Impact of Island Urbanization on Freshwater Lenses: A Case Study on a Small Coral Island

Freshwater resources on small coral islands mainly exist in the form of freshwater lenses. The freshwater lens is highly vulnerable to salinization due to natural recharge variations and urbanization construction. However, it is unclear how a freshwater lens evolves under the influence of urbanization construction and which factors control its evolution. Based on the hydrogeological data of a small coral island in China, a corresponding 3D numerical model was established by the Visual MODFLOW software to investigate the formation and evolution of freshwater lenses under natural conditions. Thereby, the island reclamation scenario and impermeable surface scenario were set up and the changes in morphology and volume of the freshwater lens were analyzed. The results show the following: (1) After island reclamation and island building, the freshwater lens would reach a stable state after 25 years and the freshwater lens would also appear in the newly added part of the island with a thickness of 9.5 m, while the volume of the total freshwater lens would increase to 1.22 times that of the original island. (2) When the impermeable surface is built at different positions of the island, the reduction in the volume of the freshwater lens, in the order from large to small, is Scenario B (northeast side), Scenario A (southwest side) and Scenario C (central); with the increase in the impermeable surface area, the volume of the freshwater lens would gradually decrease and the volume of the freshwater lens would decrease by more than 50% with the impermeable surface exceeding 30% of the island area. The study has important implications for the conservation and rational development of subsurface freshwater resources on islands.

2. Ancient reefs and islands

Reefs in deep geological time have been built by a succession of different kinds of life: plant, bacterial, and animal. Stromatolites and bryozoans were major reef-builders that persist today in minor or non-reef-building forms, sponges built entire reefs and are still important reef components, while several groups of major reef-builders flourished for a while and then became extinct: archaeocyathids which were similar to sponges, and coral-like forms including rugose and tabulate corals. Today’s reef-builders, cnidarian corals, appeared well after the great Permian-Triassic extinction event. All of these groups deposited vast quantities of limestone rock on which they live, often visible today as low mountain ranges. Reefs grow to the surface but not beyond, but upon them sand and sediments may build up, forming an island that attracts plants, then birds and other terrestrial forms of life. The sediments become cemented with the aid of rainwater too, and ‘low islands’ develop. Many islands also show their old, central volcanoes, resulting in the vast array of different combinations of coral island type. Today, however, there is a coral reef crisis due to overexploitation of a reef’s rich resources, from pollution of several kinds, and climate change.

Laboratory Physical Experiments on the Saltwater Upconing and Recovery of Island Freshwater Lenses: Case Study of a Coral Island, China

The research of saltwater upconing is crucial for the development and utilization of island freshwater resources. In this paper, a laboratory physical experiment device was developed, and the saltwater upconing and recovery of island freshwater lenses were investigated using rainfall simulations and single-well and multi-well pumping tests with various pumping intensities. The results of the experiment revealed that: (1) The thickness of the freshwater lens increased continuously and linearly during the early stages of rainfall. As the rainfall continued, the upward trend slowed and eventually leveled off. (2) Under the same pumping duration, when the pumping intensity was less than the critical pumping intensity, the increasing height rate of the saltwater upconing increased linearly at a small gradient, and when the pumping intensity was greater than the critical value, the rising height rate of the saltwater upconing increased linearly at a large gradient. (3) Under the same pumping intensity conditions, the pumping duration of the multi-well was longer than that of the single-well pumping, and the pumping volume of the multi-well was also greater than that of the single-well pumping. The experiment results can provide support for the development and utilization of island freshwater lens.

Morphodynamic Controls for Growth and Evolution of a Rubble Coral Island

Rubble islands are dynamic sedimentary features present on reef platforms that evolve under a variety of morphodynamic processes and controlling mechanisms. They provide valuable inhabitable land for small island nations, critical habitat for numerous species, and are threatened by climate change. Aiming to investigate the controlling mechanisms dictating the evolution of One Tree Island (OTI), a rubble island in the Southern Great Barrier Reef, we combined different remotely-sensed data across varying timescales with wave data extracted from satellite altimetry and cyclone activity. Our findings show that (1) OTI had expanded by 7% between 1978 and 2019, (2) significant gross planform decadal adjustments were governed by the amount, intensity, proximity, and relative position of cyclones as well as El Niño Southern Oscillation (ENSO) phases, and (3) the mechanisms of island growth involve rubble spits delivering and redistributing rubble to the island through alongshore sediment transport and wave overtopping. Frequent short-term monitoring of the island and further research coupling variations in the different factors driving island change (i.e., sediment availability, reef-wave interactions, and extreme events) are needed to shed light on the future trajectory of OTI and other rubble islands under a climate change scenario.

PULAU ABIDON: POTENSI ARKEOLOGI DI KAWASAN PULAU TERLUAR RAJA AMPAT [PULAU ABIDON: THE POTENCY OF ARCHAEOLOGY IN THE RAJA AMPAT OUTER ISLANDS]

Pulau Abidon merupakan suatu pulau karang berbukit-bukit yang berada di kawasan pulau-pulau terluar Raja Ampat di Papua Barat bagian utara. Tulisan ini membahas mengenai potensi arkeologi yang terdapat di situs gua-gua di Pulau Abidon. Tujuan dari penelitian ini adalah untuk memahami potensi arkeologi yang terdapat di Pulau Abidon. Metode yang digunakan dalam penelitian ini bersifat eksploratif. Berdasarkan data dan hasil analisis diindikasikan adanya persentuhan budaya asing yang masuk ke kawasan Papua. Potensi arkeologis tersebut dibuktikan dengan tinggalan fragmen gerabah dan lukisan dinding gua. Gambar arang di Gua Abidon 3 menggambarkan kontak budaya dengan penutur bahasa Austronesia. Lebih lanjut, hunian gua dibuktikan dengan temuan berupa alat-alat dari batu, tulang, dan kerang, fragmen gerabah, dan perhiasan kerang. Tinggalan budaya di gua-gua pulau Abidon diduga merupakan alat-alat penunjang kehidupan para penghuninya. Pulau Abidon is a hilly coral island located in the outer islands of Raja Ampat in the northern region of Papua Barat. This research discusses the potency of archaeology in cave sites on Pulau Abidon. This research was aimed to comprehend the archaeology of Pulau Abidon. The method used in this research is exploratory. Based on the data and analysis results, it is indicated that there was a cultural contact with a foreign culture that entered the Papua region. This potency of archaeology was evident by potsherds and rock arts. The charcoal drawings in Gua Abidon 3 illustrate a cultural contact with the Austronesian-language speakers. Furthermore, the cave habitation was evident also by the discovery of tools of stone, bones and shells, and shell ornaments. The cultural heritage in the caves on Pulau Abidon is suggested to be a means of supporting the life of the inhabitants.