Calcretes are indurated terrestrial carbonates that are widespread in arid and semi-arid settings and serve as important archives of present and past environments. Here, we use geochemical tools to explore the nature and origin of calcretes documented from tropical Niue Island in the Southwest Pacific. The study recognizes two types of calcretes that differ in their mineral assemblage, microfabrics, elemental chemistry, and carbon and oxygen isotopes. The calcretes common in the paleo-lagoon soils consist of 90% low-Mg calcite and ~10% highly weathered Mg-Al silicates. These pedogenic calcretes formed in the soil profiles within the vadose zone bear the following distinctions: (i) Fe/Al ratio of 0.75, identical to the ratio in soils (Fe/Al = 0.76 ± 0.5), substantiating the link between the calcretes and soils; (ii) presence of rhizoliths, root voids, micritic nodules, and clasts, which are consistent with a pedogenic calcrete fabric; and (iii) 13C and 18O depletions of −10.6‰ and −5.3‰, respectively, which are compatible with carbon sources from microbial and root respiration, as well as formation in oxygen isotope equilibrium with vadose waters. Unlike the pedogenic calcrete, a rare calcrete from the coastal terrace contains an exceptionally rare hydrotalcite [Mg6Al2(CO3)(OH)16(H2O)4] mineral (65%) coated by microbial films. We contend that the hydrotalcite-rich calcrete was deposited through interaction of dolomite with seawater, similar to the method of producing hydrotalcite in the laboratory. 13C and 18O enrichments of 0.8 to 1.7‰ and −1.0 to −1.6‰, respectively, are in agreement with (i) mixed carbon sources consisting of microbial CO2 degassing, seawater HCO3, and dolomite dissolution, and (ii) oxygen isotope equilibration with seawater-derived fluid.

A chronological framework of ocean-based methane emissions is a prerequisite to identify the factors controlling the flux and duration of gas venting into the atmosphere. Here we report the results of a study of ubiquitous barite chimneys and barite-bearing mud volcanoes documented in association with methane seeps and gas hydrates at mid-bathyal depths in the Gulf of Mexico. The barites exhibit anomalously high 226Ra and 228Ra activities, orphans in the 238U/232Th decay chains, and are well suited for the application of 210Pb/226Ra and 228Th/228Ra dating methods. In order to account for the interference of impurities with the accuracy of the dates, gamma counts from paired raw and purified aliquots were evaluated. Chimneys yield a mean 228Th/228Ra age of 1.60 +/- 0.25 yr (n = 16) and consistently younger 210Pb/226Ra ages. The biased 210Pb/226Ra ages are attributed to poor 210Pb counting efficiency, young chimneys with little 210Pb ingrowth and possibly an open system. Rapid vertical growth rates of 11.4 and 12.8 cm/yr established from 228Th/228Ra ages are comparable to barite chimney estimates from hydrothermal settings. In general mud volcanoes are distinguished from the chimneys by absence of 228Ra and 228Th and high 210Pb/226Ra ratios, occasionally >1 in the raw subsamples. A positive relation between 210Pb/226Ra and the amount of pyrite in the samples supports the contention that PbS, co-precipitated with pyrite in H2S-rich pore fluids, acts as the principal scavenger of external210Pb resulting from decay of 222Rn in the pore fluids. The mud volcanoes 210Pb/226Ra assays yield consistent ages with a mean of 17.4 +/- 1.8 yr (n = 9) after removal of the pyrite. The general absence of 228Ra and 228Th in the mud volcanoes, contrasting to their abundance in the chimneys, is attributed to the age difference between the barite geomorphic forms. The (228Ra/226Ra)o initial activities in the chimneys, representing the ratios in the fluids at the time of barite deposition, exhibit statistically distinct values of 1.084 +/- 0.015 (n = 28) and 0.752 +/- 0.023 (n = 4) at the two sampling sites separated by a 265 km stretch. These (228Ra/226Ra)o values match the range of oil-field brine values from deep siliciclastic reservoirs in the US Gulf Coast. The fluids maximum migration time from the source to the seafloor is estimated to be <20 yr on the basis of 228Ra presence in the chimneys. The assessment based on the radiometric ages supports the notion that barite deposition in the Gulf of Mexico is a relatively recent event.

The origin of soils on Niue Island, an uplifted South Pacific former atoll, has long been a subject of debate because the soils are highly radioactive. A new assessment of proposed sources shows that content in the soils of major elements such as Fe and Al rules out erosion of the carbonate buildup or pristine aragonite coral. Subaerial volcanic ash has been previously argued to be a major source and according to a newly employed diagnostic ratio of Fe2O3/Al2O3 2 O 3 /Al 2 O 3 it is confirmed as a definite minor contributor. Fe2O3/Al2O3 2 O 3 /Al 2 O 3 ratios offer evidence that the most likely major soil precursor is pumice rafted periodically from neighboring seamounts or from the volcanically active Tongan Trench. Pumice raftings likely contributed to the soil formation of other uplifted atolls having configurations similar to Niue Island.

Using a manned submersible, a 25-cm-long sediment core was acquired at upper bathyal depth in the Gulf of Mexico from a mound underlain by methane hydrates at the threshold of stability. The main objective of the core investigation was to elucidate little-understood effects of gas-hydrate dissociation on benthic foraminiferal ecology, stratigraphy, and stable isotopes. Our results show that intense methane seepage has decimated, but not obliterated, the benthic foraminiferal community that includes many facultative anaerobes, dominated by two species of Bolivina. The remnant community has continued to exist at this site for over 2000 years. A shift from a much higher foraminiferal density to persistent low values is observed in the lowermost part of the core. A major oil spill in the Gulf of Mexico in 2010 caused similar devastation to the foraminiferal community, and drastically reduced the density. Planktonic (Globigerinoides ruber) and benthic foraminiferal tests (Bolivina ordinaria and Bolivina albatrossi) yield delta C-13 and delta O-18 profiles that are inversely correlated and exhibit anomalously negative carbon and positive oxygen isotope values up to maxima of -10.2% and 3.9% (VPDB), respectively. The isotope anomalies are attributed to anaerobic oxidation of C-13-depleted biogenic methane and intake of O-18-rich fluids released during hydrate dissociation. Two sequential sulfate-methane transition zones (SMTZs), with sedimentation rates of 8.2 and 27.5 cm Ka(-1), are coeval with two distinct intervals in the G. ruber isotope profiles. Three lines of evidence attest to the complexity of reconstructing methane flow from foraminiferal records. (i) Both planktonic and benthic foraminiferal tests serve as templates for secondary carbonate overgrowths. (ii) Decoupling of the overgrowths from the primary biogenic calcite tests is intractable, and therefore stable isotopes cannot be used as a tool to confirm whether Bolivina may live and thrive in anoxic sediments.

A monitoring study at DeSoto Caverns during two years (2012 - 2013) of rainfall-contrasting variability presents the opportunity to test the response of the hydroclimate proxies in drips and active speleothems to forcing factors on intrannual and interannual time scales. The weighted monthly mean rainwater delta O-18 and delta H-2 range from - 1.2 to - 6.4 (parts per thousand V-SMOW) and - 4 to -41.6 (parts per thousand V-SMOW), respectively, and show modest interannual variation. D-excess values exhibit a large intrannual contrast suggesting a primary control by sub-cloud evaporation processes. Coeval drip-water delta O-18 and delta H-2 vary from -3.1 to -5.3 (%o V-SMOW) and -9.9 to -30.5 (parts per thousand V-SMOW), respectively, and exhibit interannual negative trends from the 2012 dry/warm year to the 2013 relatively wet/cool year. Substantial attenuation of drip-water isotope amplitudes, relative to its rainwater source, is likely caused by mixing of fresh with residual evaporated-water in the epikarst zone. Drip-water Ca, Mg, Sr and Mg/Ca and Sr/Ca ratios exhibit an inverse relation with respect to the contrasting hydroclimate years such that lower values and higher ratios occur during the dry/warm year and higher values and lower ratios occur during the wet/cool year. We assert that interannual rainfall variability exerts a dominant control on the elemental concentrations and their ratios of the drips through changes of biomass productivity in the soils overlying the cave, and prior aragonite precipitation in the epikarst. The distribution coefficients of Mg (D-Mg = 3.49 x 10(-3) +/- 1.06 x 10(-3)) and Sr (D-Sr = 1.12 +/- 0.041) between drips and aragonite speleothems estimated in this study are in broad agreement with aragonite-solute experimental values. Coeval changes of trace elements and delta O-18 in response to interannual rainfall variability confirm their usefulness to better constrain the controlling hydroclimate drivers.

Oxygen and carbon isotope time-series derived from an actively growing aragonitic stalagmite in DeSoto Caverns exhibit with unusual clarity rapid hydroclimate changes in the mid-to-late Holocene. Data consist of 1884 delta O-18 and delta C-13 determinations whose chronology is anchored on 35 Th-230/U-234 absolute dates in the interval 6.0-1.1 cal ka BP. Exceptional O-18 and C-13-enrichments centered at 4.8 +/- 0.14 cal Ka BP likely represent the imprints of a severe drought. Isotope cycles from 4.7 to 1.3 cal ka BP, exhibit a dominant periodicity of 68 +/- 4 yrs. A gradual cooling trend of -0.6 degrees C/10(3) yrs is attributed to a declining seasonal contrast in insolation. The synchronicity of the mega-drought in the Southeast US with the (1) termination of the African Humid Period; (ii) abrupt reduction of the North Atlantic Deep Water production, and (iii) rapid sea-ice expansion in the polar regions of both Hemispheres testifies to the global extent and rapidity of the "5 ka" event and points to the North Atlantic Deep Water variability as the likely controlling factor. The multidecadal cycles are consistent with alternating dry and wet summers occurring during a long-term switch in the seasonal rainfall amount dominance from winter to summer. The periodic summer droughts in the Southeast US support climate models that predict profound hydroclimate changes in the late Holocene governed by the Atlantic Multidecadal Oscillation. The relatively short and rapid hydroclimate phase transitions documented in this study introduce a complication in the correlation of late Holocene drought events that had significant societal impacts. (C) 2017 Elsevier Ltd. All rights reserved.

The giant clam, Tridacna gigas, is an important faunal component of reef ecosystems of the Indo-Pacific region. In addition to its ecological role, shells of this bivalve species are useful bioarchives for past climate and environmental reconstructions. However, the biomineralization processes involved in shell aragonite deposition are insufficiently understood. Here, we present a study of the shell microstructure of modern specimens from Palm Island, Great Barrier Reef (GBR), Australia, and Huon Peninsula, Papua New Guinea (PNG), using a combination of petrography, scanning electron microscopy, electron backscatter diffraction, Raman spectroscopy and stable carbon isotope ratios. Daily growth increments were recognizable in all specimens through ontogeny, and counting these growth lines provides a robust specimen age estimate. For the internal layers, paired increments of organized aragonitic needles and compact, oblong crystals were recognized in a specimen from PNG, whereas specimens from GBR were composed of shield-like crystals that were not definable at the microscale. The combination of nutrient availability, rainfall and solar irradiance are likely to be the most significant factors controlling shell growth and may explain the observed differences in microstructure. The external layer, identical in all specimens, was composed of dendritic microstructure that is significantly enriched in 13C compared to the internal layer, suggesting different metabolic controls on layer deposition. We propose that the mineralization of the internal and external layers is independent from each other and associated with the activity of specific mantles. Future studies using T. gigas shells as bioarchives should consider the microstructure as it reflects the environment in which the individual lived and the differences in mineralization pathways of internal and external layers.

The study area is located in the Khammouan Province of Central Laos, along the Nan Hinboun River, in the Hinboun District. The Khammouan Formation exposed in Central Laos hosts a variety of karstic landscape including limestone massifs that rise from the alluvial plains. The availability of groundwater in the Nam Hinboun region varies widely, largely due to the geologic complexity of the area. Extensive and productive karst aquifers occur in the Khammouan Formation of the upper Nam Hinboun river basin. The Khammouan Formation consisting of Carboniferous-age carbonate rocks exhibits little or no intercrystalline porosity. Groundwater flow occurs along solutionally enlarged fractures, cavities, joints, and bedding planes. Thirteen water samples were collected and analyzed for anions, cations, and oxygen and hydrogen isotopes from caves (dripping water), rainwater, streams, and sumps. The pH values range between 6.80 and 7.56 that is typical for cave waters. Specific conductance values range between 192.60 and 444.70 μS/cm, the total alkalinity as CaCO3 varies from 107 to 244 mg/l, whereas the values for the total hardness (as CaCO3) are between 58 and 266 mg/l. The water samples collected in the upstream section of Nam Hinboun River are depleted in both 18O and deuterium with a d-excess value below 10 (7.2 ‰). This suggests a long travel distance of monsoon air masses (high degree of rainout) and represents a typical example of predominantly SW circulation. The data set reported here only represents a local and time-limited snapshot of the water isotopic compositions. Further studies are required to confirm our preliminary results from Central Laos and a comprehensive regional study of the karst geology and isotope hydrology is highly desirable given the rapid economic development and population explosion of the SE Asian region.

The presence of gas hydrates on the Blake Ridge diapir, northeastern Atlantic Ocean, offers an opportunity to study the impact of methane seepage on the ecology and geochemistry of benthic foraminifera in the late Holocene. Three push cores, covering a time span of ~1000yrs, were retrieved from three distinct microhabitats at the top of the diapir at a water depth of ~2150m: (i) sediments away from seepage (control core), (ii) sediments overlain by clusters of methanotrophic and thiotrophic bivalves, and (iii) chemoautotrophic microbial mats. The foraminiferal assemblages at the two seep sites are marked by a reduction in benthic foraminiferal species diversity, coupled with a near-absence of agglutinated species. However, an opportunistic population rise in CH4- or H2S-tolerant calcareous species (e.g., Globocassidulina subglobosa and Cassidulina laevigata) that utilize the abundant trophic resources at the seeps has led to an increase in the overall assemblage density there.The δ18O and δ13C values of three species of benthic foraminifera – Gyroidinoides laevigatus, Globocassidulina subglobosa, and Uvigerina peregrina – and the planktonic species Globorotalia menardii were acquired from all three cores. The benthic species from methane seeps yield δ13C values of 0.1 to −4.2 (‰VPDB), that are distinctly more 13C-depleted relative to the δ13C of 0.4 to −1.0 (‰VPDB) at the control (off seep) site. The species from a mussel-bed site exhibit more negative δ13C values than those from microbial mats, possibly reflecting different food sources and higher rate of anaerobic oxidation of methane.The positive δ13C values in the paired planktonic species suggest that authigenic carbonate precipitation did not overprint the observed 13C depletions. Hence the probable cause of negative δ13C of benthic foraminifera is primary calcification from Dissolved Inorganic Carbon (DIC) containing mixed carbon fractions from (a) highly 13C-depleted, microbially-oxidized methane and (b) a seawater source.

A newly acquired, absolute U/Th dated, delta O-18 record archived in a stalagmite from DeSoto Caverns in Alabama renders highly resolved time series for the past four millennia. Two principal states of variability are discerned in the delta O-18 record: (1) stable state spanning the intervals before Common Era 2350 to A. D. 400 and A. D. 1700 to 2008 that exhibits significant periodicities of 30 and 60 years and (2) unstable state in between containing six major discontinuities alternating with rapid delta O-18 positive excursions. The two contrasting states are likely the manifestations of extreme rainfall events established on the basis of the imprints of anomalously high/low drip flow rates discerned in the fabrics of the discontinuities. The proxy rainfall record offers valuable insights on whether climate variability may have been implicated in the rise and demise of the Mississippian chiefdoms (A. D. 800 to 1700) in the southeastern United States. The time of emergence and growth of the Mississippian chiefdoms and their subsistence transition to a dependency on corn, a warm and wet weather crop, coincides with a period of increased rainfall over the A. D. 450-1000 interval. Overall decline in rainfall between A. D. 1000 and 1500, alternating with droughts, is contemporaneous with evidence of abandonment of towns and villages and downstream movement of populations. Thus, warmer, wetter conditions than present may have promoted corn agriculture during the rise and growth of the chiefdoms, whereas food shortages, caused by failed corn crops under drought conditions, may have played a much greater role in the demise of the Mississippian chiefdoms than previously recognized.