Spatial and seasonal distribution of adult Oithona similis in the Southern Ocean: Predictions using boosted regression trees

We applied a multivariate statistical modelling technique called boosted regression trees to derive relationships between environmental conditions and the distribution of the adult stage of the cyclopoid copepod Oithona similis in the Southern Ocean. Nearly 20 000 samples from the Southern Ocean Continuous Plankton Recorder survey (87% from East Antarctica) were used to model the probability of detection (presence) and relative abundance of adults of this zooplankton species in surface waters. We demonstrate that it is possible to obtain reasonable models for both the presence (area under the Receiver Operating Characteristic curve of 0.77) and relative abundance (28–35% variance explained) of adult O. similis between November and March in much of the Southern Ocean. No investigation was possible where the environmental characteristics were not well represented by the SO-CPR dataset, namely, the Argentine shelf, Weddell Sea, and the frontal region north of the Amundsen Sea, or under sea-ice. Our analyses support the hypothesis that adult O. similis abundance is related to environmental conditions in a broadly similar way throughout the Southern Ocean. Compared to a compilation of net-haul data from the literature, the abundance model explained 34% of the variance in surface concentrations of adult stages of this species, and 23–59% of the variance in depth-integrated abundance of copepodite and adult stages combined. The models show higher occurrence and elevated abundances in a broad circumpolar band between the Antarctic Polar Front and the southern boundary of the Antarctic Circumpolar Current (approximately 54–64°S). Evidence of diel vertical migration by adults of this species north of 65°S was found, with surface abundances 20% higher at night than during the day. There was no evidence of diel migration south of 65°S. Five potential “hotspots” of adult O. similis were identified: in the southern Scotia Sea, two areas off east Antarctica, in the frontal zone north of the Amundsen Sea, and a small area in the outer Bellingshausen Sea. We recommend that a database of all available net-haul data on Oithona similis in the Southern Ocean be created to facilitate further investigations on the circumpolar distribution of this species.     

Anomalous Biogeochemical Response to a Flooding Event in the Delaware Estuary: A Possible Typology Shift Due to Climate Change

General circulation models have suggested that the number of extreme floods and droughts will increase with climate change; recent analyses of satellite data have demonstrated that these increases have been higher than predicted. Coastal systems, like the Delaware Estuary, can be vulnerable to such extreme weather events. In analyzing the 100- and 80-year records of the two major rivers of the Delaware Estuary, we find that about 20% of the very large and 50% of the extreme daily discharges occurred in the current decade (2001?C2011), and this represents a significant increase in flood occurrence compared with the rest of the discharge record. This is consistent with predictions of increased extreme weather conditions (inundation and drought) from climate change. Previously, we had characterized the Delaware Estuary as usually well mixed in the summer without significant bottom water oxygen depletion, based on our 30-year research efforts, and a 44-year agency monitoring record. In the summer of 2006, an extreme river discharge pushed the Delaware Estuary salinity gradient further downstream than seen in our research record and induced a nutrient influx to the nutrient-poor lower bay regions. As a result, stratification apparently allowed for a rapid phytoplankton biomass increase similar to the spring bloom phenomenon. A simple modeling exercise supports the idea that although unusual for this estuary in the summer, oxygen depletion occurred in response to the bloom biomass falling and decomposing in the isolated bottom waters. Using the summer 2006 anomalous discharge event and the resultant stratification as an illustration, and considering the significant increase in large and extreme floods in the last decade, we suggest that the typology of the Delaware Estuary is shifting as a result of climate change.     

ATLAS, and Wide-Angle Tail Galaxies in ATLAS

Using the Australia Telescope Compact Array (ATCA), ATLAS (Australia Telescope Large Area Survey) is imaging two fields totalling 7 square degrees down to 10 μJy beam^− 1 at 1.4 GHz. We have found 6 wide-angle tail galaxies (WATs), 4 of which have sufficient data to identify associated galaxy overdensities. The largest WAT, at a red-shift of 0.22, appears to be associated with an overdensity of galaxies that is spread over an unusually large extent of 12 Mpc, with a velocity range of 4500 km s^− 1. Here we present the WATs in ATLAS and discuss the implications of these observations for future large-scale radio surveys such as ASKAP-EMU.     

Fluids on differentiated asteroids: Evidence from phosphates in differentiated meteorites GRA 06128 and GRA 06129

Abstract– Paired meteorites Graves Nunatak 06128 and 06129 (GRA) represent an ancient cumulate lithology (4565.9 Ma ± 0.3) containing high abundances of sodic plagioclase. Textures and stable isotope compositions of GRA indicate that superimposed on the igneous lithology is a complex history of subsolidus reequilibration and low‐temperature alteration that may have extended over a period of 150 Myr. In GRA, apatite is halogen‐rich with Cl between 4.5 and 5.5 wt% and F between 0.3 and 0.9 wt%. The Cl/(Cl+F+OH) ratio of the apatite is between 0.65 and 0.82. The Cl and F are negatively correlated and are heterogeneously distributed in the apatite. Merrillite is low in halogens with substantial Na in the 6‐fold coordinated Na‐site (≈2.5%) and Mg in the smaller octahedral site. The merrillite has a negative Eu anomaly, whereas the apatite has a positive Eu anomaly. The chlorine isotope composition of the bulk GRA leachate is +1.2‰ relative to standard mean ocean chloride (SMOC). Ion microprobe chlorine isotope analyses of the apatite range between ?0.5 and +1.2‰. Textural relationships between the merrillite and apatite, and the high‐Cl content of the apatite, suggest that the merrillite is magmatic in origin, whereas the apatite is a product of the interaction between merrillite and a Cl‐rich fluid. If the replacement of merrillite by apatite occurred at approximately 800 °C, the fluid composition is f(HCl)/f(H₂O) = 0.0383 and a HCl molality of 2.13 and f(HCl)/f(HF) = 50–100. It is anticipated that the calculated f(HCl)/f(H₂O) and a HCl molality are minimum values due to assumptions made on the OH component in apatite and basing the calculations on the apatite with the lowest X_Cl. The bulk δ³⁷Cl of GRA is a >2σ outlier from chondritic meteorites and suggests that parent body processes resulted in fractionation of the Cl isotopes.     

Oxygen isotope composition of annually banded modern and mid-Holocene travertine and evidence of paleomonsoon floods, Grand Canyon, Arizona, USA

Holocene and modern travertine formed in spring-fed Havasu Creek of the Grand Canyon, Arizona, was studied to determine the factors governing its oxygen-isotope composition. Analysis of substrate-grown travertine indicates that calculated calcite-formation temperatures compare favorably with measured water temperatures, and include silt-rich laminae deposited by monsoon-driven floods. Ancient spring-pool travertine is dated by U-series at 7380 ± 110 yr and consists of 14 travertine-silt couplets of probable annual deposition. One hundred eighty high-resolution δ¹⁸O analyses of this mid-Holocene sample average −11.0‰ PDB. The average value for modern travertine is 0.5‰ lower, perhaps because mid-Holocene temperature was higher or there was proportionally greater summer recharge. δ¹⁸O cyclicity in the mid-Holocene travertine has average amplitude of 1.9 ± 0.5‰ PDB, slightly less than the inferred modern-day annual temperature range of Havasu Creek. The annual temperature range might have been reduced during the 14-yr interval compared to present, although other non-temperature factors could account for the muted annual variation. Silt-rich laminae within isotopically lower calcite in the modern and mid-Holocene travertine verifies the seasonal resolution of both samples, and suggests that similar temperature-precipitation conditions, as well as monsoon-generated summer floods, prevailed in the mid-Holocene as they do throughout the Grand Canyon region today.     

Recent disruptions in the timing and intensity of precipitation in Calakmul,Mexico

This study addresses changes in the timing and intensity of precipitation from 1982 to 2016 from three meteorological stations around Calakmul, Mexico, a landscape balancing biodiversity conservation and smallholder agricultural production. Five methods were used to assess changes in precipitation: the Mann-Kendall test of annual and wet season trends; a fuzzy-logic approach to determine the onset of the rainy season; the Gini Index and Precipitation Concentration Index (PCI) to evaluate the temporal distribution of precipitation; Simple Precipitation Intensity Index (SDII) to evaluate precipitation intensity; and the Rainfall Anomaly Index (RAI) to identify the deficit or surplus of rainfall compared with the long-term mean. Overall, rainfall trends in Calakmul over this period indicate a slight increase, though results of the indices (Gini, SDII, PCI) all indicate that rainfall has become more intense and more unevenly distributed throughout the year. There was no significant trend in the onset date of rainfall or the RAI overall, though there were more pronounced crests and troughs from 2004 to 2016. Higher interannual variability and more pronounced rainfall anomalies, both positive and negative, suggest that rainfall in the Calakmul region has become more extreme. This research informs for management and livelihood strategies in the local region and offers insights for analyses of regional patterns of seasonal precipitation events in tropical landscapes worldwide.     

A comparison of solute fluxes and sources from glacial and non‐glacial catchments over contrasting melt seasons

Solute and runoff fluxes from two adjacent alpine streams (one glacial and one non‐glacial) were investigated to determine how the inorganic solute chemistry of runoff responded to seasonal and interannual changes in runoff sources and volume, and to differences in physical catchment properties. Intercatchment differences in solute composition were primarily controlled by differences in catchment geology and the presence of soils, whereas differences in total solute fluxes were largely dependent on specific discharge. The glacial stream catchment had higher chemical denudation rates due to the high rates of flushing (higher specific discharge). The non‐glacial Bow River had higher overall concentrations of solutes despite the greater prevalence of more resistant lithologies in this catchment. This is likely the result of both longer average water–rock contact times, and a greater supply of protons from organic soils and/or pyrite oxidation. Increases in snowpack depth/snowmelt runoff reduced the retention of nitrate in the Bow River catchment (i.e. increased nitrate export), probably by reducing net biological uptake, or by reducing the proportion of runoff that had contact with biologically active soil horizons that tend to remove nitrate. The two streams exhibited opposite solute flux responses to climate perturbations over three melt seasons (1998, 1999, and 2000). The 1998 El Niño event resulted in an unusually thin winter snowpack, and increased runoff and solute fluxes from the glacial catchment, but decreased fluxes from the Bow River catchment. Solute fluxes in the Bow River increased proportionally to discharge, indicating that increased snowmelt runoff in this catchment resulted in a proportional increase in weathering rates. In contrast, the proportional variation in solute flux in the glacial stream was only ∼70–80% of the variation in water flux. This suggests that increased ablation of glacier ice and the development of subglacial channels during the 1998 El Niño year apparently reduced the average water–rock contact time in the glacial catchment relative to seasons when the subglacial drainage system was primarily distributed in character. Copyright © 2005 John Wiley & Sons, Ltd.