Description: Modern to Pleistocene Amiantis purpurata shells collected in Bahía San Antonio (Patagonia, Argentina) were studied by X-ray diffraction (XRD), optical and electron microscopy, electron microprobe analyses, and microindentation, in order to characterize early diagenetic changes and mechanical resistance. The sole crystalline phase is twinned aragonite showing pseudohexagonal symmetry. The regularity of the crystallographic texture decreases in older samples, but average crystallite size does not increase. The microstructure, which is dominantly crossed lamellar, is progressively replaced by a more randomly oriented grain aggregate. Compositional profiles across the shell show gradients in Sr, Na, S, and Cl, whereas Mg and P are more evenly distributed. Each shell layer has a distinct chemical signature. A marked decrease in the concentration of all of these elements, along with flattening of profiles, is evident as age increases. Vickers microhardness is lowest in modern specimens, showing at the same time the least chipped regions; older shells become harder and more fragile. All of these changes are attributed to postdepositional modifications by dissolution-recrystallization processes mediated by a thin film of water in a vadose environment. Microstructural adjustments are more sluggish than chemical modifications produced by diagenetic processes, whereas microhardness rapidly reaches high values, probably due to the early degradation of organic compounds from the shell. Our study shows that aragonitic shells that retain their primary mineralogical composition have undergone subtle chemical and microstructural changes. A very small amount of calcite was produced during grinding for XRD. Care should therefore be taken when seeking calcite as evidence of diagenetic changes.

Description: The use of rocky intertidal assemblages in paleoecology and conservation paleobiology studies is limited because these environments have low preservation potential. Here, we evaluate the fidelity between living intertidal mussel bed communities (life assemblages or LAs) and mollusk shell accumulations (death assemblages or DAs) from the environmentally harsh Patagonian Atlantic Coast. LAs were sampled from rocky mid-intertidal and mussel-dominated habitats while DAs were collected from the high water mark at beaches in close proximity to the living intertidal community to assess live-dead mismatch at regional scales. DAs were restricted to the subset of species in the DAs that inhabit rocky intertidal habitats. A total of 37,193 mollusk specimens from 15 intertidal species were included in the analysis. Ten species were present in LAs, 14 in DAs, and nine were shared by LAs and DAs. DAs showed higher diversity, less dominance, and more rare species than LAs. Despite finding good agreement in species composition between DAs and LAs within the same region, smaller species are underrepresented, as shown by differences in size-frequency distributions. Our findings indicate that the composition of DAs is a result of the combined effects of spatial and temporal averaging, size-related biases, and biases related to low detectability of boring and vagile species in LAs. Thus, DAs do not accurately detect within-provincial latitudinal gradients in composition. However, DAs clearly capture differences between the Argentine–Magellanic Transition Zone and the Magellanic Province, indicating that DAs are informative tools at regional scales despite the environmental harshness to which they are subjected.

Description: Drilling predation is frequently studied in the fossil record. Less information is available from recent environments, however. Previous studies have indicated that drilling predation is usually higher in the tropics but little research has been undertaken in high latitudes. To address this hypothesis, we examine muricid-drilling predation along a 1,000 km transect in southern South America. Drilling frequencies ranged between 3% and 36%, and they were not correlated with the abundance of the predator ( Trophon geversianus ) or the abundance of its preferred prey. The only locality with exceptionally high predation (36%) was a heavily anthropogenically impacted site. Trophon exhibited different drilling strategies on different prey, and edge drilling represented 27%–56% of the drill holes in mytilids. Drilling frequencies were not correlated with latitude or water temperature. Our results, however, show that drilling frequencies are indeed lower at high latitudes compared to the tropics, and these data provide a recent baseline to compare and interpret spatial variability in muricid drilling predation from past environments. The fact that dead-shell assemblages seem to be recording human-related impacts in this system strengthens their relevance as potentially valuable conservation tools.

Description: Article Mesopelagic fishes dominate the global fishes biomass, yet there exist major uncertainties regarding their global biomass. Irigoien et al. analyse acoustic data collected during a circumglobal cruise and show that biomass estimates should be raised by an order of magnitude. Nature Communications doi: 10.1038/ncomms4271 Authors: Xabier Irigoien, T. A. Klevjer, A. Røstad, U. Martinez, G. Boyra, J. L. Acuña, A. Bode, F. Echevarria, J. I. Gonzalez-Gordillo, S. Hernandez-Leon, S. Agusti, D. L. Aksnes, C. M. Duarte, S. Kaartvedt

Description: In southern South America little is known about the biotic response of marine species to large scale climate variability during the Holocene. Fossil shells of the aragonitic bivalve Retrotapes exalbidus (previously called Eurhomalea) offer the possibility to investigate climate variability in the Beagle Channel and past seasonal dynamics of sea water temperature during the mid-to-late-Holocene. This selection is based on two reasons: extant R. exalbidus preserve annual increments in the outer shell layer (Lomovasky et al., 2002 J. Sea Research 48, 209-216); and, although not very common as other venerids, this species is well preserved in different Holocene marine outcrops along the channel. Holocene fossil shells of R. exalbidus were sectioned, polished, photographed and measured, and after examination three of them were selected for chemical sampling. In each case, one-half of the shell was used to resolve the annual growth bands and the other half was used for stable isotopes sampling. In addition, a fragment of each shell was used to C-14 dating performed in the Poznań Radiocarbon Laboratory with the accelerator mass spectrometry (AMS) technique. Ontogenetic ages were measured by counting the annual growth increments under a stereo microscope. Our results show differences between the three specimens. In the ontogenetic oldest individual (14 years), which gave a calibrated mean value age of 3839 BP, the δ18O values ranged from 1.53‰ to –1.16‰. The two other specimens (8 years), with calibrated mean ages of 5190 BP and 431 BP, gave δ18O values from 1.55‰ to 0.44‰ in the oldest specimen, and from 1.29‰ to 0.72‰, in the youngest one. Besides, we found variations in annual growth increment widths at different radiocarbon ages, probably correlated with environmental changes over the mid-to-late Holocene. We correlated the most positive δ18O values with winter and the most negative δ18O with summer. In addition, the summer values around 3800 yr BP are more negative than around 5000 years or 500 years BP. These findings correlate well with an episode of cooling at ca. 5000 BP, followed by a period of ameliorization (the Hypsithermal at ca. 4000 BP), and towards the end of the Holocene, at ca. 500 years BP, a new cooling event was recorded. This sclerochronological study of the growth patterns and the oxygen isotope ratios in fossil R. exalbidus shells demonstrated that this species clearly exhibited annual cycles showing seasonality patterns through the mid-to-late Holocene, providing an opportunity to analyze intra-seasonal time scales in the fossil record. This study was supported jointly by the CONICET and the DAAD and was made as part of the Scientific Visit of SG to the AWI and the international cooperation from MINCyT and BMBF to BL.

Description: The venerid Tawera gayi could be a suitable Holocene bioarchive for Southern South-America given that it is found in a wide distribution range from the Beagle Channel (54º 50´ S) to 33º S along the Pacific coast, and to the North Patagonia (36°S) in the South Atlantic. In the Beagle Channel, both extant T. gayi populations and shell beds of mid-Holocene origin can be found. On the other hand there is reliable life history information in modern populations (Lomovasky et al. 2005 J. Appl. Ichthyol. 21, 64-69), i.e. shell growth patterns and the confirmation of the annual periodicity of the translucent bands. Finally, the shells provide geochemical proxies, e.g., d18O for temperature reconstruction. In order to investigate climate variability in the Beagle Channel, the individual age, growth increments and isotopes analyses of modern and fossil shells of T. gayi were used. The shell cuts of T. gayi showed a pattern of alternating broad opaque and narrow translucent bands, which were confirmed by acetate peels too. In general, the translucent bands showed a pink to purple colour, similar to internal part of the shell. Both modern and fossil populations showed a maximum age of 13 years old. Radiocarbon dating revealed ages ranging between ca 3800 to 4400 years b.p. in the fossil shells corresponding to the Holocene Climate Optimum. The δ18O values obtained in fossil shells ranged from 1.316‰ to –0.064‰ We correlated the most positive δ18O values with winter forming the translucent bands and the most negative δ18O with summer. The comparison of the von Bertalanffy growth curve showed no difference in the H∞ between modern (32.50 mm; Confidence Interval (CI)=31.07, 33.94) and fossil (33.23 mm; CI=31.94, 34.51) populations, but higher values (p 〈 0.05) were observed in the growth rate k and t0 in the modern (0.37 (0.31, 0.42) and 1.12 (0.98, 1.25) respectively) than fossil shells (0.24 (0.21, 0.27) and 0.57 (0.44, 0.69) respectively). This study demonstrated that this species clearly exhibited annual cycles showing seasonality patterns from the mid-Holocene to the present with translucent bands corresponding to slow or halted growth formed in fall/winter; the growth rate was lower during the past warm epochs than the present possible related to a different productivity in the Channel and/or a lower metabolic rate of the clams exposed to a higher temperature.

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.