Holocene surface ocean conditions of the Norwegian Sea as recorded by Globigerina bulloides (Mg/Ca ratios and ä18O)

Temperature superficiali del Mare di Norvegia nell'Olocene desunte dall'analisi del Mg/Ca e degli isotopi stabili dell'ossigeno in Globigerina bulloides

Type Thesis
Date 2011
Language English, Spanish
Other localization http://tesi.cab.unipd.it/27478/
Author(s) Dei Tos Luana1
University Universita Degli Studi Di Padova
Discipline Scienze della Natura
Thesis supervisor Luca Giusberti
French abstract

During my master project I have performed stable isotopes and Mg/Ca analyses of the foraminifera Globigerina bulloides (d’Orbigny, 1826) from the core MD95-2011 located in the Vøring Plateau in the eastern Norwegian Sea (66.97 °N, 7.64 °E at 1048 m water depth). The North Atlantic represents a key area for the study of climate: here the warm Atlantic Water loses most of its heat, sinks and flow back southwards as part of the North Atlantic Deep Water. The Nordic Seas are strongly influenced by Atlantic, Arctic and Polar water. When the Atlantic water flux through the Norwegian Current is stronger, warmer conditions occur in the eastern Nordic Seas (the Norwegian Sea). The Holocene is the period that starts after the last glacial period at 11.7 ka BP and continues until the Present. This is the most recent warm period that has not been influenced by the human activity, for this reason the climate dynamics of this Epoch can reveal useful information to understand climate change dynamics and future climate. Globigerina bulloides is a planktic dwelling foraminifer that populates the upper 50 m of the water column. It has a calcite test that is built in equilibrium with the surrounding water conditions. Therefore the isotopic content of the test reflects the water conditions in which the biogenic calcite precipitated. Shallow water dwelling foraminifera were thought to reflect insolation conditions like alkenones and diatoms (Jansen et al., 2008), however recent studies (i.e. Thornalley et al., 2009) show that G. bulloides reflect temperature and salinity more than insolation, however the interpretations of the information stored in the tests of this species are still not completely clear. This dissertation aims at clarifying the possible uses of this species as a paleotemperature and paleoinsolation proxy. This target is achieved by comparing the stable isotopes and Mg/Ca results with other paleo achieves from the same core and other cores in the North Atlantic. The results show a warming trend throughout the whole Holocene in agreement with the observations made in many other studies of cores of North Atlantic. The early Holocene shows a warming trend in the oxygen isotope record. The Mg/Ca record indeed displays a warm peak at 10.8 ka followed by a cold period that has its minimum at 9.1 ka BP. This cold period is assumed to be caused by the input of meltwater in the Nordic Seas during the beginning of the deglaciation. Slowly the fresh water input stabilised and the actual circulation start to form at about 8 ka. The middle Holocene (4-8 ka BP) in the core MD95-2011 record is indeed characterised by a stable period. The late Holocene shows a slightly warming trend in the oxygen isotopes record that can be explained with a regression northwards of the Arctic front and an increase in heat and/or increased transport northwards. The Mg/Ca record is characterised by an odd warm period with mean temperature of about 15°C. This warm period recorded by Mg/Ca is seen in other records from the same core, although with lower amplitude (Andersson et al., 2003; Risebrobakke et al., 2003). This warm period is interrupted by a cooling period between 600-800 yr BP, which overlaps with colder foraminiferal SSTs. The record of Globigerina bulloides in this study shows high variability in both the stable isotopes and Mg/Ca ratios. The Mg/Ca analysis of Neogloboquadrina pachyderma from the same core shows high variability. Many hypotheses were considered to explain this high variability. According to the more reliable hypothesis the changes in the water column during the calcification season can partly explain the observed variability. In such an unstable environment the water compositions may differ a lot from year to year, and this reflects on the test isotopic content. Temperature values higher than expected are seen throughout the whole Holocene in this record. This abnormality reflects higher assimilation of Mg in the test of G. bulloides. In other studies this is not seen, therefore it is not a normal behaviour of this species, but can have been forced from environmental causes. To answer this question more detailed studies are needed both on the paleontological record and on biological behaviour of the species.

Abstract <p>Durante il tirocinio svolto presso il Bjerknes Centre dell&acirc;€™Universit&Atilde;&nbsp; di Bergen (Norvegia) mi sono occupata di ricostruzioni di paleotemperature nel Mar di Norvegia durante l&acirc;€™Olocene. Nello specifico sono stati analizzati esemplari di Globigerina bulloides (d&acirc;€™Orbigny, 1826) presenti nella carota di fondo MD95-2011 estratta dal fondale del V&Atilde;&cedil;ring Plateau (66.97 &Acirc;&deg;N, 7.64 &Acirc;&deg;E) a largo della Norvegia. G. bulloides &Atilde;&uml; un foraminifero planctonico e il carbonato di calcio (calcite) del suo guscio viene generalmente secreto in equilibrio con l&acirc;€™ambiente circostante. La composizione del suo guscio riflette quindi le condizioni dell&acirc;€™acqua in cui la calcite &Atilde;&uml; precipitata (es. temperatura e salinit&Atilde;&nbsp;). Tre proxy sono stati utilizzati per ricostruire le paleotemperature nell&acirc;€™intervallo studiato: composizione isotopica dell&acirc;€™ossigeno (&Icirc;&acute;18O/16O), composizione isotopica del carbonio (&Icirc;&acute;13C/12C) e rapporto tra Mg e Ca (Mg/Ca). Le paleotemperature ricostruite grazie a questi proxy mostrano un aumento di temperatura dalla base dell&acirc;€™Olocene (11.7 ka) fino a 0.5 ka. Quest&acirc;€™aumento di temperatura &Atilde;&uml; stato gi&Atilde;&nbsp; osservato in molti studi nel Nord Atlantico che ne confermano quindi l&acirc;€™attendibilit&Atilde;&nbsp;, ed &Atilde;&uml; spiegato dall&acirc;€™aumento dell&acirc;€™insolazione invernale nello stesso periodo. Il primo Olocene (11.7-8 ka) &Atilde;&uml; caratterizzato da un periodo freddo, centrato a 9.1 ka. Questo evento &Atilde;&uml; presente anche in altri record nel Nord Atlantico (Came et al., 2007) e testimonia che il famoso &acirc;€œ8.2 ka event&acirc;€ non &Atilde;&uml; l&acirc;€™unico abbassamento di temperatura di tale portata nel primo Olocene. Nella carota studiata G. bulloides registra nell&acirc;€™Olocene medio (8-3.6 ka) temperature relativamente stabili mentre i proxy basati su organismi fitoplanctonici (diatomee e alchenoni) evidenziano una fase calda nota come Holocene Thermal Maximum (HTM). Quest&acirc;€™anomalia &Atilde;&uml; spiegata dall&acirc;€™aumento di insolazione estiva durante l&acirc;€™Olocene inferiore-medio: il fitoplancton che abita la zona eufotica registra maggiormente i cambiamenti di insolazione rispetto allo zooplancton che abita la parte sottostante della colonna d&acirc;€™acqua. Il tardo Olocene al V&Atilde;&cedil;ring Plateau (3.6-0 ka) &Atilde;&uml; caratterizzato da un aumento di temperature (&Icirc;&acute;18O record), come confermato da altri proxy misurati nella carota studiata e in altre carote del Nord Atlantico. Quest&acirc;€™aumento di temperature &Atilde;&uml; spiegato da un intensificarsi dell&acirc;€™influsso delle acque Atlantiche e/o da un indebolimento delle acque Artiche. Nel complesso l&acirc;€™andamento dei proxy studiati nella carota MD95-2011 dimostra che G. bulloides registra in modo affidabile l&acirc;€™evoluzione climatica dell&acirc;€™Olocene. Il lavoro svolto ha evidenziato tuttavia nei gusci di G. bulloides concentrazioni di Mg anomale che si traducono in temperature mediamente pi&Atilde;&sup1; alte rispetto a quelle ottenute da altri proxy (es. Mg/Ca su Neogloboquadrina pachyderma dx; Nyland et al., 2006). Quanto emerso rende quindi necessarie ulteriori indagini sulle modalit&Atilde;&nbsp; di calcificazione in G. bulloides.</p>
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Dei Tos Luana (2011). Holocene surface ocean conditions of the Norwegian Sea as recorded by Globigerina bulloides (Mg/Ca ratios and ä18O) / Temperature superficiali del Mare di Norvegia nell'Olocene desunte dall'analisi del Mg/Ca e degli isotopi stabili dell'ossigeno in Globigerina bulloides. PhD Thesis, Universita Degli Studi Di Padova. https://archimer.ifremer.fr/doc/00505/61713/