Description: Dissolved silicon isotope compositions have been analysed for the first time in pore waters (δ30SiPW) of three short sediment cores from the Peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments. The δ30SiPW varies between +1.1‰ and +1.9‰ with the highest values occurring in the uppermost part close to the sediment–water interface. These values are of the same order or higher than the δ30Si of the biogenic opal extracted from the same sediments (+0.3‰ to +1.2‰) and of the overlying bottom waters (+1.1‰ to +1.5‰). Together with dissolved silicic acid concentrations well below biogenic opal saturation, our collective observations are consistent with the formation of authigenic alumino-silicates from the dissolving biogenic opal. Using a numerical transport-reaction model we find that approximately 24% of the dissolving biogenic opal is re-precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56 μmol Si cm−2 yr−1. The fractionation factor between the precipitates and the pore waters is estimated at −2.0‰. Dissolved and solid cation concentrations further indicate that off Peru, where biogenic opal concentrations in the sediments are high, the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino-silicate phases.

Description: The stable silicon isotopic composition (δ30Si) of waters and diatoms has increasingly been used to investigate the biogeochemical cycling of Si in the major ocean basins. Here we present the first Si isotope data set from the northern South China Sea (NSCS), a large marginal sea system in the western North Pacific to examine sources and utilization of silicic acid (Si(OH)4). During two cruises in July–August 2009 (summer) and January 2010 (winter), samples for isotope measurements of dissolved Si(OH)4 (δ30SiSi(OH)4) and of biogenic silica (δ30SiBSi) in suspended particles were collected along a transect perpendicular to the coast from the inner shelf to the deep-water slope, as well as at the South East Asian Time-series Study (SEATS) station located in the NSCS basin. Surface δ30SiSi(OH)4 generally increased from values ∼+2.3‰ on the inner shelf to ∼+2.8‰ above the deep basin, suggesting an increasing utilization of dissolved Si(OH)4 reflecting the transition from eutrophic to oligotrophic conditions. The δ30SiBSi values were systematically lower than the corresponding δ30SiSi(OH)4 in the euphotic zone (above 100 m) on the shelf and slope. In contrast at station SEATS in the NSCS basin, δ30SiBSi signatures in both seasons were within error equal to δ30SiSi(OH)4 in the surface mixed layer (above 50 m) and δ30SiBSi in waters below were significantly higher than the corresponding δ30SiSi(OH)4. By comparing the field data with the Si isotope fractionation revealed by the Rayleigh or the steady state models, we demonstrate the existence of variable Si(OH)4 origins in different areas of the NSCS. Surface waters on the inner shelf were largely fed by nutrients from the Pearl River input. While the primary source of Si(OH)4 for the euphotic zone on the outer shelf and slope was upwelling or vertical mixing from underlying waters, the Si(OH)4 in the surface mixed layer of the NSCS basin might have originated from horizontal mixing with other highly fractionated surface waters. As a consequence, the Si isotope dynamics in the NSCS are largely controlled by variable biological fractionation of Si in waters from different sources with different initial Si isotopic compositions rather than any single source water.

Description: We combine the stable silicon isotope composition (δ30Si) of diatoms and the radiogenic neodymium isotope compositions (εNd) of past seawater extracted from the authigenic fraction of the sediments (Mn–Fe coatings of particles and benthic foraminifers), as well as the radiogenic isotope compositions (Nd, Sr) of the detrital material itself to reconstruct silicic acid utilisation, water mass mixing, and upwelling intensity from the same marine sediments in the Peruvian upwelling region during the past 20,000 years. Additionally, the sedimentary signals were compared to the water column isotope compositions. Along the Peruvian shelf, the dissolved εNd in the water column ranges from −5.7 to +0.6. The corresponding εNd signatures of the coatings and the benthic foraminifers of the surface sediments range from −4.5 to +1.8 and from −2.5 to +2.2, respectively. The detrital εNd (87Sr/86Sr) signatures range from −6.3 to 0 (0.70508–0.71049). All phases show a trend from more radiogenic εNd values in the north towards less radiogenic values in the south broadly reflecting local weathering inputs and hinterland geology. The εNd signatures of the coatings extracted from sediment core SO147-106KL located in the present day centre of coastal upwelling near 12°S have been essentially constant (−1.5) throughout the past 20,000 years, while the detrital εNd (87Sr/86Sr) varied between values of −0.7 (0.70620) during the Last Glacial and −4.5 (0.70849) during the late Holocene reflecting changes in the origin of the sediment and current transport from a more northerly towards a more southerly source and from overall stronger to weaker upwelling. The δ30Si signature of both total biogenic opal (δ30Siopal) and of hand-picked diatoms (δ30Sidiatom) ranged from +0.3‰ (Last Glacial) to +1.4‰ (late Holocene) confirming large variations in upwelling intensity driving silicic acid utilisation by diatoms. Our combined bSiO2 MAR, δ30Siopal and detrital radiogenic isotope results demonstrate that the strongest El Niño-Southern Oscillation conditions off Peru of the past 20 ka have prevailed during the past 5 ka.

Description: Silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon (Si). In this study the distribution of the Si isotope composition of dissolved silicic acid (δ30Si(OH)4) was analyzed in the water column of the Eastern Equatorial Pacific (EEP) where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Samples were collected at 7 stations along two meridional transects from the equator to 14°S at 85°50′W and 82°00′W off the Ecuadorian and Peruvian coast. Surface waters show a large range in isotope compositions δ30Si(OH)4 (+2.2‰ to +4.4‰) with the highest values found at the southernmost station at 14°S. This station also revealed the most depleted silicic acid concentrations (0.2 μmol/kg), which is a function of the high degree of Si utilization by diatoms and admixture with waters from highly productive areas. Samples within the upper water column and the OMZ at oxygen concentrations below 10 μmol/kg are characterized by a large range in δ30Si(OH)4, which mainly reflects advection and mixing of different water masses, even though the highly dynamic hydrographic system of the upwelling area off Peru does not allow the identification of clear Si isotope signals for distinct water masses. Therefore we cannot rule out that also dissolution processes have an influence on the δ30Si(OH)4 signature in the subsurface water column. Deep water masses (〉2000 m) in the study area show a mean δ30Si(OH)4 of +1.2±0.2‰, which is in agreement with previous studies from the eastern and central Pacific. Comparison of the new deep water data of this study and previously published data from the central Pacific and Southern Ocean reveal substantially higher δ30Si(OH)4 values than deep water signatures from the North Pacific. As there is no clear correlation between δ30Si(OH)4 and silicic acid concentrations in the entire data set the distribution of δ30Si(OH)4 signatures in deep waters of the Pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct Si isotope signatures including Lower Circumpolar Deep Water (LCDW) and North Pacific Deep Water (NPDW).

Description: We present the first systematic study of the silicon isotope composition in the water column (δ30SiSi(OH)4) and in diatoms (δ30Sidiatom) from the underlying surface sediments in a coastal upwelling region. The surface waters upwelling on the shelf off Peru are mainly fed by southward flowing subsurface waters along the coast, which show a mean δ30SiSi(OH)4 of +1.5‰. The concentration of dissolved silicic acid (Si(OH)4) increases towards the south in these waters and with increasing water depth, suggesting lateral mixing with water masses from the south and intense remineralisation of particulate biogenic silica (bSiO2) in the water column and in the surface sediments. Surface waters in the realm of the most intense upwelling between 5°S and 15°S have only marginally elevated δ30SiSi(OH)4 values (δ30SiSi(OH)4 = +1.7‰) with respect to the source Si isotope composition, whereas further north and south, where upwelling is less pronounced, surface waters are more strongly fractionated (δ30SiSi(OH)4 up to +2.8‰) due to the stronger utilisation of the smaller amounts of available Si(OH)4. The degree of Si(OH)4 utilisation in the surface waters along the shelf estimated from the Si(OH)4 concentration data ranges from 51% to 93%. The δ30Sidiatom values of hand-picked diatoms in the underlying surface sediments vary from +0.6‰ to +2.0‰, which is within the range of the expected fractionation between surface waters and diatoms. The fractionation signal in the surface waters produced during formation of the diatoms is reflected by the δ30Sidiatom values in the underlying sediments, with the lowest δ30Sidiatom values in the main upwelling region. The silicon isotope compositions of bSiO2 (δ30SibSiO2) from the same surface sediment samples are generally much lower than the δ30Sidiatom signatures indicating a significant contamination of the bSiO2 with biogenic siliceous material other than diatoms, such as sponge spicules. This shift towards lighter δ30SibSiO2 values by up to −1.3‰ compared to δ30Sidiatom signatures for the same surface sediment samples potentially biases the interpretation of δ30Si paleorecords from sediments with low bSiO2 concentrations, and thus the reconstruction of past Si(OH)4 utilisation in surface waters.

Description: In this study we combine for the first time silicon (Si) isotope compositions of small mixed diatom species (δ30SibSiO2) and of large handpicked mono-generic (i.e. genus = Coscinodiscus) diatom samples (δ30SiCoscino) with diatom assemblages extracted from marine sediments in the Peruvian upwelling region in order to constrain present and past silicate utilisation. The extension of a previous core-top data set from the Peruvian shelf demonstrates that δ30SiCoscino values record near-complete Si utilisation, as these are similar to the isotopic composition of the subsurface source waters feeding the upwelling. In contrast, the δ30SibSiO2 of small mixed diatom species increase southward along the shelf as well as towards the shore. We attribute highest δ30SibSiO2 values partly to transient iron limitation but primarily to the gradual increase of Si isotope fractionation within the seasonal diatom succession, which are mainly recorded by small diatom species during intense bloom events. In contrast, lower δ30SibSiO2 values are related to initial Si isotope utilisation during periods of weak upwelling, when low Si(OH)4 concentrations do not permit intense blooms and small diatom species record substantially lower δ30Si signatures. As such, we propose that the intensity of the upwelling can be deduced from the offset between δ30SibSiO2 and δ30SiCoscino (Δ30Sicoscino–bSiO2), which is low for strong upwelling conditions and high for prevailing weak upwelling. We apply the information extracted from surface sediments to generate a record of the present-day main upwelling region covering the past 17,700 years and find that this location has also been characterized by a persistent offset (Δ30Sicoscino–bSiO2). By comparison with the diatom assemblages we show that the coastal upwelling system changed markedly between weak and strong upwelling conditions. In addition, our model calculations to quantify species-specific Si isotope fractionation effects based on the diatom assemblages indicate an overall minor influence that cannot explain the high amplitude in the measured δ30SibSiO2 record.

Description: Dissolved silicon isotope compositions (δ30Si) have been analysed for the first time in groundwaters of beach sediments, which represent a subterranean estuary with fresh groundwater discharge from a freshwater reservoir and mixing with recirculated seawater. The fresh groundwater reservoir has high and variable dissolved silica concentrations between 136 and 736 μM, but homogeneous δ30Si of +1.0 ± 0.15‰. By contrast, the seawater is strongly depleted in dissolved silica with concentrations of 3 μM, and consequently characterised by high δ30Si of +3.0‰. The beach groundwaters are variably enriched in dissolved silica compared to seawater (23–192 μM), and concentrations increase with depth at all sampling sites. The corresponding δ30Si values are highly variable (+0.3‰ to +2.2‰) and decrease with depth at each site. All groundwater δ30Si values are lower than seawater and most values are lower than dissolved δ30Si of freshwater discharge indicating a significant amount of lithogenic silica dissolution in beach sediments. In contrast to open North Sea sediments, diatom dissolution or formation of authigenic silica in beach sediments is very low (ca. 5 μmol Si g−1). Silica discharge from the beach to the coastal ocean is estimated as approximately 210 mol Si yr−1 per meter shoreline. Considering the extent of coastline this is, at least for the study area, a significant amount of the total Si budget and amounts to ca. 1% of river and 3.5% of backbarrier tidal flat area Si input.