Opening the South China Sea
We have drilled 2,600 feet below the sea floor and in another 500 feet, will reach the crystalline igneous basalt of the ocean crust. Though finding the age of the basalt is our main aim, the thick sediments that overly the crust also have a story to tell. As the sediments build up over time, they record the geological and climate history of the region.
There are the muds, silts, and sands, shaken loose from shallower depths and transported by gravity down-slope to the deep basin, where our first drill site is located. Ultimately, these sediments come from erosion of the surrounding land, and in this tectonically active part of the world, there is a lot of erosion going on. The island of Taiwan, for example, is being tectonically uplifted at a rate of about 0.2 inches per year, and is being eroded at about the same rate. This may not sound like much uplift, but imagine a world without erosion, Taiwan would stand 12 miles high after 4 million years. All that eroded rock ends up on the seabed, and some of it may find its way to our site.
There are the tiny shells of foraminifera and coccolithophores (familiar to us as chalk, in their pure rock form). They form a continual rain from the sea surface, and build up slowly but steadily on the seabed. The overturn of marker species shows us the age of the sediments, and their chemistry carries a record of ocean temperatures in the past.
Finally, there are volcanic sediments – from thin ash layers from distant volcanoes, to thick beds containing coarse chunks of rock exploded from nearby volcanoes. The close volcanoes are no longer active, and some have sunk beneath the sea to become seamounts. We will know from the depth of these beds in the sediment succession when the volcanoes erupted and for how long they were active.
This diversity means there is always something new and interesting to see in each 33-foot-long core that comes up from the sea bed, each another chapter in the geological history of the South China Sea. Among the 32 scientists on board, we have specialists in sedimentology, micropaleontology, volcanology and other fields. We are an international group; about half of us hail from China, a quarter from the U.S, and the rest from Australia, Brazil, France, Switzerland, Japan, Taiwan, and the Philippines (so there’s a good mix of music in the core laboratory – very nice). And that’s just the science party – the ship’s crew is almost as diverse.
Five days after leaving Hong Kong, the JOIDES Resolution is on site and drilling into the muds and silts of the South China Sea. The expedition’s main objectives are tectonic in nature, and I’m not really a tectonicist (I’m on board for the borehole logging), so for me this cruise is a crash course in the geological history of this area.
The origin of the ocean crust under the South China Sea is enigmatic, and there is ongoing scientific debate about which tectonic forces pulled apart the crust here to form the basin. In one hypothesis, the collision of India into Asia that built the Himalayas and pushed out Indochina to the southeast had the collateral effect of causing extension to form the South China Sea. The leading rival hypothesis says that the extension resulted from slab-pull from subduction at the southern edge of the basin (Borneo and Padawan). Of course, there are theories that mix the two, as well as minor-party candidates (plumes!).
The expedition aims to test the competing hypotheses by dating the earliest ocean crust (at the northern edge of the basin) and the youngest ocean crust (close to the now-inactive spreading center). If the age interval of sea floor spreading matches the age of the extrusion of Indochina (lets say 35 to 16 million years ago), then the Indochina extrusion hypothesis gains support; but if we find different ages, other hypotheses will move up the leader board. The debate and this expedition add to our understanding of the basic forces that shape the Earth’s surface.
Until now, the dating and interpretations rely on magnetic sea floor anomalies and other geophysical surveys. We will date the rocks directly for the first time, by argon-argon dating of the basalt that forms the ocean crust, and by the age of the sediments sitting on the basalt. The tricky part is that the basalt lies under 950 meters of sediments at the first site, and under 1850 meters at the second. To drill to this depth and bring back 100 meters of basalt is challenging to say the least, but there is a highly experienced drilling crew on board, so we are in with a shot. I’ll let you know how we get on!