Brine formation recorded by stable isotopes of Recent benthic foraminifera in Storfjorden, Svalbard: palaeoceanographical implications

摘要

We discuss water oxygen isotopes (delta O-18(w)) and carbon isotopes of dissolved inorganic carbon (delta C-13(DIC)) of brine-enriched shelf water (BSW) from Storfjorden (southern Svalbard) in comparison to Recent benthic foraminiferal delta O-18(c) and delta C-13(c) calcified in the same water. We determined relatively high delta O-18(w) values of 0.15 +/- 0.03 parts per thousand VSMOW in BSW below sill depth at temperatures below -1.8 degrees C, and high delta O-18(c) values of 3.90 +/- 0.18 parts per thousand VPDB. Such high BSW delta O-18(w) cannot significantly deplete O-18(w) contents of Arctic Ocean deep water; furthermore, such high delta O-18(c) cannot be distinguished from delta O-18(c) values of 3.82 +/- 0.12 parts per thousand, calcified in warmer Arctic and Nordic seas intermediate and deeper waters. Today, in Storfjorden low benthic delta C-13(c) and high delta O-18(c) reflect the low delta C-13(DIC) and relatively high delta O-18(w) of BSW. High benthic delta O-18(c) is in contrast to expected low delta O-18(c) as brine rejection is widely thought to predominantly take place in surface water diluted by meteoric water with very low delta O-18(w). Low epibenthic delta C-13(c) values of 0.50 +/- 0.12 parts per thousand partly reflect low delta C-13(DIC) caused by enhanced uptake of atmospheric low delta C-13(CO2) decreased by anthropogenic activities. An adjustment for preindustrial higher values would increase delta C-13(c) by about 0.6 parts per thousand. Therefore, in Storfjorden brine formed before the industrial era would be characterized by both high delta C-13(c) as well as high delta O-18(c) values of benthic foraminiferal calcite. Our data may cast doubt on scenarios that explain negative excursions in benthic foraminiferal stable isotope records from the Atlantic Ocean during cold stadials in the last glacial period by enhanced brine formation in Nordic seas analogously to modern processes in Storfjorden.