M. Rutgers van der Loeff

Trapping efficiencies of sediment traps from the deep Eastern North Atlantic:: the 230Th calibration

Bottom-tethered sediment traps deployed in the deep eastern North Atlantic between 54°N 20°W and 33°N 20°W (L1, L2, L3), at the European continental margin at 49°N (OMEX) and off the Canary Islands (ESTOC) were investigated for the determination of 230Th trapping efficiencies. The ratios of 230Th flux measured in the traps (Fa) to the expected 230Th flux from the production rate of 230Th in the overlying water column (Fp) ranged between 0.09 and 1.26. For the traps with deployment periods >300 days the interannual variation of Fa/Fp ratios (different years but same location and water depth) were up to 10%, suggesting that the average 230Th flux to the sediment traps did not vary significantly. The influence of lateral advection on the 230Th flux was taken into account either by applying a mass balance of 230Th and 231Pa or by assuming a constant removal rate of 230Th from the water column, an assumption based on similar 230Th concentration-depth profiles observed at most locations investigated. 230Th trapping efficiencies were between 9 and 143%, showing a trend of increasing efficiencies with increasing water depth. No relation was found between current velocities and 230Th trapping efficiencies. Our investigations suggest that the observation of constant or even increasing particle flux rates with increasing water depths in several sediment trap arrays investigated may be a result of sediment trap biases. The correction for the trapping biases is important for the understanding of the regional differences in the particle flux in the eastern North Atlantic.

Ecology and biogeochemistry of the Antarctic Circumpolar Current during austral spring : a summary of Southern Ocean JGOFS cruise ANT X/6 of R.V. Polarstern

The R.V. Polarstern cruise ANT X/6, part of the international Southern Ocean JGOFS programme, investigated phytoplankton spring bloom development and its biogeochemical effects in different water masses of the Atlantic sector of the Southern Ocean: the Polar Frontal region (PFr), the southern Antarctic Circumpolar Current zone (sACC), its boundary with the Weddell Gyre (AWB) and the marginal ice zone (MIZ). The relative roles of physical stability, iron limitation and grazing pressure in enhancing or constraining phytoplankton biomass accumulation were examined. Three sections were carried out between the PFr and the ice edge along the 6°W meridian from early October to late November 1992. This paper summarises the major findings of the cruise and discusses their implications for our understanding of Southern Ocean ecology and biogeochemistry. A major finding was the negligible build-up of plankton biomass and concomitant absence of CO2 drawdown associated with seasonal retreat of the ice cover. In striking contrast to this unexpected poverty of both the MIZ and the frontal region of the AWB, distinct phytoplankton blooms, dominated by different diatom species, accumulated in the PFr. Chlorophyll stocks in the sACC remained monotonously low throughout the study. Our findings confirm those of other studies that frontal regions are the major productive sites in the Southern Ocean and that input of meltwater and associated ice algae to the surface layer from a retreating ice edge is by itself an insufficient condition for induction of phytoplankton blooms. The blooms in the PFr developed under conditions of shallow mixing layers, high iron concentrations and relatively low grazing pressure. However, in all three blooms, high biomass extended to deeper than 70 m, which cannot be explained by either in situ growth or sinking out of a part of the population from the upper euphotic zone. Subduction of adjoining, shallower layers to explain depth distribution is invoked. Despite a clear CO2 drawdown in the Polar Frontal region, the highly variable conditions encountered render reliable estimation of annual CO2 fluxes in the Southern Ocean difficult.

Enhanced scavenging of 231Pa relative to 230Th in the South Atlantic south of the Polar Front: Implications for the use of the 231Pa/230Th ratio as a paleoproductivity proxy

Abstract The fractionation of 230Th and 231Pa was investigated throughout the Atlantic sector of the Southern Ocean. Published scavenging models generally assume that the 231Pa/230T ratio of surface sediments is primarily determined by the mass flux of particles. This relationship holds north of the Polar Front, where low primary productivity coincides with ratios of unsupported 231Pa/230Th—xs(231Pa/230Th) — in surface sediments below the production ratio of both radionuclides in the water column. However, we observed high xs231Pa/230Th ratios, conventionally interpreted as a high-productivity signal, in surface sediments south of the Polar Front, especially throughout the Weddell Sea, in contradiction with the low particle flux of this region. Measurements of both dissolved and particulate fractions of 231Pa and 230Th in the water column revealed a strong N S decrease in the Th/Pa fractionation factor, from typical open ocean values around 10 north of the Polar Front to values between 1 and 2 south of 60°S. This observation clearly indicates that the high xs231Pa/230Th ratios in surface sediments south of the Antarctic Circumpolar Current are produced by a N S increase in the relative scavenging efficiency of 231Pa relative to 230Th, most probably due to a change in the chemical composition of particulate matter, and not by a high mass flux. It is speculated that biogenic opal, suggested not to significantly fractionate231Pa and 230Th, may explain the enhanced scavenging of 231Pa to the south. This assumption is further supported by extremely high 231Pa/230Th ratios up to 0.34 in material collected with sediment traps south of the Polar Front, where fluxes are primarily determined by biogenic opal. Based on these results we conclude that, in regions where the sedimenting flux is dominated by biogenic opal, the 231Pa/230Th ratio is not a reliable indicator for the mass flux of particles, thus limiting its use as a paleoproductivity proxy in the Southern Ocean.

230Th in the eastern North Atlantic: the importance of water mass ventilation in the balance of230Th

The230Th distribution in the water column on a north-south transect in the eastern North Atlantic was measured using thermal ionization mass spectrometry (TIMS). Water samples at three locations (L1, 33°N 21.6°W; L2, 47.4°N 19.5W; and L3, 54.4°N 21.1°W) were filtered and the dissolved and particulate fractions were analysed for their230Th and232Th contents. In the upper water column ( 2000 m) at L2 suggested by our model (8-25 years) is probably too rapid in comparison with previous findings based on chlorofluorocarbon data. Enhanced scavenging of230Th would be an alternative explanation for the low concentration in NEADW at L2. With the scavenging-mixing model, we calculated an advective export flux of230Th of up to 50% of the production in the water column in the eastern North Atlantic. Advective loss of230Th has to be taken into account when using the230Th flux into sediments for paleoceanographic studies in ocean basins where deep water residence time is comparable to the230Th scavenging residence time.

Late Quaternary sediment dating and quantification of lateral sediment redistribution applying 230Thex: A study from the eastern Atlantic sector of the Southern Ocean

High-resolution records of the natural radionuclide230Th were measured in sediments from the eastern Atlantic sector of the Antarctic circumpolar current to obtain a detailed reconstruction of the sedimentation history of this key area for global climate change during the late Quaternary. High-resolution dating rests on the assumption that the230Thex flux to the sediments is constant. Short periods of drastically increased sediment accumulation rates (up to a factor of 8) were determined in the sediments of the Antarctic zone during the climate optima at the beginning of the Holocene and the isotope stage 5e. By comparing expected and measured accumulation rate of230Thex, lateral sediment redistribution was quantified and vertical particle rain rates originating from the surface water above were calculated. We show that lateral contributions locally were up to 6.5 times higher than the vertical particle rain rates. At other locations only 15% of the expected vertical particle rain rate were deposited.

Distribution of 230Th and 231Pa in the water column in relation to the ventilation of the deep Arctic basins

Distribution of 230Th and 231Pa in the water column was measured in the Eurasian basins of the Arctic Ocean, i.e. the Makarov, Amundsen and Nansen basins. The profiles obtained were compared to one previously published from the Alpha Ridge (Bacon et al., Earth and Planetary Science Letters, 95 (1989) 15–22. The Amundsen and Nansen basins are similar in their 23OTh and 231Pa distribution, whereas in the Makarov Basin and at the Alpha Ridge these radionuclides are comparatively enriched in the deep basin. This may be due to low particle flux in conjunction with low water mass renewal rates. The 230Th concentration in water depth < 1000 m in the Makarov Basin is relatively low compared with the Alpha Ridge, which may be caused by an exchange of water masses between the Amundsen and Makarov basins. In the Eurasian Basin Deep Water, 230Th profiles can be explained both by a scavenging model and by a mixing-scavenging model. Enhanced particle load caused by resuspension may explain the increased scavenging in the Eurasian Basin Bottom Water. The differences in scavenging between the Norwegian-Greenland Sea and the Eurasian basins at present are not large enough to cause significant fractionation between 230Th and 231Pa.

Shallow vs. deep-water scavenging of 231Pa and 230Th in radionuclide enriched waters of the Atlantic sector of the Southern Ocean

Abstract The scavenging of 231 Pa and 230 Th was investigated in the Atlantic Sector of the Southern Ocean by combining results from sediment trap and in situ filtration studies. We present the first high-resolution profile of dissolved 230 Th and 231 Pa in surface waters across the ACC, showing a dramatic southward increase of both radionuclides around the southern ACC Front at 53°S. High dissolved 231 Pa / 230 Th ratios combined with low 230 Th / 231 Pa fractionation factors (F) in these surface waters result in extremely high 231 Pa xs / 230 Th xs ratios of material collected in the shallow traps. Particulate 231 Pa xs / 230 Th xs ratios in a shallow trap near Bouvet Island increase continuously during the productive period in austral summer, and drop back in the low flux period. This behavior, following the Rayleigh fractionation principle, is interpreted to be due to an increase in the dissolved 231 Pa / 230 Th ratio in the euphotic zone resulting from preferential scavenging of 230 Th relative to 231 Pa , even in opal-dominated regions. In the post-bloom stage, the depleted radionuclide concentrations are replenished by upwelling of Circumpolar Deep Water. The high particulate 231 Pa xs / 230 Th xs signal is weakened during downward transport of the bloom particles in the water column by incorporation of deep suspended particles, which have a lower 231 Pa xs / 230 Th xs ratio. It is shown that under the special hydrographic conditions in the Southern Ocean scavenging from the upper water column significantly influences the budgets of 230 Th and 231 Pa in the sediment. Nevertheless, the budgets are still made up primarily by scavenging from the large standing stock of deep suspended particles.

Actinium-227 as a Deep-Sea Tracer: Sources, Distribution and Applications

AbstractActinium is one of the rarest naturally occurring elements on earth. We measured its longest-lived isotope 227Ac (half-life 21.77 years) for the first time in the water column of the Southeast Pacific, the central Arctic, the Antarctic Circumpolar Current (ACC) and the Weddell Gyre. Besides the profile in the Southeast Pacific, which confirms earlier findings about the role of diapycnal mixing for 227Ac distribution, we found three other different types of vertical profiles. These profiles point to a prominent role of advection for 227Ac distribution, especially in the Southern Ocean. Depending on the type of profile found, 227Ac is proposed as a tracer for different oceanographic questions.In the Southern Ocean, up to 4.93 ± 0.32 dpm*m﷓3 227Acex (227Ac in excess of its progenitor 231Pa) are found close to the sea floor, which are the highest concentrations ever observed in the ocean. Close to the sea surface in the Weddell Gyre (WG), 0.46 ± 0.05 dpm*m-3 227Acex are detected. We use 227Acex there to determine the upwelling velocity in the Eastern Weddell Gyre to be about 55 m*y-1.In the ACC, Upper and Lower Circumpolar Deep Water (UCDW and LCDW) are found to differ clearly in their 227Acex activity. High 227Acex activities are therefore a promising tracer for recent inputs of LCDW to the sea surface, which may help to understand the role of deep upwelling for iron inputs into Antarctic surface waters.The expected release of 227Ac is compared with 228Ra to make sure that the large near-surface excess in the water column of the Southern Ocean is not due to lateral inputs by isopycnal mixing. Results of a model indicate that the 227Ac/228Ra flux ratio is about 50 times lower on shelves than in the deep-sea. Data from the Central Arctic and from a transect across the ACC confirm that 228Ra and 227Acex differ strongly in their sources.The first measurements of 227Ac on suspended matter (less than 1.7% of total 227Ac close to the sea floor) indicate that the particle-reactivity of 227Ac is negligible in the open ocean, in agreement with earlier findings [Nozaki 1984].Despite the extremely low concentrations of 227Ac, new measurement techniques [Moore and Arnold 1996] point to a comfortable and comparably simple determination of 227Ac in the future. Finally, 227Acex may become a widely used deep-sea specific tracer.

Weddell sea contributes little to silicate enrichment in Antarctic Bottom Water

The enrichment of silicate in Antarctic Bottom Water (AABW) is associated in the Weddell Sea with a small but significant depletion in oxygen. Silicate fluxes from the sediment (33–210 and 210–480 mmol m−2 y−1 in deep-sea and shelf sediments, respectively) are far too low to account for the Si enrichment in AABW. Moreover, the ratio of silicate to nitrate and consequently to oxygen fluxes across the sediment-water interface (0.42 molar ratio Si/O2 flux) contrasts with the observed ratio of silicate enrichment to oxygen depletion in AABW (2.9 molar ratio). Silicate data in the Weddell Sea can be explained largely by conservative mixing: Si concentration and potential temperature of the cold bottom water formed by the overflow of Ice Shelf Water (ISW) from the Filchner Depression follow the conservative mixing line of ISW (θ–2.0°C) with the lower end of Warm Deep Water (θ 0.4°C) until this cold water reaches a depth of over 2000 m and flows under the AABW. The composition of the so-formed Weddell Sea Bottom Water is determined mainly by vertical mixing with AABW flowing into the Weddell Sea north of Kapp Norvegia (θ down to –0.7/–0.8°C). However, in situ dissolution of Si causes additional enrichments in the bottom water up to 20 μM towards the center of the Weddell Gyre, implying longer residence times. Siliceous sediments in the Enderby Basin produce far higher Si fluxes than the sediments of the Weddell Sea. It is likely that the Si enrichment in the AABW flowing into the Weddell Sea is mainly produced in the Enderby Basin by dissolution in the water column and in the sediments. Thus, although the Weddell Sea produces 70% (Carmack, 1977, A voyage of discovery, pp. 15–41) of the cold endmember of the AABW, it contributes little to its silicate enrichment.

Shelf‐basin exchange times of Arctic surface waters estimated from 228Th/228Ra disequilibrium

The transpolar drift is strongly enriched in 228Ra accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra activity ratio (AR = 0.8–1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that 228Th grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate 234Th) and total 234Th depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of 228Ra decay and 228Th ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.