Grain size,areal thickness distribution and controls on sedimentation of the 1991 Mount Pinatubo tephra layer in the South China Sea

The June 15, 1991 climactic eruption of Mt. Pinatubo produced an extensive, largely co-ignimbrite-derived airfall ash layer on Luzon Island and across the central South China Sea. The layer covers an area of ~4×10⁵ km² with a volume of 5.5 km³. Near the coast of Luzon, the deposit consists of two units: a normally graded basal ash bed, unimodal in grain size, and a finer-grained, internally structureless upper ash bed showing grain size bimodality. With increasing distance from the source, the coarse particle populations of the two units merge and migrate towards a near-constant fine population (~11 m); the distal region is covered by a fine-mode dominated, virtually ungraded single ash layer. The reversal of the winds from easterly directions at upper-tropospheric and stratospheric levels to westerly directions in the middle and lower troposphere indicates that both the coarse- and fine-mode components fell out from high-altitude eruption clouds. The high-velocity upper-level winds, however, would have transported fine-grained ash particles far beyond the South China Sea, which suggests that their settling was accelerated by aggregation. The boundary between the units thus marks a change from fallout of predominantly discrete pyroclasts to simultaneous fallout of aggregated fines and freely falling, coarse-grained particles. The particle populations composing the upper ash bed were almost completely removed from the proximal areas by the upper-level winds. At lower elevations, the counterclockwise circulation of a typhoon over the coastal area advected the ash south and eastward, producing a thickness maximum in the medial region (at about 160 km from source). The strong displacement of fines, possibly aided by wind turbulence, led to a break in bulk tephra thinning rates close to the coastline. In the distal region, outside the influence of the typhoon, southwest monsoonal winds caused a distinct lobe axis inflection and thickness asymmetry. Within this region, at about 420 km from source, fallout of particle aggregates created a second thickness maximum. Comparison of the field data with previous experimental observations and tephra flux records in the deep sea (Wiesner et al. 1995; Carey 1997; McCool 2002) implies that the transport of ash in the water column was largely determined by vertical density currents. Differences in the reaction of coarse and fine particles to turbulence in the descending plumes probably suppressed the segregation of fines but allowed the coarser pyroclasts to maintain their initial order of arrival at the sea surface. Considering typical fall rates of convective plumes, modifications of the initial fallout position of the particles by the South China Sea current system are on the order of only a few kilometers. The results suggest that convective sedimentation processes ensure the preservation of atmospheric particle transport directions, distances, and fallout modes in the deep sea.Editorial responsibility: R. CioniAn erratum to this article can be found at     

Early Ship Observational Data and Icoads

Many surface marine meteorological observations (∼125 million) from ships' logbooks have been assembled in the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) back to the late 18th century. We describe the makeup of the available data before 1950, and a recent update for that period incorporating a variety of US and international sources – focusing on the background, digitization, and processing of the US Maury Collection, which provides the earliest data (mainly 1830–1860) currently blended into ICOADS. We also describe planned data and metadata additions to early ICOADS. Among these, the new Climatological Database for the World's Oceans (CLIWOC) will extend and enhance coverage for 1750–1854. Prospects for data improvements and homogeneity enhancements to further benefit climate research are also discussed.     

Organic Geochemical Microanalysis by Time‐of‐Flight Secondary Ion Mass Spectrometry (ToF‐SIMS)

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) is a powerful method for the chemical analysis of solid surfaces. In this paper, the capabilities and limitations of this technique and the potential for its use in geochemical research are outlined. Using ToF‐SIMS, the chemical composition of sample structures down to 10–100 μm can be determined, without the need for pre‐selection or labelling of the analysed substances. In addition, the lateral distribution of organic and inorganic compounds can be mapped in geochemical samples at a resolution in the micrometre range. The capabilities of the technique in geochemistry are illustrated by two examples. In the first example, it is shown that ToF‐SIMS can be used to detect biomarkers in oil samples, making it a promising method for the analysis of biomarkers in fluid inclusions. In the second example, a number of specific lipid biomarkers were identified and mapped on the surface of a microbial mat cryosection surface. Post‐measurement optical microscopy correlated the localisation of the lipids with the presence of methanotrophic archaea in the microbial mat.     

Reply to Molina-Garza et al. (2019) “Discussion of: Ortega-Flores et al. (2018) provenance analysis of Oligocene sandstone from the Cerro Pelón area,southern Gulf of Mexico”

ABSTRACT

The origin of the Oligocene turbidites from the Cerro Pelón area in south Gulf Mexico proposed by Ortega-Flores et al. (2018) is in disagree with the interpretations made by Molina-Garza et al. (2019), which main criticism is based on U-Pb ages of detrital zircons from the matrix of a conglomerate unit, which they refer to as ‘Nanchital Conglomerate’, as well as on the presence of limestone, gabbros, and mafic protolith-derived clasts. Molina-Garza et al. (2019) basically interpret the Nanchital Conglomerate as Miocene in age, which was sourced mainly from metamorphic complexes including their sedimentary covers located to the west and south of the Cerro Pelón area. For some reason, Molina-Garza et al. (2019) suppose that the Nanchital Conglomerate should have the same provenance sources that the Oligocene turbidites from Cerro Pelón area, reported by Ortega-Flores et al. (2018). Based on the foregoing, we strongly disagree with Molina-Garza et al. (2019) considering that, from the beginning, they intend to compare two units of different age. Additionally, the scarce data reported from both the matrix and the clasts of the Nanchital Conglomerate are not determinant for interpreting the provenance of this conglomeratic unit and subsequently, to consider the same rock sources from the Oligocene through Miocene time.     

Settlement and colonization of epi‐endobenthic communities on calcareous substrata in an underwater archaeological site

Underwater archaeological sites represent assets of great value that are subjected to physical and biological stresses. The study of biological settlement on calcareous substrata is the first step to understanding the biodeterioration processes that take place on man‐made structures (marble, statues, columns and other architectonic elements) in marine environments. To accomplish this objective, calcareous panels (30 × 30 × 2 cm) were immersed for 3, 6, 9 and 12 months along a north?south transect in marine protected areas of Baia (Naples). Both epilithic and endolithic assemblages were analysed: the latter was observed by scanning electron microscope after the cast‐embedding technique had been used. After 1 year the epilithic community was still changing, with algae and pioneer species overgrown by encrusting organisms, and the endolithic assemblage dominated by cyanobacteria/chlorophyte traces. Changes of epilithics lead changes on endolithics. The initial phototrophic borers were replaced by more sciophilous and competitive species; moreover, the first macroboring organisms arrived after the settlement of grazers. After 12 months of immersion, the rate of bioerosion was high, the material was deeply bored and all of the limestone surface was heavily covered.     

Field evidence of hydraulic connections between bedrock aquifers and overlying granular aquifers: examples from the Grenville Province of the Canadian Shield

Field evidence of hydraulic connections between a bedrock aquifer and an overlying granular aquifer in the Canadian Shield (Grenville Province) is presented. This issue is rarely considered and investigated despite its important hydraulic and chemical consequences and its widespread occurrence worldwide. The methodology employed is based on complementary field tests conducted at specific experimental sites instrumented both in the rock and in the overlying deposits. One of the bedrock sites revealed a natural hydraulic connection with the overlying granular aquifer caused by the weathered surface of the uppermost bedrock. Another site revealed an artificial hydraulic connection between the bedrock and the granular aquifer created by an improperly sealed casing. A regional study showed that hydraulic connections yield an erroneous interpretation of the true hydraulic properties of the tested aquifer. The detection of hydraulic connections is therefore essential to properly define well-capture areas and contamination conditions. It is recommended to practitioners that pumping tests be performed as well as hydrochemical comparisons of each existing aquifer unit. Falling-head permeability tests are also helpful in verifying the quality of the seal at the bedrock–casing contact. More effective procedural controls and better well-construction practices are necessary to reduce the risks of cross-contamination induced by defective seals.     

Spatial Distribution of Nitrous Oxide (N<Subscript>2</Subscript>O) in the Reloncaví Estuary–Sound and Adjacent Sea (41°–43° S), Chilean Patagonia

Fjords and estuaries exchange large amounts of solutes, gases, and particulates between fluvial and marine systems. These exchanges and their relative distributions of compounds/particles are partially controlled by stratification and water circulation. The spatial and vertical distributions of N₂O, an important greenhouse gas, along with other oceanographic variables, are analyzed from the Reloncaví estuary (RE) (~41° 30′ S) to the gulf of Corcovado in the interior sea of Chiloé (43° 45′ S) during the austral winter. Freshwater runoff into the estuary regulated salinity and stratification of the water column, clearly demarking the surface (<5 m depth) and subsurface layer (>5 m depth) and also separating estuarine and marine influenced areas. N₂O levels varied between 8.3 and 21 nM (corresponding to 80 and 170 % saturation, respectively), being significantly lower (11.8 ± 1.70) at the surface than in subsurface waters in the Reloncaví estuary (14.5 ± 1.73). Low salinity and NO₃ ^?, NO₂ ^?, and PO₄ ^3? levels, as well as high Si(OH)₄ values were associated with low surface N₂O levels. Remarkably, an accumulation of N₂O was observed in the subsurface waters of the Reloncaví sound, associated with a relatively high consumption of O₂. The sound is exposed to increasing anthropogenic impacts from aquaculture and urban discharge, occurring simultaneously with an internal recirculation, which leads to potential signals of early eutrophication. In contrast, within the interior sea of Chiloé (ISC), most of water column was quasi homohaline and occupied by modified subantarctic water (MSAAW), which was relatively rich in N₂O (12.6 ± 2.36 nM) and NO₃ ^? (18.3 ± 1.63 μM). The relationship between salinity, nutrients, and N₂O revealed that water from the open ocean, entering into ISC (the Gulf of Corcovado) through the Guafo mouth, was the main source of N₂O (up to 21 nM), as it gradually mixed with estuarine water. In addition, significant relationships between N₂O excess vs. AOU and N₂O excess vs. NO₃ ^? suggest that part of N₂O is also produced by nitrification. Our results show that the estuarine and marine waters can act as light source or sink of N₂O to the atmosphere (air–sea N₂O fluxes ranged from ?1.57 to 5.75 μmol m^?2 day^?1), respectively; influxes seem to be associated to brackish water depleted in N₂O that also caused a strong stratification, creating a barrier to gas exchange.