Article

Investigating the use of 232 Th/ 230 Th as a dust proxy using co-located seawater and sediment samples from the low-latitude North Atlantic

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Abstract

The thorium isotope ratio ²³²Th/²³⁰Th can be measured in seawater and sediment samples, and has been used as a proxy to reconstruct lithogenic fluxes to the oceans for the modern day and the Pleistocene. There has not yet been a study testing the proxy using the ²³²Th/²³⁰Th ratio in seawater and the ratio recorded in the underlying sediment. In this study we use co-located core-top sediments and seawater samples from five seamount sites spanning the tropical North Atlantic to investigate the link between seawater and sediment ²³²Th/²³⁰Th ratios across a range of water depths. Our results indicate that a broad correlation exists between seawater and sedimentary ²³²Th/²³⁰Th ratios. Both seawater and sedimentary ²³²Th/²³⁰Th ratios record a signal consistent with decreasing lithogenic input east to west, from Africa to South America. However, calculated ²³²Th fluxes for the core-top sediment samples indicate a strong dependence on depth, with up to a factor of ∼4 difference from shallow (<600 m) to deep sites (>2900 m). This depth dependence is likely caused by either a deficit of ²³⁰Th burial at depth compared to the production in the overlying water column, through addition of ²³²Th, or by a combination of the two processes. By comparing seawater and sedimentary ²³²Th/²³⁰Th ratios we derive an apparent fractional solubility of ²³²Th of 29 ± 3%, in reasonable agreement with the upper end of existing estimates.

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... Over 50 years ' (1966-2019) worth of data have been compiled to create the global thorium database (n = 1,167) presented here (Adkins et al., 2006;Anderson et al., 2006Bausch, 2018;Böhm et al., 2015;Bohrmann, 2013;Borole, 1993;Bradtmiller et al., 2006Bradtmiller et al., , 2007Bradtmiller et al., , 2009Broecker, 2008;Broecker et al., 1993;Brunelle et al., 2007Brunelle et al., , 2010Causse & Hillaire-Marcel, 1989;Chase et al., 2003Chase et al., , 2014Chong et al., 2016;Costa, McManus, & Anderson, 2017;Crusius et al., 2004;Dekov, 1994;Denis et al., 2009;Dezileau et al., 2000Dezileau et al., , 2004Durand et al., 2017;Fagel et al., 2002;Francois et al., 1990, Francois et al., 1993Frank, Eisenhauer, Bonn, et al., 1995Fukuda et al., 2013;Galbraith et al., 2007;Geibert et al., 2005;Gherardi et al., 2005Gherardi et al., , 2009Gottschalk et al., 2016;Hickey, 2010;Hillaire-Marcel et al., 2017;Hoffmann et al., 2013Hoffmann et al., , 2018Jaccard et al., 2009Jaccard et al., , 2013Jacobel et al., 2017a;Jonkers et al., 2015;Kienast et al., 2007;Ku & Broecker, 1966;Kumar et al., 1995;Lam et al., 2013;Lamy et al., 2014;Lippold et al., 2009, Lippold et al., 2011, Lippold et al., 2012Loubere et al., 2004;Loveley et al., 2017;Lund et al., 2019;Mangini & Dominik, 1978;Marcantonio et al., 1996, Marcantonio et al., 2001Martínez-Garcia et al., 2009;McGee et al., 2007, 2010, McGee & Mukhopadhyay, 2013McManus et al., 1998McManus et al., , 2004Meier, 2015;Middleton et al., 2020;Missiaen et al., 2018;Mohamed et al., 1996;Mollenhauer et al., 2011;Moran et al., 2005;Mulitza et al., 2008Muller et al., 2012;Nave et al., 2007;Negre et al., 2010;Neimann & Geibert, 2003;Ng et al., 2018;Not & Hillaire-Marcel Claude, 2010;Nuttin, 2014 Paleoceanography and Paleoclimatology Plain, 2004;Pourmand et al., 2004, Pourmand et al., 2007Purcell, 2019;Roberts et al., 2014;Robinson et al., 2008;Rowland et al., 2017;Ruhlemann et al., 1996;Sarin et al., 1979;Saukel, 2011;Scholten et al., 1990, Scholten et al., 2008Serno et al., 2014Serno et al., , 2015Shiau et al., 2012;Shimmield et al., 1986;Shimmield & Mowbray, 1991;Shimmield & Price, 1988;Singh et al., 2011;Skonieczny et al., 2019;Studer et al., 2015;Sukumaran, 1994;Thiagarajan & McManus, 2019;Thöle et al., 2019;Thomas et al., 2007;Thomson et al., 1993, Thomson et al., 1995, Thomson et al., 1999Vallieres, 1997;Veeh et al., 1999Veeh et al., , 2000Veiga-Pires & Hillaire-Marcel, 1999;Voigt et al., 2017;Waelbroeck et al., 2018;Walter et al., 1997;Wengler et al., 2019;Williams et al., 2016;Winckler et al., 2008;Yang & Elderfield, 1990;Yang et al., 1995;Yu, 1994;Zhou & McManus, 2020). ...
... The influence of calcium carbonate (CaCO 3 ) dissolution on 230 Th-normalized mass fluxes may be investigated by looking at core top depth transects, for example, in a study from seamounts across the tropical Atlantic (Rowland et al., 2017). These sites are primarily composed of CaCO 3 and lithogenic components, and they are proximally located such that the principle deviations in apparent preserved mass flux with depth should result from increasing carbonate dissolution with increasing depth. ...
... in calcium carbonate burial. Core top mass fluxes are reconstructed from depth transects recovered from five seamounts in the tropical Atlantic(Rowland et al., 2017). Data are represented as deviations from the mass flux or calcium carbonate at the shallowest core top of each seamount; that is, a ΔCaCO 3 of −20% indicates that the calcium carbonate concentration is 20% lower than that of the shallowest core top on that seamount. ...
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... Eolian inputs are primarily estimated from the work of Mahowald et al. (2005) on global deposition of desert dust. However, 230 Th-derived lithogenic inputs are preferentially employed in assessing the dilution effect on calcite in sediments (Anderson et al., 2016;Hsieh et al., 2011;Kienast et al., 2016;Rowland et al., 2017) where these scarce measurements are available (see Table S2 in the Supporting Information S1). ...
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Plain Language Summary Biologically produced CaCO3 (calcite) both accumulates and dissolves on the ocean floor, depending on the CaCO3 saturation state of overlying water. This saturation state decreases with depth and can be lowered by the introduction of carbon dioxide (CO2), either from organic matter decomposition or from sinking and circulation of CO2‐enhanced surface waters. Here, we reveal that the distribution of sedimentary CaCO3 with ocean depth within the Southwest Atlantic Ocean is sensitive to and reflects the complexity of benthic ocean currents, the source of these bottom waters, and the nature of the processes that control the dissolution rate on the seafloor. Specifically, we find greater dissolution/lowered accumulation of CaCO3 in sediments beneath waters with Antarctic source, when compared to sediments beneath waters originating from the North Atlantic. We are able to predict quantitatively these differences in CaCO3 preservation using a simple model and available chemical data. Our approach has the potential to provide important insights into basin scale distributions and the sources of bottom currents in the past oceans.
... In the modern tropical Atlantic, there is no strong northward current; flow in the surface and subsurface is dominated by zonal currents (Brandt et al., 2006;Schmid et al., 2001;Zhang et al., 2003). The presence of zonal 232 Th flux gradients across the tropical Atlantic, despite the strong east-west currents gives us confidence that sediments do capture dust deposition patterns Rowland et al., 2017). ...
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2021. American Geophysical Union. All Rights Reserved. The flux of terrestrial material from the continents to the oceans links the lithosphere, hydrosphere, and biosphere through physical and biogeochemical processes, with important implications for Earth's climate. Quantitative estimates of terrigenous fluxes from sources such as rivers, aeolian dust, and resuspended shelf sediments are required to understand how the processes delivering terrigenous material respond to and are influenced by climate. We compile thorium-230 normalized 232Th flux records in the tropical Atlantic to provide an improved understanding of aeolian fluxes since the Last Glacial Maximum (LGM). By identifying and isolating sites dominated by aeolian terrigenous inputs, we show that there was a persistent meridional gradient in dust fluxes in the eastern equatorial Atlantic at the LGM, arguing against a large southward shift of the intertropical convergence zone during LGM boreal winter. The ratio of LGM to late-Holocene 232Th fluxes highlights a meridional difference in the magnitude of variations in dust deposition, with sites <10°N showing larger changes over time. This supports an interpretation of increased trade wind strength at the LGM, potentially combined with differential changes in soil moisture and reductions in higher altitude summer winds. Our results also highlight the persistent importance of continental margins as sources of high terrigenous flux to the open ocean. This is especially evident in the western tropical Atlantic, where study locations reveal the primary influence of the South American continent up to >700 km away, characterized by 232Th fluxes approximately twice as large as aeolian-dominated sites in the east.
... Th and Ti concentrations vary by grain size and among localities within North African dust source regions and span 10 to 37 ppm Th and 0.36 to 0.78 wt.% Ti ( Castillo et al., 2008). Alternatively, Ti-bearing sedimentary basalt chips or the deposition of authigenic Th supplied by the advection of Th-rich bottom waters (e.g., Rowland et al., 2017) could potentially supply significant components of non-dust Ti and Th to the Mid-Atlantic sediments. However, significant components of non-dust Th are not consistent with the excellent correlations between 4 He terr and Th observed in both cores. ...
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The role of iron as a limiting micronutrient motivates an effort to understand the supply and removal of lithogenic trace metals in the ocean. The long-lived thorium isotopes (232Th and 230Th) in seawater can be used to quantify the input of lithogenic metals attributable to the partial dissolution of aerosol dust. Thus, Th can help in disentangling the Fe cycle by providing an estimate of its ultimate supply and turnover rate. Here we present time-series (1994-2014) data on thorium isotopes and iron concentrations in seawater from the Hawaii Ocean Time-series Station ALOHA. By comparing Th-based dissolved Fe fluxes with measured dissolved Fe inventories, we derive Fe residence times of 6-12 months for the surface ocean. Therefore, Fe inventories in the surface ocean are sensitive to seasonal changes in dust input. Ultrafiltration results further reveal that Th has a much lower colloidal content than Fe does, despite a common source. On this basis, we suggest Fe colloids may be predominantly organic in composition, at least at Station ALOHA. In the deep ocean (〉2 km), Fe approaches a solubility limit while Th, surprisingly, is continually leached from lithogenic particles. This distinction has implications for the relevance of Fe ligand availability in the deep ocean, but also suggests Th is not a good tracer for Fe in deep waters. While uncovering divergent behavior of these elements in the water column, this study finds that dissolved Th flux is a suitable proxy for the supply of Fe from dust in the remote surface ocean.
Article
The GEOTRACES program requires the analysis of large numbers of seawater samples for Th-232, Th-230, and Pa-231. During the GEOTRACES international intercalibration exercise, we encountered unexpected difficulties with recovery and contamination of these isotopes, Th-232 in particular. Experiments were carried out to identify the source of these issues, leading to a more streamlined and efficient procedure. The two particular problems that we identified and corrected were (1) frits in columns supplied by Bio-Rad Laboratories caused loss of Th during column chemistry and (2) new batches of AG1-X8 resin supplied by Bio-Rad Laboratories released more than 100 pg of Th-232 during elution of sample. To improve yields and blanks, we implemented a series of changes including switching to Eichrom anion exchange resin (100-200 mu m mesh) and Environmental Express columns. All Th and Pa samples were analyzed on a Neptune multi-collector inductively-coupled-plasma mass spectrometer (MC-ICP-MS) using peak hopping of Th-230 and Th-229 on the central SEM, with either Th-232, U-236 (or both) used to monitor for beam intensity. We used in-house laboratory standards to check for machine reproducibility, and the GEOTRACES intercalibration standard to check for accuracy. Over a 1-y period, the 2 s.d. reproducibility on the GEOTRACES SW STD 20101 was 2.5% for Th-230, 1.8% for Th-232, and 4% for Pa-231. The lessons learned during this intercalibration process will be of value to those analyzing U-Th-Pa and rare earth elements as part of the GEOTRACES program as well as those using U-series elements in other applications that require high yields and low blanks, such as geochronology.
Article
In the nearly four decades since the discovery of deep-sea vents, one-third of the length of global oceanic spreading ridges has been surveyed for hydrothermal activity. Active submarine vent fields are now known along the boundaries of 46 out of 52 recognized tectonic plates. Hydrothermal survey efforts over the most recent decade were sparked by national and commercial interests in the mineral resource potential of seafloor hydrothermal deposits, as well as by academic research. Here we incorporate recent data for back-arc spreading centers and ultraslow- and slow-spreading mid-ocean ridges (MORs) to revise a linear equation relating the frequency of vent fields along oceanic spreading ridges to spreading rate. We apply this equation globally to predict a total number of vent fields on spreading ridges, which suggests that ~900 vent fields remain to be discovered. Almost half of these undiscovered vent fields (comparable to the total of all vent fields discovered during 35 years of research) are likely to occur at MORs with full spreading rates less than 60 mm/yr. We then apply the equation regionally to predict where these hydrothermal vents may be discovered with respect to plate boundaries and national jurisdiction, with the majority expected to occur outside of states’ exclusive economic zones. We hope that these predictions will prove useful to the community in the future, in helping to shape continuing ridge-crest exploration.
Article
The natural radionuclides 231Pa and 230Th are incorporated into the marine sediment record by scavenging, or adsorption to various particle types, via chemical reactions that are not fully understood. Because these isotopes have potential value in tracing several oceanographic processes, we investigate the nature of scavenging using trans-Atlantic measurements of dissolved (< 0.45 μm) and particulate (0.8–51 μm) 231Pa and 230Th, together with major particle composition. We find widespread impact of intense scavenging by authigenic Fe/Mn (hydr)oxides, in the form of hydrothermal particles emanating from the Mid-Atlantic ridge and particles resuspended from reducing conditions near the seafloor off the coast of West Africa. Biogenic opal was not found to be a significant scavenging phase for either element in this sample set, essentially because of its low abundance and small dynamic range at the studied sites. Distribution coefficients in shallow (< 200 m) depths are anomalously low which suggests either the unexpected result of a low scavenging intensity for organic matter or that, in water masses containing abundant organic-rich particles, a greater percentage of radionuclides exist in the colloidal or complexed phase. In addition to particle concentration, the oceanic distribution of particle types likely plays a significant role in the ultimate distribution of sedimentary 230Th and 231Pa.
Article
The 230Th method of determining mass accumulation rates (MARs) assumes that little to no fractionation occurs during sediment redistribution processes at the seafloor. We examine 230Th inventories in radiocarbon-dated multicore sediments from paired winnowed and focused sites at Cocos and Carnegie Ridges, Panama Basin. Radiocarbon-derived sand MARs, which likely represent the vertical rain of particles poorly transported by bottom currents, are similar at each of the paired sites, but are different using 230Th normalization. 230Th-normalized MARs are 60% lower focused sites, and likely underestimate vertical MARs, while the reverse is true for winnowed sites. We hypothesize that size fractionation occurs most frequently at lower current velocities, resulting in the coarse fraction being left behind and primarily the fine 230Th-rich grains being transported downslope. 230Th-normalization works well for recording fine-grained (detrital and opal), but not coarse-grained (carbonate), fluxes in regions that have undergone sediment redistribution.
Article
Aeolian dust is a key aspect of the climate system. Dust can modify the Earth's energy budget, provide long-range transport of nutrients, and influence land surface processes via erosion. Consequently, effective modeling of the climate system, particularly at regional scales, requires a reasonably accurate representation of dust emission, transport, and deposition. Here we evaluate African dust in 23 state-of-the-art global climate models used in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. We find that all models fail to reproduce basic aspects of dust emission and transport over the second half of the 20th century. The models systematically underestimate dust emission, transport and optical depth, and year-to-year changes in these properties bear little resemblance to observations. These findings cast doubt on the ability of these models to simulate the regional climate and the response of African dust to future climate change.
Article
Dissolved 232Th is added to the ocean though the partial dissolution of lithogenic materials such as aerosol dust in the same way as other lithogenically sourced and more biologically important trace metals such as Fe. Oceanic 230Th, on the other hand, is sourced primarily from the highly predictable decay of dissolved 234U. The rate at which dissolved 232Th is released by mineral dissolution can be constrained by a Th removal rate derived from 230Th:234U disequilibria, assuming steady-state. Calculated fluxes of dissolved 232Th can in turn be used to estimate fluxes of other lithogenically sourced dissolved metals as well as the original lithogenic supplies, such as aerosol dust deposition, given the concentration and fractional solubility of Th (or other metals) in the lithogenic material. This method is applied to 7 water column profiles from the Innovative North Pacific Experiment (INOPEX) cruise of 2009 and 2 sites from the subtropical North Pacific. The structure of shallow depth profiles suggests rapid scavenging at the surface and at least partial regeneration of dissolved 232Th at 100–200 m depth. This rapid cycling could involve colloidal Th generated during mineral dissolution, which may not be subject to the same removal rates as the more truly dissolved 230Th. An additional deep source of 232Th was revealed in deep waters, most likely dissolution of seafloor sediments, and offers a constraint on dissolved trace element supply due to boundary exchange.
Article
Radiocarbon and 230Thexcess data from six NE Atlantic box cores are considered. The cores form a transect from the Porcupine Abyssal Plain over the East Thulean Rise to the southern end of Feni Drift. The chronology for the cores is established from bulk sediment carbonate radiocarbon data and reveals that sections exhibiting constant accumulation rates can be identified in all the cores, with rates of 3.0-3.5 cm kyr-1 on the plain through the Holocene and late Holocene rates of 4.3-6.6 cm kyr-1 elsewhere. Five out of the six cores show accumulations of more 230Thexcess than is produced in the overlying water column, with the greatest inventories (up to 225% of production) in the cores from the rise and drift. A size fraction comparison between two cores from the plain and rise reveals that the higher overall accumulation rates and 230Thexcess inventories in the off-plain cores are due to an increased fine (<5 µm) component fraction, whereas the flux of coarser material is similar to that received on the plain. This suggests that the higher fluxes of materials observed are physically (rather than biogeochemically) driven and also that drift formation has been continuously active in the late Holocene. Sections of all the cores where regular accumulation is defined by the radiocarbon data are modeled first by a linear radiocarbon age/depth model and second by a constant rain (230Thexcess)0 model prorated for the observed core inventories. These modeling approaches yield historical mass accumulation rate estimates which are generally in reasonable agreement (±30%), but the differences observed appear to be well organized in time rather than random.
Article
Satellite datasets show higher chlorophyll concentration in the surface water of the equatorial Atlantic relative to that of the equatorial Pacific. Is the phytoplankton biomass also higher in the Atlantic? To answer this question, a basin-scale ocean circulation-biogeochemistry model that has a phytoplankton dynamic model is utilized to compare the spatial and temporal variations of phytoplankton carbon biomass. We use field data collected in the equatorial Atlantic to derive a new set of biological parameters so that the model can reproduce the general features of the phytoplankton biomass and chlorophyll in this region. For instance, the model reproduces the observed deep chlorophyll a maximum (DCM) that is much deeper to the south (~ 100 m) of the equator than to the north (~ 70 m). The simulated surface chlorophyll also compares well with the satellite derived chlorophyll at basin scale for the equatorial Atlantic. Our comparative analyses demonstrate that for the upwelling region, the phytoplankton biomass in the surface water is considerably higher in the equatorial Pacific than in the equatorial Atlantic. However, for the entire euphotic zone, the integrated phytoplankton biomass is much higher in the equatorial Atlantic than in the Pacific. The difference in the surface water simply reflects larger phytoplankton carbon to chlorophyll ratios in the equatorial Pacific, due to strong iron limitation. The difference in the subsurface water is due to a pronounced deep biomass maximum (DBM) existing in the equatorial Atlantic, which is associated with higher nitrate in the lower euphotic zone. This modeling study emphasizes the importance of using variable C:Chl ratios to estimate carbon biomass at regional to global scales.
Article
Vertical distributions of total 230Th and dissolved 232Th were determined in the mid-latitudes of the North Pacific Ocean (8°N, 95°W–40°N, 160°E). The observed 232Th concentrations are some of the lowest values reported for oceans. Because of low biogenic particle flux, high 230Th activity is exhibited in the central gyre. Most vertical distributions of 230Th in the Pacific Ocean have been described well using the reversible-scavenging model. However, we observed the depletion of total 230Th compared with a reversible scavenging model prediction for the deep layers in the mid-latitudes of the North Pacific Ocean. Considering the time-scale of the scavenging residence time of Th, horizontal eddy diffusion from the higher particle flux region could not have a major influence on 230Th depletion in deep waters. The observed depletion is partly explained by lateral advection, adopting the scavenging-mixing model. Additionally, a bottom scavenging process above the seafloor and a diffusion process are considered to explain the 230Th deficit. Taking into account the time-scale of the scavenging residence time of Th in deep waters (<100 yr), the vertical diffusion process could reach 800–1600 m above the seafloor. The enhanced vertical eddy diffusion derived from the complex topography of the western Pacific Ocean might promote bottom scavenging and transport of the low 230Th in deep water to the upper layers.
Article
The constant-flux proxies excess 230Th (230Thxs) and extraterrestrial 3He (3HeET) are commonly used to calculate sedimentary mass accumulation rates and to quantify lateral advection of sediment at core sites. In settings with significant lateral input or removal of sediment, these calculations depend on the assumption that concentrations of 230Thxs and 3HeET are the same in both advected sediment and sediment falling through the water column above the core site. Sediment redistribution is known to fractionate grain sizes, preferentially transporting fine grains; though relatively few studies have examined the grain size distribution of 230Thxs and 3HeET, presently available data indicate that both are concentrated in fine grains, suggesting that fractionation during advection may bias accumulation rate and lateral advection estimates based on these proxies. In this study, we evaluate the behavior of 230Thxs and 3HeET in Holocene and last glacial samples from two cores from the Blake Ridge, a drift deposit in the western North Atlantic. At the end of the last glacial period, both cores received large amounts of laterally transported sediment enriched in fine-grained material. We find that accumulation rates calculated by normalization to 230Th and 3He are internally consistent despite large spatial and temporal differences in sediment advection. Our analyses of grain size fractions indicate that ~ 70% of 3HeET-bearing grains are in the < 20 μm fraction, with roughly equal amounts in the < 4 and 4–20 μm fractions. 230Thxs is concentrated in <4-μm grains relative to 4- to 20-μm grains by approximately a factor of 2 in Holocene samples and by a much larger factor (averaging a factor of 10) in glacial samples. Despite these enrichments of both constant-flux proxies in fine particles, the fidelity of 230Th- and 3He-based accumulation rate estimates appears to be preserved even in settings with extreme sediment redistribution, perhaps due to the cohesive behavior of fine particles in marine settings.
Article
The extent and the time constant of dissolution of a set of inorganic tracers during the decomposition of large marine particles are estimated through in vitro experiments. Large marine particles were collected with in situ pumps at 30 m and 200 m in the Ligurian Sea at the end of summer. They were subsequently incubated under laboratory conditions with their own bacterial assemblage for 20 days in batches under oxic conditions in the dark. Some samples were initially sterilized in order to observe possible differences between biotic and abiotic samples. Particulate (>0.2 μm) and dissolved (
Article
The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing on the bank tops. These waters have significantly lower ²³²Th/²³⁰Th (∼10,000) than those immediately adjacent to the banks (24,000–31,000) and a (²³¹Pa/²³⁰Th) near the production ratio (∼0.1). The change in ²³²Th/²³⁰Th and (²³¹Pa/²³⁰Th) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a mixture of ²³⁰Th and ²³¹Pa alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both ²³²Th/²³⁰Th (∼13,000) and (²³¹Pa/²³⁰Th) (∼0.5) is observed at ∼430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa can be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35).
Article
Variations in carbonate flux and dissolution, which occurred in the equatorial Atlantic during the last 24,000 years, have been estimated by a new approach that allows the point-by-point determination of paleofluxes to the seafloor. An unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation. The method is based on observations that the flux of unsupported 230Th into deep-sea sediments is nearly independent of the total mass flux and is close to the production rate. Thus excess 230Th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated. The study was conducted at two sites (Ceará Rise; western equatorial Atlantic, and Sierra Leone Rise; eastern equatorial Atlantic) in cores raised from three different depths at each site. From measurements of 230Th and CaCO3, changes in carbonate flux with time and depth were obtained. A rapid increase in carbonate production, starting at the onset of deglaciation, was found in both areas. This event may have important implications for the postglacial increase in atmospheric CO2 by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters (and possibly the North Atlantic Deep Water). Increased carbonate dissolution occurred in the two regions during deglaciation, followed by a minimum during mid-Holocene and renewed intensification of dissolution in late Holocene. During the last 16,000 years, carbonate dissolution was consistently more pronounced in the western than in the eastern basin, reflecting the influence of Antarctic Bottom Water in the west. This trend was reversed during stage 2, possibly due to the accumulation of metabolic CO2 below the level of the Romanche Fracture Zone in the eastern basin.
Article
Between 1988 and 1994, twenty time-series sediment traps were deployed at different water depths in the Canary Island region, off Cape Blanc (Mauritania), and off Cape Verde (Senegal). Lithogenic particle fluxes and grain size distributions of the carbonate-free fraction of the trapped material show a high impact of dust transported either in the northeast trade winds or the Saharan Air Layer (SAL). Highest annual mean lithogenic fluxes (31.2–56.1mgm-2d-1) were observed at the Cape Blanc site, and largest annual mean diameters (>6μm) were found off Cape Verde (14.5–16.9μm) and off Cape Blanc (15.2–16.7μm). Lowest annual lithogenic fluxes (11.4–21.2mgm-2d-1 ) and smallest mean diameters (13.5–13.7μm) occurred in the Canary Island region. A significant correlation of organic carbon and lithogenic fluxes was observed at all sites. Off Cape Blanc, fluxes and mean diameters correlated well between upper (around 1000m depth) and lower traps (around 3500 m depth), indicating a fast and mostly undisturbed downward transport of particulate matter. In contrast, a major correlation of fluxes without correlating mean diameters occurred in the Canary Island region, which translates into a fast vertical transport plus scavenging of laterally advected material with depth at this site. The seasonality of lithogenic fluxes was highest in the Canary Island region and off Cape Verde, reflecting strong seasonal patterns of atmospheric circulation, with highest occurrence of continental winds in the trade wind layer during winter. In addition, grain size statistics reflect a dominant change of dust transport in the trade winds during winter/spring and transport in the SAL during summer 1993 at the Cape Verde site. Highest lithogenic fluxes during winter were correlated with mean diameters around 10–13μm, whereas lower fluxes during summer consisted of coarse grains around 20μm. Annual mean dust input wascalculated from lithogenic fluxes in the range 0.7×106–1.4×106tyr-1, roughly confirming both sediment accumulation rates and atmospheric model calculations reported previously from this area.
Article
Excess Thorium-230 (230Thxs) as a constant flux tracer is an essential tool for paleoceanographic studies, but its limitations for flux normalization are still a matter of debate. In regions of rapid sediment accumulation, it has been an open question if 230Thxs-normalized fluxes are biased by particle sorting effects during sediment redistribution. In order to study the sorting effect of sediment transport on 230Thxs, we analyzed the specific activity of 230Thxs in different particle size classes of carbonate-rich sediments from the South East Atlantic, and of opal-rich sediments from the Atlantic sector of the Southern Ocean. At both sites, we compare the 230Thxs distribution in neighboring high vs. low accumulation settings. Two grain-size fractionation methods are explored.We find that the 230Thxs distribution is strongly grain size dependent, and 50–90% of the total 230Thxs inventory is concentrated in fine material smaller than 10µm, which is preferentially deposited at the high accumulation sites. This leads to an overestimation of the focusing factor Ψ, and consequently to an underestimation of the vertical flux rate at such sites. The distribution of authigenic uranium indicates that fine organic-rich material has also been re-deposited from lateral sources. If the particle sorting effect is considered in the flux calculations, it reduces the estimated extent of sediment focusing. In order to assess the maximum effect of particle sorting on Ψ, we present an extreme scenario, in which we assume a lateral sediment supply of only fine material
Article
The distribution of ²³⁴Th, ²³°Th, and ²²⁸Th between dissolved and particulate forms was determined in 17 seawater samples from the Guatamala and Panama basins. Sampling was carried out in situ with battery-powered, submersible pumping systems in which the seawater first passed through a Nuclepore filter (1.0-..mu..m pore size) and then through a cartridge packed with nitex netting that was impregnated with MnOâ to scavenge the dissolved Th isotopes. Natural ²³⁴Th was used as the tracer for monitoring the efficiency of scavenging. For all three isotopes, most of the activity was found in the dissolved form. On the average 4% of the ²³⁴Th, 15% of the ²²⁸Th, and 17% of the ²³°Th occurred in the particulate form. On the average 4% of the ²³⁴Th, 15% of the ²²⁸Th, and 17% of the ²³°Th occurred in the particulate form, though the percentages were found to be strongly dependent on particle concentration. These distributions are not consistent with chemical scavenging models that assume irreversible uptake of Th on particle surfaces. the results can be explained, however, if continuous exchange of Th isotopes between seawater and the particle surfaces is assumed. Vertical profiles of both particulate and dissolved ²³°Th show increasing concentrations with depth, as required by the assumption of reversible exchange. Some of the dissolved ²³°Th profiles, however, show a reversal of this trend near the bottom, indicating accelerated scavenging near the water/sediment interface. Kinetics of both adsorption and desorption can be examined if at least two Th isotopes are measured in the same samples. Results show that reaction times are short (a few months) compared to the residence time of suspended matter in the deep ocean (several years) indicating that particles suspended in the deep sea are close to equilibrium with respect to exchange of metals at their surfaces.
Article
Measurements of the half-lives of {sup 3}H, {sup 10}Be, {sup 14}C, {sup 26}Al, {sup 40}K, {sup 39}Ar, {sup 53}Mn, {sup 87}Rb, {sup 92}Nb, {sup 129}I, {sup 138}La, {sup 147}Sm, {sup 176}Lu, {sup 174}Hf, {sup 180}Ta, {sup 187}Re, {sup 186}Os, {sup 190}Pt, {sup 204}Pb, {sup 210}Pb, {sup 210}Po, {sup 222}Rn, {sup 224}Th, {sup 226}Ra, {sup 227}Ac, {sup 228}Ra, {sup 228}Th, {sup 230}Th, {sup 232}Th, {sup 231}Pa have been compiled and evaluated. The effect of the {sup 14}C half-life value on carbon dating ages is discussed as well as the stability of {sup 204}Pb. 237 refs., 30 tabs.
Article
We present multi-collector (MC) inductively coupled plasma mass spectrometry (ICPMS) protocols developed to obtain high precision, accurate determinations of U and Th isotope ratios that are applicable to a wide range of geological materials. MC-ICPMS provides a means to make high precision measurements but a recent laboratory inter-comparison, Regular European Inter-laboratory Measurement Evaluation Programme (REIMEP)-18, indicates that accurate results for U isotope ratios are not currently achieved by all facilities using MC-ICPMS. We detail a suite of protocols that can be used for a wide variety of U and Th isotope ratios and total loads. Particular attention is devoted to instrument optimisation, instrumental backgrounds, stability and memory effects, multiplier nonlinearity and yield determinations. Our results indicate that the extent of mass fractionation of U and Th analyses run under similar instrumental conditions is 0.48% per amu and 0.45% per amu, respectively, but cannot be distinguished at per mil precision levels. However, we note that multiplier–Faraday cup gain can be significantly different for U and Th by 1% and thus a U standard should not be used to correct Th measurements. For this reason, a combination of thermal ionisation mass spectrometry (TIMS) and MC-ICPMS methods have been used to determine the isotopic ratio of an in-house Th standard (TEDDi). As part of our methods, TEDDi and the U standard NBL-112a are used as bracketing standards for Th and U samples, respectively. While the in-house Th standard has 229Th–230Th–232Th composition specific for bracketing low 232Th analyses, the methods have been also successful for silicates with 230Th/232Th
Article
Palaeo-dust records in sediments and ice cores show that wind-borne mineral aerosol ('dust') is strongly linked with climate state. During glacial climate stages, for example, the world was much dustier, with dust fluxes two to five times greater than in interglacial stages. However, the influence of dust on climate remains a poorly quantified and actively changing element of the Earth's climate system. Dust can influence climate directly, by the scattering and absorption of solar and terrestrial radiation, and indirectly, by modifying cloud properties. Dust transported to the oceans can also affect climate via ocean fertilization in those regions of the world's oceans where macronutrients like nitrate are abundant but primary production and nitrogen fixation are limited by iron scarcity. Dust containing iron, as fine-grained iron oxides/oxyhydroxides and/or within clay minerals, and other essential micronutrients (e.g. silica) may modulate the uptake of carbon in marine ecosystems and, in turn, the atmospheric concentration of CO 2 . Here, in order to critically examine past fluxes and possible climate impacts of dust in general and iron-bearing dust in particular, we consider present-day sources and properties of dust, synthesise available records of dust deposition at the last glacial maximum (LGM); evaluate the evidence for changes in ocean palaeo-productivity associated with, and possibly caused by, changes in aeolian flux to the oceans at the LGM; and consider the radiative forcing effects of increased LGM dust loadings.
Article
An inverse finite-difference model of the abyssal circulation in the North Atlantic Ocean is developed in order to evaluate the dynamical information contained in measurements of thorium-230 (230Th). The model has a very coarse resolution and is based on lowest order balances for planetary flows. The naturally occurring 230Th differs from more conventional oceanic tracers in several respects, e.g., its production (by radioactive decay) is globally uniform to a good approximation and its removal can be understood in terms of a simple reversible exchange with particles sinking slowly to the seafloor. The time required for 230Th to reach steady state with respect to particle exchange is estimated to increase with depth, reaching O(10) yr below 1000 m. In the North Atlantic 230Th activities at distant locations share a similar increase with depth in the upper —a pattern consistent with a reversible exchange—but show drastic differences in the abyssal interior. Two inversions are conducted in order to determine whether the 230Th differences reflect the effects of the circulation—by preventing the slow attainment to steady state w.r.t. particle exchange in deep water—and provide complementary information about the abyssal flow. In a first inversion, observations of density from a hydrographic compilation and of volume transports at specific locations are combined with the dynamical balances in order to infer the basin-scale flow. The inferred flow displays the western boundary current and coherent structures in the abyssal interior with low statistical significance. In a second inversion, the flow is further constrained by the 230Th measurements and the condition that 230Th divergence by the flow field and particle sinking must be locally balanced by 230Th production from decay. The addition of 230Th leads to the estimation of a larger amplitude of the integrated meridional transports below 1000 m (by 2–9 Sv), where the range reflects the uncertainties in the large scale 230Th distribution and in the radiochemical balance. This result is interpreted as a correction by 230Th for the tendency of inverse geostrophic models to lead to the inference of a vanishing circulation when horizontal density gradients are insignificant.
Article
Desert particulate samples from locations in the Sahara–Sahel dust corridor (Western Sahara, Algeria, Chad and Niger) were segregated into size fractions ranging from <1.5 to >20 μm and analysed using inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). The size-fractionated samples contain higher concentrations of Al, Na, Mg, Fe and most trace elements in the finer, phyllosilicate-rich materials. A notable exception is provided by Zr and Hf, which concentrate in the coarser fractions due to a higher abundance of coarse detrital zircon. Equally explicable by natural processes are enrichments (relative to the average upper continental crust) in incompatible trace elements with higher ionic potential (Nb, Ce, La), these being more resistant to chemical weathering. In contrast, samples show a relative loss of large ion lithophile elements (Rb, Cs and Ba), and the more calcareous samples show relative enrichment in Sr and depletion in Nb. However, not all the geochemical variations exhibited by these samples are easily explicable by invoking “natural” geochemical processes: anomalous concentrations of several transition metals (Zn, Pb, Cd) can be attributed to anthropogenic pollution, and enrichments in Cr, Ni and Mo are due to abrasive interaction between the siliceous dust sample and the fractionation equipment.
Article
The geochemistry of marine sediments is a major source of information about the past environment. Of the many measurements that provide such information, those of the U-series nuclides are unusual in that they inform us about the rate and timescales of processes. Oceanic processes such as sedimentation, productivity, and circulation, typically occur on timescales too short to be assessed using parent-daughter isotope systems such as Rb-Sr or Sm-Nd. So the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides (principally 14C) or the U-series nuclides. This makes the U-series nuclides powerful allies in the quest to understand the past ocean-climate system and has led to their widespread application over the last decade. As in other applications of the U-series, those in paleoceanography rely on fractionation of the nuclides away from secular equilibrium. In the oceanic setting, this fractionation is generally due to differences in the solubility of the various nuclides. The behavior of the U-series nuclides in the ocean environment was widely researched in the middle decades of the twentieth century. This work established knowledge of the concentrations of the nuclides in the various compartments of the ocean system, and of their fluxes between these compartments. Such understanding was comprehensively summarized in the Ivanovich and Harmon U-series volume (1992), particularly by Cochran (1992). Understanding of the behavior of the U-series nuclides has not advanced very dramatically in the decade since that summary but a major theme of research has been the use of this geochemical understanding to develop U-series tools to assess the past environment (Table 1⇓). View this table: Table 1. Summary of seawater data for U-series nuclides with paleoceanographic applications. S - soluble, I - insoluble. Full descriptions of the paleoceanographic uses and references are provided in the text. Further details of the half …
Article
We have introduced a simple particle field into an existing and well-documented ocean general circulation model. This enables us to investigate the advection and scavenging of particle-reactive species within the water column. As a first use of this model, we have assessed the advection and flux to sediment of , a nuclide with a well understood marine chemistry that exhibits extreme particle reactivity. The flux to sediment of this nuclide is of interest as it is widely assumed to be related only to water depth, and therefore to act as a constant-flux indicator for marine sediments. By assuming an average settling velocity for marine particles of 3 m/d, in good agreement with observational constraints, the model generates a particle field close to that observed. Thorium-230 is scavenged onto this particle field reversibly according to Kd values constrained by observations and incorporating a particle-concentration effect. This scavenging gives a good fit to the ≈900 literature water-column measurements of suggesting that the model is advecting and removing realistically. An exception to this is the Weddell Sea, where the model has too little ice cover and too much lateral mixing, which prevents it from duplicating the observed high values. The model confirms that significant advection of occurs and duplicates the low values seen deep in the North Atlantic due to the advection of low-230Th surface waters to depth. Model-derived maps of the flux to the sediment indicate that ≈70% of the ocean floor receives a flux within 30% of that expected from production. In extremely non-productive regions, the flux can fall to as low as 0.4 times that expected for in situ scavenging, while highly productive regions have fluxes up to 1.6 times that expected. An additional model run using glacial circulation fields suggests that glacial fluxes are similar to those in the Holocene except in regions close to sea ice. This is particularly true of the North Atlantic, where appreciably more scavenging occurs in the glacial run due to advection of from the ice-covered Arctic, and because of reduced North Atlantic Deep Water (NADW) formation. These ice-related effects mean that the area of ocean floor with fluxes within 30% of production falls to ≈60% for the glacial. The Holocene and Glacial flux maps allow an assessment of the accuracy of -derived sedimentation rates for existing and future studies.
Article
Uranium, thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the Panama Basin and by in-situ filtration of large volumes of seawater in the Panama and Guatemala Basins. Concentrations of dissolved Th and Pa isotopes were determined by extraction onto MnO2 adsorbers placed in line behind the filters in the in-situ pumping systems.Concentrations of dissolved 230Th and 231Pa in the Panama and Guatemala Basins are lower than in the open ocean, whereas dissolved 230Th/231Pa ratios are equal to, or slightly greater than, ratios in the open ocean. Particulate 230Th/231Pa ratios in the sediment trap samples ranged from 4 to 8, in contrast to ratios of 30 or more at the open ocean sites previously studied. Particles collected by filtration in the Panama Basin and nearest to the continental margin in the Guatemala Basin contained 230Th/231Pa ratios similar to the ratios in the sediment trap samples. The ratios increased with distance away from the continent.Suspended particles near the margin show no preference for adsorption of Th or Pa and therefore must be chemically different from particles in the open ocean, which show a strong preference for adsorption of Th. Ocean margins, as typified by the Panama and Guatemala Basins, are preferential sinks for 231Pa relative to 230Th. Furthermore, the margins are sinks for 230Th and, to a greater extent, 231Pa transported by horizontal mixing from the open ocean.
Article
Fluxes of continental mineral aerosols (dust) were greater during glacial periods than during interglacials throughout most regions of the Earth. The equatorial Pacific Ocean was a possible exception to this pattern in that previous studies have reported either greater dust fluxes during interglacials or no consistent glacial–interglacial pattern of dust flux. We have applied the 230Th-normalization technique to derive five new records of dust flux from central equatorial Pacific Ocean sediments. In contrast to previous studies, which relied on stratigraphic accumulation rates, the 230Th-normalization technique produces internally consistent results, revealing fluxes to this region of continental lithogenic material that were positively correlated with global ice volume throughout the past 300,000 yr. Maximum glacial fluxes of continental mineral aerosols exceed minimum interglacial fluxes by about a factor of 2, similar to changes found elsewhere at low and mid-latitudes. This amplitude of variability is substantially smaller than that seen in some recent models, and these observations provide a calibration point for future model development.
Article
The bottom 1 km of the ocean is appreciably more turbid than the two or more km above it. This bottom nepheloid layer has been mapped principally by nephelometers that are most sensitive to particles ⪅2 μm in diameter. However, the layers contain larger aggregated particles which play an important role in sedimentation. The thickness of the nepheloid layer is much greater than the height above the bed to which turbulent mixing occurs, a fact explained by lateral transport of turbid mixed layers that have detached from the bottom. The overall upwards decline in turbidity is accounted for by the layers higher above the bed having come farther and being on average older than those lower down. The most intense nepheloid layers are found in areas of strong western boundary currents and in regions where the bottom current regime is thought to be highly variable due to penetration of eddy energy from the surface. Areas under the Gulf Stream and in the Argentine basin are examples. Review of the distribution and characteristics of nepheloid layers examines features contributing critically to these hypotheses. An important contributing factor to the depth distribution of turbidity is that the ratio of water volumes to bed areas in contact with them is eight times higher for water between 4 and 5 km than that between 2 to 3 km.
Article
High-resolution (one to two samples/ka) radionuclide proxy records from core 93KL in the northeastern Arabian Sea provide evidence for millennial climate variability over the past 110 ka. We interpret 230Th-normalized 232Th fluxes as a proxy for eolian input, and authigenic uranium concentrations as a proxy for past productivity. We attribute orbital and suborbital variations in both proxies to changes in the intensity of the southwest Indian Ocean monsoon. The highest 230Th-normalized 232Th fluxes occur at times that are consistent with the timing of the Younger Dryas, Heinrich events 1–7 and cold Dansgaard–Oeschger stadial events recorded in the GISP2 ice core. Such high dust fluxes may be due to a weakened southwest monsoon in conjunction with strengthened northwesterlies from the Arabian Peninsula and Mesopotamia. Authigenic uranium concentrations, on the other hand, are highest during warm Dansgaard–Oeschger interstadials when the southwest monsoon is intensified relative to the northwesterly winds. Our results also indicate that on orbital timescales maximum average eolian fluxes coincide with the timing of marine isotopic stage (MIS) 2 and 4, while minimum fluxes occur during MIS 1, 3 and 5. Although the forcing mechanism(s) controlling suborbital variabilities in monsoonal intensity is still debated, our findings suggest an atmospheric teleconnection between the low-latitude southwest monsoon and North Atlantic climate.
Article
The deposition of aeolian, or windblown, dust is widely recognized as an important physical and chemical flux to ecosystems. Dust deposition adds exogenous mineral and organic material to terrestrial surfaces and can be important for the biogeochemical cycling of nutrients. There have been many studies that characterize the physical and chemical composition of dust. However, few studies have synthesized these observations in order to examine patterns geochemical fluxes. We have compiled observations of dust deposition rates, particle size distributions (PSD), mineralogy and bulk elemental and organic chemistry.
Article
The activity ratio of228Th/228Ra in the open surface ocean averages 0.21. This suggests that thorium is removed from surface water in about 0.7 yr. As plant matter is cycled within the surface sea on a similar time scale, the suggestion is made that highly reactive compounds are inadvertently removed by plants in their quest for the critical nutrients nitrate and phosphate. Combined with the coefficient for horizontal eddy diffusivity obtained from the distribution of228Ra in surface sea water this result provides a basis for the prediction of the distribution of “reactive” pollutants released to the surface sea from coastal areas.
Article
Key aspects of deep-ocean fluid dynamics such as basin-scale (residual) and tidal flow are believed to have changed over glacial/interglacial cycles, with potential relevance for climatic change. To constrain the mechanistic links, magnitudes and temporal succession of events analyses of sedimentary paleo-records are of great importance. Efforts have been underway for some time to reconstruct residual-flow patterns from sedimentary records. Attempts to reconstruct tidal flow characteristics from deep-sea sediment deposits, however, are at a very early stage and first require a better understanding of the reflection of modern tides in sediment dynamics. In this context internal (baroclinic) tides, which are formed by the surface (barotropic) tide interacting with seafloor obstacles, are believed to play a particularly important role. Here we compare two modern deep-sea environments with respect to the effect of tides on sediment dynamics. Both environments are influenced by kilometre-scale topographic features but with vastly different tidal forcing: (1) two sites in the Northeast Atlantic (NEA) being surrounded by, or located downstream of, fields of short seamounts (maximum barotropic tidal current velocities ∼5 cm s−1); and (2) a site next to the Anaximenes seamount in the Eastern Mediterranean (EMed) (maximum barotropic tidal current velocities ∼0.5 cm s−1). With respect to other key fluid-dynamical parameters both environments are very similar. Signals of sedimentary particle dynamics, as influenced by processes taking place in the bottom boundary layer, were traced by the vertical water-column distribution of radioactive disequilibria (daughter/parent activity ratios≠1) between the naturally occurring, short-lived (half-life: 24.1 d) particulate-matter tracer 234Th relative to its very long-lived and non-particle-reactive parent nuclide 238U. Activity ratios of 234Th/238U<1 in water samples collected near the seafloor indicate fast 234Th scavenging onto particles followed by fast settling of these particles from the sampled parcel of water and, therefore, imply active sediment resuspension and dynamics on time scales of up to several weeks. In the Northeast Atlantic study region tides (in particular internal tides) are very likely to locally push total current velocities near the seafloor across the critical current velocity threshold for sediment erosion or resuspension whereas in the Eastern Mediterranean the tides are much too weak for this to happen. This difference in tidal forcing is reflected in a difference of the frequency of the occurrence of radioactive disequilibria <1 between total 234Th and 238U: In the near-bottom water column of the Northeast Atlantic region 59% of samples had detectable 234Th/238U disequilibria whereas at the Eastern Mediterranean site this fraction was only 7% (including a few disequilibria >1). The results of this study, therefore, add to the evidence suggesting that tides in the deep sea of the open oceans are more important for sediment dynamics than previously thought. It is hypothesised that (a) tide/seamount interactions in the deep open ocean control the local distribution of erosivity proxies (e.g., distributions of sediment grain sizes, heavy minerals and particle-reactive radionuclides) in sedimentary deposits and (b) the aforementioned topographically controlled sedimentary imprints of (internal) tides are useful in the reconstruction of past changes of tidal forcing in the deep sea.
Article
Dissolved Al concentrations from the NE Atlantic, between 25°N and 35°N off the coast of NW Africa, range from 16 nM to 32 nM in a well-mixed surface layer extending to 150 m depth, increasing towards the maximum of the Saharan dust plume. Mediterranean outflow water is detected as a high Al anomaly corresponding to the salinity signal of the Mediterranean water. Other than in surface and Mediterranean waters Al concentrations are invariant with depth, but increase from 6 nM to 11 nM towards the continental margin. The magnitude of the dissolved Al signal in the surface layer is compatible with dissolution of Al from the highest of present estimates of dust input to this area. Comparison with data from other areas indicates that Al cannot be cycled through the oceans in parallel to silica as has been previously suggested.