M. Rhein

Reduction of deepwater formation in the Greenland Sea during the 1980s: Evidence from tracer data

Hydrographic observations and measurements of the concentrations of chlorofluorocarbons (CFCs) have suggested that the formation of Greenland Sea Deep Water (GSDW) slowed down considerably during the 1980s. Such a decrease is related to weakened convection in the Greenland Sea and thus could have significant impact on the properties of the waters flowing over the Scotland-Iceland-Greenland ridge system into the deep Atlantic. Study of the variability of GSDW formation is relevant for understanding the impact of the circulation in the European Polar seas on regional and global deep water characteristics. New long-term multitracer observations from the Greenland Sea show that GSDW formation indeed was greatly reduced during the 1980s. A box model of deepwater formation and exchange in the European Polar seas tuned by the tracer data indicates that the reduction rate of GSDW formation was about 80 percent and that the start date of the reduction was between 1978 and 1982.

Radiocarbon calibration data for the 6th to the 8th millennia BC.

MINZE STUIVER* , ABSTRACT. 14C calibration curves derived from South German oak tree-ring series are pre sented. They cover the interval between 4400 and 7200 BC complementing existing data sets and extending them to older periods. The atmospheric 14C level before 6200 BC no longer follows the long-term sinusoidal trend fitted to the bristlecone data. This observation is supported by a tentative match of the Main 9 series.

3He in the Bransfield Strait waters: indication for local injection from back-arc rifting

Helium data from the waters of the Bransfield Strait, the southern Drake Passage and the northwestern shelf of the Weddell Sea are presented. The 3He profiles from the eastern and central basins of the Bransfield Strait show maxima (δ3He ≈ 7%) below the sill depths that separate the strait from the surrounding open ocean. The 3He excess is interpreted as a local injection of a 3He-rich helium component into the deep waters of the Bransfield Strait from backarc rifting. Tritiogenic 3He and excess 3He from mixing with Circumpolar Deep Water are excluded as possible sources. The estimated 3He/4He ratio of the injected helium component (2.4–5.0 × 10−6) is less than that of pure mantle helium and may contain radiogenic helium from continental crustal material which underlies the Bransfield Strait.

226Ra and Ba in northeast Atlantic deep water

Abstract 226 Ra and Ba data are presented for the northeast Atlantic below 2000 m depth from a section of stations 8°S to 46°N obtained by F.S. Meteor in 1981. The precision of the 226 Ra data (±1%) allows a resolution of structures in the 226 Ra distribution that is comparable to the resolution for Ba and nutrients. Linear property—property correlations in the upper parts of the depth range appear to be uniform within the observational error along the section for 226 Ra, Ba [Ba(nmol kg −1 ) = 3.88 Ra (dpm 100 kg −1 ) + 12.3], and alkalinity, whereas the slopes of these properties versus Si increase northwards from about 10 to 25 × 10 −4 dpm Ra μmol Si −1 . From this observation, and from studying the property distributions across the Romanche Fracture Zone, it is concluded that CaCO 3 dissolution must be an important process of 226 Ra and barium regeneration, while opal dissolution appears to be of minor importance. The water overflowing eastward over the sill of the Romanche Fracture Zone has 17.2 ± 0.3 dpm 100 kg −1 226 Ra, 79.5 ± 1.5·nmol kg −1 Ba and 2338 ± 4 μ eq kg −1 alkalinity. Within 800–1400 m above the sea floor, excess 226 Ra relative to the linear correlation with Ba further up in the water column is generally present, but there is also a reflection of 226 Ra decay at intermediate depths at the northern stations of the section. The latter effect appears to amount to only 3 ± 1.5%, but this magnitude is consistent with the 14 C distribution reported previously ( Schlitzer et al., Journal of Geophysical Research , 90 , 6945–6952, 1985). From the excess 226 Ra in the lowest 800–1400 m of the water column, the 226 Ra flux from the sediments is estimated to be 7.7 ± 4 × 10 −21 mol Ra (m 2 s) −1 , a value in accordance with results by Cochran ( Earth and Planetary Science Letters , 49 , 381–392, 1980).

High-precision measurement of oceanic 226Ra

Abstract A system capable of oceanic 226 Ra measurements with a precision of ±1% is described, which represents an improvement of approximately a factor of three over existing techniques. 222 Rn grown-in from 226 Ra decay in 14-l seawater samples is quantitatively transferred to, and measured in, proportional gas counters. Errors other than counting statistics are estimated not to exceed ±0.5%, which is consistent with repeated 226 Ra measurements on the same samples. A NE Atlantic 226 Ra depth profile (2000–5000 m) is reported as an example. It is found that with the precision reported here, certain hitherto unresolved features of the 226 Ra distribution in deep water become apparent.