Testing theories of the vertical stratification of the ACC against observations

Recent theories of the Antarctic Circumpolar Current (ACC) suggest that its lateral and vertical stratification is controlled by its baroclinic instability: eddies in the ACC not only feed-off the available potential energy stored in sloping isopycnals but play a central role is setting up that stratification. Simple theory makes predictions about how the depth of the thermocline in the ACC depends on the surface winds, the air–sea buoyancy flux and transfer by baroclinic eddies. By examining gridded hydrographic data, here we test some of these predictions against observations. We show that, to a remarkable degree, the buoyancy field in the ACC decays exponentially with depth beneath the mixed layer. The e-folding depth increases equatorward, from less than 500 m on the poleward flank of the ACC to greater then 1000 m on its equatorial flank, in a manner that is broadly consistent with the theory.     

The antiquity of aroid pit agriculture and significance of buried A horizons on Pacific atolls

The cultivation of aroids (herbaceous plants with starchy corms) is the foundation of Oceanic societies, yet the study of prehistoric atoll agriculture (utilizing Cyrtosperma chamissonis) has been almost totally neglected. Aroid pit agricultural features, some measuring up to 100 m long and 20 m wide, excavated in the Marshall Islands (center ca. 8°N latitude, 170°E longitude) provide the first chronometric dates (1910 ± 70 B.P., Beta-79576) for this type of cultivation practice associated with coral atolls found throughout the Pacific. Excavations through an aroid pit cultivation pit rim identified a stratigraphic sequence beginning with the sterile subsoil, an A horizon deeply buried under pit spoil dirt, and a prehistoric midden deposit beginning below the surface A horizon. Granulometric analysis of sediments and identification of foraminifers documented the nearshore lagoon as the source for all non-cultural sediments. Anthropophilic land snails (Gastrocopta pediculus and Lamellidea pusilla) in the dated, buried A horizon is a firm basis for confirming the presence of humans near initial colonization (ca. 2000 B.P.) and anchors the culture-historical sequence for the long-term study of human impacts to low coral islands. Consequently, on-going analyses of plant opal phytoliths, starch grains, and charcoal from the buried A horizon, should document the nature of early atoll ecology prior to significant human modification. As aroid pit construction is associated, in many examples, with traditional property boundaries, detailed mapping and dating of these cultivation systems should relate to changes in land tenure and prehistoric social organization. © 1999 John Wiley & Sons, Inc.     

From drought to flood: A water balance analysis of the Tuolumne River basin during extreme conditions (2015–2017)

The degree to which the hydrologic water balance in a snow-dominated headwater catchment is affected by annual climate variations is difficult to quantify, primarily due to uncertainties in measuring precipitation inputs and evapotranspiration (ET) losses. Over a recent three-year period, the snowpack in California's Sierra Nevada fluctuated from the lightest in recorded history (2015) to historically heaviest (2017), with a relatively average year in between (2016). This large dynamic range in climatic conditions presents a unique opportunity to investigate correlations between annual water availability and runoff in a snow-dominated catchment. Here, we estimate ET using a water balance approach where the water inputs to the system are spatially constrained using a combination of remote sensing, physically based modelling, and in-situ observations. For all 3 years of this study, the NASA Airborne Snow Observatory (ASO) combined periodic high-resolution snow depths from airborne Lidar with snow density estimates from an energy and mass balance model to produce spatial estimates of snow water equivalent over the Tuolumne headwater catchment at 50-m resolution. Using observed reservoir inflow at the basin outlet and the well-quantified snowmelt model results that benefit from periodic ASO snow depth updates, we estimate annual ET, runoff efficiency (RE), and the associated uncertainty across these three dissimilar water years. Throughout the study period, estimated annual ET magnitudes remained steady (222 mm in 2015, 151 mm in 2016, and 299 mm in 2017) relative to the large differences in basin input precipitation (547 mm in 2015, 1,060 mm in 2016, and 2,211 mm in 2017). These values compare well with independent satellite-derived ET estimates and previously published studies in this basin. Results reveal that ET in the Tuolumne does not scale linearly with the amount of available water to the basin, and that RE primarily depends on total annual snowfall proportion of precipitation.     

Origins and maturity of organic matter in mid-Cretaceous black shales from ODP Site 1138 on the Kerguelen Plateau

We have conducted elemental, isotopic, and Rock-Eval analyses of Cenomanian–Santonian sediment samples from ODP Site 1138 in the southern Indian Ocean to assess the origin and thermal maturity of organic matter in mid-Cretaceous black shales found at this high-latitude location. Total organic carbon (TOC) concentrations range between 1 and 20 wt% in black to medium-gray sediments deposited around the Cenomanian–Turonian boundary. Results of Rock-Eval pyrolysis indicate that the organic matter is algal Type II material that has experienced modest alteration. Important contributions of nitrogen-fixing bacteria to the amplified production of organic matter implied by the high TOC concentrations is recorded in δ¹⁵N values between −5 and 1‰, and the existence of a near-surface intensified oxygen minimum zone that favored organic carbon preservation is implied by TOC/TN ratios between 20 and 40. In contrast to the marine nature of the organic matter in the Cenomanian–Turonian boundary section, deeper sediments at Site 1138 contain evidence of contributions land-derived organic matter that implies the former presence of forests on the Kerguelen Plateau until the earliest Cenomanian.     

Fluid expulsion features associated with sand waves on Australia’s central North West Shelf

Multibeam swath bathymetric data collected in 95–120 m water depth on Australia’s North West Shelf revealed two distinct populations of sand waves: a laterally extensive, low-amplitude composite form comprising superimposed dunes and ripples, and a laterally restricted form which has unusually high bedform heights and slopes. These large subaqueous sand waves comprise bioclastic ooid/peloid sand. Significantly, evidence of seabed fluid flow was detected in association with the high-amplitude sand waves. This evidence includes seabed pockmarks approximately 2–15 m in diameter imaged with side-scan sonar, tubular and massive carbonate concretions dredged from the seabed, and potential active venting of a fluid plume from the seabed observed during an underwater camera tow. Molecular and isotopic analyses of carbonate concretions collected from within pockmarks associated with the high-amplitude sand waves indicate that the fluids from which they precipitated comprise modern seawater and are not related to thermogenic fluids or microbial gases. The fluid flow is interpreted to be driven by macrotidal currents flowing over the relatively steep slopes of the high-amplitude sand waves. Pockmarks and carbonate concretions then develop where the interstitial flows are confined and focused by subsurface ‘mounds’ in a shallow seismic reflector.     

A new coral reef province in the Gulf of Carpentaria, Australia: Colonisation, growth and submergence during the early Holocene

Multibeam sonar mapping, drill cores and underwater video data have confirmed the existence of a previously unknown coral reef province in the Gulf of Carpentaria, Australia. Seven reefs, comprised of coral limestone that support living corals have been mapped so far and as many as 50 other reefs may exist in the region. U/Th ages show that reef growth commenced shortly after limestone pedestals were submerged by rising sea level around 10.5 kyr BP, making them the oldest Holocene reefs known in Australia. Reef growth persisted for ~ 2.0 kyr but it had ceased at most locations by ~ 7.0 kyr BP. Measurements of reef growth rates (0.95 to 4 m kyr^− 1), indicate that the reefs were unable to keep pace with contemporaneous rapid sea level rise (> 10 m kyr^− 1), which is consistent with a “give up” reef growth history. Core samples from reef platforms demonstrate that Pleistocene limestone is exposed at depths of 27 and 30 m below present mean sea level. These depths represent regionally significant phases of reef growth during a prolonged sea level still stand. We conclude that the reefs are therefore mostly relict features, whose major phase of growth and development relates to an earlier, pre-Holocene sea level still stand.     

A GIS-based Investigation of Gaming Venue Catchments

Globally, the growth of the gambling industry in recent decades has raised questions about the potential for community costs associated with gambling development. In order to minimise the harms and maximise the gains for local, state and national economies, in many jurisdictions the industry is heavily regulated. Regulators rely upon a wide range of information sources to assist in decision making and are often concerned with issues that have socio‐spatial characteristics. Despite this, there have been few applications of geographical information systems (GIS) to gambling behaviour. This paper trials the use of a GIS‐based approach to investigate one aspect of gambling regulation, namely the spatial characteristics of gaming venue catchments. The research was conducted in a suburban region of Canberra, Australia. The results from the study show that the catchment areas of gaming venues can vary considerably and are more complex than previous studies have suggested. The regulatory implications of the findings are discussed as well as possible avenues for further GIS‐based research into gambling.     

Influence of non-linear effects upon surge elevations along the west coast of Britain

A two-dimensional vertically integrated hydrodynamic finite-element model of the west coast of Britain is used to examine the response of the region to extreme meteorological forcing. The extent to which tide–surge interaction modifies the computed surge elevation and current distributions is examined in detail. The nature of the finite-element model with its ability to refine the mesh in nearshore regions is ideal for examining the influence of non-linear effects upon surges in these regions. Calculations using spatially uniform orthogonal wind stresses show that the surge elevation and current in shallow water are particularly sensitive to the method used to remove the tide as a result of the highly non-linear nature of the tide–surge interaction in these regions. The most accurate means of de-tiding the solution is by subtracting a tide derived by harmonic analysis of the tide and surge time series at the time of the surge. Subtracting a tide-only solution (the usual approach) leads to tidal energy leaking into the surge solution. Calculations show that this arises because the surge modifies the tidal amplitude and phase in shallow-water regions to such an extent that they are appreciably different to those found in the tide-only calculation. Results suggest that this problem becomes more important, as nearshore meshes are refined in an attempt to improve surge prediction. This suggests that in the future, highly accurate fine-mesh models will be required to compute total water levels without the present linear separation into tidal and surge signal used in operational surge prediction.     

Phytoplankton index of biotic integrity for Chesapeake Bay and its tidal tributaries

A Phytoplankton Index of Biotic Integrity (P-IBI) was developed from data collected during 18 yr 91985–2002) of the Chesapeake Bay Water Quality Monitoring Program. Dissolved inorganic nitrogen (DIN), orthophosphate (PO₄), and Secchi depth were used to characterize phytoplankton habitat conditions. Low DIN and PO₄ concentrations and high Secchi depths characterized least-impaire conditions. Thirty-eight phytoplankton metrics were tested for their ability to discriminate between impaired and least-impaired habitat conditions. Twelve discriminatory metrics were chosen, and different combinations of these twelve metrics were scored and used to create phytoplankton community indexes for spring and summer in the four salinity regimes in Chesapeake Bay. The scoring criteria for each metric were based on the distribution of the metric’s values in least-impaired conditions relative to the distribution in impaired conditions. An independent data set and jackknife validation procedure were used to examine P-IBI performance. The P-IBI correctly classified 70.0–84.4% of the impaired and least-impaired samples, grouped by season and salinity, in the calibration data set. The P-IBI is a management tool to assess phytoplankton community status relative to estuarine nutrient and light conditions.