Description: Highlights • We present a Ba isotope data set of seawater, river waters and biogenic particles. • Ba isotope signatures of upper ocean waters are heavier than river and deep waters. • Adsorption of lighter Ba isotopes on biogenic particles induces the fractionation. • Ba isotopes trace land–sea interactions and ocean mixing processes. • Decoupling of Ba from major nutrients confirms Ba to be a biointermediate element. Abstract The distribution of barium (Ba) concentrations in seawater resembles that of nutrients and Ba has been widely used as a proxy of paleoproductivity. However, the exact mechanisms controlling the nutrient-like behavior, and thus the fundamentals of Ba chemistry in the ocean, have not been fully resolved. Here we present a set of full water column dissolved Ba (DBa) isotope (δ137BaDBa) profiles from the South China Sea and the East China Sea that receives large freshwater inputs from the Changjiang (Yangtze River). We find pronounced and systematic horizontal and depth dependent δ137BaDBa gradients. Beyond the river influence characterized by generally light signatures (0.0 to +0.3‰+0.3‰), the δ137BaDBa values in the upper water column are significantly higher (+0.9‰+0.9‰) than those in the deep waters (+0.5‰+0.5‰). Moreover, δ137BaDBa signatures are essentially constant in the entire upper 100 m, in which dissolved silicon isotopes are fractionated during diatom growth resulting in the heaviest isotopic compositions in the very surface waters. Combined with the decoupling of DBa concentrations and δ137BaDBa from the concentrations of nitrate and phosphate this implies that the apparent nutrient-like fractionation of Ba isotopes in seawater is primarily induced by preferential adsorption of the lighter isotopes onto biogenic particles rather than by biological utilization. The subsurface δ137BaDBa distribution is dominated by water mass mixing. The application of stable Ba isotopes as a proxy for nutrient cycling should therefore be considered with caution and both biological and physical processes need to be considered. Clearly, however, Ba isotopes show great potential as a new tracer for land–sea interactions and ocean mixing processes.

Description: One of the key activities during the initial phase of the international GEOTRACES program was an extensive international intercalibration effort, to ensure that results for a range of trace elements and isotopes (TEIs) from different cruises and from different laboratories can be compared in a meaningful way. Here we present the results from the intercalibration efforts on neodymium isotopes and rare earth elements in seawater and marine particles. Fifteen different laboratories reported results for dissolved 143Nd/144Nd ratios in seawater at three different locations (BATS 15 m, BATS 2000 m, SAFe 3000 m), with an overall agreement within 47 to 57 ppm (2σ standard deviation of the mean). A similar agreement was found for analyses of an unknown pure Nd standard solution carried out by 13 laboratories (56 ppm), indicating that mass spectrometry is the main variable in achieving accurate and precise Nd isotope ratios. Overall, this result is very satisfactory, as the achieved precision is a factor of 40 better than the range of Nd isotopic compositions observed in the global ocean. Intercalibration for dissolved rare earth element concentrations (REEs) by six laboratories for two water depths at BATS yielded a reproducibility of 15% or better for all REE except Ce, which seems to be the most blank-sensitive REE. Neodymium concentrations from 12 laboratories show an agreement within 9%, reflecting the best currently possible reproducibility. Results for Nd isotopic compositions and REE concentrations on marine particles are inconclusive, and should be revisited in the future.

Description: Previous studies have demonstrated the potential for the Li content of coral aragonite to record information about environmental conditions, but no detailed study of tropical corals exists. Here we present the Li and Mg to Ca ratios at a bimonthly to monthly resolution over 25 years in two modern Porites corals, the genus most often used for paleoclimate reconstructions in the tropical Indo-Pacific. A strong relationship exists between coral Li/Ca and locally measured SST, indicating that coral Li/Ca can be used to reconstruct tropical SST variations. However, Li/Ca ratios of the skeleton deposited during 1979-1980 do not track local SST well and are anomalously high in places. The Mg/Ca ratios of this interval are also anomalously high, and we suggest Li/Ca can be used to reconstruct tropical SST only when Mg/Ca data are used to carefully screen for relatively rare biological effects. Mg/Li or Li/Mg ratios provide little advantage over Li/Ca ratios, except that the slope of the Li/Mg temperature relationship is more similar between the two corals. The Mg/Li temperature relationship for the coral that experienced a large temperature range is similar to that found for cold water corals and aragonitic benthic foraminifera in previous studies. The comparison with data from other biogenic aragonites suggests the relationship between Li/Mg and water temperature can be described by a single exponential relationship. Despite this hint at an overarching control, it is clear that biological processes strongly influence coral Li/Ca, and more calibration work is required before widely applying the proxy

Description: The radiogenic neodymium (Nd) isotope composition of foraminiferal shells provides a powerful archive to investigate past changes in sources and mixing of water masses. However, seawater Nd isotope ratios extracted from foraminiferal shells can be biased by contaminant phases such as organic matter, silicates, or ferromanganese coatings, the removal of which requires rigorous multiple step cleaning of the samples. Here we investigate the efficiency of Flow Through and batch cleaning methods to extract seawater Nd isotope compositions from planktonic foraminifera in a shelf setting in the Gulf of Guinea that is strongly influenced by riverine sediment inputs. Nd isotope analyses of reductively and oxidatively cleaned mono-specific planktonic foraminiferal samples and reductively cleaned mixed benthic foraminifera were complemented by analyses of non-reductively cleaned mono-specific planktonic foraminiferal samples, Fe–Mn coatings of de-carbonated bulk sediment leachates, and the residual detrital fraction of the same sediment. Al/Ca and Mn/Ca ratios of fully cleaned foraminiferal samples reveal indistinguishable levels of cleaning efficiency between the batch and the Flow Through methods and the Nd isotope compositions obtained from application of both methods are identical within error. Furthermore, non-reductively cleaned foraminiferal samples have the same Nd isotope composition as reductively cleaned foraminifera at our study sites. Close to the Niger River mouth the Nd isotope composition of the foraminifera agree with the seawater Nd isotope composition of nearby stations. Based on the combined extracted Nd isotope signatures and element to calcium ratios, as well as rare earth element distribution patterns, we infer that the planktonic foraminiferal Nd isotope signatures reflect bottom water/pore water signatures. The isotopic composition of the bulk de-carbonated sediment leachates (Fe–Mn coatings) differs significantly from the foraminiferal data at this site and probably reflects particles that acquired their ferromanganese/pre-formed pre-formed/ferromanganese coatings in nearby rivers. Therefore, in such river influenced shelf settings foraminiferal shells should be used to obtain unbiased bottom seawater signatures.

Description: The recrystallisation (dissolution–precipitation) of carbonate sediments has been successfully modelled to explain profiles of pore water Sr concentration and radiogenic Sr isotope composition at different locations of the global ocean. However, there have been few systematic studies trying to better understand the relative importance of factors influencing the variability of carbonate recrystallisation. Here we present results from a multi-component study of recrystallisation in sediments from the Integrated Ocean Drilling Program (IODP) Expedition 320/321 Pacific Equatorial Age Transect (PEAT), where sediments of similar initial composition have been subjected to different diagenetic histories. The PEAT sites investigated exhibit variable pore water Sr concentrations gradients with the largest gradients in the youngest sites. Radiogenic Sr isotopes suggest recrystallisation was relative rapid, consistent with modelling of other sediment columns, as the 87Sr/86Sr ratios are indistinguishable (within 2σ uncertainties) from contemporaneous seawater 87Sr/86Sr ratios. Bulk carbonate leachates and associated pore waters of Site U1336 have lower 87Sr/86Sr ratios than contemporaneous seawater in sediments older than 20.2 Ma most likely resulting from the upward diffusion of Sr from older recrystallised carbonates. It seems that recrystallisation at Site U1336 may still be on-going at depths below 102.5 rmcd (revised metres composite depth) suggesting a late phase of recrystallisation. Furthermore, the lower Sr/Ca ratios of bulk carbonates of Site U1336 compared to the other PEAT sites suggest more extensive diagenetic alteration as less Sr is incorporated into secondary calcite. Compared to the other PEAT sites, U1336 has an inferred greater thermal gradient and a higher carbonate content. The enhanced thermal gradient seems to have made these sediments more reactive and enhanced recrystallisation. In this study we investigate stable Sr isotopes from carbonate-rich deep sea sediments for the first time. Pore water δ88/86Sr increases with depth (from 0.428‰ to values reaching up to 0.700‰) at Site U1336 documenting an isotope fractionation process during recrystallisation. Secondary calcite preferentially incorporates the lighter Sr isotope (86Sr) leaving pore waters isotopically heavy. The δ88/86Sr values of the carbonates themselves show more uniform values with no detectable change with depth. Carbonates have a much higher Sr content and total Sr inventory than the pore waters meaning pore waters are much more sensitive to fractionation processes than the carbonates. The δ88/86Sr results indicate that pore water stable Sr isotopes have the potential to indicate the recrystallisation of carbonate sediments.

Description: Highlights: • High-precision glacial–interglacial 87Sr/86Sr record from planktic foraminifera. • Major oceans yield indistinguishable 87Sr/86Sr values at ±5 ppm level of precision. • Foraminifera show no resolvable 87Sr/86Sr variation since last glacial interval. • These data accommodate a ±12% variation for the steady-state weathering flux. • A short-term weathering pulse during de-glaciation is not accommodated by the data. Abstract Existing strontium radiogenic isotope (87Sr/86Sr) measurements for foraminifera over Quaternary glacial–interglacial climate cycles provide no evidence for variations in the isotope composition of seawater at the ±9–13 ppm level of precision. However, modelling suggests that even within this level of uncertainty significant (up to 30%) variations in chemical weathering of the continents are permitted, accounting for the longer-term rise in 87Sr/86Sr over the Quaternary, and the apparent imbalance of Sr in the oceans at the present-day. This study presents very high-precision 87Sr/86Sr isotope data for modern seawater from each of the major oceans, and a glacial–interglacial seawater record preserved by planktic foraminifera from Ocean Drilling Program (ODP) Site 758 in the north-east Indian ocean. Strontium isotope 87Sr/86Sr measurements for modern seawater from the Atlantic, Pacific and Indian Oceans are indistinguishable from one another (87Sr/86Sr = 0.7091792 ± 0.0000021, n=17n=17) at the level of precision obtained in this study (±4.9 ppm 2σ). This observation is consistent with the very long residence time of Sr in seawater, and underpins the utility of this element for high precision isotope stratigraphy. The 87Sr/86Sr seawater record preserved by planktic foraminifera shows no resolvable glacial–interglacial variation (87Sr/86Sr = 0.7091784 ± 0.0000035, n=10n=10), and limits the response of seawater to variations in the chemical weathering flux and/or composition to ±4.9 ppm or less. Calculations suggest that a variation of ±12% around the steady-state weathering flux can be accommodated by the uncertainties obtained here. The new data cannot accommodate a short-term weathering pulse during de-glaciation, although a more a diffuse weathering pulse accompanying protracted ice retreat is permissible. However, these results still indicate that modern weathering fluxes are potentially higher than average over the Quaternary, and such variations through glacial cycles can also account for the longer-term rise in 87Sr/86Sr over this time interval. The very high-precision measurements made for the marine 87Sr/86Sr record in this study place clear limits on the magnitude and timing of changes in the chemical weathering flux during glacial–interglacial cycles. Further, constraints must be sought from even higher precision measurement or elements with shorter residence times in the ocean, such as osmium (Os), that have the capacity to respond to short-term variations in input.

Description: Radiogenic isotopes of hafnium (Hf) and neodymium (Nd) are powerful tracers for water mass transport and trace metal cycling in the present and past oceans. However, due to the scarcity of available data the processes governing their distribution are not well understood. Here we present the first combined dissolved Hf and Nd isotope and concentration data from surface waters of the Atlantic sector of the Southern Ocean. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during RV Polarstern expeditions ANTXXIV/3 and ANTXXIII/3 in the frame of the International Polar Year (IPY) and the GEOTRACES program. The general distribution of Hf and Nd concentrations in the region is similar. However, at the northernmost station located 200 km southwest of Cape Town a pronounced increase of the Nd concentration is observed, whereas the Hf concentration is minimal, suggesting much less Hf than Nd is released by the weathering of the South African Archean cratonic rocks. From the southern part of the Subtropical Front (STF) to the Polar Front (PF) Hf and Nd show the lowest concentrations (〈0.12 pmol/kg and 10 pmol/kg, respectively), most probably due to the low terrigenous flux in this area and efficient scavenging of Hf and Nd by biogenic opal. In the vicinity of landmasses the dissolved Hf and Nd isotope compositions are clearly labeled by terrigenous inputs. Near South Africa Nd isotope values as low as εNd = −18.9 indicate unradiogenic inputs supplied via the Agulhas Current. Further south the isotopic data show significant increases to εHf = 6.1 and εNd = −4.0 documenting exchange of seawater Nd and Hf with the Antarctic Peninsula. In the open Southern Ocean the Nd isotope compositions are relatively homogeneous (εNd ∼ −8 to −8.5) towards the STF, within the Antarctic Circumpolar Current, in the Weddell Gyre, and the Drake Passage. The Hf isotope compositions in the entire study area only show a small range between εHf = + 6.1 and +2.8 support Hf to be more readily released from young mafic rocks compared to old continental ones. The Nd isotope composition ranges from εNd = −18.9 to −4.0 showing Nd isotopes to be a sensitive tracer for the provenance of weathering inputs into surface waters of the Southern Ocean.

Description: Dissolved rare earth element (REE) concentrations and Nd isotope compositions were measured for surface waters and full water column profiles of the Gulf of Alaska (GoA), and compared to water mass properties and circulation in order to better understand the mechanisms controlling the input and transport of REEs in the ocean. The REEs display a typical open-ocean range of concentrations (i.e., La: 12-66 pM; Lu: 0.2-2.5 pM) and depth distributions (i.e., surface ocean depletion and enrichment with water depth). Nd isotope signatures are highly radiogenic, as expected for the North Pacific margin (ranging from -3.8 to +0.2 epsilon Nd). The most radiogenic values were found in the coastal waters but also in the cores of eddies, indicating efficient export of REEs from the margins and across the mixed layer. This is the first time that distinct Nd isotope distributions in near surface waters can be directly assigned to offshore eddy transport. A distinct mid-depth (similar to 2200 m) Nd isotope signal was found that most likely reflects advection of a water mass that formed through past down-welling in the Northern Pacific. Subsurface Nd isotope compositions appear to behave conservatively and can be explained through a REE distribution model proposed here. This model is based on multivariate analysis of the REEs and invokes two distinct "pools" of dissolved REEs: a "passive pool" complexed by carbonate ions, and a "bio-reactive pool" that is microbially manipulated. The latter "pool" is only significant in the upper water column and most likely reflects the indirect effects of microbial cycling of iron. Our model of the open ocean REE distribution contributes to explaining the conservative nature of Nd isotopes and provides a mechanism linking surface ocean and pore water REE dynamics

Description: The East Scotia subduction zone, located in the Atlantic sector of the Southern Ocean, hosts a number of hydrothermal sites in both back-arc and island-arc settings. High temperature (〉348 °C) ‘black smoker’ vents have been sampled at three locations along segments E2 and E9 of the East Scotia back-arc spreading ridge, as well as ‘white smoker’ (〈212 °C) and diffuse (〈28 °C) hydrothermal fluids from within the caldera of the Kemp submarine volcano. The composition of the endmember fluids (Mg = 0 mmol/kg) is markedly different, with pH ranging from 〈1 to 3.4, [Cl−] from ∼90 to 536 mM, [H2S] from 6.7 to ∼200 mM and [F−] from 35 to ∼1000 μM. All of the vent sites are basalt- to basaltic andesite-hosted, providing an ideal opportunity for investigating the geochemical controls on rare earth element (REE) behaviour. Endmember hydrothermal fluids from E2 and E9 have total REE concentrations ranging from 7.3 to 123 nmol/kg, and chondrite-normalised distribution patterns are either light REE-enriched (LaCN/YbCN = 12.8–30.0) with a positive europium anomaly (EuCN/Eu∗CN = 3.45–59.5), or mid REE-enriched (LaCN/NdCN = 0.61) with a negative Eu anomaly (EuCN/Eu∗CN = 0.59). By contrast, fluids from the Kemp Caldera have almost flat REE patterns (LaCN/YbCN = 2.1–2.2; EuCN/Eu∗CN = 1.2–2.2). We demonstrate that the REE geochemistry of fluids from the East Scotia back-arc spreading ridge is variably influenced by ion exchange with host minerals, phase separation, competitive complexation with ligands, and anhydrite deposition, whereas fluids from the Kemp submarine volcano are also affected by the injection of magmatic volatiles which enhances the solubility of all the REEs. We also show that the REE patterns of anhydrite deposits from Kemp differ from those of the present-day fluids, potentially providing critical information about the nature of hydrothermal activity in the past, where access to hydrothermal fluids is precluded.

Description: Integrated Ocean Drilling Program Expedition 320/321, "Pacific Equatorial Age Transect" (Sites U1331–U1338), was designed to recover a continuous Cenozoic record of the equatorial Pacific by coring above the paleoposition of the Equator at successive crustal ages on the Pacific plate. These sediments record the evolution of the equatorial climate system throughout the Cenozoic. As we gained more information about the past movement of plates and when in Earth's history "critical" climate events took place, it became possible to drill an age transect ("flow-line") along the position of the paleoequator in the Pacific, targeting important time slices where the sedimentary archive allows us to reconstruct past climatic and tectonic conditions. The Pacific Equatorial Age Transect (PEAT) program cored eight sites from the sediment surface to basement, with basalt aged between 53 and 18 Ma, covering the time period following maximum Cenozoic warmth, through initial major glaciations, to today. The PEAT program allows the reconstruction of extreme changes of the calcium carbonate compensation depth (CCD) across major geological boundaries during the last 53 m.y. A very shallow CCD during most of the Paleogene makes it difficult to obtain well-preserved carbonate sediments during these stratigraphic intervals, but Expedition 320 recovered a unique sedimentary biogenic sediment archive for time periods just after the Paleocene/Eocene boundary event, the Eocene cooling, the Eocene–Oligocene transition, the "one cold pole" Oligocene, the Oligocene–Miocene transition, and the middle Miocene cooling. Expedition 321, the second part of the PEAT program, recovered sediments from the time period roughly from 25 Ma forward, including sediments crossing the Oligocene/Miocene boundary and two major Neogene equatorial Pacific sediment sections. Together with older Deep Sea Drilling Project and Ocean Drilling Program drilling in the equatorial Pacific, we can delineate the position of the paleoequator and variations in sediment thickness from ~150°W to 110°W longitude.