The top surface layers of 28 Fe-Mn crust samples and 2 Fe-Mn nodules (1–3 mm in thickness) were analysed. Some samples were obtained from various collections in the Pacific Ocean. Most of them were collected by dredging during the China Ocean Mineral Resources R&D Association cruises. The geographic location of the samples is shown in Fig. 1. In the laboratory, the outermost layer of each crust was cut and ground into powders using an agate mortar.

All acids (GR grade) used in this study were further purified by sub-boiling distillation systems (DST-1000, Savillex, USA). Acids and standards were prepared using >18.2 MΩ H₂O from a Milli-Q water system (Millipore Corporation, USA). Sample digestion and ion exchange chromatographic separation of Cu and Zn were conducted in an ultra-clean laboratory using a Savillex^® PFA beaker. Total procedural blanks were less than 10 ng for Zn and less than 15 ng for Cu.

Zn isotope ratios were measured relative to the standard reference material IRMM-3702, which has an offset of 0.30‰±0.05‰ with respect to the Johnson Matthey (JMC) Lyon standard (Archer et al., 2017; Moeller et al., 2012). Cu isotope ratios were measured relative to the standard reference material IRMM-633, which has an offset of 0.01‰±0.05‰ with respect to the NIST SRM 976 Cu (Moeller et al., 2012). The δ⁶⁶Zn (‰) and δ⁶⁵Cu (‰) values in this paper are reported with respect to JMC-Lyon Zn and the NIST SRM 976 Cu standards using the following equations:

All sample preparation, isotopic measurements and element concentration measurements were performed at the Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, Ministry of Natural Resources, China.

Approximately (50±0.5) mg of powdered samples was weighed in Teflon pressure vials and digested in a mixture of 1 mL concentrated HNO₃ and 0.3 mL concentrated HCl in the oven at 190°C for 48 h. After evaporation at 150°C on hot plates, the solid residue was dissolved with 1 mL concentrated HNO₃ and then evaporated to dryness at 150°C. The solid residue was dissolved in 1 mL concentrated HNO₃. Meanwhile, 0.5 mL of 1.0 μg/g Rh solution was added as an internal standard. Then, the Teflon pressure vials were kept in the oven at 150°C for 12 h.

Major and trace element concentrations were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) (iCAP6300, Thermofisher Scientific, USA) and quadrupole inductively coupled plasma mass spectrometry (ICP-MS) (X-Series 2, Thermofisher Scientific, USA). Precision was generally better than 6%. Geological reference materials of manganese crusts GBW07337 and GBW07338 were systematically analysed along with the samples, and the measured concentrations were within 5% uncertainty of the published concentration values (Gao et al., 2017).

Approximately 30 mg of each sample was dissolved in 2 mL aqua regia and evaporated to dryness and then dissolved in 1 mL HF+HNO₃ mixture in order to eliminate silica. After evaporation, the residue was taken up in 0.2 mL concentrated HCl, and insoluble particles were centrifuged out. The supernatant was transferred into a clean beaker. The pellet was dissolved in HCl, evaporated to dryness at 110°C, and dissolved in 0.1 mL concentrated HCl, and the centrifugation procedure was repeated. Finally, all the supernatant was evaporated to dryness and dissolved in 1 mL 8.2 mol/L HCl.

The Cu and Zn fractions of all samples were separated from matrix components by ion exchange chemistry using a strongly basic anion resin (Bio-Rad macro-porous AG MP-1 M resin, 200–400 mesh). The PE columns (Bio-Rad, total volume of 10 mL) were filled with a total volume of 2 mL anion exchange resin, washed alternately with Milli-Q water and 0.5 mol/L HNO₃, and then preconditioned with 4 mL 8.2 mol/L HCl. Cu and Zn were eluted from the resin following a chromatographic procedure that was modified from Maréchal et al. (1999) (Table 1) (He et al., 2016). The purified Cu and Zn were evaporated to dryness, and 0.2 mL concentrated HNO₃ was added to remove residual organics that may have leached from the column. Cu and Zn recovery was measured by ICP-MS, and the recovery of all samples was 100%±5%.

Samples were re-dissolved in 2% HNO₃ at Cu or Zn concentrations of 200 ng/g. For Cu isotope measurements, samples were doped with 200 ng/g IRMM 3702 Zn as an internal calibrator element to correct instrument mass bias. Similarly, samples were doped with IRMM 633 Cu for Zn isotope measurements. The Cu and Zn isotope compositions were measured on a Nu Plasma multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) in low-resolution mode. Samples were introduced via a desolvation nebulizer system (Nu DSN-100) at an uptake rate of 0.2 mL/min. Signal intensity was measured on masses 62, 63, 64, 65, 66, 67 and 68. Ni sampling cones were used, and ⁶⁴Ni was subtracted from ⁶⁴Zn by monitoring the signal from ⁶²Ni. Samples were bracketed with a mixed standard of IRMM 3702 Zn and IRMM 633 Cu (mixture ratio around 1). Each measurement was preceded by an analysis of a 2% HNO₃ blank. These “on-peak zeroes” were used to correct sample signals for acid blanks. Data collection consisted of 2 blocks of 24 cycles with 4 s integration time.

Instrumental mass bias correction for Zn was achieved using a combination of sample-standard bracketing and Cu internal spike with spike-sample ratios around 1. If f_Cu=f_Zn (f represents mass bias factor), the slope of ln(⁶⁵Cu/⁶³Cu)_meas vs. ln(⁶⁶Zn/⁶⁴Zn)_meas should be 1.015 (Maréchal et al., 1999; Mason et al., 2004). The slope of ln(⁶⁵Cu/⁶³Cu)_meas vs. ln(⁶⁶Zn/⁶⁴Zn)_meas for a mixture of IRMM 3702 Zn standard and IRMM 633 Cu standard remains constant within a period of several hours. It may vary slightly from day to day, but it remains remarkably constant in a measurement session of several hours (Fig. 2). Long-term measurements show that the slope ranges from 0.93 to 1.10, normally less than 10%. Therefore, we consider that the mixture standards form a linear relationship with a slope the same as f_Cu= f_Zn. Zn data were corrected using Cu. Similarly, the mass bias for Cu was corrected using Zn.

The long term reproducibilities of the results were 0.08‰ for Cu and 0.04‰ for Zn. Cu isotope reproducibility was assessed by repeated measurements of the international standard IRMM 647 Cu, with δ⁶⁵Cu/⁶³Cu = 0.11‰±0.08‰ (n = 75). The reproducibility for Zn was assessed based on repeated analyses of an in-house standard MR-Zn. Zn reproducibility was very good for MR-Zn, and δ⁶⁶Zn/⁶⁴Zn = –9.48‰±0.04‰ (n = 57).

Element contents and selected element ratios are presented in Table 2. Zn content is in the range of 484.3–1 039 μg/g, and Cu content varies between 0.02% and 0.79%. The Cu content in the Fe-Mn crusts varies little, and the Cu content in several Fe-Mn nodules is extremely high. The contents of other transition metals such as Co are around 800 μg/g, and the Ni content is homogenous in the range of 0.32%–1.06%. The average Mn content is around 24.35%, whereas the Fe content is around 15.92%. Phosphorus content varies between 0.21% and 0.78%, and Ca content is in the range of 1.67%–4.57%. It can be seen that P and Ca contents display a perfect correlation. This correlation is likely related to the fact that these two elements are essentially hosted in the authigenic phosphatic phase. Lithogenic element contents are in the range of 0.30% to 2.18% for Al, and Ti content yields an average value of 1.02%.

Zn/Mn ratios for Fe-Mn crusts and Fe-Mn nodules are similar. The mean Zn/Mn ratio for all the samples is 0.002 8. The mean Cu/Mn ratio for Fe-Mn crusts is 0.004 1. The Cu/Mn ratios for Fe-Mn nodules vary between 0.004 and 0.031, which are slightly higher than those for Fe-Mn crusts. As observed in other studies, Zn/Mn and Cu/Mn ratios for Fe-Mn crusts exhibit more scatter. This feature is likely due to the non-conservative distribution of these two elements in ocean waters. As a widely used geochemical source tracer, the Fe/Mn ratio varies between 0.36 and 0.90, which is consistent with the values already reported for other Fe-Mn crusts in the ocean. Al/Fe ratios, which are used as an estimate of the lithogenic contribution, are in the range of 0.014 to 0.228. The other element ratio details can be seen in Table 2.

Cu and Zn isotope data, together with sample locations and water depth, are listed in Table 3. Overall, the δ⁶⁶Zn values are in the range of 0.71‰ to 1.08‰, with a mean value of 0.94‰±0.21‰ at the 95% confidence level. Cu isotope compositions seem slightly more variable. Sample D27-1a* shows a very heavy Cu isotope composition with δ⁶⁵Cu=0.91‰. Overall, the δ⁶⁵Cu values range between 0.33‰ and 0.91‰, with a mean value of 0.58‰±0.20‰ at the 95% confidence level. Although the observed variations for Cu and Zn isotope compositions exceed the reproducibility of the measurements (i.e., 0.08‰ for Cu and 0.04‰ for Zn), the ranges fall within the ranges of previously published data.

One of the striking results from this study is that the Cu and Zn isotope compositions of the top surface of Fe-Mn crusts and Fe-Mn nodules. The δ⁶⁶Zn values of the two Fe-Mn nodules (D27-1a* and D43-1*) are 1.06‰ and 0.89‰, which are consistent with previous studies. The Zn isotope compositions of the top surface of Fe-Mn crusts are in the range of 0.71‰ to 1.08‰, with a mean value of 0.94‰±0.21‰. There is no significant variation in the isotopic compositions of Zn in Fe-Mn crusts and Fe-Mn nodules. However, the deviation from Fe-Mn crusts is significant compared to the analytical uncertainty associated with the long-term reproducibility. The sample with the lowest δ⁶⁶Zn is D008-1, which is a platy crust, with pale yellow bedrock, loose and granular surface, and mixed with yellow impurities. The sample with the highest δ⁶⁶Zn (D02-1-2) is a pebble crust, with dark brown surface. The appearance of these two crust samples is quite different.

For Cu, the sample with abnormally high δ⁶⁵Cu (D27-1a*, δ⁶⁵Cu=0.91‰) is a Fe-Mn nodule. This nodule is oval, about 2 cm in diameter, with nodular protrusions, and the surface of the nodules is bright black with traces of biological activity (Fig. 3). The abnormally high Cu isotope ratio may be linked to marine biological activity on the surface of the nodules. Another Fe-Mn nodule sample has a Cu isotope ratio of 0.44‰. The δ⁶⁵Cu values of all Fe-Mn crusts samples range from 0.33‰ to 0.73‰, with a mean value of 0.58‰±0.20‰. This shift is over two times greater than the analytical uncertainty on the measurements.

The precise reason why the Cu and Zn isotopic compositions of the top surface of Fe-Mn crusts vary is not clear. Because Fe-Mn crusts grow very slowly, typically at a rate of millimetres per thousand years, the significance of their geochemical properties is distorted by the long-time coverage even by the smallest samples. Maréchal et al. (2000) reported that higher values of δ⁶⁶Zn in ferromanganese nodules are preferentially associated with temperate and subpolar latitudes. They ascribed the isotopic fluctuations to the seasonal variation of primary productivity. However, in this study, the δ⁶⁶Zn and δ⁶⁵Cu values of the top surface of Fe-Mn crusts do not show visibly variability with latitudes. Chlorophyll concentration of phytoplankton in seawater is an important observational parameter in these investigation stations. It is not only an index characterizing phytoplankton biomass, but also can be used to estimate the primary productivity of a given sea area. In this study, most of higher δ⁶⁶Zn and δ⁶⁵Cu values are associated with higher chlorophyll concentration. However, in some stations, the chlorophyll concentration is relatively low but the δ⁶⁵Cu and δ⁶⁶Zn values are high. Considering that fractionation also occurs during the formation of Fe-Mn crusts, this study infers that the local productivity and remineralization may account for the Cu and Zn isotopic variations in Fe-Mn crusts. However, it is difficult to explain the distribution of δ⁶⁶Zn and δ⁶⁵Cu in Fe-Mn crusts with these limited data. More related research is needed.

Fe-Mn crusts and Fe-Mn nodules represent useful analogues for the dispersed Fe-Mn oxide phases presented throughout oxic marine sediments. They grow very slowly, typically at rates of millimetres per thousand years. Dissolved Cu and Zn in the seawater are adsorbed and incorporated into Fe-Mn crusts during their formation. Do the Cu and Zn isotope compositions of the top surface of Fe-Mn crusts reflect the average isotopic composition of ocean deep water? It is necessary to compare the isotopic compositions of the top surface of Fe-Mn crusts with those of seawater. The first dataset of dissolved δ⁶⁵Cu in seawater from Pacific Ocean, India Ocean and English channel was published by Vance et al. (2008) and reported δ⁶⁵Cu ranging from 0.90‰ to 1.44‰. More recently, three Pacific Ocean δ⁶⁵Cu vertical profiles were measured, ranging from 0.61‰ to 0.78‰ (Boyle et al., 2012; Thompson et al., 2013; Thompson and Ellwood, 2014). Takano et al. (2014) also investigated several profiles in the Pacific and focused mostly on surface layers. However, these profiles show that most of the seawater δ⁶⁵Cu composition variability occurs in the photic zone but is relatively homogenised in the deeper waters. Overall, previous studies give the δ⁶⁵Cu of seawater at around 0.9‰±0.3‰. In general, the δ⁶⁵Cu value is higher than 0.9‰ when samples below 1 000 m. Compared to those for Cu, there are more published data for the Zn isotope composition of seawater. The repeated findings to date in studies of the Zn isotope composition of seawater, representing nearly 700 isotope analyses from Southern Ocean and NE Pacific values (Vance et al., 2012) and 6 depth profiles from the equatorial Atlantic (Boyle et al., 2012) demonstrate that the deep ocean (beneath 1 000 m) is extremely homogeneous. Excluding data that have been specifically identified by the authors as impacted by local hydrothermal and benthic sources, nearly 300 analyses of this dominant deep ocean give a δ⁶⁶Zn=0.51‰±0.14‰. A comparison of these results with the data of this study for Fe-Mn crusts provides interesting observations. First, the Cu isotope compositions of the Fe-Mn crusts range from 0.33‰ to 0.73‰, with a mean value of 0.58‰±0.20‰. Even the highest δ⁶⁵Cu value (0.73‰) is distinctly isotopically lighter than the Cu isotopic composition in deep seawater at around 0.9‰. On the other hand, the δ⁶⁶Zn values of the Fe-Mn crusts (0.94‰±0.21‰) appear to be isotopically heavy compared to deep seawater (0.51‰±0.14‰). Overall, the isotopic separation between seawater and the top surface of Fe-Mn crusts (∆_{Fe-Mn crusts-seawater}) is -0.3‰ for Cu and 0.4‰ for Zn. This also implies that the Cu and Zn isotopes in the outmost layer of Fe-Mn crusts are not a direct reflection of the Cu and Zn isotopes in modern seawater, but only a function of the Cu and Zn isotopes in modern seawater.

The source effects on the ∆_{Fe-Mn crusts-seawater} are almost certainly negligible, as all samples derived from the continental crust and the mantle fall with a relatively narrow range of δ⁶⁶Zn at approximately 0.3‰ and δ⁶⁵Cu at 0.00‰. According to the element geochemistry, Co/Mn, Cu/Mn, Fe/Mn, and Zn/Mn are similar between the crusts, which suggest that growth rates or seawater mass parameters are not the major controls on the enrichments in Cu and Zn in these crusts. Some of the variations in Zn/Al, Cu/Al and Zn and Cu enrichment factors in the Fe-Mn crusts may be due to differences in the accumulation rates of Cu and Zn, perhaps associated with differing deep ocean Cu and Zn concentrations. However, Cu and Zn isotope ratios do not show any variations with depth (Fig. 4), thus once again proved that Cu and Zn isotopes are homogenous in deep water.

Possible explanations for the difference between Cu and Zn isotope compositions for Fe-Mn crusts and seawater are (1) biologically mediated fractionation of Cu and Zn in seawater; (2) adsorption and incorporation of Cu and Zn in seawater during formation of Fe-Mn crusts accompanied by isotopic fractionation; (3) fractionation associated with differences in Cu and Zn speciation in seawater or (4) a combination of these factors. However, the precise fractionation mechanisms are not clear.

Cu and Zn are both strongly cycled between the surface water and the deep ocean. Zn concentrations are approximately 2 nmol/L and 10 nmol/L in the deep Pacific (e.g., Bruland, 1980; Bruland et al., 1994; Lohan et al., 2002; Boyle et al., 2012). However, in the photic zone, Zn concentrations are often less than 0.1 nmol/L. The strong depletion in the surface ocean is due to biological uptake (Morel and Price, 2003), and regeneration from the deep ocean drives up the concentrations. Similarly, the Cu concentration profile shows depletion in the surface water and enrichment in the deep ocean. Both Cu and Zn are bioessential elements and dominated by complexation with strong organic ligands in the surface ocean (Coale and Bruland, 1988; Bruland, 1989; Donat and Bruland, 1990; Moffett and Dupont, 2007). Biological processes are usually accompanied by kinetic fractionation, where the lighter isotope is preferentially concentrated in the biomass, forcing the residual reservoir to form a heavier isotopic composition (Valley and Cole, 2001). Fe-Mn crusts are formed through the slow precipitation of Fe-Mn colloids in seawater. Metal cations in seawater, such as Cu and Zn, are incorporated with Fe-Mn colloids and can further precipitate through slow uptake onto the Fe-Mn crust surface. In this study, Zn content in the Fe-Mn crusts is in the range of around 484 μg/g to 1039 μg/g, and Cu content varies from 200 μg/g to 7 900 μg/g. A plot of the Cu and Zn isotope compositions of Fe-Mn crusts against their contents is shown in Fig. 5. The Cu and Zn isotope ratios do not visibly vary with Cu and Zn contents, which may imply that isotopic fractionation due to biological activity in the surface ocean is not responsible for Cu and Zn isotope ratios in Fe-Mn crusts. Previous studies also indicate that Cu and Zn are isotopically homogeneous in deep seawater, so it is impossible to cause large variations in Cu and Zn isotope ratios in Fe-Mn crusts. The Cu and Zn isotope compositions of the Fe-Mn crusts are unrelated to their Co contents (Fig. 5). Therefore, the Cu and Zn isotope compositions are independent of the growth rate (Halbach et al., 1983).

The Cu and Zn isotope variations may be associated with equilibrium partitioning between the sorption to crusts and the organic-ligand-bound Cu and Zn in seawater. Stable isotope fractionations are ordered by the rule that heavy isotopes tend to accumulate in substances with stronger bonds. This process means that heavy isotopes tend to bind with smaller coordination numbers (Schauble, 2004). Fe-Mn crusts are formed through slow precipitation of Fe-Mn colloids in seawater. The surface of the crust is exposed to seawater for a long time. It is possible that adsorbed Cu and Zn on the Fe-Mn crust surface are continuously incorporated into the mineral structure. This process would allow isotopic exchange of Cu and Zn with seawater. It has been shown that free Cu²⁺ in seawater is dominated by V-fold coordination, forming a hydrate [Cu (H₂O)₅ ]²⁺ complex (Pasquarello et al., 2001; Amira et al., 2005). Cu on Fe-Mn crusts is mainly in Ⅲ- to Ⅳ-fold coordination (around 65%), with limited Ⅵ-fold structural incorporation (around 35%) (Little et al., 2014a). The balance of surface complexation and structural incorporation indicates that Fe-Mn crusts favour sorption of heavy Cu isotopes. Free Zn²⁺ in seawater is Ⅵ-fold coordinated in the [Zn (H₂O)₆]²⁺ complex and Ⅳ-fold coordinated in Fe-Mn crusts. Thus, Fe-Mn crusts also favour sorption of heavy Zn isotopes. If all the Cu and Zn elements in the Fe-Mn crust come from free Cu²⁺ and free Zn²⁺ in seawater, then the δ⁶⁵Cu and δ⁶⁶Zn values in the Fe-Mn crust should be higher than that in seawater. However, Cu and Zn are dominantly complexed to organic ligands in seawater. Free Cu²⁺ is less than 0.1% of the total dissolved fraction. Free Zn²⁺ is also very limited in seawater. Therefore, the isotopic fractionation of Cu and Zn in crusts is only a part of the total isotope separation between sorbed Cu and Zn and that dissolved in seawater relative to free Cu²⁺ and free Zn²⁺. Partitioning between the free phase and the ligand-bound phase is equally important. The organic-ligand complexations favour heavy isotopes. Cu is strongly complexed to organic ligands in seawater, which can explain why Cu in seawater is isotopically heavier than Fe-Mn crusts. For Zn, the strong organic-ligand complexations appear to be restricted in the surface ocean. Half of dissolved inorganic Zn is complexed by chloride ions, and the chloride ions prefer to complex light Zn isotopes. Thus, heavy Zn in seawater is incorporated on the surface of Fe-Mn crusts, making the Zn isotope composition of Fe-Mn crusts heavier.

What exactly controls the fractionation of Cu and Zn isotopes in Fe-Mn crusts is still under research. In summary, possible explanations are (1) changes in Cu and Zn isotopic composition in seawater, and (2) changes in marine environment during the formation of Fe-Mn crusts. The relatively long oceanic residence time of Cu and Zn in comparison to the around 1 500 a mixing time of oceans, making the distribution of Cu and Zn in deep waters is relatively well-mixed and homogenous. Regardless of the fractionation mechanism, the isotopic composition of Cu and Zn in Fe-Mn crusts is a function of Cu and Zn isotopes in seawater. The δ⁶⁶Zn and δ⁶⁵Cu values of each layer in Fe-Mn crust actually reflect the isotopic composition of Cu and Zn in seawater during their respective formation periods. Considering the relatively long residence times of Cu and Zn in the deep ocean, changes in either Cu and Zn input to or output from seawater have been recorded in crusts. One may presume that different Cu and Zn isotope records between different layers from the same Fe-Mn crusts reflect either local sources or changes in the Cu and Zn isotope fractionation factors. Assuming that the known functional relationship between the Cu and Zn isotopes in seawater and the Cu and Zn isotopes in the Fe-Mn crust, it would be more convenient to understand ancient ocean water by analysis the Cu and Zn isotope compositions in the Fe-Mn crusts. The potential of Cu and Zn isotope compositions for palaeoceanography through Fe-Mn crust analysis becomes quite appealing.