Impact of hydrothermal activity on marine barium isotope composition: a case study from the Southwestern Indian Ocean

Abstract

The cycling of barium (Ba) is closely linked to marine biogeochemical processes. Barium and its isotopes are commonly used as tracers for marine productivity, seawater alkalinity, and ocean circulation. Mid-ocean ridge hydrothermal systems significantly impact marine chemistry, acting as key sources of trace elements in deep seawater. However, the overall contribution of hydrothermal Ba to the global Ba cycle remains poorly quantified, and studies on hydrothermal Ba isotopes are limited, hindering a comprehensive understanding of the marine Ba cycle. This study investigated the concentration of dissolved Ba and other elements, along with Ba isotope composition (δ¹³⁸Ba), in the hydrothermal influenced water and sediment samples collected near the Longqi and Tiancheng vents in the southwestern Indian Ocean. This constitutes the first such investigation in this region. The vertical profiles of dissolved Ba and its isotope compositions mirrored each other in the southwestern Indian Ocean, consistent with prior observations in other ocean basins. For near-field water samples, Ba isotope compositions (-0.10‰ to 0.05‰) are significantly lower than background seawater (~0.29‰). In addition, Hydrothermal sediments exhibited Ba isotopic values (-0.16‰ to 0.01‰) markedly lower than background sediments (0.01‰ to 0.14‰). The depleted δ138Ba values of near-field water samples indicate preferential removal of lighter Ba isotopes during the mixing of hydrothermal fluids with seawater. Consequently, precipitated particles acquire lighter Ba isotope signatures, explaining the low values in hydrothermal sediments. This demonstrates that sediments effectively capture and preserve hydrothermal Ba signals. Collectively, these findings provide new insights into hydrothermal influences on the marine Ba cycle.

1 Introduction

The marine barium (Ba) cycle is closely linked to marine biogeochemistry (Horner and Crockford, 2021). The primary sources of Ba in the ocean are rivers, groundwater, and hydrothermal vents (Kumagai et al., 2008; Cao et al., 2016; Carter et al., 2020; Hsieh et al., 2021). Once introduced into seawater, the dissolved Ba is removed from seawater and accumulated in marine sediments primarily in the form of the barite and in association with iron and manganese oxides (Dymond et al., 1992; Paytan and Kastner, 1996; Carter et al., 2020). The concentration of dissolved Ba in seawater is found to be associated with the concentration of the major nutrient silicon (Lea and Boyle, 1989; Jacquet et al., 2005; Roeske et al., 2012). Its sinking flux is closely linked with organic carbon flux, and it is commonly used as a tracer for marine productivity (Eagle et al., 2003; McManus et al., 1999; McCulloch et al., 2003; Paytan and Griffith, 2007). In addition, Ba displays a positive correlation with ocean alkalinity and dissolved inorganic carbon. The Ba/Ca ratio in foraminifera, for instance, can serve as a proxy for past seawater alkalinity (Lea, 1993; McManus et al., 1999).

Recently, Ba isotopes (δ¹³⁸Ba) have been recognized as a valuable tool for exploring the oceanic Ba cycle, providing important insights into its processes and behavior, such as, to trace the movement and mixing of deep-water masses, to serve as a proxy for marine biological productivity and to indicate riverine inputs (Horner et al., 2015; Bates et al., 2017; Hsieh and Henderson, 2017; Bridgestock et al., 2018; 2021; Crockford et al., 2019; Cao et al., 2021; Middleton et al., 2023). The Ba isotope composition in seawater ranges from 0.24‰ – 0.65‰, and the primary factor influencing the observed variations in seawater δ¹³⁸Ba is the formation and dissolution of barite (Horner et al., 2015; Bates et al., 2017; Hsieh and Henderson, 2017; Cao et al., 2021; Whitmore et al., 2022). Modern oceans show a significant δ¹³⁸Ba gradient from surface to depth, characterized by higher δ¹³⁸Ba values in surface water masses and lighter δ¹³⁸ Ba values in deep water masses (Hsieh and Henderson, 2017; Bridgestock et al., 2018; Carter et al., 2020). This type of vertical distribution is a result of the precipitation process as barite from seawater, which is marked by significant isotopic fractionation, with lighter Ba isotopes preferentially incorporated into biogenic barite crystals (Horner et al., 2015; Bates et al., 2017; Hsieh and Henderson, 2017; Bridgestock et al., 2018). Similarly, adsorption onto biological particles preferentially removes lighter Ba isotopes from ambient seawater (Cao et al., 2020). Both adsorption and barite precipitation increase the δ¹³⁸Ba values of seawater in the upper ocean (~ 600 m), whereas the dissolution processes re-introduce lighter Ba isotopes into deeper waters.

The degree to which Ba isotopes behave conservatively in the deep ocean remains uncertain. Moreover, available published data suggest an imbalance between the sources and sinks of Ba isotopes in the modern ocean. Specifically, Ba isotope compositions from terrestrial sources range from 0.1‰ to 0.2‰, while sediment values, representing sinks, range from 0‰ to 0.1‰ (Horner and Crockford, 2021). Therefore, it is proposed that lighter Ba isotope sources or heavier isotope sinks may be missing and not fully accounted for (Horner and Crockford, 2021; Zhang et al., 2024). Bates et al. (2017) suggested that Ba isotopes exhibit conservative behavior during the mixing of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) in the Atlantic. However, Hsieh and Henderson (2017) observed that in deep water between 2000 and 3000 meters, Ba isotope compositions exceeded conservative mixing predictions, reflecting additional inputs or sinks of Ba during transport, a phenomenon later explained by Hsieh et al. (2021) as resulting from hydrothermal vents imparting distinct Ba isotopic signatures to deep water masses.

Hydrothermal systems at mid-ocean ridges play a critical role in trace element recycling processes in marine system (Elderfield and Schultz, 1996; Coogan et al., 2019). Barium in source rocks is released into hydrothermal fluids through water-rock interactions under high temperatures and pressures (Von Damm, 1985), resulting in fluids highly enriched in dissolved Ba. Barium concentrations in hydrothermal fluids (1-119 μM) can be up to three orders of magnitude higher than those in seawater (30-150 nM) (Butterfield and Massoth, 1994; Kumagai et al., 2008; Seyfried et al., 2011). However, when these fluids mix with seawater, Ba precipitates as barite and is largely removed (Jamieson et al., 2016; Gartman et al., 2019). Ba also co-precipitates with metals like Fe and Cu, resulting in minimal or no net Ba flux into seawater (Carter et al., 2020). Additionally, the ultramafic lithology contrasts with the mafic-dominated settings in mid-ocean ridge vents, which may lead to unique fluid-rock interactions that modulate Ba release and isotopic fractionation (Hsieh et al., 2021; Zhang et al., 2024). To date, the research on Ba isotopes in hydrothermal systems primarily focuses on hydrothermal fluids and non-buoyant plumes, with very limited empirical data available.

To advance the understanding of the marine Ba cycle, this study investigates the geochemical characteristics and Ba isotope compositions of hydrothermal fluids, ambient seawater, and sediments from the Longqi and Tiancheng hydrothermal fields along the Southwest Indian Ridge. This research fills a gap in the study of Ba isotopes in Indian Ocean seawater and reveals the Ba isotope composition of hydrothermal end-member inputs. It is also the first investigation of Ba isotopes in hydrothermal sediments, exploring how these sediments record hydrothermal Ba inputs. The study systematically examines the mechanisms driving Ba cycling within the Southwest Indian Ridge hydrothermal system.

2 Materials and methods

2.1 Study region

The study area is located along the Southwest Indian Ridge (SWIR), an ultra-slow spreading mid-ocean ridge (Figure 1) (Demets et al., 1994; Cannat et al., 2008). The SWIR is characterized by distinctive tectonic features, including a series of large north-south trending transform faults that formed due to lateral spreading during ridge evolution. This study focuses on the Longqi and Tiancheng hydrothermal fields. The Longqi Hydrothermal Field, located at 37°47′S, 49°39′E, at a depth of 2775 m, is the first active high-temperature hydrothermal field (> 300°C) discovered on the SWIR, situated on a mafic rock substrate (Tao et al., 2012). The Tiancheng Hydrothermal Field, situated at 27°51′S, 63°55′E, at a depth of 2750 meters, is the first confirmed area of low-temperature hydrothermal activity on the SWIR, with fluid temperatures around 13.2°C (Tao et al., 2014). This region experiences limited magmatic supply and intense tectonic activity, with fractured basalt as the dominant substrate (Chen et al., 2018). As shown in Figue 1, the sub-surface, intermediate, and deep layers of seawater in the study area are respectively influenced by water masses Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), Circumpolar Deep Water (CDW), and Antarctic Bottom Water (AABW) (Rippert et al., 2015; Zhang et al., 2022).

Figure 1

2.2 Sample collection

Water samples were collected during the DY-52 and DY-78 expeditions aboard the R/V Da Yang 1 in April 2019 and R/V Da Yang in 2023, respectively. Three full-depth seawater stations (S1, S2, S3) were collected (Figure 2). Among them, S1 and S3 located directly above the Longqi and Tiancheng hydrothermal vents, respectively, and S2 represents background seawater between the two vent sites. The samples were obtained using Niskin bottles mounted on a stainless-steel rosette equipped with a CTD system (Sea-Bird^® 911plus). Immediately after collection, the seawater was filtered through AcroPak capsule filters (0.2 μm, Pall Corporation^®) and transferred to acid-clean low-density polyethylene (LDPE) bottles. The samples were then acidified to pH ~ 2 using high-purity nitric acid (Fisher Scientific^®, Optima grade) and stored at room temperature in sealed containers. All sampling equipment, including tubes, filters, and bottles, were pre-cleaned following the protocols of Li et al. (2015).

Figure 2

Near-vent seawater samples (LQ-1, LQ-2, LQ-3, TC-1, TC-2, TC-3, TC-4) were collected using a custom-designed sampler mounted on the Remotely Operated Vehicle (ROV) Sea Dragon III during DY-52. The sampler was composed of a hydraulic piston, an acid-clean polycarbonate bottle, and a Teflon inlet attached to the silicone sampling tube. During DY-78, hydrothermal fluids (LQ-LT) were sampled with a titanium alloy “Pressure-Resistant Fluid Sampler” and the titanium inlet collected the fluid within the vent. Immediately after recovery of the ROV sampler, samples were filtered through polyethersulfone (PES) membranes (0.45 μm, 47 mm, Pall) and filtrate transferred to acid-cleaned LDPE bottles. They were then acidified to pH ~2 with high-purity nitric acid (Fisher Scientific^®, Optima grade) and stored at room temperature. Filter membranes were preserved and stored in sealed bags for subsequent analysis.Surface sediment samples (P1–P15) were obtained from the China Ocean Sample Repository, collected during various expeditions (DY-19, 20, 21, 26, 30, 34, 35, 40, 49) using a TV grab sampler. These samples were stored in clean, sealed bags and frozen at -20°C for preservation.

2.3 Sample analysis

2.3.1 Analysis of seawater and hydrothermal fluid dissolved samples

Seawater and hydrothermal fluid samples were diluted 20-fold with 2% HNO₃ (Optima grade) and spiked with indium (In) as an internal standard with a final concentration of 10 ppb. Concentrations of dissolved Fe, Mnand Bawere determined using an inductively coupled plasma mass spectrometer (ICP-MS, iCAP–RQ, Thermo Fisher Scientific) in collision cell-mode at the First Institute of Oceanography of China. The precision and accuracy of Ba, Fe, Mn measurements were verified using certified reference seawater CASS-5 (National Research Council^®, Canada), and the analytical results are shown in Table 1. Since no Ba data are reported in certified values of the reference seawater, the analytical results reported here are compared with data published previously by (Mori et al., 2019).

Approximately 60 ng of Ba was extracted from each stock solution and spiked with a suitable amount of the ¹³⁰Ba-¹³⁵Ba double spike. Following the methods outlined by Lin et al. (2020), the spike-sample mixtures were dried, redissolved in 2.5 mol/L HCl, and purified via column separation using AG 50 W-X8 cation-exchange resin. Procedural blanks were measured for each batch, with Ba concentrations < 0.6 ng, which was <0.1% of the total Ba in the samples. The purified samples were then dried and redissolved in 3% HNO₃ (with Ba concentrations ~ 60 ppb), and Ba isotope ratios were analyzed using a Thermo Neptune XT MC-ICP-MS at the Key Laboratory of Surficial Geochemistry, Nanjing University.

Ba isotope compositions in this study are expressed as ‰ deviations (δ¹³⁸Ba) relative to the international Ba standard NIST SRM 3104a (Equation 1):

The standard solution NIST SRM3104a and a seawater standard IAPSO were analyzed after every five samples. The Ba isotope compositions (δ¹³⁸Ba) of the NIST 3104a standard and IAPSO standard seawater were 0.00 ± 0.05‰ (2SD, n=11) and -0.04 ± 0.05‰, respectively. The δ¹³⁸Ba value for IAPSO in this study is consistent with the 0.05 ± 0.04‰ reported in a previous study (Pretet, 2013). We determined the error of the Ba isotope compositions (δ¹³⁸Ba) for all samples to be 0.05‰, based on the measurements of the standard SRM3104a for δ¹³⁸Ba (n=11).

2.3.2 Analysis of sediments

The sediment samples were freeze-dried for 48 hours and then ground into a fine powder (200-mesh). For bulk concentration analysis, approximately 50 mg of sediments (weighed to a precision of 0.1 mg) were digested in a concentrated acid mixture (HNO₃/HF, 1:1). The major elements were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 6300, Thermo Fisher Scientific^®) with RSD < 5%, and trace elements and rare earth elements (REEs) were analyzed using ICP-MS. Sediment reference standards GBW07309 (China Reference Material) were analyzed to evaluate the reproducibility of this method (n=3). The measured values agreed well with certified values for all targeted elements (96 ± 0.6% – 105 ± 1%) and detailed information can be found in the supplementary material (Supplementary Table S1).

Approximately 100 ng of Ba was extracted from each stock digest solution for Ba isotope analysis, following the same procedure as for seawater and hydrothermal samples (Section 2.3.1). The standard solution NIST SRM3104a and a coral standard SH-1 were analyzed after every five samples. The Ba isotope compositions (δ¹³⁸Ba) of the coral standard SH-1 was 0.29 ± 0.04‰ (2SD, n=8), which was consistent with the published value of 0.31 ± 0.04‰ (Lin et al., 2022).

2.3.3 Analysis of filter membrane

The hydrothermal filter membrane was analyzed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX; FEI Quanta 200, Netherlands). A piece of the filter membrane (~ 0.5 cm × 0.5 cm) was attached to conductive adhesive using tweezers. The sample was then coated with gold for 25 seconds under vacuum conditions, after which it was placed under the scanning electron microscope for observation, with the aim of identifying as many different types of mineral particles as possible.

3 Result and discussion

3.1 Dissolved Ba concentrations and its isotope compositions of seawater

3.1.1 Characteristics of barium concentrations and isotopic distributions in this study

Vertical distribution of dissolved Ba concentrations ([Ba]_diss) at stations S1, S2 and S3 are shown in Figure 3 and Table 2. The dissolved Ba concentrations range from approximately 37 to 107 nM, with a typical increasing pattern from surface to bottom. The concentration varied slightly within the surface 1000 m (37 – 50 nM), and increased sharply to over 80 nM at 2000 m. S3, the station located above the Tiancheng hydrothermal vent, had a higher [Ba]_diss (~ 90 nM) compared to the ones further south (S1 and S2). Barium is considered a bio-intermediate element, with its dissolved concentrations ([Ba]_diss) in the water column typically following a nutrient-like depth profile. In surface waters, [Ba]_diss is reduced due to its removal via association with marine particles. However, [Ba]_diss becomes enriched in deep water, primarily as a result of the decomposition and remineralization of settling particles (Lea and Boyle, 1991; Hsieh and Henderson, 2017; Carter et al., 2020; Yu et al., 2022).

Figure 3

In contrast, δ¹³⁸Ba values decrease from 0.65‰ near the surface to about 0.20‰ at greater depths (Figure 3). This inverse relationship between Ba concentration and isotope composition mirrors the vertical distributions that have also been observed in the Atlantic and Pacific Oceans (Figure 4) (Horner et al., 2015; Hsieh and Henderson, 2017). The observed profiles are attributed to isotopic fractionation that occurs during pelagic barite precipitation while its dissolution process shows negligible isotopic fractionation. Different from [Ba]_diss in which all three water profiles shown similar trends and values with depth, the vertical profiles of δ¹³⁸Ba for stations S2 and S3 are quite similar, and δ¹³⁸Ba for stations S1 and S2 are also exactly similar (S1: 0.68‰;S2: 0.67‰) while the ones for S1 are heavier by ~ 0.1―0.2 ‰ within the top 2000 m. At 2500 m, the δ¹³⁸Ba at S1 (0.18 ± 0.05‰) is marginally lower than at S2 (0.31± 0.05‰) and S3 (0.27 ± 0.05‰), though the difference is within analytical uncertainty (Figure 3). The relationship between dissolved Ba concentrations and Ba isotope compositions (δ¹³⁸Ba) is displayed in Supplementary Figure S1, revealing a statistically significant negative correlation (p < 0.01). In addition, the temperature and salinity of the seawater samples are plotted in the T-S diagram (Figure 5). With the exception of a few surface seawater samples, all data points form distinct curves or clusters, and no extreme temperature or salinity characteristics are observed. These results indicate that the analyzed seawater samples do not exhibit significant influence from hydrothermal input.

Figure 4

Figure 5

Hsieh and Henderson (2017).

3.1.2 Comparison with global barium cycling trends

The comparison of the vertical profile of station S2 with the ones in other ocean basins is shown in Figure 4. Except for the vertical distribution in the Southern Ocean, [Ba]_diss in S2 in this study always lie between those from the Pacific Ocean and Atlantic Ocean, both in surface waters and deep waters. In surface waters, the [Ba]_diss are relatively higher (~ 45 nM) in the Atlantic Ocean and lower (~ 30 nM) in the Pacific Ocean, while the opposite is true in deep waters. The [Ba]_diss in deep waters are relatively higher (~130 nM) in the Pacific Ocean and lower in the Atlantic Ocean (~ 70 nM), with the ones found in this study similar to the deep water in the Southern Ocean (~ 90 nM).

While the study by Jacquet et al. (2005) on barium in the Crozet–Kerguelen Basin demonstrated that barite precipitation can induce surface water alterations in the Southern Ocean, recent investigations of dissolved Ba concentrations in water column profiles from the high-latitude Southern Ocean (Weddell Sea) have revealed that the Southern Ocean exhibits relatively high [Ba]_diss in surface waters, with minimal variation between surface and deep waters (Yu et al., 2022). This limited variation is attributed, in part, to the region’s classification as a high nutrient, low chlorophyll (HNLC) zone, where restricted barite formation leads to minor changes in surface water [Ba]_diss (Fu and Wang, 2022; Yu et al., 2022). Additionally, the strong upwelling of Circumpolar Deep Water (CDW) from below further reduces the extent of [Ba]_diss variation throughout the water column (Hsieh and Henderson, 2017; Yu et al., 2022).

Overall, deep-water [Ba]_diss follows the sequence: North Pacific > South Pacific > Southern Ocean > Southwest Indian Ocean > South Atlantic > North Atlantic. This trend correlates positively with the age of the water masses, i.e., the older the water mass, the higher the [Ba]_diss, which again is the accumulation result of the dissolution of barite in deep water.

The comparison of the δ¹³⁸Ba values of the vertical profile of station S2 with the ones in other ocean basins is shown in Figure 4. Similar to the distribution of [Ba]_diss, the δ¹³⁸Ba found in S2 in this study is between those in the Pacific and Atlantic Oceans in deep waters. The δ¹³⁸Ba values in deep waters are relatively higher (~ 0.45‰) in Atlantic Oceans and lower in Pacific Oceans (~ 0.25‰), and the ones found in this study is ~ 0.30‰. Likewise, The Southern Ocean exhibits relatively light Ba isotope compositions in deep water, with minimal variation between surface and deep waters. The δ¹³⁸Ba values show an inverse relationship to water mass age, indicating that lighter Ba isotopes are released into seawater during barite particle dissolution.

3.2 Geochemical processes near hydrothermal vents

3.2.1 Dissolved Mg, Fe and Mn distributions

Deep-sea hydrothermal vent fluids are in a reduced state and highly enriched in dissolved trace metals such as Fe, Mn, Cu, and Zn, with concentrations several orders of magnitude higher than those in surrounding seawater (Rouxel et al., 2008; Findlay et al., 2019; Gartman et al., 2019). When these high-temperature, reducing hydrothermal fluids mix with cooler, oxygenated seawater, most trace metals precipitate as sulfides or oxides near the vent (Rouxel et al., 2016; Yuan et al., 2018). Recently, GEOTRACES studies have shown that a portion of these metals (Fe, Zn, Cu) released from the hydrothermal vents can be stabilized as organic complexes or nanoparticles, remaining in the dissolved phase, and transported over thousands of miles (Bennett et al., 2009; Hawkes et al., 2013; Li et al., 2014; Fitzsimmons et al., 2017; Lough et al., 2019).

Dissolved Mg, Fe, Mn, Ba concentrations and Ba isotope compositions in samples collected in and near the hydrothermal vents in this study are reported in Table 3. Dissolved Mg concentrations are commonly used to indicate the extent of seawater mixing with hydrothermal fluids. During hydrothermal fluid-rock interactions, Mg is typically removed as magnesium silicates, so that Mg concentration in hydrothermal fluids that is not mixed with seawater (termed unmixed hydrothermal fluids hereafter) should approach zero theoretically (Alt, 1995; Mottl et al., 1978). This inference is supported by Ji et al. (2017) who reported that Mg concentration in unmixed hydrothermal fluids from the Longqi hydrothermal field is 1.41 mM. Conversely, the background seawater adjacent to Longqi hydrothermal vent has a concentration of Mg of 52.5 mM (Gallant and Von Damm, 2006), significantly higher than that in unmixed hydrothermal fluids.

In this study, the Mg concentrations were found to range from 40.9 to 54.2 mM. One sample, referred to as “hydrothermal fluid” (LQ-LT), collected within the vent during the DY-78 expedition, had a Mg concentration of 40.9 mM, suggesting that mixing with background seawater had already occurred within the vent. In addition, the plume samples (LQ-1, LQ-2, LQ-3, TC-1, TC-2, TC-3, TC-4), collected near hydrothermal vents, exhibited Mg concentrations between 51.8 and 54.2 mM, indicating that these samples were highly mixed with seawater.

Despite the high degrees of mixing with background seawater, dissolved Fe and Mn concentrations in these samples were significantly higher than those reported in background seawater (Fang and Wang, 2021), indicating a notable contribution from hydrothermal sources. The relationship between Fe and Mn concentrations and their distance from hydrothermal vents is shown in Figure 6. The results reveal that among the hydrothermal samples from the Longqi vent, the fluid sample LQ-LT collected from the vent interior exhibits the highest dissolved Fe and Mn concentrations, with concentrations rapidly declining in the remaining plume samples. All hydrothermal samples from the Tiancheng vent belong to plume samples, showing significantly lower Fe and Mn concentrations compared to the fluid sample (LQ-LT). Furthermore, Fe and Mn data from both vents indicate that only two samples (LQ-LT and TC-3) collected in close proximity to the hydrothermal vents display higher Fe concentrations than Mn concentrations. In all other plume samples, Mn concentrations decline at a slower rate relative to Fe. Previous studies have reported that during the mixing of hydrothermal fluids and seawater, dissolved Mn tends to be more stabilized in dissolved phase compared to Fe, the latter of which is removed as particles quite quickly during this process (Resing et al., 2015; Fitzsimmons et al., 2017). The distribution characteristics of these metals provide critical fluid mixing context for understanding barium isotope fractionation in hydrothermal systems.

Figure 6

3.2.2 The dissolved Ba distributions and Ba isotope composition

The hydrothermal samples exhibited dissolved barium concentrations ([Ba]_diss) ranging from 93 to 869 nM (Figure 6). Notably, two samples showed exceptionally high values: the hydrothermal fluid LQ-LT (869 nM) and the plume sample TC-3 (738 nM). Excluding these, the remaining of plume samples displayed a narrower concentration range (93-198 nM). Given the limited reported [Ba]diss data for the Indian Ocean, we calculated a background seawater value (~89 nM) by averaging measurements from station S2 (located away from hydrothermal vents) at depths≥2000 m. All hydrothermal samples exceeded this background value, demonstrating a clear influence of hydrothermal input on Ba enrichment.

The δ¹³⁸Ba in hydrothermal samples ranged from -0.10 to 0.27‰ and it exhibits a positive correlation with the distance from the hydrothermal vent (Figure 6). The two samples with the nearest distance and highest [Ba]_diss, LQ-LT and TC-3, had lightest isotope composition -0.01‰ and -0.10‰, respectively, significantly lower than δ¹³⁸Ba values found in background seawater (0.29 ‰) in this study. The station with the third highest Ba concentration, TC-4, also had a light Ba isotope composition, being 0.05‰. The Ba isotopic compositions (δ¹³⁸Ba) of samples from the Tiancheng (TC) hydrothermal vent exhibit relatively lower values compared to those from the Longqi (LQ) vent, albeit within analytical uncertainty. Furthermore, the δ¹³⁸Ba values of TC samples display a slower rate of change with increasing distance from the vent, whereas LQ samples show a rapid increase in δ¹³⁸Ba values as distance from the vent increases. This contrast likely reflects differences in Ba isotope behavior under distinct geological settings. The Longqi Hydrothermal Field is an active high-temperature system (>300°C) hosted in mafic rocks (Tao et al., 2012). In contrast, the Tiancheng Hydrothermal Field is characterized by low-temperature hydrothermal activity (fluid temperatures ≈13.2°C; Tao et al., 2014), limited magmatic supply, and intense tectonic activity, with fractured basalt dominating the substrate (Chen et al., 2018). These geological disparities—specifically, variations in thermal regimes, host rock composition, and tectonic stress—may govern the observed differences in Ba isotope fractionation and transport dynamics between the two vent fields. Overall, the hydrothermal samples closer to the vents with high [Ba]_diss (LQ-LT, TC-3, TC-4) have a mean δ¹³⁸Ba value of -0.02‰(SD=0.07‰), significantly lower than that of background seawater (0.29‰, SD=0.04‰), which suggests that the hydrothermal vent introduce lighter Ba into surrounding seawater. The Ba isotope compositions of hydrothermal fluids from the Longqi vent field (δ¹³⁸Ba = −0.01‰) and plume samples near the Tiancheng vent field (δ¹³⁸Ba = −0.10‰ to +0.05‰) on the Southwest Indian Ridge (SWIR) align closely with the range of initial endmember vent fluids (−0.17‰ to +0.09‰) reported by Hsieh et al. (2021) for mid-ocean ridge (MOR) systems. This consistency suggests that the Ba isotopic signatures of the SWIR fluids are primarily controlled by water-rock interactions with oceanic crustal lithologies, similar to other MOR hydrothermal systems. The Longqi fluid value (−0.01‰) approaches the range of mid-ocean ridge basalts (MORBs: +0.02‰ to +0.15‰; Nielsen et al., 2018) and falls within the range of pelagic sediments (−0.21‰ to +0.11‰; Bridgestock et al., 2018), supporting Hsieh et al.’s conclusion that Ba isotopes are not significantly fractionated during high-temperature fluid-rock reactions. Notably, the SWIR data lack the extremely heavy δ¹³⁸Ba values (up to +0.91‰) observed in sediment-influenced systems like the Main Endeavour Field (MEF) (Hsieh et al., 2021). This absence reinforces Hsieh et al.’s finding that sediment interaction can elevate fluid Ba isotope ratios, a process unlikely in the ultramafic- or basalt-hosted SWIR vents. Instead, the SWIR results align more closely with basalt-dominated systems (e.g., East Pacific Rise), further emphasizing the role of host rock composition in shaping initial fluid signatures. These comparisons highlight the utility of Ba isotopes in tracing hydrothermal contributions to oceanic Ba cycling, while underscoring the need to account for local geological and physicochemical conditions when interpreting isotopic variability.

3.2.3 Ba isotopic fractionation during hydrothermal fluids-seawater mixing

Previous studies on Ba isotopes in hydrothermal systems remain limited (Hsieh et al., 2021; Zhang et al., 2024). Hsieh et al. (2021) examined Ba isotopes in hydrothermal fluids and particulates from multiple vents. After correcting for particulate data, they reported Ba isotope compositions in endmember hydrothermal fluids ranging from -0.17‰ to 0.09‰, which are lighter than background seawater. They calculated an isotope fractionation factor (Δ¹³⁸/¹³⁴Ba _Dregs-fluid) of -0.35‰ during the mixing of hydrothermal fluids with seawater, suggesting preferential removal of lighter Ba isotopes. Modeling with this fractionation value, they further inferred that the Ba isotope composition would increase to +1.7 ± 0.7‰ when barite becomes undersaturated, indicating a potential contribution of heavier Ba isotopes from hydrothermal systems to the ocean. In contrast, Zhang et al. (2024) performed direct measurements on non-buoyant plume samples from the Rainbow hydrothermal field and found the δ¹³⁸Ba values of hydrothermally affected deep seawater exhibit a lighter isotopic signature (~ 0.3‰) compared to those found at similar depths at distant locations in the Atlantic Ocean (~ 0.45‰). Interestingly, they found that dissolved Ba undergoes conservative mixing between Rainbow hydrothermal endmember fluids and the non-buoyant plume, as evidenced by a significant linear correlation between dBa and ³He. However, the Rainbow is an ultramafic system, distinct from most mafic-hosted vents. The linear regression used for Rainbow is based on a single sample and whether the conservative Ba behavior observed in the Rainbow field also occurs in other hydrothermal systems is unknown. To better constrain the global hydrothermal Ba inputs, more observations in hydrothermal plumes are required.

In this study, we also found the hydrothermal fluid and plume samples with high concentration of Ba exhibited lighter Ba isotope compositions, ranged from -0.10‰ to 0.05 ‰, which is consistent with the result reported in the previous study (Zhang et al., 2024). The relationship between dissolved Ba concentrations and Ba isotope compositions (δ¹³⁸Ba) is displayed in Supplementary Figure S2. One station in our samples (TC-4) exhibited a dissolved Ba concentration of 198 nM, closely matching the effective input Ba concentration of 200 nM calculated in Hsieh et al. (2021). However, the Ba isotope composition at TC-4 was 0.05‰, substantially lighter than the model-based value (+1.7 ± 0.7‰) reported by Hsieh et al. (2021). This significant discrepancy between the observed isotopic composition and the model-predicted endmember values raises a critical question: what mechanisms drive the marked deviation, particularly given the opposing trends?

It is critical to consider that Ba may be sequestered from hydrothermal fluids via coprecipitation with iron-manganese oxides and sulfide minerals, as well as adsorption onto authigenic particulates, processes that can significantly influence its isotopic fractionation (Rouxel et al., 2016; Yuan et al., 2018). Previous studies indicate that in high-temperature hydrothermal environments, anhydrite has a higher saturation index than barite, which suggests that anhydrite may precipitate firstly, thereby limiting barite precipitation (Jamieson et al., 2016). Zhang et al. (2024) also highlighted the complexity of barite precipitation and Ba removal in hydrothermal systems. Therefore, assuming the Ba removal is solely due to barite formation, as proposed by Hsieh et al. (2021), may not be correct. As reported by previous studies, trace elements are often precipitated with particles released from hydrothermal vents (Rouxel et al., 2016; Yuan et al., 2018). Analysis of filter membranes from Longqi and Tiancheng hydrothermal vents samples using Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy (SEM-EDX; FEI Quanta 200) revealed that, various mineral species exist in addition to barite, such as Fe oxides, FeS, FeS₂, Fe_(1-x) Sx, CeO₂, ZnS, and SiO₂, etc. (Figures 7A; 7B). Therefore, barium may co-precipitate with iron-manganese oxides and metal sulfide particles in addition to forming barite; these various processes could result in different isotopic fractionation. Further studies should be conducted to investigate the Ba isotopic fractionation associated with different removal processes.

Figure 7

3.3 Sediments near hydrothermal field

3.3.1 Geochemical characteristics

As discussed in the previous section, some elements (e.g. Fe, Mn, Cu, Zn) enriched in hydrothermal fluids typically precipitate with particles, and eventually settle to the seafloor near the vents (Liao et al., 2018). Even after alteration during early diagenesis processes, the hydrothermal elements that are enriched in sediments in general remain stable (Feely et al., 1987; Li et al., 2015; Agarwal et al., 2020). These sediments preserve geochemical information related to hydrothermal activities, offering valuable insights into complex hydrothermal systems (Dias and Barriga, 2006; Liao et al., 2018).

The measured major and minor element contents in selected sediment samples are reported in Table 4A. Visual observation on the selected surface sediment samples revealed that samples P1 to P9 are light gray, while P10 to P15 are reddish-brown, likely due to higher metal content from hydrothermal influence. The analytical results show that sediments P10–P15 have Fe contents ranging from 8.8% to 32.3%, with an average of 20.8%, significantly higher than those in sediments P1–P9 (0.4% – 3.2%, with an average of 1.21%). The Cu and Zn contents in sediments P10–P15 range from 182 μg/g to 130 mg/g (averaged to be 47 mg/g) and 74 μg/g to 33 mg/g (averaged to be 8.8 mg/g), respectively. These values are substantially higher than those in sediments P1–P9, where Cu and Zn contents range from 19 to 100 μg/g (averaged to be 52.8 μg/g) and 15 to 66 μg/g (averaged to be 40.6 μg/g), respectively.

The principal component analysis of the geochemistry data set was performed using IBM SPSS Statistics 25.0 software to further constrain the factors influencing the geochemistry data for the surface sediments in the study area (Supplementary Table S2). Three major factors with eigen values greater than 1 that together explain approximately 80% of the variance of the data set were extracted. And the PCA scores and loading Plot of surface sediment geochemistry is showed in Supplementary Figure S3. The dominant factor (Factor 1 (F1)) explains approximately 42% of the total variance and is loaded primarily with lithogenic elements, e.g., Fe, Cu, Pb, S, V, Zn, which frequently reflect the hydrothermal component. Factor 2 (F2), explaining a significant portion of the remaining variance, was predominantly associated with elevated Al and Ti concentrations, which are commonly used as lithogenic tracers. Factor 3 (F3) highlighted Ba enrichment, likely linked to marine authigenic processes, such as biogenic barite precipitation or scavenging by sinking organic matter. This tripartite structure underscores the interplay of hydrothermal activity, terrestrial sediment supply, and pelagic marine influences in shaping the geochemical signatures of the studied sediments.

The elemental ratios serve as a robust proxy for assessing hydrothermal input intensity (Qiu et al., 2023). To evaluate hydrothermal influence on sediments along the Southwest Indian Ridge, two geochemical indices were employed: (1) the metal enrichment index Al/(Al + Fe + Mn) and (2) the Fe/Ti ratio. These indices effectively differentiate hydrothermal contributions from detrital components in marine sediments.

The Al/(Al + Fe + Mn) index distinguishes metal-rich hydrothermal sediments from background sediments. Previous studies (e.g., Fisher et al., 1969; Dias and Barriga, 2006) established that background deep-sea sediments, devoid of hydrothermal input, typically exhibit values >40% due to higher detrital aluminosilicate content. In contrast, hydrothermal sediments near vent systems are characterized by lower values, reflecting Fe-Mn oxide dominance. Concurrently, the Fe/Ti ratio quantifies hydrothermal metal enrichment relative to detrital inputs, with ratios >20 indicating significant hydrothermal influence (Zhou et al., 2020). By cross-plotting these indices (Figure 8A), hydrothermal and detrital sediment sources can be systematically discriminated (Liao et al., 2018).

Figure 8

Boynton (1984).

Application of this dual-index approach to 15 surface sediment samples revealed distinct geochemical signatures. Six samples (P2, P10–P13, P15) exhibited Fe/Ti >20 coupled with Al/(Al + Fe + Mn) <40%, unambiguously identifying them as hydrothermal-impacted sediments (Figure 8A). These results align with established thresholds and underscore the utility of combining elemental ratios to disentangle complex sedimentary provenance.

Another way to identify the significance of hydrothermal activity on surface sediments is by looking into the rare earth element (REE) composition in sediments. The hydrothermal fluids from mid-ocean ridges are characterized by Light REE (LREE) enrichment and positive Eu anomalies (Douville et al., 1999; Allen and Seyfried, 2005; Cao et al., 2012). Consequently, sediments affected by hydrothermal activity often exhibit similar characteristics (Gillis and Thompson, 1993; Liao et al., 2018; Zhou et al., 2020). The Chondrite-normalized REE patterns of the sediment samples are shown in Figure 8B. In general, all sediment samples showed a trend of LREE enrichment. Whilst, several samples (from P10 to P15), to different extents, showed apparent positive Eu anomaly ((Eu/Eu^*)_N = 0.90–3.39) compared to the value (0.71) reported by Liao et al. (2018) for the background sediments.

It is specifically noted that although the metal ratios of P14 exceed the predicted range, they fall near the threshold boundaries. Additionally, P14 exhibits elevated Fe, Cu, and Zn concentrations compared to samples P1–P9, while its Ca content is significantly lower. Visually, the sample displays a reddish-brown coloration—a characteristic indicative of hydrothermal activity. Based on this integrated evidence, we have classified P14 as hydrothermal-impacted sediment. The sediment types are explicitly detailed in Table 4B. Overall, by taking into consideration of the results, we considered that, among the 15 surface sediment samples, P10 to P15 can be identified as hydrothermal-impacted sediments, and that P1 to P9 represent background sediments.

3.3.2 Ba contents and Ba isotope compositions

Following sediment classification, we measured the Ba contents across all samples and selected eight representative samples for Ba isotope analysis. This subset included three background sediments (P3, P5, P7) and five hydrothermal sediments (P11 to P15). The Ba contents and its isotopic compositions in sediment samples are reported in Table 4B. The Ba content ranged from 110–1109 µg/g, and there is no apparent elevation in Ba content in hydrothermal influenced samples. Paytan and Griffith (2007) reported that the content of Ba in marine sediments can be significantly affected by dilution effects due to the accumulation of other sedimentary components.

To the contrary, the δ¹³⁸Ba values in background samples (P3,P5,P7) ranged from 0.01‰ to 0.14‰, similar to the deep-sea sediments reported in previous study (0 – 0.10‰) (Bridgestock et al., 2018). In hydrothermal-impacted sediments (P11 to P15), δ¹³⁸Ba values ranged from -0.16‰ – 0.01‰. The average δ¹³⁸Ba of hydrothermal sediments (-0.08‰, SD=0.07‰, n=5) is notably lower than that of the background sediments (0.09‰, SD=0.07‰, n=3). The relationship of Ba contents and δ¹³⁸Ba is shown in Supplementary Figure S4. The results show that the data points do not follow a distinct fractionation trend (i.e., lighter Ba isotopes correlating with higher Ba concentrations). Instead, samples with lighter Ba isotopes exhibit both high and low Ba concentrations. We therefore propose that the Ba concentrations in hydrothermal-impacted sediments are influenced by dilution effects from co-occurring hydrothermal metal elements.

As we discussed in section 3.2.3, during the mixing process between hydrothermal fluid and seawater, lighter Ba isotopes are preferentially removed, resulting the settling particles compose lighter Ba isotope. This is consistent with our observation in the hydrothermally influenced sediment samples in this study. As conclusion, while the Ba content of hydrothermal sediments cannot be distinguished from the background sediments, the Ba isotope retains lighter isotopic signature, potentially can serve as an indicator of hydrothermal influence on sediment of the study region.

4 Conclusion

Although the overall Ba cycle in the ocean is relatively well understood, studies focusing on specific processes, such as hydrothermal activities, remain limited. This study presents the first Ba isotope study in hydrothermal fluid, seawater, and sediment near Southwest Indian Ocean hydrothermal vent field.

The vertical profiles of dissolved Ba and its isotope compositions is similar with previous results of other oceans. The [Ba]_diss in several water samples collected using ROV near hydrothermal vents are significantly higher than background seawater, while the Ba isotope compositions in those samples(-0.10 – 0.05‰) are distinctly lower than those of background seawater(~ 0.29‰), suggesting that the hydrothermal fluids in the study region bearing lighter Ba isotopes.

Analysis of filter membranes from Longqi and Tiancheng hydrothermal vents samples using Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy revealed that, various mineral species exist, not only barite, but also Fe oxides, FeS, FeS₂, Fe_(1-x) Sx, CeO₂, ZnS, and SiO₂, etc. Therefore, barium may co-precipitate with iron-manganese oxides and metal sulfide particles in addition to forming barite, which processes could result in different isotopic fractionation.

Analysis of hydrothermally influenced sediment showed that the Ba isotope compositions are significantly lighter (-0.16‰ – 0.01‰) compared to those in background sediments (0.01 – 0.14‰), which is consistent with lighter isotope composition found in hydrothermal fluid and also align with the calculation result of the Rayleigh fractionation model, suggesting lighter isotopes are preferable removed during the mixing process between hydrothermal fluid and seawater. Therefore, the Ba isotope may serve as a valuable indicator of hydrothermal influence in future studies on sediment.

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