The accelerated melting of the Antarctic ice sheet due to global warming profoundly influences adjacent marine ecosystems. The delivery of fresh water from the ice sheet potentially represents a new source of key nutrients, including iron (Fe), a micronutrient known to limit phytoplankton primary production and the biological pump of CO2 in the Southern Ocean. But the bioavailability of Fe, provided mainly in the form of colloids and particles by glacial ice, is poorly understood. The main objectives of PIANO are to elucidate the role of marine prokaryotes as key engineers in the transformation of Fe delivered by Antarctic ice sheet melting and its consequences on their interactions with phytoplankton. PIANO aims to elucidate 1- the processes by which prokaryotes transform and acquire glacial Fe 2- the response of phytoplankton to Fe processed by prokaryotes and 3- the nature and strength of the Fe-mediated prokaryote-phytoplankton interactions. To fill these knowledge gaps the multidisciplinary team of PIANO will combine experimental observations using model organisms of prokaryotes and phytoplankton, a mechanistic model, and in situ observations throughout one year at a coastal Antarctic site influenced by ice melting. This latter will be achieved by the deployment of two innovative autonomous samplers. The identification of the pathways involved in the acquisition of glacial Fe by prokaryotes and the resulting interactions with phytoplankton together with the seasonal in situ dynamics of the respective genes and transcripts will provide original insights to the metabolic routes and microbial taxa involved in this key process. PIANO will thereby provide a better understanding of a key mechanism likely to influence the Southern Ocean ecological trajectory under climate change conditions.

Over a large part of Antarctica, the surface mass balance (SMB) is controlled by a few extreme events, resulting in a high natural variability of this parameter. In particular, extreme moisture intrusions linked to Southern Ocean Atmospheric Rivers (ARs) have been recently demonstrated to be major sources of both snow accumulation, heating and surface melt. Despite their key role, there is a general omission of AR variability, and more broadly of extreme events, in studies of past and future Antarctic climate and SMB. ARCA will assess the impact of ARs on the surface mass balance of Antarctica and will explore to what extent past AR activity can be recorded in ice cores. To reach this goal, ARCA is organized in 4 working packages. 1) ARCA will use recent novel numerical methodologies for identifying ARs applied to global and regional circulation models (GCMs and RCMs respectively). New algorithms will be applied to historical, present and future climate simulations. 2) ARCA will provide new field measurements of water stable isotopes and chemistry composition of snow precipitation and air masses from Adelie and Wilkes Lands, and 3) apply a regional scale modeling of water stable isotopes to interpret the signal observed in the field. 4) ARCA will finally revisit data from existing ice cores (aerosol content, e.g. sea salt, insoluble particles, water isotopes). Following this methodology, ARCA proposes to: 1) understand how natural variability and external forcings control the AR activity. 2) quantify AR moisture and heat transport towards Antarctica and their impacts on the SMB of Antarctica. 3) describe AR impact on the isotopic and aerosol contents of air masses transported through East Antarctica, 4) analyze the processes (e.g., moisture origin, sublimation of hydrometeors) producing characteristic signals in air masses during ARs, 5) estimate the induced bias in ice core records in regards to past temperature reconstructions. 6) Evaluate (qualitatively) past AR variability and the resulting bias in current estimates of past millenium climate in Antarctica. The ARCA project will deliver products that describe AR climatology and variability (occurrence maps, statistics), their atmospheric moisture signature (time-series of isotopes and aerosol content), and their impacts on Antarctic climate and SMB (through maps of induced melting and accumulation). Results will be presented for the 20th and 21st centuries, aiming in particular at projecting observationally constrained impacts of ARs on the SMB. ARCA will define a multi proxy approach to define how past AR could be retrieved in ice cores and provide a metric using water isotopic composition in ice cores to qualitatively define periods of higher and lower AR activity over the past millennium. Finally, ARCA will define the regions of Adelie and Wilkes Lands where ice cores should be drilled to best capture the AR and their influence in past climate variability. The ARCA consortium presents recognized experts from the IGE, LSCE and LOCEAN in particular in atmospheric modeling with polar-Regional Circulation Models and General Circulation Models, AR detection and estimation surface mass balance for Antarctica. The project will also rely on the broad expertise of the group in the interpretation of water isotopes and aerosol contents in air samples and ice cores.

The Western Tropical South Pacific (WTSP) Ocean has recently been identified as a hotspot of N2 fixation and harbors among the highest rates reported in the global ocean. N2-fixing organisms have high iron (Fe) quotas relative to non-diazotrophic plankton and their success in the WTSP has been attributed to the alleviation of Fe limitation in this region. However, our knowledge on Fe sources and distribution in the WTSP remains limited. During the OUTPACE cruise in 2015, the proposed team identified a shallow (<500 m) hydrothermal Fe source in the WTSP close to the Tonga volcanic Arc, which resulted in high concentrations (4-60 nM) of dissolved Fe (DFe) up to the photic (~0-150 m) layer. Such inputs are suspected (together with high sea surface temperature ~27-29°C) to trigger diazotroph blooms in the WTSP. However, the potential impact of such hydrothermal input on plankton communities and biogeochemical cycles of biogenic elements (carbon (C), Nitrogen (N), Phosphorus (P)) remains to be studied. In this context, the main objectives of the TONGA project are: -To accurately quantify Fe (and other biogeochemically relevant compounds) input from shallow (<500 m) submarine volcanoes and associated hydrothermal vents along the Tonga volcanic arc for the photic zone in comparison with atmospheric deposition, -To study the fate of shallow hydrothermal plumes in the water column at the local and regional scales, -To investigate the bioavailability and the potential impact of such hydrothermal inputs on planktonic communities and biogeochemical cycles in the WTSP To achieve this goal, we propose a multidisciplinary approach based both on a 37-day oceanographic cruise (R/V L’Atalante, approved for 2019 by the TGIR FOF) in the WTSP and modeling work. The TONGA consortium involves 85 scientists from 19 international institutions among which hydrothermal geochemists, physical oceanographers, trace element chemists, biogeochemists, biologists and modelers. The cruise will consist of 31 short (6h) stations and two 5-day process studies stations above 2 known active shallow submarine volcanoes. Hydrothermal tracers and the full suite of physical/hydrological/biogeochemical/biological parameters will be measured together with atmospheric survey. We will also deploy drifting and fixed mooring lines, turbulence profilers and ARGO floats to determine how shallow hydrothermal venting impacts trace elements and isotopes distributions in the mixed layer (Task 1), and how physical parameters impact the plume dilution and transport (Task 2). The potential effect of those inputs on plankton communities (fertilization vs toxicity) depends on their bioavailability that will be assessed through trace metal chemistry investigations. The impacts on ecosystem functioning and biogeochemical fluxes (Task 3) will be investigated based on stocks, fluxes and plankton diversity measurements across hydrothermal gradients, and complemented by means of mixing experiments with hydrothermal fluids in trace metal clean Climate Reactors recently-developed by the proposed team. These observations will fuel several modelling approaches and vice-versa: (1) a multi-scale modelling approach will allow to better characterize the local and regional dispersion of hydrothermal nutrients in the WTSP, (2) a new Lagrangian modelling approach will be used to quantify the transport of hydrothermal Fe to the photic layer and (3) a coupled dynamical PISCES model version that includes dynamic Fe binding ligands, colloidal Fe, alongside hydrothermal input will allow to characterize the role of hydrothermal Fe on primary production and export. TONGA has been endorsed as a GEOTRACES process study and received a letter of support from the IMBER international programme. It will significantly improve understanding several fundamental processes linked to the impacts of shallow hydrothermal sources on ecosystem functioning, a question that have not been addressed so far.