[pt] A reconstrução da temperatura da superfície do mar (TSM) através de proxies geoquímicos orgânicos tem sido um tema central das pesquisas paleoclimáticas nos últimos anos. Determinações da variabilidade da paleotemperatura no Atlântico Sul são de grande interesse dado o papel da TSM no transporte de calor global, mas este tema ainda requer estudos mais aprofundados. O sistema de ressurgência de Cabo Frio (SRCF) representa uma área de estudo particularmente interessante por ser um sistema de ressurgência de contorno oeste sujeito a vários processos oceanográficos peculiares. Este projeto tem como objetivo a utilização de proxies geoquímicos orgânicos como indicadores de paleotemperatura, tais como: o índice de insaturação das alquenonas (U37K), o índice dos tetraéteres (TEX86H) e o índice dos diois de cadeira longa (LDI) registrados em sedimentos da margem continental sudeste brasileira. Também foram testados os índices de metilação e de ciclização dos tetraéteres ramificados (MBT e CBT) como indicadores da temperatura média anual do ar (TMAA) e do pH do solo. O presente trabalho possui dois focos: (i) investigar a aplicabilidade dos índices U37K, TEX86H e MBT/CBT considerando as condições ambientais locais e regionais observadas na região da Bacia de Campos utilizando 53 amostras de sedimento superficiais; (ii) aplicar uma abordagem multiproxy (U37K, TEX86H e LDI) para reconstruir a paleotemperatura durante o período do Holoceno a partir de dois testemunhos sedimentares localizados nas plataformas adjacentes a Cabo Frio e Rio de Janeiro. Os resultados revelam que U37K e TEX86H são proxies de temperatura adequados para estudos paleoclimáticos em escala regional na margem continental do sudeste brasileiro. Exceções na forma de desvios negativos entre a TSM média anual reconstruída e observada (World Ocean Atlas 2013) foram observados na porção sul da região estudada e ressaltam que, em escala local, os efeitos da ressurgência devem ser levados em consideração ao aplicar esses proxies na mesma região. Em relação aos índices MBT/CBT, os resultados se encaixam na faixa superior do pH do solo e na faixa inferior da TMAA encontrados na bacia de drenagem adjacente para a maioria das amostras. No entanto, a distribuição dos GDGTs ramificados apontam para a existência de diferentes fontes e efeitos pós deposicionais, e.g. degradação seletiva e produção in situ, que comprometem a aplicabilidade desses proxies nos sedimentos marinhos na Bacia de Campos. Ao aplicar os proxies U37K, TEX86H e LDI na reconstrução da temperatura para os últimos 14,5 kyr no testemunho do Rio de Janeiro e 7,1 kyr no testemunho de Cabo Frio, as variações observadas estão em acordo com diversos dados paleoclimáticos pré-existentes para a mesma região e período. Na plataforma do Rio de Janeiro, uma fase distinta antes de 7,5 cal kyr BP coincide com o período em que o nível do mar se encontrava baixo e passou a aumentar gradualmente. Na plataforma de Cabo Frio, a transição entre o Holoceno médio e o tardio é bem marcada pelo aumento na TSM-U37K e da TSMLDI provavelmente relacionados a uma atividade mais intensa da Corrente do Brasil (BC) sobre a plataforma, enquanto a temp-TEX86H apresentou uma tendência oposta de resfriamento que pode ser um resultado da ressurgência das Águas Centrais do Atlântico Sul em subsuperfície. Uma diminuição na temperatura reconstruída após cerca de 2,0 cal kyr BP pode ser explicada pelo estabelecimento do SRCF às condições modernas. Diferenças entre o Rio de Janeiro e Cabo Frio provavelmente estão relacionadas às instabilidades locais da CB próximo à região do SRCF. De forma geral, para o período investigado em ambos os testemunhos, a TSM-LDI mostrou-se mais quente, a TSM-TEX86H obteve uma clara tendência a temperaturas mais frias de subsuperfície e a TSM-U37K foi a mais afetada pela variação no nível do mar e pelo fluxo da CB. As diferenças entre os proxies forneceram informações valiosas sobre as mudanças paleoceanográficas locais durante o Holoceno, contribuindo para os estudos paleoclimáticos na margem sudeste do Brasil. / [en] The reconstruction of past sea surface temperature (SST) using organic geochemical proxies has been a central theme of paleoclimatic research in recent years. Paleotemperature variability determinations in the South Atlantic are of great interest given the SST role in the global heat transport, but still requires further studies. The Cabo Frio Upwelling System (CFUS) is a particularly interesting study area for being a western boundary upwelling system subject to several peculiar oceanographic processes. This thesis aims at using organic geochemical proxies as indicators of paleotemperature, namely: the alkenoneunsaturation index (U37K), the tetraether index (TEX86 H ) and the long-chain diols index (LDI) as registered in sediments in the south-eastern Brazilian continental margin. The methylation of branched tetraether index (MBT’) and the cyclization of branched tetraether index (CBT) were also tested as potential indicators of continental mean annual air temperature (MAAT) and soil pH. There are two focuses: (i) to investigate the applicability of U37K, TEX86H and MBT’/CBT proxies under local and regional environmental conditions in the Campos Basin region using 53 surface sediments; (ii) to use a multiproxy approach (U37K, TEX86H and LDI) to reconstruct the paleotemperature during the Holocene period from two sediment cores located in the shelves adjacent to Cabo Frio and Rio de Janeiro. The results reveal that U37K and TEX86H are suitable temperature proxies for paleoclimate studies in regional scale in the SE Brazilian continental margin. Exceptions in the form of strong negative biases between reconstructed and observed mean annual SST (World Ocean Atlas 2013) were observed in the southern portion of the Campos Basin and highlight that, on a local scale, the effects of upwelling must be taken in consideration when applying these proxies to the same region. Regarding the MBT/CBT proxies, the results for most of the marine sediments agree with the upper range of soil pH and lower range of MAAT found in the adjacent drainage basin. However, the spatial distribution of the branched GDGTs points to the existence of different sources and post-depositional effects, e.g. selective degradation and in situ production, that compromise the applicability of these proxies to the marine sediments in the Campos Basin, especially more offshore. When applying the U37K, TEX86H and LDI proxies to reconstruct the SST for the past 14.5 kyr in the Rio de Janeiro core and 7.1 kyr in the Cabo Frio core, the variations observed agreed with several pre-existing paleoclimatic data for the same region and period. In the Rio de Janeiro shelf, a distinct phase before 7.5 cal kyr BP coincides with the period when the sea level was low and gradually increased. In the Cabo Frio shelf, the transition between mid- and late Holocene is well marked by a warming in SST-U37K and SST-LDI, probably related to a more intense Brazil Current (BC) activity over the shelf, whereas temp-TEX86H showed an opposite cooling trend that may be a result of the subsurface upwelling of the cold South Atlantic Central Waters. A decrease in reconstructed temperatures after ca 2.5 kyr BP may be related to the establishment of the CFUS to modern conditions. Differences between Rio de Janeiro and Cabo Frio can be explained by the local instabilities of the BC near de CFUS region. Overall, for the entire covered period in both sediment cores, SST-LDI was warmer, SST-TEX86H was clearly biase towards colder subsurface temperatures and SST-U37K was the most affected by the sea level variation and BC flow. The differences between the proxies signal provided valuable information about the local paleoceanographic changes during the Holocene period, contributing to the paleoclimatology studies in the southeastern Brazilian margin. / [de] Die Rekonstruktion Oberflächenwassertemperaturen in der Vergangenheit mit organisch geochemischen Proxies ist in den vergangenen Jahren ein wichtiges Thema der Palaoklimaforschung gewesen. Die Rekonstruktion von Palaotemperaturvariabilitat im Südatlantik ist hierbei in Anbetracht der wichtigen Rolle der Ozeane im globalen Wärmetransport von zentraler Bedeutung, erfordert jedoch noch weiterführende Untersuchungen. Das Auftriebssystem von Cabo Frio ist in diesem Zusammenhang wegen seiner speziellen Lage auf der westlichen Seite eines Ozeanbeckens und der damit zusammenhangenden besonderen ozeanographischen Prozesse ein interessantes Arbeitsgebiet. Diese Arbeit hat zum Ziel, die organisch-geochemischen Proxies U37K (Alkenon-UntersattigungsIndex), TEX86H (Tetraether-Index an sogenannten GDGTs, Glycerol Dialkyl Glycerol Teteraether) und LDI (Langkettige-Diole-Index) aufgezeichnet in Sedimenten des sudostlichen brasilianischen Kontinentalrands zu verwenden. Zudem wurden die Indices MBT und CBT, die die Methylierung bzw. Zyklisierung verzweigter GDGTs quantifizieren, als Proxies fur die Jahresmitteltemperatur an Land und fur den pHWert der Boden getestet. Der Fokus der Arbeit liegt hierbei einerseits darauf i) die Anwendbarkeit der Proxies U37K, TEX86H , MBT/CBT im unter den lokalen und regionalen Umwelteinflussen des Campos Bessens auf Grundlage von 53 Oberflachensedimentproben zu uberprufen und ii) eine sogenannte multi-proxy Rekonstruktion der Palaoumweltbedingungen anhand der U37K, TEX86H und LDI Proxies an zwei Sedimentkernen vom Schelf vor Cabo Frio und Rio de Janeiro zu erstellen. Es zeigt sich, dass sich sowohl U37K als auch TEX86H fur regionale Palaoumweltstudien am sudostlichen brasilianischen Kontinentalrand eignen. Ausnahmen in Form von starken negativen Abweichungen zwischen rekonstruierter und instrumentell beobachteter (World Ocean Atlas 2013) Oberflachenwassertemperatur treten im sudlichen Teil des Campos Beckens auf und zeigen, dass sehr lokal die Auswirkungen von Auftrieb bei der einer regionalen Anwendung dieser Proxies berucksichtigt werden mussen. Die Ergebnisse der MBT/CBT Proxies an marinen Sedimenten stimmen mit den Gegebenheiten an Land weitgehend uberein, wobei die pH-Rekonstruktionen im oberen Bereich der Beobachtungen im angrenzenden Einzugsgebiet liegen, wahrend die Rekonstruktion der Jahresmitteltemperatur an Land zu den niedrigeren Werten der Beobachtungen neigen. Die raumliche Verteilung der verzweigten GDGTs in deutet zudem auf unterschiedliche Quellen sowie diagenetische Effekte wie selektiven Abbau und in-situ Produktion von verzweigten GDGTs hin, die die Anwendbarkeit der MBT/CBT Proxies an marinen Sedimenten des Campos Beckens, insbesondere in den kustenfernen Regionen, einschranken. Die Rekonstruktion der Palao-Oberflachenwassertemperaturen mithilfe von U37K, TEX86H und LDI fur die letzten 14.5 kyr vor Rio de Janeiro und fur die letzten 7.1 kyr vor Cabo Frio ergab eine Variabilitat, die mit Ergebnissen aus mehreren vorherigen palaoklimatischen Studien fur die Region ubereinstimmt. Auf dem Schelf vor Rio de Janeiro kann eine Phase vor 7.5 kyr deutlich vom Rest der Zeitreihe unterschieden werden, wahrend derer der Meeresspiegel niedriger war und allmahlich anstieg. Auf dem Schelf vor Cabo Frio ist der ubergang vom mittleren ins spate Holozan von deutlichen Anstiegen der U37K - und LDI-basierten Temperaturrekonstruktionen gepragt, die wahrscheinlich mit der Intensivierung des Brasilstroms auf dem Schelf in Verbindung stehen. Im Gegensatz dazu verzeichnet die TEX86H -basierte Temperaturrekonstruktion eine deutliche Abkuhlung, die das Ergebnis von verstarktem Auftrieb von kaltem sudatlantischem Zentralwasser sein kann. Eine Abnahme der rekonstruierten Temperaturen nach 2.5 kyr vor heute deutet auf die Etablierung moderner Bedingungen im Cabo Frio Auftriebssystem hin. Unterschiede zwischen den Temperaturrekonstruktionen fur die Regionen Cabo Frio und Rio de Janeiro konnen mit lokaler Instabilitat des Brasil-Stroms im Bereich des Cabo Frio Auftriebssystems zusammenhangen. Insgesamt sind über die gesamte Untersuchungsperiode die Temperaturrekonstruktionen auf Grundlage von LDI am warmsten, die auf TEX86H basierenden von kalteren Temperaturen in etwas groberen Wassertiefen beeinflusst und diejenigen auf Grundlage von U37K am starksten von Meeresspiegelanderungen und Variabilitat im Brasil-Strom betroffen. Diese Unterschiede zwischen den einzelnen Proxies liefern wertvolle Informationen uber die lokalen palaozeanographischen Veranderungen wahrend des Holozans und somit einen Beitrag zum Verstandnis des Palaoklimas am sudostlichen brasilianischen Kontinentalrand.

Environmental change in the Arctic proceeds at an unprecedented rate. The Pliocene epoch (5-2.65 million years ago) represents an analog for future climate conditions, with pCO2 and continental configurations similar to present. Yet conditions in the Pliocene Arctic are poorly characterized because of sparse sampling. The records that do exist indicate periods of extreme warmth, as well as the first expansion of large ice-sheets in the Northern Hemisphere, took place from the end of the Pliocene into the early Pleistocene. Understanding these deposits and their implications for our future requires developing a sense of climatic evolution across the Plio-Pleistocene transition and especially during the intensification of Northern Hemisphere Glaciation (iNHG) ~2.7 million years ago. Here we reconstruct environmental change in the Arctic using a suite of organic geochemical proxies in a sedimentary archive recovered from Lake El'gygytgyn, Arctic Northeast Russia. We use the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) and the hydrogen isotopic composition (δD) of plant leaf-waxes (n-alkanes) to reconstruct relative temperature change across the interval spanning 2.8 to 2.4 million years ago. Our work demonstrates that, following the first major glaciation of the Northern Hemisphere, it took multiple glacial cycles for the Arctic to become synchronized with the climatic changes recorded in the deep ocean. This work has implications for understanding the role of sea-level, sea-ice, vegetation and carbon-cycle feedbacks in a changing Arctic.

Rapid tropical glacier retreat over the last 50 years has been well documented, and has received significant media attention. Many studies suggest these changes are due to rising global surface air temperatures, however disentangling the effects of temperature and precipitation has hampered scientific consensus. Furthermore, because of the shortness of the instrumental record, it is difficult to assess the larger significance of the climate changes associated with the decline of tropical glaciers. Here, we present a locally calibrated, independent temperature reconstruction for the past 1,500 years from Laguna Chingaza, Colombia based on distributions of branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) in order to assess the controls on long term temperature variability in the tropical Andes, and their relationship with growth and demise of Andean glaciers. Comparison of reconstructed temperatures with the instrumental record suggests that our proxy record faithfully records decadal to century scale trends in temperature. The largest temperature decline over the last 1,500 years was a decrease of 2.5 ± 0.3 °C during the Little Ice Age (LIA), reaching lowest temperatures during the mid-17th century, and is broadly consistent with terrestrial temperature reconstructions throughout the tropics and the higher latitudes. The structure and timing of temperature changes at Laguna Chingaza are remarkably similar to recent terrestrial temperature reconstructions from elsewhere in the tropics, including sites in the tropical Pacific and equatorial Africa, suggesting that these changes are widespread in the tropics. Together, these records suggest that warming over the last few decades is unprecedented over the last 1,500 years, including the Medieval Climate Anomaly (800-1150 AD). Comparison of these temperature changes with records of Andean glacier limits suggests that temperature is the dominant driver of glacial retreat, particularly over the past few decades. Additionally, paleotemperatures inferred from LIA and recent glacial equilibrium line altitudes (ELAs) underestimate past changes in temperature when compared with brGDGTs reconstructions, suggesting that changes in precipitation complicate the use of glacier ELAs to reconstruct past temperatures. The coupling of temperature and ice extent in South America suggests that with projected future warming, the health of tropical glaciers could be in jeopardy, significantly impacting the communities and ecosystems that depend on them. / text

La Province Volcanique du Rungwe (PVR) en Afrique de l’Est abrite diverses archives continentales (séquence de loess-paléosols, tourbières, lacs) permettant d’étudier les changements environnementaux passés des zones tropicales continentales. Ce travail s’est intéressé à l’applicabilité des alkyl tetraéthers de glycérol (tetraéthers) et des n-alcanes comme biomarqueurs de ces changements passés. Leur analyse le long de transects altitudinaux a permis de rendre compte de la capacité des tetraéthers à enregistrer de faibles changements de température de l’air dans la région. En conséquence, une calibration régionale a été établie afin d’améliorer les reconstructions de température de l’air en Afrique de l’Est. Au contraire, il est apparu que la composition isotopique de l’hydrogène des n-alcanes (δ²Hwax) était impactée par des paramètres physiques et biologiques supplémentaires ce qui complique son utilisation pour suivre les changements de température à l’échelle régionale. L’étude des variations de ces deux biomarqueurs le long d’une carotte de tourbe et d’une carotte de sédiment lacustre prélevées dans la PVR a révélé l’existence synchrone d’une période climatique plus chaude au cours de l’Holocène tardif. Par ailleurs, il a été observé dans la carotte de tourbe une transition d’un lac à une tourbière il y a ca. 2000 ans BP. Le présent travail démontre donc que les tetraéthers et les n-alcanes sont des biomarqueurs prometteurs, notamment parce qu’ils sont sensibles à de faibles variation climatiques ; cependant, afin de les interpréter correctement, il est nécessaire d’adopter une approche multi-marqueurs. / The Rungwe Volcanic Province (RVP) in East Africa offers a wide diversity of continental archives (loess-paleosols sequences, peatlands, lakes), allowing the investigation of past environmental changes in tropical continents. This work focused on the validation of glycerol dialkyl glycerol tetraether- (GDGT) and n-alkane-derived proxies. Analyses of these two biomarkers along altitudinal transects allowed assessing their ability to record temperature changes in the region. GDGTs were found to be robust temperature proxies in East Africa, and a regional calibration was established to improve temperature reconstructions from soil archives. In contrast, investigation of n-alkane hydrogen isotopic composition (δ²Hwax) in surface soils revealed that its variations seemed to be impacted by additional biological and physical parameters than temperature, preventing its generalization at a regional scale. Application of GDGT and n-alkane proxies to continental archives revealed past environmental changes over the Late Holocene in the RVP. A late Holocene synchronous temperature maximum was especially observed in a peat core and sediment core from two sites of the RVP. A multi-proxy approach was applied to the peat core, revealing that an ecosystem change from a lake to a peatland occurred ca. 2000 yrs BP ago. The present work showed that GDGTs and n-alkanes are promising biomarkers because of their sensitivity to slight climate variations, but that they need to be combined with other proxies to accurately reconstruct environmental changes.

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