Late Holocene (0–6 ka) sea-level changes in the Makassar Strait, Indonesia

The Spermonde Archipelago, off the coast of southwest Sulawesi, consists of more than 100 small islands and hundreds of shallow-water reef areas. Most of the islands are bordered by coral reefs that grew in the past in response to paleo relative sea-level changes. Remnants of these reefs are preserved today in the form of fossil microatolls. In this study, we report the elevation, age, and paleo relative sea-level estimates derived from fossil microatolls surveyed in five islands of the Spermonde Archipelago. We describe 24 new sea-level index points, and we compare our dataset with both previously published proxies and with relative sea-level predictions from a set of 54 glacial isostatic adjustment (GIA) models, using different assumptions on both ice melting histories and mantle structure and viscosity. We use our new data and models to discuss Late Holocene (0–6 ka) relative sea-level changes in our study area and their implications in terms of modern relative sea-level estimates in the broader South and Southeast Asia region

Fossil Java Sea corals record Laurentide ice sheet disappearance

The Laurentide ice sheet was the largest late Pleistocene ice mass and the largest contributor to Holocene pre-industrial sea-level rise. While glaciological dates suggest final ice sheet melting between 8 and 6 ka, inversion of sea-level data indicates deglaciation at ca. 7 ka. Here, we present new chronostratigraphic constraints on Laurentide ice sheet disappearance based on Holocene relative sea-level observations from the tectonically stable north coast of Java, Indonesia. Age-elevation data from the flat upper surfaces of 13 fossil intertidal corals (i.e., microatolls) indicate that the Java Sea experienced a relative sea level of 1.3 ± 0.7 m above present between 6.9 and 5.3 ka. To determine uncaptured relative sea-level trends within the observational uncertainties of this apparently constant highstand, we analyzed the internal structure of three sliced microatolls from the same site to produce a high-resolution data set. These data were used to statistically model relative sea-level rates and trends. Employing the data with the model provided evidence for a short-lived rise of relative sea level from 1.0 ± 0.3 m above present at 6.7 ± 0.1 ka to 1.9 ± 0.3 m above present at 6.4 ± 0.1 ka. The end of this rise likely represents the last input of meltwater from the vast Laurentide ice sheet, which, consequently, collapsed at least 400 yr later than assumed by some widely used models of glacial isostatic adjustment. Incorporating these new results into such predictive models will help to better understand the geographical variability of future sea-level rise as a result of global warming

Seagrass meadows on tropical Indo-Pacific reef islands: How do water motion and water depth relate to seagrass species composition, and are seagrass communities really controlled by shrimp bioturbation?

The present study investigated the distribution pattern of six tropical seagrass species on two coral islands in the Spermonde Archipelago, Indonesia. Shoot density, leaf area index, biomass and rhizome length of Halophila ovalis, Halodule uninervis, Syringodium isoetifolium, Cymodocea rotundata, Thalassia hemprichii and Enhalus acoroides were related to water motion and water depth. At research sites with high water motion the seagrass sucession was frequently disrupted by blowouts and moving sandbars, and small seagrass species were dominant. In areas with moderate water motion bare sand patches produced by alpheid shrimp became more prominent, and all six seagrass species were sometimes found forming mixed stands. In sheltered areas mounds and funnels produced by callianassid shrimp were abundant and only the two species T. hemprichii and E. acoroides were present. Assemblages in intertidal areas were dominated by T. hemprichii and C. rotundata. The highest densities of callianassid shrimps were found in deep, sheltered, seagrass-free areas. Shrimp were excluded from experimental plots in one such area and all six seagrass species were transplanted into exclusion, procedural control and zero treatment plots. At the end of a two-year monitoring period, T. hemprichii and S isoetifolium had disappeared from all plots; however C. rotundata, H. uninervis and H. ovalis had successfully established themselves on some of the exclusion plots. There was a marked seasonality, with decreasing water quality during the Northeast monsoon seasons being followed by a decline in shoot numbers inside the plots. Also, seagrass shoots numbers where higher in plots located in shallower water

Dynamics of seagrasses in a heterogeneous tropical reef ecosystem

In tropical Southeast Asia seagrasses can be found in three types of habitats: "River estuaries" (including mud flats), "Shallow Coastal / Back Reef" (including reef flats and lagoons) and "Deep Water / Deep Coastal / Fore Reef". Especially those seagrass meadows growing on reef flats are characterized by high temporal and spatial dynamics. Research conducted in the Spermonde Archipelago, Southwest Sulawesi, Indonesia, revealed that water motion and water depth are important structuring agents which influence the species composition of mixed meadows. Furthermore, burrowing alpheid and callianassid shrimp fulfill important roles in the meadows by enhancing nutrient recycling and controlling the lower boundary of the meadows. It is concluded that both abiotic (water motion and depth) and biotic (e.g. shrimp) factors affect the capacity of top-reef seagrass meadows to process and sequester carbon. The potential impact of sea level rise and ocean acidification on these meadows is also discussed

The impact of climate change on coral reefs, and the mitigation potential of seagrasses

Due to human activities atmospheric carbon dioxide concentrations have increased from 280 to almost 400 parts per million since the industrial revolution. This has resulted in global warming and a sea surface temperature increase of approximately 0.7 degrees Celsius. About half of the excess carbon is now dissolved in the oceans, leading to a 30 % increase in ocean water acidity. Warming and acidification combined have potentially disastrous consequences for coral reefs if the current carbon dioxide emission trend continues. In tropical Southeast Asia several species of seagrasses can often be found growing together on reef flats where they form dense meadows. Seagrasses have the potential to mitigate the effects of global warming on coral reefs by taking up carbon and increasing seawater pH. They also accumulate organic matter with a high residence time in below-ground deposits, thereby taking carbon out of the global cycle for extended periods of time. However, most of what is known about tropical seagrasses stems from research conducted in the Caribbean or Australia. A series of experiments and studies is planned in collaboration between the Alfred-Wegener-Institute for Polar and Marine Research, Germany, and the Research and Development Center for Marine, Coastal and Small Islands (RDC MacSI) of Hasanuddin University Makassar, Indonesia. The studies will be conducted in the Spermonde Archipelago, Makassar Strait, Indonesia. Dissolved carbon dioxide concentrations will be elevated in in-situ mesocosms to study the reaction of tropical seagrasses to the concentrations expected in the year 2100. Sediment organic content will be determined in soil and sediment cores collected in a variety of seagrass habitats, ranging from “river estuary / mud flats” over “shallow coastal and back reef / coral reef” to “deep water / deep costal / fore reef”. The results will be compared to nearby mangrove and bare sand areas