The summertime plankton community at South Georgia (Southern Ocean): Comparing the historical (1926/1927) and modern (post 1995) records

The earliest comprehensive plankton sampling programme in the Southern Ocean was undertaken during the early part of last century by Discovery Investigations to gain a greater scientific understanding of whale stocks and their summer feeding grounds. An initial survey was carried out around South Georgia during December 1926 and January 1927 to describe the distribution of plankton during the summer, and to serve as a baseline against which to compare future surveys. We have reanalysed phytoplankton and zooplankton data from this survey and elucidated patterns of community distribution and compared them with our recent understanding of the ecosystem based on contemporary data. Analysis of Discovery data identified five groups of stations with characteristic phytoplankton communities which were almost entirely consistent with the original analysis conducted by [Hardy A.C., Gunther, E.R., 1935. The plankton of the South Georgia whaling grounds and adjacent waters 1926-1927. Discovery Report 11, 1-456]. Major groupings were located at the western end of the island and over the northern shelf where Corethron spp. were dominant, and to the south and east where a more diverse flora included high abundances of Nitzschia seriata. Major zooplankton-station groupings were located over the inner shelf which was characterised by a high abundance of Drepanopus forcipatus and in oceanic water >500 m deep that were dominated by Foraminifera, Oithona spp., Ctenocalanus vanus, and Calanoides acutus. Stations along the middle and outer shelf regions to the north and west, were characterised by low overall abundance. There was some evidence that groupings of stations to the north of the island originated in different water masses on either side of the Southern Antarctic Circumpolar Current Front, the major frontal system in the deep ocean close to South Georgia. However, transect lines during 1926/1927 did not extend far enough offshore to sample this frontal region adequately. Interannual variability of zooplankton abundance was assessed from stations which were sampled repeatedly during seven recent British Antarctic Survey cruises (1995-2005) to the region and following taxonomic harmonization and numerical standardization (ind. m⁻³), a subset of 45 taxonomic categories of zooplankton (species and higher taxa) from 1926/1927, were compared with similar data obtained during the BAS cruises using a linear model. Initially comparisons were restricted to BAS stations that lay within 40 km of Discovery stations although a comparison was also made using all available data. Despite low abundance values in 1926/1927, in neither comparison did Discovery data differ significantly from BAS data. Calculation of the percentage similarity index across cruises did not reveal any systematic differences in species composition between 1926 and 1927 and the present. In the light of ocean warming trends, the existence of more subtle changes in species composition is not ruled out, but an absence of finely resolved time-series data make this impossible to determine.     

A stochastic model of SST for climate simulation experiments

 This study describes the implementation of a statistical method to simulate a multi-century sequence of global sea surface temperature (SST) fields. A multi-variable auto-regressive (AR) model is trained on the observed time series of SST from the data set compiled at the Hadley Centre (GISST 2.0). To reduce the dimensionality of the model, the stochastic process is in practice fitted to empirical orthogonal function (EOF) time coefficients of the SST series, retaining the first 14 EOFs. Selected lag cross-covariances among the EOF time series are retained, based on the structure of the cross-correlation matrix and lags up to 64 months are included. Though the resulting system is quite large (a 14-dimensional AR process, with 400 parameters to be determined) the calculation is possible and a stable process is obtained. The process can then be used to investigate some statistical properties of the SST data set and to generate synthetic SST data that could be used in very long numerical experiments with atmospheric or ocean models in which only the main features of the observed statistics of the SST must be retained. Results indicate that the synthetic SST data set seems to be of usable quality as boundary condition for the atmosphere or the ocean in climate experiments. Analysis of extreme events and extreme decades in the synthetic SST data confirms the exceptional character of the 1980s, but also provides circumstantial evidence that the 1980s were indeed within the limits of the statistics of the previously observed record. Received: 6 August 1996 / Accepted: 29 September 1997     

Metamorphism of mineral matter in coal from the Bukit Asam deposit, south Sumatra, Indonesia

The coal of the Miocene Bukit Asam deposit in south Sumatra is mostly sub-bituminous in rank, consistent with regional trends due to burial processes. However, effects associated with Plio–Pleistocene igneous intrusions have produced coal with vitrinite reflectance up to at least 4.17% (anthracite) in different parts of the deposit. The un-metamorphosed to slightly metamorphosed coals, with Rv_max values of 0.45–0.65%, contain a mineral assemblage made up almost entirely of well-ordered kaolinite and quartz. The more strongly heat-affected coals, with Rv_max values of more than 1.0%, are dominated by irregularly and regularly interstratified illite/smectite, poorly crystallized kaolinite and paragonite (Na mica), with chlorite in some of the anthracite materials. Kaolinite is abundant in the partings of the lower-rank coals, but is absent from the partings in the higher-rank areas, even at similar horizons in the same coal seam. Regularly interstratified illite/smectite, which is totally absent from the partings in the lower-rank coals, dominates the mineralogy in the partings associated with the higher-rank coal beds. A number of reactions involving the alteration of silicate minerals appear to have occurred in both the coal and the associated non-coal lithologies during the thermal metamorphism generated by the intrusions. The most prominent involve the disappearance of kaolinite, the appearance of irregularly interstratified illite/smectite, and the formation of regular I/S, paragonite and chlorite. Although regular I/S is identified in all of the non-coal partings associated with the higher-rank coals, illite/smectite with an ordered structure is only recognised in the coal samples collected from near the bases of the seams. The I/S in the coal samples adjacent to the floor of the highest rank seam also appears to have a greater proportion of illitic components. The availability of sodium and other non-mineral inorganic elements in the original coal to interact with the kaolinite, under different thermal and geochemical conditions, appears to be the significant factor in the formation of these new minerals, and distinguishes the mineralogical changes at Bukit Asam from those developed more generally with rank increases due to burial, and from the effects of intrusions into coals that were already at higher rank levels.     

Stratigraphic correlation of the Devonian sequence in the Blantyre Sub-basin,Darling Basin,western New South Wales∗

Seismic sections and the analysis of lithostratigraphic units from well-log data were used to develop a new stratigraphic correlation of the Winduck, Snake Cave and Ravendale Intervals for the Blantyre Sub-basin. The stratigraphic boundaries of the intervals were defined at marked changes in well-log characteristics, and depth estimates of the boundaries were derived from the well-log data in Mt Emu 1, Blantyre 1 and Kewell East 1. Six seismic-stratigraphic boundaries have been identified in the seismic sections to show the continuity of the latest Silurian to Holocene sediments throughout the Blantyre Sub-basin; from bottom to top they are: H-1, base of the Winduck Interval; H-2, base of the Snake Cave Interval; H-3, base of the Ravendale Interval; H-4/5 base of the undifferentiated Upper Carboniferous/Permian sediments; and H-6 base of the undifferentiated Cenozoic sediments. All stratigraphic boundaries are based on good continuous markers, with strong amplitudes throughout the whole sub-basin. A three-dimensional geological model was developed from the seismic data to map out the geometry of the key reflectors, and hence the structure and stratigraphy of the Winduck, Snake Cave and Ravendale Intervals in the areas where these intervals have been preserved. This model has better defined the Wilcannia High and two smaller highs around the Mt Emu 1 and Snake Flat 1 wells, and further defines the relationships between the stratigraphy, sub-basin geometry and development of complex structures in the Blantyre Sub-basin.     

Measurement of Martian atmospheric winds by the O2 1.27 μm airglow observations using Doppler Michelson Interferometry: A concept study

Science China Earth Sciences - China’s Mars exploration mission has stimulated tremendous interest in planetary science exploration recently. To propose potential scientific research...     

Last interglacial western camel (Camelops hesternus) from eastern Beringia

Western camel (C. hesternus) fossils are rare from Eastern Beringia, thus there is little available information on their chronology, paleoecology, and biogeography in this region. In August of 2010, a partial proximal phalanx of a western camel was recovered from a sedimentary exposure along the White River, in the formerly glaciated terrain of southwest Yukon, northwest Canada. The fossil specimen was recovered in situ from sediments that are correlated by stratigraphic, tephra and radiocarbon data to the Marine Isotope Stage (MIS) 5 interglacial period (Sangamonian). Associated paleoenvironmental data indicates that this western camel inhabited a shrub tundra ecosystem that did not include spruce trees or boreal forest during a relatively cold interval between MIS 5e and 5a. This is the oldest reliably dated western camel fossil from Eastern Beringia and supports the model of range expansion for this species to the high latitudes of northwest North America during the last interglacial (sensu lato).     

Soil water content in southern England derived from a cosmic‐ray soil moisture observing system – COSMOS‐UK

Cosmic‐ray soil moisture sensors have the advantage of a large measurement footprint (approximately 700 m in diameter) and are able to operate continuously to provide area‐averaged near‐surface (top 10–20 cm) volumetric soil moisture content at the field scale. This paper presents the application of this technique at four sites in southern England over almost 3 years. Results show the soil moisture response to contrasting climatic conditions during 2011–2014 and are the first such field‐scale measurements made in the UK. These four sites are prototype stations for a UK COsmic‐ray Soil Moisture Observing System, and particular consideration is given to sensor operating conditions in the UK. Comparison of these soil water content observations with the Joint UK Land Environment Simulator 10‐cm soil moisture layer shows that these data can be used to test and diagnose model performance and indicate the potential for assimilation of these data into hydro‐meteorological models. The application of these large‐area soil water content measurements to evaluate remotely sensed soil moisture products is also demonstrated. Numerous applications and the future development of a national COsmic‐ray Soil Moisture Observing System network are discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of Non-Uniform Crosswind Fields on Scintillometry Measurements

The effects of a non-uniform wind field along the path of a scintillometer are investigated. Theoretical spectra are calculated for a range of scenarios where the crosswind varies in space or time and compared to the ‘ideal’ spectrum based on a constant uniform crosswind. It is verified that the refractive-index structure parameter relation with the scintillometer signal remains valid and invariant for both spatially and temporally-varying crosswinds. However, the spectral shape may change significantly preventing accurate estimation of the crosswind speed from the peak of the frequency spectrum and retrieval of the structure parameter from the plateau of the power spectrum. On comparison with experimental data, non-uniform crosswind conditions could be responsible for previously unexplained features sometimes seen in observed spectra. By accounting for the distribution of crosswind, theoretical spectra can be generated that closely replicate the observations, leading to a better understanding of the measurements. Spatial variability of wind speeds should be expected for paths other than those that are parallel to the surface and over flat, homogenous areas, whilst fluctuations in time are important for all sites.     

The SOLAS air-sea gas exchange experiment (SAGE) 2004

The SOLAS air-sea gas exchange experiment (SAGE) was a multiple-objective study investigating gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the expansive subpolar zone of the southern oceans. This paper provides a general introduction and summary of the main experimental findings. The release site was selected from a pre-voyage desktop study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the south-east of New Zealand and the experiment was conducted between mid-March and mid-April 2004. In common with other mesoscale iron addition experiments (FeAX’s), SAGE was designed as a Lagrangian study, quantifying key biological and physical drivers influencing the air-sea gas exchange processes of CO₂, DMS and other biogenic gases associated with an iron-induced phytoplankton bloom. A dual tracer SF₆/³He release enabled quantification of both the lateral evolution of a labelled volume (patch) of ocean and the air-sea tracer exchange at tenths of kilometer scale, in conjunction with the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the gas exchange coefficient on windspeed that is widely applicable and describe air-sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties and windspeed were quantified to further assist the development of gas exchange models for high-wind environments.There was a significant increase in the photosynthetic competence (F_v/F_m) of resident phytoplankton within the first day following iron addition, but in contrast to other FeAX’s, rates of net primary production and column-integrated chlorophyll a concentrations had only doubled relative to the unfertilised surrounding waters by the end of the experiment. After 15 days and four iron additions totalling 1.1 ton Fe²⁺, this was a very modest response compared to other mesoscale iron enrichment experiments. An investigation of the factors limiting bloom development considered co-limitation by light and other nutrients, the phytoplankton seed-stock and grazing regulation. Whilst incident light levels and the initial Si:N ratio were the lowest recorded in all FeAXs to date, there was only a small seed-stock of diatoms (less than 1% of biomass) and the main response to iron addition was by the picophytoplankton. A high rate of dilution of the fertilised patch relative to phytoplankton growth rate, the greater than expected depth of the surface mixed layer and microzooplankton grazing were all considered as factors that prevented significant biomass accumulation. In line with the limited response, the enhanced biological draw-down of pCO₂ was small and masked by a general increase in pCO₂ due to mixing with higher pCO₂ waters. The DMS precursor DMSP was kept in check through grazing activity and in contrast to most FeAX’s dissolved dimethylsulfide (DMS) concentration declined through the experiment. SAGE is an important low-end member in the range of responses to iron addition in FeAX’s. In the context of iron fertilisation as a geoengineering tool for atmospheric CO₂ removal, SAGE has clearly demonstrated that a significant proportion of the low iron ocean may not produce a phytoplankton bloom in response to iron addition.     

Surface layer mixing during the SAGE ocean fertilization experiment

Vessel-based observations of the oceanic surface layer during the 14-day 2004 SAGE ocean fertilization experiment were conducted using ADCP, CTD and temperature microstructure in a frame of reference moving with a patch of injected SF₆ tracer. During the experiment the mixed layer depth z_mld ranged between 50 and 80 m, with several re-stratifying events that brought z_mld up to less than 40 m. These re-stratifying events were not directly attributable to local surface-down development of stratification and were more likely associated with horizontal variation in density structure. Comparison between the CTD and a one-dimensional model confirmed that the SAGE experiment was governed by 3-d processes. A new method for estimating z_mld was developed that incorporates a component that is proportional to density gradient. This highlighted the need for well-conditioned near-surface data which are not always available from vessel-based survey CTD profiles. A centred-displacement scale, L_c, equivalent to the Thorpe lengthscale, reached a maximum of 20 m, with the eddy-centroid located at around 40 m depth. Temperature gradient microstructure-derived estimates of the vertical turbulent eddy diffusivity of scalar (temperature) material yielded bin-averaged values around 10⁻³ m² s⁻¹ in the pycnocline rising to over 10⁻² m² s⁻¹ higher in the surface layer. This suggests transport rates of nitrate and silicate at the base of the surface layer generate mixed layer increases of the order of 38 and 13 mmol/m²/day, respectively, during SAGE. However, the variability in measured vertical transport processes highlights the importance of transient events like wind mixing and horizontal intrusions.