The marine geochemistry of the rare earth elements

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1983Novel methods were developed for the determination of 12 of the 14 Rare Earth Elements (REE) in seawater. Initial extractions of the REE by chelating ion exchange chromatography is followed by cation exchange for removal of co-extracted U and remaining traces of major ions. Finally traces of U are removed by anion exchange before irradiation for 8 hours at a flux of 5 x 1013 neutrons.cm-2.sec-l. After post-irradiation separation of 24 Na, the gamma spectra are recorded over four different time intervals with a Ge(Li) detector. An internal standard (144Ce) is carried all along the procedure for improved precision by avoidance of counting geometry errors. Vertical profiles are reported for three stations in respectively the Northwest Atlantic Ocean, the Eastern Equatorial Pacific Ocean and the Cariaco Trench, an anoxic basin. This data set represents the first detailed profiles of Pr, Tb, Ho, Tm and Lu in seawater, together with profiles of La, Ce, Nd, Sm, Eu, Gd and Yb. The first observations of positive Ce anomalies in seawater are ascribed to regeneration of Ce under reducing conditions. The first reported positive Gd anomalies are ascribed to the unique chemical properties of the Gd(III)-cation, which has an exactly half-filled 4f electron shell. Concentrations of the REE range from 0.3 pmol.kg-l (Lu) to 86 pmol.kg-l (Ce) and are among the lowest reported so far for trace elements in seawater. The REE as a group typically exhibit a quasi-linear increase with depth. In the deep water there appears to be some degree of correlation with silicate. Concentration levels in the deep Pacific Ocean are 2-4 times those in deep Atlantic waters. Ce has an opposite behaviour, with very strong depletions in deep Pacific waters. In the Cariaco Trench all REE, but especially Ce, are strongly affected by the chemical changes across the oxic/anoxic interface. The REE distributions normalized versus shales (crustal abundance) exhibit four major features: i) a gradual enrichment of the heavy REE, most strongly developed in the deep Pacific Ocean. This is compatible with the stabilization of heavy REE by stronger inorganic complexation in seawater as predicted by the TURNER- WHITFIELD-DICKSON speciation model. ii) the first description of positive Gd anomalies, in agreement with the anomalously strong complexation of the Gd(III)-cation predicted by the same speciation model. iii) most commonly negative, but sometimes positive, Ce anomalies. iv) a linear Eu/Sm relation for all samples. Distributions of the dissolved REE in ocean waters seem to be dominated by their internal cycling within the ocean basins. With a few notable exceptions, the ultimate external sources (riverine, aeolian, hydrothermal) and sinks (authigenic minerals) appear to have little impact on the spatial distribution of the REE in oceanic water masses. Analogies with distributions of other properties within the oceans suggest that the REE as a group are controlled by two simultaneous processes: A) cycling like or identical to opal and calcium-carbonate, with circumstantial evidence in support of the latter as a possible carrier. B) adsorptive scavenging, possibly by manganese-oxide phases on settling particles. The latter mechanism is strongly supported by the parallels between REE(III) speciation in seawater and the 'typical 1 seawater REE pattern. This general correspondence is highlighted by the very distinct excursions of Gd in both Gd(III) speciation and the observed seawater REE patterns. Combination of both apparent mechanisms, for instance scavenging of REE by adsorptive coatings (Mn oxides) on settling skeletal material, is very well conceivable. Upon dissolution of the shells at or near the seafloor the adsorbed REE fraction would be released into the bottom waters. The observations of positive Ce anomalies in Northwest Atlantic surface waters, enhanced Ce anomalies and Mn levels in the OZ-minimum zone of the Eastern Equatorial Pacific Ocean, and enhanced Ce concentrations in anoxic waters all support the contention that a vigorous cycling driven by oxidation and reduction reactions dominates both Ce and Mn in the ocean basins. Under conditions of thermodynamic equilibrium, Ce tends to become depleted in well-oxygenated open ocean waters, and normal or enriched in waters below a pOZ threshold of about 0.001-0.010 atm partial pressure. The latter threshold level generally lies below the sediment/water interface. However, the kinetics of oxidation (and reduction) of Ce appears to be slow relative to various transport processes. This leads to disequilibria, i.e. a major uncoupling of the pOZ threshold level and the Ce anomaly distribution. The REE are definitely non-conservative in seawater and in general the REE pattern or 143Nd/144Nd isotopic ratio cannot be treated as ideal water mass tracers. The continuous redistribution of Ce within the modern ocean, combined with the likelihood of active diagenesis, precludes the use of Ce anomalies as indicators of oxic versus anoxic conditions in ancient oceans. On the other hand, the Eu/Sm ratio, possibly combined with 143Nd/144Nd , would have potential as a tracer for understanding modern and ancient processes of hydrothermal circulation.This research was supported by Department of Energy contract DE-AS02-76EV03566 and Office of Naval Research Contract NOOOl 4-82-C-00l 9 NR 083-004

The dependence on temperature and salinity of dissolved

Recurring latitudinal patterns of the dissolved inorganic carbon (DIC) content and the fugacity of CO2 (fCO2) were observed in East Atlantic surface waters with strong gradients at hydrographic fronts. The dissolved inorganic carbon chemistry clearly displayed the effects of oceanic circulation and of persistent surface water processes. In two cases inorganic carbon components could be used as an indicator of the origin of hydrographic features. Surface water fCO2 below the atmospheric value, low DIC and low salinity north of the equator were ascribed to a combination of high rainfall and low wind speed in the Intertropical Convergence Zone and of biological uptake of CO2. Low surface water DIC and salinity delineated the Congo outflow. Along the cruise tracks calculated titration alkalinity (TA) had an almost linear relationship with salinity, while DIC had an apparent dependence on temperature and salinity. The latter dependence was tested by comparing observed DIC to DIC estimated from fCO2 and a reference value of TA normalised to salinity. Different scenarios of temperature, salinity, fCO2 and nutrient contents were applied. Changes of DIC were found to be indeed related to both temperature and salinity. The latitudinal distribution of DIC could be inferred with an accuracy of 17 μmol kg−1 and a standard deviation of 13 μmol kg−1 from in situ salinity, in situ temperature and the reference values of TA and nutrient contents normalised to in situ salinity (scenario D). The applied technique of estimating DIC from temperature and salinity is a powerful diagnostic tool to evaluate the spatial distribution of DIC.

Factors controlling phytoplankton ice-edge blooms in the marginal ice-zone of the northwestern Weddell Sea during sea ice retreat 1988:field observations and mathematical modelling

The factors controlling phytoplankton bloom development in the marginal ice zone of the northwestern Weddell Sea were investigated during the EPOS (Leg 2) expedition (1988). Measurements were made of physical and chemical processes and biological activities associated with the process of ice-melting and their controlling variables particularly light limitation mediated by vertical stability and ice-cover, trace metal deficiency and grazing pressure. The combined observations and process studies show that the initiation of the phytoplankton bloom, dominated by nanoplanktonic species, was determined by the physical processes operating in the marginal ice zone at the time of ice melting. The additional effects of grazing pressure by protozoa and deep mixing appeared responsible for a rather moderate phytoplankton biomass (4 mg Chl a m−3) with a relatively narrow geographical extent (100–150 km). The rôle of trace constituents, in particular iron, was minor. The importance of each factor during the seasonal development of the ice-edge phytoplankton bloom was studied through modelling of reasonable scenarios of meteorological and biological forcing, making use of a one-dimensional coupled physical-biological model. The analysis of simulations clearly shows that wind mixing events – their duration, strength and frequency – determines both the distance from the ice-edge of the sea ice associated phytoplankton bloom and the occurrence in the ice-free area of secondary phytoplankton blooms during the summer period. The magnitude and extent of the ice-edge bloom is determined by the combined action of meteorological conditions and grazing pressure. In the absence of grazers, a maximum ice-edge bloom of 7.5 mg Chl a m−3 is predicted under averaged wind conditions of 8 m s−1. Extreme constant wind scenarios (4-14 m s−1) combined with realistic grazing pressure predict maximum ice-edge phytoplankton concentrations varying from 11.5 to 2 mg Chl a m−3. Persistent violent wind conditions (≥ 14 m s−1) are shown to prevent blooms from developing even during the brightest period of the year

Vertical flux of fatty acids in the North Atlantic Ocean

The quantitative and qualitative composition of fatty acids in particulate material collected in traps deployed during 98 days at 389, 988, 3755 and 5068 m depths in the equatorial North Atlantic was determined. The fatty acid composition indicates a predominantly marine source (14:0, 16:0, 16:1, 18:0, 18:1, 20:5, 20:4, 22:6, 22:5) with possibly a minor terrigenous component in the bathypelagic traps. The vertical fluxes of fatty acids and lipids decrease rapidly with depth. The rate of net loss of carboxylic acids increases with number of double bonds and decreases with number of carbon atoms. Iso- and anteiso- as well as some monoenoic fatty acids are more persistent, probably due to enhanced microbial synthesis during settling which counteracts degradation

Calcium carbonate saturation states along the West Antarctic Peninsula

The waters along the West Antarctic Peninsula (WAP) have experienced warming and increased freshwater inputs from melting sea ice and glaciers in recent decades. Challenges exist in understanding the consequences of these changes on the inorganic carbon system in this ecologically important and highly productive ecosystem. Distributions of dissolved inorganic carbon (CT), total alkalinity (AT) and nutrients revealed key physical, biological and biogeochemical controls of the calcium carbonate saturation state (Ωaragonite) in different water masses across the WAP shelf during the summer. Biological production in spring and summer dominated changes in surface water Ωaragonite (ΔΩaragonite up to +1.39; ∼90%) relative to underlying Winter Water. Sea-ice and glacial meltwater constituted a minor source of AT that increased surface water Ωaragonite (ΔΩaragonite up to +0.07; ∼13%). Remineralization of organic matter and an influx of carbon-rich brines led to cross-shelf decreases in Ωaragonite in Winter Water and Circumpolar Deep Water. A strong biological carbon pump over the shelf created Ωaragonite oversaturation in surface waters and suppression of Ωaragonite in subsurface waters. Undersaturation of aragonite occurred at < ∼1000 m. Ongoing changes along the WAP will impact the biologically driven and meltwater-driven processes that influence the vulnerability of shelf waters to calcium carbonate undersaturation in the future

Winter-summer differences of carbon dioxide and oxygen in the Weddell Sea surface layer

Mid-winter total inorganic carbon (TCO2) and oxygen measurements are presented for the central fully ice-covered Weddell Sea. Lateral variations of these properties in the surface layer of the central Weddell Sea were small, but significant. These variations were caused by vertical transport of Warm Deep Water into the surface layer and air-sea exchange before the ice cover. Oxygen saturation in the surface layer of the central Weddell Sea was near 82%, whereas in the eastern shelf area this was 89%. Surprisingly, pCO2, as calculated under the assumption of (reported) conservativeness of alkalinity, was also found to be below saturation (86-93%). This was not expected since ongoing Warm Deep Water entrainment into the surface layer tends to increase the pCO2. Rapid cooling and subsequent ice formation during the previous autumn, however, might have brought about a sufficiently low undersaturation of CO2, that as to the point of sampling had not yet been replenished through Warm Deep Water entrainment.In the ensuing early summer the measurements were repeated. In the shelf area and the central Weddell Sea, where the ice-cover had almost disappeared, photosynthesis had caused a decrease of pCO2 and an increase of oxygen compared to the previous winter. Inbetween these two regions there was an area with significant ice-cover where essentially winter conditions prevailed.Based on the summer-winter difference a (late-winter) entrainment rate of Warm Deep Water into the surface layer of 4-5 m/month was calculated. A complete surface water balance, including entrainment, biological activity and air-sea exchange, showed that between the winter and summer cruises CO2 and oxygen had both been absorbed from the atmosphere. The TCO2 increase due to entrainment of Warm Deep Water was partly countered by (autumn) cooling, and partly through biological drawdown. Part of the CO2 removed through biological activity sinks down the water column as organic material and is remineralised at depth. It is well-known that bottom water formation constitutes a sink for atmospheric CO2. However, whether the Weddell Sea as a whole is a sink for CO2 depends on the ratio of two counteracting processes, i.e. entrainment, which increases CO2 in the surface and the biological pump, which decreases it. As deep water is not only entrained into the surface, but also conveyed out of the Weddell Sea, the relative importances of these (CO2-enriched) deep water transports are important as well