Temporal and spatial variability in the hydrothermal signature of sinking particles and sediments in the Western Tropical South Pacific Ocean

Abstract

Iron (Fe) is an essential micronutrient for phytoplankton, particularly diazotrophs, which are abundant in the Western Tropical South Pacific Ocean (WTSP). Their success depends on the numerous trace metals, particularly iron, released from shallow hydrothermal vents along the Tonga Arc. This study aimed to explore the impact of hydrothermal fluids on particulate trace metal concentrations and biological activity. To identify the composition of sinking particles across a wide area of the WTSP, we deployed sediment traps at various depths, both close and further west of the Tonga Arc. Seafloor sediments were cored at these deployment sites, including at a remote location in the South Pacific Gyre. The sinking particles were composed of a large amount of biological material, indicative of the high productivity of the Lau Basin. A significant portion of this material was lithogenic of hydrothermal origin, as revealed through Al-Fe-Mn tracing. The sinking material showed similar patterns between lithogenic and biogenic fractions, indicating that hydrothermal input within the photic layer triggered surface production. A hydrothermal fingerprint was suggested in the sediments due to the high sedimentation rates and the presence of large, heterogeneous, trace metal-rich particles. The presence of nearby active deep hydrothermal sources was suspected near the Lau Ridge due to the large particle size and the significant enrichment of Fe and Mn. Overall, this study revealed that deep and shallow hydrothermal sources along with submarine volcanism have a significant influence on the biogeochemical signature of particles in the Lau Basin at large spatial and temporal scales.

Quantifying N2 fixation and its contribution to export production near the Tonga-Kermadec Arc using nitrogen isotope budgets

“…The associated distance and time components of sinking PN to the deep trap potentially underestimates the importance of N 2 fixation to export production. Similar flux and isotopic composition offsets have been observed between shallow and deep sediment traps in the ETSP (Berelson et al, 2015;Knapp et al, 2016a;Tilliette et al, 2023).…”

“…Additionally, horizontal advection of particles generated at locations not impacted by the shallow hydrothermal vents potentially decouples the euphotic zone diazotrophic abundance and/or importance from the PN collected in the moored trap (Waniek et al, 2000). Indeed, the exported material in the deep trap was observed to be compositionally different from that captured in the shallow, short-term traps, where the hydrothermal signature of the particles was less evident due to organic matter remineralization while being transported to depth (Tilliette et al, 2023). The associated distance and time components of sinking PN to the deep trap potentially underestimates the importance of N 2 fixation to export production.…”

“…Unsurprisingly, the mass flux collected in the shallower, short-term PIT traps was higher and the d 15 N PNsink was lower than that collected in the deeper, moored trap. However, we note that the moored trap was deployed ~200 km west of the shallow traps and hydrothermal vents (Table 2) (Figures 1, 4), and the conical shape of the PPS5 has been observed to undersample (Baker et al, 2020;Tilliette et al, 2023). The d 15 N PNsink in the shallow PIT traps collected in the late spring at stations 5-2019 and 10-2019 ranged from -0.2 to -0.6‰ compared to a seasonal average d 15 N PNsink of 2.9 ± 0.5‰ and 1.5 ± 0.7‰ observed in the moored PPS5 trap during the spring and summer, respectively.…”

“…Additionally, a longterm Technicap PPS5 trap (1 m 2 collecting area, aspect ratio of 5.3) was deployed (containing the same buffered formaldehyde brine solution described above) at station 12-2019/4-2020 at 1000 m, collecting samples every 14 days (bimonthly) for 12 months from November 2019 to October 2020. Although sediment traps are a standard tool used to capture sinking particles, uncertainties remain in their collection efficiency within the water column and between trap designs (Buesseler et al, 2007;Baker et al, 2020;Tilliette et al, 2023). ] was determined using colorimetric analysis (Aminot and Kerouel, 2007) with a detection limit of 0.05 μM and a standard deviation (S.D.)…”