Abstract
We report on rare earth element and neodymium isotopic compositions in a series of grain-size fractions separated from river suspended matter in the Murray-Darling Basin (MDB) and a nearby marine sediment core (MD03-2607) offshore south-eastern Australia. This source-to-sink approach was used to further investigate the extent to which sediment sorting may control the geochemistry of suspended loads in rivers, and to provide constraints on the source of the detrital sediment material exported to the ocean. Our results provide further compelling evidence that significant size-dependent geochemical decoupling can occur in river systems, accounting here for Nd isotopic (Nd) differences of up to 8 epsilon-units between silt (>25µm) and colloidal (0.2-0.006 µm; 0.006-0.003 µm) fractions. All suspended particulate samples from the River Murray watershed display a trend towards more radiogenic (higher Nd) Nd signatures with decreasing grain-size, in addition to differing REE signatures, which collectively point towards a preferential volcanogenic origin for the fine-grained inorganic particles transported by MDB rivers. Furthermore, we show that the same river-borne volcanogenic material dominates in the fine-grained detrital fractions extracted from core MD03-2607 at the south-eastern Australian margin; a finding corroborated by REE signatures in a series of copepod faecal pellet separates from the same core. Collectively, our results suggest that river sediment discharge is accompanied by preferential export of fine-grained volcanogenic particles to the ocean. This potential source of bioavailable trace metals and nutrients in ocean surface waters could impact marine productivity.
Highlights
On continents, hydrological and transport processes can result in significant grain size sorting in river systems, which controls both the geochemical and mineralogical composition of suspended particulates and bedloads (e.g., Garzanti et al, 2009, 2011; Bouchez et al, 2011)
Except for the finest colloids (0.006−0.003 μm), all size-fractions separated from the Darling River sample display similar World River Average Silt (WRAS)-normalized Rare earth elements (REE) patterns, characterized by light and heavy REE (LREE and HREE, respectively) depletion relative to mid REE (MREE)
By analogy with results obtained on our series of river suspended particulates, we propose that all above-mentioned geochemical characteristics mainly reflect the presence of abundant volcanogenic particles in the fine-grained detrital fractions extracted from core MD03-2607
Summary
Hydrological and transport processes can result in significant grain size sorting in river systems, which controls both the geochemical and mineralogical composition of suspended particulates and bedloads (e.g., Garzanti et al, 2009, 2011; Bouchez et al, 2011). Rare earth element abundances for the same set of river suspended particulate samples investigated during this study were partially published and briefly discussed in previous studies (Douglas, 1993; Douglas et al, 1995; Douglas et al, 1999) that examined changes in strontium (Sr) isotope signatures as a function of grain size All three samples from the Murray River watershed display similar size-specific REE patterns, with: (1) silt-size fractions (>25 μm; 25−1 μm) being characterized by pronounced HREE depletion; (2) clay-size fractions (1−0.2 μm) displaying relatively flat WRAS-normalized patterns; and (3) colloidal-size fractions (0.2−0.006 μm; 0.006−0.003 μm) being generally characterized by much lower REE abundances, the presence of both positive and negative Eu- and Ce- anomalies, respectively, and a LREE depletion relative to MREE and HREE (Figure 2; Table 2)
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