The Hunga Tonga-Hunga Ha'apai (HTHH) volcano's January 2022 eruption was one of the largest volcanic events ever recorded, discharging an estimated 2,900 teragrams of ejecta. A significant portion of this material entered the South Pacific Ocean, raising concerns about its potential impact on the region's delicate biogeochemical balance. The South Pacific Gyre (SPG), known for its oligotrophic nature and iron limitation, presents a particularly interesting case study. Previous research has established a link between major volcanic eruptions and ocean fertilization, particularly through the release of bioactive trace metals like iron (Fe) and manganese (Mn) from volcanic ash. These metals can stimulate phytoplankton growth, either directly by fertilizing nutrient-replete waters or indirectly by promoting nitrogen fixation in nitrate-depleted regions. The Pacific Ring of Fire's abundance of active volcanoes suggests airborne volcanic ash contributes significantly to atmospheric and oceanic iron supply. However, quantifying the large-scale impact of a specific eruption, particularly a submarine one like HTHH, on the SPG biogeochemistry remains a challenge due to difficulties in distinguishing volcanic inputs from other sources like eolian dust and weathering. This study aims to address this gap by examining the biogeochemical consequences of the HTHH eruption on the SPG using a combination of observational data and modeling.

Prior studies have demonstrated the potential for volcanic ash to fertilize the ocean surface. Frogner et al. (2001) and Duggen et al. (2007) highlighted the fertilizing potential of volcanic ash, linking it to phytoplankton growth. Olgun et al. (2011) quantified the role of airborne volcanic ash as an iron source for the Pacific Ocean, emphasizing its comparable contribution to mineral dust inputs. Other research, like that by Du et al. (2022), has explored links between past volcanic eruptions and broader climate changes. The challenge, however, is disentangling the effects of volcanic inputs from other sources affecting the SPG. While eolian aluminosilicate dust from continents and weathering inputs from volcanic islands are known factors, their contributions are difficult to separate from those of volcanic eruptions. Radiogenic neodymium isotopes and dissolved rare earth elements (REEs) are useful tools for tracing specific trace element sources in the ocean (Tachikawa et al., 2017; Lacan & Jeandel, 2001; Grenier et al., 2013; Van De Flierdt et al., 2016). The distinct isotopic signatures of different lithogenic materials allow researchers to distinguish between various sources of input to the ocean.

The study leveraged data from the GEOTRACES cruise GP21, conducted from February to April 2022, 39–79 days after the HTHH eruption. Samples were collected along a zonal transect across the SPG (26–32.5°S) at various depths using Niskin bottles and a trace-metal-clean near-surface sampling device. The NOAA HYSPLIT volcanic ash dispersion and trajectory models were employed to characterize ash deposition and atmospheric transport. The models provided insights into the spatial and temporal distribution of volcanic ash following the eruption, helping to pinpoint areas impacted by the event. Dissolved neodymium isotope compositions (εNd) and REE concentrations were determined using Multicollector-Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) to trace volcanic inputs. The researchers meticulously followed GEOTRACES protocols for sample handling and processing to minimize contamination and ensure the accuracy of the trace metal measurements. These protocols include filtering samples, acidification, co-precipitation with iron hydroxide, and chromatographic separation. In addition to the neodymium isotopes and REE, the team measured dissolved aluminum (dAl), manganese (dMn), and iron (dFe) concentrations using a combination of shipboard and laboratory methods. Surface chlorophyll-a (Chl-a) concentrations, a proxy for phytoplankton biomass, were analyzed using high-performance liquid chromatography (HPLC) to assess the biological response to the volcanic input of nutrients. Helium isotope data from selected stations were also collected and analyzed. Statistical analyses and correlation tests were used to explore the relationships between different geochemical parameters.

The study revealed a strong geochemical impact of the HTHH eruption in the western SPG, which appears to have extended to the central SPG. Surface water εNd values showed pronounced variability, with the most unradiogenic (negative) signal observed at 90°W, consistent with currents transporting subantarctic water. However, the western SPG showed more radiogenic (positive) εNd than in earlier studies, indicating a significant external input. This radiogenic signal is associated with a weakening of the Cerium (Ce) anomaly and strengthening of the Heavy REE enrichment (HREE/LREE), which are consistent with the dissolution of volcanic material and not other sources like Australian dust or weathering from volcanic islands. Concentrations of dissolved Al, Mn, and Fe showed increasing concentrations from East to West, consistent with the volcanic input. Dissolved Al exhibited a strong positive correlation with εNd, indicating a common volcanic source. However, dissolved Fe showed a depletion near the main ash deposition area, due to rapid scavenging. A weak positive correlation was found between surface εNd and total chlorophyll-a (TChl-a) inventory, suggesting a potential biological response to volcanic inputs. Quantitative estimates suggest the HTHH eruption released up to 0.16 kt of neodymium and 32 kt of iron into the SPG, amounts comparable to annual global dust fluxes.

The findings strongly implicate the HTHH eruption as a dominant source of trace metals in the western SPG. The more radiogenic εNd values, weaker Ce anomaly, and stronger HREE enrichment in surface waters are consistent with the dissolution of volcanogenic material and its redistribution by surface currents. The observed depletion of dissolved Fe, despite high input, is attributed to its rapid scavenging and biological uptake, highlighting the dynamic interplay between physical and biological processes. The positive correlation between surface εNd and TChl-a suggests that the volcanic input of trace metals, especially Fe which is essential for nutrient limitation, stimulated phytoplankton growth, potentially through enhanced nitrogen fixation. The study acknowledges that the time elapsed between the eruption and sampling might have affected the observation of peak phytoplankton biomass. Nevertheless, the results offer substantial evidence for a significant and widespread biogeochemical impact of the HTHH eruption on the SPG.

This study provides compelling evidence for a substantial input of trace metals from the 2022 Hunga Tonga-Hunga Ha'apai eruption into the South Pacific Gyre. The findings demonstrate the significant biogeochemical consequences of large volcanic eruptions, even submarine ones, emphasizing their influence on ocean productivity and nutrient cycling. Future research could focus on longer-term monitoring to capture the full extent of the eruption's impact, including the long-term release of iron from pumice. Further investigation into the interaction of volcanic inputs with existing nutrient limitation within the SPG, and the detailed exploration of species-specific responses to the eruption, will allow for a more complete understanding of the complex interplay.

The study's findings are based on a single GEOTRACES cruise conducted approximately 9-10 weeks after the eruption. This temporal gap might limit the ability to observe the peak biological responses and other short-lived processes following the event. The model used for extrapolation of the total metal release depends on several assumptions, including the extent of the ash deposition area, and the soluble fraction of metals contained in the ejecta. While the study uses various methods to minimize contamination, it acknowledges the inherent challenges in precisely measuring trace metal concentrations in seawater. These factors influence the uncertainty associated with quantitative estimates of trace metal release.