Brine formation in the Nordic seas: Stable isotope signals in foraminifera from Holocene and glacial sediments

During the last glacial period the climate of the Nordic seas shifted repeatedly on millennial time scales between warm interstadial periods and cold stadial periods including the socalled Heinrich events. The δ18O values of benthic foraminifera from the cold periods are low, whereas they are high from the warm interstadials. The low values have been attributed to brine formation carrying the signal from the low salinity surface water into deep water. Therefore, brines are considered to have played a major role for the abrupt millennial scale climate shifts during the last glaciation. However, very little is known of the isotopic composition of modern brines in the Nordic seas, limiting the interpretation of past data. The aim of the study is to contribute to a better understanding of the benthic 18O and 13C signals of brines from the Nordic seas and to increase our knowledge of the abrupt shifts in δ18O values during the cold stadials and Heinrich events. We have measured the oxygen and carbon isotope composition of benthic foraminifera in two cores from a brine-influenced shelf environment in Storfjorden, Svalbard. The brines flow down-slope and may reach 2000 m into the deep-intermediate water of the Greenland Sea. The results from Storfjorden are compared to deep sea millennial scale records of stable isotopes in benthic foraminfera from the last glacial period in the Nordic seas. The data show that in the Arctic Ocean and Nordic seas only brines formed from cold, salty surface waters can obtain sufficient density to contribute significantly to intermediate and deep water and they have high δ18O values. They would not significantly change the δ18O composition of the deep water. Therefore, dense brines would probably go undetected in the isotope records and they cannot be responsible for the very low δ18O values measured in the benthic foraminifera during stadials. Brines with low δ18O values are formed from cold, fresher water and have relatively low density and they are not able to penetrate through the Atlantic Water layer and into the deeper parts of the Nordic seas.