High resolution color reflectance and hematite and goethite record of ODP Hole 184-1143A in the South China Sea

Precipitation has a larger variability than temperature in tropical monsoon regions, thus it is an important climate variable. However, reconstructions of long-term rainfall histories are scarce because of the lack of reliable proxies. Here we document that iron oxide minerals, specifically the ratio of hematite to goethite (Hm/Gt), is a reasonable precipitation proxy. Using diffuse reflectance spectrophotometry, we measured samples from Ocean Drilling Program (ODP) 1143 drilling site (9°21.72'N, 113°17.11'E, 2777 m water depth) for hematite and goethite, whose formation processes are favored by opposing climate conditions. In order to determine the content of hematite and goethite we produced a set of calibration samples by removing the iron oxides to generate the natural matrix to which hematite and goethite in known percentages were added. From these calibration samples we developed a transfer function for determining hematite and goethite concentration from a sample's spectral reflectance. Applying this method to ODP 1143 sediments (top 34 m of a 510 m core with sampling interval of 10 cm) we were able to reconstruct a continuous precipitation history for SE Asia of the past 600 kyr using the Hm/Gt ratio as a proxy of the precipitation variability of Asian monsoon. The reliability of this Hm/Gt proxy is corroborated by its consistency with the stalagmite delta18O data from South China. Comparing long-term Hm/Gt records with the surface temperature gradient of equatorial Pacific Ocean, we found that monsoon precipitation and El Niño are correlated for the last 600 kyr. The development of El Niño-like conditions decreased SE Asia precipitation, whereas precipitation increases in response to La Niña intensification

Depth and ages of calcareous nannofossil events from ODP Leg 184 sites, South China Sea

Sites 1146 and 1148 of Ocean Drilling Program Leg 184, in the South China Sea (SCS), comprise long sediment sections with a time span from the early Oligocene to the Pleistocene. Calcareous nannofossils from these two sites were biostratigraphically studied. We recognized 53 early Oligocene to Pleistocene events that are commonly found in open sea areas and can therefore be correlated within a large geographic range. This study also revealed that a few conventionally used nannofossil events are not suitable for the SCS, and further evaluation is needed. The lower Oligocene to Pleistocene sequences recovered at Sites 1146 and 1148 were subdivided into the 4 Paleogene zones and 21 Neogene to Quaternary zones of Martini, in correlation with the Paleogene to Quaternary zones of Okada and Bukry. This provided a lower Oligocene through Pleistocene nannofossil biostratigraphic framework. A significant unconformity was recognized in the Oligocene-Miocene transition, in which the upper part of Oligocene Zone NP25 and lower part of Miocene Zone NN1 were missing. The time span of the unconformity was estimated to be ~1 m.y. Very high sedimentation rates were seen in the Oligocene, relative low values were seen in the Miocene, and the highest values were seen in the Pleistocene, which was believed to be the result of tectonic and sedimentation history of the SCS.