Crozet Plateau Research Articles

A New Tectonic Model Between the Madagascar Ridge and Del Cano Rise in the Indian Ocean

AbstractThe southern Indian Ocean has several prominent aseismic ridges recognized as oceanic large igneous provinces (i.e., the Madagascar Ridge, Del Cano Rise, Crozet Plateau, and Conrad Rise) in the off‐axis areas of the Southwest Indian Ridge (SWIR). However, previously obtained magnetic survey lines are sparse and not correctly aligned with the seafloor spreading direction; thus, the detailed spreading history, including the formation of these aseismic ridges, remains an open question. We reconstructed the tectonic history of two segments between the Discovery II and Gallieni fracture zones in the SWIR using newly obtained magnetic data (total and vector magnetic field) and an open‐source magnetic data set. We revealed that the southern Madagascar Ridge and the Del Cano Rise once formed a single bathymetric high and separated by at least Chron 30y, which is quite different from the global age model. In addition, the rises may have formed before Chron 34y, assuming an extinct ridge south of the Del Cano Rise. The two rises have been recognized as having formed by Marion hotspot plume‐induced excess volcanism around the active spreading ridge of the SWIR, which can explain locally isostatically compensated thicker‐than‐normal crust. However, linear magnetic anomalies have not been observed across the main part of these rises, suggesting that magmatic activity controlled by seafloor spreading is unlikely. Like other aseismic ridges in the southern Indian Ocean, these two rises may possibly have been formed partly by continental fragments rather than plume‐induced excess volcanism.

Evidence of a southward eddy corridor in the South-West Indian ocean

Mesoscale eddies and meanders have been shown to be one of the dominant sources of flow variability in the world׳s ocean. One example of an isolated eddy hotspot is the South-West Indian Ridge (SWIR). Several investigations have shown that the SWIR and the corresponding planetary potential vorticity field (f/H) exert a strong influence on the location and dynamics of the Antarctic Circumpolar Current (ACC), resulting in substantial fragmentation of the jets downstream of the ridge. The easterly extension of this eddy corridor appears to be restricted to the deep channel separating the Conrad Rise from the Del Cano and Crozet Plateau. However, while the fate of eddies formed at the SWIR has been widely investigated and the frontal character of this eastward extension is well known, the zone of diminishing variability that extends southwards to approximately 60°S remains poorly sampled. Using a combination of Argo, AVISO and NCEP/NCAR datasets, the character of this eddy corridor as a conduit for warm core eddies to move across the ACC into the Antarctic zone is investigated. In this study, we track a single warm-core eddy as it moves southwards from an original position of 31°E, 50°20′S to where it dissipates 10 months later in the Enderby Basin at 56°20′S. An Argo float entrained within the eddy confirms that its water masses are consistent with water found within the Antarctic Polar Frontal Zone north of the APF. Latent and sensible heat fluxes are on average 8W/m2 and 10W/m2 greater over the eddy than directly east of this feature. It is estimated that the eddy lost an average of 5W/m2 of latent heat and 5W/m2 of sensible heat over a 1-year period, an amount capable of melting approximately 0.92m of sea ice. In addition, using an eddy tracking algorithm a total of 28 eddies are identified propagating southwards, 25 of which are anti-cyclonic in rotation. Based on the new Argo float data, combined with AVISO and NCEP/NCAR datasets, these results suggest that the southward passage of warm-core eddies act as vehicles transporting heat, salt and biota southwards across the ACC and into the eastern boundary of the Weddell gyre.

Abyssal scavenging demersal fauna at two areas of contrasting productivity on the Subantarctic Crozet Plateau, southern Indian Ocean

The Crozet Plateau is situated below typical high-nutrient, low-chlorophyll (HNLC) waters of the southern Indian Ocean. The area to the east of the Crozet Islands experiences high levels of surface productivity during the austral summer due to natural iron enrichment from terrestrial sources and favourable light conditions. The demersal scavenging fauna at two areas of contrasting productivity, to the east and south of the islands, were investigated using two landers equipped with baited cameras and traps. Five species of scavenging fish were observed along with five groups of invertebrates during a single deployment of the RObust BIOdiversity (ROBIO) lander. Further deployments of the Fish RESPirometry (FRESP) video lander yielded no additional scavenging fish species. A modelled arrival and departure curve for the abyssal grenadier Coryphaenoides armatus suggests a region of low food availability compared to other regions worldwide. The ROBIO-derived abundance estimate for C. armatus of 187 ind. km–2 is comparable with published trawl-derived estimates. Significantly more amphipods were collected to the south of the islands, which was subject to lower organic matter supply. Reasons for this are unknown but may be due to differing current direction/velocities, or increased fish predation at the enriched site. The numerically dominant amphipod species present was Paralicella caperesca, followed by Eurythenes gryllus and Orchomenella gerulicorbis. A further five species were observed in low numbers, some occurring only once. One, Paracallisoma sp. nov, was a new species.

Geochemical heterogeneities within the Crozet hotspot

The Crozet Plateau is a 54 Ma-old volcanic plateau that supports five islands characterized by recent volcanic manifestations that are the surface expression of a deep-mantle plume. Due to their remote location and difficult access, the Crozet Islands are poorly sampled. Both the petrological descriptions and geochemical data are scarce. Thus, the sources of the Crozet plume are still under debate. Similarly, the interactions between the Southwest Indian Ridge (SWIR) and the Crozet plume remain questioned. Here, we present a new set of isotopes (Pb, Sr, Nd and He), major and trace elements data on basalts from three islands of the Crozet Archipelago: Penguins, East, and Possession Islands. Our main purpose is to characterize the sources of the Crozet plume and to test its influence at regional scale. Two groups of lavas can be distinguished based on the isotopic data: East and Possession lavas, and Penguins lavas. Principal component analyses on our high-precision Pb isotopes data and literature data show that two mantle sources can explain most of the geochemical variability measured in Crozet lavas. A third minor contribution is however needed to fully explain the data. The entire set of isotopic compositions (Pb, Sr, Nd and He) can be explained by a mixing between three mantle sources: (1) a FOZO (Focus Zone) component, with 206Pb/204Pb higher than 19.5 and high 207Pb/204Pb, 208Pb/204Pb 87Sr/86Sr, 143Nd/144Nd and R/Ra (R/Ra=(He3/He4)sample/(He3/He4)atmosphere) ratios, that is mainly sampled Penguins lavas, (2) a component called “East–Possession” that is mostly sampled by the East–Possession lava group and which presents Pb, Sr and Nd isotope signatures similar to those of the Reunion–Mauritius Islands, and (3) a third minor contribution of the local Depleted MORB Mantle (DMM). The new He isotopes data on the Crozet plume allow us to propose that Crozet plume material is present in the segment of the Southwest Indian Ridge located between the Indomed (ITF) and Gallieni (GTF) transform faults. This hypothesis is confirmed by a mixing model based on trace-elements and isotopes data from the ITF–GTF segment of the SWIR and Crozet. We propose that the shallow mantle below the ITF–GTF segment of the SWIR is contaminated by deep material from the Crozet plume.

The reproductive ecology of deep-sea ophiuroids around the Crozet plateau, Southern Indian ocean, under contrasting productivity regimes

The reproductive characteristics of six ophiuroid species, Ophiura irrorata loveni, Ophiura lienosa, Amphioplus daleus, Ophiacantha cosmica, Ophiernus quadrispinus and Ophioplexa condita, were studied at two localities in the vicinity of the Crozet Islands in the Southern Indian Ocean. The two localities were notable in having almost identical environmental characteristics other than major differences in surface-water primary productivity and organic matter flux. The two localities were only 460km apart. Three species were sufficiently abundant at the two sites to compare their reproductive biology under different productivity regimes. Ophiura irrorata loveni showed significant differences in oocyte size distributions and population size distributions between the two sites. The differences appeared to be related to the characteristics of organic matter flux at the two localities. Ophiacantha cosmica and Ophioplexa condita showed differences in abundance between the two sites. This appeared to be related to the availability of suitable substrata.

Signature of organic matter exported from naturally Fe-fertilised oceanic waters

Biomarker distributions and organic carbon and nitrogen isotopic signatures of organic matter (OM) produced in surface waters around the Crozet Plateau (Southern Ocean) are significantly different between a Fe-fertilised region (north) and a high nutrient low chlorophyll region (HNLC, south). If these OM signatures are exported to and preserved in surface sediments, they could potentially be used as palaeo-proxies for identifying Fe-fertilisation events in the past. Here, we assess the alteration of the OM signature through the water column and at the sediment–water interface by comparing organic signatures in surface waters, sediment traps and surface sediments. Our results suggest that there is significant degradation of OM during transport to the sea floor, which causes reduced fluxes of biomarkers to sediments north and east of the Crozet Plateau. Sterols, alkenones and C27 and C29 12-hydroxymethyl alkanoates, and C28 1,14-diols appear to be less labile than total organic carbon (TOC), except to the north, where alkenones and sterols are more rapidly degraded than TOC. Sedimentary bulk and compound specific δ13C values also reflect surface water productivity patterns, with elevated values occurring in sediments underlying the Fe-fertilised waters. In contrast, δ15N values appear to be strongly biased by degradation and grazing during export and burial. Thus, only some of the differences observed in surface waters between the Fe- fertilised and HNLC areas are exported to deep waters and preserved in the sedimentary record, suggesting that caution is required in the application of these proxies to studies of ocean palaeoproductivity.

Abyssal demersal fish fauna composition in two contrasting productivity regions of the Crozet Plateau, Southern Indian Ocean

Superimposed on the topographically uniform Crozet Plateau is an annually occurring pronounced gradient of surface productivity during the austral summer. Primary productivity is enhanced due to leached dissolved iron from the Crozet Islands resulting in high nutrient input to the seafloor east of the islands (+Fe; M5), and low nutrient input in the south (HNLC; M6). The demersal fish community at the +Fe site, 4160m, depth, and the HNLC site, 4190m depth, were sampled by trawl. Demersal fish species richness was greater at the +Fe site with 14 vs. 10 demersal fish species. The same major taxonomic groups were present at both sites, with the macrourids dominating both abundance and biomass. At both sites the macrourids and Bathypterois oddi dominate numerically, and the macrourids dominate biomass. Biomass was 374kgkm−2 at +Fe and 199kgkm−2 at HNLC and abundances were 1336 fishkm−2, and 881 fishkm−2, respectively. Despite elevated demersal fish biomass, biodiversity and abundance at +Fe compared with HNLC, an imbalance in sampling regime resulted in no statistical difference between the two sites, calling in to question current deep demersal fish sampling on multidisciplinary cruises.

Quantifying export production in the Southern Ocean: Implications for the Baxs proxy

The water column and sedimentary Baxs distribution around the Crozet Plateau is used to decipher the controls and timing of barite formation and to evaluate how export production signals are recorded in sediments underlying a region of natural Fe fertilization within the Fe limited Southern Ocean. Export production estimated from preserved, vertical sedimentary Baxs accumulation rates are compared with published export fluxes assessed from an integrated study of the biological carbon pump to determine the validity of Baxs as a quantitative proxy under different Fe supply conditions typical of the Southern Ocean. Detailed assessment of the geochemical partitioning of Ba in sediments and the lithogenic end‐member allows appropriate correction of the bulk Ba content and determination of the Baxs content of sediments and suspended particles. The upper water column distribution of Baxs is extremely heterogeneous spatially and temporally. Organic carbon/Baxs ratios in deep traps from the Fe fertilized region are similar to other oceanic settings allowing quantification of the inferred carbon export based on established algorithms. There appears to be some decoupling of POC and Ba export in the Fe limited region south of the Plateau. The export production across the Crozet Plateau inferred from the Baxs sedimentary proxy indicates that the Fe fertilized area to the north of the Plateau experiences enhanced export relative to equivalent Southern Ocean settings throughout the Holocene and that this influence may also have impacted the site to the south for significant periods. This interpretation is corroborated by alternative productivity proxies (opal accumulation, 231Paxs/230Thxs). Baxs can be used to quantify export production in complex settings such as naturally Fe‐fertilized (volcanoclastic) areas, providing appropriate lithogenic correction is undertaken, and sediment focusing is corrected for along with evaluation of barite preservation.

Productivity variation around the Crozet Plateau: A naturally iron fertilised area of the Southern Ocean

The Crozet Plateau is an area of interest as it is a region of the Southern Ocean where annual primary productivity is relatively high. We describe spatial and temporal variations in the biomarker distributions and carbon and nitrogen isotopic signatures of organic matter formed within surface waters around the plateau during an algal bloom. In the centre of the bloom, diatom-derived sterols dominate, whereas alkenones are relatively more abundant outside and at the edge of the bloom. Bulk organic δ 13C and δ 15N values are enriched inside the bloom area, ranging from −18.7‰ to −17.6‰ and +1.6‰ to +3.7‰, respectively, compared to −25.7‰ to −20.9‰ and −2.0‰ to +1.3‰, respectively, outside the bloom area. Variation in bulk δ 15N values is attributed primarily to NH 3 or dissolved organic N uptake, with only a minor contribution from Rayleigh fractionation of the NO 3 - pool. Compound specific δ 13C values (cholest-5-en-3β-ol, cholesta-5,22-dien-3β-ol, 24-methylcholesta-5,22-dien-3β-ol, 24-methylcholesta-24(28)-dien-3β-ol and alkenones) are also enriched in the Fe fertilised region relative to other areas, presumably reflecting higher phytoplankton growth rate, consumption of dissolved inorganic carbon and perhaps more intense use of carbon concentrating mechanisms. The observations confirm previous interpretations that Fe fertilisation in the Crozet region has affected nutrient utilisation and the algal community structure; it also suggests that lipid biomarkers and their δ 13C values record a specific bloom signature that may be recorded in underlying sediments.

Особенности структуры поля температуры и крупномасштабных фронтов на поверхности Индоокеанского сектора Южного океана в районах неоднородности рельефа дна

Abstract On the base of monthly averaged SST satellite data (AVHRR Ocean Pathfіnder Data JPL NOAA/NASA) the space-time variability of SST field and large-scale fronts on the South Indian Ocean surface is investigated in the regions with pronounced bottom configuration inhomogeneities. Shown, that the structure and characteristics of climatic seasonal cycle of SST field and hydrological fronts have the regional features above the Kerguelen ridge, Crozet plateau, Australian-Antarctic rise and others bottom relief forms.

Bathymetry and frontal system interactions influence seasonal foraging movements of lactating subantarctic fur seals from Marion Island

Sixteen lactating subantarctic fur seals Arctocephalus tropicalis were satellite-tracked during the winter of 2006 (n = 6), summer of 2006/07 (n = 6) and autumn/winter (n = 4) of 2007, from Marion Island, Southern Ocean. Despite varied individual movement patterns, a favoured foraging area lay to the northeast of the island. In contrast to findings for populations at similar latitudes, seals from Marion Island did not undertake short overnight foraging trips, but trips consistently went beyond 300 km from the island. This aligns with the at-sea duration of lactating seals’ foraging trips from temperate Amsterdam Island, but differs from subantarctic Crozet and Macquarie islands. Time spent at sea, maximum distances travelled and movement variation of tracks from the island varied seasonally. Faecal analysis suggests the diet comprised primarily myctophid fish with limited seasonal variation. Well-defined areas of restricted movement coincided with significant bathymetric features to the west/northwest of the Crozet Plateau, with the Del Cano Rise clearly being important. Positive and negative sea-surface height anomalies (compared to the mean) appeared to be preferred by most seals across seasons. Higher summer sea-surface temperatures correlated with the movements of some seals. Higher chlorophyll a concentrations dictated transit and foraging areas during summer. Bathymetrically influenced oceanographic variables likely explain these preferred long-distance eastward movements. The Iles Crozet and Marion island subantarctic fur seals differ in their foraging ecology despite being neighbours. Conversely, the subantarctic fur seal populations from the distant Amsterdam and Marion islands appear to be similarly influenced by such environmental factors.

Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 1. Mean circumpolar paths

High resolution hydrographic sections and maps of the gradient of sea surface height (SSH) reveal that the Antarctic Circumpolar Current (ACC) consists of multiple jets or frontal filaments. Here we use a 15 year time series of SSH observations to determine the circumpolar structure and distribution of the ACC fronts. The jets are consistently aligned with particular streamlines along the entire circumpolar path, confirming and extending the results of an earlier study restricted to the region south of Australia. The intensity of the fronts (as measured by the cross‐front gradient of SSH) varies along the fronts and the individual branches merge and diverge, often in response to interactions with bathymetry. Maps of absolute velocity at 1000 m depth derived from Argo trajectories confirm the existence of multiple current cores throughout the Southern Ocean. High resolution hydrographic sections and profiles of temperature and salinity from Argo floats are used to show that the front locations derived from fitting SSH contours to maps of SSH gradient are consistent with locations inferred from the traditional criteria based on water mass properties, suitably modified to account for multiple frontal branches. Three regions are examined in detail: the Crozet Plateau, the Kerguelen Plateau and the Scotia Sea. These examples show how recognition of the multiple jets of the ACC can help resolve discrepancies between previous studies of ACC fronts.

Pore-fluid Fe isotopes reflect the extent of benthic Fe redox recycling: Evidence from continental shelf and deep-sea sediments

Pore-fluid Fe isotopes may be a unique tracer of sediment respiration by dissimilatory Fe-reducing bacteria, but to date, pore-fluid Fe isotope measurements have been restricted to continental shelf settings. Here, we present δ56Fe values of pore fluids from two distinct sedimentary settings: (1) a riverine-dominated site on the northern California margin (Eel River shelf; 120 m water depth) and (2) biogenic opal-rich volcaniclastic deep-sea sediments from the Southern Ocean (north and south of the Crozet Plateau; 3000–4000 m water depth). The Fe isotope compositions of Crozet region pore fluids are significantly less fractionated (δ56Fe = +0.12‰ to −0.01‰) than the Eel River shelf (δ56Fe = −0.65‰ to −3.40‰) and previous studies of pore-fluid Fe isotopes, relative to average igneous rocks. Our data represent the first measurements of Fe isotope compositions in pore fluids from deep-sea sediments. A comparison of pore-fluid δ56Fe with the relative abundance of highly labile Fe in the reactive sedimentary Fe pool demonstrates that the composition of Fe isotopes in the pore fluids reflects the different extent of sedimentary Fe redox recycling between these sites.

Psychropotid holothurians (Echinodermata: Holothuroidea: Elasipodida) collected at abyssal depths from around the Crozet Plateau in the Southern Indian Ocean

Seven species of holothurians of the family Psychropotidae were collected during the cruise D300 of RRS Discovery to Crozet plateau. Three of them, Benthodytes wolffi, Psycheotrephes discoveryi and Psychropotes xenochromata are described as new to science. One species, Benthodytes abyssicola Théel, regarded recently as incertae sedis, is redescribed with designation of a lectotype. Additional notes are provided to descriptions of Psychropotes scotiae (Vaney) known from two specimens and Psychropotes longicauda Théel, the cosmopolitan highly variable species.

Gebrukothuria profundus, a new genus and species of laetmogonid holothurian (Elasipodida, Laetmogonidae) from around the Crozet Plateau in the Southern Indian Ocea

A new genus and species of laetmogonid holothurian (Elasipodida, Laetmogonidae), collected from around the Crozet Plateau in the Southern Indian Ocean, is described. It differs from other members of the family in that the body wall lacks the wheel-shaped calcareous deposits completely. Instead only rods are present. The genus is also distinguished by the combination of other morphological characters lacking in other known genera: absence of circum-oral and ventrolateral papillae together with development of midventral tube feet. All other members of the family Laetmogonidae are known to have wheel-shaped deposits, therefore diagnosis of the family is refined.

Algal biomarkers in surface waters around the Crozet plateau

The region around the Crozet plateau (Southern ocean) exhibits enhanced annual primary productivity, occurring during a distinct spring bloom, due to natural iron fertilisation of the macro nutrient-replete polar frontal zone. We describe differences in algal biomarker abundances and distributions, with particular emphasis on alkenones and sterols, around the Crozet plateau during an algal bloom (December 2005 to January 2006). In the centre of the bloom, diatom-derived sterols dominate, whereas alkenones are proportionally more abundant at lower productivity times or sites. This is consistent with biomarker analyses for other settings and suggests that high productivity and high nutrient, low chlorophyll (HNLC) regimes can be distinguished on the basis of algal community structure and the associated lipid biomarker assemblages.

Phytoplankton community composition around the Crozet Plateau, with emphasis on diatoms and Phaeocystis

Phytoplankton community composition has important implications for upper-ocean biogeochemistry and export production. The species composition of the phytoplankton community (diatoms, Phaeocystis antarctica) was examined around the Crozet Plateau (Southern Ocean) during the CROZEX project in order to address spatial changes in species composition. Phytoplankton biomass was dominated by P. antarctica to the north of the Crozet Plateau, while south of the plateau a diverse community including several large diatom species (e.g., Corethron pennatum) occurred. The phytoplankton community in the vicinity of the Crozet Plateau and Crozet islands included a mixture of P. antarctica and Thalassionema nitzschioides. High numbers of empty diatom frustules characterised many of the sampling sites, with similar species present in the water column as both full and empty cells. Multivariate analysis was used to analyse both the level of similarity between the different sampling sites and the co-occurrence of species. Cluster analysis of the sampling sites showed distinct clusters that differed in terms of species composition and environmental parameters. Recurrent species analysis showed groupings which relate to phytoplankton cell size; groupings separated (i) small (∼15μm) diatom species, (ii) medium (∼35μm) and large (∼90μm) diatom species as well as P. antarctica colonies, and (iii) giant (∼300μm) diatoms. The abundance and distribution of the different floral groups identified by multivariate analysis are linked to the different environmental regimes, although the relationships are different for small and large species due to the interaction between factors important for phytoplankton growth (e.g., light, nutrients, iron) and mortality (e.g., grazing).

Large-scale circulation around the Crozet Plateau controls an annual phytoplankton bloom in the Crozet Basin

The circulation in the vicinity of the Crozet Plateau in the Southwest Indian Ocean sector of the Southern Ocean is examined using hydrographic sections, Argo floats, surface drifters, and satellite altimetry. All four techniques confirm that a major branch of the Antarctic Circumpolar Current, the SubAntarctic Front (SAF), flows anticyclonically round the Del Cano Rise west of the Crozet Plateau, i.e. eastward to the south of the Del Cano Rise, then northward and sometimes northwestward into the Crozet Basin, before turning back eastward in a combined front with the Agulhas Return Current and the SubTropical Front. This S-bend in the SAF is a permanent feature, controlled by the bathymetry, as has been inferred previously by Pollard and Read [Pollard, R.T., Read, J.F., 2001. Circulation pathways and transports of the Southern Ocean in the vicinity of the Southwest Indian Ridge. Journal of Geophysical Research, 106(C2), 2881–2898]. Similar, but much weaker, anticyclonic flow is found round the Crozet Plateau itself, with no more than 5?10×106 m3 s?1 turning north to the east of the Crozet Islands. Circulation north of the Crozet Plateau, between the Plateau and the S-bend of the SAF, is extremely weak, fed only by anticyclonic meanders breaking off the SAF into the area from the west or north, and occasional input from the northward, partially wind-driven (i.e. Ekman) flow south and east of the islands. In consequence of the weak circulation, dissolved iron from the land or sediments of the Crozet Plateau and Islands can build up during the winter in the Polar Frontal Zone between Crozet and the SAF, which gives rise to an annual bloom in this area. Biological evidence from satellite images, and from phytoplankton and zooplankton distributions, supports the circulation pattern we develop. This pattern confirms that patchiness of productivity in the bloom area results from close juxtaposition of water that has entered the area from the west from the SAF and from the south and east after flowing past the islands. Flow from the south and east has had a chance to entrain iron from the islands and sediments whereas flow from the west has not.

Phytoplankton productivity and community structure in the vicinity of the Crozet Plateau during austral summer 2004/2005

The CROZet natural iron bloom and EXport experiment (CROZEX) was carried out around the Crozet Plateau (Southern Ocean) in late austral spring-early summer 2004/2005. The primary aim of this study was to test the hypothesis that natural Fe fertilisation from the islands enhanced primary production to the north of the plateau, leading to elevated carbon export. During the 12-week shipboard study (November–January), size-fractionated chlorophyll-a (chl-a), primary productivity and phytoplankton pigments were determined at 20 stations around the Crozet Plateau, both inside and outside the expected bloom region. Satellite-derived chl-a data from August 2004 to April 2005 revealed that the cruise period covered the declining stage of a large phytoplankton bloom north of the Crozet Plateau. Productivity derived from satellite chl-a data showed a strong north–south gradient, with productivity reaching a maximum of 1.9 g C m?2 d?1 in the north (late October), 0.6 over the plateau (late March) and 0.4 in the south (early December). From November to early December, measured chl-a and productivity were still relatively high northwest of the plateau (54–74 mg chl-a m?2 and 0.5–1.1 g C m?2 d?1), the assemblage being dominated by Phaeocystis sp. and microplankton representing 50% of total chl-a. Declining biomass and productivity to the northwest in mid-December was associated with a picoplankton (52%) dominated community (19–38 mg chl-a m?2 and 0.1–0.4 g C m?2 d?1). In contrast, at the north eastern edge of the plateau, a mixed diatom/Phaeocystis dominated bloom (229 mg chl-a m?2 and 3 g C m?2 d?1) developed in mid-January. South of the plateau in the control high-nutrient low-chlorophyll (HNLC) region, biomass and productivity were always significantly lower, with only a small chl-a peak in December, and a community composed of microflagellates, cyanobacteria and some giant diatoms. Particulate organic carbon (POC):chl-a ratios provided physiological evidence for the impact of Fe fertilisation north of the plateau. The assimilation rate (i.e. integrated productivity normalised to integrated chl-a, ?P/?chl-a) did not show significant differences between the north and south, probably due to the late sampling period relative to the bloom peak. Overall, measurements of primary productivity during CROZEX supported the hypothesis of enhanced biomass and production due to natural iron enrichment of the region north of the Crozet Plateau, while south of the plateau, low biomass and productivity resulted primarily from Fe limitation.

The island mass effect and biological carbon uptake for the subantarctic Crozet Archipelago

Marine productivity is often higher downstream than upstream of islands. This so-called island mass effect was tested and quantified with respect to biological carbon uptake and air–sea exchange of carbon dioxide (CO2) at the Crozet Plateau between November 2004 and January 2005 during two CROZEX cruises. The remote plateau is situated at 45.5–47.0°S 49.0–53.0°E, south of the Subantarctic Front (SAF) in the Polar Frontal Zone (PFZ). Surface waters upstream (south) of the plateau had high nutrient and low chlorophyll (HNLC) concentrations. The fugacity of carbon dioxide (fCO2) in surface water was just below the atmospheric value and oceanic CO2 uptake was small (0.2±0.1 mol m−2) throughout CROZEX. The mixed-layer concentration of dissolved inorganic carbon (DIC) decreased by 15 μmol kg−1 from November to January in these HNLC waters, indicating significant biological carbon uptake. Extensive phytoplankton blooms occurred downstream (north) of the plateau in austral spring. These reduced surface water fCO2 by 30–70 μatm and DIC by 30–60 μmol kg−1 and created an important oceanic sink for atmospheric CO2 of 0.6–0.8±0.4 mol m−2, corresponding to a total uptake of 1.3±0.8 Tg C (1 Tg=1012 g). The reduction of DIC in the upper 100 m was much larger downstream (2–3 mol m−2) than upstream (1 mol m−2) of the plateau in January, further confirming the existence of the island mass effect for the Crozet Archipelago. An additional finding is the sizeable DIC deficit in the HNLC waters upstream (south) of the plateau, suggesting that some HNLC waters of the PFZ are more productive than commonly thought. Deep mixed layers of 60–90 m may hide such sustained, modest marine productivity from detection by satellite.