The strontium (88Sr/86Sr) stable isotope composition of seawater preserved in sedimentary foraminifera potentially provides key information on variations in the composition of material delivered by continental weathering and on changes in carbonate productivity over time. However, recent studies suggest a significant temperature dependent fractionation of Sr stable isotopes during the precipitation of calcium carbonate, which must be quantified before seawater records can be accurately retrieved [1, 2].
This study presents high-precision stable Sr isotope data (±10 ppm; 2 s.d.) for core-top planktonic foraminifera from sites in the South Atlantic covering a sea surface temperature range of ca. 18 - 28°C, and quaternary marine foraminiferal records from the SE Indian Ocean (core ). These results indicate that there is no signficiant variation in the stable isotope composition of an individual species across the temperature range studied here, but there are resolvable differences in the offset from seawater between species. In this case, seawater stable Sr isotope records can be reconstructed without the necessity of a temperature correction. The preliminary results for a glacial-interglacial planktonic foraminiferal record indicate that there are no resolvable variations in the stable isotope ratios over this time interval, indicating that there are no significant variations in the Sr isotope composition of continental runoff or carbonate productivity in the oceans over this time interval.
[1] Fietzke, J., Eisenhauer, A. (2006), Geochem. Geophys. Geosyst., 7, (8), 1-6 . [2] Ruggerburg, A., Fietzke, J., Liebetrau, V. Eisenhauer, A., Dullo, W-C., Freiwald, A. (2008). Earth Plan. Sci. Lett. 269, 570-575
The strontium (88Sr/86Sr) stable isotope composition of seawater reflects input from continental weathering and hydrothermal exchange at mid-ocean ridges, and output in carbonate sediments. Variations in the stable isotope composition of seawater over time reflect either changes in the balance of carbonate to silicate weathering or changes in carbonate productivity and sedimentation in the oceans. However, temperature and species dependent fractionation of Sr stable isotopes during incorporation into marine carbonate has to be quantified in order to reconstruct past seawater compositions accurately. This study presents high-precision δ88Sr data, obtained using double-spike TIMs technique, for seawater, modern foraminifera from core-top samples and a record through the last glacial maximum (LGM). Present-day seawater yields a δ88Sr composition of 0.356±0.007 (2σm) with no resolvable difference between Pacific, Atlantic and Indian Oceans. Globigerinoides sacculifer (in the 350-450μm size range) from sites in the South Atlantic, covering a mixed layer temperature range of ~10°C, show no systematic variation with temperature, and have an average δ88Sr value of 0.22±0.07. Both G. sacculifer and G. menardii show systematic variations with growth rate (shell size) with heavier compositions in the larger size fractions. By contrast, G. aequilateralis and G. ruber yield δ88Sr values of -0.056±0.067 and -0.023±0.008 respectivley, with no systematic variation with shell size. These observations indicate that for G. sacculifer, at least, there is no effect of temperature on Sr stable isotope uptake, but the species specific, and/or shell size effects need to be considered in order to retrieve seawater δ88Sr values from foraminiferal tests. Preliminary δ88Sr data for Pulleniatina obliquiloculata covering the last 43 kyr indicate that there was no resolvable change in the δ88Sr composition of seawater across the LGM and deglaciation. In this case the postulated enhanced weathering of shelf carbonates during glacial intervals, delivering light Sr isotopes to the ocean may not have been as significant as predicted or else was offset by increased production and preservation of carbonates, driving seawater to heavier δ88Sr values. Alternatively the very long residence time of Sr in the oceans may simply buffer the changes in input or output such that no changes are resolved at the level of precision of this study.
Abstract A large portion of freshwater and sediment is exported to the ocean by a small number of major rivers. Many of these megarivers are subject to substantial anthropogenic pressures, which are having a major impact on water and sediment delivery to deltaic ecosystems. Due to hydrodynamic sorting, sediment grain size and composition vary strongly with depth and across the channel in large rivers, complicating flux quantification. To account for this, we modified a semi‐empirical Rouse model, synoptically predicting sediment concentration, grain‐size distribution, and organic carbon (%OC) concentration with depth and across the river channel. Using suspended sediment depth samples and flow velocity data, we applied this model to calculate sediment fluxes of the Irrawaddy (Ayeyarwady) and the Salween (Thanlwin), the last two free‐flowing megarivers in Southeast Asia. Deriving sediment‐discharge rating curves, we calculated an annual sediment flux of Mt/year for the Irrawaddy and Mt/year for the Salween, together exporting 46% as much sediment as the Ganges‐Brahmaputra system. The mean flux‐weighted sediment exported by the Irrawaddy is significantly coarser ( D 84 = 193 ± 13 μ m) and OC‐poorer ( 0.29 ± 0.08 wt%) compared to the Salween ( 112 ± 27 μ m and 0.59 ± 0.16 wt%, respectively). Both rivers export similar amounts of particulate organic carbon, with a total of Mt C/year, 53% as much as the Ganges‐Brahmaputra. These results underline the global significance of the Irrawaddy and Salween rivers and warrant continued monitoring of their sediment flux, given the increasing anthropogenic pressures on these river basins.
Upon permafrost thaw, the volume of soil accessible to plant roots increases which modifies the acquisition of plant-available resources. Tundra vegetation is actively responding to the changing environment with two major directions for vegetation shift across the Arctic: the expansion of deep-rooted sedges and the widespread increase in shallow-rooted shrubs. Changes in vegetation composition, density and distribution have large implications on the Arctic warming and permafrost stability by influencing the albedo, the snow accumulation and the litter decomposition rate. A better understanding of these cumulated effects of changing vegetation on warming and permafrost requires assessing the changes in plant nutrient sources upon permafrost thaw, nutrient access being a limiting factor for the Arctic tundra vegetation development. In this study, we determined the influence of permafrost degradation on the base cation sources for plant uptake by using the radiogenic Sr isotope ratio as a tracer of source, along a permafrost thaw gradient at Eight Mile Lake in Interior Alaska (USA). As plants take up Sr from the exchangeable soil fraction with no measurable fractionation, we determined the differences in 87Sr/86Sr ratio of the exchangeable Sr between shallow and deeper soil horizons, and we compared the 87Sr/86Sr ratio of foliar samples for three Arctic tundra species with contrasted rooting depths (Betula nana, Vaccinium vitis-idaea, and Eriophorum vaginatum) upon different permafrost thaw conditions. The higher foliar 87Sr/86Sr ratios of shallow-rooted Arctic tundra shrubs (B. nana, V. vitis-idaea) was consistent with a shallow source of soil exchangeable Sr from surface soil horizons, whereas the lower foliar 87Sr/86Sr ratios of deep-rooted Arctic tundra sedges (E. vaginatum) reflected a source of Sr from deeper soil horizons. There is a shift between poorly and highly thawed soil profiles towards lower foliar 87Sr/86Sr ratios in both deep- and shallow-rooted plant species. This shift supports that micro-landscape variability in the exchangeable base cation reserve with soil depth represents a key source of readily available nutrients for both shallow- and deep-rooted plant species upon permafrost thaw. This study highlights a key change in plant nutrient source to consider upon thaw. This finding lies beyond the common view that nutrient release at the permafrost thaw front preferentially benefits deep-rooted plant species.
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