Shallow forearc mantle dynamics and geochemistry: New insights from IODP Expedition 366

The Mariana forearc is a unique setting on Earth where serpentinite mud volcanoes exhume clasts originating from depths of 15 km and more from the forearc mantle. These peridotite clasts are variably serpentinized by interaction with slab derived fluid, and provide a record of forearc mantle dynamics and changes in geochemistry with depth. During International Oceanic Discovery Program (IODP) Expedition 366, we recovered serpentinized ultramafic clasts contained within serpentinite muds of three different mud volcanoes located at increasing distance from the Mariana trench and at increasing depth to the slab/mantle interface: Yinazao (distance to the trench: 55 km / depth to the slab/mantle interface: 13 km), Fantangisña (62 km / 14 km) and Asùt Tesoru (72 km / 18 km). Four different types of ultramafic clasts were recovered: blue serpentinites, lizardite-serpentinites, antigorite/lizardite- and antigorite-serpentinites. Lizardite-serpentinites are primarily composed of orange serpentine, forming mesh and bastite textures. Raman and microprobe analyses revealed that these textures contain a mixture of Fe-rich brucite (XMg ~ 0.84) and lizardite/chrysotile. Antigorite/lizardite- and antigorite-serpentinites record the progressive recrystallization of mesh and bastite textures to antigorite, magnetite and pure Fe-poor brucite (XMg ~ 0.92). Oxygen isotope compositions of clasts and pore fluids showed that the transition from lizardite to antigorite is due to the increase in temperature from 200 °C to about 400 °C within the forearc area above the slab/mantle interface. Lizardite-, antigorite/lizardite- and antigorite-serpentinites displayed U-shaped chondrite normalized Rare Earth Element (REE) patterns and are characterized by high fluid mobile element concentrations (Cs, Li, Sr, As, Sb, B, Li) relative to abyssal peridotites and/or primitive mantle. The recrystallization of lizardite to antigorite is accompanied by a decrease in Cs, Li and Sr, and an increase in As and Sb concentrations in the bulk clasts, whereas B concentrations are relatively constant. Some clasts are overprinted by blue serpentine, often in association with sulfides. Most of these blue serpentinites were recovered at Yinazao and the uppermost units of Fantangisña and Asùt Tesoru suggesting alteration in the shallower portions of the forearc, possibly during exhumation of the clasts. This episode of alteration resulted in a flattening of REE spectra and an increase of Zn concentrations in serpentinites. Otherwise, no systematic changes of ultramafic clasts chemistry or mineralogy were observed with increasing depth to the slab. The samples document previously undescribed prograde metamorphic events in the shallow portions of the Mariana subduction zone, consistent with a continuous burial of the serpentinized forearc mantle during subduction. Similar processes, induced by the interaction with fluids released from the downgoing slab, likely occur in subduction zones worldwide. At greater depth, breakdown of brucite and antigorite will result in the massive transfer of fluids and fluid mobile elements, such as As, Sb and B, to the source of arc magmas

Freshwater bivalve shells as hydrologic archives in the Congo Basin

We test the applicability of bivalve shell oxygen isotope composition to reconstruct hydrological dynamics in four riverine sites in the Congo River basin. Twenty-three specimens from the Unionoida order were collected from locations where long-term discharge data are available, and in situ measurements and water samples were collected over several years. Due to the highly variable (species-specific) shell morphology, various sampling techniques were used to analyze the shell sections; however, every specimen recorded the seasonality of the host water oxygen stable isotope composition (δ 18 O w ) in its δ 18 O shell record. Discharge data showed an inverse relationship with δ 18 O w values, which was well described with a logarithmic fit. An exception was the Kasai River, where the δ 18 O w record shows an additional peak occurring during the high discharge period, which renders the discharge-δ 18 O w relationship more complex than in the other systems investigated. Low ratios of maximum to minimum discharge (Q max /Q min ) were found to result in a low δ 18 O w amplitude, which was reflected as low δ 18 O shell variability. The Congo and Kasai rivers had Q max /Q min ratios ~2 to 2.5, while the Oubangui showed a much higher Q max /Q min (~19). Shells correspondingly showed a large δ 18 O shell range (amplitude between 2.4 and 5.0‰) for individual Oubangui shells, and lower amplitude for other sites (1.0 to 2.2‰). Thus, shells have a high resolving power to be used to record hydrological variability, since long-term changes in precipitation pattern, discharge, land-use change, or other hydrological changes have an influence on δ 18 O w values. Shells with wide range of δ 18 O values reflect high seasonal variability in rivers, while shells with lower δ 18O amplitude correspond to sites with more steady river conditions over the year. Our study illustrates that fossil shell δ 18 O values could indicate Q max /Q min values in ancient African river systems.AFRIVA

Rapid Environmental Change over the Past Decade Revealed by Isotopic Analysis of the California Mussel in the Northeast Pacific

The anthropogenic input of fossil fuel carbon into the atmosphere results in increased carbon dioxide (CO2) into the oceans, a process that lowers seawater pH, decreases alkalinity and can inhibit the production of shell material. Corrosive water has recently been documented in the northeast Pacific, along with a rapid decline in seawater pH over the past decade. A lack of instrumentation prior to the 1990s means that we have no indication whether these carbon cycle changes have precedence or are a response to recent anthropogenic CO2 inputs. We analyzed stable carbon and oxygen isotopes (δ13C, δ18O) of decade-old California mussel shells (Mytilus californianus) in the context of an instrumental seawater record of the same length. We further compared modern shells to shells from 1000 to 1340 years BP and from the 1960s to the present and show declines in the δ13C of modern shells that have no historical precedent. Our finding of decline in another shelled mollusk (limpet) and our extensive environmental data show that these δ13C declines are unexplained by changes to the coastal food web, upwelling regime, or local circulation. Our observed decline in shell δ13C parallels other signs of rapid changes to the nearshore carbon cycle in the Pacific, including a decline in pH that is an order of magnitude greater than predicted by an equilibrium response to rising atmospheric CO2, the presence of low pH water throughout the region, and a record of a similarly steep decline in δ13C in algae in the Gulf of Alaska. These unprecedented changes and the lack of a clear causal variable underscores the need for better quantifying carbon dynamics in nearshore environments

Alteration of the bZIP60/IRE1 Pathway Affects Plant Response to ER Stress in Arabidopsis thaliana

The Unfolded Protein Response (UPR) is elicited under cellular and environmental stress conditions that disrupt protein folding in the endoplasmic reticulum (ER). Through the transcriptional induction of genes encoding ER resident chaperones and proteins involved in folding, the pathway contributes to alleviating ER stress by increasing the folding capacity in the ER. Similarly to other eukaryotic systems, one arm of the UPR in Arabidopsis is set off by a non-conventional splicing event mediated by ribonuclease kinase IRE1b. The enzyme specifically targets mature bZIP60 RNA for cleavage, which results in a novel splice variant encoding a nuclear localized transcription factor. Although it is clear that this molecular switch widely affects the transcriptome, its exact role in overall plant response to stress has not been established and mutant approaches have not provided much insight. In this study, we took a transgenic approach to manipulate the pathway in positive and negative fashions. Our data show that the ER-resident chaperone BiP accumulates differentially depending on the level of activation of the pathway. In addition, phenotypes of the transgenic lines suggest that BiP accumulation is positively correlated with plant tolerance to chronic ER stress

Trace element fingerprinting of cockle (Cerastoderma edule) shells can reveal harvesting location in adjacent areas

Determining seafood geographic origin is critical for controlling its quality and safeguarding the interest of consumers. Here, we use trace element fingerprinting (TEF) of bivalve shells to discriminate the geographic origin of specimens. Barium (Ba), manganese (Mn), magnesium (Mg), strontium (Sr) and lead (Pb) were quantified in cockle shells (Cerastoderma edule) captured with two fishing methods (by hand and by hand-raking) and from five adjacent fishing locations within an estuarine system (Ria de Aveiro, Portugal). Results suggest no differences in TEF of cockle shells captured by hand or by hand-raking, thus confirming that metal rakes do not act as a potential source of metal contamination that could somehow bias TEF results. In contrast, significant differences were recorded among locations for all trace elements analysed. A Canonical Analysis of Principal Coordinates (CAP) revealed that 92% of the samples could be successfully classified according to their fishing location using TEF. We show that TEF can be an accurate, fast and reliable method to determine the geographic origin of bivalves, even among locations separated less than 1 km apart within the same estuarine system. Nonetheless, follow up studies are needed to determine if TEF can reliably discriminate between bivalves originating from different ecosystems

Contrasting biogeochemical characteristics of the Oubangui River and tributaries (Congo River basin)

The Oubangui is a major tributary of the Congo River. We describe the biogeochemistry of contrasting tributaries within its central catchment, with watershed vegetation ranging from wooded savannahs to humid rainforest. Compared to a 2-year monitoring record on the mainstem Oubangui, these tributaries show a wide range of biogeochemical signatures, from highly diluted blackwaters (low turbidity, pH, conductivity, and total alkalinity) in rainforests to those more typical for savannah systems. Spectral analyses of chromophoric dissolved organic matter showed wide temporal variations in the Oubangui compared to spatio-temporal variations in the tributaries, and confirm that different pools of dissolved organic carbon are mobilized during different hydrological stages. δ(13)C of dissolved inorganic carbon ranged between -28.1‰ and -5.8‰, and was strongly correlated to both partial pressure of CO2 and to the estimated contribution of carbonate weathering to total alkalinity, suggesting an important control of the weathering regime on CO2 fluxes. All tributaries were oversaturated in dissolved greenhouse gases (CH4, N2O, CO2), with highest levels in rivers draining rainforest. The high diversity observed underscores the importance of sampling that covers the variability in subcatchment characteristics, to improve our understanding of biogeochemical cycling in the Congo Basin.status: publishe

Calibration of hydroclimate proxies in freshwater bivalve shells from Central and West Africa

Freshwater bivalve shell oxygen and carbon stable isotope ratios (d18O, d13C) may act as recorders of hydroclimate (e.g., precipitation-evaporation balance, discharge) and aquatic biogeochemistry. We investigate the potential of these hydroclimate proxies measured along the growth axis of shells collected from the Oubangui River (Bangui, Central African Republic) and the Niger River (Niamey, Niger). Biweekly water samples and in situ measurements collected over several years, along with daily discharge data from both sites allowed a direct comparison with proxies recorded in the shells. Data from a total of 14 unionid shells, including three species (Chambardia wissmanni, Aspatharia dahomeyensis, and Aspatharia chaiziana), confirmed that shells precipitate carbonate in oxygen isotope equilibrium with ambient water. Because water temperature variations were small, shell d18O values (d18Oshell) also accurately record the seasonality and the range observed in water d18O (d18Ow) values when calculated using an average temperature. Calculated d18Ow values were in good agreement over the entire record of measured d18Ow values, thus d18Oshell records can be reliably used to reconstruct past d18Ow values. Discharge and d18Ow values from both rivers fit a logarithmic relationship, which was used to attempt reconstruction of past hydrological conditions, after calculating d18Ow values from d18Oshell values. A comparison with measured discharge data suggests that for the two rivers considered, d18Oshell data are good proxies for recording discharge conditions during low(er) discharge levels, but that high discharge values cannot be accurately reconstructed due to the large scatter in the discharge-d18Ow relationship. Moreover, periods of bivalve shell growth cessation due to high turbidity or air exposure should be taken into account. While d13C values of dissolved inorganic carbon in both rivers showed clear seasonality and correlated well with discharge, most of the shells analyzed did not record these variations adequately, likely due to the complication of vital effects including the variable contribution of metabolic CO2. Thus, tropical African unionid d18Oshell values can be used to reconstruct d18Ow values with high confidence to provide insight on past hydroclimate such as precipitation-evaporation balance and periods of low discharge.status: publishe