Description: The Rare Earth Elements (REEs) have been widely used to investigate marine biogeochemical processes as well as the sources and mixing of water masses. However, there are still important uncertainties about the global aqueous REE cycle with respect to the contributions of highly reactive basaltic minerals originating from volcanic islands and the role of Submarine Groundwater Discharge (SGD). Here we present dissolved REE concentrations obtained from waters at the island-ocean interface (including SGD, river, lagoon and coastal waters) from the island of Tahiti and from three detailed open ocean profiles on the Manihiki Plateau (including neodymium (Nd) isotope compositions), which are located in ocean currents downstream of Tahiti. Tahitian fresh waters have highly variable REE concentrations that likely result from variable water–rock interaction and removal by secondary minerals. In contrast to studies on other islands, the SGD samples do not exhibit elevated REE concentrations but have distinctive REE distributions and Y/Ho ratios. The basaltic Tahitian rocks impart a REE pattern to the waters characterized by a middle REE enrichment, with a peak at europium similar to groundwaters and coastal waters of other volcanic islands in the Pacific. However, the basaltic island REE characteristics (with the exception of elevated Y/Ho ratios) are lost during transport to the Manihiki Plateau within surface waters that also exhibit highly radiogenic Nd isotope signatures. Our new data demonstrate that REE concentrations are enriched in Tahitian coastal water, but without multidimensional sampling, basaltic island Nd flux estimates range over orders of magnitude from relatively small to globally significant. Antarctic Intermediate Water (AAIW) loses its characteristic Nd isotopic signature (-6 to-9) around the Manihiki Plateau as a consequence of mixing with South Equatorial Pacific Intermediate Water (SEqPIW), which shows more positive values (-1 to -2). However, an additional Nd input/exchange along the pathway of AAIW, eventually originating from the volcanic Society, Tuamotu and Tubuai Islands (including Tahiti), is indicated by an offset from the mixing array of AAIW and SEqPIW to more radiogenic Nd isotope compositions.

Description: We present the first set of dissolved silicon isotope data in seawater (delta Si-30(Si(OH)4)) from the East China Sea, a large and productive marginal sea significantly influenced by the Kuroshio Current and freshwater inputs from the Changjiang (Yangtze River). In summer (August 2009), the lowest surface delta Si-30(Si(OH)4) signatures of +2.1 parts per thousand corresponding to the highest Si(OH)(4) concentrations (similar to 30.0 mu mol L-1) were observed nearshore in Changjiang Diluted Water. During advection on the East China Sea inner shelf, surface delta Si-30(Si(OH)4) increased rapidly to +3.2 parts per thousand while Si(OH)(4) became depleted, indicating increasing biological utilization of the Si(OH)(4) originating from the Changjiang Diluted Water. This is also reflected in the water column profiles characterized by a general decrease of delta Si-30(Si(OH)4) and an increase of Si(OH)(4) with depth on the East China Sea mid-shelf and slope. In winter (December 2009-January 2010), however, the delta Si-30(Si(OH)4) was nearly constant at +1.9 parts per thousand throughout the water column on the East China Sea shelf beyond the nearshore, which was a consequence of enhanced vertical mixing of the Kuroshio subsurface water. Horizontal admixture of Kuroshio surface water, which is highly fractionated in Si isotopes, was observed only beyond the shelf break. Significant seasonal differences in delta Si-30(Si(OH)4) were detected in the surface waters beyond the Changjiang Diluted Water-influenced region on the East China Sea shelf, where the winter values were similar to 1.0 parts per thousand lower than those in summer, despite the same primary Si(OH)(4) supply from the Kuroshio subsurface water during both seasons. This demonstrates significantly higher biological consumption and utilization of Si(OH)(4) in summer than in winter.

Description: We present a comparison of the dissolved stable isotope composition of silicate (δ30Si(OH)4) and nitrate (δ15 inline image) to investigate the biogeochemical processes controlling nutrient cycling in the upwelling area off Peru, where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Besides strong upwelling of nutrient rich waters mainly favoring diatom growth, an anticyclonic eddy influenced the study area. We observe a tight coupling between the silicon (Si) and nitrogen (N) cycles in the study area. Waters on the shelf showed high Si(OH)4 concentrations accompanied by diminished inline image concentration as a consequence of intense remineralization, high Si fluxes from the shelf sediments, and N-loss processes such as anammox/denitrification within the OMZ. Correspondingly, the surface waters show low δ30Si(OH)4 values (+2‰) due to low Si utilization but relatively high δ15 inline image (+13‰) values due to upwelling of waters influenced by N-loss processes. In contrast, as a consequence of the deepening of the thermocline in the eddy center, a pronounced Si(OH)4 depletion led to the highest δ30Si(OH)4 values (+3.7‰) accompanied by high δ15 inline image values (+16‰). In the eddy center, high inline image: Si(OH)4 ratios favored the growth of non-siliceous organisms (Synechococcus). Our data show that upwelling processes and the presence of eddies play important roles controlling the nutrient cycles and therefore also exert a major influence on the phytoplankton communities in the Peruvian Upwelling. Our findings also show that the combined approach of δ30Si(OH)4 and δ15 inline image can improve our understanding of paleo records as it can help to disentangle utilization and N-loss processes.