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International audience; The queen conch, Strombus gigas, is an important fisheries resource in the Western Tropical Atlantic. In order to maintain harvesting success, improve fisheries management and contribute to mariculture pursuits, a detailed understanding of the life history traits of this species is required. Traditionally, this has been achieved by tedious and time-consuming long-term field observations. This study presents a highly versatile and rapid technique to estimate the timing and rate of shell growth based on sclerochronology. The Belizean S. gigas specimens (N = 2) from the offshore atoll, Glovers Reef, reached their final shell size (maximum shell height: 22.7 and 23.5 cm, respectively; completed formation of the flared lip) after only two years. However, seasonal growth rates varied considerably. Shells grew up to 6 mm d−1 during spring (April-June) and fall (September-November) but only 1 to 2 mm d−1d uring July and August. Furthermore, shell growth ceased between December and March. Fastest shell growth occurred nearly contemporaneously with times of maximum precipitation which probably resulted in increased food availability. Slowest shell growth however, occurred during times of reduced rainfall and reduced riverine runoff, i.e. during times of reduced food supply. Sea-water temperature apparently did not exert a major control on shell growth. Notably, the slow winter growth was marked by a distinct purple-colored growth line in the cross-sectioned flared lip. Formation of a second major growth line (brown) fell together with the main reproduction period (late October/early November). Shell microgrowth patterns potentially represent daily or semidiurnal periods but cannot be used to assign exact calendar dates to each shell portion, because they were not visible across the entire cross-section of the whorl. Also, the protruding spines developed on the outer shell surface do not function as time gauges. The time represented by the shell portion between consecutive spines varies greatly from 1 to 72 days. Sclerochronology can potentially facilitate maricultural strategies and aid in site pre-testing and selection to grow S. gigas.

[1] The creation of networks of shell-based chronologies which can provide regionally extensive high-resolution proxies for the marine environment depends on the spatial extent of the common environmental signal preserved in the shell banding and on the reliability of the dating model. Here Arctica islandica chronologies from five neighboring sites in the North Sea are compared, and the strength of the common environmental signal across distances up to 80 km is analyzed using statistical techniques derived from dendrochronology. The signal is found to be coherent across these distances. In a linked study, chronologies based on one of the same sites but constructed by two different research teams are compared. Methodological differences in increment interpretation are found to lead to slippage in the dating models. Systematic inclusion or exclusion of intermittently occurring increments results in the two chronologies becoming misaligned by 4 years over a 70-year period. Comparisons with neighboring chronologies indicate that such increments can generally be regarded as genuine annual increments even if they are not visible in all shells.

To investigate environmental variability during the late Holocene in the western Gulf of Maine, USA, we collected a 142-year-old living bivalve (Arctica islandica) in 2004, and three fossil A. islandica shells of the Medieval Warm Period (MWP) and late MWP / Little Ice Age (LIA) period (corrected 14CAMS = 1030 ± 78 ad; 1320 ± 45 ad; 1357 ± 40 ad) in 1996. We compared the growth record of the modern shell with continuous plankton recorder (CPR) time-series (1961–2003) from the Gulf of Maine. A significant correlation (r2 = 0.55; p < 0.0001) exists between the standardized annual growth index (SGI) of the modern shell and the relative abundance of zooplankton species Calanus finmarchicus. We therefore propose that SGI data from A. islandica is a valid proxy for paleo-productivity of at least one major zooplankton taxa. SGIs from these shells reveal significant periods of 2–6 years (NAO-like) based on wavelet analysis, multitaper method (MTM) analysis and singular spectrum analysis (SSA) during the late Holocene. Based on established physical oceanographic observation in the Gulf of Maine, we suggest that slope water variability coupled with North Atlantic Oscillation (NAO) dynamics is primarily responsible for the observed SGI variability.

[1] Climate archives contained in shells of the European limpet, Patella vulgata, accumulated in archaeological deposits can potentially provide much needed information about Holocene environmental change in midlatitude coastal areas. Before reconstructing climate information preserved in these zooarchaeological records, we studied the controls on oxygen and carbon isotope ratios (δ18O and δ13C, respectively) in modern specimens. We tested the hypothesis that P. vulgata precipitates its shell in isotopic equilibrium with the ambient water by comparing δ18OSHELL with predicted values. Predicted δ18OSHELL was constructed using observed sea surface temperature (SST) records and the equilibrium fractionation equation for calcite and water. We assumed a constant δ18OWATER value of +0.10‰ (VSMOW) based on published regional measurements. Comparison of δ18OSHELL with predicted values revealed that δ18OSHELL values were higher than expected by +1.01 ± 0.21‰. Consequently, estimated SST calculated from δ18OSHELL was 4.2 ± 2.3°C lower than observed SST. However, because of the relatively uniform offset between observed and expected δ18O, an adjustment can be made to account for this predictable vital effect. Thus past climate can be reliably reconstructed using this temperature proxy once the offset is taken into account. δ13C values have a similar cyclicity to the δ18O variation and therefore vary seasonally. However, δ13C is slightly out of phase relative to δ18O. An overall negative shift in δ13CSHELL over the lifetime of the individual indicates a vital effect associated with ontogeny. Further study of environmental and ecological factors that influence shell δ13C is required to evaluate fully the potential of carbon isotope ratios as a useful environmental proxy.

[1] While recent changes in subarctic North Pacific climate had dramatic effects on ecosystems and fishery yields, past climate dynamics and teleconnection patterns are poorly understood due to the absence of century-long high-resolution marine records. We present the first 117-year long annually resolved marine climate history from the western Bering Sea/Aleutian Island region using information contained in the calcitic skeleton of the long-lived crustose coralline red alga Clathromorphum nereostratum, a previously unused climate archive. The skeletal δ18O-time series indicates significant warming and/or freshening of surface waters after the middle of the 20th century. Furthermore, the time series is spatiotemporally correlated with Pacific Decadal Oscillation (PDO) and tropical El Nino-Southern Oscillation (ENSO) indices. Even though the western Bering Sea/Aleutian Island region is believed to be outside the area of significant marine response to ENSO, we propose that an ENSO signal is transmitted via the Alaskan Stream from the Eastern North Pacific, a region of known ENSO teleconnections.

Annual growth increments were examined from shells of the ocean quahog (Arctica islandica L.) from northwest Norway and from tree-ring samples of the Scots pine (Pinus sylvestris L.) from nearby coastal areas. The reconstructed annual growth increments were used to compare growth variability in marine and terrestrial ecosystems. Spatiotemporal comparison of the growth records showed statistically significant correlation during the 19th century A.D., indicative of ecosystem-independent response to pre-anthropogenic climate variations. Geographical correlation between marine and terrestrial records was only observed at the local scale. Years with particularly low winter or high summer North Atlantic Oscillation (NAO) indices showed the best synchronization of marine and terrestrial growth. Despite strong correlation during historical time, our palaeoecological evidence suggests that marine and terrestrial ecosystems may show dissimilar growth reaction to recently observed positive winter-NAO phases.

Nares Strait is one of three main passages of the Canadian Archipelago that channel relatively fresh seawater from the Arctic Ocean through Baffin Bay to the Labrador Sea. Oxygen isotopic profiles along the growth axis of bivalve shells, collected live over the 5 – 30 m depth range from the Greenland and Ellesmere Island sides of the strait, were used to reconstruct changes in the hydrography of the region over the past century. The variability in oxygen isotope ratios is mainly attributed to variations in salinity and suggests that the northern end of Nares Strait has been experiencing an increase in freshwater runoff since the mid 1980s. The recent changes are most pronounced at the northern end of the strait and diminish toward the south, a pattern consistent with proximity to the apparently freshening Arctic Ocean source in the north and mixing with Baffin Bay waters as the water progresses southward. This increasing freshwater signal may reflect changes in circulation and ice formation that favor an increased flow of relatively fresh waters from the Arctic Ocean into Nares Strait.

The possibility of applying absolute dating techniques to annual growth increments from the hard parts of aquatic animals was examined. This was done using the theory of cross-dating, which was adopted from dendrochronological principles. Using two mollusc species as examples, the practical issues of the method were demonstrated. Empirical data were used to evaluate the different time series analysis techniques as follows. Biological growth trends were first captured from original time series using cubic splines. Dimensionless growth indices were obtained by extracting the observed growth values from the values of spline curves as ratios. The common growth signal among the index series was quantified visually and statistically. In statistical analysis, correlations between all possible pairs of indexed sample series and, alternatively, between sample series and master chronology (the average of all other remaining time series) were calculated. It was demonstrated that sample–master correlations were consistently higher than sample–sample correlations. Sclerochronologically cross-dated time series were proved to provide absolute dating of high-resolution proxy records that assessed environmental change in marine and freshwater settings. The wider applicability of the associated techniques is discussed, and it is suggested that use of the term ‘sclerochronology’ be restricted to refer only to material or studies for which careful cross-dating has been successfully applied.

Existing reconstructions of the winter North Atlantic Oscillation (WNAO) are based on terrestrial proxies and historical documents. No direct high-resolution, long-term rec ords from marine settings are available for this major climate-dictating phenomenon, which severely affects a variety of economic aspects of our society. Here we present a 245 yr proxy WNAO index based on shells of the long-lived marine bivalve mollusk Arctica islandica . Variations in annual rates of shell growth are positively correlated with WNAO-related changes in the food supply. Maximum amplitudes in frequency bands of 7–9 and 5–7 yr fall exactly within the range of instrumental and other proxy WNAO indices. These estimates were obtained for specimens collected live, 2000 km apart, in the central North Sea and on the Norwegian Shelf. Hence, the WNAO influences hydrographic regimes of large regions of the ocean. Our study demonstrates that A. islandica can reliably reconstruct WNAO dynamics for time intervals and regions without instrumental records. Our new tool functions as a proxy for the WNAO index prior to the twentieth-century greenhouse forcing and has the potential to further validate other proxy-based WNAO records.

A concerted effort has begun to gather and preserve archives of marine samples and descriptive data, giving scientists ready access to insights on ancient environments.

International audience; This study aimed to investigate the environmental controls on the oxygen isotope composition of shells of the European abalone, Haliotis tuberculata. Seasonal delta O-18 profiles from the outer prismatic layer of four abalone shells, collected live in northwest Brittany (France) in 2002 and 2012, were compared to local temperatures and salinities. According to the findings herein, delta O-18 variations in abalone shells corresponded to seasonal variations, and thus, shell composition represented a reliable tool for aging and growth studies. Seawater temperatures estimated from the abalone collected in 2012 reflected the in situ measured temperatures, but the reconstructed temperatures from shells of the three specimens collected in 2002 deviated from measured temperatures by 2.5 degrees C. This overestimation of temperatures corresponded to a "kinetic effect" related to very high annual abalone growth rates; thus, it could be corrected by applying +053%. to the delta O-18(shell). This methodology was then applied to a fossil (6000 cal yr BP) collected in the Bay of Biscay. Given the worldwide distribution of both live and fossilized abalones, the results of the present study showed that this genus represents a promising paleoclimatic tool.

The seasonal pattern of shellfish foraging at the archaeological site of Haua Fteah in the Gebel Akhdar, Libya was investigated from the Epipaleolithic to the Neolithic via oxygen isotope (δ 18 O) analyses of the topshell Phorcus (Osilinus) turbinatus . To validate this species as faithful year-round palaeoenvironmental recorder, the intra-annual variability of δ 18 O in modern shells and sea water was analysed and compared with measured sea surface temperature (SST). The shells were found to be good candidates for seasonal shellfish forging studies as they preserve nearly the complete annual SST cycle in their shell δ 18 O with minimal slowing or stoppage of growth. During the terminal Pleistocene Early Epipaleolithic (locally known as the Oranian, with modeled dates of 17.2–12.5 ka at 2σ probability, Douka et al., 2014), analysis of archaeological specimens indicates that shellfish were foraged year-round. This complements other evidence from the archaeological record that shows that the cave was more intensively occupied in this period than before or afterwards. This finding is significant as the period of the Oranian was the coldest and driest phase of the last glacial cycle in the Gebel Akhdar, adding weight to the theory that the Gebel Akhdar may have served as a refugium for humans in North Africa during times of global climatic extremes. Mollusc exploitation in the Latest Pleistocene and Early Holocene, during the Late Epipaleolithic (locally known as the Capsian, c. 12.7 to 9 ka) and the Neolithic ( c . 8.5 to 5.4 ka), occurred predominantly during winter. Other evidence from these archaeological phases shows that hunting activities occurred during the warmer months. Therefore, the timing of Holocene shellfish exploitation in the Gebel Akhdar may have been influenced by the seasonal availability of other resources at these times and possibly shellfish were used as a dietary supplement when other foods were less abundant.

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.