Bioturbational structures record environmental changes in the upwelling area off Vietnam (South China Sea) for the last 150,000 years

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Abstract

The sediments in the upwelling area off central Vietnam are totally bioturbated and display a low-diverse assemblage of bioturbational structures. During interglacial times (Marine Isotope Stage MIS 1, 5a, 5c, 5e), summer monsoon leads to pronounced upwelling and seasonally pulsed arrival of organic matter on the seafloor. These deposits are characterized by a 4-tier bioturbated zone with Zoophycos. Zoophycos producers used this seasonal food source and show a cache behavioral strategy. During glacial periods upwelling was weak and oxygenation of bottom water decreased as evidenced by bioturbational structures that show a decrease in size, penetration depth and diversity. Enlarged freshwater influx during glacial times fuelled primary production and led to estuarine circulation. The resultant oxygen minimum layer favored an increased deposition of organic matter that in turn affected sediment properties. During MIS 2 and 5b oxygenation was probably a little lower than today. During MIS 3 and 5d Chondrites-like burrows point to a stronger oxygen deficiency. During MIS 4 bottom water oxygenation was even lower and only biodeformational structures occur in soft sediment rich in benthic food. Based on these findings an ichnofabric model for food-rich, fairly soft deposits experiencing oxygen deficiency is suggested having biodeformational structures as low-oxygen end member.

Highlights

► Ichnofabrics record fluctuating oxygenation off central Vietnam for last 150 ky. ► Pronounced oxygen minimum layer developed off central Vietnam during glacial times. ► De-oxygenation of bottom water is recorded by a succession of ichnofabrics. ► Biodeformational structures represent end member for dysaerobic conditions. ► Zoophycos is produced only during times of seasonal organic matter deposition.

Introduction

Biogenic sedimentary structures are autochthonous indicators of environmental conditions and are advantageous for paleoecological and sedimentological analyses. Trace fossils have been used for ecological studies for a long time (e.g., Abel, 1935, Ekdale et al., 1984, Pemberton et al., 2001). A biotope can be characterized often better by its bioturbation structures than by other paleontologic constituents, especially in the deep sea (e.g., Leszczyński, 1991, Wetzel, 1991, Wetzel and Uchman, 1998, Uchman and Wetzel, 2011). By studying modern analogs, the significance of marine trace fossils, ichnofabrics, and ichnofacies as ecologic proxies and environmental indicators can be improved.

The ecologic interpretation of trace-fossil communities (ichnocoenoses) is today mainly based on three concepts. Ichnofacies refers to recurrent trace-fossil associations that characterize the environmental setting in a general sense (Seilacher, 1967). Index trace fossils indicate a specific environmental situation, for instance, some types of Chondrites point to lowered oxygenation (e.g., Bromley and Ekdale, 1984). Ichnofabrics result from bioturbation at all scales and they include all aspects of the texture and internal structures of marine sediments (Bromley and Ekdale, 1986).

The various endobenthic organisms normally occupy different depth levels within the seafloor while sediment consistency, food availability, oxygenation and other factors change with depth in sediment; the subdivision of the available ecospace results in a tiering of the burrows (e.g., Wetzel, 1981, Wetzel, 1984, Wetzel, 1991, Wetzel, 2010, Ausich and Bottjer, 1982, Ekdale et al., 1984, Bromley, 1996). Crosscutting relationships provide information about tiered bioturbation or a succession of burrows in ichnofabrics.

The deep South China Sea is an appropriate area to study organism–sediment interactions because the environmental factors and their variation are well known; in particular, bathyal sediments provide a long-term continuous record of environmental changes at a reasonable resolution in time (e.g., Sarnthein and Wang, 1999, Clemens et al., 2003). However, only a few studies address aspects of bioturbation in the deep South China Sea (Wetzel, 2002, Wetzel, 2008, Wetzel, 2009, Löwemark et al., 2004a, Löwemark et al., 2006). The latter author focused on the long-term record of the trace fossil Zoophycos, the former analyzed the modern situation after deposition of the 1991 Mount Pinatubo ash layer (Wiesner et al., 2004). However, a study addressing general aspects of a long-term bioturbational record is missing.

In the semi-enclosed marginal basin of the South China Sea the change of the environmental conditions from glacial to interglacial times offers the chance to study the response of the burrowing organisms: Do the ichnofabrics differ from bathyal sediments not affected by upwelling? How do the ichnofabrics off Vietnam compare with those found in other upwelling areas, for instance, off NW Africa? How do the ichnofabrics record the effects of a pronounced oxygen minimum during glacial times when enhanced freshwater influx led to estuarine conditions? Is the Zoophycos producer a good indicator for seasonal organic matter deposition in slowly accumulating settings as in the northern South China Sea, where it is only present in glacial deposits characterized by a strongly seasonal, upwelling-related deposition of organic matter (Löwemark et al., 2006)?

Section snippets

Regional setting

The South China Sea is a western marginal sea of the Pacific Ocean, surrounded by the Southeast Asian mainland in the north and west and the islands of Borneo, Palawan, Luzon and Taiwan to the south and the east (Fig. 1). The South China Sea has large shelf regions, such as the Sunda Shelf and the Gulf of Tonkin, and deep basins as between the Philippines and Vietnam.

The most prominent connection between the South China Sea and the Pacific Ocean is the Bashi Channel, between Taiwan and Luzon

Material and methods

Cores were collected during cruise 187 of the German research vessel Sonne in 2006 to the South China Sea (Wiesner et al., 2006). The present study is based on 7 cores, comprising one long 12-cm diameter gravity core and 6 short, 50 × 50 cm2 box cores (Fig. 2; Table 1). Station 187-61 was found to be representative for all these cores and hence, only this site is discussed in detail. Additional information was provided by already published core data (Fig. 2; Table 1) and a core from the northern

Biogenic sedimentary structures

The biogenic sedimentary structures in the studied deposits differ in fill (composition, grain size, color), geometry, and boundary to the surrounding sediment; two general types are distinguished. Biodeformational structures have no distinct outlines or recognizable geometry, which would allow their classification (Schäfer, 1956). They ‘mottle’ pre-existing sedimentary structures. Trace fossils exhibit sharp outlines and possess a characteristic geometry that allows classification in terms of

Sediment data

Ichnological and core data were related to a stratigraphic time frame by using the lg(Ti/Ca) of the XRF core scanning measurements. Off Vietnam, the element Calcium (Ca) reflects predominantly the marine carbonates, whereas the conservative element Ti reflects the lithogenic sediment fraction that resides in Ti-oxides and is enriched in tropical soils (Tjallingii et al., 2010). Here, the lithogenic fraction is defined by the sediment fraction free of CaCO3, organic matter and opal (lithogenics = 

Interpretation

The Zoophycos producers were active during interglacial or relatively warmer periods when strong summer monsoon enhanced upwelling and led to seasonally pulsed arrival of organic matter on the seafloor. The Zoophycos producers fed on this seasonal food source and stored some material deep below sediment surface as evidenced by the recently produced spreiten (see above). Consequently, a cache behavioral strategy of the producer is inferred (e.g., Bromley, 1991, Löwemark et al., 2006; cf. Shields

Discussion

The Zoophycos producers were found to respond to seasonally pulsed arrival of organic matter on the seafloor during times of slow sedimentation in the northern South China Sea (Löwemark et al., 2006). There, such conditions resulted from winter monsoon which intensified during glacial periods. In contrast, off central Vietnam Zoophycos was produced during interglacial periods when, once again, upwelling and hence seasonally pulsed organic matter deposition became enhanced. Therefore, Zoophycos

Conclusions

Today in the upwelling area off central Vietnam organic-rich sediments accumulate and the bottom water is moderately oxygenated. However, compared to other upwelling affected areas, such as the northern South China Sea during glacial times or the area off NW Africa, the burrows off central Vietnam are smaller, penetrate less deep and the deeper ones coin the fabric less intense. All these observations imply an impact of restricted oxygen availability for the burrowing fauna in an environment

Acknowledgments

This study would not have been possible without the support of various persons and institutions. Captain and crew of RV Sonne Cruise 187 are thanked for their assistance in retrieving the cores. All participants on RV Sonne Cruise 187 provided enthusiasm and an encouraging atmosphere. D. Unverricht (Kiel, Germany) helped with the X-ray radiographs of core Sonne 187-61 and W. Rehder (Kiel, Germany) provided prints of X-ray radiographs of cores taken during Sonne Cruise 95 to the South China Sea.

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