Search

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2008.06.002 Byline: M.J. Whitehouse (a), M.P. Meredith (a), P. Rothery (b), A. Atkinson (a), P. Ward (a), R.E. Korb (a) Keywords: Rapid ocean warming; South Georgia; Southern Ocean; Antarctic Circumpolar Current; Southern Annular Mode; Euphausia superba; Phytoplankton Abstract: The Southern Ocean is known to have warmed considerably during the second half of the 20th century but there are few locations with data before the 1950s. In addition, assessments of change in this region are hampered by the strong seasonal bias in sampling, with the vast majority of data collected during the austral summer. However, oceanographic measurements near South Georgia span most of the last century, and we here consider almost year-round data from this location over an 81-year period (1925-2006). We observe significant warming between the early and late 20th century, with differential warming between summer and winter months and an indication that late 20th century summer temperatures peaked [approximately equal to]6 days earlier. To quantify the long-term warming trend in this highly variable data, a mixed model utilising a Residual Maximum Likelihood (REML) method was used. Over the 81-year period, a mean increase of [approximately equal to]0.9[degrees]C in January and [approximately equal to]2.3[degrees]C in August was evident in the top 100m of the water column. Warming diminished below 100m and approached 0 at 200m. Thus the long-term warming around South Georgia is substantial -- more so than documented previously for the circumpolar warming of the Southern Ocean. We examine potential causal effects of this trend, including local atmospheric and cryospheric change, the influence of upstream waters and the role of coupled modes of climate variability such as El NiA[+ or -]o/Southern Oscillation and the Southern Annular Mode (SAM). It is likely that all of these play a part in the observed temperature increase. However, the role of the SAM is strongly indicated, via its likely role in the circumpolar warming trend in the Southern Ocean, and also by the atypical response of the South Georgia region to changes in heat fluxes associated with the SAM. Furthermore, the combination of a regional decline in ice extent and strong upstream warming likely explains a significant part of the strong seasonal variation apparent in the warming trend. In addition, we consider the implications that long-term warming has for South Georgia's lower trophic levels. For Euphausia superba, at their northern limit, we find a significant negative relationship between summer South Georgia water temperatures and mean summer density of E. superba across the southwest Atlantic sector of the Southern Ocean. Simple abundance and growth rate relationships with our long-term temperature data appear to show declining habitat suitability for E. superba. In contrast, the warming trend is likely to favour other macro- and mesozooplankton species that occupy the more northerly parts of the Antarctic Circumpolar Current, and it is likely to promote phytoplankton growth. Author Affiliation: (a) British Antarctic Survey, Natural Environmental Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK (b) Centre for Ecology and Hydrology, Natural Environmental Research Council, Monks Wood, Abbots Ripton, Huntingdon PE28 2LS, UK Article History: Received 24 September 2007; Revised 9 June 2008; Accepted 12 June 2008

Spatial and temporal variability of salinity, temperature, phosphate, nitrate, silicate, and chlorophyll 'a' in the surface waters around South Georgia Island in the South Atlantic are described. A comparative analysis shows that the Weddell Sea is colder than the Antarctic Circumpolar Current water during summer. Salinity measurements show marginal differences between the two waters, but indicates more salinity in oceanic waters than shelf-slope and shelf water. Silicate levels show a correlation with seawater temperature. The two waters exhibit little difference in phosphate or nitrate concentrations, and nutrient levels.

Abstract The enriched basaltic (martian) shergottite Ksar Ghilane (KG) 002, discovered in 2010, is exceptionally rich in coexisting but discrete apatite and merrillite crystals. It has been selected to better constrain the formation conditions and post-crystallization processes, and thus the evolution of martian rocks based on Ca-phosphates. A petrological, chemical, chronological and microstructural approach using a series of high-spatial resolution techniques including Raman spectroscopy, electron microscopy (SEM, EPMA, CL-imaging) and secondary ion mass spectrometry (SIMS) analysis has been applied to a representative number of Ca-phosphate grains. Analytical results for apatite and merrillite reveal: (i) zoning in F, Cl, Br and I concentrations, (ii) elevated Cl concentrations in the range of ∼11,900–35,300 µg/g and halogen ratios, i.e., Cl/Br and Cl/I, as well as stable chlorine isotope composition, reported as δ37Cl values rel. to Standard Mean Ocean Chloride (SMOC, defined as 0‰) with a value of +0.67 ± 0.14‰ (1σ), distinguishing KG 002 phosphates from that of other enriched and depleted shergottites. The halogen and heavier δ37Cl record indicate a slightly higher degree of ∼3.5% assimilation of Cl-rich and isotopically heavier crustal reservoir on Mars when compared to other enriched shergottites. (iii) Structural investigations together with the chemical and petrological context of the grains confirm the occurrence of hydroxyl-poor merrillite, indicate weak if any alteration effects induced by metamictization, only minor structural modifications due to shock metamorphism, and absence of replacement reactions. Therefore, igneous crystallization of Ca-phosphates from a fractionated, hydrous and ferrous mantle source, rich in volatiles including the halogens and Na and lithophile rare earth-elements, and absence of interaction with crustal fluids/brines of the sample is deduced. (iv) The Pb isotopic composition of six apatite and three merrillite grains is highly unradiogenic and the 238U-206Pb record yields a phosphate crystallization time at 395 ± 240 Ma (2σ), which is similar to those of other enriched shergottites. [ABSTRACT FROM AUTHOR]

Rare Hadean zircon grains represent the only direct sample of the Earth older than 4.0 Ga. As such, they have tremendous potential to illuminate our knowledge of this period of Earth's history for which there is no extant rock record. In this study we revisit the existing dataset, supplemented by new analyses, to identify those grains from which a robust age may be inferred. This rigorous filtering approach identifies four distinct zircon growth events in the Hadean between ca. 4.4 Ga and 4.0 Ga, and allows a reassessment of conclusions made from the determination of the O- and Hf-isotope systematics in these grains. Notably, we find no firm evidence for involvement of supracrustal reservoirs in zircon genesis prior to 4.15 Ga and, while our filtered Hf-isotope data support interpretations for a mafic protocrust, there are insufficient analyses to constrain its evolution accurately. Clearly, further work is required and needs to be conducted in a systematic manner that first seeks to establish both the age and homogeneity of any given grain before proceeding to other types of analysis. [ABSTRACT FROM AUTHOR]

Asteroid 4 Vesta is the largest known differentiated body in the asteroid belt, and is thought to be the likely parent body of howardite, eucrite, and diogenite (HED) meteorites. Howardites likely represent the vestan surface, eucrites likely crystallized as lavas in the upper crust, and diogenites likely originated from a plutonic layer deep in the crust. HEDs are potentially linked to another group of meteorites: the stony-iron mesosiderite group, due to mineralogical, geochemical, and isotopic similarities. Collision and disruption processes in the asteroid belt are key to understanding the evolution of asteroids, with impact events generating significant volumes of melt, which, if dated, can provide information on the timing and nature of such events. We performed in situ lead-lead (Pb-Pb) dating using Secondary Ionization Mass Spectrometry (SIMS) on melt clasts (mainly comprised of pyroxene, plagioclase, ± iron-nickel metal and/or glass) in two eucrites (Serra Pelada and Northwest Africa (NWA) 2696) and phosphates targeted within three mesosiderites (Vaca Muerta, Hainholz, and Estherville), respectively. The eucrite melt clasts yielded ages of 4520 ± 11 Ma and 4528.6 ± 6.3 Ma, in Serra Pelada and NWA 2696, respectively, and are likely indicative of a major heating event, such as an impact, metamorphism due to burial, or prolonged magmatism on the parent body. Our results from targeted mesosiderite phosphate analysis yielded a younger age range of ∼3968 – 4112 Ma, similar to ages reported previously for phosphate analysis in eucrites, as well as the broad range observed previously for many HEDs, and towards the upper end of the age range observed in lunar materials. These data may suggest a period of increased impact flux or possibly several higher-magnitude impacts on the mesosiderite parent body within that timeframe. Our results add further support to the likelihood of the existence of (at least) two different parent bodies for the HEDs (Vesta) and mesosiderites (Mesosiderite Parent Body – MPB). [ABSTRACT FROM AUTHOR]

Abstract: The Scotia Sea area has high productivity relative to the Southern Ocean as a whole, but this displays strong latitudinal and longitudinal gradients. Elucidating the extent of these from a single cruise is problematic, given the high variability of bloom timing and location in this region. Therefore, this study used data from transects across the central Scotia Sea in spring, summer and autumn of 2006, 2008 and 2009, combined with satellite data, to obtain a larger-scale appreciation of the latitudinal contrasts in phytoplankton standing stock and primary production across the region. Concentrations of nitrate, phosphate and particularly silicic acid increased towards the south of the transect with the latter showing a step change at the Southern Antarctic Circumpolar Current Front (SACCF). Changes in seasonal nutrient concentrations indicated increasing phytoplankton uptake north of ∼57°S that peaked at ∼53°S in the Georgia Basin. Based on seasonal depletions of nitrate relative to phosphate, the highest relative nitrate uptake occurred northwest of South Georgia on the periphery of the Georgia Basin, indicating efficient nitrate use here due to iron-replete conditions. An integrative approach to examine these gradients was with the use of 10-year satellite climatology data. These showed that the lowest mean chlorophyll a (chl-a) values were in the central/northern Scotia Sea, but these were still substantial values, 67% of values within the Georgia Basin bloom. Cruise data on chl-a and on microplankton biomass from cell counts support this finding of substantial biomass in the central Scotia Sea; since these averaged half of values in the iron-fertilised bloom of the Georgia Basin downstream of South Georgia. Given that our transect was nearly 1000km long and in parts was land remote with low iron concentrations, the relatively high production in the central and northern Scotia Sea is surprising. Iron levels may be maintained here by efficient recycling and irregular injections, possibly for example from dust deposition or shelf-derived inputs from the south. The moderate chl-a concentration across the mid-Scotia Sea and southwest of South Georgia reflect periodic, non-ice-associated blooms that occur in some years and not others. These may provide a connection between the large populations of krill inhabiting the northern and southern fringes of the Scotia Sea. [Copyright &y& Elsevier]

Abstract: In this study we examined the hypothesis that, under conditions of replete macronutrients and iron in the Southern Ocean, phytoplankton abundance and specific N uptake rates are influenced strongly by the processes of grazing and NH4 regeneration. NH4 and NO3 uptake rates by marine phytoplankton were measured to the northeast and northwest of the island of South Georgia during January–February 1998. Mean specific uptake rate for NO3 (vNO3) was 0.0026h−1 (range 0.0013–0.0065h−1) and for NH4 (vNH4) was 0.0097h−1 (0.0014–0.0376h−1). vNH4 was related positively with NH4 availability, which ranged from 0.1 to 1.5mmolm−3 within the upper mixed layer. Ambient NH4 concentrations and vNH4 were both positively related to local krill biomass values, computed from mean values along acoustic transect segments within 2km of the uptake measurement stations. These biomass values ranged from ∼1gkrill fresh massm−2 in the northwest to >4kgkrill wet massm−2 in the northeast. In contrast to the variability found with NH4 concentrations and uptake rates, vNO3 was more uniform across the sampling sites. Under these conditions, increasing NH4 concentration appeared to represent an additional N resource. However, high vNH4 tended to be found for stations with lower phytoplankton standing stocks, across a total range of 0.24–20mgchlorophyll a m−3. These patterns suggest a coupling between phytoplankton biomass, vNH4 and krill in this region of variable but high krill biomass. Locally high concentrations of krill in parts of the study area appeared to have two opposing effects. On the one hand they could graze down phytoplankton stocks, but on the other hand, their NH4 excretion supported enhanced uptake rates by the remaining, ungrazed cells. [Copyright &y& Elsevier]

Abstract: Highly radiogenic Pb isotope compositions determined for volcanic glass beads from the Apollo 14 soil sample 14163 are similar to those commonly determined for mare basalts and are correlated with chemical variations observed in the beads. This indicates that Pb unsupported by in-situ U decay has a similar origin in both glass beads and mare basalt samples and is likely to reflect variations of 238U/204Pb (μ) in the lunar mantle. An alternative explanation that this Pb is a result of late equilibration with the radiogenic Pb present in soil is less likely as it would imply that all other characteristics of glass beads such as their chemistry must also be a consequence of equilibration near the lunar surface. Regardless of the origin of unsupported Pb, observed variations of Pb isotope compositions in the glass beads and mare basalts appear to be a result of two component mixing between a primitive reservoir with a μ-value similar to the Earth’s mantle and KREEP with a μ-value in excess of several thousand. This range cannot be explained by the fractionation of major rock forming minerals from the crystallising Lunar Magma Ocean and instead requires substantial extraction of sulphide late in the crystallisation sequence. The proportion of sulphide required to produce the inferred range places limits on the starting μ of the Moon prior to differentiation, demanding a relatively high value of about 100–200. Low μ indicated by several basalt samples and previously analysed volcanic glass beads can be explained by the preservation of an early (but post Ferroan Anorthosite) sulphide rich reservoir in the lunar mantle, while a complete range of Pb isotope compositions observed in the glass beads and mare basalts can be interpreted as mixing between this sulphide rich reservoir and KREEP. [Copyright &y& Elsevier]