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 Δδ¹³C and δ¹⁸O 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.