Land to ocean transfer of material largely controls the chemical composition of seawater and the global element cycles. Overall this transfer is dominated by the riverine transport of particulate ...material to the oceans. A large number of isotopic tracers including 143Nd/144Nd, 87Sr/86Sr, 30Si/28Si, 56Fe/54Fe, and 232Th/230Th, demonstrate that a significant fraction of this particulate material dissolves in seawater after its arrival to the oceans. Laboratory experiments confirm that these particles dissolve readily in seawater; 0.5 to 10% of the Sr and Nd in riverine transported particulate material is found to dissolve in seawater over time scales ranging from weeks to months. Noting that the mass of most elements arriving to the oceans via particulates exceeds that of the elements arriving via dissolved transport by at least a factor of 50, it follows that 1) particulate material dissolution in the ocean may be the dominant mechanism contributing numerous elements to the oceans, and 2) estimates based on dissolved riverine transport alone may significantly underestimate the global element fluxes to the oceans.
The role of particulate material dissolution in seawater may be most significant in the cycles of sparingly soluble elements, which are far more concentrated in particulate material than more soluble elements. As such, particulate material dissolution and transport likely play a major role in the availability of those elements limiting marine primary productivity. This effect will be most significant at the ocean margins, as a large fraction of the products of particulate material dissolution are re-precipitated locally via reversible scavenging. The major effect of particulate dissolution in the open-ocean will be on element isotope compositions. Moreover, as the transport of particulates to the oceans is far more sensitive to temperature and runoff than dissolved transport, the dissolution of particulate in seawater and subsequent reactions may provide a strong yet underappreciated link between continental weathering and climate.
Continental margins are, via river sediment discharges, the major source of a number of elements to the ocean. They are also, for several reactive elements, sites of preferential removal from the ...water column, due to enhanced scavenging 1 M.P. Bacon, Tracers of chemical scavenging in the ocean: Boundary effects and large-scale chemical fractionation, Philos. Trans. R. Soc. Lond., A 325 (1988) 147–160.. They can therefore be understood as sources of elements for the ocean, sinks or both. Although exchanges of matter are suspected to occur at the continent/ocean interface 2 P.H. Santschi, L. Guo, I.D. Walsh, M.S. Quigley, M. Baskaran, Boundary exchange and scavenging of radionuclides in continental margin waters of the Middle Atlantic Bight: implications for organic carbon fluxes, Cont. Shelf Res. 19 (1999) 609–636. and despite their probable importance for the ocean chemistry, closed budgets have still yet to be determined. Here, based on neodymium isotopic composition data obtained during the past 6 yr, we document and quantify significant neodymium exchange at ocean boundaries, in areas covering a large spectra of hydrographical, biological and geochemical characteristics : Eastern Indian Ocean, Western Equatorial Pacific, Western Tropical Pacific and Northwestern Atlantic, with neodymium removal fluxes accounting for 74±23%, 100±38%, 62±54% and 84±45% of the neodymium input fluxes, respectively. Recognition of boundary exchange and its potential globalization have important implications for (1) our understanding of margin/ocean interactions and their influence on the oceanic isotopic chemistry, and (2) geochemical cycling of reactive elements (including pollutants) at ocean margins.
Human activity is leaving a pervasive and persistent signature on Earth. Vigorous debate continues about whether this warrants recognition as a new geologic time unit known as the Anthropocene. We ...review anthropogenic markers of functional changes in the Earth system through the stratigraphic record. The appearance of manufactured materials in sediments, including aluminum, plastics, and concrete, coincides with global spikes in fallout radionuclides and particulates from fossil fuel combustion. Carbon, nitrogen, and phosphorus cycles have been substantially modified over the past century. Rates of sea-level rise and the extent of human perturbation of the climate system exceed Late Holocene changes. Biotic changes include species invasions worldwide and accelerating rates of extinction. These combined signals render the Anthropocene stratigraphically distinct from the Holocene and earlier epochs.
A global compilation of the neodymium isotopic composition of seawater is presented. With 880 data points, it confirms the gradual εNd increase for intermediate/deep water masses from the northwest ...North Atlantic, via the Austral and Indian oceans, to the Pacific. This confirms the usefulness of this tracer for studying large scale oceanic circulation. The compilation stresses the need for documenting the oceans south of 30°S, from which less than 4% of the data are derived. The associated neodymium concentrations display heterogeneous vertical gradients among major oceanic basins. In addition to particle remineralization along the global thermohaline circulation, the database suggests that basin size differences also contribute to that heterogeneity.
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► For the first time, a global database of seawater neodymium isotopes is presented. ► It confirms the gradual εNd increase along the global thermohaline circulation. ► It stresses the need for documenting the oceans south of 30°S. ► It suggests that basin sizes play a role in neodymium surface concentrations.
The exchange of material between particulates and seawater along the continental margins, a process commonly referred to as boundary exchange, is thought to play a significant role in controlling the ...neodymium (Nd) isotope and Rare Earth Element (REE) composition of the oceans. This study provides experimental verification of this concept by quantifying the effect of particulate dissolution in seawater on dissolved εNd and REE compositions. Three closed-system experiments were performed using basaltic particulate material of riverine, estuarine and marine origin. The release of Nd from this basaltic material increased the εNd composition of seawater in all three experiments, with a εNd value close to that of the associated sediment being achieved within 80 days in all experiments. Mass balance indicates that up to 0.4% of Nd from the particulate phase was released to the seawater over the duration of these experiments, and that the rate of release varied according to particulate origin and surface area. Progressive variations in the PAAS normalised REE patterns, as well as the Eu and Ce anomalies and La/Yb ratio, demonstrate that REEs were also transferred from the basaltic particulates to seawater during the experiments. Despite evidence for the release of REEs from the particulate material, dissolved REE abundances decreased during the experiments, and are thought to reflect incorporation into the REE-phosphate mineral rhabdophane. Together these experimental results confirm that elemental release from basaltic sediments on the ocean margins is a significant marine flux that can have a major control on the composition of seawater.
•We experimentally assess the nature of basaltic particulate dissolution in seawater.•We quantify the importance of this process for marine εNd and REE compositions.•Particulate released Nd dominates seawater εNd by the end of the experiment.•REE patterns reflect particulate dissolution and scavenging into secondary phases.•These results verify the importance of boundary exchange on the ocean margins.
Land to ocean transfer of material largely controls the chemical composition of seawater and the global element cycles. Oceanic isotopic budgets of chemical species, macro- and micronutrients (e.g. ...Nd, Sr, Si, Mg, Zn, Mo and Ni) have revealed an imbalance between their sources and sinks. Radiogenic isotope budgets underlined the importance of taking into account continental margins as a source of elements to oceans. They also highlighted that the net land-ocean inputs of chemical species probably result from particle-dissolved exchange processes, named 'Boundary Exchange'. Yet, locations where 'Boundary Exchange' occurs are not clearly identified and reviewed here: discharge of huge amount of freshly weathered particles at the river mouths, submarine weathering of deposited sediments along the margins, submarine groundwater discharges and subterranean estuaries. As a whole, we conclude that all of them might contribute to 'Boundary Exchange'. Highlighting their specific roles and the processes at play is a key scientific issue for the second half of GEOTRACES.
This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.
Rare earth element (REE) concentrations and neodymium isotopic composition (ɛNd) are tracers for ocean circulation and biogeochemistry. Although models suggest that REE release from lithogenic ...sediment in river discharge may dominate all other REE inputs to the oceans, the occurrence, mechanisms and magnitude of such a source are still debated. Here we present the first simultaneous observations of dissolved (<0.45 μm), colloidal and particulate REE and ɛNd in the Amazon estuary. A sharp drop in dissolved REE in the low-salinity zone is driven by coagulation of colloidal matter. At mid-salinities, total dissolved REE levels slightly increase, while ɛNd values are shifted from the dissolved Nd river endmember (-8.9) to values typical of river suspended matter (-10.6). Combining a Nd isotope mass balance with apparent radium isotope ages of estuarine waters suggests a rapid (3 weeks) and globally significant Nd release by dissolution of lithogenic suspended sediments.
Hydrothermal activity is significant in regulating the dynamics of trace elements in the ocean. Biogeochemical models suggest that hydrothermal iron might play an important role in the iron-depleted ...Southern Ocean by enhancing the biological pump. However, the ability of this mechanism to affect large-scale biogeochemistry and the pathways by which hydrothermal iron reach the surface layer have not been observationally constrained. Here we present the first observational evidence of upwelled hydrothermally influenced deep waters stimulating massive phytoplankton blooms in the Southern Ocean. Captured by profiling floats, two blooms were observed in the vicinity of the Antarctic Circumpolar Current, downstream of active hydrothermal vents along the Southwest Indian Ridge. These hotspots of biological activity are supported by mixing of hydrothermally sourced iron stimulated by flow-topography interactions. Such findings reveal the important role of hydrothermal vents on surface biogeochemistry, potentially fueling local hotspot sinks for atmospheric CO
by enhancing the biological pump.
•High surface water REE of zonal eastward equatorial Pacific currents.•Shelf inputs are the key source of REEs to the equatorial Pacific.•Preformed seawater REE of westward currents in the tropical ...West Pacific.•Temporal and spatial variability of tropical West Pacific surface water REE.
Continental sources and current transport play a major role in rare earth element (REE, and other trace element) input and distribution in the Tropical Western Pacific. Here, we present spatially highly resolved distributions of dissolved REE concentrations (REE) along three transects in the zonal (extra-)equatorial current system and the Solomon Strait of the Tropical Western Pacific. We use seawater REE in combination with direct physical oceanographic observations (e.g., current velocity data) to characterize the geochemical composition, origin and pathways of the complex surface and upper layer currents of the Tropical Western Pacific and to quantify the input fluxes of REEs. We identify Papua New Guinea (PNG) volcanic rocks, sediments, and/or river particles as the key source adding trace elements to the equatorial eastward zonal currents of the Tropical Western Pacific. Our and published data indicate temporal and spatial variability of this input and transport in the PNG source area and the equatorial eastward currents. The westward currents, on the other hand, lack this REE input signal suggesting lateral transport of preformed seawater REE. At the transition between these zonal eastward and westward currents, our data indicate lateral mixing of Eastern and Western Pacific source waters.
Following a brief description of a typical GEOTRACES oceanographic cruise, this article illustrates the benefits of the multidisciplinary and multi-tracer approach that has characterized GEOTRACES. ...The value and organization of this type of research cruise—which requires many people and large vessels—are discussed in relation to the resulting carbon footprint and the potential alternatives.
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