Description: The central Baltic Sea is a marginal brackish basin which comprises anoxic bottom waters and is surrounded by geological source terrains with a wide variety of compositions and ages. This allows the investigation of water mass mixing using radiogenic isotope compositions of Nd and Hf as well as their geochemical cycling across varying redox conditions in the water column. In this study, we present the distribution of Nd and Hf concentrations and their isotopic compositions for 6 depth profiles and 3 surface water sites obtained during a cruise in the central Baltic Sea onboard the RV Oceania as a part of the international GEOTRACES program. The results obtained indicate that Nd isotopes effectively trace the mixing between more radiogenic saline waters from the south and unradiogenic fresh waters from the north, which helps to understand the reliability of Nd isotopes as water mass tracer in the open ocean. In surface waters, Nd shows higher concentrations and less radiogenic isotope compositions at the northern stations, which are progressively diluted and become more radiogenic to the south, consistent with the counterclockwise circulation pattern of central Baltic Sea surface waters. In contrast to the variable Nd concentrations, Hf shows much less variability. At the Gotland Deep station, the Nd concentrations of the euxinic waters are higher by a factor 〉10 than those of the overlying oxygen-depleted waters, whereas Hf only shows small concentration variations. This indicates faster removal of Hf from the water column than Nd. Moreover, the dissolved Hf isotope signatures document great variability but no consistent mixing trends. Our explanation is that Hf has a lower residence time than Nd, and also that the Hf isotope signatures of the sources are highly heterogeneous, which is attributed to their differing magmatic and tectonic histories as well as incongruent post-glacial weathering around the central Baltic Sea.

Description: Geochemical data were gained for short sediment cores from Maxwell Bay and Potter Cove (King George Island, Antarctica) in 2012 and 2013. We investigated the influence of different iron transport pathways and their effect on the early diagenetic cycling of this nutrient. The dataset includes pore water data (alkalinity, sulfate, Fe2+, NH4+) for 14 sites and solid phase data (including stable iron isotope data for sequentially extracted Fe pools) as well as pore water stable iron isotope data for two contrasting sites: one located at the glacier terminus and the other one at the discharge of a surficial oxic meltwater stream draining the ice-free Potter Peninsula.

Description: Reactive iron (oxyhydr)oxide minerals preferentially undergo early diagenetic redox cycling which can result in the production of dissolved Fe(II), adsorption of Fe(II) onto particle surfaces, and the formation of authigenic Fe minerals. The partitioning of iron in sediments has traditionally been studied by applying sequential extractions that target operationally-defined iron phases. Here, we complement an existing sequential leaching method by developing a sample processing protocol for d56Fe analysis, which we subsequently use to study Fe phase-specific fractionation related to dissimilatory iron reduction in a modern marine sediment. Carbonate-Fe was extracted by acetate, easily reducible oxides (e.g. ferrihydrite and lepidocrocite) by hydroxylamine-HCl, reducible oxides (e.g. goethite and hematite) by dithionite-citrate, and magnetite by ammonium oxalate. Subsequently, the samples were repeatedly oxidized, heated and purified via Fe precipitation and column chromatography. The method was applied to surface sediments collected from the North Sea, south of the Island of Helgoland. The acetate-soluble fraction (targeting siderite and ankerite) showed a pronounced downcore d56Fe trend. This iron pool was most depleted in 56Fe close to the sediment-water interface, similar to trends observed for pore-water Fe(II). We interpret this pool as surface-reduced Fe(II), rather than siderite or ankerite, that was open to electron and atom exchange with the oxide surface. Common extractions using 0.5 M HCl or Na-dithionite alone may not resolve such trends, as they dissolve iron from isotopically distinct pools leading to a mixed signal. Na-dithionite leaching alone, for example, targets the sum of reducible Fe oxides that potentially differ in their isotopic fingerprint. Hence, the development of a sequential extraction Fe isotope protocol provides a new opportunity for detailed study of the behavior of iron in a wide-range of environmental settings.

Description: We present new isotopic data for sedimentary planktonic foraminifera, as well as for potential water column and sedimentary sources of neodymium (Nd), which confirm that the isotopic composition of the foraminifera is the same as surface seawater and very different from deep water and sedimentary Nd. The faithfulness with which sedimentary foraminifera record the isotopic signature of surface seawater Nd is difficult to explain given their variable and high Nd/Ca ratios, ratios that are often sedimentary foraminifera, ratios that are often much higher than is plausible for direct incorporation within the calcite structure. We present further data that demonstrate a similarly large range in Nd/Ca ratios in plankton tow foraminifera, a range that may be controlled by redox conditions in the water column. Cleaning experiments reveal, in common with earlier work, that large amounts of Nd are released by cleaning with both hydrazine and diethylene triamine penta-acetic acid, but that the Nd released at each step is of surface origin. While further detailed studies are required to verify the exact location of the surface isotopic signature and the key controls on foraminiferal Nd isotope systematics, these new data place the use of planktonic foraminifera as recorders of surface water Nd isotope ratios, and thus of variations in the past supply of Nd to the oceans from the continents via weathering and erosion, on a reasonably sure footing.

Description: This dataset includes stable iron isotope data (δ56Fe ) of dissolved iron (Fe) as well as for sequentially extracted Fe pools of sediment cores that were retrieved from Site C0023 (Hole A) during International Ocean Discovery Program (IODP) Expedition 370 in the Nankai Trough offshore Japan in the Pacific Ocean (Drilling vessel Chikyu). Site C0023 was established on 17 September 2016. Coring terminated on 3 November 2016. Furthermore, the dataset comprises solid-phase data of discrete ash layers from Site C0023. The solid-phase data include bulk aluminum (Al), titanium (Ti) and Fe contents, sequentially extracted Fe contents as well as stable Fe isotope data for the sequentially extracted Fe pools.

Description: The 14C ages of planktonic foraminifers Globigerinoides sacculifer bracketing the Younger Dryas in a d18O record of Globigerinoides ruber from a laminated sediment core on the Pakistani continental margin suggest that surface reservoir ages in the Arabian Sea were in excess of 1000 years during the deglaciation. A least squares error fit of a detailed 14C chronology to the (atmospheric) tree ring record gave variable early Holocene reservoir ages between 780 and 1120 years, well above the prebomb value of 640 years. Mid-Holocene reservoir ages are less well constrained but were probably closer to the prebomb value. The method used to fit individual core sections to the tree ring record was designed to require only a rough a priori estimate of the time spans, which in the core presented here were taken from each section's range of 14C ages. A significant 220-year quasi-oscillation was present in the d18O record during the early Holocene but not thereafter. This frequency and amplitude pattern resembles an early Holocene 207- to 227-year oscillation previously observed in the atmospheric 14C record, which is generally interpreted as reflecting solar irradiance variability. An early Holocene climate event at 8150-8400 calendar years B.P. observed elsewhere within the Asian-East African monsoon system was again found in our record, suggesting a reduction in precipitation over NW India and Pakistan.

Description: Iron stable isotope signatures (d56Fe) in hemolymph (bivalve blood) of the Antarctic bivalve Laternula elliptica were analyzed by Multiple Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS) to test whether the isotopic fingerprint can be tracked back to the predominant sources of the assimilated Fe. An earlier investigation of Fe concentrations in L. elliptica hemolymph suggested that an assimilation of reactive and bioavailable Fe (oxyhydr)oxide particles (i.e. ferrihydrite), precipitated from pore water Fe around the benthic boundary, is responsible for the high Fe concentration in L. elliptica (Poigner et al., 2013, doi:10.1016/j.ecss.2013.10.027). At two stations in Potter Cove (King George Island, Antarctica) bivalve hemolymph showed mean d56Fe values of -1.19 ± 0.34 per mil and -1.04 ± 0.39 per mil, respectively, which is between 0.5 per mil and 0.85 per mil lighter than the pool of easily reducible Fe (oxyhydr)oxides of the surface sediments (-0.3 per mil to -0.6 per mil). This is in agreement with the enrichment of lighter Fe isotopes at higher trophic levels, resulting from the preferential assimilation of light isotopes from nutrition. Nevertheless, d56Fe hemolymph values from both stations showed a high variability, ranging between -0.21 per mil (value close to unaltered/primary Fe(oxyhydr)oxide minerals) and -1.91 per mil (typical for pore water Fe or diagenetic Fe precipitates), which we interpret as a "mixed" d56Fe signature caused by Fe assimilation from different sources with varying Fe contents and d56Fe values. Furthermore, mass dependent Fe fractionation related to physiological processes within the bivalve cannot be ruled out. This is the first study addressing the potential of Fe isotopes for tracing back food sources of bivalves.

Description: This data product contains dissolved silicon concentrations and isotopic composition, major ion concentrations and discharge for streams in Potter Peninsula, King George Island and Commonwealth Stream, McMurdo Dry Valleys, Antarctica.