Xavier Crosta

• Ph.D.
• Directeur de Recherche at University of Bordeaux

The nitrogen isotopic composition of diatom frustule‐bound organic matter (δ¹⁵NDB) is often used to study changes in high latitude biological pump efficiency across glacial‐interglacial cycles, but the proxy may be biased by species‐specific effects. The genus Chaetoceros is of particular interest because of its abundance throughout ocean basins, i...

We here describe a distinctive weakly calcified holococcolithophore Crotalia jafari gen. et sp. nov. from the Southwest Indian Ocean phytoplankton. The coccospheres of Crotalia jafari measure 8–15 ?m in diameter, and possess 430–538 coccoliths, with each coccolith measuring less than 1 ?m in diameter. The coccoliths are tightly attached to each oth...

Antarctic fjords and coastal bays are excellent traps for sediment and represent key areas for high-resolution investigation of past environmental conditions. Robertson Bay is an understudied coastal area located at the confluence of the Ross Sea and the Southern Ocean. Recently obtained seafloor morphology data indicate the presence of a cross-she...

This paper describes a new monospecific polycystine radiolarian genus and its single species so far, Pirumosphaera armandae, found in the Southwest Pacific and Indian sectors of the Southern Ocean. The new spumellarian form is rarely found as a complete specimen, as only its first and second shells are commonly found. However, partially complete sp...

The Antarctic Circumpolar Current plays a pivotal role in global climate through its strong influence on the global overturning circulation, ocean heat and CO2 uptake. However, when and how the Antarctic Circumpolar Current reached its modern-like characteristics remains disputed. Here we present neodymium isotope and sortable silt records from sed...

The nitrogen isotopic composition of diatom frustule-bound organic matter (δ15NDB) is often used to study changes in high latitude biological pump efficiency across glacial-interglacial cycles, but the proxy may be biased by species-specific effects. The genus Chaetoceros is of particular interest because of its abundance throughout ocean basins, i...

The Pikialasorsuaq (North Water polynya) is an area of local and global cultural and ecological significance. However, over the last decades, the region has been subject to rapid warming, and in some recent years, the seasonal ice arch that has historically defined the polynya's northern boundary has failed to form. Both factors are deemed to alter...

Fires in Africa account for more than half of global fire-carbon emissions but the long-term evolution of fire activity and its link to global climate change remains elusive. Paleofire records provide descriptive information about fire changes through time, going beyond the range of satellite observations, although fire regime characteristics are c...

Radiolarians are microzooplankton that produce siliceous shells that preserve well in sediments and allow for paleo-reconstructions. Previous studies have used them for sea surface temperature (SST) reconstructions. However, radiolarians peak in abundances between 100 and 400 m in the Southern Ocean (SO), suggesting that their assemblages are more...

Antarctic sea ice forms a critical part of the Southern Ocean and global climate system. The behaviour of Antarctic sea ice throughout the last glacial-interglacial (G-IG) cycle (12 000–130 000 years) allows us to investigate the interactions between sea ice and climate under a large range of mean climate states. Understanding both temporal and spa...

Antarctic sea ice plays a critical role in the Earth system, influencing energy, heat and freshwater fluxes, air–sea gas exchange, ice shelf dynamics, ocean circulation, nutrient cycling, marine productivity and global carbon cycling. However, accurate simulation of recent sea-ice changes remains challenging and, therefore, projecting future sea-ic...

Southern hemispheric sea-ice impacts ocean circulation and the carbon exchange between the atmosphere and the ocean. Sea-ice is therefore one of the key processes in past and future climate change and variability. As climate models are the only tool available to project future climate change, it is important to assess their performance against obse...

Antarctic sea ice plays a critical role in the Earth system, influencing energy, heat, and freshwater fluxes, air-sea gas exchange, ice shelf dynamics, ocean circulation, nutrient cycling, marine productivity, and global carbon cycling. However, accurate simulation of recent sea-ice changes remains challenging, and therefore projecting future sea-i...

Cosmogenic nuclide dating of glacial landforms on the Kerguelen Archipelago (49°S, 69°E) gives the opportunity to study multi-millennial glacier fluctuations within the sub-Antarctic sector of the Indian Ocean. We here dated such geomorphic features to provide time constraints over the last 17,000 years using in situ-produced 36Cl in three glacial...

Sea ice expansion in the Southern Ocean is believed to have contributed to glacial–interglacial atmospheric CO2 variability by inhibiting air–sea gas exchange and influencing the ocean's meridional overturning circulation. However, limited data on past sea ice coverage over the last 140 ka (a complete glacial cycle) have hindered our ability to lin...

Antarctic sea ice forms a critical part of the Southern Ocean and global climate system. The behaviour of Antarctic sea ice throughout the last glacial-interglacial (G-IG) cycle (12,000–130,000 years) allows us to investigate the interactions between sea ice and climate under a large range of mean climate states. Understanding both temporal and spa...

Environmental conditions during Marine Isotope Stage (MIS) 5e (130–116 ka) represent an important “process analogue” for understanding the climatic responses to present and future anthropogenic warming. The response of Antarctic sea ice to global warming is particularly uncertain due to the short length of the observational record. Reconstructing A...

Radiolarians (holoplanktonic protozoa) preserved in marine sediments are commonly used as palaeoclimate proxies for reconstructing past Southern Ocean environments. Generating reconstructions of past climate based on microfossil abundances, such as radiolarians, requires a spatially and environmentally comprehensive reference dataset of modern cens...

Marine Isotope Stage (MIS) 5e (130–116 ka) represents an important ‘process analogue’ for understanding the climatic feedbacks and responses likely active under future anthropogenic warming. Reconstructing the Southern Ocean (SO) palaeoenvironment during MIS 5e and comparing it to the present day provides insights into the different responses of th...

The Antarctic coastal zone is an area of high primary productivity, particularly within coastal polynyas, where large phytoplankton blooms and drawdown of CO2 occur. Reconstruction of historical primary productivity changes and the associated driving factors could provide baseline insights on the role of these areas as sinks for atmospheric CO2, es...

Antarctic sea-ice extent, primary productivity and ocean circulation represent interconnected systems that form important components of the global carbon cycle. Subdecadal to centennial-scale variability can influence the characteristics and interactions of these systems, but observational records are too short to evaluate the impacts of this varia...

Sea ice expansion in the Southern Ocean is believed to have contributed to glacial-interglacial atmospheric CO2 variability by inhibiting air-sea gas exchange and influencing the ocean’s meridional overturning circulation. However, limited data on past sea ice coverage over the last 140 ka (a complete glacial cycle) have hindered our ability to lin...

Environmental conditions during Marine Isotope Stage (MIS) 5e (130–116 ka) represent an important ‘process analogue’ for understanding the climatic responses to present and future anthropogenic warming. The response of Antarctic sea ice to global warming is particularly uncertain due to the short length of the observational record. Reconstructing A...

High Arctic ecosystems and Indigenous livelihoods are tightly linked and exposed to climate change, yet assessing their sensitivity requires a long-term perspective. Here, we assess the vulnerability of the North Water polynya, a unique sea ice ecosystem that sustains the world’s northernmost Inuit communities and several keystone Arctic species. W...

Radiolarians (holoplanktonic Protozoa) preserved in marine sediments are commonly used as palaeoclimate proxies for reconstructing past Southern Ocean environments. Generating reconstructions of past climate based on microfossil abundances, such as radiolarians, requires a spatially and environmentally comprehensive reference dataset of modern cens...

Changes in water mass distribution are considered to be a significant contributor to the atmospheric CO2 concentration drop to around 186 ppm recorded during the Last Glacial Maximum (LGM). Yet simulating a glacial Atlantic Meridional Overturning Circulation (AMOC) in agreement with paleotracer data remains a challenge, with most models from previo...

In the Southern Ocean (SO), climate-driven latitudinal migrations of the Antarctic Circumpolar Current (ACC) frontal system impact large-scale ocean circulation and primary productivity. Latitudinal migrations may not have been identical in all SO basins due to the presence or absence of regional bathymetric obstacles. The Antarctic Polar Front (AP...

Plain Language Summary Documenting the causes and consequences of climate variability during past warmer‐than‐present periods is essential to provide benchmarks to scenarios of future, projected climate variability. It is especially true for the Southern Ocean (SO) region, where the atmosphere, ocean, sea ice, cryosphere, and carbon cycle interact...

Antarctic sea ice has paradoxically become more extensive over the past four decades despite a warming climate. The regional expression of this trend has been linked to changes in vertical redistribution of ocean heat and large-scale wind-field shifts. However, the short length of modern observations has hindered attempts to attribute this trend to...

Over recent decades Antarctic sea-ice extent has increased, alongside widespread ice shelf thinning and freshening of waters along the Antarctic margin. In contrast, Earth system models generally simulate a decrease in sea ice. Circulation of water masses beneath large-cavity ice shelves is not included in current Earth System models and may be a d...

Sea-ice cover over the Southern Ocean responds to and impacts Southern Ocean dynamics and, thus, mid to high latitude climate in the Southern Hemisphere. In addition, sea-ice cover can significantly modulate the carbon exchange between the atmosphere and the ocean. As climate models are the only tool available to project future climate changes, it...

The oceanography of the western Indian sector of the Southern Ocean is extremely complex due to the presence of several subantartic islands and plateaus that alter the zonal flow of the Antarctic Circumpolar Current. The circulation is even more complex around the Kerguelen Islands (KI) as the hydrological fronts merge with the Agulhas Return Curre...

Changes in water mass distribution are considered to be a significant contributor to the atmospheric CO2 concentration drop to around 186 ppm recorded during the Last Glacial Maximum (LGM). Yet simulating a glacial Atlantic Meridional Overturning Circulation (AMOC) in agreement with paleotracer data remains a challenge, with most models from previo...

Climate warming is rapidly reshaping the Arctic cryosphere and ocean conditions, with consequences for sea ice and pelagic productivity patterns affecting the entire marine food web. To predict how ongoing changes will impact Arctic marine ecosystems, concerted effort from various disciplines is required. Here, we contribute multi‐decadal reconstru...

Centennial- and millennial-scale variability of Southern Ocean temperature over the Holocene is poorly known, due to both short instrumental records and sparsely distributed high-resolution temperature reconstructions, with evidence for past temperature variations in the region coming mainly from ice core records. Here we present a high-resolution...

Antarctic sea ice plays a vital role in global climate via its impact on ocean circulation, biological productivity and CO2 partitioning between the ocean and the atmosphere. However, very little is known about its past history, especially in the southwestern Indian sector of the Southern Ocean (SO). We here provide new quantitative records of wint...

Ice core data have shown that atmospheric CO2 concentrations during interglacials were lower before the Mid‐Brunhes Event (MBE, ~430 ka), than after the MBE by around 30 ppm. To explain such a difference, it has been hypothesized that increased bottom water formation around Antarctica or reduced Atlantic Meridional Overturning Circulation (AMOC) co...

Abstract. The Antarctic coastal zone is an area of high primary productivity, particularly within coastal polynyas where large phytoplankton blooms and drawdown of CO₂ occur. Reconstruction of historical primary productivity changes, and the associated driving factors, could provide baseline insights on the role of these areas as sinks f...

Changes in ocean circulation and the biological carbon pump have been implicated as the drivers behind the rise in atmospheric CO2 across the last deglaciation; however, the processes involved remain uncertain. Previous records have hinted at a partitioning of deep ocean ventilation across the two major intervals of atmospheric CO2 rise, but the co...

The Last Abundant Appearance Datum of the endemic Southern Ocean diatom Hemisdiscus karstenii Jousé is dated at around 190,000 years Before Present, which provides a robust and convenient biostratigraphic marker to Late Pleistocene paleoceanographic studies. However, no prior studies have sought to understand what processes may have driven H. karst...

Abstract. Centennial and millennial scale variability of Southern Ocean temperature is poorly known, due to both short instrumental records and sparsely distributed high-resolution temperature reconstructions, with evidence for past temperature variability instead coming mainly from ice core records. Here we present a high-resolution (~ 60 year), d...

Abstract. Over recent decades Antarctic sea-ice extent has increased, alongside widespread ice shelf thinning and freshening of waters along the Antarctic margin. In contrast, Earth system models generally simulate a decrease in sea ice. Circulation of water masses beneath large cavity ice shelves is not included in current models and may be a driv...

The high abundances and high silicification levels of the diatoms Fragilariopsis kerguelensis (O'Meara) Hustedt and Thalassiosira lentiginosa (Janisch) Fryxell make them crucial silica carriers to the Southern Ocean (SO) sea-floor. However, their efficiency as silica vectors to the ocean bottom might be modulated by important variations in their si...

Dramatic changes in sea ice have been observed in both poles in recent decades. However, the observational period for sea ice is short, and the climate models tasked with predicting future change in sea ice struggle to capture the current Antarctic trends. Paleoclimate archives, from marine sedimentary records and coastal Antarctic ice cores, provi...

We present ²³⁰Th-normalized dust and export production fluxes for two contrasted marine sediment cores spanning the Antarctic Polar Front, close to the Kerguelen Plateau in the Southern Indian Ocean, covering the last glacial cycle. We report glacial lithogenic fluxes comparable to the South Atlantic and higher than in the South Pacific sectors of...

Water masses and depositional environments over the last 500 ka were reconstructed using absolute and relativeabundances of lithogenous, biogenous and redox-sensitive elements in four sediment cores from two channel-levee systems of the Wilkes Land continental slope (East Antarctica). Sediments older than the Mid-Bruhnesevent (MBE, 430 ka BP) show...

ABSTRACT: The families Calciosoleniaceae, Syracosphaeraceae and Rhabdosphaeraceae belong to the order Syracosphaerales and constitute a significant component of extant coccolithophore species, sharing similar ultrastructural bauplans. The literature on Calciosoleniaceae is reviewed and combined light- and scanning electron microscope studies of fou...

The recent thinning and retreat of Antarctic ice shelves has been attributed to both atmosphere and ocean warming. However, the lack of continuous, multi-year direct observations as well as limitations of climate and ice shelf models prevent a precise assessment on how the ocean forcing affects the fluctuations of a grounded and floating ice cap. H...

Figure S3. Relationship between valve width and apical length shows a much clearer separation than aspect ratio, and substantially less dependence on apical length.

Table S2. Valve width ranges of the three species when considering (a) only specimens identified in full agreement (unequivocal); (b) specimens identified as belonging to the species considered by the majority of participants (majority); and (c) by any single participant (single vote).

Figure S1. Validation of striae density measurement by SHERPA (on the x‐axis) versus measured manually (on the y‐axis). Black line: y=x. Red line: least squares regression line.

Figure S2. Dependence of identification agreement on apical valve length. The gray line represents the percentage of specimens within a 10 μm broad apical length range which received at least 90% identical taxonomic labels; the black dotted line depicts the absolute number of these cases within the 10 μm size window. The solid black line depicts th...

Figure S4. Eccentricity of the broadest valve position along the apical axis hardly depends on apical length, and is slightly higher (away from 0.5 on the y‐axis) in Fragilariopsis ritscheri than in the other two species.

Table S1. Apical valve length ranges of the three species when considering (a) only specimens identified in full agreement (unequivocal); (b) specimens identified as belonging to the species considered by the majority of participants (majority); and (c) by any single participant (single vote).

Table S3. Striae density ranges of the three species when considering (a) only specimens identified in full agreement (unequivocal); (b) specimens identified as belonging to the species considered by the majority of participants giving an identification for that specimen (majority); and (c) by any single participant (single vote).

Semi‐automated methods for microscopic image acquisition, image analysis and taxonomic identification have repeatedly received attention in diatom analysis. Less well studied is the question whether and how such methods might prove useful for clarifying the delimitation of species that are difficult to separate for human taxonomists. To try to answ...

A new method merges conventional and quantitative biostratigraphic approaches, supported by magnetic polarity data, to develop a chronological framework for sediment core MD03-2595 retrieved on the continental rise off Wilkes Land, East Antarctica before proceeding to paleoceanographic and paleoclimatic analysis. This combined method helped to iden...

Atmospheric CO2 levels during interglacials prior to the Mid-Brunhes Event (MBE, ∼ 430 kaBP) were around 40 ppm lower than after the MBE. The reasons for this difference remain unclear. A recent hypothesis proposed that changes in oceanic circulation, in response to different external forcings before and after the MBE, might have increased the ocea...

The causes of the recent increase in Antarctic sea ice extent, characterised by large regional contrasts and decadal variations, remain unclear. In the Ross Sea, where such a sea ice increase is reported, 50% of the sea ice is produced within wind-sustained latent-heat polynyas. Combining information from marine diatom records and sea salt sodium a...

We here investigate the spatial and temporal variability of eolian dust particle sorting recorded in the Dome B (77� 050 S, 94� 55’ E) ice core, central East Antarctica, during Marine Isotope Stage (MIS) 2. We address the question whether such changes reflect variable transport pathways from a unique source area or rather a variable apportionment f...

In the open Southern Ocean (SO), both modern and past size changes of the diatom Fragilariopsis kerguelensis appear to be strongly controlled by iron availability. Conversely, sea surface temperatures (SST) and sea ice seasonal dynamics take over in the seasonal sea-ice zone where iron is not limiting. No information exists on F. kerguelensis biome...

The Humboldt Current Ecosystem (HCE) off Peru yields about 10% of the global fish catch, producing more fish per unit area than any other region in the world. The high productivity is maintained by the upwelling of cold, nutrient-rich water from the oxygen minimum zone (OMZ), driven by strong trade winds. However, the potential impacts of climate c...

Changes in siliceous productivity in the eastern Equatorial Atlantic Ocean, off the Western African margin, over the last several glacial cycles have been either related to global-to-regional oceanographic changes (upwelling intensity) or climate changes (precipitation and river discharge). Based on diatom assemblages in core KZAI-02, located to th...

Atmospheric CO2 levels during interglacials prior to the Mid Bruhnes Event (MBE, ~ 430 ka BP) have lower values of around 40 ppm than after the MBE. The reasons for this difference remain unclear. A recent hypothesis proposed that changes in oceanic circulation, in response to differences in external forcing before and after the MBE, might have inc...

Understanding the causes of recent climatic trends and variability in the high-latitude Southern Hemisphere is hampered by a short instrumental record. Here, we analyse recent atmosphere, surface ocean and sea-ice observations in this region and assess their trends in the context of palaeoclimate records and climate model simulations. Over the 36-y...

The diatom Fragilariopsis kerguelensis (O′Meara) Hustedt is thought to be the main carrier of biogenic silica to the Southern Ocean sea-floor, thereby attracting most investigations of diatom biometry. Thalassiosira lentiginosa (Janisch) Fryxell, a large centric diatom which is the second most abundant species in the Southern Ocean, conversely rece...

Managing scientific data is a crucial issue for modern science. This concerns particularly the conservation of high value geological samples: cores. International scientific programs are leading an intense effort to solve this problem and propose detailed high standard work- and dataflows including core handling and curating. However, there is curr...

Diatoms account for a large proportion of primary productivity in Antarctic coastal and continental shelf zones. Diatoms, which have been used for a long time to infer past sea surface conditions in the Southern Ocean, have recently been associated with diatom-specific biomarkers (highly branched isoprenoids, HBI). Our study is one of the few sedim...