ALBERT

Description: Environmental conditions and anthropogenic impacts are key influences on ecological processes and associated ecosystem services. Effective management of Tonga's marine ecosystems therefore depends on accurate and up-to-date knowledge of environmental and anthropogenic variables. Although many types of environmental and anthropogenic data are now available in global layers, they are often inaccessible to end users, particularly in developing countries with limited accessibility and analytical training. Furthermore, the resolution of many global layers might not be sufficient to make informed local decisions. While the near-shore marine ecosystem of Tonga is extensive, the resources available for its management are limited and little is known about its current ecological state. Here we provide a marine socio-environmental dataset covering Tonga's near-shore marine ecosystem as compiled from various global layers, remote sensing projects, local ministries, and the 2016 national census. The dataset consists of eleven environmental and six anthropogenic variables summarized in ecologically relevant ways, spatially overlaid across the near-shore marine ecosystem of Tonga. The environmental variables selected include: bathymetry, coral reef density, distance from deep water, distance from land, distance from major terrestrial inputs, habitat, land area, net primary productivity, salinity, sea surface temperature, and wave energy. The anthropogenic variables selected include: fishing pressure, management status, distance to fish markets, distance from villages, population pressure, and a socioeconomic development index based on population density, growth, mean age, mean education level, and unemployment. This extensive and accessible dataset will be an essential tool for future assessment and management of marine ecosystems in Tonga.

Description: Quantitative morphometric analyses were carried out for each mound following the workflows presented by Purkis et al. (2007) The coral mound base was defined following the methodological approach of Correa et al. (2012) using the dip angle map, generated from the digital elevation model (DEM), to extract closed polygons that follow the 3°-contour line. This 3°-cutoff has been qualitatively validated with a comparison between the DEM and the dip angle (Fig. 2). Small-scaled polygons within mound perimeters and resulting from bathymetric artifacts were filtered out. Manual editing was applied to split simple merged mound structures (e.g. twin-peak mounds) based on higher cut-off slope values (4-5°). Furthermore, polygons describing the mound footprint have been corrected to remove unrealistic shapes especially common for the CBM. The DEM was subsequently re-gridded to generate hypothetical bathymetric maps without mounds, for which the vertical relief beneath each removed mound was interpolated from the mound perimeters. The newly interpolated surfaces were then subtracted from the original DEMs to evaluate the volume and heights of the coral mounds. Only features with a footprint area greater than 900 squared meters (corresponding to a two-dimensional array of 3 × 3 DEM grid cells) and with a height of 〉2 m above the surrounding seafloor (4 × 0.5 m of vertical precision) were considered as coral mounds and quantitatively analyzed.

Description: Quantitative morphometric analyses were carried out for each mound following the workflows presented by Purkis et al. (2007) The coral mound base was defined following the methodological approach of Correa et al. (2012) using the dip angle map, generated from the digital elevation model (DEM), to extract closed polygons that follow the 3°-contour line. This 3°-cutoff has been qualitatively validated with a comparison between the DEM and the dip angle. Small-scaled polygons within mound perimeters and resulting from bathymetry artefacts were filtered out. Manual editing was applied to split simple merged mound structures (e.g. twin-peak mounds) based on higher cut-off slope values (4-5°). Furthermore, polygons describing the mound footprint have been corrected to remove unrealistic shapes especially common for the CBM. The DEM was subsequently re-gridded to generate hypothetical bathymetric maps without mounds, for which the vertical relief beneath each removed mound was interpolated from the mound perimeters. The newly interpolated surfaces were then subtracted from the original DEMs to evaluate the volume and heights of the coral mounds. Only features with a footprint area greater than 900 suared meters (corresponding to a two-dimensional array of 3 × 3 DEM grid cells) and with a height of 〉2 m above the surrounding seafloor (4 × 0.5 m of vertical precision) were considered as coral mounds and quantitatively analyzed.

Description: The search for, and extraction of, hydrocarbons in carbonate rocks demands a thorough understanding of their depositional anatomy. The complexity of carbonate systems, however, hinders detailed direct characterization of their volumetric heterogeneity. Information with which to construct a reservoir model must therefore be based on information gathered from wells or outcrops transecting the sequence of interest. Most (particularly exploration wells) are vertical, presenting a problem for geostatistical modeling. While understanding vertical stratal stacking is straightforward, it is difficult to obtain lateral facies information. Though in some situations outcrop surfaces, seismic data, and horizontal wells may somewhat mitigate this bias, the likelihood remains that the lateral dimension of a buried system will be vastly undersampled with respect to the vertical. However, through the principle of Walther's Law (Walther 1894) or due to the geometry of basinward-inclined beds, comparable facies frequencies and transition probabilities may link vertical and lateral stratal arrangements, the implication being that a reservoir model, competent at least in terms of transition statistics, could be built against information harvested down-core. Taking an interpreted outcrop panel from Lewis Canyon (Albian, Pecos River, Texas), we use Markov-chains to first ascertain that vertical and lateral stratal ordering is nonrandom. Second, we show lithofacies transition probabilities in the outcrop as being interchangeable between the vertical and lateral directions. The work concludes by demonstrating the utility of an existing 3-D Markov random field simulation to volumetrically model the Lewis Canyon outcrop on the basis of vertical facies transition tendencies. Statistical interrogation of the 3-D model output reveals the simulation to contain realistic facies associations compared to the outcrop. This suggests that the reconstruction process, based on Markov chains, produces a useful representation of 3-D heterogeneity in this Lower Cretaceous carbonate succession. Markov random field simulation might provide an important tool for prediction and simulation of subsurface carbonate reservoirs.

Description: Great Bahama Bank (GBB) is the modern example of a flat-topped, isolated carbonate platform. It is a major modern location of carbonate deposition that stands behind much of the understanding of modern processes of carbonate sedimentation, serves as a training venue for academia and industry, is the basis for numerous geological models, and is commonly used as a reservoir analog. GBB also provides valuable insight into the extent and patterns of sediment fill of accommodation space atop an isolated carbonate platform. Satellite imagery (Landsat TM and ETM+) and an extensive set of water-depth measurements ( n = 5,723) were used to map bathymetry across GBB and derive a digital terrain model (DTM). Analyzing the extent, patterns, and nature of sediment fill of accommodation space was facilitated by partitioning a depositional facies map on the basis of the DTM to show that 18% of accommodation on the GBB is “overfilled and filled” (emergent to −1.5 m water depth), 52% is “underfilled” (−1.5 to −6.0 m), and 30% is “unfilled” (〈 −6.0 m). Considering the bathymetric variation, the DTM shows that only 10% (10,800 km2) of the awash platform has aggraded to sea level in the form of sand shoals or mud flats (〉 −1.5 m water depth) and therefore the greater part of available accommodation (88,000 km2) remains incompletely filled with sediment. Areas of filled accommodation mostly extend platformward from the western coastlines of islands, which in turn are preferentially distributed along the eastern (windward) margin of the GBB. Seventy percent of sediment in the areas of filled accommodation is rudstone, high-energy grainstone, grainstone, and mud-poor packstone. Although dominated by grainstones (45%), since it occupies so much space (55,000 km2), the underfilled sector also contains the most heterogeneous facies mosaic and by definition, therefore, the greatest and most complex lateral facies variations. Although islands are numerous ( n = 1,430) they occupy only 8%, or 8,700 km2, of the platform top. Despite their small proportional occupancy of the platform, islands play a direct role in the accumulation of muddy fabrics and exert a sphere of influence over the character of sedimentation for many tens of kilometers from their coastlines. Further, since islands represent the portion of the bank where accommodation has been overfilled, their rarity emphasizes the challenges that such a large platform faces in filling accommodation space, even given the diverse grain factories producing carbonate sediment (mud precipitation through whitings, ooids and reefs along the platform margins, skeletal sediments and nonskeletal grains such as fecal pellets, peloids, and pelletoids). Two factors can be evoked to explain the inability of GBB to fill accommodation space. First, falling sea level during the Pleistocene appears to have repeatedly aborted the filling process. By analogy, it will not be able to do so in the Holocene either, a situation exasperated by the second factor, which is that most of the Holocene GBB, particularly areas away from islands, presently lack any appreciable filling. Instead, facies analysis suggest reduced sedimentation and enhanced cementation. Through regional mapping, the work delivers a platform-wide assessment of how accommodation is filled and the facies responsible for the filling. Amongst other trends, by highlighting how the thickest accumulations of Holocene sediment are skewed to the windward margin of the GBB and related to the presence of islands, whereas the thinner accumulations, which dominate the platform interior and are more grainy but also more heterogeneous than the thicker deposits, these data might contain useful guidelines as to how depositional cycles might vary laterally in ancient systems.