# Light Zn and Cu isotope compositions recorded in ferromanganese crusts during the Cenozoic as evidence for hydrothermal inputs in South Pacific deep seawater

Bleuenn Gueguen, Olivier Rouxel, Yves Fouquet

► To cite this version:

Bleuenn Gueguen, Olivier Rouxel, Yves Fouquet. Light Zn and Cu isotope compositions recorded in ferromanganese crusts during the Cenozoic as evidence for hydrothermal inputs in South Pacific deep seawater. Geochimica et Cosmochimica Acta, 2022, 333, pp.136-152. 10.1016/j.gca.2022.06.038. insu-03868832

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1 *Light Zn and Cu isotope compositions recorded in ferromanganese crusts during the Cenozoic as evidence for hydrothermal inputs in South Pacific deep seawater*

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4 Bleuenn Gueguen¹,²*, Olivier Rouxel³ and Yves Fouquet³

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7 ¹Univ Brest, CNRS, UMS 3113, IUEM, Place Nicolas Copernic, 29280 Plouzane, France

8 ²Univ Brest, CNRS, UMR 6538, LGO UMR 6538, IUEM, Place Nicolas Copernic, 29280 Plouzane, France

9 Plouzane, France

10 ³IFREMER, Centre de Brest, Unité Géosciences Marines, F-29280 Plouzane, France

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19 *Corresponding author : bleuenn.gueguen@univ-brest.fr

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This study presents a high-resolution record of Cu and Zn isotopes in four Fe-Mn crusts from the North and South Pacific oceans. North Pacific crusts were collected on the Apuupuu seamount south of the Hawaiian archipelago and South Pacific crusts were recovered near Rurutu Island in the Tahiti archipelago. Major and trace element compositions suggest that Cu and Zn in these crusts is of hydrogenous origin, i.e., precipitated from seawater, and they may therefore mirror deep seawater metal isotope. We show that Cu and Zn display different isotopic patterns between the North and the South Pacific Oceans but show similar temporal evolution within each geographical area. Copper and Zn isotope composition of both North Pacific crusts vary between 0.57 ‰ to 0.73 ‰ for δ⁶⁵/⁶³Cu_NIST976 and 0.97 ‰ to 1.25 ‰ for δ⁶⁶/⁶⁴Zn_JMC-Lyon. In contrast, South Pacific crusts show resolvable temporal variations, with Cu and Zn isotopic ratios increasing sharply over the last ~ 6 Ma from 0.16 ‰ to 0.51 ‰ and 0.67 ‰ to 1.09 ‰ respectively. Notably, we observed a positive correlation between δ⁶⁵/⁶³Cu_NIST976 and δ⁶⁶/⁶⁴Zn_JMC-Lyon values in Fe-Mn crusts from the South Pacific. The correlation suggests mixing between two components in Fe-Mn crusts, a hydrothermal component with δ⁶⁵/⁶³Cu_NIST976 ~ 0.2 ‰ and δ⁶⁶/⁶⁴Zn_JMC-Lyon ~ 0.7 ‰, and a Pacific deep seawater component with δ⁶⁵/⁶³Cu_NIST976 ~ 0.7 ‰ and δ⁶⁶/⁶⁴Zn_JMC-Lyon ~ 1.2 ‰. These values are fractionated from modern dissolved Cu and Zn by a factor of -0.3 ‰ and 0.5 ‰ respectively. We suggest that the deep Southern Pacific Ocean received sustained hydrothermal input during the last 6 Ma, which was recorded in the Cu and Zn isotope composition of Fe-Mn crusts precipitated thousands of kilometers away. Our study highlights that hydrothermal venting may be a significant source of Cu and Zn in the deep oceans despite their extensive precipitation within hydrothermal vent fields. We show that this source could be persistent through time, and thus, it could have a significant impact on the
47 biogeochemical cycling of Cu and Zn in seawater which would ultimately be recorded by Fe-
48 Mn crusts.

49

50 **1. Introduction**

51 Our understanding of metal biogeochemical cycles in the oceans has increased
52 significantly since the development of methods for measuring low trace metal concentrations
53 in seawater (e.g., Bruland et al., 1994; Sohrin et al., 1998; Sohrin et al., 2008; Sohrin and
54 Bruland, 2011; Biller and Bruland, 2012; Minami et al., 2015; Yang et al., 2020). Surface
55 bioproductivity has long been recognized as an important parameter influencing distribution
56 of transition metals in seawater because many of those trace elements are essential, and in
57 some cases bio-limiting, for primary productivity (e.g., Fe, Zn, Cd, Ni; Morel and Price,
58 2003). However, our knowledge of the deep ocean biogeochemical cycling of metals is
59 relatively incomplete, in particular with respect to benthic sources and oceanic water masses
60 dynamics. For instance, the importance of hydrothermal inputs for the oceanic Mn, Fe, Zn and
61 Cu budgets has been demonstrated to be important in the South Pacific, notably because of
62 hydrothermal vents along the fast spreading South East Pacific Rise (EPR) that contribute to
63 large hydrothermal fluxes in seawater (Boström et al., 1969; Lupton and Craig, 1981; Baker
64 and Massoth, 1986; Baker et al., 1995; Urabe et al., 1995; Lupton, 1998; Tagliabue et al.,
65 2010; Hannington, 2013; Fitzsimmons et al., 2014; Resing et al., 2015; Roshan et al., 2016;
66 John et al., 2018). Variations of these fluxes through time could potentially have an influence
67 on global metal biogeochemical cycles.

68 Among these trace metals, particular attention is now given to Cu and Zn as new
69 biogeochemical tools (e.g., see review by Moynier et al., 2017). These elements are mostly
70 supplied to oceans via riverine input from the continents, whereas the most important sink is
71 associated with the precipitation of Fe-Mn-oxides at the seafloor (Little et al., 2014; Little et
al., 2016; Vance et al., 2016). Dissolved Zn exhibits a nutrient-type profile in seawater as a result of biological uptake in surface waters and recycling of settling organic matter in deep waters (Bruland, 1980; Bruland, 1983; Morel and Price, 2003). The importance of Zn for biological activity is also shown by the close relationships between Zn cycling and phosphate in the Southern Ocean for example (Sieber et al., 2020), and silicate cycling (e.g., Bruland, 1980; Samanta et al., 2017; Vance et al., 2017; Weber et al., 2018; Lemaitre et al., 2020). By contrast, dissolved Cu behaves as a “hybrid-type” element, i.e. a particle-reactive element that can be partially scavenged and partially used in biological activity (Bruland et al., 2014), and its concentration in the water column increases with depth (Boyle et al., 1977; Bruland, 1980).
In addition, most dissolved Zn and Cu in the marine environment are complexed to strong organic ligands (e.g., Coale and Bruland, 1988; Bruland, 1989; Moffett and Dupont, 2007; Horner et al., 2021b).

Subsequent to the development of efficient methods for measuring metal concentration in seawater, many studies have reported seawater Cu and Zn isotope compositions of the dissolved phase (e.g., Bermin et al., 2006; Vance et al., 2008; Conway et al., 2013; Conway and John, 2014; Zhao et al., 2014; Little et al., 2018; Baconnais et al., 2019; Vance et al., 2019; Lemaitre et al., 2020; Liao et al., 2020). Biological activity fractionates Zn isotopes and may impart heavier isotope signatures to surface waters in the water column (Zhu et al., 2002; John et al., 2007; Andersen et al., 2011; Samanta et al., 2017; Köbberich and Vance, 2019).
Some studies therefore suggest that Zn isotopic signatures might be used to trace primary productivity in the sedimentary archive, e.g., in carbonates (Pichat et al., 2003; Kunzmann et al., 2013). However, recent studies of Zn isotopes in modern seawater have revealed a more complex picture. For example, in the Southern Ocean, uptake of Zn by diatoms does not necessarily produce an isotopically heavy surface seawater (Zhao et al., 2014), while in the North Pacific Ocean light Zn isotope compositions in some surface waters may be attributed
97 to intracellular regeneration of light Zn isotopes (Conway and John, 2014; Zhao et al., 2014).

98 Scavenging and adsorption of Zn onto organic matter may also have an important role in the

99 cycling of Zn isotopes in surface waters (John and Conway, 2014; John et al., 2018; Weber et al., 2018) in addition to aerosol and anthropogenic sources (Conway and John, 2014;

101 Lemaitre et al., 2020; Liao et al., 2020). No clear and systematic effects have been observed due to biological uptake in the water column for Cu isotopes (e.g., Vance et al., 2008; Takano et al., 2014; Little et al., 2018; Baconnais et al., 2019; Yang et al., 2020), but light Cu isotope compositions in surface waters have been attributed to local sources such as aeolian particles and particulates from rivers and sediments (Takano et al., 2014; Little et al., 2018).

106 In the deep ocean, despite relatively homogenous isotopic composition for Zn with an average value of 0.5 ±0.1 ‰ (e.g.; Boyle et al., 2012; Conway et al., 2013; Conway and John, 2014, 2015; Takano et al., 2017; John et al., 2018; Vance et al., 2019), and Cu with an average value of ~0.7 ‰ (e.g.; Boyle et al., 2012; Takano et al., 2013, 2014; Takano et al., 2017; Little et al., 2018; Baconnais et al., 2019); these studies show that deep sources of Zn and Cu may have an impact on isotopic signatures. For example, the influence of hydrothermal inputs on Zn in seawater has been demonstrated by a few studies in the North Atlantic Ocean (Conway and John, 2014; Lemaitre et al., 2020) and in the South Pacific Ocean (Roshan et al., 2016; John et al., 2018). They show that isotopically light Zn is released in the deep ocean and could be transported over a few thousands of kilometers, and this input flux could, in fact, be a predominant source of Zn.

117 Experimental results of Syverson et al. (2021) showed that chalcopyrite precipitates at equilibrium conditions at 350°C and is isotopically fractionated from dissolved Cu by -0.22 ±0.16 ‰. This suggests that the Cu remaining in the hydrothermal fluids released in seawater is enriched in heavy isotopes. Furthermore, studies show that Cu could be transported far from the hydrothermal source producing significant Cu enrichment in the sediment
(Hannington, 2013). This shows that deep local sources such as hydrothermal venting could contribute significantly to the biogeochemical cycling of Cu and Zn through time. However, how Cu can be transported from the hydrothermal vent is not clear yet. Models indicate that Cu can be transported through complexation in organic ligands (Sander and Koschinsky, 2011), but this has not been observed on the field yet. Observations indicate that most of the dissolved Cu is precipitated near the vent (Roshan and Wu, 2015), and GEOTRACES data only indicate an increase in particulate Cu (GEOTRACES Intermediate Data Product Group, 2021).

A promising target for investigating metal biogeochemical cycling and isotopic compositions of deep waters over longer time scale is the hydrogenetic ferromanganese (Fe-Mn) crusts archive. These metalliferous deposits are composed of a mineralogical assemblage of Mn- and Fe-oxides, which are highly enriched in Ni, Co, Cu, Zn, Mo, Te, Pt, Tl relative to the average continental crust (e.g.; Li and Schoonmaker, 2003; Hein et al., 2010; Hein et al., 2013). The well accepted model for explaining metal enrichment is that metals are exclusively derived from seawater through precipitation of hydrogenous Fe- and Mn-oxyhydroxides. The process of Fe-Mn crusts formation is promoted by the presence of an Oxygen Minimum Zone (OMZ) in the water column, which allows accumulation of soluble reduced Mn²⁺ due to low oxygen concentrations (Koschinsky and Halbach, 1995). The term Oxygen Minimum Zone (OMZ) has been used to refer to regions, typically along upwelling continental margins, with very depleted oxygen concentrations of < 20 µM (Paulmier and Ruiz-Pino, 2009). However, low oxygen concentrations, of about 50-100 µM, are found throughout the oceans at 1000 to 1500 m water depths, and have also classically been termed “OMZs”. Here, we adopt the term OMZ to refer to the latter, i.e., low oxygen regions throughout intermediate depths of the world oceans. Upon recycling of the dissolved Mn out of the OMZ, Mn is re-oxidized and form colloids to scavenge dissolved trace metals. Colloids provide nucleation sites for the
147 formation of Mn-oxides at the surface of rocks of the seafloor, and then ferromanganese
148 crusts grow through incorporation of dissolved metals. They are generally precipitated at very
149 slow growth rates between 1 and 6 mm/Ma on the flank of seamounts (but some are formed
150 on a sedimentary substrate), which serves as a substrate for Fe-Mn encrustations at depths
151 between 1000 and 3000 meters. Ferromanganese crusts grow in low sedimentation rates areas
152 in order to prevent transition metals dilution by terrigenous and calcareous particles (Halbach
153 et al., 1983; Hein et al., 1988; Hein et al., 1992; Koschinsky and Halbach, 1995; Koschinsky
154 et al., 1997). To date, Fe-Mn crusts deposits have been intensively investigated for their
155 radiogenic isotope composition (Pb, Nd, Hf) for tracing the evolution of oceanic circulation
156 and its relation to climate variations and continental fluxes of material to oceans (see for
157 example the review by Frank, 2002). During the last decade, studies focusing on transition
158 metal isotope systematics records in Fe-Mn crusts have blossomed (e.g., see reviews by; Fu,
159 2020; Horner et al., 2021a), including Fe (Zhu et al., 2000; Levasseur et al., 2004; Chu et al.,
160 2006; Horner et al., 2015), Tl (Rehkämper et al., 2002; Rehkämper et al., 2004), Cd (Schmitt
161 et al., 2009; Horner et al., 2010), Mo (Siebert et al., 2003), Ni (Gall et al., 2013; Gueguen et
162 al., 2016) and Cu and Zn (Maréchal et al., 2000; Albarède, 2004; Little et al., 2014).

163 Published data on Cu and Zn isotopes in oceanic metalliferous deposits display a significant
164 range of values, i.e., δ<sup>65/63</sup>Cu<sub>NIST976</sub> = 0.31± 0.23 ‰ and δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> = 0.90 ± 0.28 ‰ in
165 Mn-nodules (Maréchal et al., 2000; Albarède, 2004); and δ<sup>65/63</sup>Cu<sub>NIST976</sub> = 0.44± 0.23 ‰ and
166 δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> = 1.04 ± 0.21 ‰ in Fe-Mn crusts (Little et al., 2014), although the causes of
167 such isotopic variations remain largely unresolved. Therefore, in Fe-Mn deposits Cu isotopes
168 are on average 0.3 ‰ lighter and Zn isotopes are 0.5 ‰ heavier compared to seawater. In
169 addition to source variations, adsorption mechanisms may be also important (Little et al.,
170 2014b; Bryan et al., 2015; Sherman and Little, 2020), complicating paleo-oceanographic
171 reconstructions.
Here, we investigated spatial and temporal variations of Cu and Zn isotope compositions in four hydrogenetic Fe-Mn crusts from the Pacific Ocean in order to understand the possible factors responsible for Cu and Zn isotope variations in the deep ocean through time.

176

## 2. Materials and methods

178

### 2.1. Sample description

#### 2.1.1. Microsampling of Fe-Mn crusts from the North and South Pacific Ocean

181

Four Fe-Mn crusts were selected for generating high-resolution depth profiles through microsampling of Fe-Mn crusts layers. Detailed description of the samples has already been published in Gueguen et al. (2016). Briefly, the sample set comprises two Fe-Mn crusts collected in the North Pacific on Apuupuu seamount ~50 km south of Hawaii during FeMO 2009 cruise on the R/V Kilo Moana (University of Hawaii) in October 2009, and two Fe-Mn crusts collected in the South Pacific (Tahiti, Austral archipelago of French Polynesia) near Rurutu Island by dredges during ZEPOLYF 2 cruise on the R/V L'Atalante (Ifremer-Genavir) in July-August 1999 (Table 1). The North Pacific Fe-Mn crusts (J2-480 and J2-480-R14) were collected at 2079 m depth while the South Pacific Fe-Mn crusts were collected at shallower depths at 1826 m and 1530 m (ZEP2-DR-05-04 and ZEP2-DR-06-03 respectively).

Each crust was split and a representative cross section was taken off (including whenever it was possible the top surface) and embedded into epoxy resin and polished.
Growth layers of Fe-Mn crusts were subsampled at mm-scale using a microdrilling device (Micromill®). Lines perpendicular to the direction of growth were drilled to obtain sufficient crust powder for geochemical analyses. The total amount of powder collected in each line was
generally between ~30 and 50 mg. Details on sample processing can be found in Gueguen et al. (2016) as well as dating of the crusts which was made using cosmogenic $^{10}$Be isotope measurements (Segl et al., 1984; Bourles et al., 1989). Fe-Mn crust ZEP2-DR05-04 could not be dated with $^{10}$Be because the base of the crust has been affected by post-deposition alteration which has reset the chronometer. ZEP2-DR05-04 was therefore dated using Co accumulation rates in the Fe-Mn crust (see explanations in Gueguen et al., 2016).

203

## 2.1.2. Bulk ferromanganese crusts

205

Additional bulk Fe-Mn crusts samples from various locations including Pacific and Atlantic Oceans completed the dataset of this study. Description of samples is reported in Gueguen et al. (2021). Fe-Mn crusts from the Atlantic Ocean were sampled at different locations, along the Ascension Fracture Zone (FZ), VEMA FZ, Gloria FZ and the Azores Triple Junction (TJ). Even if these samples do not strictly reflect the bulk crusts because they consist of subsamples collected at different depths within the crust and this depth is different according to samples, the resolution of sampling and the size of sample are large enough to be considered as a bulk sample. As a consequence, no attempts were made to give age range for these samples. Two additional Fe-Mn crusts collected in the North Pacific from Apuupuu seamounts were also selected as bulk Fe-Mn crust samples (recovered during the same cruise).

217

## 2.3. Cu and Zn isotopes analyses

219

Copper and Zinc isotope compositions were measured at Pôle-Spectrométrie-Océan (PSO, Ifremer, Brest, France) by MC-ICP-MS (Neptune, Thermofisher Scientific). Copper
and Zinc isotope ratios were corrected with internal isotopic standards of Zn NIST SRM 3168a and Cu NIST SRM 976 respectively, doped in samples prior to analysis coupled to a standard-sample-bracketing procedure (Marechal et al., 1999; Albarède, 2004).

The fractionation factor calculated for the doped element for which the isotope composition is known is applied to the isotopic ratios of the measured element assuming that isotopes of both the doped element and the element to be measured follow a similar exponential mass fractionation law (Marechal et al., 1999; Albarède, 2004). Prior to isotope analyses on the mass spectrometer, Cu and Zn were separated with ion-exchange chromatography columns using AG-MP1 resin. Two milliliters of resin were loaded in polypropylene columns. A split from the sample archive solutions was taken off and evaporated to dryness on hot Teflon plates and redissolved in 1.2 mol/L HCl and then loaded on the resin. Both Cu and Zn were purified twice through the columns. Samples were prepared to target 100 ng of Cu and Zn for MC-ICP-MS measurements. Procedural blanks measured were <0.3 ng for Cu and <3 ng for Zn.

Isotopic compositions are expressed as $\delta^{65/63}\text{Cu}_{\text{NIST976}}$ and $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}} (\text{‰})$ using respectively the following equations (1) and (2):

$$ \delta^{65/63}\text{Cu}_{\text{NIST976}} = [({}^{65}\text{Cu}/{}^{63}\text{Cu}_{\text{sample}})/({}^{65}\text{Cu}/{}^{63}\text{Cu}_{\text{NIST976}}) - 1] \times 1000 \quad (1) $$

$$ \delta^{66/64}\text{Zn}_{\text{JMC-Lyon}} = [({}^{66}\text{Zn}/{}^{64}\text{Zn}_{\text{sample}})/({}^{66}\text{Zn}/{}^{64}\text{Zn}_{\text{JMC-Lyon}}) - 1] \times 1000 \quad (2) $$

Precisions on the Cu and Zn isotope delta values are reported as a two-standard deviation (2sd) calculated on replicate measurements of the isotopic standards Cu NIST SRM 976 and Zn NIST SRM 3168a respectively (Table 1), following a standard-sample bracketing method. Zinc isotope values are generally reported relatively to the JMC Lyon Zn standard
and Archer et al. (2017) proposed to keep this notation to allow for a direct comparison with all published data. Based on an inter-calibration exercise (Archer et al., 2017), JMC Lyon Zn isotopic standard is fractionated relative to Zn AA-ETH standard by 0.28‰ and to Zn NIST SRM 3168a standard by 0.94 ‰.

We also performed replicate analyses of Geological Reference Materials (GRMs) Nod-A-1 and Nod-P-1 which yield, respectively, average values of 0.29 ±0.09 ‰ (2sd, n=4) and 0.34 ±0.05 ‰ (2sd, n=5) for δ65/63CuNIST976 and 0.95 ±0.07 ‰ (2sd, n=4) and 0.80 ±0.12 ‰ (2sd, n=5) for δ66/64ZnJMC-Lyon (Table 1). The values are consistent with published data (Chapman et al., 2006; Bigalke et al., 2010; Little et al., 2017). Errors reported for the GRMs in Table 1 correspond to the two-standard deviation of the replicate analyses of each GRM. These replicates include different digested aliquots and duplicates from the chromatography column separation procedure.

259

## 3. Results

261

Description of the trace element data and determination of the age the Fe-Mn crusts are reported in Gueguen et al. (2016). The four Fe-Mn crusts are typical hydrogenetic Fe-Mn crusts and fall in the hydrogenetic field of the ternary diagram of Bonatti et al. (1972). Copper and Zn concentrations covary with Mn concentrations suggesting that Cu and Zn are hosted by the Mn-phase. Therefore, we chose to present in this study elemental ratios with elements normalized to Mn in order to avoid potential dilution by other mineral phases present in the crusts. Mn/Fe ratios (Table 1 and supplementary information) in North Pacific Fe-Mn crusts show a significant increase through time between ~10 Ma and ~8 Ma from 0.68 to 2.22 in J2-480 and from 1.25 to 1.79 in J2-480-R14. Fe-Mn crusts from South Pacific also exhibit an increase in Mn/Fe ratios from ~4 Ma to the top of the crust from 0.98 to 1.45 in ZEP2-DR06-
272 03 and from 0.99 to 1.91 in ZEP2-DR05-04. Zn/Mn and Cu/Mn ratios are similar in both
273 South Pacific Fe-Mn crusts, and display a continuous decrease from the “bottom” region (i.e.,
274 older portion of the crust) to the “top” region (i.e., younger portion of the crust). In contrast,
275 North Pacific Fe-Mn crusts display opposite trends in Zn/Mn ratios although both crusts show
276 similar features with a sharp increase ca. 6 Ma (Figure 1). Fe-Mn crusts from the North
277 Pacific on the one hand, and South Pacific on the other hand show similar decrease in Cu/Mn
278 ratios. The highest Cu/Mn ratios are observed for the oldest sub-samples of J2-480 (>10Ma),
279 a time period not covered by South Pacific Fe-Mn crusts (Figure 1).

280 The Cu and Zn isotope compositions in both North Pacific Fe-Mn crusts show a
281 relatively restricted range of variations with values of 0.57 ±0.06 ‰ to 0.67 ±0.06 ‰ for
282 δ65/63CuNIST976 and 0.97 ±0.04 ‰ to 1.25 ±0.04 ‰ for δ66/64ZnJMC-Lyon in J2-480, and from 0.56
283 ±0.06 ‰ to 0.72 ±0.06 ‰ for δ65/63CuNIST976 and 1.15 ±0.04 ‰ to 1.25 ±0.04 ‰ for
284 δ66/64ZnJMC-Lyon in J2-480-R14 (Table 1 and Figure 2). In contrast to North Pacific Fe-Mn
285 crusts, South Pacific Fe-Mn crusts show significant isotopic variability over the last 10 Ma.
286 First, there is a concomitant increase in δ65/63CuNIST976 and δ66/64ZnJMC-Lyon at about 6 Ma to 4
287 Ma, from 0.16 ±0.03 ‰ to 0.51 ±0.03 ‰ for δ65/63CuNIST976 and from 0.67 ‰ ±0.03 to 1.09
288 ±0.03 ‰ for δ66/64ZnJMC-Lyon values (Table 1 and Figure 2). This translates into a strong
289 positive correlation between δ65/63CuNIST976 and δ66/64ZnJMC-Lyon values (Figure 3) with a
290 correlation coefficient R²=0.90 (Figure 3). Second, the Cu and Zn isotope compositions of the
291 South Pacific Fe-Mn crusts (which essentially control the positive correlation in Figure 3)
292 become heavier with time. Figure 3 also includes the isotopic compositions of bulk Fe-Mn
293 crusts. δ65/63CuNIST976 values in bulk Fe-Mn crusts are comprised between 0.15 ‰ and 0.54 ‰
294 with an average value of 0.28 ±0.13 ‰ (2sd, n=20) while δ66/64ZnJMC-Lyon values range from
295 0.58 ‰ to 1.03 ‰ around an average value of 0.80 ±0.15 ‰ (2sd, n=20) which encompasses
296 the range measured in the four Fe-Mn crusts time-series of this study.
Figure 4 shows that light $\delta^{65/63}\text{Cu}_{\text{NIST976}}$ and $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}}$ values in South Pacific Fe-Mn crusts (ZEP2-DR05-04 and ZEP2-DR06-03) are correlated with higher Cu/Zn ratios whereas such correlation does not exist in North Pacific Fe-Mn crusts (J2-480 and J2-480-R14) displaying constant Cu and Zn isotope values for variable Cu/Zn ratios. The range of Cu/Zn ratios (0.75 to 2.66) is similar in both South Pacific and North Pacific Fe-Mn crusts and the four crusts show decreasing Cu/Zn ratios from older to younger crust (Table 1). In a previous paper, Gueguen et al. (2016), it was shown that the last four samples of ZEP2-DR05-04 have been subjected to post-deposition alteration. Although the Cu and Zn isotope patterns are not changed by these alteration processes (see Table 1), we have chosen not to represent these four data-points in the figures of the manuscript.

307

## 4. Discussion

309

### 4.1 Significance of Cu and Zn isotope co-variations in ferromanganese crusts

311

Ferromanganese crusts are precipitated from trace metals dissolved in seawater. Thus, isotopic variations recorded in the crusts are the result of either in-situ fractionation during precipitation of the crust (e.g., adsorption, growth rates) or late alteration of the crust, or to variable seawater isotopic composition through time (e.g., oceanic circulation, sources of trace metals to seawater). Since a positive correlation between Cu and Zn isotopes is observed in the two South Pacific Fe-Mn crusts, it is likely that the causes of both Cu and Zn isotope variations are intimately coupled. The correlation may result either from, (1) isotopic fractionation during growth (e.g., sorption) or post-depositional alteration of the crusts, with preferential fractionation of the light (or heavy) isotopes of both Cu and Zn, or, (2) mixing of
two sources with distinct Cu and Zn isotope compositions. The relative importance of each process is discussed below.

The primary Zn and Cu hosting Mn oxide phases in hydrogenetic Fe-Mn crusts are poorly crystallized phyllomanganate phases, typically vernadite (δ-MnO₂, also often called birnessite). Experimental results of Zn isotope fractionation during sorption to Mn-oxides birnessite indicate that isotopically heavy Zn (up to ~3 ‰) is sorbed to the mineral surface (Bryan et al., 2015), consistent with the fact that Zn isotopes in ferromanganese crusts (Little et al., 2014) are on average 0.5 ‰ heavier than seawater (Boyle et al., 2012; Conway and John, 2014; Zhao et al., 2014; Conway and John, 2015; Lemaitre et al., 2020). Adsorption of Cu onto synthetic birnessite results in preferential fractionation of the light isotopes on the solid phase by a factor of -0.45 ±0.18 ‰ (Ijichi et al., 2018). On the other hand, Sherman and Little (2020) found that dissolved Cu isotope composition should be 0.94 ‰ (at 5°C) heavier than Cu sorbed to birnessite. The latter value is substantially greater than the observed fractionation factor between ferromanganese crusts and seawater which is ~ -0.3 ‰, but both studies confirm that Cu isotope fractionation during adsorption occurs in opposite direction to Zn isotope fractionation. Hence, the concomitant shifts in Cu and Zn isotope compositions in the same direction occurring ca. 6-4 Ma in the South Pacific Fe-Mn crusts cannot be explained by simple sorption-desorption processes associated with, for example, changes of growth rates or post-depositional processes. Cu/Mn and Zn/Mn ratios in South Pacific Fe-Mn crusts show a slight decrease with decreasing time (Figure 1), thus Cu/Mn and Zn/Mn decreases with increasing δ<sup>65/63</sup>Cu<sub>NIST976</sub> and δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> values. By contrast, large variations in Cu/Mn and Zn/Mn ratios observed in North Pacific Fe-Mn crusts are not associated with any significant Cu and Zn isotope variations, thus this likely precludes the effect of a mineralogical control on Cu and Zn isotope fractionation. We further estimated the potential effect of Fe-Mn crust growth on the isotopic composition using Co concentrations
normalized to Mn in Fe-Mn crusts. Cobalt is commonly used for dating Fe-Mn crusts and nodules layers because the flux of Co to the oceans is assumed to be constant through time (Halbach et al., 1983; Puteanus and Halbach, 1988), and thus variations in Co/Mn ratios of the crust would reflect variations in growth rate of the different layers of the crust. The absence of clear co-variations between Co/Mn ratios and $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}}$ and $\delta^{65/63}\text{Cu}_{\text{NIST976}}$ in South Pacific Fe-Mn crusts (Table 1) likely suggests that growth rates are not a factor controlling Cu and Zn isotope fractionation. We therefore suggest that the positive correlation observed between $\delta^{65/63}\text{Cu}_{\text{NIST976}}$ and $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}}$ reflects variable composition of deep seawater rather than in-situ processes, such as adsorption, precipitation or effect of the mineralogy in Fe-Mn crusts.

Because changes in $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}}$ and $\delta^{65/63}\text{Cu}_{\text{NIST976}}$ recorded in South Pacific crusts occur within an age window of 4 to 6 Ma, which is not recorded in North Pacific crusts, we speculate that regional, rather than global oceanographic events affected deep seawater composition. Figure 4 shows that high Cu/Zn ratios (from 0.97 to 2.33) in both South Pacific Fe-Mn crusts are correlated with light Cu and Zn isotope compositions. North Pacific Fe-Mn crusts display a similar range of Cu/Zn ratio (and even higher ratios) than South Pacific Fe-Mn crusts but do not display any co-variations with isotopic compositions. In fact, decreasing Cu/Zn ratios with time are observed in North Pacific Fe-Mn crusts but they are not contemporaneous to any Cu and Zn isotope variations. The correlation observed between Cu/Zn ratios and Cu and Zn isotope compositions in South Pacific crusts suggests mixing between two end-members with different Cu/Zn ratios and isotopic compositions.

In the following discussion, we evaluate the possible factors resulting in variations of Cu and Zn isotope signatures in the deep Pacific Ocean seawater. Because most Cu and Zn isotope variations occur in the first ~500 m depth, we first evaluate the influence of biogeochemical processes occurring in the water column such as, bioproductivity in surface
waters, influence of oxygen minimum areas, continental runoff and continental margin inputs,
and volcanic activity and associated hydrothermal inputs; and the impacts of these processes
on the deep Pacific Ocean seawater. Then, we compare the results observed in our Fe-Mn
crusts with other Fe-Mn deposits, and conclude the discussion by evaluating the impact of our
findings on the global oceanic Cu and Zn budgets.

376

377 4.2. Biogeochemical processes in the water column

378

379 4.2.1. Effects of surface bioproductivity and export in deep seawater

380

381 Zinc is a micronutrient in seawater (Morel and Price, 2003) and it has been shown that
382 Zn isotopes are fractionated during biological uptake with preferential uptake of light Zn
383 isotopes by microorganisms (John et al., 2007; Andersen et al., 2011; Samanta et al., 2017;
384 Köbberich and Vance, 2019). Therefore, one would expect that dissolved Zn in surface
385 seawater is isotopically heavy compared to deep waters. However, this pattern is not
386 systematically observed (e.g., in the Southern Ocean, Zhao et al., 2014). Studies suggest that
387 Zn isotopes in surface waters are dominated by scavenging processes where heavy Zn
388 isotopes are preferentially adsorbed on organic matter resulting in light Zn isotope signatures
389 in seawater (John and Conway, 2014; John et al., 2018; Weber et al., 2018), and intracellular
390 regeneration of light Zn isotopes is also observed in the North Pacific Ocean (Vance et al.,
391 2019). In addition, anthropogenic inputs may also impart light isotope signatures to surface
392 seawaters (Lemaitre et al., 2020; Liao et al., 2020). Thus, suggested relationships between Zn
393 isotopes in the marine geological archive and past paleoproductivity (Pichat et al., 2003;
394 Kunzmann et al., 2013) may in fact be more complex.
In addition, while Zn has clearly a nutrient-type profile in the water column (micronutrient), Cu behavior in the water column is not only affected by biological uptake but also by scavenging and dissolution processes as exemplified by continuous increase in Cu concentrations with depth as a result of dissolution of sinking particles or a benthic flux from sediments (Boyle et al., 1977; Little et al., 2013; Roshan and Wu, 2015; Richon and Tagliabue, 2019). Thus, variations of surface productivity are unlikely to explain the relationships observed between Cu and Zn isotope compositions in the two South Pacific Fe-Mn crusts.

403

404

### 4.2.2. Influence of oxygen minimum areas

405

Scavenging of trace metals during Mn-oxides colloids formation is probably more efficient at the base of the OMZ (defined as low oxygen concentrations areas, i.e., about 50-100 µM, located between 1000 to 1500 m water depths) due to enhanced oxidation of Mn from the OMZ where dissolved reduced Mn accumulates (Klinkhammer and Bender, 1980; Koschinsky and Halbach, 1995). Thus, Fe-Mn crusts formed close to the OMZ could be preferentially enriched in transition metals in comparison to Fe-Mn crusts precipitated in deeper waters, thus increasing their vertical offset from OMZ depths and resulting in lower Mn/Fe ratios. The subsidence and drift of seamounts during plate motion imply that Fe-Mn crusts have deepened through time, which kept them away from the OMZ. The influence of the depth of OMZ base relatively to the depth of Fe-Mn crusts formation has already been suggested by previous authors to explain highest enrichment in Co, Ni and Mn in the shallowest crusts (Halbach et al., 1984; Aplin and Cronan, 1985; De Carlo et al., 1987). If the location of the crusts relative to the base of an OMZ has varied through time as a consequence of seafloor subsidence and plate motion, so that older parts of the crusts precipitated in
420 shallower waters were more proximate to the base of an OMZ than their present-day location,
421 then one would expect to have higher Mn/Fe ratios in older part of the crusts (i.e., closer to
422 the rock substrate) than in the younger part (i.e., closer to the crust surface/seawater
423 interface). In fact, an inverse trend is observed in South Pacific Fe-Mn crusts where
424 increasing Mn/Fe ratios are found in the youngest parts of the crusts, suggesting that distance
425 from the OMZ is not responsible for geochemical variations.

426 Most Zn isotope variations in the water column occur between 0 and ~500 m depth,
427 and more specifically within the first 100 meters depth (Conway and John, 2014; Zhao et al.,
428 2014; Liao et al., 2020), and exhibit lighter values with decreasing depth (Boyle et al., 2012;
429 Vance et al., 2016; John et al., 2018; Vance et al., 2019; Lemaitre et al., 2020). Small Zn
430 isotope variations in the water column are associated with the presence of low oxygen
431 concentrations (< 100 µM), where lighter δ66/64ZnJMC-Lyon values are likely related to Zn
432 scavenging onto particles (Conway and John, 2014). However, in deep waters below ~1000 m
433 depth, δ66/64ZnJMC-Lyon values remain relatively homogenous in the Pacific Ocean with an
434 average value of ~ 0.5 ‰ (Conway and John, 2015; John et al., 2018; Vance et al., 2019).
435 Copper isotopes are not affected by low oxygen conditions associated with an OMZ and most
436 Cu isotope variations occur in surface waters well above 500 meters depth (Takano et al.,
437 2014), implying that Cu isotope compositions in deep waters (>1000 m depth) should also be
438 homogenous in the Pacific Ocean. Thus, for both Cu and Zn most isotopic variations in the
439 water column occur above 1000 m water depth. This feature is important to highlight because
440 the four crusts studied here are located below the limit where Cu and Zn isotope variations
441 were observed in the water column (1826 and 1530 m for ZEP2-DR05-04 and ZEP2-DR06-
442 03 respectively, and ~2000 and 2079 m for J2-480 and J2-480-R14 respectively) and below
443 the depth of the OMZ. Recording an isotopic signal in Fe-Mn crusts related to processes
444 occurring in the OMZ would require transport of Cu and Zn from the OMZ to deep waters,
445 for example, via sinking particles. However, this process remains speculative since we cannot
446 demonstrate it with our study. Therefore, the influence of OMZs on elemental composition
447 and Cu and Zn isotope composition of Fe-Mn crusts is not a relevant explanation to the
448 positive correlation observed, but a change in local or global Cu and Zn sources to the oceans,
449 or change in water mass composition is most likely.

450

451 **4.2.3. Inputs from continental runoff and continental margins**
452

452

453 Continental runoff via riverine inputs and atmospheric particles is the main source of
454 dissolved Cu and Zn to seawater (Little et al., 2014). Conway and John (2014) also identified
455 a sedimentary margin Zn source in seawater samples collected near the North American and
456 Iberian margins, while Lemaitre et al. (2020) observed a light Zn benthic source in the North
457 Atlantic Ocean. In the Northwestern Pacific Ocean, light δ66/64ZnJMC-Lyon values in deep
458 seawater are also attributed to benthic input of Zn (Liao et al., 2020). Conway and John
459 (2014) show that continental margins are a source of isotopically light Zn with δ66/64ZnJMC-Lyon
460 values between ~ -0.7 ‰ and ~ -0.5 ‰ for the Iberian and North American margins
461 respectively. The authors proposed two explanations for this isotopically light Zn source, (1)
462 release of Zn in the sediment during phytoplankton degradation, or (2) release of Zn in the
463 sediment from sulfides formed in the water column. They also suggest that this sedimentary
464 source could be a sink for isotopically light Zn in environments where, for example, Zn is
465 retained in sulfidic or reducing porewaters instead of being released in the water column. This
466 interpretation was also further proposed by Little et al. (2016). A significant source of
467 sedimentary Cu from continental margins has not been clearly demonstrated yet, and the
468 isotope composition of this flux also remains unclear (Little et al., 2017). But the lithogenic
469 Cu has an isotope composition of ~ 0 - 0.1 ‰ (Little et al., 2017), thus lighter than average
470

seawater Cu isotope composition and lighter than the oldest Fe-Mn crust end-member. The

471

benthic flux have been hypothesized to be a source of dissolved Cu to the deep ocean (Boyle

472

et al., 1977; Roshan and Wu, 2015), but the Cu isotope composition of this flux is not known

473

yet.

474

The four Fe-Mn crusts investigated here are located a few thousands of kilometers

475

away from continental margins. This would imply either transport of this sedimentary source

476

to the deep ocean or that the crusts were located closer to the continents some time during

477

their growth. The latter could possibly be consistent with lighter Zn isotope composition

478

recorded in the older part of Fe-Mn crusts. The position of Fe-Mn crusts formed in the Pacific

479

Ocean has varied through time in a northwestward direction during plate motion (Meynadier

480

et al., 2008), in which case the position of the crusts relatively to the continents surrounding

481

the Pacific Ocean may have changed. However, based on the model of Meynadier et al.

482

(2008) South Pacific Fe-Mn crusts would have moved from a distance of < 400 km. This

483

distance is too small to consider that Fe-Mn crusts formed close to the continents in their early

484

growth and would have been directly impacted by inputs from continental margins.

485

486

### 4.2.4. Impact of volcanic activity and hydrothermal fluxes in the Pacific Ocean

487

488

A study showed that interaction of seawater with fresh lava flows released isotopically

489

light Cu (~-1.6 ‰) and Zn (~-0.1‰) in seawater from the basalts (Hawco et al., 2020). The

490

Society Islands exhibit an age progression to the West from Mehetia (less than 1 Ma) to Tahiti

491

(0.6–1.2 Ma), Moorea (1.5–2.0 Ma) Bora Bora (3.1–3.5 Ma) and Maupiti (3.9–4.5 Ma) (Blais

492

et al., 2002; Yamamoto et al., 2002; Uto et al., 2007). The Marquesas Archipelago is the

493

northernmost linear volcanic chain in French Polynesia. It comprises eight main islands and a

494

few islets and seamounts formed between 5.5 and 0.4 Ma (Legendre et al., 2006). By
comparison, the Hawaiian Islands are all younger than 5.1 Ma, with most recent volcanism
less than 1 Ma, but most of the older islands have since subsided below sea level. This
indicates that the timing of regional volcanic activity (i.e., volcanic seamounts are younger
than 6 Ma) is not consistent with the observed Fe-Mn crusts Cu and Zn isotope patterns (i.e.,
light Cu and Zn isotope compositions are older than 6 Ma). Hence, it does not seem that
regional hotspot-related volcanic activity affects Fe-Mn crusts composition. In particular, the
formation of Society Islands and Marquesas marked a period of less than 5.5 Ma, which is
associated with Cu and Zn isotope signatures consistent with modern deep seawater. The
volcanic chain extending from the Rurutu Island and nearby seamounts was dated at 12 Ma
(Clouard and Bonneville, 2001). Hence, it is unlikely that low δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> - δ<sup>65/63</sup>Cu<sub>NIST976</sub>
end-members, located at the base of the Fe-Mn crusts are affected by the latest stages of the
formation of the volcanic seamounts nearby Rurutu Island and to the weathering of fresh lava
flows.

The influence of hydrothermal activity on transition metal enrichment in South
Eastern Pacific sediments was first mentioned by Boström et al. (1969). Although most
transition metals delivered by hydrothermal fluids are precipitated in the vicinity of vent sites,
a significant proportion of Cu and Fe, for example, may enter the deep ocean through
complexation with organic ligands (Coale and Bruland, 1988; Toner et al., 2009; Sander and
Koschinsky, 2011; Resing et al., 2015). Hannington (2013) recently highlighted that
accumulation of Cu in South Pacific deep-sea sediments through time may exceed the total
amount of Cu deposited at ridge axis. Modelling of Cu transport from hydrothermal vents by
complexation with organic ligands may account for 14% of the Cu dissolved in seawater
(Sander and Koschinsky, 2011), especially in ultramafic-hosted hydrothermal sites where
abiotic production of organic compounds is enhanced. Nevertheless, measurements of
dissolved Cu near hydrothermal sources yield contrasted results. Seawater samples from the
GA03 transect in the North Atlantic indicate removal of Cu near the TAG hydrothermal vent (Roshan and Wu, 2015). Copper concentrations in seawater at the EPR in the South Pacific does not show any removal nor increase in dissolved Cu, but an increase in particulate Cu just above hydrothermal sources is observed (GEOTRACES Intermediate Data Product Group, 2021). Therefore, how Cu can be transported over long distances from the ridge-axis remains unclear to date and must be investigated in the future. Transport of hydrothermal Zn in seawater on long distance of thousands of kilometers has been demonstrated in the South Pacific Ocean (Roshan et al., 2016; John et al., 2018) and in the North Atlantic Ocean (Conway and John, 2014; Lemaitre et al., 2020). Organic complexation of Zn from hydrothermal fluids is not specifically documented in the literature, but it was demonstrated that Zn forms strong organic ligands in seawater (e.g., Mackey, 1983; Bruland, 1989; Donat and Bruland, 1990; Ellwood and Van den Berg, 2000; Kim et al., 2015), or that it could be transported as Zn sulphide nanoparticulates (Conway and John, 2014).

The Tahiti archipelago, where the South Pacific Fe-Mn crusts were collected, is clearly within the area of the Pacific Ocean (between 10-20°S) characterized by large-scale $ ^{3}He $ enrichment at 2500 m depth (Lupton and Craig, 1981; Lupton, 1995; Urabe et al., 1995). One can suggest that, for a long period of time, the South Pacific area has been receiving large hydrothermal inputs from the superfast East Pacific Rise (EPR) segments (Resing et al., 2015). Local hydrothermal sources have been also proposed, for example through active volcanic seamounts such as Teahitia potentially associated with reactivation of the Society Islands Hotspots (German et al., 2020). Hence, we interpret the difference in Cu and Zn isotope records between the South Pacific and the North Pacific Fe-Mn crusts as being related to local contribution of trace metals from hydrothermal sources that would have affected water masses of the South Pacific Ocean from more than 6 Ma ago until 4 Ma.
The effect of hydrothermal Zn on the deep seawater Zn have been directly observed in the Atlantic Ocean where $\delta^{66/64}$Zn$_{JMC-Lyon}$ values of ~ -0.5 ‰ have been measured in deep waters near TAG hydrothermal vent in comparison with the average value of ~ 0.5 ‰ for deep waters (Conway and John, 2014; John et al., 2018). In addition, light Zn isotopes (~ -0.3 ‰) measured in seawater near the Reykjanes Ridge was attributed to hydrothermal inputs transported by the Labrador Sea Water from the MAR (Mid-Atlantic Ridge) over 1000 km (Lemaitre et al., 2020). The authors calculated a hydrothermal end-member of ~ -0.5 ‰ responsible for lighter Zn isotopes values (Lemaitre et al., 2020), which is consistent with the value of ~ -0.5 ‰ measured near the TAG hydrothermal field (Conway and John, 2014).

However, in the Pacific Ocean, Zn in hydrothermal fluids from EPR hydrothermal vent sites is enriched in heavy isotopes from 0.0 ‰ to 1.0 ‰ relatively to the source rock as a result of precipitation of isotopically light Zn in sulfides (John et al., 2008). In other words, high temperature hydrothermal vents have probably Zn isotope values close to basalt values, but when fluids cool and sulfides precipitate, the fluids become enriched in isotopically heavy Zn. John et al. (2018) calculated the Zn isotope composition of a hydrothermal end-member in South Pacific seawater of ~ 0.2 ‰. The authors argued that hydrothermal fluids might possibly affect Zn isotopes budget of seawater, but notably the Zn isotope composition of the Pacific Ocean hydrothermal end-member is significantly heavier than the average value calculated for the Atlantic Ocean (~ -0.5 ‰). Copper isotope composition of hydrothermal fluids has been recently reported in the literature and suggests slightly heavier values (i.e., ~ 0-0.4 ‰) than basaltic values for high-temperature hydrothermal fluid end-members, although heavier $\delta^{65/63}$Cu$_{NIST976}$ have been also reported in vent fluids (i.e., ~ 1.1 ‰, Syverson et al., 2021) and active hydrothermal chimneys (i.e., up to ~ 3 ‰, Rouxel et al., 2004).

In our Figure 3, taking a hydrothermal end-member for the South Pacific seawater (lower end of the trend) with values of ~ 0.2 ‰ for $\delta^{65/63}$Cu$_{NIST976}$ and ~ 0.7 ‰ for $\delta^{66/64}$Zn$_{JMC}$-
Lyon, and a "Pacific seawater" end-member represented by the average North Pacific Fe-Mn crusts ($\delta^{65/63}\text{Cu}_{\text{NIST976}} \sim 0.7\,\text{‰}$ and $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}} \sim 1.2\,\text{‰}$), we can reconstruct the seawater isotope composition using Cu and Zn isotope fractionation factors between Fe-Mn crusts and seawater of $-0.3\,\text{‰}$ and $0.5\,\text{‰}$ respectively. This yields a Pacific seawater with $\delta^{65/63}\text{Cu}_{\text{NIST976}} \sim 1\,\text{‰}$ and $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}} \sim 0.7\,\text{‰}$, and a hydrothermal fluid end-member with $\delta^{65/63}\text{Cu}_{\text{NIST976}} \sim 0.5\,\text{‰}$ and $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}} \sim 0.2\,\text{‰}$. These values are slightly offset from the average modern seawater values ($0.7\,\text{‰}$ for Cu, and $0.5\,\text{‰}$ for Zn) and from known values for Pacific hydrothermal fluids ($0-0.4\,\text{‰}$ for Cu, and $0.2\,\text{‰}$ for Zn). Possible reasons are the uncertainty in the determination of the Cu and Zn isotope compositions of Pacific hydrothermal fluids and the fractionation factors between seawater and ferromanganese crusts.

Despite these uncertainties, the trend observed in Figure 3 is consistent with the fact that the source of Cu and Zn in the older Fe-Mn crusts tends towards a source end-member dominated by hydrothermal inputs. We interpret the results shown by South Pacific Fe-Mn crusts as the result of sustained hydrothermal influence from the EPR during the early stages of Fe-Mn crust formations, 4-6 Ma ago.

### 4.3. Comparison with other oceanic Fe-Mn deposits

High Cu/Zn ratios (between ~2 and ~4) were also reported for Bauer Basin Fe-Mn crusts, yet these Fe-Mn oxide deposits (including Fe-Mn crusts and nodules) are known to have been affected by disseminated hydrothermal particles remaining in the water column after inputs from the EPR that did not precipitate near the vent (Elderfield and Greaves, 1981). Therefore, increasing Cu/Zn ratios correlating with decreasing $\delta^{65/63}\text{Cu}_{\text{NIST976}}$ and $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}}$ in South Pacific Fe-Mn crusts is consistent with hydrothermal inputs in seawater.
As illustrated in Figure 3, Cu and Zn isotope composition of Fe-Mn crusts and nodules from different oceanic basins do not all plot on the linear trend defined by North and South Pacific Fe-Mn crusts. Mn-nodules show more scatter in $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}} - \delta^{65/63}\text{Cu}_{\text{NIST976}}$ relationships because they are probably influenced by diagenetic processes and metal cycling occurring in sediment porewaters. Hence, the isotope composition of the surface scrapings of nodules reported by Maréchal et al. (2000) and Albarède (2004) are not accurate proxies of seawater isotopic composition through time. The dataset shown in Figure 3 also includes isotopic compositions of bulk Fe-Mn crusts samples which composition represents an average of the composition through time. Little et al. (2014) found no variations through time in the Cu and Zn isotope compositions of the three Fe-Mn crusts from the Atlantic, Indian and Pacific Oceans. If we look at Figure 3, one can see that most Fe-Mn crusts samples from the Central Pacific Ocean (Little et al., 2014) (located Southwestern of the North Pacific Fe-Mn crusts from this study) plot in the trend defined by our Fe-Mn crusts time-series, and are all very close to the Pacific seawater end-member. Outlier samples with lighter (-0.16 ‰) and heavier (1.19 ‰) $\delta^{65/63}\text{Cu}_{\text{NIST976}}$ values, and lighter (0.60 ‰) and heavier (1.42 ‰) $\delta^{66/64}\text{Zn}_{\text{JMC-Lyon}}$ value reported for the Indian Fe-Mn crust by Little et al. (2014) were interpreted as the result of a lower enrichment in Cu or Zn in the Fe-Mn crusts, implying that the Cu and Zn isotopic signal in the Fe-Mn phases could have been biased by the presence of other mineral phases in the crust.

### 4.4. Implication for global Cu and Zn isotope budget

Our results suggest that contribution of hydrothermal inputs as a source of dissolved marine Cu and Zn is significant and could be persistent through time. Little et al. (2014) assumed that rivers are the only important source of marine Cu and Zn. They also pointed out
that the mass balance for Zn was not achieved which they attributed to a missing sink. By contrast, there is a missing isotopically light input flux necessary to balance the marine Cu isotopic budget (Little et al., 2017). These missing fluxes could be accounted for by the hydrothermal flux to the oceans of Cu and Zn as recently reevaluated for Zn by Roshan et al. (2016). Our results bolster recent studies arguing for the non-negligible role of hydrothermalism on the oceanic pool of dissolved transition metals such as Fe (Tagliabue et al., 2010; Saito et al., 2013; Horner et al., 2015; Resing et al., 2015) in particular Fe in the South Pacific Ocean (Fitzsimmons et al., 2014), Ni (Gall et al., 2013; Gueguen et al., 2021), Cu (Hannington, 2013) and Zn (Roshan et al., 2016). For instance, Horner et al. (2015) argued that deeply sourced Fe including Fe from hydrothermal venting has a strong influence on the distribution of deep Fe contrasting with previous studies suggesting that oceanic Fe is predominantly supplied by continental aeolian dust and particles. This finding calls into question the effective role of hydrothermal inputs on the global marine biogeochemical cycling of trace metals suggesting that, besides continental sources (rivers and dusts), mass balance of key transition metals in seawater may also be strongly controlled by hydrothermal sources. This picture is particularly relevant for the South Pacific Ocean where hydrothermal venting from the East Pacific Rise has likely a significant influence on the distribution of trace metals in seawater.

637

## 5. Summary and concluding remarks

639

Copper and Zinc isotope patterns during the last ~17 Ma are distinct for Fe-Mn crusts formed in the North Pacific and those in the South Pacific Ocean. North Pacific Fe-Mn crusts show limited Cu and Zn isotope variations through time with values between 0.57 and 0.73 ‰ for δ$^{65/63}$Cu$_{NIST976}$ and between 0.97 ‰ to 1.25 ‰ for δ$^{66/64}$Zn$_{JMC-Lyon}$. The range in isotopic
644

variations is larger in South Pacific crusts than in North Pacific crusts with Zn isotopes

645

varying from 0.67 to 1.09 ‰ and Cu isotopes are comprised in the 0.16 - 0.51 ‰ range.

646

The major finding of this study is the positive correlation between δ<sup>65/63</sup>Cu<sub>NIST976</sub>

647

values and δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> values in South Pacific Fe-Mn crusts, which has not been observed
in Fe-Mn crusts from other oceanic regions (Atlantic, Indian and Antarctic Oceans).

648

649

Variations are not related to precipitation processes or mineralogical effects, but we interpret
these patterns as being the result of hydrothermal inputs from the East Pacific Rise where
transport of Cu and Zn with organic ligands or sulphide nanoparticulates may have favored
their dispersion in deep seawater. We show that significant hydrothermal inputs 6 Ma ago
affected both Cu and Zn isotope compositions in the South Pacific Ocean. The four Fe-Mn
crusts investigated here plot along a trend defined by a hydrothermal end-member

650

651

652

653

654

655

(δ<sup>65/63</sup>Cu<sub>NIST976</sub> ~ 0.2 ‰ and δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> ~ 0.7‰) and a “Pacific seawater” end-member

656

(δ<sup>65/63</sup>Cu<sub>NIST976</sub> ~ 0.7 ‰ and δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> ~ 1.2 ‰). The values of the end-members are

657

fractionated from seawater by a factor of -0.3 ‰ and 0.5 ‰ for Cu and Zn respectively. Our
study further suggests that hydrothermal venting could be an important source of Cu and Zn
in seawater, in particular in the South Pacific Ocean, and that it could persist during several
million years. Investigation of additional Fe-Mn crusts from other locations in the South
Pacific will confirm the extent of hydrothermal influence on the chemical composition of the
South Pacific Ocean.

658

659

660

661

662

663

664

**Acknowledgments:**

665

666

We thank the FeMO team, K. Edwards, C. Moyer, H. Staudigel, B. Tebo, D. Emerson, and B.

667

Glazer and R/V Kilo Moana and ROV Jason II for facilitating access to samples from Hawaii.

668

We thank Philippe Fernagu (Ifremer Brest, France) for his help during preparation of Fe-Mn
669 crusts samples for microdrilling. We thank Yoan Germain for his assistance in the clean lab,

670 Emmanuel Ponzevera for his assistance during MC-ICP-MS analyses and Marie-Laure

671 Rouget and Claire Bassoulet for analyses on the ICP-MS. We thank Yazhou Fu, Tim Conway

672 and two anonymous reviewers and for helpful comments on our paper. We also thank the

673 associate editor Susan H. Little and executive editor Jeffrey G. Catalano for very constructive

674 and thorough reviews and additional comments on our manuscript. The study was supported

675 by funding from the European Reintegration grant #FP7 #247837, and the LabexMer ANR-

676 10-LABX-19-01.

677

678 **Figure captions:**

679

680 **Figure 1:** Cu/Mn (panels A and B), Zn/Mn (panels C and D) ratios time-series in South

681 Pacific Fe-Mn crusts and North Pacific Fe-Mn crusts respectively. Cu/Mn and Zn/Mn ratios

682 in South Pacific Fe-Mn crusts show limited temporal variations. Cu/Mn ratios in North

683 Pacific Fe-Mn crusts display a decreasing pattern from older to younger part of the crust

684 whereas the opposite trend is observed for Zn/Mn ratios.

685

686 **Figure 2:** Cu and Zn isotopes (‰) time-series in South Pacific Fe-Mn crusts (panels A and C)

687 and North Pacific Fe-Mn crusts (panels B and D). The latter displays constant Cu and Zn

688 isotope compositions through time whereas similar temporal Cu and Zn isotope variations are

689 observed in both South Pacific Fe-Mn crusts.

690

691 **Figure 3:** Plot showing δ<sup>65/63</sup>Cu<sub>NIST976</sub> versus δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> values (‰) in South Pacific and

692 North Pacific Fe-Mn crusts time-series (large red symbols), smaller red triangles correspond

693 to bulk Fe-Mn crusts. Literature data for Cu and Zn isotope composition in Fe-Mn crusts
694 (Little et al., 2014) (circles) and nodules (Maréchal et al., 2000; Albarède, 2004) (triangles) is
695 shown by colored symbols. The positive trend displayed by our time-series Fe-Mn crusts
696 (large red symbols) is exemplified by the dotted red line (r²=0.90). The blue ellipse outlines
697 the cluster of values for North Pacific Fe-Mn crusts which would represent the Pacific
698 seawater end-member of the trend, and the light green box represents the hydrothermal end-
699 member. Mixing between these two end-members could account for the correlation between
700 Cu and Zn isotopes in South Pacific Fe-Mn crusts. Note that these data include both Fe-Mn
701 crusts and nodules (see text for more explanation).

702

703 **Figure 4:** δ<sup>65/63</sup>Cu<sub>NIST976</sub> and δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> values (‰) versus Cu/Zn ratios in South Pacific
704 (A and B) and North Pacific (C and D) Fe-Mn crusts. δ<sup>65/63</sup>Cu<sub>NIST976</sub> and δ<sup>66/64</sup>Zn<sub>NIST3168a</sub>
705 values in North Pacific Fe-Mn crusts do not vary with varying Cu/Zn ratios, whereas South
706 Pacific Fe-Mn crusts display decreasing Cu/Zn ratios with increasing δ<sup>65/63</sup>Cu<sub>NIST976</sub> and
707 δ<sup>66/64</sup>Zn<sub>JMC-Lyon</sub> values in both Fe-Mn crusts. Higher Cu/Zn ratios with lighter Cu and Zn
708 isotope values are compatible with scattered hydrothermal inputs in the deep South Pacific
709 Ocean.

710

711 **Table captions:**

712

713 **Table 1:** Mn/Fe, Cu/Mn, Zn/Mn and Cu/Zn ratios, and Cu and Zn isotope composition (in ‰)
714 of ferromanganese crusts time-series from the South Pacific, the North Pacific, and bulk
715 ferromanganese crusts from various locations.

716
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<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 4</figcaption></figure>
<table><caption>Table 1: Mn/Fe, Cu/Mn, Zn/Mn, Cu/Zn and Co/Mn ratios (µg/µg), and Cu and Zn isotopes (%) in ferromanganese crusts time-series from South Pacific and North Pacific, and bulk Fe-Mn crusts.</caption><thead><tr><th>Sample name</th><th>Location</th><th>Depth (mm) in the crust<sup>a</sup></th><th>Age (Ma)<sup>b</sup></th><th>Mn/Fe<sup>a</sup></th><th>Cu/Mn</th><th>Zn/Mn</th><th>Cu/Zn</th><th>Co/Mn</th><th>δ<sup>55/53</sup>Cu<sub>NIST676</sub></th><th>2sd<sup>b</sup></th><th>δ<sup>56/54</sup>Zn<sub>NIST3168a</sub></th><th>2sd<sup>b</sup></th><th>δ<sup>56/54</sup>Zn<sub>JMC-Lyon</sub></th></tr></thead><tbody><tr><td colspan="14"><strong>ZEP2-DR05-04 Fe-Mn crust</strong></td></tr><tr><td>ZEP2-DR05-04-L1</td><td>South Pacific</td><td>0.5</td><td>1.1</td><td>1.91</td><td>0.0022</td><td>0.0023</td><td>0.97</td><td>0.073</td><td>0.52</td><td>0.03</td><td>2.03</td><td>0.03</td><td>1.09</td></tr><tr><td>ZEP2-DR05-04-L2</td><td>South Pacific</td><td>1</td><td>1.7</td><td>1.66</td><td>0.0030</td><td>0.0025</td><td>1.22</td><td>0.066</td><td>0.51</td><td>0.03</td><td>2.04</td><td>0.03</td><td>1.10</td></tr><tr><td>ZEP2-DR05-04-L3</td><td>South Pacific</td><td>2.5</td><td>3.3</td><td>1.79</td><td>0.0036</td><td>0.0027</td><td>1.35</td><td>0.054</td><td>0.49</td><td>0.03</td><td>2.01</td><td>0.03</td><td>1.07</td></tr><tr><td>ZEP2-DR05-04-L4</td><td>South Pacific</td><td>3.5</td><td>3.9</td><td>1.52</td><td>0.0040</td><td>0.0030</td><td>1.33</td><td>0.050</td><td>0.51</td><td>0.03</td><td>2.00</td><td>0.03</td><td>1.06</td></tr><tr><td>ZEP2-DR05-04-L5</td><td>South Pacific</td><td>4.5</td><td>4.1</td><td>1.12</td><td>0.0044</td><td>0.0031</td><td>1.43</td><td>0.041</td><td>0.43</td><td>0.03</td><td>1.88</td><td>0.03</td><td>0.94</td></tr><tr><td>ZEP2-DR05-04-L6</td><td>South Pacific</td><td>6.5</td><td>4.6</td><td>1.05</td><td>0.0051</td><td>0.0031</td><td>1.62</td><td>0.045</td><td>0.34</td><td>0.03</td><td>1.83</td><td>0.03</td><td>0.89</td></tr><tr><td>ZEP2-DR05-04-L7</td><td>South Pacific</td><td>8.5</td><td>5.9</td><td>1.07</td><td>0.0053</td><td>0.0032</td><td>1.63</td><td>0.046</td><td>0.32</td><td>0.03</td><td>-</td><td>-</td><td>-</td></tr><tr><td>ZEP2-DR05-04-L8</td><td>South Pacific</td><td>10</td><td>6.5</td><td>1.04</td><td>0.0059</td><td>0.0032</td><td>1.69</td><td>0.049</td><td>0.27</td><td>0.03</td><td>1.74</td><td>0.03</td><td>0.80</td></tr><tr><td>ZEP2-DR05-04-L9</td><td>South Pacific</td><td>11.5</td><td>7.1</td><td>1.08</td><td>0.0058</td><td>0.0034</td><td>1.72</td><td>0.050</td><td>0.27</td><td>0.03</td><td>1.75</td><td>0.03</td><td>0.81</td></tr><tr><td>ZEP2-DR05-04-L10</td><td>South Pacific</td><td>12.5</td><td>7.7</td><td>1.03</td><td>0.0054</td><td>0.0032</td><td>1.69</td><td>0.055</td><td>0.32</td><td>0.03</td><td>1.76</td><td>0.03</td><td>0.82</td></tr><tr><td>ZEP2-DR05-04-L11</td><td>South Pacific</td><td>13.5</td><td>8.0</td><td>0.99</td><td>0.0058</td><td>0.0032</td><td>1.83</td><td>0.052</td><td>0.23</td><td>0.03</td><td>1.73</td><td>0.03</td><td>0.79</td></tr><tr><td>ZEP2-DR05-04-L12</td><td>South Pacific</td><td>15.5</td><td>8.6</td><td>1.02</td><td>0.0068</td><td>0.0036</td><td>1.87</td><td>0.052</td><td>0.24</td><td>0.03</td><td>1.74</td><td>0.03</td><td>0.80</td></tr><tr><td>ZEP2-DR05-04-L13</td><td>South Pacific</td><td>16.5</td><td>8.9</td><td>0.95</td><td>0.0063</td><td>0.0034</td><td>1.88</td><td>0.050</td><td>0.21</td><td>0.03</td><td>1.68</td><td>0.03</td><td>0.74</td></tr><tr><td>ZEP2-DR05-04-L14</td><td>South Pacific</td><td>17.5</td><td>9.6</td><td>1.14</td><td>0.0073</td><td>0.0041</td><td>1.79</td><td>0.049</td><td>0.21</td><td>0.03</td><td>1.76</td><td>0.03</td><td>0.82</td></tr><tr><td>ZEP2-DR05-04-L15</td><td>South Pacific</td><td>19</td><td>9.9</td><td>1.08</td><td>0.0074</td><td>0.0045</td><td>1.65</td><td>0.042</td><td>0.21</td><td>0.03</td><td>1.76</td><td>0.03</td><td>0.82</td></tr><tr><td>ZEP2-DR05-04-L16</td><td>South Pacific</td><td>20</td><td>10.4</td><td>1.27</td><td>0.0080</td><td>0.0047</td><td>1.71</td><td>0.051</td><td>0.19</td><td>0.03</td><td>1.76</td><td>0.03</td><td>0.82</td></tr><tr><td>ZEP2-DR05-04-L17</td><td>South Pacific</td><td>22</td><td>10.9</td><td>1.31</td><td>0.0101</td><td>0.0061</td><td>1.66</td><td>0.050</td><td>0.16</td><td>0.03</td><td>1.76</td><td>0.03</td><td>0.82</td></tr><tr><td colspan="14"><strong>ZEP2-DR06-03 Fe-Mn crust</strong></td></tr><tr><td>ZEP2-DR06-03-L1</td><td>South Pacific</td><td>0.5</td><td>0.93</td><td>1.45</td><td>0.0026</td><td>0.0022</td><td>1.15</td><td>0.076</td><td>0.51</td><td>0.03</td><td>2.00</td><td>0.03</td><td>1.06</td></tr><tr><td>ZEP2-DR06-03-L2</td><td>South Pacific</td><td>2.5</td><td>2.14</td><td>1.27</td><td>0.0036</td><td>0.0025</td><td>1.48</td><td>0.063</td><td>0.44</td><td>0.03</td><td>1.99</td><td>0.03</td><td>1.05</td></tr><tr><td>ZEP2-DR06-03-L3</td><td>South Pacific</td><td>4</td><td>3.05</td><td>1.25</td><td>0.0039</td><td>0.0025</td><td>1.52</td><td>0.058</td><td>0.50</td><td>0.03</td><td>2.02</td><td>0.03</td><td>1.08</td></tr><tr><td>ZEP2-DR06-03-L4</td><td>South Pacific</td><td>5.5</td><td>3.96</td><td>1.14</td><td>0.0041</td><td>0.0027</td><td>1.55</td><td>0.057</td><td>0.46</td><td>0.03</td><td>2.02</td><td>0.03</td><td>1.08</td></tr><tr><td>ZEP2-DR06-03-L5</td><td>South Pacific</td><td>7.5</td><td>5.17</td><td>0.77</td><td>0.0054</td><td>0.0042</td><td>1.28</td><td>0.051</td><td>0.47</td><td>0.03</td><td>-</td><td>-</td><td>-</td></tr><tr><td>ZEP2-DR06-03-L6</td><td>South Pacific</td><td>9.5</td><td>5.54</td><td>1.17</td><td>0.0049</td><td>0.0027</td><td>1.84</td><td>0.051</td><td>0.48</td><td>0.03</td><td>1.90</td><td>0.03</td><td>0.96</td></tr><tr><td>ZEP2-DR06-03-L7</td><td>South Pacific</td><td>11</td><td>5.81</td><td>1.10</td><td>0.0056</td><td>0.0028</td><td>2.01</td><td>0.042</td><td>0.40</td><td>0.03</td><td>1.81</td><td>0.03</td><td>0.87</td></tr><tr><td>ZEP2-DR06-03-L8</td><td>South Pacific</td><td>12.5</td><td>5.88</td><td>1.02</td><td>0.0060</td><td>0.0029</td><td>2.06</td><td>0.047</td><td>0.34</td><td>0.03</td><td>1.73</td><td>0.03</td><td>0.79</td></tr><tr><td>ZEP2-DR06-03-L9</td><td>South Pacific</td><td>14</td><td>5.95</td><td>0.98</td><td>0.0067</td><td>0.0031</td><td>2.16</td><td>0.051</td><td>0.25</td><td>0.03</td><td>1.71</td><td>0.03</td><td>0.77</td></tr><tr><td>ZEP2-DR06-03-L10</td><td>South Pacific</td><td>16</td><td>6.05</td><td>0.94</td><td>0.0071</td><td>0.0033</td><td>2.14</td><td>0.051</td><td>0.27</td><td>0.03</td><td>1.63</td><td>0.03</td><td>0.69</td></tr><tr><td>ZEP2-DR06-03-L11</td><td>South Pacific</td><td>17.5</td><td>6.12</td><td>0.92</td><td>0.0075</td><td>0.0033</td><td>2.28</td><td>0.053</td><td>-</td><td>-</td><td>1.62</td><td>0.03</td><td>0.68</td></tr><tr><td>ZEP2-DR06-03-L12</td><td>South Pacific</td><td>19</td><td>6.29</td><td>0.99</td><td>0.0072</td><td>0.0032</td><td>2.26</td><td>0.051</td><td>0.21</td><td>0.03</td><td>1.63</td><td>0.03</td><td>0.69</td></tr><tr><td>ZEP2-DR06-03-L13</td><td>South Pacific</td><td>21</td><td>6.51</td><td>0.91</td><td>0.0076</td><td>0.0033</td><td>2.29</td><td>0.055</td><td>0.22</td><td>0.03</td><td>1.61</td><td>0.03</td><td>0.67</td></tr><tr><td>ZEP2-DR06-03-L14</td><td>South Pacific</td><td>23.5</td><td>6.91</td><td>0.98</td><td>0.0078</td><td>0.0034</td><td>2.33</td><td>0.052</td><td>0.18</td><td>0.03</td><td>1.61</td><td>0.03</td><td>0.67</td></tr><tr><td colspan="14"><strong>J2-480 Fe-Mn crust</strong></td></tr><tr><td>J2-480-R1</td><td>North Pacific</td><td>0.2</td><td>7.13</td><td>1.18</td><td>0.0061</td><td>0.0053</td><td>1.14</td><td>0.025</td><td>0.73</td><td>0.06</td><td>2.10</td><td>0.04</td><td>1.16</td></tr><tr><td>J2-480-R2</td><td>North Pacific</td><td>2.5</td><td>7.40</td><td>1.44</td><td>0.0075</td><td>0.0041</td><td>1.81</td><td>0.023</td><td>0.54</td><td>0.06</td><td>2.19</td><td>0.04</td><td>1.25</td></tr><tr><td>J2-480-R3</td><td>North Pacific</td><td>5</td><td>7.69</td><td>1.40</td><td>0.0080</td><td>0.0046</td><td>1.74</td><td>0.023</td><td>0.59</td><td>0.06</td><td>2.10</td><td>0.04</td><td>1.16</td></tr><tr><td>J2-480-R4</td><td>North Pacific</td><td>7</td><td>7.93</td><td>1.55</td><td>0.0081</td><td>0.0041</td><td>1.96</td><td>0.029</td><td>0.66</td><td>0.06</td><td>2.13</td><td>0.04</td><td>1.19</td></tr><tr><td>J2-480-R5</td><td>North Pacific</td><td>9</td><td>8.16</td><td>2.22</td><td>0.0081</td><td>0.0033</td><td>2.41</td><td>0.024</td><td>0.65</td><td>0.06</td><td>2.19</td><td>0.04</td><td>1.25</td></tr><tr><td>J2-480-R6</td><td>North Pacific</td><td>11</td><td>8.39</td><td>1.72</td><td>0.0071</td><td>0.0035</td><td>2.04</td><td>0.023</td><td>0.67</td><td>0.06</td><td>2.18</td><td>0.04</td><td>1.24</td></tr><tr><td>J2-480-R7</td><td>North Pacific</td><td>12.5</td><td>8.57</td><td>1.69</td><td>0.0073</td><td>0.0039</td><td>1.88</td><td>0.020</td><td>0.64</td><td>0.06</td><td>2.14</td><td>0.04</td><td>1.20</td></tr><tr><td>J2-480-R8</td><td>North Pacific</td><td>14</td><td>8.74</td><td>1.75</td><td>0.0076</td><td>0.0038</td><td>1.97</td><td>0.017</td><td>0.66</td><td>0.06</td><td>2.13</td><td>0.04</td><td>1.19</td></tr><tr><td>J2-480-R9</td><td>North Pacific</td><td>15</td><td>9.40</td><td>1.28</td><td>0.0086</td><td>0.0042</td><td>2.03</td><td>0.018</td><td>0.63</td><td>0.06</td><td>2.15</td><td>0.04</td><td>1.21</td></tr><tr><td>J2-480-R10</td><td>North Pacific</td><td>16.5</td><td>10.39</td><td>0.69</td><td>0.0111</td><td>0.0054</td><td>2.05</td><td>0.014</td><td>0.56</td><td>0.06</td><td>1.97</td><td>0.04</td><td>1.03</td></tr><tr><td>J2-480-R12</td><td>North Pacific</td><td>19</td><td>12.04</td><td>0.68</td><td>0.0126</td><td>0.0048</td><td>2.66</td><td>0.015</td><td>0.57</td><td>0.06</td><td>1.91</td><td>0.04</td><td>0.97</td></tr><tr><td colspan="14"><strong>J2-480-R14 Fe-Mn crust</strong></td></tr><tr><td>J2-480-R14-L1</td><td>North Pacific</td><td>0.2</td><td>3.69</td><td>1.32</td><td>0.0046</td><td>0.0055</td><td>0.84</td><td>0.030</td><td>0.65</td><td>0.06</td><td>2.19</td><td>0.04</td><td>1.25</td></tr><tr><td>J2-480-R14-L10</td><td>North Pacific</td><td>3</td><td>5.47</td><td>1.21</td><td>0.0055</td><td>0.0073</td><td>0.75</td><td>0.029</td><td>0.63</td><td>0.06</td><td>2.16</td><td>0.04</td><td>1.22</td></tr><tr><td>J2-480-R14-L9</td><td>North Pacific</td><td>5</td><td>6.75</td><td>1.35</td><td>0.0057</td><td>0.0063</td><td>0.91</td><td>0.032</td><td>0.66</td><td>0.06</td><td>2.06</td><td>0.04</td><td>1.12</td></tr><tr><td>J2-480-R14-L8</td><td>North Pacific</td><td>7</td><td>7.81</td><td>1.67</td><td>0.0053</td><td>0.0036</td><td>1.45</td><td>0.031</td><td>0.65</td><td>0.06</td><td>2.14</td><td>0.04</td><td>1.20</td></tr><tr><td>J2-480-R14-L7</td><td>North Pacific</td><td>9</td><td>8.87</td><td>1.71</td><td>0.0059</td><td>0.0036</td><td>1.65</td><td>0.037</td><td>0.72</td><td>0.06</td><td>2.18</td><td>0.04</td><td>1.24</td></tr><tr><td>J2-480-R14-L6</td><td>North Pacific</td><td>10</td><td>9.40</td><td>1.79</td><td>0.0060</td><td>0.0042</td><td>1.41</td><td>0.036</td><td>0.67</td><td>0.06</td><td>2.18</td><td>0.04</td><td>1.24</td></tr><tr><td>J2-480-R14-L5</td><td>North Pacific</td><td>12</td><td>10.46</td><td>1.30</td><td>0.0073</td><td>0.0043</td><td>1.70</td><td>0.038</td><td>0.67</td><td>0.06</td><td>2.12</td><td>0.04</td><td>1.18</td></tr><tr><td>J2-480-R14-L4</td><td>North Pacific</td><td>14</td><td>11.51</td><td>1.64</td><td>0.0065</td><td>0.0046</td><td>1.40</td><td>0.034</td><td>0.69</td><td>0.06</td><td>2.20</td><td>0.04</td><td>1.26</td></tr><tr><td>J2-480-R14-L3</td><td>North Pacific</td><td>16</td><td>12.37</td><td>1.57</td><td>0.0064</td><td>0.0035</td><td>1.84</td><td>0.024</td><td>0.63</td><td>0.06</td><td>2.09</td><td>0.04</td><td>1.15</td></tr><tr><td>J2-480-R14-L2</td><td>North Pacific</td><td>18</td><td>13.23</td><td>1.26</td><td>0.0082</td><td>0.0042</td><td>1.95</td><td>0.026</td><td>0.60</td><td>0.06</td><td>2.05</td><td>0.04</td><td>1.11</td></tr><tr><td>J2-480-R14-L1</td><td>North Pacific</td><td>20</td><td>16.98</td><td>1.25</td><td>0.0085</td><td>0.0042</td><td>2.02</td><td>0.025</td><td>0.56</td><td>0.06</td><td>2.03</td><td>0.04</td><td>1.09</td></tr><tr><td colspan="14"><strong>Bulk Fe-Mn crusts</strong></td></tr><tr><td>CAP BREST 03-03</td><td>Atlantic (Ascension FZ)</td><td>-</td><td>-</td><td>0.53</td><td>0.0061</td><td>0.0036</td><td>1.68</td><td>0.023</td><td>0.22</td><td>0.02</td><td>1.66</td><td>0.05</td><td>0.72</td></tr><tr><td>DR01-003 4B-5</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.59</td><td>0.0080</td><td>0.0039</td><td>2.06</td><td>0.026</td><td>0.31</td><td>0.01</td><td>1.52</td><td>0.05</td><td>0.58</td></tr><tr><td>DR01-005 16-A-12</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.66</td><td>0.0061</td><td>0.0036</td><td>1.68</td><td>0.038</td><td>0.23</td><td>0.01</td><td>1.56</td><td>0.02</td><td>0.62</td></tr><tr><td>DR01-005 16-A-6</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.61</td><td>0.0071</td><td>0.0040</td><td>1.79</td><td>0.022</td><td>0.23</td><td>0.00</td><td>1.56</td><td>0.05</td><td>0.62</td></tr><tr><td>DR01-005 16-B-11</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.66</td><td>0.0051</td><td>0.0036</td><td>1.40</td><td>0.028</td><td>0.25</td><td>0.02</td><td>1.62</td><td>0.05</td><td>0.68</td></tr><tr><td>DR01-005 16-B-12</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.72</td><td>0.0047</td><td>0.0036</td><td>1.30</td><td>0.036</td><td>0.52</td><td>0.01</td><td>1.69</td><td>0.05</td><td>0.75</td></tr><tr><td>DR01-005 6C-13</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.79</td><td>0.0052</td><td>0.0037</td><td>1.39</td><td>0.039</td><td>0.16</td><td>0.02</td><td>1.73</td><td>0.05</td><td>0.79</td></tr><tr><td>DR03-003 17-C-4</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.60</td><td>0.0048</td><td>0.0042</td><td>1.15</td><td>0.025</td><td>0.54</td><td>0.02</td><td>1.65</td><td>0.05</td><td>0.71</td></tr><tr><td>DR03-025 15#5</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.61</td><td>0.0055</td><td>0.0040</td><td>1.37</td><td>0.022</td><td>0.26</td><td>0.03</td><td>1.68</td><td>0.05</td><td>0.74</td></tr><tr><td>DR03-025 15-D-10</td><td>Atlantic (VEMA FZ)</td><td>-</td><td>-</td><td>0.56</td><td>0.0075</td><td>0.0044</td><td>1.68</td><td>0.025</td><td>0.21</td><td>0.03</td><td>1.60</td><td>0.05</td><td>0.66</td></tr><tr><td>DR12-24 32-A-6</td><td>Atlantic (Gloria FZ)</td><td>-</td><td>-</td><td>0.67</td><td>0.0068</td><td>0.0044</td><td>1.53</td><td>0.021</td><td>0.21</td><td>0.02</td><td>1.77</td><td>0.05</td><td>0.83</td></tr><tr><td>DR12-24 32-B-2</td><td>Atlantic (Gloria FZ)</td><td>-</td><td>-</td><td>0.74</td><td>0.0056</td><td>0.0038</td><td>1.47</td><td>0.030</td><td>0.21</td><td>0.02</td><td>1.76</td><td>0.05</td><td>0.82</td></tr><tr><td>DR12-24 32-B-3-1</td><td>Atlantic (Gloria FZ)</td><td>-</td><td>-</td><td>0.67</td><td>0.0067</td><td>0.0043</td><td>1.55</td><td>0.028</td><td>0.30</td><td>0.03</td><td>1.75</td><td>0.05</td><td>0.81</td></tr><tr><td>DR12-24 32-B-3-2</td><td>Atlantic (Gloria FZ)</td><td>-</td><td>-</td><td>0.69</td><td>0.0062</td><td>0.0043</td><td>1.44</td><td>0.027</td><td>0.15</td><td>0.02</td><td>1.73</td><td>0.05</td><td>0.79</td></tr><tr><td>DR24-09 34-A-2-1</td><td>Atlantic (Azores TJ)</td><td>-</td><td>-</td><td>2.58</td><td>0.0023</td><td>0.0023</td><td>0.98</td><td>0.047</td><td>0.25</td><td>0.02</td><td>1.95</td><td>0.05</td><td>1.01</td></tr><tr><td>DR24-09 34-A-2-2</td><td>Atlantic (Azores TJ)</td><td>-</td><td>-</td><td>2.56</td><td>0.0022</td><td>0.0023</td><td>0.97</td><td>0.046</td><td>0.19</td><td>0.01</td><td>1.96</td><td>0.05</td><td>1.02</td></tr><tr><td>DR24-09 34-C-2-1</td><td>Atlantic (Azores TJ)</td><td>-</td><td>-</td><td>2.43</td><td>0.0027</td><td>0.0023</td><td>1.17</td><td>0.053</td><td>0.53</td><td>0.00</td><td>1.94</td><td>0.05</td><td>1.00</td></tr><tr><td>DR24-09 34-C-2-1</td><td>Atlantic (Azores TJ)</td><td>-</td><td>-</td><td>2.48</td><td>0.0030</td><td>0.0024</td><td>1.26</td><td>0.054</td><td>0.17</td><td>0.01</td><td>1.94</td><td>0.05</td><td>1.00</td></tr><tr><td>J2-480-R11</td><td>North Pacific (Hawaii)</td><td>-</td><td>-</td><td>1.12</td><td>0.0069</td><td>0.0035</td><td>1.98</td><td>0.015</td><td>0.28</td><td>0.04</td><td>1.86</td><td>0.05</td><td>0.92</td></tr><tr><td>J2-480-R13A</td><td>North Pacific (Hawaii)</td><td>-</td><td>-</td><td>1.24</td><td>0.0080</td><td>0.0027</td><td>2.92</td><td>0.022</td><td>0.48</td><td>0.01</td><td>1.97</td><td>0.05</td><td>1.03</td></tr><tr><td>Nod-A-1<sup>c</sup></td><td>Atlantic</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.29</td><td>0.09</td><td>1.89</td><td>0.07</td><td>0.95</td></tr><tr><td>Nod-P-1<sup>c</sup></td><td>Pacific</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.34</td><td>0.05</td><td>1.74</td><td>0.12</td><td>0.80</td></tr></tbody></table>

<footnote><sup>a</sup>Age (Ma) were calculated using cosmogenic <sup>10</sup>Be isotope, except sample ZEP2-DR05-04 dated with the Co-method. The age data and elemental data are already published in Gueguen et al. (2016).</footnote>

<footnote><sup>b</sup>2sd stands for two-standard deviation calculated on replicate measurements of NIST976 for Cu isotopes and NIST3168a for Zn isotopes, except for Nod-A-1 and Nod-P-1 which corresponds to digested and column procedure duplicates and replicate measurements on the mass spectrometer.</footnote>

<footnote><sup>c</sup>Average of digested and column procedure duplicates of Nod-A-1 and Nod-P-1.</footnote>