Continental bedrock and riverine fluxes of strontium and neodymium isotopes to the oceans

Abstract

[1] Realistic models of past climate and ocean chemistry depend on reconstructions of the Earth's surface environments in the geologic past. Among the critical parameters is the geologic makeup of continental drainage. Here we show, for the present, that the isotope composition of dissolved strontium in rivers increases linearly with the age of bedrock in drainage basins, with the notable exception of the drainage area of Arabia, India, and Southeast Asia that is affected by unusually radiogenic dissolved Sr from the Himalaya. We also demonstrate that the neodymium isotope compositions of suspended matter in rivers as well as clastic sediments deposited along the ocean margins decrease linearly with the bedrock ages of river drainage basins and large-scale continental drainage regions, as determined from digital geologic maps. These correlations are used to calculate the present-day input of dissolved Sr (4.7 × 10 10 mol yr −1 , 87 Sr/ 86 Sr of ∼0.7111) and particulate Nd isotopes (" Nd of approximately −7.3 ± 2.2) to the oceans. The fact that the regionally averaged " Nd of the global detrital input to the global coastal ocean is identical to globally averaged seawater (" Nd of −7.2 ± 0.5) lends credence to the importance of "boundary exchange" for the Nd isotope composition of water masses. Regional biases in source areas of detrital matter and runoff are reflected by the observation that the average age of global bedrock, weighted according to the riverine suspended sediment flux, is significantly younger (∼336 Myr) than the age of global bedrock weighted according to water discharge (394 Myr), which is younger than the average bedrock age of the nonglaciated, exorheic portions of the continents (453 Myr). The observation that the bedrock age weighted according to Sr flux is younger (339 Myr) than that weighted according to water flux reflects the disproportionate contribution from young sedimentary and volcanic rocks to the dissolved Sr load. Neither the isotope composition of the dissolved nor the particulate continental inputs to the ocean provide unbiased perspectives of the lithologic makeup of the Earth's surface. Temporal changes in bedrock geology as well as the shifting focal points of physical erosion and water discharge will undoubtedly have exerted strong controls on temporal and spatial changes in the isotope chemistry of past global runoff and thus seawater.

Reactivity of neodymium carriers in deep sea sediments: Implications for boundary exchange and paleoceanography

“…The neodymium (Nd) isotopic composition of seawater is spatially heterogeneous, reflecting isotopically distinct weathering inputs from the surrounding continents (Goldstein and Jacobsen, 1987;Peucker-Ehrenbrink et al, 2010). Nd isotopes are also described as a quasi-conservative tracer for inter-and intra-basin water mass mixing (Piepgras et al, 1979;Goldstein and Hemming, 2003) because the oceanic residence time of Nd ($200-1000 yrs; Tachikawa et al, 1999Tachikawa et al, , 2003Siddall et al, 2008;Rempfer et al, 2011) is shorter than the mixing time of the deep ocean ($1500 yrs; Broecker and Peng, 1982).…”

“…However, our understanding of the controls on the dissolved e Nd distribution in the deep ocean is incomplete, leading to a degree of uncertainty in such interpretations. Weathering and erosion of the continents represent the ultimate source of dissolved Nd to the oceans (Peucker-Ehrenbrink et al, 2010), but the relative importance of dissolved versus particulate inputs remains uncertain . In particular, there is emerging evidence that exchange between particulate and dissolved fractions, particularly along ocean margins, may exert an important control on the deep ocean Nd budget and e Nd distribution; a process (or multitude of processes) that has been termed 'boundary exchange' (Jeandel et al, 1995;Tachikawa et al, 1999;Jeandel, 2001, 2005).…”

Climate induced temporal change in Sr-Nd isotope ratios in the valley-fill deposits of the Ganga river

“…The 87 Sr/ 86 Sr of the major global rivers water ranges from 0.7089 to 0.7291 in which the Ganga river water is characterized by the highest Sr isotopic ratio (0.7291) whereas the Danube river water have lowest Sr isotope ratio (0.7089) (Goldstein and Jacobsen, 1987;Krishnaswami et al, 1992;Palmer and Edmond, 1992;Pande et al, 1994;Trivedi et al, 1995;Yang et al, 1996;Gaillardet et al, 1999;Millot et al, 2003;Tipper et al, 2006;Moon et al, 2007Moon et al, , 2009Pattanaik et al, 2007;Xu and Marcantonio, 2007;Peucker-Ehrenbrink et al, 2010;Noh et al, 2009). The rivers like Amazon, Orinoco etc.…”

Clay mineralogy, strontium and neodymium isotope ratios in the sediments of two High Arctic catchments (Svalbard)

“…The radiogenic isotope tracers 143 Nd / 144 Nd (expressed as εNd) and 87 Sr / 86 Sr are often used together to understand where and how sediment is generated and weathered, enabling source regions to be characterised (Goldstein and Jacobsen, 1988;Cameron and Hattori, 1997;Tricca et al, 1999;Peucker-Ehrenbrink et al, 2010;Lupker et al, 2013;Clinger et al, 2016). εNd is particularly suited to being a source tracer because, unlike Sr and Rb which are fluid mobile, Sm and Nd are immobile and behave very similarly during chemical weathering such that the Sm / Nd ratio does not fractionate during weathering (McCulloch and Wasserburg, 1978) and therefore variations in εNd are predominantly controlled by age (Goldstein and Jacobsen, 1988) rather than the weathering of specific minerals which can affect the Rb-Sr system (e.g.…”