Elena Dunca

North Atlantic Oscillation dynamics recorded in shells of a long-lived bivalve mollusk

Existing reconstructions of the winter North Atlantic Oscillation (WNAO) are based on terrestrial proxies and historical documents.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.

Constructing long-term proxy series for aquatic environments with absolute dating control using a sclerochronological approach: introduction and advanced applications

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.

Siphuncular structure in the Recent Nautilus, compared with that in Mesozoic nautilids and ammonoids from Madagascar

The connecting ring in the Recent Nautilus is composed of two layers: a fibrous, organic inner layer, and a porous, spherulitic-prismatic outer layer. The organic layer is an uncalcified continuation from the nacreous layer of the septal neck. In the Mesozoic nautilids Cymatoceras and Paracenoceras from Madagascar, that have the Nautilus type of siphuncles, the inner fibrous-organic layer of the connecting ring has been destroyed by diagenesis. The outer spherulitic-prismatic layer is twice as thick as in Nautilus, which indicates that, in addition to jet-powered swimming, they also used short-term buoyancy changes for vertical migrations, and that the mode of life of these nautilids was somewhat different from that of the Recent Nautilus. Moreover, the Madagascar nautilids lack the auxiliary ridge by which the connecting ring in Nautilus is firmly attached to the inner surface of the preceding septal neck. The siphuncle in Nautilus can therefore withstand a much higher hydrostatic pressure than that in the fossil nautilids that lacked this deposit. In ammonoids the fibrous-organic layer of the connecting ring has a solid, semi elastic structure. In all 11 ammonoid taxa from Madagascar here studied, this layer is preserved whereas the nautilids from the same stratigraphic horizons have lost this layer. This clearly indicates that the fibrous-organic layer of the connecting ring in nautilids is chemically different from that in ammonoids.

Structural relationship between interlamellar organic sheets and nacreous tablets in gastropods and the cephalopodNautilus

The nacreous tablets in gastropods and the cephalopodNautilus are composed of three calcareous layers: a principal, thick, finely granular layer and two thin, coarse-granular layers, one covering the upper surface of the principal layer and another the lower surface of this layer. The granules on the surface layers inNautilus differ from those in gastropods by their much more elongated shape and larger size. The central portion of the nacreous tablet of gastropods andNautilus is more or less elevated forming the central elevation. The granules on this portion usually are larger, irregularly shaped and more crowded than on the main, peripheral portion of the tablet. The untreated, dry interlamellar organic sheets on upper surfaces of immature nacreous tablets are uncalcified and elastic. Narrow thicker parts of the sheet, the trabeculae, Surround large intertrabecular spaces where the sheet is thin. In places it can be observed that each calcareous granula on the surface layer of the nacreous tablet is situated within the intertrabecular space. The size, shape and distribution of the intertrabecular Spaces correspond those of the surface granules. No mineral bridges were observed between the consecutive nacreous tablets.KurzfassungDie Perlmutt-Tafeln in der Schale von Gastropoden und dem CephalopodenNautilus bestehen aus drei kalkigen Lagen, einer dicken, fein-granularen Hauptlage und zwei dünnen grob-granularen Lagen, von denen eine die äußere Oberfläche, die andere die innere Oberfläche der Hauptlage bedeckt. Die Körner der Decklagen beiNautilus unterscheiden sich von denen der Gastropoden durch ihre starke Verlängerung und Größe. Der zentrale Bereich der Perlmutt-Tafeln von Gastropoden undNautilus ist mehr oder weniger aufgewölbt und bildet die zentrale Wölbung. Normalerweise sind die Körner in diesem Bereich größer, unregelmäßig geformt und dichter gedrängt als im peripheren Bereich der Tafel. Die unbehandelten, trockenen interlamellaren organischen Lagen auf der äußeren Oberfläche von immaturen Perlmutt-Tafeln sind unverkalkt und elastisch. Schmale dickere Bereiche der Lagen, die Trabeculae, umgeben große intertrabeculäre Bereiche, in denen die Lagen dünn bleiben. An einigen Stellen zeigt sich, dass jedes kalkige Korn der Oberflächen-Lage einer Perlmutt-Tafel innerhalb des intertrabeculären Bereichs liegt. Größe, Form und Verteilung der intertrabeculären Räume entsprechen denen der Oberflächen-Körner. Zwischen aufeinander folgenden Perlmutt-Tafeln wurden keine Mineral-Brücken beobachtet.

Siphonal zone structure in the cuttlebone of Sepia officinalis

The evolutionary process through which the siphonal zone of the cuttlebone of Sepia replaced the tubular siphuncle seen in other shelled cephalopods is poorly understood. Recently, porous connecting stripes, interpreted as homologous to connecting rings of tubular siphuncles, were revealed in Sepia (Acanthosepion) cf. savignyi (Geobios, 45:13–17, 2012). New data on the siphonal zone structure are herein demonstrated through SEM testing of 16 beach-collected cuttlebones of Sepia officinalis from Vale do Lobo, southern Portugal. In examined cuttlebones, the organic connecting stripes are mineralized along their peripheries where they are attached to septa by inorganic–organic porous contacting ridges. The contacting ridges consist of globular crystalline units within an organic matrix; each globule is a stack of rounded alternating organic and mineralized microlaminas parallel to the septal surface; mineralized microlaminas contain carbonate microgranules. Porous connecting stripes together with the contacting ridges may serve as transport routes for the cameral liquid used in buoyancy regulation. The contacting ridges appear to reinforce contacts between the connecting stripes and septa, and may strengthen shell resistance to changing environments. Lamella–fibrillar nacre in septa is demonstrated in Sepia for the first time. Comparison of Sepia and Spirula reveals the common character of their phragmocones, the slit-like shape of the permeable zones between chambers and the siphuncle. Narrowing of the permeable zones may provide shell resistance to high hydrostatic pressure; however, the essentially dissimilar relative length of the permeable zones may results in different capabilities of two genera for buoyancy regulation. In Sepia, long narrow porous inorganic–organic permeable connecting stripes and contacting ridges may allow for rapid buoyancy regulation which would lead to environmental plasticity and higher species diversity.