Methodology:
Various types of iron (Fe) and manganese (Mn) material, crusts and nodules, were analyzed to determine the fractionation from the starting sewawter value to better understand the oxic sink of vanadium. The Fe-Mn crusts and nodules studied here were collected at 2013 from three seamounts or guyots at the West Pacific Ocean using the Chinese manned submersible Jiaolong during DY125-31 cruise. Samples were from Caiwei Guyot (155°E, 15°N) and Caiqi Guyot (154°E, 15°N) from the Magellan Seamount cluster, and Weiyuan Seamount (154°W, 9°N) from Clarion-Clipperton Fe-Mn Nodule Zone (C-C zone) at water depths of 1720–4420 m, 2000–2400 m, and 5200 m, respectively. The samples studied here were obtained from the surface of the seafloor to avoid the potential diagenetic process that have occurred post burial. Some samples are from the surface The Fe-Mn crusts and nodules studied here were collected at 2013 from three seamounts or guyots at the West Pacific Ocean using the Chinese manned submersible Jiaolong during DY125-31 cruise. Samples were from Caiwei Guyot (155°E, 15°N) and Caiqi Guyot (154°E, 15°N) from the Magellan Seamount cluster, and Weiyuan Seamount (154°W, 9°N) from Clarion-Clipperton Fe-Mn Nodule Zone (C-C zone) at water depths of 1720–4420 m, 2000–2400 m and 5200 m, respectively. The samples studied here were obtained from the surface of the seafloor to avoid the potential diagenetic process that have occurred post burial. All samples from Caiwei Guyot and Caiqi Guyot are Fe-Mn crusts, while samples from Weiyuan Seamount include both Fe-Mn crusts and nodules. Some samples are from the surface layer of the ferromanganese crust to study the spatial variations of V isotope of 'recently' or near modern precipitated ferromanganese crust. Furthermore, some Fe-Mn crust samples were removed perpendicular to growth layers, which best represents a stratigraphic record of ocean chemistry.

Sampling and analytical procedures:
Elemental Quantification: The collected Fe-Mn crust and nodule samples were first cleaned by ultrasonication with deionized water and then dried in an oven at 105°C for six hours. Then samples were powdered using an agate mortar and pestle until the sample passed through a 200 mesh sieve. Prior to powdering, crust samples were cut into layers using a hacksaw with a vertical thickness of 1–3 cm. Nodule samples with diameters of 2–5 cm were cut in half from center prior to powdering. Whole-rock major and trace element measurements of Fe-Mn crusts and nodules were conducted in Guangzhou, China (http://www.alsglobal.net.cn/). The sample powders were combined with lithium borate flux and fused in a furnace at 1000°C. The resulting melt was then cooled and dissolved in nitric acid and hydrochloric acid. This solution was then analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) for major elements measurements. For trace element measurements, the powders were digested with a mixture of perchloric, nitric, and hydrofluoric acids, and then dissolved with dilute hydrochloric acid. The prepared solutions were then analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

Vanadium Purification: Approximately 10–20 mg of sample powders containing 5–10 mg of V were digested for V isotope analysis. Samples were first treated with aqua regia. After evaporation to dryness, samples were dissolved with a 3:1 (v/v) mixture of concentrated HF and HNO₃ to digest the remaining oxide and silicate phases. Then, samples were dried and further treated with aqua regia and concentrated HNO₃ and dried down in between each step. All the samples were finally dissolved in 1 mL of 1 M HNO₃ for ion-exchange chromatography. Briefly, V was purified with a multistep ion-exchange procedure by coupling cation- and anion-exchange columns. The cation resin AG50W-X12 (200–400 mesh) was applied to remove Fe, Ti, and other main matrix elements (e.g., Al, Ca, Mn, and Cr). Then AG1-X8 (200–400 mesh) chloride-form anion resin was used to further remove residual matrix and isobaric elements (e.g., K, Na, Mg, and trace amount of Cr). The total procedural blank was less than 2 ng, thus is negligible compared with the loaded amount of V (5–10 mg). The V yields of the total chemical procedures are >99%.

Isotope Instrumental Configuration: Vanadium isotopic ratios were measured using a sample-standard bracketing method on a Thermo Scientific Neptune plus MultiCollector Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS). Measurements were performed on the flat-topped shoulder on the lower mass side of the overlapping V and molecular interference peaks in medium-resolution mode (resolution > 4000) to resolve all interfering molecular species representing combinations of C, N, O, S, Ar and Cl (such as ³⁶Ar¹⁴N+, ³⁶Ar¹⁶O+, and ³⁸Ar¹⁴N+). Jet sample and Ni X-skimmer cones were used to obtain the highest possible V transmission efficiency. In addition, the amplifiers with 10¹⁰Ω and 10¹¹Ω resistor were applied to monitor ⁵¹V and ⁵⁰V signal, respectively. We applied dry plasma inlet system with Aridus II desolvator (CETAC Technologies).