Barium stable isotopes as a fingerprint of biological cycling in the Amazon River Basin

Abstract

<jats:p>Abstract. Although biological cycling of rock-derived nutrients is a major operator of element cycles at the Earth surface, its magnitude still remains elusive. The isotope composition of rock-derived nutrients, which can be fractionated during biological uptake, is a powerful tool to quantify biological cycling. In this paper we use the elemental and isotopic composition of such a rock-derived nutrient, the trace element barium (Ba), measured in river dissolved and sediment load samples collected across the Amazon Basin. We show that dissolved Ba derives mainly from silicate rocks, while a correlation between dissolved Ba and K abundances suggests a strong role of biological cycling on the Ba river budget. The isotope composition of Ba (δ138Ba) of the dissolved load is significantly different from that of silicate rocks and is affected by i) formation of secondary phases and ii) biological uptake and release from dead organic matter. Results from an isotope mass balance model applied to the river dissolved load data indicate that after its release to solution by rock weathering, Ba is partitioned between the dissolved load, the secondary weathering products such as those found in soils and river sediments, and the biota. In most sub-catchments of the Amazon, river dissolved Ba abundance and isotope composition are significantly affected by biological cycling. Relationships between estimates of Ba cycling and independent metrics of ecosystem dynamics (such as Gross Primary Production and Terrestrial Ecosystem Respiration) allows us to discuss the role of erosion rates on the cycling of rock-derived nutrients. In addition, river catchment-scale mass and isotope budgets of Ba show that the measured riverine export of Ba is lower than the estimated delivery of Ba to the Earth surface through rock alteration. This indicates the existence of a missing Ba component, that we attribute to the formation of a Ba-bearing particulate organic component, possibly accumulating as soil organic matter or currently growing biomass within the catchments; and to organic-bound exported as unsampled river particulate organic matter. Given our findings on the minor nutrient Ba, we explore whether the river fluxes of most major rock-derived nutrients (K, Mg, Ca) might also be significantly affected by biological uptake or release. A first-order correction of river-derived silicate weathering fluxes from biological cycling shows that, at the Amazon at mouth, the CO2 consumption by silicate weathering should be 20 % higher than the yet-reported value. Overall, our study clearly shows that the chemical and isotope composition of the Amazon (and most likely of most rivers) bears a biological imprint. </jats:p>

Otolith Sr/Ca ratio complements Sr isotopes to reveal fish migration in large basins with heterogeneous geochemical landscapes

“…Although little is known about the controls on Ba abundance in freshwater, lithology is expected to play a control over water Ba/Ca ratio. Unlike Sr, river dissolved Ba is mostly derived from silicate rocks (Charbonnier et al 2020;Gou et al 2020). In addition, Ba is much more sensitive than Sr to secondary processes including the formation of clays and oxides (Dalai et al 2002;Das and Krishnaswami 2006;Gong et al 2019) or biological uptake (Bullen and Chadwick 2016;Charbonnier et al 2020).…”

“…Unlike Sr, river dissolved Ba is mostly derived from silicate rocks (Charbonnier et al 2020;Gou et al 2020). In addition, Ba is much more sensitive than Sr to secondary processes including the formation of clays and oxides (Dalai et al 2002;Das and Krishnaswami 2006;Gong et al 2019) or biological uptake (Bullen and Chadwick 2016;Charbonnier et al 2020). Although it is beyond the scope of this work to clearly identify the controls on river dissolved Ba/Ca ratios, it can be hypothesized that the presence/absence of carbonate rocks in the studied basins (leading to high conductivity but low Ba/Ca ratios) exerts the strongest influence on the widespread inverse relationships between conductivity and Ba/Ca ratios.…”