Improving the age control of eastern Borneo's Miocene sedimentary record

Abstract

The Indonesia Archipelago is a complex region that both harbors the today’s most biodiverse marine environments and plays an important role in climate and ocean regulation. Additionally, the area contains a large human population and noteworthy fossil fuel industries. However, there is a dearth of knowledge about the region’s natural history. The vast majority of fossil specimens available today were collected by British and Dutch explorers in the 19th and very early 20th centuries, which are now housed in the Natural History Museum of London and Naturalis Museum of Leiden. These specimens lack modern taxonomic and environmental context that is important for judging what caused the flourishing of biodiversity and how climate events of the past influenced the biosphere. Traditionally, it was thought that sea-level fluctuations (1 Ma to 11 Ka) of the last ice ages led to the high level of species in the Indonesian area. However, recent studies show evidence that the current biodiversity hotspot was forming during the Miocene (23-5 Ma). Borneo is at the heart of the hotspot and is furthermore situated centrally within the Indonesian Throughflow (ITF), which acts as a complex ‘faucet’ controlling the mixing of the Pacific and Indian Oceans. Today, the ITF is a vital modulator for several globally important climate/ocean features including the Asian Monsoons and El Niño Southern Oscillation. Hence, Borneo is pivotal to understanding many mysterious about the regional and global biosphere and climate operations. Because of this, a consortium of European research institutions, funded by the EU, formed the “Throughflow project” which aimed to better understand this region’s geological and biological history and how past events alterted the coarse of life on Earth. But, what good is a fantastic story without an element of timing? As part of the Throughflow project, my PhD undertook a collaborative role in adding age control and environmental context to the largely paleobiological data being collected by my fellow researchers. My research focuses on eastern Borneo during the Miocene. My PhD covered an array of studies including: paleoenvironmental reconstruction, magnetostratigraphy, strontium isotope stratigraphy, cyclostratigraphy, and paleomagnetic rotational studies. The improved dating provided by my research has helped tie down the arrival of the current biodiversity hotspot in the region to at least the middle Miocene. Furthermore, by better dating the growth of the Mahakam River Delta, it is now better realized that biodiverse reefs flourished in very dynamic tectonic and muddy environments, not typical (pehaps even antithetical) for today. My research shows that cyclic sedimentary alternations in the region match Earth’s orbital oscillations which is perhaps one of the best examples globally of orbital forces manifested in an equatorial and riverine sedimentary record. It also appears that these oscilations played an important role in allowing biodiverse reefs to form or be killed. Also, a study of the microscopic magnetic minerals in rocks from eastern Borneo indicate that the island probably did not rotate drastically since at least around 40 million years ago, which is contrary to current tectonic models for the region.