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Muschelschalen eignen sich hervorragend für raumzeitlich hochaufgelöste Paläoumweltrekonstruktionen. Umweltbedingungen der Vergangenheit können aus dem Schalenzuwachs und den stabilen Sauerstoff- und Kohlenstoffisotopenverhältnissen rekonstruiert werden, welche Archive für Temperatur, Süßwassereintrag und Primärproduktivität darstellen. Das Ziel dieser Doktorarbeit besteht darin, holozäne Klima- und Umweltschwankungen im nordpazifischen Sektor mit einer hohen räumlichen und zeitlichen Auflösung mittels Muschelschalen zu rekonstruieren. Diese Arbeit fokussiert sich dabei auf verschiedene holozäne Zeitabschnitte und mehrere Regionen im Nordpazifik, darunter Japan, Alaska (AK), Britisch-Kolumbien (BC) und Washington, welche vom Monsun, der Pazifischen Dekadischen Oszillation (PDO) und der El Niño/Southern Oscillation (ENSO) beeinflusst werden. Derartig hochauflösende Proxydaten aus dem marinen Bereich der mittleren und hohen Breiten sind jedoch selten. Somit trägt diese Arbeit zur Optimierung und Verifikation von Klimamodellen bei. Bevor Muschelschalen allerdings zur Umweltrekonstruktion und in Studien zur Saisonalität verwendet werden können, müssen life-history-Eigenschaften untersucht werden, um die geochemischen Daten zeitlich anzuordnen und zu interpretieren. Solche Kalibrierungsstudien sind notwendig, um die Nutzbarkeit ausgewählter Muschelarten als Paläoklimaarchive zu untersuchen. Im Fokus dieser Arbeit stehen zwei Muschelarten: die kurzlebige Buttermuschel Saxidomus gigantea und die langlebige Elefantenrüsselmuschel Panopea abrupta. Verschiedene Schalenlagen von P. abrupta wurden sklerochronologisch und sauerstoffisotopisch untersucht, um die Eignung dieser Art als Klimaarchiv zu prüfen. Die jährlichen Inkrementbreiten sind im Schalenwirbel klar erkennbar und sollten dort gemessen werden. Eine verlässliche Rekonstruktion von Paläotemperaturen kann nur durch die ausschließliche Beprobung der äußeren Schalenlage von mehreren kontemporären Muschelschalen erreicht werden. Eine detaillierte Untersuchung der life-history-Eigenschaften (z.B. Timing der Wachstumslinien, Dauer der Wachstumssaison und Wachstumsraten) und der stabilen Isotopenverhältnisse erfolgte für rezente S. gigantea Schalen aus AK und BC. Des Weiteren wurde ein Wachstums-Temperatur-Modell basierend auf S. gigantea Schalen aus AK aufgestellt, welches ein besseres Verständnis der hydrologischen Veränderungen der Strömung entlang der Küste Alaskas (Alaska Coastal Current, ACC) bietet. Dieser Ansatz erlaubt die unabhängige Messung von Wassertemperatur und Salinität aus Variationen in der Breite der Tagesinkremente in S. gigantea. Der Schalenzuwachs ist bis zu 70% temperaturgesteuert. Das Modell wurde mit rezenten Muschelschalen kalibriert und getestet und dann auf archäologische Schalen angewandt. Die Sommer in der Zeit zwischen 988 und 1447 Jahren vor heute waren kälter (~1-2°C) und viel trockener (2-5 PSU) und der ACC floss wahrscheinlich viel langsamer als heute. Im Gegensatz dazu waren die Sommer zwischen 599-1014 Jahren vor heute kälter (bis zu 3°C) und frischer (1-2 PSU) als heute. Das Aleutentief war wahrscheinlich stärker zu dieser Zeit und der ACC floss voraussichtlich schneller., Bivalve mollusk shells are useful tools for multi-species and multi-proxy paleoenvironmental reconstructions with a high temporal and spatial resolution. Past environmental conditions can be reconstructed from shell growth and stable oxygen and carbon isotope ratios, which present an archive for temperature, freshwater fluxes and primary productivity. The purpose of this thesis is the reconstruction of Holocene climate and environmental variations in the North Pacific with a high spatial and temporal resolution using marine bivalve shells. This thesis focuses on several different Holocene time periods and multiple regions in the North Pacific, including: Japan, Alaska (AK), British Columbia (BC) and Washington State, which are affected by the monsoon, Pacific Decadal Oscillation (PDO) and El Niño/Southern Oscillation (ENSO). Such high-resolution proxy data from the marine realm of mid- and high-latitudes are still rare. Therefore, this study contributes to the optimization and verification of climate models. However, before using bivalves for environmental reconstructions and seasonality studies, life history traits must be well studied to temporally align and interpret the geochemical record. These calibration studies are essential to ascertain the usefulness of selected bivalve species as paleoclimate proxy archives. This work focuses on two bivalve species, the short-lived Saxidomus gigantea and the long-lived Panopea abrupta. Sclerochronology and oxygen isotope ratios of different shell layers of P. abrupta were studied in order to test the reliability of this species as a climate archive. The annual increments are clearly discernable in umbonal shell portions and the increments widths should be measured in these shell portions. A reliable reconstruction of paleotemperatures may only be achieved by exclusively sampling the outer shell layer of multiple contemporaneous specimens. Life history traits (e.g., timing of growth line formation, duration of the growing season and growth rates) and stable isotope ratios of recent S. gigantea from AK and BC were analyzed in detail. Furthermore, a growth-temperature model based on S. gigantea shells from Alaska was established, which provides a better understanding of the hydrological changes related to the Alaska Coastal Current (ACC). This approach allows the independent measurement of water temperature and salinity from variations in the width of lunar daily growth increments of S. gigantea. Temperature explains 70% of the variability in shell growth. The model was calibrated and tested with modern shells and then applied to archaeological specimens. The time period between 988 and 1447 cal yrs BP was characterized by colder (~1-2°C) and much drier (2-5 PSU) summers, and a likely much slower flowing ACC than at present. In contrast, the summers during the time interval of 599-1014 cal yrs BP were colder (up to 3°C) and fresher (1-2 PSU) than today. The Aleutian Low may have been stronger and the ACC was probably flowing faster during this time.

International audience; Interannual and intra-annual variations in lithium-to-calcium ratio were investigated with high temporal resolution in the aragonitic outer shell layer of juvenile Arctica islandica (Mollusca; Bivalvia) collected alive in 2006 off northeast Iceland. Li/Cashell ranged between 7.00 and 11.12 µmol mol-1 and presented well-marked seasonal cycles with minimum values recorded at the annual growth lines; a general pattern was a progressive increase in Li/Cashell from March to May, followed by a plateau in June and a decrease down to minimum values in July-August. Li/Cashell was correlated with d18Oshell-derived temperature, but the strength of this relationship was weak (r2 < 0.25 and p < 0.05). It covaried significantly with microgrowth increment width and with the discharge from one of the closest rivers. Seasonal variations of Li/Cashell in A. islandica may most likely be explained (1) by calcification rate and/or (2) by significant river inputs of Li-rich silicate particles flowing to the sea as soon as snow melts. In the first case, Li/Cashell may be a useful proxy for addressing seasonal variations of growth rate in bivalves that lack discernable microgrowth patterns. Abrupt decreases of Li/Cashell may, in turn, help identify growth retardations due to harsh environmental conditions. Alternatively, if Li/Cashell variations are linked to particulate Li inputs by rivers, this could be a new proxy for the intensity of mechanical weathering of Icelandic basalts, with interesting perspectives for the reconstruction of frequency and intensity of past jökulhlaups (subglacial outburst floods). Further works, including experimental studies, are needed to test these hypotheses.

International audience; Coral-based reconstructions of sea surface temperatures (SSTs) using Sr/Ca, U/Ca and δ 18 O are important tools for quantitative analysis of past climate variabilities. However, post-depositional alteration of coral aragonite, particularly early diagenesis, restrict the accuracy of calibrated proxies even on young corals. Considering the diagenetic effects, we present new Mid to Late Holocene SST reconstructions on well-dated (U/Th: ∼70 yr to 5.4 ka) fossil Porites sp. collected from the Society Islands, French Polynesia. For few corals, quality pre-screening routines revealed the presence of secondary aragonite needles inside primary pore space, resulting in a mean increase in Sr/Ca ratios between 5-30%, in contrast to the massive skeletal parts. Characterized by a Sr/Ca above 10 mmol/mol, we interpret this value as the threshold between diagenetically altered and unaltered coral material. At a high-resolution, observed intra-skeletal variability of 5.4 to 9.9 mmol/mol probably reflects the physiological control of corals over their trace metal uptake, and individual variations controlled by CaCO 3 − precipitation rates. Overall, the Sr/Ca, U/Ca and δ 18 O trends are well correlated, but we observed a significant offset up to ± 7 • C among the proxies on derived palaeo-SST estimates. It appears that the related alteration process tends to amplify temperature extremes, resulting in increased SST-U/Ca and SST-Sr/Ca gradients, and consequently their apparent temperature sensitivities. A relative SST reconstruction is still feasible by normalizing our records to their individual mean value defined as SST. This approach shows that SST records derived from different proxies agree with an amplitudinal variability of up to ± 2 • C with respect to their Holocene mean value. Higher SST values than the mean SSTs (Holocene warm periods) were recorded from ∼1.8 to ∼2.8 ka (Interval I), ∼3.7 to 4.0 ka (Interval III) and before ∼5 ka, while lower SST values (Holocene cold periods, Interval II and IV) were recorded in between. The ensuing SST periodicity of ∼1.5 ka in the Society Islands record is in line with the solar activity reconstructed from 10 Be and 14 C production (Vonmoos et al., 2006), emphasizing the role of solar activity on climate variability during the Late Holocene.

Marine Isotope Stage 5 deposits have been reported on many tropical Pacific islands. This paper presents a database compiled through the review of MIS 5e (last interglacial – LIG) coral reef records from islands belonging to French Polynesia (Anaa, Niau, Makatea, Moruroa, Takapoto, Bora Bora), the Hawaiian Islands (Oahu, Lanai, Midway Atoll), Tuvalu, Kiribati (Christmas Island, Tarawa), the Cook Islands (Mangaia, Atiu, Mitiaro, Mauke, Pukapuka, Rakahanga, Rarotonga), Tonga, Samoa, the Federal States of Micronesia, the Mariana Islands, the Marshall Islands (Enewetak, Bikini), New Caledonia, Papua New Guinea, the Solomon Islands, Vanuatu, Fiji and Niue. Studies reporting other sea-level indicators dated to other Pleistocene interglacials and Holocene sea-level indicators were not inserted in the database but are included in this data description paper for completeness. Overall, about 300 studies concerning Pleistocene and Holocene sea-level indicators have been reviewed, and finally 163 age data points and 94 relative sea-level (RSL) data points from 38 studies on the MIS 5e have been inserted in the database. An additional 155 age data points have been reviewed; i.e. the tropical Pacific islands database contains 318 age data points. The main sea-level indicators include emerged coral reef terraces, but also reef units recovered in drill cores from a few islands, thus reflecting the diversity of tectonic settings and sampling approaches. Future research should be directed towards better constrained RSL reconstructions, including more precise chronological data, more accurate elevation measurements and a better refinement of the palaeo-water-depth significance of coralgal assemblages. The database for tropical Pacific islands is available open access at this link: https://doi.org/10.5281/zenodo.3991672 (Hallmann and Camoin, 2020).

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