Strontium, lead and zinc isotopes in marine cores as tracers of sedimentary provenance: A case study around Taiwan orogen

Abstract

The active collision zone around Taiwan orogen receives a variety of sedimentary inputs, including terrigenous flux from Taiwan and Chinese rivers, oceanic currents and atmospheric dust. In order to determine the present-day respective contributions of these different sources, we analysed the Pb and Sr isotopic compositions of late Quaternary sediments accumulated offshore east Taiwan. Samples from the Taiwan Coastal Range (andesites, sandstones, conglomerate and Peinan River terraces and suspended loads) have also been analysed to constrain the mean Taiwan isotopic signature. Mixing lines between the different sources of material indicate that the core located in the Okinawa Trough represents a mixture of about 60% of Taiwan, 30% of Chinese Loess and 10% of Yangtze River contributions. The southernmost core, located close to Luzon Arc, is influenced by Taiwan (60%) and volcanic material (40%). The Sr and Pb isotopic compositions of samples cored in the Manila Trench and in the Huatung Basin are strongly influenced by the Taiwan signature, while the Ryukyu Trench core samples point to significant but variable contributions of Chinese Loess and Yangtze River.

This work also reports Zn isotopic variations in the silicate fraction of these marine core samples. The overall variation in Zn isotopic compositions (expressed as δ⁶⁶Zn) is greater than 0.3 per mil (‰) for an analytical precision of ± 0.05‰. The Zn isotopic signal for most of the marine core samples is similar to the literature data measured on sedimentary material, except for the Manila Trench core samples and the older sample from the Huatung Basin: they present values heavier by 0.2–0.3‰, close to those obtained on all Taiwan rocks and Peinan River particulates. These results show: 1– the particular characteristics of Taiwan Zn isotopic compositions on a large scale; and: 2– the possible use of Zn isotopes as a tracer of material sources.

Introduction

Taiwan Island is located at the collision boundary between the Philippine Sea Plate and the Asian Continental Plate. The Philippine Sea Plate is advancing northwestward at a mean velocity of approximately 7 cm/yr (Seno et al., 1993, Lallemand et al., 1997), plunging down the Ryukyu Trench and leading to the Ryukyu Arc in the front and the Okinawa Trough at the back. The major part of Taiwan Island results from the strong convergence between the two plates, whose convergence boundary is along the Longitudinal Valley. The main structures are parallel to the strike of the suture in a NNE–SSW direction. East of the Longitudinal Valley, the Coastal Range is a manifestation on Taiwan of the Luzon Arc of the Philippine Sea plate. West of the suture, the main geological units are the Eastern Central Range (Tananao schist), the Western Central Range, the Western Foothills and the Coastal Plains. Crustal activities at this plate boundary are to some extent responsible for the high uplift and denudation rates of Taiwan, making the island an important source of sediments in this continental margin accretion wedge, as evidenced by high sedimentation rates in the Okinawa Trough (Lee, 2001).

The Okinawa Trough setting presents an active hydrodynamic flow due to the passage of the northward Kuroshio Current and its interaction with the highly rugged topography. After passing the Luzon Arc, this major western boundary current of the North Pacific Ocean flows in the NNE direction, with its axis lying very close to the east coast of Taiwan (Fig. 1a).

The volume transport and width of the Kuroshio Current increase on its path from northern Philippines to Taiwan (Nitani, 1972). At 22°N–25°N, the Kuroshio Current is about 300 m deep and 170 km wide, with a maximum velocity of 1 m/s and a volume transport between 15 and 25 Sv (Liang et al., 2003). As it flows to the northeast of Taiwan, it loses some of its energy and speed due to collision with the zonally trending East China Sea (ECS), which includes a broad continental shelf and the adjacent Okinawa Trough. Therefore, it is a major current, transporting large quantities of dissolved and particulate matter from low to high latitudes in the western Pacific. Chen (1998) reported that the upwelling and intrusion of the Kuroshio Current onto the continental shelf constitute the major source for nutrients in the ECS, driving biological productivity in this marginal sea. Surface-water circulation in the ECS is characterized by an interaction between the Kuroshio and the coastal currents, which are diluted by the Huanghe (i.e. Yellow) and Yangtze (i.e. Changjiang) Rivers draining the Chinese mainland (Katoh et al., 1996, Wang et al., 1988). The Yangtze and the Huanghe (Fig. 1a), the two largest rivers in China, deliver annually 0.5 and 1.08 billion tons of sediments to their mouths, respectively (Hay, 1998). These huge amounts of sediments, constituting about 10% of the world river sediment discharge, govern much of the sedimentary, aquatic, and ecological environments of western Pacific marginal seas such as the Bohai, the Yellow and East China Seas (Milliman et al., 1985, Alexander et al., 1991, Zhang, 1995, Zhang, 1999). The Yangtze delivers 478 Mt/yr of sediment onto the ECS (Milliman and Meade, 1983), of which a substantial fraction (~ 60%) is transported southward along the mainland Chinese coast by coastal currents (Milliman et al., 1985). Several studies have attempted to differentiate the origin of the present-day coastal plain sediments and nearby tidal sand ridges in the south-western Yellow Sea: the results vary widely between Yangtze (Yang, 1989, Zhu and An, 1993), Huanghe (Zhang and Chen, 1992, Yang et al., 2002), or both rivers (Li et al., 2001). Unfortunately, there are few studies using detailed comparison of Sr and Pb isotopic compositions between the Yangtze and the Huanghe particulates although this approach would be useful to better constrain the present-day relative contributions of these two major rivers to the Yellow Sea.

Because of the high exhumation rate and ample rainfall, Taiwan's rivers deliver large quantities of material to the Sea (e.g., 185 Mt/yr of sediments to the China Sea; Milliman and Meade, 1983). This kind of eroded material transported to the east off Taiwan could be carried northward by the Kuroshio Current. The large supply of terrestrial sediments from Taiwan and China thus appears to be largely sufficient to account for the accumulation of material in the high-sedimentation rate part of the Okinawa Trough, which is less than 10 Mt/yr (Hsu, 1998). Nevertheless, the sources of the south Okinawa Trough sediments have been debated for over two decades. Earlier efforts to identify the sources (e.g., the Yangtze, Huanghe, Chinese Loess, and/or Taiwan's rivers) and transport pathways of the Okinawa Trough sediments were based on chemical characteristics, mineralogy and physical properties of the sediments (Lin and Chen, 1983, Chen et al., 1992, Li, 1994, Chung and Chang, 1995, Chung and Chang, 1996).

In the first part of this study, we intend to: 1– characterize the Sr and Pb isotopic variability of the detrital fraction of the present-day marine sediments accumulated off eastern Taiwan; and: 2– use these geographical isotopic variations to identify and quantify the sedimentary contributions of Taiwan orogen versus those by the oceanic currents (i.e. Kuroshio), the suspended loads of large rivers (i.e. Huanghe and/or Yangtze) and the atmospheric dusts (i.e. Chinese Loess).

Radiogenic isotopes are well known as powerful tracers. Variations in the ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb ratios in continental rocks and riverine suspended loads are mirrored in the isotopic compositions of adjacent marine sediments (e.g., Biscaye, 1974, Grousset et al., 1988, France-Lanord et al., 1990, Nakai et al., 1993, Asahara et al., 1995, Allègre et al., 1996): whole rock Pb isotope analyses have been applied by several authors to sedimentary provenance studies, and it appears that the Pb isotopic signature of the source is commonly preserved during sedimentation (McDaniel et al., 1994, Hemming et al., 1995). However, the abundant seawater Sr precipitated in the marine carbonate greatly affects the ⁸⁷Sr/⁸⁶Sr ratio of the whole sediment, and the provenance of the silicate component in sediment cannot be deduced by using the isotopic ratio of the whole sediment. Therefore, for the marine sediments, we report and discuss only the Sr and Pb isotopic compositions of carbonate-free residues (i.e. silicate fractions).

Some authors show the existence of Zn isotopic variations of up to several per mil among natural samples of silicates and biological materials, which offers a potential geochemical and biochemical tracer (Maréchal, 1998, Maréchal et al., 1999, Ben Othman et al., 2001, Pichat et al., 2003, Bermin et al., 2006, Gélabert et al., 2006). Thus, the second goal of this study is to address the following questions: 1– what are the Zn isotopic compositions of marine sediment silicate fractions accumulated off eastern Taiwan?; 2– is there any variation between the different samples?; and: 3– is it possible to use this Zn isotope variability as a tracer of material?