The Mediterranean Sea during the Pleistocene - bivalve shells and their potential to reconstruct decadal and seasonal climate signals of the past

Understanding the climate of the past and past seasonal temperature amplitudes is essential to evaluate the effects of future climate change on marine ecosystems. The Mediterranean Sea is of great importance due to its crucial role in modern atmospheric phenomena such as the North Atlantic Oscillation (NAO). Fossil shells of the bivalve Arctica islandica were collected from three different Pleistocene successions in Italy. The seasonal water temperature amplitude was reconstructed using stable oxygen isotope (δ18Oshell) analysis. Samples were derived by the micro-milling approach and measured by isotope ratio mass spectrometer. Results show a low seasonality scenario (~3°C). This is in sharp contrast to the assumption that the simultaneous occurrence of boreal and warm-water species in the middle Calabrian Mediterranean Sea can be explained by high seasonality (~10°C). A prominent 6-year cyclicity was identified in the shell growth time-series by means of spectral analysis. This signal might be linked to the NAO whose periodicity ranges between 5-9 years. However, a connection to the Mediterranean Oscillation cannot be excluded. The low seasonality (~3°C) and the relatively low mean water temperature (9-10°C) indicate that the middle Calabrian Mediterranean Sea was characterized by colder climatic conditions compared to nowadays, indicating a maximum glacial phase

Temperature selectivity in Icelandic and Northeast-Arctic cod

Increasing water temperatures are predicted worldwide, with high amplitudes in the Arctic and sub-Arctic regions exceeding predictions for other regions. An understanding how Atlantic cod (Gadus morhua) reacted to changing environmental conditions in the past is essential for predicting re-distribution under climate change. In this thesis, I examined the temperature selectivity of Icelandic and Northeast-Arctic (NEA) cod in response to fluctuating temperature conditions and changes in the stock dynamics. Multiple century-long data time series and linear mixed-effect models were used to investigate the effect of fluctuating water temperatures and changes in stock dynamics on the temperature selectivity of cod, using stable oxygen isotope composition in otoliths as a proxy of ambient temperature. Icelandic cod δ18Ootolith values were significantly correlated with water temperature time series, indicating that they were exposed to fluctuating water temperatures during the past 100 years and did not move appreciably in response to increasing ocean temperatures. Furthermore, abundance changes have affected the temperature selectivity of Icelandic and NEA cod as a density-driven response; however, the response of the two populations was different. Increasing abundance resulted in increasing intraspecific competition and decreasing individual fitness levels, which expanded the distribution area of both cod stocks into previously unfavorable thermal habitats. To validate the accuracy of high-resolution otolith isotopic records as a temperature proxy, stable oxygen isotope records of wild, free-swimming Icelandic cod tracked with data-storage tags (DST) were analyzed with high-resolution secondary-ion mass spectrometry (SIMS). Results demonstrated that the method is well-suited as a relative index of temperature exposure, although maximum ambient temperatures were often overestimated. Using century-long data time series and oxygen isotope chronologies of cod otoliths have markedly improved our understanding of the Icelandic and NEA cod temperature selectivity. Thus, this thesis provides important information that helps to predict how two commercially important cod stocks might respond to global warming and fluctuating stock dynamics.Hækkandi sjávarhita er nú spáð um allan heim og á norðurslóðum er auk þess búist við meiri hitasveiflum en annars staðar. Betri skilningur á viðbrögðum Atlantshafsþorsks (Gadus morhua) við breyttum umhverfisaðstæðum á liðnum áratugum opnar möguleika á að spá fyrir um mögulega breytta dreifingu þorskins á tímum loftslagsbreytinga. Hér er greint frá rannsóknum þar sem leitast var við að skýra hvort og hvernig þorskar af íslenska stofninum og norðaustur heimsskautsstofninum bregðast við breytingum á hitastigi sjávar og stofnstærð. Gagnasett með mælingum 100 ár aftur í tíman ásamt línulegum líkönum með blönduðum áhrifaþáttum voru notuð til þess að meta hvort stofnstærð og sjávarhiti hafi áhrif á hitastigsval einstaklings, sem hægt er að meta með stöðugum ísótópum súrefnis. Marktæk fylgni fannst milli δ18O gilda íslenska þorskins og sjávarhita yfir tíma, sem bendir til þess að þorskurinn hafi verið útsettur fyrir sveiflukenndum sjávarhita síðastliðin 100 ár, en virðist ekki hafa hörfað að marki frá hækkandi hitastigi. Aukinn stofnþéttleiki á svæðum með kjörhita getur leitt til aukinnar samkeppni og minnkandi hæfni einstaklinga en slíkt getur valdið því að sumir einstaklingar leiti á önnur óhagstæðari hitastigs mið. Gögn okkar benda til þess að slíkt hafi átt sér stað í báðum stofnunum, þrátt fyrir ólík viðbrögð. Til frekari staðfestingar á notagildi kvarna til að varpa ljósi á hitastig sjávar á mismunandi stigum lífsferils voru ísótópar súrefnis mældir í kvörnum þorska sem á voru fest gagnasöfnunarmerki (DST-tag) sem mæla hitastig og dýpi. Í þessu tilfelli voru ísótópar í vaxtarhringjum kvarna mældir með meiri upplausn í nákvæmum massalitrófsgreini (SIMS) sem staðfesti að þessar aðferðir eru vel til þess fallnar að meta hitastig sjávar sem fiskar hafa dvalið í aftur í tíman, þó svo að hámarks hiti sé gjarnan ofmetin. Rannsóknirnar sýna hvernig nota má mælingar á ísótópum súrefnis í árhringjum kvarna ásamt sambærilegum langtímagögnum um hitastig sjávar og stofnsveiflur til þess að greina hvernig þessir þættir hafa áhrif á hitastigsval þorsks. Slíkar upplýsingar munu reynast gagnlegar við að spá fyrir um hvernig þorskstofnar bregðast við hnattrænni hlýnun og stofnsveiflum

Stable oxygen isotope reconstruction of temperature exposure of the Icelandic cod (Gadus morhua) stock over the last 100 years

Publisher's version (útgefin grein)Increasing water temperatures are predicted around the globe, with high amplitudes of warming in Subarctic and Arctic regions where Atlantic cod (Gadus morhua) populations currently flourish. We reconstructed oxygen isotope and temperature chronologies from Icelandic cod otoliths, one of the largest cod stocks in the world, to determine if cod moved or migrated over the last 100 years to avoid increasing water temperatures. For δ18Ootolith analysis, individual annual growth increments from immature and mature life history stages were micromilled from adult otoliths, which were collected in southern Iceland. Linear mixed-effect models confirmed that stable oxygen isotope time series of immature and mature cod differ significantly between both life stages (p < 0.001). Overall, cod otolith δ18O was significantly correlated with water temperature (sea surface temperature: p < 0.001, temperature at 200 m depth: p < 0.01), indicating that Atlantic cod were exposed to fluctuating water temperatures during the past 100 years and did not move as a response to increasing ocean temperatures. All of the alternate physical factors that were considered for the isotope-based variation in the temperature exposure of Icelandic cod were rejected.Funding for this study was provided by Icelandic Research Fund Grant 173906-051. Establishment funding for FARLAB (Facility for advanced isotopic research and monitoring of weather, climate, and biogeochemical cycling) by Research Council Norway Grant 245907 is also acknowledged.Peer reviewe

Temperature exposure in cod driven by changes in abundance

Animals actively select the most suitable habitat in terms of fitness, much of which is mediated by temperature. We reconstructed population abundance, oxygen isotope and temperature chronologies for the Icelandic and the Northeast Arctic (NEA) cod (Gadus morhua) populations to determine if their temperature selectivity over the last 100 years was driven by rising water temperatures and (or) changes in abundance. Individual annual growth increments from immature and mature life history stages of cod collected in southern Iceland and the Lofoten area (Norway) were micromilled from adult otoliths and then assayed for stable oxygen isotopes (δ18Ootolith). Linear mixed effect models were used to identify and quantify the density-dependent temperature exposure of both cod populations. The results indicated that Icelandic cod migrated into warmer waters with increasing abundance (p < 0.05), whereas NEA cod moved into colder waters (p < 0.001). Our results suggest that thermal preferences and density-dependent effects can be used to forecast potential redistribution scenarios of fish as oceans warm.publishedVersio

Was the Mediterranean Sea during the Calabrian (Early Pleistocene) a low seasonality environment?

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

Growth portfolios buffer climate-linked environmental change in marine systems

Large-scale, climate-induced synchrony in the productivity of fish populations is becoming more pronounced in the world's oceans. As synchrony increases, a population's “portfolio” of responses can be diminished, in turn reducing its resilience to strong perturbation. Here we argue that the costs and benefits of trait synchronization, such as the expression of growth rate, are context dependent. Contrary to prevailing views, synchrony among individuals could actually be beneficial for populations if growth synchrony increases during favorable conditions, and then declines under poor conditions when a broader portfolio of responses could be useful. Importantly, growth synchrony among individuals within populations has seldom been measured, despite well-documented evidence of synchrony across populations. Here, we used century-scale time series of annual otolith growth to test for changes in growth synchronization among individuals within multiple populations of a marine keystone species (Atlantic cod, Gadus morhua). On the basis of 74,662 annual growth increments recorded in 13,749 otoliths, we detected a rising conformity in long-term growth rates within five northeast Atlantic cod populations in response to both favorable growth conditions and a large-scale, multidecadal mode of climate variability similar to the East Atlantic Pattern. The within-population synchrony was distinct from the across-population synchrony commonly reported for large-scale environmental drivers. Climate-linked, among-individual growth synchrony was also identified in other Northeast Atlantic pelagic, deep-sea and bivalve species. We hypothesize that growth synchrony in good years and growth asynchrony in poorer years reflects adaptive trait optimization and bet hedging, respectively, that could confer an unexpected, but pervasive and stabilizing, impact on marine population productivity in response to large-scale environmental change.publishedVersio

Thin-sections of marine bivalve shells: a window to environmental reconstructions on daily scale?

“Bioarchives” are organisms, which form hard parts over the course of their lifetime that remain even after the death of the organism. Environmental conditions prevailed during the lifetime of the bioarchives can be approximated from anatomical, morphological and geochemical properties on the shell. For instance, shell growth rates constitute a “proxy” of general living conditions, oxygen isotope ratios (δ18O) are an established proxy of water temperature, and shell content of heavy metals or of organic constituents can be indicative specific pollution histories. Due to their high resolution, bivalve shells are well suited for sclerochronological studies. Generally, this science focuses on growth rates and chemical properties of hard parts. The ocean quahog Arctica islandica is suited as a bioarchive due to its broad geographic distribution and longevity. This study looks at growth patterns in the shells of the bivalve A. islandica (marine) and Unio sp. (freshwater). The objective was to establish standard procedures for shell preparation to visualize shell increments formed on a daily basis (“microincrements”). In order to visualize microincrements thin-sections of the marine bivalve A. islandica and the freshwater bivalve Unio sp. were prepared. Therefore, different attempts for embedding, etching, bleaching and visualization were tested. Microincrements are visible in thin-sections of both genera. The microincrements of the freshwater mussel Unio sp. are significantly smaller (1.5 μm on average) than those of A. islandica (12.5 μm on average). However, microincrements in Unio sp. are more easily recognizable and can be measured consecutively over a range of more than one year. The visualization of microincrements in A. islandica remained more challenging and therefore additional attempts such as bleaching, etching and additional visualization techniques were tested for their potential to improve the visualization of microincrements. The visualization of microincrements in A. islandica still needs further improvement before measured microincrement widths can be correlated to environmental data. However, Unio sp. seems to have great potential and can be used as a window to reconstruct environmental data on a daily scale in the future

The Mediterranean Sea during the Pleistocene – bivalve shells and their potential to reconstruct decadal and seasonal climate signals of the past

Understanding the climate of the past, in particular seasonal temperature amplitudes, is essential to evaluate the effects of future climate change on marine ecosystems. The Mediterranean is of particular importance, because of its crucial role in modern ocean atmosphere phenomena such as the North Atlantic Oscillation (NAO). We analyzed fossil shells of the bivalve Arctica islandica collected from Pleistocene successions in Central and Southern Italy (i.e., Rome, Lecce and Sicily). According to preliminary biostratigraphic data the studied deposits belong to the middle Calabrian, between 1.2-0.9 Ma for the Sicily outcrop and 1.4-1.2 Ma for the Rome and Lecce outcrops. Prior to isotope geochemical analysis confocal Raman microscopy measurements were conducted to detect potential diagenetic alterations (e.g., from aragonite to calcite). The seasonal water temperature amplitude was reconstructed using stable oxygen isotope (δ18O) values, which were derived by micro-milling and Isotope Ratio Mass Spectrometry. Analysis of the growth patterns (on-going research) revealed ontogenetic ages of up to 210 years. These time series are used for the identification of multi-year (i.e., decadal) patterns, such as the NAO. First results of our study indicate that seasonality was remarkably low during the studied geological epoch. This is in sharp contrast to previous assumptions according to which the simultaneous occurrence of boreal (A. islandica) and warm-water species in the Mediterranean Sea during the Pleistocene can be explained by high seasonality. Different links and scenarios on a regional as well as a bigger scale will be discussed

A low seasonality scenario in the Mediterranean Sea during the Calabrian (Early Pleistocene) inferred from fossil Arctica islandica shells

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 water temperature amplitudes from stable oxygen isotope (\uce\ub418Oshell) values of fossil shells of Arctica islandica (assuming \uce\ub418Owater= + 0.9 \uc2\ub1 0.1\ue2\u80\ub0 V-SMOW). Specimens were collected from three Pleistocene successions (Emilian and Sicilian substages of the Calabrian) in Central and Southern Italy (i.e., Rome, Lecce and Sicily). Biostratigraphic analyses from Rome Quarry deposits indicate an age between 1.6 and 1.2 Ma, whereas Sicily and Lecce successions are slightly more recent (between 1.1 and 0.62 Ma). Prior to carbonate geochemical analysis, we checked the shells for potential diagenetic alterations (e.g., from aragonite to calcite) using confocal Raman microscopy. \uce\ub418Oshelltransects indicate an annual temperature amplitude of about 3 \uc2\ub0C during the Early Pleistocene. This is in sharp contrast to reconstructions based on faunal assemblages, according to which the simultaneous occurrence of boreal and warm-water species in the Calabrian Mediterranean Sea suggests a much higher seasonality (ca. 10 \uc2\ub0C). The low seasonality and the relatively cold water (9\ue2\u80\u9310 \uc2\ub0C) indicate the outcrops represent colder climatic conditions compared to modern times, and suggest the occurrence of a maximum glacial phase

Temperature exposure in cod driven by changes in abundance

Animals actively select the most suitable habitat in terms of fitness, much of which is mediated by temperature. We reconstructed population abundance, oxygen isotope and temperature chronologies for the Icelandic and the Northeast Arctic (NEA) cod (Gadus morhua) populations to determine if their temperature selectivity over the last 100 years was driven by rising water temperatures and (or) changes in abundance. Individual annual growth increments from immature and mature life history stages of cod collected in southern Iceland and the Lofoten area (Norway) were micromilled from adult otoliths and then assayed for stable oxygen isotopes (δ18Ootolith). Linear mixed effect models were used to identify and quantify the density-dependent temperature exposure of both cod populations. The results indicated that Icelandic cod migrated into warmer waters with increasing abundance (p < 0.05), whereas NEA cod moved into colder waters (p < 0.001). Our results suggest that thermal preferences and density-dependent effects can be used to forecast potential redistribution scenarios of fish as oceans warm