Substantial trace metal input from the 2022 Hunga Tonga-Hunga Ha’apai eruption into the South Pacific

Domain: earth_science
Published: 2024-10-18
Editorial outcome: minor_revision

Abstract
The January 2022 eruption of the Hunga Tonga-Hunga Ha’apai (HTHH) volcano discharged 2,900 teragrams of ejecta, most of which was deposited in the South Pacific Ocean. Here we investigate its impact on the biogeochemistry of the South Pacific Gyre (SPG) using samples collected during the GEOTRACES cruise GP21 in February-April 2022. Surface water neodymium isotopes and rare earth element compositions showed a marked volcanic impact in the western SPG, potentially extending to the eastern region. Increasing trace metal concentrations in surface waters and chlorophyll-a inventories in euphotic layers between the eastern and western SPG further suggest that the volcanic eruption supplied (micro)nutrients potentially stimulating a biological response. We estimate that the HTHH eruption released up to 0.16 kt of neodymium and 32 kt of iron into the SPG, which is comparable to the annual global dust-borne Nd flux and the annual dust-borne Fe flux to the entire SPG, respectively.

Introduction
Volcanic ash deposited into the surface ocean readily releases bioactive trace metals (e.g., iron, Fe; manganese, Mn), which can stimulate phytoplankton growth, either directly via Fe fertilization of waters replete in macronutrients, or indirectly via Fe fertilization of nitrogen fixation in waters where nitrate is depleted and phosphate available in excess 1–3. Consequently, major volcanic eruptions driving surface ocean Fe fertilization have been linked to atmospheric CO2 drawdown and climatic changes in the past 4–7. Airborne volcanic ash has been suggested as an important atmospheric Fe source for the surface Pacific Ocean, due to the abundance of active and explosive volcanoes surrounding it (Pacific Ring of Fire) 3. The estimated input of ash into the Pacific Ocean of 128–221 × 10^12 g/yr, is comparable to estimates of mineral dust inputs of non-volcanic origin (39–519 × 10^12 g/yr) 8, resulting in a total soluble Fe flux of 3–75 × 10^6 mol/yr to the Pacific Ocean 3. Indeed, dispersed ash forms a substantial fraction of the South Pacific Ocean (SPO) pelagic clay deposits, often exceeding 50% by mass, and records episodes of southern hemisphere volcanism 3,9. The SPO is dominated by the vast oligotrophic South Pacific Gyre (SPG), which is characterized by extremely low concentrations of nitrate, relatively elevated phosphate, and expected Fe limitation of nitrogen fixation rates 10.

The underwater Hunga Tonga-Hunga Ha’apai (HTHH) volcano (20.536°S, 175.382°W; 150 m below sea-level), located in the Tonga-Kermadec volcanic arc, erupted violently on 15 January 2022 (Fig. 1a). The eruption is among the largest volcanic activity ever recorded with modern geophysical instrumentation, with a volcanic explosivity index (VEI) of ~6 11. The eruption produced a large volcanic plume that reached an altitude of up to 57 kilometers 12 and resulted in tephra covering an area of at least 600 km in diameter 13. The eruption expelled vast (2.9 × 10^15 g) amounts of basaltic-andesitic volcanogenic material, such as ash and pumice, into the SPG 14. Following the deposition of the ash-laden volcanic plumes of HTHH, satellite observations revealed elevated surface water chlorophyll-a concentrations (Chl-a) in the affected nearby region 15,16.

Volcanic ash from the HTHH eruption has the potential to exert widespread effects on the SPG via long-distance (thousands of kilometers) atmospheric transport by prevailing winds 17–19. Although suspended ash was still observed in the water column around the HTHH volcano months later 20, the volcanic ash overall sank rapidly (hours to days) through the water column, depending on particle size 21. In contrast, pumice can float for periods of months to years due to its high porosity and trapped gas bubbles and can thus be transported over thousands of kilometers via surface ocean currents 22,23. The dispersal of volcanic ash and floating pumice can enhance the impacts of a volcanic eruption on surface ocean biogeochemistry over extended temporal and spatial scales 23. However, so far, the large-scale impact of the HTHH eruption on SPG biogeochemistry has not been explored.

Tracing volcanic inputs into the SPG is challenging due to the difficulty in distinguishing them from other external sources such as eolian aluminosilicate dust derived from continents 24 and weathering inputs from volcanic islands 25. However, radiogenic neodymium isotope compositions (143Nd/144Nd, expressed as εNd) and dissolved rare earth element (REE) concentrations in seawater can be used to trace specific sources of trace elements to the ocean 26–28. The εNd signatures of lithogenic materials vary as a function of age and lithology and surface waters acquire their εNd signatures via aeolian input, volcanic input, and continental weathering input from rivers and submarine groundwater discharge 26,29. The REEs are divided into three categories based on their atomic weight: light (LREE), medium (MREE), and heavy (HREE). To eliminate natural abundance variations, REE concentrations in seawater are normalized to Post-Archean Australian Shale (PAAS) which approximates average continental crust 30. The REE patterns in the open ocean increase in abundance from LREE to HREE because of preferential LREE scavenging in seawater, which is known as HREE enrichment (HREE/LREE > 1) 31. The REEs also typically exhibit a cerium (Ce) depletion in comparison to neighboring REEs with similar mass due to Ce oxidation to insoluble forms in seawater, which is referred to as the “Ce anomaly” (Ce/Ce* < 1) 31. The dissolution of lithogenic material weakens the Ce anomaly (Ce/Ce* closer to 1) and modifies the dissolved HREE/LREE ratio, depending on the composition of the dissolving material.

To investigate the biogeochemical impacts of the HTHH eruption on the surface SPG, we characterized the magnitude and location of the ash deposition using a volcanic ash dispersal model and an air mass trajectory model. The volcanic inputs were traced with dissolved εNd and REE signals in surface seawater and biogeochemical impacts were examined via the concentrations of trace metals (dissolved aluminum (Al), Mn, Fe) and chlorophyll-a, which is a proxy for phytoplankton biomass. Samples were collected along a zonal transect at 26-32.5°S across the entire SPO during GEOTRACES cruise GP21 in February-April 2022 (Fig. 1a). We find that the western SPG exhibited strong (bio)geochemical impacts following the volcanic inputs originating from the HTHH, which likely extended to the central SPG via atmospheric ash dispersal and surface current transport.

Results
Expedition GP21 and the HTHH eruption
The Pacific GEOTRACES GP21 expedition (Chile to New Caledonia) with the research vessel SONNE (SO289) took place between 23 February and 4 April 2022, which was 39 to 79 days after the HTHH eruption (Fig. 1a). The transect was aligned with the southern limb of the anticyclonic SPG, which is formed by the broad, eastward-flowing South Pacific Current 32. Underway data from the upper ocean Acoustic Doppler Current Profiler (75 kHz-ADCP) showed that the prevailing surface current velocities were eastward, with typical surface speeds of 10-40 cm per second (Supplementary Fig. 1). However, a small region (166-174°W) in the vicinity of the Tonga-Kermadec Ridge exhibited predominantly westward surface currents. A large amount of floating tephra, mostly pumice, was observed and collected before crossing the Tonga-Kermadec Ridge (~175°W) on 30 March 2022, 73 days after the major HTHH eruption (Fig. 1a,d; Supplementary Fig. 2).

The NOAA HYSPLIT volcanic ash dispersion model indicated that volcanic ash particles were mainly deposited in the western region of the SPG (20°−50°S, 175°E−150°W), south of the HTHH volcano, within 3 days of the eruption (Fig. 1b; Supplementary Fig. 3). This area covered the western extent of the GP21 transect. NOAA HYSPLIT forward air mass trajectories for 13 days following the HTHH eruption indicated a general eastward atmospheric transport of volcanic ash across the entire SPG (Fig. 1c; Supplementary Fig. 4), which overlapped with the GP21 cruise track.

Radiogenic Nd isotopic compositions and REE patterns in surface waters
Surface seawater εNd values exhibited pronounced and systematic variability (Fig. 2a). The εNd values decreased from −0.9 at the Chilean coast to −6.2 at the eastern boundary of the SPG (90°W), followed by a gradual increase to +0.9 near the Tonga-Kermadec Arc. The most unradiogenic (negative) εNd signal (−6.2) at 90°W is consistent with a reported surface value at a nearby location (εNd = −6.4; station EGY: 32°S, 91.4°W) 33 and reflect the influence of surface currents. Specifically, as the South Pacific Current approaches South America, the Humboldt Current system between the SPG and the Chilean coast transports subantarctic water equatorward at approximately 90°W 34. The surface water in the Subantarctic Zone of the southeastern Pacific Ocean carries an εNd signal ranging between −5.7 and −8.4 35 resulting in the unradiogenic εNd signal at ~90°W along the GP21 section. The surface εNd signal became more radiogenic (positive) between 90°W (−6.2) and the Chilean coast (−0.9; Fig. 2a). This was accompanied by a pronounced increase in Nd concentration from ~4 to ~20 pmol/kg (Fig. 2b) and a weakening of the Ce anomaly towards the Chilean coast (Fig. 2c). Collectively these geochemical seawater signatures were due to lithogenic input from the Chilean coast, where outcropping continental rocks and sediments exhibit highly radiogenic εNd signatures (0 to +5) due to their volcanic nature 33.

Within the SPG, the surface water εNd signal became more radiogenic from the eastern edge of the gyre at 90°W (−6.2) towards the Tonga-Kermadec Arc at 180°W (+0.9) (Fig. 2a). The εNd signature in the western SPG (~0 to +1) was more radiogenic than reported surface εNd signatures at nearby locations. This includes two stations at 170°W (SX-22: 20°S; SX-20: 30°S) sampled in January 2005 (εNd = −1.9 and −2.3) 36 and one station at 174°E (GeoB17019: 15°S) sampled in September-October 2012 (εNd = −1.7) 37. The more positive values observed in our study indicate pronounced external inputs into the surface ocean prior to our sampling in the western SPG that were enriched in radiogenic εNd. However, surface water Nd concentrations in the western SPG remained consistently low (~4 pmol/kg), at levels comparable to previous measurements 36,37 (Fig. 2b). It can be concluded that scavenging and/or exchange processes occurred to maintain a constant Nd concentration while modifying the Nd isotopic composition. This will be discussed in greater detail in a later section (Rapid release and scavenging of Nd following the volcanic input). Accompanying the εNd increases, the Ce anomaly weakened (Fig. 2c) while the HREE enrichment strengthened (Fig. 2d) from eastern to western SPG.

Potential sources of external inputs to SPG surface waters
The positive correlation between Ce anomaly and HREE enrichment values (R^2 = 0.56, p < 0.05; Fig. 3) in the SPG (170°E-90°W) indicates that the radiogenic source of external inputs in the western SPG were characterized by a weaker (or absent) Ce anomaly and stronger HREE enrichment. Possible external source materials for the surface water of the western SPG include volcanic material of the HTHH eruption, such as ash or pumice, weathering of basaltic volcanic islands, aeolian dust from Australia, and/or shallow hydrothermal inputs along the Tonga-Kermadec Arc, each of which have particular εNd signatures and REE ratios (Table 1).

The dissolution of volcanogenic materials and also the weathering of basaltic volcanic islands 38 result in the release of Nd with a highly radiogenic signature. This is because oceanic crust, such as mid-ocean ridge basalts (MORBs), ocean island basalts (OIBs), and island arc rocks all have positive εNd values between ~0 and +10 39. Consistent with that, our pumice sample collected near the Tonga-Kermadec arc yielded a highly radiogenic εNd signature of +7.5. The weathering of volcanic islands, for example Papua New Guinea, also supplies radiogenic Nd, resulting in εNd values of both surface and subsurface waters in the equatorial Western Pacific reaching up to +0.7 37. However, basalts formed in different tectonic settings have distinct chemical characteristics, primarily determined by the source type, tectonic environment and magma generation processes 40. These factors lead to systematic compositional differences between MORBs and OIBs. Therefore, the Primitive Mantle-normalized REE pattern of the pumice closely resembled that of normal MORBs 40, and differed from that of OIBs 40 (Supplementary Fig. 5). In addition, basalts formed within island arcs exhibit geochemical diversity as a function of different mantle origins and/or varying melting processes, which result in distinguishable elemental compositions 41. Lavas from different Tonga-Kermadec islands display distinctive REE patterns 42, and our collected pumice has a Primitive Mantle-normalized REE pattern similar to that of the lava from the HTHH volcano (Supplementary Figs. 5, 6). Therefore, the geochemical composition of our collected pumice strongly suggests that it originated from the HTHH eruption.

When the REE concentrations were normalized to PAAS, the pumice and HTHH lava exhibited a more pronounced enrichment in HREE (Fig. 4a) and no Ce anomaly compared to surface seawater (Fig. 4b). This is consistent with the REE characteristics of the external source to the western SPG and explain the correlated changes in Ce anomaly and HREE enrichment in the surface waters between 90°W and 170°E (Fig. 3). OIBs exhibit a pronounced enrichment of MREEs, a crustal Ce signal (without anomaly), and little fractionation between light and heavy REEs (i.e. no HREE enrichment) (Fig. 4). Volcanic island weathering imprints an OIB-like REE pattern in local fresh or brackish waters at the island-ocean interface, although this signal is typically rapidly lost in coastal seawaters due to scavenging 25,28,38. Overall, input of volcanic island weathering cannot account for the observed changes in REE ratios of surface seawater in the SPG (Fig. 3). Therefore, it can be concluded that volcanic island weathering was not the source of the material driving the observed geochemical changes.

Another possible external source to the western SPG is Australian dust 43,44. However, the εNd signatures of the Australian dust (−29 to +2) 43 are generally too unradiogenic to account for the observed radiogenic signature (+1) near the Kermadec arc. In addition, Australian dust has an overall shale-like REE pattern lacking a Ce anomaly and a HREE enrichment in contrast to surface seawater (Fig. 3). Furthermore, our western part of the transect is located at a distance from the region receiving long-range dust transport from Australia, which follows a southeastward trajectory 44. Therefore, it is unlikely that Australian dust is a major component of the external source.

Shallow hydrothermal vents (<500 m depth) along the Tonga-Kermadec arc potentially increase Fe concentrations in the photic layer through vertical transport 45. Although dissolved εNd may show a slightly more radiogenic signature within hydrothermal plumes (one ε unit shift) 46, REEs are typically immediately immobilized near the vents upon contact between hot fluids and cold bottom waters 47. Furthermore, no significant excess of 3helium (3Hexs), a conservative tracer for hydrothermal plumes 48, was detected at our station above the Monowai Seamount on the Kermadec ridge. The measured 3He levels were even lower than the background 3He value (~2.38 fmol/kg) (Fig. 5c). In addition, hydrothermal fluids commonly exhibit a pronounced, positive europium (Eu) anomaly (Eu/Eu* > 1) 49. The absence of a positive Eu anomaly in the water column near the Tonga-Kermadec arc (Fig. 5b) suggests a negligible shallow water hydrothermal input. Moreover, εNd signatures become more radiogenic from depths of 600 m towards the surface (Fig. 5a), suggesting external inputs into surface seawaters from above rather than vertical supply from below.

In summary, we conclude that the dissolution of volcanogenic material originating from the HTHH eruption, which occurred 39 to 79 days prior to our sampling, is predominantly responsible for the radiogenic εNd signature, weak Ce anomaly and strong heavy REE enrichment observed in the western SPG along GP21. The area of the strongest geochemical signal largely overlaps with the main region of post-eruption volcanic ash deposition as well as the location of floating pumice (Fig. 2; Supplementary Fig. 3). In addition, surface currents may redistribute the volcanic input signal up to 600-2400 km away in 70 days. The gradual change in εNd signatures and REE ratios between the western and eastern SPG may be indicative of the eastward extension of the influence of the volcanic eruption through atmospheric ash dispersion (Fig. 1c; Supplementary Fig. 4) and/or the predominant direction of the surface ocean currents (Fig. 1a).

Rapid release and scavenging of Nd following the volcanic input
The SPG is known for the lowest surface water REE concentrations in the global ocean, likely due to its remoteness with a limited amount of terrestrial inputs and a lack of vertical supply due to strong water column stratification 50. Despite the clear evidence for partial dissolution of volcanogenic materials discussed above, surface water Nd concentrations in the western SPG remained consistently low (~4 pmol/kg) at levels comparable to previous measurements 36,37 (Fig. 2b). Assuming an initial surface seawater Nd concentration of ~4 pmol/kg with an εNd value ~−2 in the western SPG, an addition of 0.9-1.9 pmol/kg (of seawater) of Nd from the volcanogenic material with an εNd value of +7.5 is required to obtain a surface εNd value of ~0 to +1. In the absence of subsequent scavenging from the surface water 51, this would result in a 22–46% increase in Nd concentration. Given that the samples were collected 9-10 weeks after the eruption, subsequent particle scavenging was likely to have occurred, which had offset the increase in concentration. The preferential removal of Nd and other LREEs during particle scavenging, following the dissolution of volcanic material, resulted in a downward shift in the Ce/Ce* to HREE/LREE slope. This explains the deviation of the linear regression between Ce anomaly and HREE enrichment from the pumice endmember (Fig. 3).

Surface layer Nd residence times were suggested to range from 1–4 years in the eastern Indian Ocean and eastern Atlantic Ocean 39,52. However, the mean residence time of Nd in the mixed layer is dependent on various factors such as REE inventory, supply rate or export flux, and particle density 39,53. Trace metals are shown to exhibit a markedly shorter residence time in response to large dust deposition events due to enhanced particle scavenging 54–56. Similarly, ash particles have residence times ranging from hours to days in the mixed layer, depending on their particle sizes 21. Following ash deposition, the rapid downward flux of these particles may facilitate rapid Nd scavenging, resulting in a short residence time of REE in the mixed layer. This enables the surface water REE pattern to be rapidly altered in response to local external sources (Fig. 3).

Trace metal fertilization and biological response in the SPG
The concentrations of surface dissolved Al (dAl), Mn (dMn) and Fe (dFe) all increased between the eastern and western SPG (Fig. 6a,b), accompanying the overall increase in εNd values (Fig. 2a). Previous studies have attributed elevated dFe concentrations in the western South Pacific to (i) shallow inputs of Fe of hydrothermal origin from the vents along the Tonga-Kermadec arc 45,57, or (ii) direct interaction of seawater with the islands or Australian continent, which can subsequently be advected elsewhere by currents 10,58. However, as discussed above, volcanic material was the dominant external source of trace metals in the western SPG during our cruise, with a greater influence than other external sources. In addition, a comparison of surface trace metal concentrations in the western SPG between our western transect and other two earlier GEOTRACES cruises, GP13 and GP19, revealed elevated levels in the region 175°E-175°W for our cruise, with a notable increase in dAl over an extended area (Supplementary Fig. 7). It must be acknowledged that the temporal gap between our sampling and the eruption (9–10 weeks) requires consideration. For instance, approximately two to three months after the 2010 Eyjafjallajökull eruption, Achterberg et al. 21 observed no elevated surface dFe in offshore waters of the Iceland Basin. This mid-sized eruption, however, emitted tephra in an amount (~3.8 × 10^14 g) that was one magnitude lower than the major HTHH eruption (~2.9 × 10^15 g) 14. Consequently, the HTHH eruption appeared to exert a stronger and more far-reaching geochemical impact in comparison to those previously observed for mid-sized eruptions.

In the SPG, the dAl closely followed the change in surface εNd, showing a highly significant positive correlation (n=19, R^2 = 0.88, p < 0.001, Fig. 6c). This suggests that dAl had the same source as the radiogenic εNd, that is, the volcanic aluminosilicate input. However, there was a slight deviation in the increase exhibited by dMn compared to the near linear increase observed by dAl (Fig. 6a). This is documented by its weaker positive correlation with surface εNd (n=18, R^2 = 0.53, p < 0.001, Fig. 6d). In contrast, the concentrations of dFe decreased to very low levels (~0.1 nmol/L) in the main ash deposition area (shaded in Fig. 6b). As a result, we observe insignificant correlations between surface εNd and dFe (n=18, R^2 = 0.11, p = 0.19, Fig. 6e).

Given the time gap of 9–10 weeks between the HTHH eruption and our observation in the western SPG, the observed differences between dAl, dFe, and dMn can be attributed to their scavenging behavior and biological requirements, which result in a markedly shorter residence time for dFe compared to dAl and dMn 59. In open ocean surface waters, dAl distributions are primarily controlled by aeolian inputs, as it is not known to be actively assimilated by organisms, while dMn is essential for organisms and surface ocean concentrations are further buffered by photochemical reduction 60,61. In contrast, dFe is scavenged from seawater more rapidly than dAl or dMn and has a greater requirement for phytoplankton growth than dMn 61,62. Dust-seeding mesocosm experiments have indicated that a large dust deposition event can accelerate the export of Fe from the water column through scavenging, while a remarkable increase of dissolved Al and Mn was observed 56. Extremely short residence times, with timescales of days to weeks, were reported for Fe in Atlantic surface waters receiving enhanced atmospheric inputs from Saharan origins 54,55. Consequently, the more pronounced depletion of dFe in the main ash-deposition area (Fig. 6b), despite the anticipated high input fluxes, is primarily a result of scavenging subsequent to the release and/or biological removal 21.

A weak positive correlation was observed between surface εNd and the total chlorophyll-a (TChl-a) inventory in the euphotic layer measured on the research cruise (n=18, R^2 = 0.32; p < 0.05, Fig. 6f). In addition, a positive satellite-derived Chl-a anomaly for March 2022 was evident in the western part of the transect compared to the March average for the period between 2002 and 2023 (Supplementary Fig. 8). Together these indicate a potential increase in phytoplankton growth in response to the volcanic input of dFe via stimulation of N2 fixation, particularly in the western SPG, as shown in Fig. 6b. It should be noted, however, that the time interval after the eruption may have precluded the observation of the most elevated phytoplankton biomass in the western SPG. Given that multiple sources of trace metals are known in the western SPG, confidently ascribing this elevated phytoplankton biomass to volcanic dFe fertilization over and above other sources is challenging. However, as discussed above, volcanic input was identified as the dominant TM source to surface waters, therefore we consider a linkage between volcanic input from the HTHH eruption into the western SPG and the enhanced TChl-a inventory a plausible scenario.

Constraints on volcanic Nd and Fe supply to the SPG
Extrapolating the Nd concentration in the pumice (4.18 µg/g) to the total ejecta of the HTHH eruption (2.9 × 10^15 g) 14, the ejecta contain a total of 1.2 × 10^10 g of Nd. The NOAA HYSPLIT volcanic ash dispersion model indicated a mass loading area coverage of 5.4 × 10^6 km^2 three days after the eruption (Fig. 1b), which can be considered to represent the lower boundary of the ash depositional area. Meanwhile, our data demonstrate a significant increase of 2–3 εNd units between stations 33 and 44 (152°W to 172°E) in the western SPG. Based on this longitudinal coverage and the latitudinal range of 30° (20°S to 50°S) indicated by the HYSPLIT model (Fig. 1b), we can estimate the upper limit of the depositional area to be 1.2 × 10^7 km^2. On the basis of an estimated overall release of 0.9–1.9 pmol/kg Nd within the surface mixed layer (~50 m), we calculate a total release of 0.4–1.6 × 10^8 g of Nd. The upper limit is comparable to, although slightly lower than, the annual global dust input of Nd (2–4 × 10^8 g/yr) 63,64. It represents 1.3% of the total Nd contained in the ejecta from HTHH, which is lower than the estimated fraction of soluble Nd in dust (2–50%) 64. Although Nd scavenging is clearly evident based on the lack of a Nd concentration enrichment in surface waters of the depositional area, this release still had a significant impact on the Nd isotope composition and REE characteristics of surface waters, making them sensitive tracers of volcanic activity in the present and past ocean.

Volcanic ash also rapidly supplies significant amounts of soluble Fe to seawater, with a global mean estimate of 200 ± 50 nmol Fe/g ash for subduction zone volcanic ash 3. Multiplying this mean value by the total amount of ejecta from the HTHH eruption (2.9 × 10^15 g) 14 results in a potential release of 5.8 ± 1.5 × 10^8 mol (3.2 ± 0.8 × 10^10 g) of Fe. This estimate likely represents an upper limit, as some types of ejecta, such as pumice, release Fe less efficiently than volcanic ash upon contact with seawater 65. However, the long-lived buoyancy of pumice potentially promotes a continuous release of Fe into surface waters for months to years before finally sinking 22,23. The long-term Fe release efficiency of pumice to surface waters should be further investigated to test this possibility. This estimate is comparable to the Fe fertilization amount from the similar giant eruption of Mount Pinatubo in June 1991 (VEI 6; 7.2 × 10^8 mol Fe), which was associated with atmospheric carbon/oxygen anomalies 7, but it is much larger than the estimated Fe release from previous mid-sized eruptions, including the Kasatochi eruption in August 2008 (VEI 4; ~1 × 10^8 mol Fe), which stimulated a massive phytoplankton bloom 66, and the Eyjafjallajökull eruption in April-May 2010 (VEI 4; 1.8 × 10^6 mol Fe), which resulted in enhanced major nutrient drawdown 21.

The New Zealand–Tonga–Kermadec arc is estimated to emit 20–26 × 10^12 g/yr volcanic ash 3, corresponding to 30–78 × 10^5 mol/yr Fe, suggesting that Fe release from HTHH approximates the integrated Fe contribution of the New Zealand–Tonga–Kermadec arc volcanic input over a century. Pavia et al. 24 estimated a dust-borne Fe flux to the central region of the SPG at 3.5–11 μmol/m^2/yr. Multiplying this estimate with the surface area of the SPG (37 million km^2) yields an estimated dust-borne Fe flux to the SPG of 1.3–4.1 × 10^8 mol/yr. Consequently, the quantity of Fe released from the HTHH eruption is equivalent to the annual dust-borne Fe flux into the entire SPG. In order to improve the accuracy of biogeochemical models in the Pacific Ocean, it is, therefore, necessary to incorporate the episodic volcanic input of trace metals and to improve parameterization of the key chemical-biological processes that respond to volcanic inputs in the models.

Methods
Volcanic ash dispersion model and trajectory model
The NOAA HYSPLIT volcanic ash dispersion model 67 was used to calculate the transport and dispersion of volcanic ash from the Hunga Tonga-Hunga Ha’apai (HTHH) volcanic eruption. Information about the HTHH volcano was extracted from the Smithsonian Institution “Volcanoes of the World” database. Archived GDAS (1 degree, global, 2006-present) data was used for meteorology. Eruption source parameters (ESPs) for the HTHH volcanic eruption, as assigned by the United States Geological Survey (USGS), were used for the mass eruption rate. The HYSPLIT model was run for 72 hours with 3 concentration layers after the eruption (15th January 2022, 09:00 UTC) with a 24 hours output interval. Model output displays results of one-hour averages at the given snapshot interval for 3 altitude layers: 6000 m, 12000 m, and 18000 m.

The NOAA HYSPLIT trajectory model 67 was used to calculate forward trajectories after the HTHH eruption (15th January 2022, 09:00 UTC) from the eruption location. A maximum of 315 hours can be specified for archive datasets on the website. Archived GDAS (1 degree, global, 2006-present) data was used for meteorology. “Model vertical velocity” that uses the vertical velocity field from meteorological data was chosen for the type of vertical motion method the trajectory model uses in its calculation.

Radiogenic neodymium isotopes sampling and analyses
A total of 28 surface seawater samples were collected for the radiogenic Nd isotope measurements. Approximately 20 L of surface seawater was collected for stations 1-14 using 10 L Niskin bottles attached to a stainless steel CTD rosette. Starting from station 16, ~40 L of surface seawater was collected per sample from a towed, trace-metal-clean near-surface seawater sampling device equipped with acid-washed PVC tubing, and pumping provided by a Teflon diaphragm pump (i.e. tow-fish), either immediately before or after the station, to allow isotopic composition measurements at anticipated low concentrations. Between stations 39 and 44, an additional 17 surface samples (1-2 L) were collected from the tow-fish for REE measurements.

The samples were treated onboard strictly following recommended GEOTRACES protocols 68. Each 20 or 40 L sample was filtered through a nitro-cellulose acetate filter (0.45 μm pore diameter) into one or two acid-cleaned LDPE-cubitainers (20 L) with a peristaltic pump within 2 h after collection, and subsequently acidified to pH ~1.9 with concentrated, distilled HCl. For the determination of Nd (using isotope dilution) and REE concentrations, 1-2 L aliquots from each filtered sample were collected in acid-cleaned 1 or 2 L PE-bottles and acidified to pH ~1.9. To each large volume sample, 25 μL of FeCl3 solution (∼200 mg Fe/mL) was added per 1 L of sample, and the sample was left to equilibrate for 24 h. Ammonia solution (25%, Merck Suprapur®) was next added to increase the pH to 7.8-8.2. After 48 h the trace elements co-precipitated with the iron hydroxide precipitates and settled to the bottom of the cubitainers, and the supernatant was syphoned off. The precipitates were then transferred into 2 L PE-bottles and transported to GEOMAR for analysis.

Precipitates were dissolved and purified for measurement with Multicollector-Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) in the home laboratory at GEOMAR. Detailed purification procedures can be found in Rahlf et al. 69. Briefly, the precipitates were dissolved in 6 M HCl/0.5 M HF and dried down, and then treated with aqua regia at 120 °C for 24 h to remove organic compounds. Then pre-cleaned di-ethyl ether was used to remove most Fe via liquid–liquid extraction. Neodymium was chromatographically separated from matrix elements using first a cation exchange resin AG 50W-X8 (1.4 ml, 200–400 μm) and then from the other REE using Eichrom® LN-Spec resin (2 ml, 50–100 μm) on a second chromatographic column. The 143Nd/144Nd ratios were determined using a Neptune Plus MC-ICP-MS at GEOMAR and corrected for instrumental mass bias to 146Nd/144Nd = 0.7219 and to 142Nd/144Nd = 1.141876 70. The 143Nd/144Nd ratios of all samples were normalized using bracketing analyses of the JNdi-1 standard, with a value of 0.512115 71. The total procedural laboratory blanks for water samples (n=3) were negligible at <20 pg for Nd and contributed a maximum of 0.3% to the seawater samples.

εNd is defined by the equation:
εNd = [(143Nd/144Nd)sample / (143Nd/144Nd)CHUR − 1] × 10^4 (1)
where the 143Nd/144Nd of CHUR (Chondritic Uniform Reservoir) are 0.512639 72.

The external reproducibility (2 × standard deviations, 2SD) of the Nd isotope measurements was assessed by repeated purification and measurement of USGS reference material NOD-A-1 at concentrations matched to those of the samples (12-20 ppb, εNd = −9.35 ± 0.15, n=7 for repeated purification, n=35 for repeated measurement), which is used to demonstrate the error of the measured εNd in all figures (2SD = 0.15). If the internal error (2 × standard errors, 2SE) of a sample was larger than the external error, the internal error is used.

Pumice was rinsed with Milli-Q water (Millipore) three times and then dried and homogenized prior to alkaline fusion following Bayon et al. 73. Neodymium in the digestion solution was then purified by chromatography following the same procedure as for seawater. The accuracy and reproducibility of the fusion technique and measurement were monitored by processing reference materials with each batch of samples including USGS reference material BHVO-2 (εNd = +6.80 ± 0.24, n=3) and AGV-2 (εNd = +2.99 ± 0.13, n=3). The larger 2SD from BHVO-2 measurement is used to demonstrate the error of the measured εNd in the pumice (2SD = 0.24).

Rare earth element concentration analyses
For the precise determination of Nd concentrations, 1 L samples were spiked with a pre-weighed 150Nd spike, after which the Nd was precipitated with FeCl3 solution and purified with cation exchange resin AG 50W-X8 (1.4 ml, 200–400 μm). The Nd concentration was then measured on the Neptune Plus MC-ICPMS via the isotope dilution (ID) method based on 150Nd/144Nd. External reproducibility (2SD) was better than 0.4% for Nd according to repeated treatment and measurement of the same sample (n=4).

The rare earth elements (REEs) were pre-concentrated offline using a SeaFAST system (model M5 from Elemental Scientific) which employs a NOBIAS PA-1 resin column to preconcentrate the REEs and other trace metals 74. A 10 ng/g thulium solution (25 μL) was added to each 25 mL acidified seawater sample prior to preconcentration to monitor yields. The same was done with acidified pure Milli-Q water blanks, and reference seawaters (pH ~1.9) which were preconcentrated like the samples. Using the M5 SeaFast system, 24 mL of sample was precisely loaded onto the column, the matrix was washed away, and then the REEs were eluted with 400 μL of 1.5 M HNO3. Before measurement, 200 μL of 10 ng/g Re in 0.1% HNO3 were added to each sample as an internal standard and to account for any evaporation of the samples in the meantime. All samples were analyzed on a Thermo Element XR ICP-MS coupled with a CETAC “Aridus 2” desolvating nebulizer to reduce oxide formation. Oxide formation was monitored at the start of each analytical session using single element solutions 74 and was negligible at <0.05%.

GEOTRACES inter-calibration seawater samples BATS 15 m and 2000 m 75, as well as seawater samples from this cruise (Station 36, 5 m and 5289 m), were repeatedly measured to monitor external reproducibility and accuracy. In this work, the 2SD of repeated measurements of the surface seawater at station 36 (n=6) is used to represent the uncertainties of REEs of all the surface samples, which is <10% for all REEs, except for Ce (14.9%). The average total procedural onboard blank (n=3) was ≤3.54% for all REEs, except for Ce (8.59%), compared to the surface sample containing the lowest REE concentrations along the transect. Blanks, mean values and 2SD for the reference seawaters are provided in Supplementary Data 1. A comparison of Nd concentrations measured by SeaFast and by the isotope dilution method in this study shows consistency between the two methods for Nd (Nd-SeaFast = 1.01 × Nd-ID, R^2 = 0.99; Supplementary Fig. 9).

The REE concentrations of pumice digestion solution was measured on an Agilent 7500cx Quadrupole-ICP-MS at GEOMAR. Oxide formation was corrected by measuring element solutions of barium (Ba), Ce, praseodymium (Pr) + Nd, and samarium (Sm) + europium (Eu) + gadolinium (Gd) + terbium (Tb) at the start of the analytical session. The reproducibility was monitored by repeated measurements of USGS reference material BHVO-2. Mean values and 2SD for the BHVO-2 measurements are given in Supplementary Data 1.

Surface dissolved manganese (dMn), dissolved iron (dFe), and dissolved aluminum (dAl)
Surface seawater for trace metal measurements was collected from the trace-metal-clean underway tow-fish and filtered through a 0.8/0.2 μm filter cartridge (AcroPak 500, Pall) into pre-cleaned 125 mL LDPE bottles (Nalgene), then acidified with hydrochloric acid (Ultrapure, Romil) for storage and stabilization (pH ~1.9). All sample and standard preparations were completed under a laminar flow bench, and laboratory and analytical work was undertaken in an ISO Class 5 clean lab. All LDPE and FEP bottles used for reagents, standards, and sample preparation were acid washed according to GEOTRACES protocols 68.

Dissolved Mn and Fe analytes were initially pre-concentrated and buffered online to pH 6.4 using SeaFast (SC-4DX; ESI), then measured on the ICP-MS (Element-XR, Thermo Fisher) at GEOMAR. The ammonium acetate buffer was prepared using Optima grade (Fisher) glacial acetic acid and ammonium hydroxide. Subboiled distilled nitric acid was used to make up the 1 M elution acid, with a 250 ng L−1 indium spike added to manage any potential drift correction. Additional details about this methodology are reported in Rapp et al. 76. Quantification and accuracy of trace metals was assessed using standard addition (R^2 = 0.99) as well as certified reference materials (SAFe D2; Cass-6 and Nass-7, National Research Council Canada), achieved to good agreement with reported consensus values (weighted mean absolute percentage error (WMAPE): <5.5% for dFe and <11% for dMn). Several seawater samples were run as duplicates or triplicates and returned relative standard deviations (RSDs) of <6.7% (dFe) and <9% (dMn), which are used to represent the error of each sample. The limit of detection was 0.005 nmol/L for dMn, and 0.05 nmol/L for dFe.

Dissolved Al was analyzed shipboard following the methodology of Ren et al. 77. Briefly, a working reagent consisting of 40 mg/L lumogallion and 2 M ammonium acetate (pH ~6) was prepared and stored in dark at the fridge. Working reagent (250 µL) was added into the 5 mL acidified seawater (pH ~1.9), resulting in a final pH of 5.0-5.5. Samples were then placed in an oven at 80 °C for 3 h to accelerate Al-lumogallion complexation. An eight-point calibration line (0, 1, 2, 4, 8, 12, 15, 20 nmol/L Al standard additions to surface seawater from South Atlantic Gyre), blank determination and an internal reference sample were pretreated in the same manner as samples and measured daily. All samples were analyzed using a Cary Eclipse fluorimeter; emission and excitation wavelengths were set to 507−575 nm, with a 10 nm slit width. Blank contributions were determined as two separate parts, the manifold blank and the reagent blank. The manifold blank was assessed by the average counts of two acidified seawater samples (one surface and one deep water) without reagents. The reagent blank was determined using two different methods: (1) by analyzing three acidified seawater samples spiked with 1x, 2x, and 3x reagent volume and (2) using two calibration series with 1x and 2x reagent volume, respectively. The reagent blank was then assessed from the difference in the intercepts of two series of calibration lines. For each seawater sample, duplicates or triplicates were performed, and the error of each sample is reported as the 1 SD of repeated measurements.

Total chlorophyll-a (TChl-a) concentrations
Seawater samples for phytoplankton pigment analysis using high-performance liquid chromatography (HPLC) were collected in 10 L opaque carboys at six depths throughout the euphotic zone. The depths were identified by the photosynthetically active radiation (PAR) intensity using a PAR sensor and an in vivo fluorescence sensor attached to the CTD (i.e., 100%, 50%, 25%, 10%, 1%, 0.1% surface PAR). Samples (4 L) were filtered onto 25 mm diameter Fisher MF300 GF/F filters and frozen immediately at −80 °C. Upon return to land, pigments were extracted in 90% acetone in plastic vials by homogenization of the filters using glass beads in a cell mill, centrifuged (10 minutes, 4500 g, 4 °C), then the supernatant was filtered through 0.2 μm polytetrafluoroethylene filters (VMR International) and subsequently quantified by reverse-phase HPLC (Dionex UltiMate 3000 LC system, Thermo Scientific) 78. Pigment standards were from Sigma-Aldrich (USA) and the International Agency for 14C Determination (Denmark). The total chlorophyll-a (TChl-a) concentration was calculated as the sum of chlorophyll-a and divinyl chlorophyll-a concentrations. The TChl-a inventory in the euphotic zone was calculated through the trapezoidal integration method.

Helium (He) isotopes
Seawater samples for He isotopes were collected using a copper pipe connected to the Niskin bottle, with water flowing until all bubbles in the tube were purged. Subsequently, the pipe was securely sealed using an electrical drill and a ratchet to ensure a tight seal. Additionally, 0.5 L samples for tritium analyses were collected at full depth at Station 40 to account for tritium concentrations in the deep water, thus correcting the helium isotope measurements. The helium isotopes and tritium samples were analyzed at the University of Bremen following the procedures described by Sültenfuß et al. 79.

The helium 3He/4He isotope ratio (R), is expressed relative to the atmospheric ratio (Ra = 1.38 × 10^−6), using the delta notation:
δ3He (%) = [(R / Ra) − 1] × 100 (2)

The background 3He concentration (~2.38 fM) in the deep ocean was calculated from the background δ3He (~−1.7%) and equilibrium He concentrations (~1.75 nM) at the observed in situ salinity (~35 PSU) and temperature (~3.5 °C). The excess of 3He (3Hexs) was then calculated by subtracting the background 3He and 3H levels from the measured 3He concentrations.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I enjoyed reading the manuscript by Zhang et al. on trace metal input from the 2022 Hunga Tonga-Hunga Ha’apai eruption to the South Pacific Gyre. The dataset is impressive, experimental design was strong, and the manuscript was well-written. I recommend moderate revisions but feel that the paper is publishable in Nature Communications – and indeed that the authors have some connections to draw at the end of the paper to make its impact even higher.\nThe difference in eNd and trace metals before and after the eruption was striking. I feel convinced that indeed, this eruption altered the REE and trace metal budgets of the surface of the SPG.\nI had one most significant issue reading the paper. It’s discussed in lines 218-228 and Figure 3. It seems like there’s a problem with the authors’ proposed hypothesis of scavenging followed by volcanic material dissolution, because the trend of the data doesn’t lead towards either the HTHH lava or the pumice endmember. The authors did not really explain why this is the case, or if they did I didn’t follow the argument. There must be additional processes at play to explain their data. One option is that continuous scavenging pulls the Ce/Ce* to HREE/LREE slope downward away from the apparent HTTH lava/pumice endmembers. Another is that volcanic ash has a distinct Ce/Ce* to HREE/LREE ratio from the lava and pumice.\nLine by line comments:\nLines 80-82: Please add a citation for the scavenging signature on LREE/HREE patterns\nLine 110: Add “A” before “Large” and change amount to amounts\nLine 119 and Figure 1: I found this trend difficult to see on Figure 1, especially figure 1c. Could the stations be added to this plot with the airmass trajectories in Figure 1c? Even if they aren’t labeled?\nLine 145: At least a sentence discussing that scavenging and/or exchange processes must be occurring is needed here to maintain constant Nd concentrations while modifying the Nd isotopic composition – this is discussed in more detail later, but a sentence here would be helpful as well.\nLine 151: This sentence doesn’t make sense – scavenging must be playing an important role here in order to have the eNd signature persist as far east as it does even though Ce/Ce* drops precipitously in this same region (eastern portion of the shaded region in Figure 2).\nLine 196-198: Seems like Australian dust range overlaps with the observed eNd\nLine 295-297: Pavia et al. 2020 (cited later by the authors in this paper) derive an Fe residence time on the order of 0.75-6.5 years in the South Pacific Gyre – this would be inconsistent with the authors’ argument for days to weeks residence times for Fe in this region, and I don’t think their argument is correct\nLines 297-300: The authors do not have supporting evidence to say that scavenging is more important than biological uptake for Fe, or if they do, it isn’t discussed to support this statement.\nSection beginning on line 308: The authors include error bars on their Fe inputs from the HTHH eruption, but not Nd – the Nd error bars should be included. The authors should also include error bars on their estimate of the depositional area, which is not an error free determination.\nThere is a units problem lurking somewhere in this section, it seems. On line 328, the authors state there the HTTH eruption released 2.0e15 g of ejecta, and calculate a potential release of 5.8e8 moles of Fe. First, the authors end up with a ~30% error, but there’s multiple orders of magnitude error in the Fe fractional solubility estimate. How is this incorporated and/or why isn’t it incorporated?\nLines 341-348: I think the authors have even more they could say about this – possibly connecting to Dunlea et al. 2015’s estimates based on geochemical multivariate modeling that SPG sediments near the core top have 20-25% rhyolitic ash in them, possibly speculating on the role episodic Fe input (from volcanic eruptions) might play in setting variable nitrogen fixation in the SPG, which responds to short-term Fe supply (Guieu et al. 2014, Pavia et al. 2020) – this could cause a huge difference in long, time-integrated estimates of nitrogen fixation here (say, from inversions of nutrient distributions) compared to short-term nitrogen fixation rates derived from incubations.\nFigure 3 legend: Change Astralian to Australian", "reviewer_2": "The manuscript submitted by Zhang et al. investigates the biogeochemical impact of the eruption of the Hunga Tonga-Hunga Ha’apai volcano in January 2022. In this study, the authors report surface concentrations and isotope composition of Neodymium (Nd), rare earth elements (REE) and trace metals (TMs) collected in the South Pacific Ocean to demonstrate the impact of the eruption on the surface ocean biogeochemistry. The dataset presented in this manuscript allows clearly demonstrating the link between the HTHH volcano eruption and change - over a large area - in Nd and REE characteristics. However, due to the time lag between the eruption and sample collection (~1-3 months), the trend in TMs concentrations is less clear. Furthermore, the link between TMs inputs and a biological response is not convincing, reducing the overall impact of this study.\nThat said, the authors have done a great job to demonstrate Nd and REE inputs from this volcanic event over a large area. Overall, it is an interesting study and I have only few concerns that need to be addressed (see below).\nWhile a short-term biological response has been observed using satellite observations few days after the eruption (e.g. Barone et al. 2023), the authors claim that “Increasing trace metal concentrations in surface waters and chlorophyll-a inventories in euphotic layers between the eastern and western SPG further suggest that the volcanic eruption supplied (micro)nutrients stimulating a biological response” (L. 23-26).\nI think that this statement of a long-term biological response (+1-3 months) is too speculative. It is mainly based on the relationship between the Chl-a inventory and surface Nd (Fig. 6f). However, the authors did not report any anomalies in Chl-a inventory relative to the pre-eruption period. Second, multiple sources of TMs are known in the study area, especially shallow hydrothermal vents that supply large amount of dissolved iron to the area, sustaining large phytoplankton blooms at the regional scale (Bonnet et al. 2023).\nA map showing the dispersion of the ash particles in the surface waters 1 to 3 months after deposition (the period studied here) would be much more useful than the map presented in Fig. 1b showing ash deposition 72-h after the eruption. A relationship between the volcanic ash distribution (after 1-3 month of dispersion) vs. Nd, Ce and/or REE anomalies would be helpful to show that the eruption and ash deposition represent the main mechanism responsible the Nd and REE anomalies observed in the area after 1-3 months.\nTable 1 – Nd, Ce anomaly and HREE enrichment values for shallow hydrothermal vents are missing in Table 1 (while stated L158-162). I am wondering if it’s an oversight, or if no data has been published yet? Please specify.\nLine 314-316. The estimation of the Nd input in the surface mixed layer requires vertical profiles of Nd in this 0-50 m layer to assess the vertical distribution of Nd.\nFig. 3. Please add the details of the linear model in the caption.\nAll the figures. Please define the depth range considered in the caption.\nL205. “increasing” instead of “increase”.\nReferences\nBarone, B., Letelier, R. M., Rubin, K. H. & Karl, D. M. Satellite Detection of a Massive Phytoplankton Bloom Following the 2022 Submarine Eruption of the Hunga Tonga‐Hunga Haʻapai Volcano. Geophys. Res. Lett. 49, e2022GL099293 680 (2022).\nBonnet, S. et al. Natural iron fertilization by shallow hydrothermal sources fuels diazotroph blooms in the ocean. Science, 380, 812–817 (2023).", "reviewer_3": "Dear authors,\nI have enjoyed reading a well written article presenting your results from the HTHH eruption.\nThe article has a great Nature Communications vibe, where the science meets a nice blend with communication to a more diverse public, thanks to it well written and thought process.\nNature Comms provides us reviewers with some questions and concerns which we should think about. In your case, the methodology is widely accepted and has a strong backing.\nI have made some short comments within the pdf file I was able to download from the application. I will also proceed to list them below.\nL44:I do not like the term \"ultra-oligotrophic\".\nI agree with the comment than the surface water of the SPG is the most oligotrophic in global oceans, with the lowest sea surface chl a concentrations (i.e. McClain et al., 2004; Morel et al., 2007). But the word oligotrophic already defines the lack of nutrients.\nL52: I have found some references stating 5 and some stating 6. Please check.\nL83: Change \"Ce\" to \"cerium, Ce\" like you did in L31 for Fe and Mn.\nL102: Please check if you also need to add a reference to: SO289\nL159: Could it possible be from ongoing submarine eruptions and hydrothermal activity i.e. during a 2018 cruise in the same area, we observed submarine bubbling in numerous sites, with bubbles reaching the surface in many locations.\nI guess not if we consider comment from L181? But with so many sources can we ever be certain?\nL224: Please round it to 600-2400 km. If you want, here you could comment on the signal compraed to other trace metal i.e. Fe in the EPR from Fitzsimmons or in the South Atlantic by Saito\nL297: This was also determined in hydrothermal plumes i.e. González-Santana et al., (2020) and Rusakov (2007).\nL306: Could it be a normal bloom in the area like the one observed by Tilliette et al., 2022 (i.e. Fig. 1)?\nL315: As an English as a second language reader, I will need a bit of help with this unit. Would you please refer to it as pmol/kg of SW or as pmol/kg of pumice (or ash).\nL326: The volcano will source Fe as Fe(II) which should be soluble (both in the aerial deposition and in the emission with the lava in contact with SW). This Fe is thought to be more bioavailable (at least it requires less energy to capture).\nL331: I think this should be \"potentially\" or \"could\", since there could be sorption associated to organic compounds attaching to the pumice.\nL382: Any reason of not filtering on-line directly from the teflon pump? So as to not have an extra step which could cause contramination (we use a T piece where one of the exits contains no filter but a valve where we can increase or decrease the flux through the second exit which contains the filter).\nL452: Do you also have the information for Limits of Detection?\nL455: Any idea on why this 1% continuos displacement?\nL471: Old nomenclature, change to \"ISO class 5\", page 59 of the GEOTRACES cookbook.\nL494: Why did you use water from another oceanic basin? There are potential changes caused by the water composition.\nCheers\nDavid González-Santana"}, "author_response": "Responses to reviewers for the manuscript: “Substantial trace metal input from the 2022 Hunga\nTonga-Hunga Ha’apai eruption into the South Pacific Gyre” by Zhang et al. Please note that\nresponses to reviewer comments are provided in blue below. All line numbers in responses refer\nto those in the tracked change version of the revised manuscript. References added during the\nrevision process are highlighted in yellow in the revised reference list.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nI enjoyed reading the manuscript by Zhang et al. on trace metal input from the 2022 Hunga\nTonga-Hunga Ha’apai eruption to the South Pacific Gyre. The dataset is impressive, experimental design\nwas strong, and the manuscript was well-written. I recommend moderate revisions but feel that the\npaper is publishable in Nature Communications – and indeed that the authors have some connections\nto draw at the end of the paper to make its impact even higher.\nThe difference in eNd and trace metals before and after the eruption was striking. I feel convinced that\nindeed, this eruption altered the REE and trace metal budgets of the surface of the SPG.\nWe thank the reviewer for their positive evaluation and constructive comments.\nI had one most significant issue reading the paper. It’s discussed in lines 218-228 and Figure 3. It\nseems like there’s a problem with the authors’ proposed hypothesis of scavenging followed by volcanic\nmaterial dissolution, because the trend of the data doesn’t lead towards either the HTHH lava or the\npumice endmember. The authors did not really explain why this is the case, or if they did I didn’t follow\nthe argument. There must be additional processes at play to explain their data. One option is that\ncontinuous scavenging pulls the Ce/Ce* to HREE/LREE slope downward away from the apparent\nHTTH lava/pumice endmembers. Another is that volcanic ash has a distinct Ce/Ce* to HREE/LREE\nratio from the lava and pumice.\nWe agree with the reviewer that the trend of the data does not pass exactly through either the HTHH\nlava nor the pumice endmember, and that additional processes may be considered to explain the data.\nWe regret that this was not addressed in the initially submitted version.\nAs the reviewer proposed, the preferential scavenging of LREE will result in an increase of the\nHREE/LREE ratio, which is reflected by a less steep slope of the Ce/Ce* versus HREE/LREE data\n(Fig.3). This is also consistent with our proposed hypothesis of Nd scavenging following volcanic\nmaterial dissolution, given that Nd is one of the LREE.\nWe consider the second mechanism proposed by the reviewer, namely that volcanic ash has a Ce/Ce*\nto HREE/LREE ratio distinct from the lava and pumice, is less likely the correct explanation. Given that\nboth the volcanic ash and pumice originate from the same source, a closely similar chemical\ncomposition can be expected.\nIn light of our observation of Nd scavenging following the dissolution of volcanic material, we favor the\nfirst mechanism put forth by the reviewer. This explanation has now also been incorporated into the\nmain text (Line 252-255):\n“The preferential removal of Nd and other LREEs during particle scavenging, following the\ndissolution of volcanic material, resulted in a downward shift in the Ce/Ce* to HREE/LREE slope.\nThis explains the deviation of the linear regression between Ce anomaly and HREE enrichment\nfrom the pumice endmember (Fig. 3).”\nLine by line comments:\nLines 80-82: Please add a citation for the scavenging signature on LREE/HREE patterns\nA reference to the publication “Elderfield, H. and Greaves, M.J. \"The rare earth elements in seawater.\"\nNature 296.5854 (1982): 214-219.” has been added (Line 83, 85).\nLine 110: Add “A” before “Large” and change amount to amounts\nRevised (Line 110).\nLine 119 and Figure 1: I found this trend difficult to see on Figure 1, especially figure 1c. Could the\nstations be added to this plot with the airmass trajectories in Figure 1c? Even if they aren’t labeled?\nWe used a purple line to illustrate the cruise track in Figure 1c, hence we consider it unnecessary to\nadd the stations to this small sub-figure. The airmass trajectories overall demonstrate an eastward\nmovement of the airmass providing evidence of the eastward dispersion of volcanic ash. This supports\nthe argument that the volcanic ash may have influenced a large area of the SPG surface waters\ntowards the east, which may also have been captured by our cruise.\nLine 145: At least a sentence discussing that scavenging and/or exchange processes must be\noccurring is needed here to maintain constant Nd concentrations while modifying the Nd isotopic\ncomposition – this is discussed in more detail later, but a sentence here would be helpful as well.\nWe thank the reviewer for this suggestion. Two sentences have been added to this section (Line 149-\n154) to provide a brief explanation of this observation:\n“However, surface water Nd concentrations in the western SPG remained consistently low (~4\npmol/kg), at levels comparable to previous measurements 36,37 (Fig. 2b). It can be posited that\nscavenging and/or exchange processes occurred to maintain a constant Nd concentration while\nmodifying the Nd isotopic composition. This will be discussed in greater detail in section “Rapid\nrelease and scavenging of Nd following the volcanic input”.”\nLine 151: This sentence doesn’t make sense – scavenging must be playing an important role here in\norder to have the eNd signature persist as far east as it does even though Ce/Ce* drops precipitously\nin this same region (eastern portion of the shaded region in Figure 2).\nWe agree with the reviewer here that scavenging also played a role following the dissolution of volcanic\nmaterial, as also outlined in our response to the reviewer’s major concern above. The sentence in\nquestion (Line 159-162) has now been deleted, and a discussion of the scavenging processes has\nbeen incorporated into the discussion (lines 252-255).\nLine 196-198: Seems like Australian dust range overlaps with the observed eNd\nThe εNd range of Australian dust (−29 to +2) exhibits a very small overlap with the observed surface\nwater εNd signatures (~0 to +1), due to the presence of samples from the Darling sub-basin (εNd ~-5\nto +2), which, when considered in aggregate, still displays a negative value in comparison to the\npositive surface seawater signal found in the western SPO. The εNd value for dust from all other areas\nof Australia is significantly lower than −2 due to the prevalence of Precambrian source rocks.\nEven with the upper bound of the εNd value (+2), assuming an initial surface seawater Nd\nconcentration of ~4 pmol/kg with an εNd value ~−2 in the western SPG, an addition of 12 pmol/kg of\nNd from the dust with an εNd value of +2 is required to obtain a surface εNd value of +1. This is,\nhowever, considered highly unlikely given that the observed Nd concentrations remained at a relatively\nlow level.\nFurthermore, the REE data do not corroborate a significant source contribution from the dust (Fig. 3)\nand, as discussed in Lines 208-211, the western part of the transect is located at a distance from the\nregion receiving long-range dust transport from Australia, which follows a southeastward trajectory\n(Wengler et al., 2019).\nLine 295-297: Pavia et al. 2020 (cited later by the authors in this paper) derive an Fe residence time\non the order of 0.75-6.5 years in the South Pacific Gyre – this would be inconsistent with the authors’\nargument for days to weeks residence times for Fe in this region, and I don’t think their argument is\ncorrect\nIn the estimation of Pavia et al. 2020, the Fe residence time is considerably shorter at the edge of the\ngyre (station 2; 0.75-2.0 years) compared to the centre of the gyre (station 8; 2.0-6.5 year). This\ndiscrepancy can be attributed to higher dust-derived dFe flux observed at the edge of the gyre. This\nindicates that in areas with high ash deposition, a large depositional flux of particles will result in a\nshorter dFe residence time.\nThe short residence times, with timescales of days to weeks, mentioned in Lines 310-312, correspond\nto dFe residence times reported for Atlantic surface waters that receive enhanced atmospheric inputs\nfrom the Saharan desert (Croot et al., 2004; Sarthou et al., 2003). Our intention was to utilize these\nexamples to posit the potential for a short residence time of dFe in the main ash deposition area.\nLines 297-300: The authors do not have supporting evidence to say that scavenging is more important\nthan biological uptake for Fe, or if they do, it isn’t discussed to support this statement.\nWe agree with the reviewer and have changed the sentence to (Lines 312-315):\n“Consequently, the more pronounced depletion of dFe in the main ash-deposition area (Fig. 6b),\ndespite the anticipated high input fluxes, is primarily a result of scavenging subsequent to the\nrelease and/or biological removal.”\nSection beginning on line 308: The authors include error bars on their Fe inputs from the HTHH\neruption, but not Nd – the Nd error bars should be included. The authors should also include error\nbars on their estimate of the depositional area, which is not an error free determination.\nWe concur with the reviewer and thank for this suggestion. An error bar has now been included for the\nNd flux considering both the uncertainty in the concentration of released Nd and the uncertainty in the\nsize of the depositional area.\n1) In evaluating the uncertainty of the concentration of the release Nd, the variability of the\nmeasured dissolved Nd isotope composition in the western SPG needs to be taken into\nconsideration. Assuming an initial surface seawater Nd concentration of ~4 pmol/kg with an\nεNd value ~−2 in the western SPG, an addition of 0.9-1.9 pmol/kg of Nd from the volcanogenic\nmaterial with an εNd value of +7.5 is required to obtain a surface εNd signaturew of ~0 to +1.\nLines 245-250 are revised accordingly.\n2) The NOAA HYSPLIT volcanic ash dispersion model indicates a mass loading area coverage\nof 5.4×106 km2 three days after the eruption (Fig. 1b), which can be considered to represent\nthe lower boundary of the ash deposition area. At the same time, our data demonstrate a\nsignificant increase of 2-3 εNd units between stations 33 and 44 (152°W to 172°E) in the\nwestern SPG. Based on this longitudinal coverage and the latitudinal range of 30° (20°S to\n50°S) indicated by the HYSPLIT model (Fig. 1b), we arrive at a maximum extent of the\ndeposition area of 1.2×107 km2. Lines 334-344 are revised accordingly.\nAccordingly, the range of the estimated Nd concentration increase as a consequence of the release\n(0.9-1.9 pmol/kg) combined with that of the ash deposition area (0.54-1.2×107 km2) indicates that the\nash-derived Nd flux was likely between 0.4-1.6 × 10⁸ g. Lines 344-346 are revised accordingly.\nThere is a units problem lurking somewhere in this section, it seems. On line 328, the authors state\nthere the HTTH eruption released 2.0e15 g of ejecta, and calculate a potential release of 5.8e8 moles\nof Fe. First, the authors end up with a ~30% error, but there’s multiple orders of magnitude error in the\nFe fractional solubility estimate. How is this incorporated and/or why isn’t it incorporated?\nThe potential release of Fe is estimated on the basis of a comprehensive assessment of the global\nmean Fe release of 200 ± 50 nmol Fe per gram of ash for subduction zone volcanic ash, as reported\nby Olgun et al. (2011). This estimation represents the amount of Fe that can be mobilized/solubilized\nupon contact with seawater per gram of ash. It is suggested that this figure is largely independent of\nthe bulk composition of the ash. The estimated fractional Fe solubility of 0.003-0.2% was derived from\ntheir estimation of the Fe release and is not relevant to our calculation.\nWe have now omitted citing the fractional solubility of iron to avoid any potential confusion (Line 356).\nLines 341-348: I think the authors have even more they could say about this – possibly connecting to\nDunlea et al. 2015’s estimates based on geochemical multivariate modeling that SPG sediments near\nthe core top have 20-25% rhyolitic ash in them, possibly speculating on the role episodic Fe input\n(from volcanic eruptions) might play in setting variable nitrogen fixation in the SPG, which responds to\nshort-term Fe supply (Guieu et al. 2014, Pavia et al. 2020) – this could cause a huge difference in\nlong, time-integrated estimates of nitrogen fixation here (say, from inversions of nutrient distributions)\ncompared to short-term nitrogen fixation rates derived from incubations.\nWe thank the reviewer for the suggestion to expand the implications of our budget calculation of trace\nmetals.\nEstablishing a link between large-scale iron budgets and nitrogen fixation rates has proven challenging\nso far. For example, the amount of nitrogen fixation facilitated by HTHH Fe input can be estimated\nusing the same calculation as the one employed by Pavia et al. (2020), in which the authors estimated\nthe nitrogen fixation rate stimulated by dust input to the SPG:\nWe assume all the solubilized/released Fe (5.8±1.5×108 mol) from the HTHH eruption is bioavailable.\nAs illustrated in Pavia et al. (2020), using cellular stoichiometries of Crocosphera as a representative\nof the dominant diazotroph in the SPG for (Fe in nitrogenase):C and C:N, we can derive an upper\nbound on N ‐fixation (N ) that can be supported by a given Fe flux:\n2 fix\nmoles cellular C moles cellular N\nN =F * *\nfix Fe,bio µmol Fe in Nitrogenase moles cellular C\n1 mol 1mol\n=5.8±1.5 ×108 mol* * =4.1±1.1×1011 mol =60±15 Tg N\n15.8 µmol 8.6 mol\nHowever, a number of the parameters employed in this calculation are subject to significant uncertainty,\nincluding the fraction of bioavailable erupted Fe, the dominant N₂-fixing organisms present, the\nproportion of Fe input that is bound in nitrogenase, the cellular stoichiometries, etc.\nWe therefore decided to not extend our estimation of Fe flux from the HTHH eruption to the nitrogen\nfixation rate, given the considerable uncertainty involved and the speculative nature of any conclusions\nthat might be drawn without further data on nitrogen fixation itself. Delving deeply into the implications\nfor nitrogen fixation is beyond the scope of the current manuscript. Further work is required to gain a\ndetailed understanding of the response of nitrogen fixation to such episodic volcanic input, in order to\nestimate the large-scale integrated nitrogen fixation rate. Nevertheless, we have added a sentence at\nthe end of this section:\n“In order to improve the accuracy of biogeochemical models in the Pacific Ocean, it is therefore\nnecessary to incorporate the episodic volcanic input of trace metals and to improve\nparameterization of the key chemical-biological processes that respond to volcanic inputs in the\nmodels.” (Line 379-382)\nFigure 3 legend: Change Astralian to Australian\nSorry for the mistake. We have fixed it.\nReviewer #2 (Remarks to the Author):\nThe manuscript submitted by Zhang et al. investigates the biogeochemical impact of the eruption of\nthe Hunga Tonga-Hunga Ha’apai volcano in January 2022. In this study, the authors report surface\nconcentrations and isotope composition of Neodymium (Nd), rare earth elements (REE) and trace\nmetals (TMs) collected in the South Pacific Ocean to demonstrate the impact of the eruption on the\nsurface ocean biogeochemistry. The dataset presented in this manuscript allows clearly demonstrating\nthe link between the HTHH volcano eruption and change - over a large area - in Nd and REE\ncharacteristics. However, due to the time lag between the eruption and sample collection (~1-3\nmonths), the trend in TMs concentrations is less clear. Furthermore, the link between TMs inputs and\na biological response is not convincing, reducing the overall impact of this study.\nThat said, the authors have done a great job to demonstrate Nd and REE inputs from this volcanic\nevent over a large area. Overall, it is an interesting study and I have only few concerns that need to\nbe addressed (see below).\nWe thank the reviewer for the positive comment on our work.\nIt is acknowledged that the time lag between the eruption and sample collection in the western SPG\n(9-10 weeks) likely resulted in a less clear trend in TMs concentrations, due to scavenging and/or\nbiological uptake. However, as shown in Figure S7, a comparison of surface trace metal\nconcentrations in the western SPG between our western transect and the two earlier GEOTRACES\ncruises, GP13 and GP19, revealed elevated surface TMs concentrations in the region 175°E-175°W\nfor our cruise, with a notable increase in dAl over an extended area. This documents that elevated\nTMs concentrations from the eruption were still present in the area during our cruise.\nWith regard to the biological response, we address this concern in the following two paragraphs.\nWhile a short-term biological response has been observed using satellite observations few days after\nthe eruption (e.g. Barone et al. 2023), the authors claim that “Increasing trace metal concentrations in\nsurface waters and chlorophyll-a inventories in euphotic layers between the eastern and western SPG\nfurther suggest that the volcanic eruption supplied (micro)nutrients stimulating a biological response”\n(L. 23-26).\nI think that this statement of a long-term biological response (+1-3 months) is too speculative. It is\nmainly based on the relationship between the Chl-a inventory and surface Nd (Fig. 6f). However, the\nauthors did not report any anomalies in Chl-a inventory relative to the pre-eruption period. Second,\nmultiple sources of TMs are known in the study area, especially shallow hydrothermal vents that supply\nlarge amount of dissolved iron to the area, sustaining large phytoplankton blooms at the regional scale\n(Bonnet et al. 2023).\nWe acknowledge that linking the chlorophyll-a concentrations measured on our research cruise to\neruption-derived micronutrient supply is challenging and subject to caveats and we felt that this was\nto some extent reflected in the wording in our initial manuscript. In order to add some additional support\nto the statements made, we conducted a further analysis of satellite-derived chlorophyll-a for the cruise\nperiod (March 2022) relative to a multi-decade March climatological average. A map of chlorophyll-a\nanomalies is provided below, which shows the relative change in chlorophyll-a concentrations in March\n2022 compared to the March average for the period 2002 to 2023 (i.e., (March_2022 - March_Average)\n/ March_Average). As illustrated in the map, a broad region with a positive chlorophyll-a anomaly is\nobserved along the western transect overlapping with our western stations, where the most significant\nvolcanic input signal was identified based on geochemical evidence. This map has now been included\nin the supplementary material as additional support for our statement that micronutrient supply from\nthe eruption may have led to enhanced chlorophyll-a concentrations (Fig. S8, Line 318-320):\n“In addition, a positive satellite-derived Chl-a anomaly for March 2022 was evident in the western\npart of the transect compared to the March average for the period between 2002 and 2023 (Fig.\nS8).”\nFigure S8. Map of satellite-derived chlorophyll-a anomalies showing the relative change in chlorophyll-a\nconcentrations in March 2022 compared to the March average for the period 2002 to 2023 in the South\nPacific Ocean. The satellite product employed was Aqua-MODIS Chlorophyll-a from NASA\n(https://oceancolor.gsfc.nasa.gov/).\nOur statement in the main text reads “Together these indicate a potential increase in phytoplankton\ngrowth in response to the volcanic input of dFe…” (Lines 320-321), which we feel reflects uncertainty\nin interpreting the observed chlorophyll-a variability. Furthermore, we have added a discussion to\nfurther emphasize the uncertainty in the main text (Lines 324-330):\n“Given that multiple sources of trace metals are known in the western SPG, confidently ascribing\nthis elevated phytoplankton biomass to volcanic dFe fertilization over and above other sources\nis challenging. However, as discussed above, volcanic input was identified as the dominant TM\nsource to surface waters, therefore we consider a linkage between volcanic input from the HTHH\neruption into the western SPG and the enhanced TChl-a inventory a plausible scenario.”\nIn addition, and for the reasons outlined above, the sentence in the abstract related to the biological\nresponse has been amended to include the term \"potentially\" (Line 26):\n“Increasing trace metal concentrations in surface waters and chlorophyll-a inventories in\neuphotic layers between the eastern and western SPG further suggest that the volcanic eruption\nsupplied (micro)nutrients potentially stimulating a biological response.”\nA map showing the dispersion of the ash particles in the surface waters 1 to 3 months after deposition\n(the period studied here) would be much more useful than the map presented in Fig. 1b showing ash\ndeposition 72-h after the eruption. A relationship between the volcanic ash distribution (after 1-3 month\nof dispersion) vs. Nd, Ce and/or REE anomalies would be helpful to show that the eruption and ash\ndeposition represent the main mechanism responsible the Nd and REE anomalies observed in the\narea after 1-3 months.\nIn addition to modelling the ash deposition 72 hours after the eruption, we have also modelled the air\nmass forward trajectories within two weeks after the HTHH eruption. This should represent the general\ndirection of ash dispersion following the air masses and covers the maximum time period that the\nNOAA HYSPLIT model is capable of addressing. It is not possible to run the model for an extended\nperiod of time, namely between one and three months.\nA map estimating the dispersion of the ash particles in surface ocean waters 1 to 3 months after\ndeposition would require the use of a more complex coupled atmospheric-ocean model, which is\nbeyond the scope of this work. In light of our extensive discussion on potential external inputs to SPG\nsurface waters, it can be concluded with a high degree of confidence that the volcanic input from the\nHTHH eruption represents the main mechanism responsible for the radiogenic Nd isotope signatures\nand heavy REE enrichment in the western SPG.\nTable 1 – Nd, Ce anomaly and HREE enrichment values for shallow hydrothermal vents are missing\nin Table 1 (while stated L158-162). I am wondering if it’s an oversight, or if no data has been published\nyet? Please specify.\nThe chemical compositions of fluids from different submarine hydrothermal systems along the\nKermadec arc show a large variability, both spatially and temporally (Kleint et al., 2019). Nd isotope\ncomposition and REE patterns of vent fluids from shallow hydrothermal vent fluids at the Kermadec\narc are not available, which is why this item is not included in Table 1.\nLine 314-316. The estimation of the Nd input in the surface mixed layer requires vertical profiles of Nd\nin this 0-50 m layer to assess the vertical distribution of Nd.\nThe vertical distribution of Nd isotopes and concentrations within the mixed layer is generally uniform\ndue to rapid mixing which homogenizes physical and chemical properties. In other words, due to wind-\ndriven mixing, the mixed layer generally shows uniform concentrations of nutrients and trace metals.\nConsequently, high-resolution large volume water sampling for Nd isotopes and corresponding Nd\nconcentrations throughout the mixed layer was deemed unnecessary and only surface water samples\nwere taken from the mixed layer.\nFig. 3. Please add the details of the linear model in the caption.\nThis information has been added to the caption of Fig.3 (Lines 621-622):\n“The dashed blue line and dotted blue lines represent the linear regression of Ce/Ce* and\nPAAS\nHREE/LREE in the surface water of the South Pacific Gyre (this study) and the 95%\nPAAS\nconfidence interval of the regression, respectively.”\nAll the figures. Please define the depth range considered in the caption.\nThe information has been added.\nL205. “increasing” instead of “increase”.\nWe think that it should say “increase” here, as it is a verb (Line 214).\nReferences\nBarone, B., Letelier, R. M., Rubin, K. H. & Karl, D. M. Satellite Detection of a Massive Phytoplankton\nBloom Following the 2022 Submarine Eruption of the Hunga Tonga‐Hunga Haʻapai Volcano. Geophys.\nRes. Lett. 49, e2022GL099293 680 (2022).\nBonnet, S. et al. Natural iron fertilization by shallow hydrothermal sources fuels diazotroph blooms in\nthe ocean. Science, 380, 812–817 (2023).\nReviewer #3 (Remarks to the Author):\nDear authors,\nI have enjoyed reading a well written article presenting your results from the HTHH eruption.\nThe article has a great Nature Communications vibe, where the science meets a nice blend with\ncommunication to a more diverse public, thanks to it well written and thought process.\nNature Comms provides us reviewers with some questions and concerns which we should think about.\nIn your case, the methodology is widely accepted and has a strong backing.\nI have made some short comments within the pdf file I was able to download from the application. I\nwill also proceed to list them below.\nWe are pleased with the positive assessment of this article by the reviewer.\nL44: I do not like the term \"ultra-oligotrophic\".\nI agree with the comment than the surface water of the SPG is the most oligotrophic in global oceans,\nwith the lowest sea surface chl a concentrations (i.e. McClain et al., 2004; Morel et al., 2007). But the\nword oligotrophic already defines the lack of nutrients.\nThe term “ultra-oligotrophic” has been modified to “oligotrophic” (Line 45).\nL52: I have found some references stating 5 and some stating 6. Please check.\nWe only find one peer-reviewed article (Terry et al., 2022) indicating a VEI of 5, which was referred to\na YouTube video “Jan 2022 eruption of Hunga Volcano, Tonga - first insights. by Shane Cronin”.\nIn the Global Volcanism Program (Smithsonian Institution), the VEI of the 2022 January 15 eruption is\nreported as 5 (https://volcano.si.edu/volcano.cfm?vn=243040&vtab=Eruptions). However, it is noted\nthat this estimation is “based on news reports/interviews describing work by Cronin et al.; will revise if\nneeded based on final published maps or papers.”\nAccordingly, we are inclined to cite the VEI estimate (5.8) from the peer-reviewed article (Poli and\nShapiro, 2022), which is based on analyses of seismic data from global seismic networks.\nL83: Change \"Ce\" to \"cerium, Ce\" like you did in L31 for Fe and Mn.\nRevised (Line 83)\nL102: Please check if you also need to add a reference to: SO289\nWe thank the reviewer for this reminder! It has been added now (Line 103).\nL159: Could it possible be from ongoing submarine eruptions and hydrothermal activity i.e. during a\n2018 cruise in the same area, we observed submarine bubbling in numerous sites, with bubbles\nreaching the surface in many locations.\nI guess not if we consider comment from L181? But with so many sources can we ever be certain?\nSubmarine bubble plumes, often occur above active submarine hydrothermal systems and cold seeps,\nare formed by bubbles of free gas rising through the water column. These are distinct from the floating\ntephra plume (e.g., pumice) that we observed in the surface water. Pumice is commonly produced by\nthe eruption of explosive volcanoes, and is created when super-heated, highly pressurized rock is\nrapidly ejected from a volcano.\nIndeed, silicic effusive eruptions have also been observed to produce pumice rafts, as for example\nwas the case during the 2012 Havre submarine eruption. However, as has been clearly discussed and\ndepicted in Manga et al. (2018) about the effusive submarine eruptions:\n“Large clasts (>1 m) rose to the sea surface without ingesting enough water to sink. Those large clasts\nwith sufficient isolated vesicles and/or trapped gas remained afloat in the raft. Smaller clasts would\nnot have reached the surface, ingesting water quickly and settling close to the vent, or were\ntransported by currents if small enough” (illustrated in figure below)\nThe pumice we observed, collected, and measured were small clasts (sizes of up to several\ncentimeters). Thus, it is more probable that they originated from a subaerial tephra plume and were\nproduced by an explosive volcanic eruption.\nL224: Please round it to 600-2400 km. If you want, here you could comment on the signal compared\nto other trace metal i.e. Fe in the EPR from Fitzsimmons or in the South Atlantic by Saito\nRevised as suggested (Line 233).\nWe thank the reviewer for suggesting such comparison. Nevertheless, we consider it unnecessary to\nmake a comparison between the redistribution of volcanic input by surface currents and the\nredistribution of mid-ocean ridge hydrothermal input by ocean circulation as these are two distinct\nprocesses.\nL297: This was also determined in hydrothermal plumes i.e. González-Santana et al., (2020) and\nRusakov (2007).\nWe thank the reviewer for this suggestion. However, the intention here is to create an analogue for\nthe Fe residence time in the surface mixed layer subsequent to intensive ash deposition (Lines 310-\n312). Therefore, we consider it inappropriate to use examples from hydrothermal plumes as analogues\nin this context.\nL306: Could it be a normal bloom in the area like the one observed by Tilliette et al., 2022 (i.e. Fig. 1)?\nAs discussed in Lines 213-226, no geochemical evidence for significant shallow hydrothermal inputs\nnear the Tonga-Kermadec arc was identified during our cruise. Therefore, we consider a linkage\nbetween volcanic input from the HTHH eruption into the western SPG and the enhanced TChl-a\ninventory more likely. However, we cannot attribute the increase in Chl-a inventory exclusively to\nvolcanic input, nor ascribe this elevated phytoplankton biomass to volcanic dFe fertilization over and\nabove other sources. Accordingly, we have toned down our assertion on the linkage between volcanic\ninput and the biological response. For further details, please refer to our response to Reviewer 2’s\nmajor comment.\nL315: As an English as a second language reader, I will need a bit of help with this unit. Would you\nplease refer to it as pmol/kg of SW or as pmol/kg of pumice (or ash).\nHere it means “of seawater”. This information has now been added (Line 247).\nL326: The volcano will source Fe as Fe(II) which should be soluble (both in the aerial deposition and\nin the emission with the lava in contact with SW). This Fe is thought to be more bioavailable (at least\nit requires less energy to capture).\nThe fractional Fe solubility of 0.003-0.2% was estimated by Olgun et al. (2011) based on geochemical\nexperiments conducted to determine the rapid release of Fe upon contact of pristine volcanic ash with\nseawater. In response to a comment from Reviewer 1, we have deleted this information (Line 356).\nL331: I think this should be \"potentially\" or \"could\", since there could be sorption associated to organic\ncompounds attaching to the pumice.\nWe have changed “likely” to “potentially” (Line 362).\nL382: Any reason of not filtering on-line directly from the Teflon pump? So as to not have an extra step\nwhich could cause contamination (we use a T piece where one of the exits contains no filter but a\nvalve where we can increase or decrease the flux through the second exit which contains the filter).\nThe online filtration of trace metals (Fe, Mn, Al, etc.) is described in a later section. This section focuses\non the sampling of large-volume samples for Nd isotope measurements (20-40L), which necessitate\na lengthy online filtration process. Additionally, Nd is less susceptible to contamination than other trace\nmetals, as evidenced by the low whole-procedure blank for Nd concentration (Line 439). The offline\nfiltration of Nd isotope samples is a widely accepted procedure, as outlined in the GEOTRACES\nprotocol (Cutter et al., 2010).\nL452: Do you also have the information for Limits of Detection?\nYes. We have added the Limits of Detection for all REEs to the dataset (Supplementary Data 1).\nL455: Any idea on why this 1% continues displacement?\nIsotope dilution combined with MC-ICP-MS and Sea-Fast preconcentration coupled with ICP-MS are\ntwo distinct methods to measure Nd concentrations. The former method is more precise and exhibits\na significantly smaller error (2SD<0.4%) compared to the latter (2SD=7.6%). However, the latter\nmethod is capable of measuring the concentrations of all rare earth elements simultaneously.\nThe objective of the comparison between these two different methods is to demonstrate the accuracy\nof our measurements and to evaluate the precision of the REE measurements based on the Sea-Fast\npreconcentration method. Our results show that the REE measurements are precise, with a difference\nof only 1% between the two methods. The discrepancy of 1% is estimated based on a linear regression\nmodel and is attributed to the inherent differences between the two methods. This small discrepancy\nmay be caused by a 1% evaporation during the SeaFast method or may be attributable to other factors.\nL471: Old nomenclature, change to \"ISO class 5\", page 59 of the GEOTRACES cookbook.\nRevised (Line 506).\nL494: Why did you use water from another oceanic basin? There are potential changes caused by the\nwater composition.\nA large volume of the surface water from the South Atlantic Gyre has been stored in our home lab. It\nis typically what is used as the base seawater for the standard addition method of Al concentration\nmeasurement due to its low concentrations of trace metals. Using this seawater ensures the\nconsistency of the matrix between samples and the standard curve.\nCheers\nDavid González-Santana"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am satisfied with the authors' revision and feel the manuscript is ready for publication.", "reviewer_2": "As stated in my first review report, this study brings important new insights into the impact of volcanic eruption on the surface ocean biogeochemistry. The authors have responded to all my comments and concerns. I therefore recommend publication of this study in Nature Communication."}, "author_response": ""}]

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