Reinhold Bayer

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.

Wind-driven transport pathways for Eurasian Arctic river discharge

Distributions of temperature, salinity, and barium in near-surface waters (depth ≤ 50 m) of the Laptev Sea and adjacent areas of the Arctic Ocean are presented for the summers of 1993, 1995, and 1996. The tracer data indicate that while fluvial discharge was largely confined to the shelf region of the Laptev Sea in the summer of 1993, surface waters containing a significant fluvial component extended beyond the shelf break and over the slope and basin areas north of the Laptev Sea in the summers of 1995 and 1996. These distributions of fluvial discharge are consistent with local winds and suggest two principal pathways by which river waters can enter the central Arctic basins from the Laptev Sea. When southerly to southeasterly wind conditions prevail, river waters are transported northward beyond the shelf break and over the slope and adjacent basin areas. These waters can then enter the interior Arctic Ocean via upper layer flow in the vicinity of the Lomonosov Ridge. Under other wind conditions, river waters are steered primarily along the inner Laptev shelf and into the East Siberian Sea as part of the predominantly eastward coastal current system. These waters then appear to cross the shelf and enter the interior Arctic Ocean via upper layer flow aligned roughly along the Mendeleyev Ridge. The extent to which either pathway is favored in a given year is largely determined by local wind patterns during the summer months, when fluvial discharge is greatest and shelf waters are at the lowest salinity of their annual cycle.

Studies of deep water formation and circulation in the Weddell Sea using natural and anthropogenic tracers

The application of natural and anthropogenic trace substances in oceanographie studies of the Weddell Sea is reviewed. The potential of some steady-state and transient tracers (tritium, CFC-11 and CFC-12, 18O, and helium isotopes) for studies of deep water formation and circulation is discussed on the basis of data sets collected mainly on cruises of R/V “Polarstern” to the Weddell Sea during the 1980s. CFC/tritium ratio dating of young water masses is applied to estimate mean age and transit times of water involved in Weddell Sea Bottom Water formation. The history of the CFC-11/tritium ratio through time is derived for Weddell Sea shelf waters.

Suppression of bottom water formation in the southeastern Weddell sea

Abstract The lack of bottom water formation in the southeastern Weddel Sea is investigated on the basis of CTD, current meter, and oxygen isotope data obtained in 1986 during the Winter Weddell Sea Project and in summer 1989 during the European Polastern Study. The principal underlying factor in suppressing the formation of bottom water is the narrow continental shelf in the region. This leads to two consequences not obtained in the western Weddel Sea: (1) the coastal polynya is able to extend out well over deep water and over th eswift-moving Antarctic Coastal Current, which acts to inhibit the acculmulation of salt released freezing in the polynya; and (2) the upper portions of Warm Deep Water come into close proximity with the glacial ice shelf floating above the continental shelf, thus providing heat for melting at the base of the ice shelf. Budgets for heat and salt derived from the winter data, along with measurements of δ18O, indicate that this melting occurs at rates more than sufficient to compensate the combined effects of brine released by freezing in the polynya and the upward flux of salt from the Warm Deep Water. As a result, the Eastern Sshelf Water cannot acquire the salt concentrations needed fopr the formation of bottom water.

Mid 1980s distribution of tritium, 3He, 14C, and 39Ar in the Greenland/Norwegian seas and the Nansen Basin of the Arctic Ocean

Abstract The distributions of tritium 3 He , 14C and 39Ar observed in the period between 1985 and 1987 in the Greenland/Norwegian Seas and the Nansen Basin of the Arctic Ocean are presented. The data are used to outline aspects of the large-scale circulation and the exchange of deep water between the Greenland/Norwegian Seas and the Nansen Basin. Additionally, semi-quantitative estimates of mean ages of the main water masses found in these regions are obtained. Apparent tritium 3 He ages of the upper waters (depth 1,500m depth) of the Greenland/Norwegian Seas show apparent tritium 3 He ages between about 17 years in the Greenland Sea and 30 years in the Norwegian Sea. 39Ar based estimates of the Nansen Basin intermediate, deep and bottom water ages are 91+26−23, 161+50−44 and 277+33−31 years for Arctic Intermediate Water (AIW), Eurasian Basin Deep Water (EBDW) and Eurasian Basin Bottom Water (EBBW), respectively. Within the errors, age estimates of EBDW and EBBW based on 14 C tritium correlations are consistent with those derived from 39Ar (163 to 287 years for EBDW and 244 to 368 years for EBBW). A quantitative evaluation of the data in terms of deep water formation and exchange rates based on box model calculations is presented in an accompanying paper.

HYDROTHERMAL GASES OFFSHORE MILOS ISLAND, GREECE

Hydrothermal fluids emerge from the seafloor of Paleohori Bay on Milos. The gases in these fluids contain mostly CO2 but CH4 concentrations up to 2% are present. The stable carbon isotopic composition of the CO2 (near 0%) indicates an inorganic carbon source (dissociation of underlying marine carbonates). The carbon and hydrogen isotopes of most CH4 samples are enriched in the heavy species ([delta]13C = -9.4 to -17.8[per mille sign]; [delta]D = -102 to -189[per mille sign]) which is believed to be characteristic for an abiogenic production of CH4 by CO2-reduction (Fischer-Tropsch reactions). Depletions in the deuterium content of three CH4 samples (to -377%) are probably caused by unknown subsurface rock alteration processes. Secondary hydrogen isotope exchange processes between methane, hydrogen and water are most likely responsible for calculated unrealistic methane formation temperatures. We show that excess helium, slightly enriched in 3He, is present in the hydrothermal fluids emerging the seafloor of Paleohori Bay. When the isotopic ratio of the excess component is calculated a 3He/4Heexcess of 3.6 · 10-6 is obtained: This indicates that the excess component consists of about one third of mantle helium and two thirds of radiogenic helium. We infer that the mantle-derived component has been strongly diluted by radiogenic helium during the ascent of the fluids to the surface.

Constraints on origin and evolution of Red Sea brines from helium and argon isotopes

Brines from three depressions along the axis of the Red Sea, the Atlantis II, the Discovery and the Kebrit Deep, were sampled and analyzed for helium and argon isotopes. We identified two principally different geochemical fingerprints that reflect the geological setting of the deeps. The Atlantis II and the Discovery brines originating from locations in the central Red Sea show 4 He concentrations up to 1.2U10 35 cm 3 STP g 31 and a 3 He/ 4 He ratio of 1.27U10 35 . The MORB-like 3 He/ 4 He ratio is typical of an active hydrothermal vent system and clearly indicates a mantle origin of the helium component within the brines. 40 Ar/ 36 Ar ratios are as high as 305 implying that mantle-derived 40 Ar excesses of up to 3% of the total argon concentration are present in the brines and transported along with the mantle helium signal. The mean ( 4 He/ 40 Ar)excess ratio of 2.1 is in agreement with the theoretical mantle production ratio. In the Kebrit Deep, located in the northern Red Sea, we found a helium excess of 5.7U10 37 cm 3 STP g 31 . The low 3 He/ 4 He ratio of 1U10 36 points to a predominantly radiogenic source of the helium excess with only a minor mantle contribution of approximately 9%. We propose a new scenario assuming that the Kebrit brine accumulates a diffusive helium flux that migrates from deeper sedimentary or crustal horizons. In contrast to the Atlantis II and Discovery Deep, the Kebrit brine shows no sign of an active hydrothermal input. fl 2001 Elsevier Science B.V. All rights reserved.

The distribution of tritium and CFCs in the Weddell Sea during the mid-1980s

Abstract Transient tracer data (tritium, CFC11 and CFC12) from the southern, central and northwestern Weddell Sea collected during Polarstern cruises ANT III-3, ANT V-2/3/4 and during Andenes cruise NARE 85 are presented and discussed in the context of hydrographic observations. A kinematic, time-dependent, multi-box model is used to estimate mean residence times and formation rates of several water masses observed in the Weddell Sea. Ice Shelf Water is marked by higher tritium and lower CFC concentrations compared to surface waters. The tracer signature of Ice Shelf Water can only be explained by assuming that its source water mass, Western Shelf Water, has characteristics different from those of surface waters. Using the transient nature of tritium and the CFCs, the mean residence time of Western Shelf Water on the shelf is estimated to be approximately 5 years. Ice Shelf Water is renewed on a time scale of about 14 years from Western Shelf Water by interaction of this water mass with glacial ice underneath the Filchner-Ronne Ice shelf. The Ice Shelf Water signature can be traced across the sill of the Filchner Depression and down the continental slope of the southern Weddell Sea. On the continental slope, new Weddell Sea Bottom Water is formed by entrainment of Weddell Deep Water and Weddell Sea Deep Water into the Ice Shelf Water plume. In the northwestern Weddell Sea, new Weddell Sea Bottom Water is observed in two narrow, deep boundary currents flowing along the base of the continental slope. Classically defined Weddell Sea Bottom Water (θ ≤ −0.7°C) and Weddell Sea Deep Water (−0.7°C ≤ θ ≤ 0°C) are ventilated from the deeper of these boundary currents by lateral spreading and mixing. Model-based estimates yield a total formation rate of 3.5Sv for new Weddell Sea Bottom Water (θ = −1.0°C) and a formation rate of at least 11Sv for Antarctic Bottom Water (θ = −0.5°C).

Origin of trace gases in submarine hydrothermal vents of the Kolbeinsey Ridge, north Iceland

Two hydrothermal fields of the Kolbeinsey Ridge area, north of Iceland, show vent gas characteristics which can be related to the subsurface conditions. Helium isotopes (R/R-air = 9.8, 10.9) indicate a mantle-derived origin and can be considered as a mixture of MORE helium and a deep-mantle plume helium component. The carbon isotope composition of CO2 ranges between -2.4 and -7.8 parts per thousand. The less negative delta(13)C-CO2 values were-found at Grimsey. The data from Grimsey are very similar to those previously published and regarded as being characteristic for the Icelandic magmatic source. However, small amounts of biogenic CO2 and/or subsurface calcite precipitation are responsible for the lighter isotope values of CO2 from Kolbeinsey. CH4/He-3 ratios which are higher than in MORB indicate an additional (sedimentary) methane source for Kolbeinsey and Grimsey hydrothermal gases. The presence of higher hydrocarbons up to butane, together with the carbon isotope values of methane (delta(13)C = -26.1 to -39.8 parts per thousand) suggest a probably high-mature organic source within thick sediments of the Tjornes Fracture Zone and smaller depressions on the west side of the Kolbeinsey Ridge crest. Geochemical characteristics of hydrocarbons present in KR hydrothermal fluids are, however, typical for a mixed (thermogenic and high-temperature hydrothermal, e.g. EPR-type) origin. Moreover, it is likely that secondary processes such as bacterial oxidation and thermal cracking determined the geochemical characteristics of the gases

Sub sea floor boiling of Red Sea Brines: New indication from noble gas data

Hydrothermal brines from the Atlantis II Deep, Red Sea, have been sampled in situ and analyzed for noble gases. The atmospheric noble gas concentrations (Ne, Aratm, Kr, Xe) in the deepest layer (LCL) are depleted by 20 to 30% relative to the initial concentrations in ambient Red Sea Deep Water without a systematic mass fractionation between the different noble gases. Sub surface boiling during the hydrothermal circulation and subsequent phase separation is shown to be a consistent explanation for the observed depletion pattern. Using a conceptual model of phase separation under sub-critical conditions, in which gases are partitioned according to Henrys Law, we reconstruct the fluid history before injection into the Atlantis II Deep: after having circulated through evaporites and young oceanic crust, where it becomes enriched in HeMORB and ArMORB, the ascending fluid boils, and the residual liquid becomes depleted in noble gas concentrations. The depleted fluid rises to the sediment surface and feeds the Atlantis II basin. The relatively low boiling degree of about 3% (i.e., the percentage of fluid removed as vapor) derived from the model indicates that the Atlantis II system represents an early stage of boiling with relatively small gas loss, in contrast to hydrothermal systems at sediment-free mid-ocean ridges.