An improved method for extracting marine sediment fractions and its application to Sr and Nd isotopic analysis
Introduction
The decay systematics of Sm–Nd and Rb–Sr isotope systems have been applied to marine sediments since the late 1960s, establishing radiogenic isotope geochemistry as an important tool in this field (see, e.g. Dasch, 1969, Faure, 1986). To date, investigations into paleoceanography have fallen along two distinct but complementary paths. In the first of these, isotopic analysis of the ‘continental’ detrital fraction of a sediment (i.e., that clay-rich fraction which contains the minerals weathered from the continents and delivered to the oceans by rivers, aeolian transport and/or advection by deep currents) provides information on both the geographical provenance of that material and its transport mechanism Grousset et al., 1988, Grousset et al., 1998, Nakai et al., 1993, Jones et al., 1994, Revel et al., 1996, Innocent et al., 1997, Parra et al., 1997, Hemming et al., 1998, Asahara et al., 1999. This approach is based on the assumption that detrital phases retain the isotopic signature of their source rocks throughout all of continental weathering, sediment transport and diagenesis. A particularly good example of the utility of this approach has been the study of aeolian dust from Antarctic ice cores: when compared to potential Antarctic, Australian, southern African and South American sources, Sr and Nd isotope ratios show, clearly, that this dust has a Patagonian provenance Grousset et al., 1992, Basile et al., 1997. Similarly, the high-resolution study of marine sediment cores may be used to deduce sediment input variations and, therefore, to constrain fluctuations in (e.g.) wind-regimes and deep-ocean circulation. Recent studies, using Sr isotopic analysis of southeast Atlantic sediment cores, have highlighted past leakage of the Algulhas Current (southwest Indian Ocean) into the Atlantic Ocean, demonstrating how powerful this approach can be Rutberg, 2000, Goldstein et al., submitted.
The second area in which isotopes are used in paleoceanography relies upon the analysis of the authigenic components (i.e., those minerals which are precipitated directly from seawater) in marine sediments. When an authigenic mineral precipitates, it is assumed to incorporate any Nd and Sr with an isotopic composition identical to that of the host water mass. Because the residence time of Nd in seawater is short (∼200–1000 years, e.g. Elderfield, 1988, Tachikawa et al., 1999) compared to the oceanic mixing time (∼1500 years), Nd is not isotopically homogenized throughout the deep ocean and each water mass possesses its own unique isotopic signature (e.g., Piepgras et al., 1979, Jeandel, 1993). The same is not the case for Sr, which has a residence time of several million years and, therefore, exhibits a constant isotopic composition in the present-day open ocean (87Sr/86Sr=0.7091–0.7092, e.g. Palmer and Elderfield, 1985a). In recent years, many advances in paleoceanography have been achieved using the Nd isotopic composition of Fe–Mn nodules and encrustations (e.g., Abouchami et al., 1997, Burton et al., 1997, Burton et al., 1999, Christensen et al., 1997, Ling et al., 1997, Frank and O'Nions, 1998, O'Nions et al., 1998, Reynolds et al., 1999, von Blackenburg, 1999, Claude-Ivanaj et al., 2001). Burton et al. (1997), for example, suggested that the closure of the Central American Isthmus, some 4 Ma ago, cut off the Atlantic Ocean from the input of Pacific water and, consequently, shifted Atlantic deep-water composition toward less radiogenic Nd isotope values. However, because crusts and nodules only grow at the rate of a few mm/Ma, at most, they can only provide information on globally integrated changes that have affected deep ocean circulation over the order of millions of years. In order to resolve fluctuations over shorter-term periods, e.g. those relevant to Quaternary climate change, it is necessary to study authigenic minerals that are deposited at higher resolution within the continuous records of marine sediment cores. In one novel approach, Burton and Vance (2000) have used cleaned planktonic foraminifera. A second alternative, recently proposed by Rutberg et al. (2000), involves the analysis of the Nd isotopic composition of the dispersed Fe–Mn oxide fraction within the marine sediment record. The latter approach led to the successful demonstration, where other proxies had failed, that the influence of NADW in the South Atlantic Ocean was considerably reduced during the last glacial maximum (LGM), some 20,000 years ago.
Here, we present a sequential leaching procedure which produces the separation of both a detrital fraction and a Fe–Mn oxide fraction, from the same marine sediment sample, which are both suitable for Nd and Sr isotopic analysis. For any given sample, this procedure allows one to make use of two independent paleoceanographic proxies: (1) the isotopic composition of the detrital fraction as a tracer of the provenance and flow trajectory of local bottom-water at the time of deposition and, (2) the isotopic composition of the Fe–Mn component as a direct record of the contemporaneous composition of deep-water.
Section snippets
Classification of marine sediment fractions
Following the classification of Tessier et al. (1979), trace elements in marine sediments can be partitioned into five distinct fractions. Elements derived from seawater can be: (1) adsorbed onto mineral surfaces (exchangeable fraction), (2) associated with carbonates, or (3) scavenged by iron and manganese oxides. The remaining trace metals in any sediment sample can be considered either (4) to be bound to organic compounds, or (5) to belong to the detrital fraction, also called the detritus.
Analytical methods
A range of different sample treatments and preparations are discussed later in the paper but, ultimately, all final ‘mother’ solutions were analysed using a common mass spectrometry methodology which we describe below. Sr was isolated from mother solutions using Sr resin (Sr spec, Eichrom Industries, IL, USA). For Nd isotopic analysis, the REE were initially separated from major elements by cation exchange, before isolation of Nd on Teflon powder columns coated with HDEHP. Sr and Nd isotope
Experimental procedures
All experimental operations conducted during the development of our final procedure have been performed upon replicates of a single freeze-dried sample from an Angola Basin sediment core (core MD96-2091, 14°53.42′S, 10°23.33′E, 3566-m depth). Since collection, the core had been stored wet in a purpose-built refrigerated core store. This giant piston Calypso core (length: 24.5 m) was collected in 1996 during the NAUSICAA-IMAGES II cruise of the research vessel ‘Marion-Dufresne’ (Chief Scientist:
Results and discussion
Results from our experimental procedures are listed in Table 1. Throughout the discussion which follows (also in Fig. 2, Fig. 3), we present the means of the replicates listed in Table 2, for brevity.
Synthesis—recommended analytical procedure
Our final procedure is schematized in Fig. 4. From the results of the experimental analysis described above, the following procedure has been adopted. Between 300 and 1000 mg of freeze-dried sediment are used, according to biogenic content, to yield a predicted detrital fraction (after carbonate and Fe–Mn ‘oxide’ removal) of 150–200 mg. For sediments with ∼10–40 wt.% carbonates, the carbonate fraction and any exchangeable fraction are first removed by placing the pre-weighed, ground bulk
Overview
In order to evaluate the chemical procedure developed, we have studied samples from a sediment core situated in the Cape Basin, southeast Atlantic Ocean. Core MD96-2086 was recovered during NAUSICAA-IMAGES II cruise at 25°48.8′S, 12°7.7′E; 3606-m water depth. The core is situated at the present-day boundary between North Atlantic Deep Water (NADW, ∼1200–3600-m water depth), and underlying Lower Circumpolar Deep Water (LCDW, below 3600 m), which fills the deepest parts of the Cape Basin. The
Conclusions
We have developed a sequential leaching procedure which allows the separation of both detrital and Fe–Mn fractions from marine sediments for Nd and Sr isotopic analysis. A series of tests has been performed on a representative marine sediment sample to assess the effects of each chemical treatment on the isotopic composition of the resultant residues. Results show that extraction of both carbonates and Fe–Mn oxyhydroxides has a significant impact on the 87Sr/86Sr and 143Nd/144Nd composition of
Acknowledgements
We would like to thank Philippe Bertrand, chief scientist of the NAUSICAA-IMAGES II cruise, and Jacques Giraudeau, from the University of Bordeaux, who generously provided us with the samples for this study. Catherine Pierre (University of Jussieu—Paris VI) is thanked for providing the benthic δ18O stratigraphy. We thank Darryl Green for help and assistance during the lab work and Valerie Chavagnac for discussions during preparation of the manuscript. We also thank Sidney Hemming and Friedhelm
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