14 research outputs found
Benthic foraminifera as tracers of brine production in the Storfjorden "sea ice factory"
Abstract. The rapid response of benthic foraminifera to environmental factors
(e.g. organic matter quality and quantity, salinity, pH) and their
high fossilisation potential make them promising bio-indicators for
the intensity and recurrence of brine formation in Arctic seas. Such
an approach, however, requires a thorough knowledge of their modern
ecology in such extreme settings. To this aim, seven stations along
a northâsouth transect across the Storfjorden (Svalbard archipelago) have been
sampled using an interface multicorer. This fjord is an area of
intense sea ice formation characterised by the production of
brine-enriched shelf waters (BSW) as a result of a recurrent
latent-heat polynya. Living (rose bengal-stained) foraminiferal
assemblages were analysed together with geochemical and
sedimentological parameters in the top 5âcm of the
sediment. Three major biozones were distinguished. (i) The "inner
fjord" zone, dominated by typical glacier proximal calcareous species, which
opportunistically respond to fresh organic matter inputs. (ii) The
"deep basins and sill" zone, characterised by glacier distal agglutinated
fauna; these are either dominant because of the mostly
refractory nature of organic matter and/or the brine persistence that
hampers the growth of calcareous species and/or causes their
dissolution. (iii) The "outer fjord" zone, characterised by typical North
Atlantic species due to the intrusion of the North Atlantic water in
the Storfjordrenna. The stressful conditions present in the deep
basins and sill (i.e. acidic waters and low food quality) result in
a high agglutinatedâââcalcareous ratio (AâC). This supports the
potential use of the AâC ratio as a proxy for brine persistence and
overflow in Storfjorden
Changements environnementaux liĂ©s Ă la dynamique de la glace dans les fjords de lâArctique : nouvelles connaissances apportĂ©es par les foraminifĂšres benthiques
Le changement climatique menace les rĂ©gions polaires avec des consĂ©quences majeures sur la dynamique des glaces et les Ă©cosystĂšmes associĂ©s. Les simulations de fonte glaciaire reposent sur des reconstitutions palĂ©o environnementales, qui complĂštent les mesures directes sur la pĂ©riode actuelle pour rĂ©duire l'incertitude des prĂ©visions. Des proxies basĂ©s sur lâĂ©cologie des foraminifĂšres benthiques (FB) sont dĂ©veloppĂ©s dans cette thĂšse pour suivre la dynamique des glaces de mer et le retrait des glaciers cĂŽtiers. Dans le Storfjorden, les FB montrent une rĂ©ponse aux eaux enrichies en CO2 (saumures) libĂ©rĂ©es pendant les processus de formation de glace de mer. Le rapport entre les FB agglutinĂ©s et calcaires (A/C) est proposĂ© comme proxy de la persistance sur les fonds de ces saumures qui provoquent la dissolution des tests calcaires des FB. Bien que le signal A/C soit affectĂ© par des processus taphonomiques, il permet de souligner les diffĂ©rences entre les zones affectĂ©es par la persistance de saumures et celles sous influence intermittente. Dans le Kongsfjorden, des gradients environnementaux abrupts (e.g., salinitĂ©, turbiditĂ© de l'eau, flux organiques) sont provoquĂ©s par la dynamique des glaciers cĂŽtiers. En Ă©tĂ©, diffĂ©rents assemblages de FB sâinstallent en fonction de lâĂ©loignement au front du glacier, avec une augmentation vers le large de la diversitĂ© taxonomique et fonctionnelle. Un indicateur combinant plusieurs mesures de diversitĂ© a Ă©tĂ© proposĂ© comme proxy du recul des glaciers cĂŽtiers, et son efficacitĂ© a Ă©tĂ© testĂ©e sur une archive sĂ©dimentaire des 60 derniĂšres annĂ©es. Les deux proxies proposĂ©s ici sont donc applicables dans tout environnement arctique similaire.Climate change threatens polar regions with major consequences on ice-related dynamics and linked ecosystems. Predictions of future sea ice cover and glacier retreat rely on paleoenvironmental reconstructions (through proxiesâ application) to extend the instrumental period and thus reduce modelsâ uncertainty. This PhD thesis aimed to develop and test ecological proxies for sea ice formation and glacier melting using benthic foraminifera. In the Storfjorden âsea ice factoryâ, foraminiferal responses to brine presence (i.e., CO2-enriched waters released during sea ice formation processes) were investigated. The ratio between Agglutinated and Calcareous (A/C) foraminifera was proposed as potential proxy for brine persistence, because of the brine-induced dissolution on calcareous foraminiferal shells. Although the A/C signal was affected by taphonomic processes, the differences between areas of brine persistence and those with intermittent influence was preserved, thus supporting the proxy. Likewise, foraminiferal responses to steep environmental gradients (e.g., water salinity and turbidity, organic fluxes) created by tidewater glacier dynamics in summer, were investigated in Kongsfjorden. The glacier induced disturbance resulted in the establishment of different foraminiferal assemblages and increased taxonomic and functional diversity with increased distance from the glacier front. A combination of diversity metrics was proposed as potential proxy for glacier retreat and its effectiveness was tested on a sedimentary archive covering the last 60 years. The two proxies proposed here are therefore applicable in any similar Arctic environments
STEP2016 living foraminifera in surface sediments of a N-S transect in the Storfjorden
Living benthic foraminiferal faunas of a N-S transtect in the Storfjorden (Svalbard)
Implication of size fraction on benthic foraminiferal-based paleo-reconstructions: A case study from the Bay of Biscay (NE Atlantic)
International audienceMany paleoenvironmental studies based on benthic foraminiferal assemblages use different protocols for sample analysis. A standardized protocol has been recently established for biomonitoring applications, but for paleostudies, the influence of size fraction on benthic foraminiferal composition and biodiversity is poorly documented. We studied fossil foraminiferal assemblages along two paleorecords (BOBGEO-CS05 and SU81â44) from the Bay of Biscay covering the last ~35âŻkaâŻcal BP. We investigated diversity and community composition to compare the impact of each size fraction (63-150âŻÎŒm, >150âŻÎŒm, >63âŻÎŒm) on environmental interpretations. Foraminiferal diversity was affected by the accumulation of small opportunistic species. In terms of faunal composition, both paleorecords displayed a different pattern depending on the size fraction selected. While in both cores, the 63-150âŻÎŒm fraction blurred the signal of some rare indicator species, our results show that i) in BOBGEO-CS05, it yielded no extra ecological information compared to the large fraction whereas ii) in SU81â44, it contained small opportunistic species that were not present in the >150âŻÎŒm, impacting therefore paleoenvironmental interpretations. According to these findings, we recommend: i) to focus on the large fraction for a thorough taxonomic determination and a detailed analysis of benthic assemblages, and ii) to analyse the small fraction separately after a taxonomical identification of major species and strategic selection of studied samples. Although the 125âŻÎŒm size limit was not tackled in this study, we recommend to use it for the limit between the small and large fractions instead of 150âŻÎŒm for harmonization with the previously published standardized protocol for living faunas
Early taphonomy of benthic foraminifera in Storfjorden âseaâice factoryâ: the agglutinated/calcareous ratio as a proxy for brine persistence
The recurrent latent-heat polynya characterizing Storfjorden (Svalbard, Norway) triggers seasonal formation of thin first-year sea ice. This leads to the production of dense, salty, and corrosive brines that cascade towards the sea floor and mix with shelf waters. The bottom topography of the fjord is responsible for the retention of these dense waters in two central deep basins throughout the year. Recent studies show that living benthic foraminifera in Storfjorden are particularly affected by the persistence of brines on the sea floor, with a strong dominance of agglutinated (A) species and high degrees of dissolution of calcareous(C) faunas. Therefore, the A/C ratio, calculated on living faunas, was proposed as a proxy for brine persistence. In the present study we analyse the fossil faunas, found below the taphonomically active zone, to investigate the residual signal of the A/C proxy after the intense early taphonomic processes and challenge its applicability in sedimentary archives. Our results show that despite the generally high taphonomic loss inside the fjord, a high proportion of agglutinated species is still visible in fossil faunas at the stations experiencing regular and/or persistent presence of brine-enriched shelf waters. These results support the application of the A/C ratio in historical records to reconstruct the persistence of brines and indirectly the first-year sea ice formation in Storfjorden. This can be further applied to other Arctic fjords with similar settings and characterized by the production of brines during the winterâearly spring season
Seasonal and interannual variability of benthic foraminiferal faunas under the influence of a tidal glacier
International audienc
nïŹuence of modern environmental gradients on foraminiferal faunas in the inner Kongsfjorden (Svalbard)
International audienc