Sclerochronology uses shell growth lines or bands for the construction of environmental time-series and the measurement of organism growth, but more study is needed to constrain the triggers of the dark cessation bands observed in many bivalve groups. We constructed a database of direct observations of modern growth seasonality across the class Bivalvia and compared the occurrence of seasonal growth bands to environmental data including latitude, temperature, and chlorophyll-a concentration. Bivalves with cold-season (winter) cessations are more common towards the poles, with logistic regression showing that temperature, followed by latitude of occurrence, displays the strongest relationship with occurrence of winter cessation. Remotely sensed and directly measured chlorophyll-a concentration show no significant relationship. Summer cessations are sparse and only weakly associated with environmental controls but are concentrated at the subtropical latitudes among temperate bivalves at their equatorial extremes. The rarity of summer cessations can be explained by the limited annual ranges of temperature in the tropics, combined with the exponential relationship of metabolic rate to temperature leading to a narrow window between normal functioning and mortality at high temperatures. This data suggests that, unless annual temperatures have low variability like in equatorial or polar regions, the season of growth cessation across bivalves is primarily a function of temperature tolerance through restriction of scope for growth. At most latitudes, growth bands can be interpreted as being primarily triggered by temperature stress, rather than seasonal starvation.

No abstract

Abstract. The chemical composition of mollusk shells is a useful tool in (paleo)climatology since it captures inter- and intra-annual variability in environmental conditions. Trace element and stable isotope analyses with improved sampling resolution now enable the use of mollusk shells for paleoenvironmental reconstructions at a daily to sub-daily resolution. Here, we discuss hourly resolved Mg/Ca, Mn/Ca, Sr/Ca and Ba/Ca profiles measured by laser ablation ICP-MS through shells of photosymbiotic giant clams (Tridacna maxima, Tridacna squamosa and Tridacna squamosina) and the non-photosymbiotic scallop Pecten maximus. Precise sclerochronological age models and spectral analysis allowed us to extract daily and tidal rhythms in the trace element composition of these shells. We find significant expression of these periodicities but conclude that this cyclicity explains less than 10 % of the sub-annual variance in trace element profiles. Tidal and diurnal rhythms explain variability of at most 0.2 mmol/mol (~10 % of mean value) in Mg/Ca and Sr/Ca, while Mn/Ca and Ba/Ca cyclicity has a median amplitude of less than 2 µmol/mol (~40 % and 80 % of the mean of Mn/Ca and Ba/Ca, respectively). Daily periodicity in Sr/Ca and Ba/Ca is stronger in Tridacna than in Pecten, with Pecten showing stronger tidal periodicity. One T. squamosa specimen which grew under a sunshade exhibits some of the strongest diurnal cyclicity. Daily cycles in trace element composition of giant clams are therefore unlikely to be driven by variations in direct insolation itself but reflect an inherent biological rhythmic process affecting element incorporation. Finally, the large amount of trace element variability unexplained by periodic variability highlights the dominance of aperiodic processes in mollusk physiology and/or environmental conditions on shell composition at the sub-daily scale. Future studies should aim to investigate whether part of this aperiodic variability in shell chemistry reliably records weather patterns or circulation changes in the paleoenvironment.

Current climate change projections anticipate that global warming trends will lead to changes in the distribution and intensity of precipitation at a global level. However, few studies have corroborated these model-based results using historical precipitation records at a regional level, especially in our study region, California. In our analyses of 14 long-term precipitation records representing multiple climates throughout the state, we find northern and central regions increasing in precipitation while southern regions are drying. Winter precipitation is increasing in all regions, while other seasons show mixed results. Rain intensity has not changed since the 1920s. While Sacramento shows over 3 more days of rain per year, Los Angeles has almost 4 less days per year in the last century. Both the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) greatly influence the California precipitation record. The climate change signal in the precipitation records remains unclear as annual variability overwhelms the precipitation trends. OPEN ACCESSClimate 2014, 2 169

The Gulf of Aqaba is home to three giant clam species with differing ecological niches and levels of photosymbiotic activity. Giant clams grow a two‐layered shell where the outer layer is precipitated in close association with photosymbiont‐bearing siphonal mantle, and the inner layer is grown in association with the light‐starved inner mantle. We collected 39 shells of the three species (the cosmopolitan Tridacna maxima and T. squamosa, as well as the rare endemic T. squamosina) and measured carbon and oxygen isotope ratios from inner and outer shell layers, to test for differences among species and between the layers of their shells. T. squamosina records higher temperatures of shell formation as determined by oxygen isotope paleothermometry, consistent with its status as an obligately shallow‐dwelling species. However, the known negative fractionation imparted on tissue carbon isotopes by photosymbiotic algae did not produce measurable offsets in the carbonate δ13C values of the more symbiotic T. squamosina and T. maxima compared to the more heterotrophic T. squamosa. Across all species, outer shell layers recorded mean growth temperatures 1.8°C higher than corresponding inner layers, which we propose is a function of the high insolation, low albedo microenvironment of the outer mantle, and potentially the activity of the symbionts themselves. Population‐wide isotopic sampling of reef‐dwelling bivalve shells can help constrain the ecological niches of rare taxa and help reconstruct their internal physiology.

The health of reef-building corals has declined due to climate change and pollution. However, less is known about whether giant clams, reef-dwelling bivalves with a photosymbiotic partnership similar to that found in reef-building corals, are also threatened by environmental degradation. To compare giant clam health against a prehistoric baseline, we collected fossil and modern Tridacna shells from the Gulf of Aqaba, Northern Red Sea. After calibrating daily/twice-daily growth lines from the outer shell layer, we determined that modern individuals of all three species ( Tridacna maxima , T. squamosa and T. squamosina ) grew faster than Holocene and Pleistocene specimens. Modern specimens also show median shell organic δ 15 N values 4.2‰ lower than fossil specimens, which we propose is most likely due to increased deposition of isotopically light nitrate aerosols in the modern era. Nitrate fertilization accelerates growth in cultured Tridacna , so nitrate aerosol deposition may contribute to faster growth in modern wild populations. Furthermore, colder winter temperatures and past summer monsoons may have depressed fossil giant clam growth. Giant clams can serve as sentinels of reef environmental change, both to determine their individual health and the health of the reefs they inhabit.

No abstract

No abstract