Ongoing and Archived
PROJECTS
GEOTRACES - GP17 (New)
We propose to measure silicon isotope distributions along a transect from Tahiti, French Polynesia to Punta Arenas, Chile scheduled for November 2022 to January 2023. Our goal is to test hypotheses related to predictions from models that processes in the Southern Ocean dictate silicon isotope distributions throughout the global ocean. The meridional overturning circulation both traps inflowing silicic acid within the Southern Ocean and partitions Si isotopes between mode and deep waters. Heavy Si isotopes are distilled out of the Southern Ocean in relatively shallow mode waters while light isotopes ride the northward flow of deep waters.
Individual aspects of these predictions have been examined, however, measurements of Si isotopes in GP17 will provide the first section that samples all relevant water masses synopticaly. Those distributions together with other data collected on GP17 will allow a more comprehensive evaluation of these mechanisms partitioning Si isotopes in the Southern Ocean.
GEOTRACES - GP15 (Ongoing)
In this project, δ30Si was measured along a north-south transect which passes directly through the North Pacific Silicic Acid Plume and key water masses of the Pacific Ocean. The hypothesis we are testing is that the global ocean δ30Si distribution is controlled by the interplay between the biological fractionation of Si isotopes by diatoms in surface waters, the remineralization of sinking opal and the meridional overturning circulation. Sampling the Northwest Pacific and the Cascadia Basin is key to resolve the apparent basin-scale variability in deep water δ30Si, also referred to as the deep Pacific “conundrum” due to their strong deviation from model predictions.
GP15 is ideal for testing this concept as it samples the ‘end’ of the MOC traversing the oldest waters with the highest silicic acid concentrations in the global ocean.
SNOWBIRDS (Ongoing)
Nitrogen and Silicon isotopes reconstructions of nutrient utilization in the Southern Ocean provide key constraints on the biological pump and large scale overturning circulation changes. During this project, I was in charge of the study of Si isotopes distribution both in the water column and sediments. I investigated the processes affecting the Si isotope fractionation of bSiO2 and porewater silicic acid during early diagenesis were investigated in marine sediments of the Southern Ocean by comparing the silicic acid and bSiO2 concentration data with their isotopic compositions from five sediment cores influenced by different biological production and sedimentation regimes.
I discovered that silicic acid concentrations and δ30Si signatures of Southern Ocean porewaters are the result of a dynamic balance between the dissolution of bSiO2, reactive lithogenic silica phases and Si re-precipitation that can be quantified. The relative contribution of the different processes involved differs significantly between regions.
Robinson et al. (2020)
Closset et al. (2022)
GEOTRACES - GN01 (Ongoing)
δ30Si of seawater samples was examined along a transect from the Bering Strait to the North Pole to evaluate mechanisms controlling the Si isotopic composition of the Arctic and waters exchanged with adjacent oceans. A preliminary isotope budget incorporating all available Arctic δ30Si data confirms the importance of isotopically heavy inflows in creating the anomalous deep Arctic δ30Si signature and reveals a surprising similarity in the δ30Si of the major inflows compared to outflows across the main gateways connecting the Arctic with the Pacific and the Atlantic. That balance implies a major role of biological productivity and opal burial in removing light isotopes entering the Arctic Ocean from rivers.
Data on the δ30Si of silicic acid obtained during GN01 greatly expands the Si isotope data set for the Arctic Ocean allowing further assessment of the processes controlling Si isotope distributions in this unique ocean.
Brzezinski et al. (2021)
The Blob (Archived)
Combining results from three different cruises, the relationship between bSiO2 production and mesoscale upwelling dynamics was investigated along the central region of the California Current System between 2013 and 2015, a period affected by a warm anomaly known as “the Blob”. Changes in the upwelling phenology caused by the Blob were investigated using an innovative index and related to patterns of diatom production during upwelling events to evaluate diatom resilience. Evaluating the relationship between production and environmental conditions reveals persistent biological hotspots characterized by high biomass, depleted nutrients, and high specific production rates.
Lower observed biogenic silica to Chlorophyll a ratios during the Blob suggested a taxonomic shift from siliceous to non-siliceous phytoplankton and/or lightly silicified diatoms signaling a change at the base of the food chain that could have ramifications for productivity in this eastern boundary coastal upwelling system.
Closset et al. (2021)
GEOTRACES - GP16 (Archived)
The distribution of dissolved silicon isotopes (δ30Si) was examined along the US GEOTRACES East Pacific Zonal Transect (GP16) extending from Peru to Tahiti (10°S and 15°S latitude). δ30Si was measured at two laboratories (GEOMAR and UCSB) and inter-calibration between samples with high and low [DSi] resulted in very good agreement (generally < 0.2 ‰; 2 s.d.). This study significantly expands the δ30Si data set for the Pacific and confirms past observations and models that suggest a homogenous δ30Si distribution below 1000 m in this region. This high degree of homogeneity is at odds with measurements in the North Pacific that show values that are both higher and lower than expectation.
Grasse et al. (2020)
ACIDD
The Across the Channel: Investigating Diel Dynamics (ACIDD) cruise abord R/V Sally Ride, was designed to examine fluctuations in microbial, chemical, and biological cycles on daily timescales in the Santa Barbara Channel, a site with decades of oceanographic research carried out on monthly timescales. Just one week before the cruise, the breakout of the Thomas Fire - at the time, it was the largest wildfire in California's recorded history - presented a rare opportunity to incorporate a direct investigation of how dry ash deposition impacts the Southern California coastal oceanic ecosystem, with a focus on microbial processes.
Bisson et al. (2019)
ALBION
This program focuses on the dynamic of dense water formation above the Adélie Land continental shelf and all the associated biogeochemical processes. I was in charge of the study of the influence of sea-ice on Si and C biogeochemical cycles using a multi-disciplinary and multi-proxy approach. Using isotopes, I discovered that the flux of particles above the shelf was composed by a mix between sea-ice and planktonic organisms and quantified for the first time their relative contribution to this export. The simultaneous analysis of seasonal evolution of C and N isotopic signatures of all these sample strengthen our observations with Si isotopes and provide insights in the biogeochemistry of the area.
These results have great implications on our understanding of the role of sea-ice in the control of polar biological activity and the answer of phytoplankton communities to warmer and fresher conditions in the context of past and future climate changes.
KEOPS-2 (Archived)
This international and multidisciplinary project focused on the impact of natural iron (Fe) fertilization on ecosystem and biogeochemical cycles in the Southern Ocean. During the 2-month cruise onboard the R/V Marion Dufresne in the vicinity of the Kerguelen Plateau (8 Oct - 30 Nov 2011), I simultaneously performed onboard incubations with 30Si labeling and sampled water and particles for natural Si isotopes analysis using Niskin bottles and in-situ pumps. My general objectives were to quantify Si fluxes and identify the seasonal variations of processes occurring both in the mixed layer and deep ocean in contrasted regions of the Southern ocean that undergo different level of Fe fertilization.
I have combined results from different isotopic approaches and sampling techniques to propose a closed budget of Si biogeochemical seasonal cycle in the Fe-fertilized area based on direct measurements.
Closset et al. (2014)
Cavagna et al. (2015)
Jacquet et al. (2015)
Closset et al. (2016)
SAZ-project (Archived)
This program, starting in 1997 by the Antarctic Cooperative Research Centre, is composed by repeated sections south of Australia focusing on the study of physical and biogeochemical processes that play a key role in our climate. In this context, I studied the seasonal evolution of the Si isotopic composition of sinking particles in different zones of the Southern Ocean. I have adapted existing technical methods to sediment trap samples for routine Si isotopes measurements and gave qualitative and quantitative information about the biological (e.g. consumption by phytoplankton) and physical (e.g. vertical mixing) processes that bring or remove Si from Antarctic surface waters.
I used for the first time Si isotopes to estimate and monitor seasonal variations of particles sinking velocities and develop a box model to investigate the processes controlling the Si isotopic signature of sinking diatoms and their significance related to paleo-oceanographic interpretations.
Closset et al. (2015)
Rigual-Hernandez et al. (2015)
Closset et al. (2019)