Continental shelves and shelf seas play a central role in the global carbon cycle. However,

their importance with respect to trace element and isotope (TEI) inputs to ocean basins

is less well understood. Here, we present major findings on shelf TEI biogeochemistry

from the GEOTRACES programme as well as a proof of concept for a new method to

estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI

cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment

micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The

proposed shelf flux tracer is 228-radium (T1/2 =5.75 yr), which is continuously supplied to

the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf

228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the

western North Atlantic margin. The results from this new approach agree well with previous

estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric

deposition by factors of approximately 3–23. Lastly, recommendations are made for additional

GEOTRACES process studies and coastal margin-focused section cruises that will help refine

the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes

to the ocean.

This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element

chemistry’.

The oxygenation of the atmosphere ∼2.45–2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth’s redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (δ65CuERM-AE633) in organic carbon-rich shales spanning the period 2.66–2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in 65Cu, along with the preferential removal of 65Cu by iron oxides, left seawater and marine biomass depleted in 65Cu but enriched in 63Cu. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy δ65Cu values coincides with a shift to negative sedimentary δ56Fe values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary δ65Cu values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.

Strontium isotope analysis of skeletal material as a means to reconstruct prehistoric residential patterns has previously mainly been applied to populations with terrestrial diets. Here we present a model for populations with mixed marine/terrestrial diets, which is based on two-component mixing of strontium isotopes. Applying this model, we can estimate the original strontium isotope value of the terrestrial component of the diet. Accordingly it is possible to identify non-local individuals even if they had a mixed marine/terrestrial diet. The model is applied to tooth enamel samples representing nine individuals recovered from a passage grave in Resmo, on the island of Öland in the Baltic Sea, where at least five non-local individuals, representing at least two different geographical regions of origin, were identified. Non-local individuals were more frequent during the Bronze Age than during previous phases.

The first inter-calibration study of the stable silicon isotope composition of dissolved silicic acid in seawater, d³⁰Si(OH)₄, is presented as a contribution to the international GEOTRACES program. Eleven laboratories from seven countries analyzed two seawater samples from the North Pacific subtropical gyre (Station ALOHA) collected at 300 m and at 1000 m water depth. Sampling depths were chosen to obtain samples with a relatively low (9 mmol L^-1, 300 m) and a relatively high (113 mmol L^-1, 1000 m) silicic acid concentration as sample preparation differs for low- and high concentration samples. Data for the 1000 m water sample were not normally distributed so the median is used to represent the central tendency for the two samples. Median d³⁰Si(OH)₄ values of +1.66‰ for the low-concentration sample and +1.25‰ for the high-concentration sample were obtained. Agreement among laboratories is overall considered very good; however, small but statistically significant differences among the mean isotope values obtained by different laboratories were detected, likely reflecting inter-laboratory differences in chemical preparation including preconcentration and purification methods together with different volumes of seawater analyzed, andthe use of different mass spectrometers including the Neptune MC-ICP-MS (Thermo Fisher™, Germany), the Nu Plasma MC-ICP-MS (Nu Instruments™, Wrexham, UK), and the Finnigan™ (now Thermo Fisher™, Germany) MAT 252 IRMS. Future studies analyzing d³⁰Si(OH)₄ in seawater should also analyze and report values for these same two reference waters in order to facilitate comparison of data generated among and within laboratories over time.

Submarine groundwater discharge (SGD) has been identified as an

important source of Sr to the ocean and the SGD-driven Sr flux to the

global ocean has been recently re-evaluated (Beck et al. 2013). However,

the uncertainty of this value is still high because of the uncertainties

related to the determination of SGD flow rates and the paucity of

87Sr/86Sr data in SGD end-members. As carbonates have high Sr

concentrations and are subjected to intense heightened weathering, they

might significantly influence the SGD input of Sr to the ocean. Here we

present data on Sr concentrations and 87Sr/86Sr ratios in three carbonate

dominated sites of the western area of the Mediterranean Sea, a semienclosed

basin characterized by abundant coastal carbonates. The

87Sr/86Sr ratios in groundwater were lower compared to modern seawater (~

0.70916), as expected for areas dominated by carbonate lithologies.

Concentrations of Sr and 87Sr/86Sr ratios in groundwater showed

conservative mixing in the studied subterranean estuaries. By using SGD

flow rates reported in the literature for the study areas, a meteoric

SGD-driven Sr flux of (0.12 - 2.1)·103 mol d-1 km-1 was calculated for

the region, with a fresh SGD end-member characterized by a Sr

concentration of 27 - 30 μM and a 87Sr/86Sr ratio of 0.708020 - 0.707834.

Integrating these Sr data with literature data (i.e. values of Sr

concentration and 87Sr/86Sr ratio from other lithologies as well as SGD

flow rates), we also calculated the fresh SGD-driven Sr flux to the

entire Mediterranean Sea, obtaining a value of (0.34 - 0.83)·109 mol y-1,

with a 87Sr/86Sr of 0.7086 - 0.7081. Thus, for the entire Mediterranean

basin, SGD is globally a source of Sr less radiogenic compared to

seawater. The SGD Sr flux to the Mediterranean Sea represents 5-6% of the

SGD Sr flux to the global ocean and the Mediterranean SGD end-member has

higher Sr concentration (5.0 - 12 μM) than the global SGD end-member (2.9

μM). This confirms the significant role of carbonate lithologies on SGDdriven

Sr fluxes to seawater.

The fresh SGD-driven Sr flux to the Mediterranean Sea is about 20 - 50%

of the riverine Sr input and significantly higher than the input through

atmospheric dust deposition. Therefore SGD should be considered as an

important continental source of Sr to the basin.

Summer iron (Fe) fertilization in the Ross Sea has previously been observed in association with diatom productivity, lithogenic particles and excess Fe in the water column. This productivity event occurred during an early breakout of sea ice via katabatic winds, suggesting that aeolian dust could be an important source of lithogenic Fe required for diatom growth in the Ross Sea. Here we investigate the provenance of size-selected dust deposited on sea ice in McMurdo Sound, south-western (SW) Ross Sea. The isotopic signature of McMurdo Sound dust (0.70533< 87Sr/86Sr< 0.70915 and -1.1 < εNd(0) <3.45)confirms that dust is locally sourced from the McMurdo Sound debris bands and comprises a two-component mixture of McMurdo Volcanic Group and southern Victoria Land lithologies. In addition, the provenance of lithogenic sediment trapped in the water column was investigated, and the isotopic signature (εNd(0) =3.9, 87Sr/86Sr = 0.70434) is differentiated from long-range transported dust originating from South America and Australia. Elevated lithogenic accumulation rates in deeper sediment traps in the Ross Sea suggest that sinking articles in the water column cannot simply result from dust input at the surface. This discrepancy can be best explained by significant upwelling and remobilization of lithogenic Fe from the sea floor.

The Ross Sea, Antarctica, is a highly productive region of the Southern Ocean. Significant new sources of iron (Fe) are required to sustain phytoplankton blooms in the austral summer. Atmospheric deposition is one potential source. The fractional solubility of Fe is an important variable determining Fe availability for biological uptake. To constrain aerosol Fe inputs to the Ross Sea region, fractional solubility of Fe was analyzed in a snow pit from Roosevelt Island, eastern Ross Sea. In addition, aluminum, dust, and refractory black carbon (rBC) concentrations were analyzed, to determine the contribution of mineral dust and combustion sources to the supply of aerosol Fe. We estimate exceptionally high dissolved Fe (dFe) flux of 1.2 × 10⁻⁶ g m⁻² y⁻¹ and total dissolvable Fe flux of 140 × 10⁻⁶ g m⁻² y⁻¹ for 2011/2012. Deposition of dust, Fe, Al, and rBC occurs primarily during spring-summer. The observed background fractional Fe solubility of ~0.7% is consistent with a mineral dust source. Radiogenic isotopic ratios and particle size distribution of dust indicates that the site is influenced by local and remote sources. In 2011/2012 summer, relatively high dFe concentrations paralleled both mineral dust and rBC deposition. Around half of the annual aerosol Fe deposition occurred in the austral summer phytoplankton growth season; however, the fractional Fe solubility was low. Our results suggest that the seasonality of dFe deposition can vary and should be considered on longer glacial-interglacial timescales.

The Kalix River shows distinct temporal variations in the Sr-isotope ratio in filtered water (0.726 to 0.732). During base flow in winter the ⁸⁷Sr/⁸⁶Sr ratio is on average 0.730. When discharge increases and peaks during spring flood the ⁸⁷Sr/⁸⁶Sr ratio shows the most radiogenic (0.732) values. The temporal variations in the ⁸⁷Sr/⁸⁶Sr ratio in the Kalix River can be explained by mixing of water from the woodlands and the mountain areas.

During high water discharge in May the ⁸⁷Sr/⁸⁶Sr ratios are more radiogenic in the suspended phase (1 kDa - 70 µm) compared to the truly dissolved phase (<1 kDa). The difference in ⁸⁷Sr/⁸⁶Sr ratio between the two phases (Δ ⁸⁷Sr/⁸⁶Sr) is linearly correlated with the suspended iron concentration. During spring flood Sr and Fe derived from an additional source, reach the river. Deep groundwater has a more radiogenic ⁸⁷Sr/⁸⁶Sr isotope ratio than the Kalix River during spring flood and thus, represents a possible source for the suspended Fe and the associated Sr. Strontium can be coprecipitated with and adsorbed to different types of Fe aggregates. We propose that the Sr-isotope ratio in the suspended phase reflects the isotopic composition of the water at the interface between anoxic groundwater and oxic stream water in the riparian zone, where the Fe aggregates are formed. These particles dominate the suspended phase in the river and the mixing with mountain waters, poor in Fe, produces the difference in the isotopic signature. The different signatures in suspended and truly dissolved fraction indicate that these aggregates are relatively stable during stream-river transport. As such the ⁸⁷Sr/⁸⁶Sr can be used to trace the origin of the non-detrital suspended phase.