Description: A comprehensive understanding of past climates and environmental conditions is essential for our ability to successfully predict the likely impacts of future climate change and to effectively employ adaptation and mitigation strategies. The continued improvement and verification of numerical global circulation models (GCMs) is our best tool for predicting future climate scenarios. However, longer time-scale reconstructions particularly rely on archives (e.g., ice cores, sediment cores) that contain preserved records of past conditions (proxies) in their biogenic hard parts (e.g., shells, bones, teeth) or non-biogenic deposits/accumulations (e.g., lake varves, layers in stalagmites or ice cores). The marine bivalve Arctica islandica is one such versatile and commonly used biogenic archive. Its key advantages are its longevity (up to 500 years old), its wide distribution throughout the North Atlantic and its abundance in the fossil record (back to 20 Ma). Variability in shell growth and the biogeochemistry of its shell carbonate (aragonite) enable reconstructions of, for example, large-scale ocean-atmosphere phenomena such as the North Atlantic Oscillation and sub-annual (seasonal) absolute water temperatures (based on stable oxygen isotopes). This well understood and wellcalibrated archive has most commonly been used for environmental reconstructions in the last 2,000 years but its full potential for climate reconstructions further back in time has not yet been fully exploited. Therefore, this thesis aims to thoroughly assess the potential of sub-fossil (up to 10,000 years old) and fossil A. islandica shell specimens as a palaeo-archive that records the environmental conditions of the North Atlantic during past warm phases. The use of any proxy data from fossil specimens presents challenges that can significantly impact upon palaeo-environmental interpretations. I firstly present a new methodology using confocal Raman microscopy (CRM) as a powerful sclerochronological tool for assessing preservation and identifying taphonomic alterations in the original shell carbonate. Diagenetic modifications to the shell make it challenging to visualize the internal growth patterns and typically render the specimen unusable for biogeochemical analysis (e.g., stable isotope or trace elemental analyses). The CRM mapping approach outlined here proves that CRM leads to comparable and even superior results when compared to commonly used growth increment visualization techniques on both modern and particularly on fossil shell material (A. islandica and Pygocardia rustica; both Pliocene). I also clearly demonstrate the significant impact that the recrystallization of metastable shell aragonite to calcite can have on isotopic signatures within the shell carbonate. δ18O and δ13C signatures in recrystallized shell sections of a Pliocene A. islandica (Iceland) show tremendous changes from the pristine shell. Preservation state, which is often overlooked, therefore has a substantial impact on isotope records and serious implications for environmental and climatic interpretation. I therefore strongly emphasise the crucial need for preservation assessment prior to sampling for any proxy record and that CRM is an ideal tool with which to do this. This thesis presents a reconstruction of high latitude (Svalbard) seasonality during the last warmer-than-today phase based on the archive A. islandica. Meticulously discussing all relevant considerations (e.g., shell preservation, palaeowater depth, palaeo-water chemistry, ice volume effect, etc.), I show that seasonal temperatures in a fjord setting during the Holocene Climate Optimum (HCO) were considerably higher than today (+6°C on average), which impressively identifies past polar amplification (expected HCO global mean temperature +3°C). This presents a first indication of the possible impacts of future climate change in high latitude marine regions (IPCC, 2013 stating up to +3.7°C global temperatures until CE 2100). The further analysis of internal shell growth patterns of HCO A. islandica specimens from Svalbard detect a pronounced and significant 11-year oscillation using tools of spectral frequency analysis. Such a signal has been previously reported in other archives and has most often been associated to the 11-year solar sunspot cycle (Schwabe cycle). Solar-climate interactions remain a debated issue, as present data is temporally and spatially insufficient to decipher any hypothetical links. Therefore no previous studies propose any explanation for a mechanistic link between shell growth in biogenic marine archives and solar activity. In this thesis I propose a first possible link between solar activity and shell growth via a biological amplifying mechanism − UV radiation and phytoplankton productivity. I emphasize that the strongly simplified hypothetical explanation presented does not claim to be conclusive. It is instead a first explanation that is intended to fuel debate and discussion on this vitally important topic that has been so far overlooked. This study shows that growth and biogeochemical proxies recorded in the shells of A. islandica are a powerful archive of past climate conditions and variability on sub-annual to multi-centennial timescales. The seasonal environmental record of past warm intervals presented is a particularly valuable result, showing that northern North Atlantic shallow marine ecosystems may experience amplified warming under future global mean climate scenarios. The importance of understanding the mechanistic links between climate drivers and growth in biogenic archives cannot be underestimated and is a priority for future research. It is hoped that the methodological advances presented here will additionally lead to significant improvements in the quality of geochemical and growth increment based biogenic proxy data produced by the sclerochronological community by facilitating preservation assessment prior to any analysis.

Description: Amiantis purpurata is a typical warm-temperate water species distributed from southern Brazil to northern Patagonia, Argentina. Recent and well preserved fossil specimens were recovered from San Matías Gulf in northern Patagonia. Holocene (shell age 3630 ±100 years BP) and interglacial Pleistocene (MIS 5, 100 ka years BP) marine sediments were used for a comparative analysis of stable isotopic profiles (δ18O; δ13C). The values range of Pleistocene A. purpurata was from -0,93‰ to 0,85‰ for δ18O and from -1,02‰ to 1,9‰ for δ13C. Holocene shell was from -0,34‰ to 1,13‰ for δ18O and from 1,45‰ to 2,44‰ for δ13C. And Recent shell was from -0,66‰ to 1,56‰ for δ18O and from 0,7‰ to 2,6‰ for δ13C. The δ18O values indicate warmer waters in Pleistocene compared to Holocene and Recent. The intra-annual δ18O shell temperature is higher today (Δδ18ORecent=2,22‰) compared to the Holocene (Δδ18OHolocene=1,47‰) and the Pleistocene (Δδ18OPleistocene=1,76‰). Pleistocene δ13C range value was the widest (Δδ13CPleistocene=2,92‰), but Holocene (Δδ13CHolocene=0,98‰) and Recent (Δδ13CRecent=1,9‰) values were tighter and more positive than Pleistocene. This could be explained by changes in ocean circulation since San Matías Gulf would has been formed approaching 12 ka years BP (after MIS 5, Ponce et al. 2011 Biol. J. Linn. Soc. 103, 363–379). These shells showed a clear marine environment but with a difference in sea surface temperature and ocean circulation through geological time in northern Patagonia. Our findings indicate that A. purpurata is a suitable candidate for detailed paleoenvironment reconstructions in North Patagonia. Further analyses will show whether some notable events that occurred during the Holocene, such as the Neoglacial (early Holocene), the Hypsithermal (Middle Holocene) and the Little Ice Age (Late Holocene) have been recorded in fossil A. purpurata shells.

Description: Since the 1970s tremendous changes have been observed in the Arctic region. As such, the surface air temperature within this region has increased twice the global average and according to existing climate model predictions, this trend will continue in the future (IPCC, 2007). However, interpretation of such transformation, which results from greenhouse warming, is still difficult. This is due to a lack of knowledge about the influence of multi-annual to decadal climate variations and the fact that climatic data from this region are usually temporally and spatially biased. Therefore, a better understanding and further research on the effects and predictability of climate variability is needed. We examined the growth variability in shells of the bivalve Arctica islandica which is affected by environmental factors, mainly temperature or food supply. We compare shells from two sampling sites, the northern Norwegian coast and Kola Peninsula coast (SW Barents Sea). Both localities are in the realm of the Norwegian Coastal Current (after crossing the border to Russia it is called the Murman Coastal Current). For the investigation of the annual and inter-annual growth variability all collected shells were cut parallel to the line of strongest growth (LSG) and 3 mm thick-sections were attached to a glass slide. After grinding and polishing, the cross-sections were stained in Mutvei´s solution. Annual growth bands were identified and measured. Samples for the stable oxygen isotope (δ18O) analysis and the seasonality approach were taken using a hand drill and the milling technique. As our prime objective we compared the shell growth of the Norwegian and the Russian populations and determined the external factors controlling the annual shell growth variability in A. islandica. Furthermore, the shells from both populations have been checked for decadal oscillations (NAO? ACRI?). Finally, stable oxygen isotope ratio (δ18O) profiles have been measured to identify seasonal signals and to reconstruct regional water temperature variability at a sub-annual level.

Description: Amiantis purpurata is a common warm-temperate water bivalve species distributed from southern Brazil to northern Patagonia, Argentina, which has a rich and well preserved fossil record in the San Matias Gulf (SMG) dating back to the late Quaternary. This study aims to establish A. purpurata shells as a new palaeoarchive of past marine conditions in South America. We compared the stable oxygen and carbon profiles (818O,heu; 813CS hcii) of eleven specimens of A. purpurata from different geological times (modem, Late Holocene and interglacial Late Pleistocene), and additionally present in situ oxygen isotope values of seawater within SMG (S,8Owatcr). Using both sets of information, we calculated and reconstructed palaeowater temperatures for the Late Holocene and compared them to modem water temperatures. Our findings indicate that A. purpurata records past environmental parameters such as water temperatures on a seasonal scale and can therefore be considered a suitable candidate for future palaeoenvironmental reconstructions in Northern Patagonia. This study is the first step towards further stable isotope analyses on fossil A. purpurata shells, which will show whether and if so, to what extent, important global climate events such as the Neoglacial (Early Holocene), the Hypsithermal (Middle Holocene) and the Little Ice Age (Late Holocene) occurred in South America.

Description: Understanding past seasonal temperature variability in the ocean is essential to evaluate the effects of future climate change on marine ecosystems. Here, we estimate seasonal amplitudes and average water temperature from stable oxygen isotope (δ18Oshell) values assuming δ18Owater values of 0.9±0.1permill (V-SMOV). Fossil valves of the bivalve Arctica islandica were collected from three Pleistocene successions (middle-late Calabrian) in Italy. Biostratigraphic analyses from Tacconi Quarry deposits (Rome) indicate an age between 1.6 and 1.2 Ma, while Augusta and Cutrofiano (Lecce) successions are slightly more recent (1.1 and 0.62 Ma, respectively). Prior to carbonate geochemical analysis, we checked the shells for potential diagenetic alterations (e.g., from aragonite to calcite). Stable oxygen isotope (δ18Oshell) profiles of eleven fossil A. islandica valves all depict a relatively low seasonality scenario. δ18Oshell amplitudes vary between 0.4permill and 1.1permill implying a reconstructed seasonal water temperature amplitude of 1.7 ̊C to 4.8 ̊C. The reconstructed average water temperature for the Sicilian population (i.e., 9 valves) is 9.5±0.47 ̊C for δ18Owater 0.9±0.1permill and coincides well with temperature requirements for modern A. islandica. The low seasonality scenario (ca. 3 ̊C) represented by the shells and the low reconstructed water temperatures, colder than modern water temperatures let to the conclusion that the shells lived during a maximum glacial phase when relatively constant water temperatures prevailed throughout the year.

Description: Biological hard parts and skeletons of aquatic organisms often archive information of past environmental conditions. Deciphering such information forms an essential contribution to our understanding of past climate conditions and thus our ability to mitigate the climatic, ecological, and social impacts of a rapidly changing environment. Several established techniques enable the visualization and reliable use of the information stored in anatomical features of such biogenic archives, i.e., its growth patterns. Here, we test whether confocal Raman microscopy (CRM) is a suitable method to reliably identify growth patterns in the commonly used archive Arctica islandica and the extinct species Pygocardia rustica (both Bivalvia). A modern A. islandica specimen from Norway has been investigated to verify the general feasibility of CRM, resulting in highly correlated standardized growth indices (r〉0.96; p〈0.0001) between CRM-derived measurements and measurements derived from the established methods of fluorescence microscopy and Mutvei’s solution staining. This demonstrates the general suitability of CRM as a method for growth pattern evaluation and cross-dating applications. Moreover, CRM may be of particular interest for paleoenvironmental reconstructions, as it yielded superior results in the analysis of fossil shell specimens (A. islandica and P. rustica) compared to both Mutvei staining and fluorescence microscopy. CRM is a reliable and valuable tool to visualize internal growth patterns in both modern and fossil calcium carbonate shells that notably also facilitates the assessment of possible diagenetic alteration prior to geochemical analysis without geochemically compromising the sample. We strongly recommend the CRM approach for the visualization of growth patterns in fossil biogenic archives, where conventional methods fail to produce useful results.

Description: Understanding the climate of the past is essential for anticipating future climate change. Palaeoclimatic archives are the key to the past, but few marine archives (including tropical corals) combine long recording times (decades to centuries) with high temporal resolution (decadal to intra-annual). In temperate and polar regions carbonate shells can perform the equivalent function as a proxy archive as corals do in the tropics. The bivalve Arctica islandica is a particularly unique bio-archive owing to its wide distribution throughout the North Atlantic and its extreme longevity (up to 500 years). This paper exemplifies how information at intra-annual and decadal scales is derived from A. islandica shells and combined into a detailed picture of past conditions. Oxygen isotope analysis (δ18O) provides information on the intra-annual temperature cycle while frequency analysis of shell growth records identifies decadal variability such as a distinct 5-year signal, which might be linked to the North Atlantic Oscillation.

Description: Future global warming will impact coastal marine ecosystems significantly. These changes are expected to be particularly dramatic in the sub-Arctic and Arctic regions which have experienced similar conditions during warm periods in the past (e.g., Holocene Optimum, mid-Pliocene Warm Period). Palaeo-climatic information from these periods will facilitate our predictions of future climate change. Fossil shells of the bivalve Arctica islandica constitute reliable bio-archives for coastal regions of the North Atlantic over geological time scales. We analysed sub-fossil shells of A. islandica from Svalbard for their potential to reconstruct local palaeoenvironmental conditions. These shells were collected from raised beach deposits in Dicksonfjorden, a branch of Isfjorden on the western coast of Spitsbergen. Radiocarbon dating (14CAMS) confirms that the analysed specimens lived at about 8800 yr BP, i.e., during the Holocene Climate Optimum, which was characterized by summer sea surface temperatures 1-3°C warmer than today. This difference in SST also explains today’s extinction of A. islandica on the Svalbard archipelago, as modern water temperatures fall below its thermal tolerance. Analysis of the growth patterns revealed ontogenetic ages of up to 90 years for individual specimens. In addition, the shell growth patterns yield evidence of significant decadal oscillations within the Holocene Climate Optimum in the Arctic. The excellent state of preservation of these shells has been confirmed using Raman microscopy. Since all the sub-fossil shells have been preserved remarkably well, it was considered appropriate to conduct stable oxygen and carbon isotope (δ18O & δ13C) analysis. Results for δ18O show well-defined seasonal cycles, ranging from 1.6‰ to 4.5‰. Ice-volume corrected δ18O values for seawater have been used to calculate palaeo-water temperatures on a sub-annual scale.

Description: Throughout the last decades the anatomical and/or geochemical properties of marine biogenic hard-parts (e.g., mollusc shells, fish otoliths or coralline algae) became a valuable source for palaeoenvironmental information. Regular growth patterns are an important characteristic of these bioarchives, which allow proxy-information to be generated with distinct and high temporal resolution. However, standard methods of growth pattern visualization may fail in fossil bio-archives owing to alterations of the organic compounds within the biogenic materials with time. We demonstrate that confocal Raman microscopy (CRM) can identify and visualize growth patterns of mollusc shells from different geological ages with high spatial resolution (300 nm). In contrast to standard staining techniques (e.g., Mutvei's solution) CRM has been be applied successfully to samples in which the organic components are altered. Furthermore, CRM is ideal to identify mineral (and organic) phases and potential taphonomic alterations (e.g., recrystallization from aragonite to calcite) in marine biogenic carbonates. Checking for such alterations should be a mandatory step prior to any kind of biogeochemical analysis (e.g., stable isotopes or trace elemental ratios) of fossil samples. Therefore, CRM can play an important role in the quality control of biogenic carbonate studies. Here we use CRM to visualize growth structures in the umbonal and the ventral shell portion of the marine bivalve Arctica islandica at different spatial resolutions (μm to mm). The reliability of the method has been tested and proven by comparing the growth structures in Mutvei and CRM derived images of the same modern A. islandica specimen. In addition, CRM has been applied to fossil samples in which staining techniques failed. Derived growth trends are shown and all CRM results are compared to reflected light microscopy and staining methods in the same specimens.