Description: Boron isotopes in marine carbonates are increasingly used to reconstruct seawater pH and atmospheric pCO2 through Earth’s history. While isotope ratio measurements from individual laboratories are often of high quality, it is important that records generated in different laboratories can equally be compared. Within this Boron Isotope Intercomparison Project (BIIP), we characterised the boron isotopic composition (commonly expressed in δ11B) of two marine carbonates: Geological Survey of Japan carbonate reference materials JCp‐1 (coral Porites) and JCt‐1 (giant clam Tridacna gigas). Our study has three foci: (i) to assess the extent to which oxidative pre‐treatment, aimed at removing organic material from carbonate, can influence the resulting δ11B; (ii) to determine to what degree the chosen analytical approach may affect the resultant δ11B, and (iii) to provide well‐constrained consensus δ11B values for JCp‐1 and JCt‐1. The resultant robust mean and associated robust standard deviation (s*) for un‐oxidised JCp‐1 is 24.36 ± 0.45‰ (2s*), compared with 24.25 ± 0.22‰ (2s*) for the same oxidised material. For un‐oxidised JCt‐1, respective compositions are 16.39 ± 0.60‰ (2s*; un‐oxidised) and 16.24 ± 0.38‰ (2s*; oxidised). The consistency between laboratories is generally better if carbonate powders were oxidatively cleaned prior to purification and measurement.

Description: In this paper we present chemical composition data for major elements in rivers from three islands of the Lesser Antilles. The Lesser Antilles are a tropical volcanic subduction arc and are characterized by steep gradients of relief, bedrock age and precipitation. They constitute a natural laboratory where the response of the weathering engine to large variations of runoff can be understood. Data indicate that the Lesser Antilles are characterized by extremely variable chemical weathering (40-430 t/km2/a) and CO2 consumption (300-3500.103 mol/km2/a) rates, amongst the highest found on Earth and consistent with the previous studies on the weathering of volcanic rock. A noteworthy observation is that, along the runoff gradient, concentrations of rock-derived solutes do not follow a pure dilution law and that a buffering mechanism exists stabilizing solute concentrations. As a result concentrations vary much less than runoff and chemical weathering rates are mainly controlled by runoff. Precipitation patterns in the Lesser Antilles are essentially orographic and controlled by the adiabatic decompression of the water-saturated Atlantic air masses. The production of acidity by volcanic degassing is an additional factor that modulates the runoff effect. Two main conclusions can be drawn from this study. First, chemical weathering fluxes of oceanic islands are strongly dependent upon relief repartition, which cautions the use of regional mean values to compare volcanic islands. Second, volcanic activity in the Lesser Antilles subduction arc, by creating relief, promotes high orographic precipitation and/or infiltration regimes, that in turn results in elevated chemical weathering and atmospheric CO2 consumption fluxes. This feedback mechanism, implying mainly precipitation and relief, is proposed to act in complement to the temperature-related feedback proposed by previous authors for stabilizing the atmospheric CO2 content of the atmosphere in response to volcanic CO2 degassing. This study highlights the importance of the water cycle in controlling chemical weathering of volcanic arc islands and associated CO2 consumption rates.

Description: To further develop boron isotopes as a tool for understanding shale weathering, we explored patterns of boron concentrations and isotopes across the forested Susquehanna Shale Hills Critical Zone Observatory (CZO). We present boron measurements for all watershed components that provided a foundation for examining water-rock interactions in a shale dominated watershed, including water compartments (e.g., precipitation, stream water, groundwater) and solid compartments (e.g., soil, bedrock, stream sediments, suspended load, and leaf litter). Results show boron isotopes (δ11B) in the bedrock (− 4.6‰) and soil (− 5.9 to - 4.2‰) were very similar. All waters were enriched in 11B by comparison: precipitation (7.2 to 22.6‰), stream (10.3 to 15.5‰), and groundwater (2.2 to 17.4‰). Modeling revealed that isotopic fractionation observed in the surface water and groundwater could mainly be explained by water-rock interactions including clay mineral dissolution (e.g., chlorite) and coprecipitation/adsorption processes (e.g., coatings on illite particles), likely in the near surface soils (~2 m deep). We found that leaching, the loss of boron from vegetation to stream water, plays a secondary role. Specifically, such leaching likely contributes the equivalent of 10 to 26% of the B fluxes from the watershed outlet. Boron mass balance between bedrock and precipitation inputs and the exported flux of dissolved and solid pools identified a “missing” isotopically light solid flux (δ11B of −12.2 ± 5.3‰ at ~4.4 ± 3.8 mol/ha/y of B; uncertainty reported as 2 SD). We did not sample any pool with this isotopic signature. Here our data suggest the composition of this pool is more likely related to precipitation of secondary clays rather than adsorption or (co)precipitation on Fe oxides. We propose two hypotheses to explain the missing light B pool: 1) a significant portion of the particles carrying the missing 10B are not sampled because they enter groundwater at depth and are transported out of the catchment under the stream; and/or 2) the inputs and outputs of boron are not operating at steady state in the catchment today, suggesting that the missing boron particles were lost in the past in proportions higher than today. When this B budget is paired with studies of δ26Mg and δ56Fe from Shale Hills, both of which also show missing isotopic pools, the pattern indicates a fundamental gap in understanding of shale weathering. We concluded that light B particles, presumably generated in the upper soils, are likely transported deep beneath the surface in the groundwater system or episodically in the past through riverine fluxes.

Description: The boron isotopic composition (δ11B) of benthic foraminifera provides a valuable tool to reconstruct past deep-water pH. As the abundance of monospecific species might be limited in sediments, microanalytical techniques can help to overcome this problem, but such studies on benthic foraminiferal δ11B are sparse. In addition, microanalytics provide information on the distribution of δ11B at high spatial resolution to increase the knowledge of biomineralization processes, for example. For this study, we investigated the intra- and inter-shell δ11B variability of the epibenthic species Cibicidoides wuellerstorfi, which is widely used in paleoceanography, by secondary ion mass spectrometry (SIMS) and femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). While the average δ11B values obtained from these different techniques agree remarkably well with bulk solution values to within ±0.1 ‰, a relatively large intra-shell variability was observed. Based on multiple measurements within single shells, the SIMS and LA data suggest median variations of 4.8 ‰ and 1.3 ‰ (2σ), respectively, while the larger spread for SIMS is attributed to the smaller volume of calcite being analyzed in each run. When analytical uncertainties and volume-dependent differences in δ11B variations are taken into account for these methods, the intra-shell variability is estimated to be on the order of ∼3 ‰ and ∼0.4 ‰ (2σ) on a ∼20 and 100 µm scale, respectively. In comparison, the δ11B variability between shells exhibits a total range of ∼3 ‰ for both techniques, suggesting that several shells need to be analyzed for accurate mean δ11B values. Based on a simple resampling method, we conclude that ∼12 shells of C. wuellerstorfi must be analyzed using LA-MC-ICPMS to obtain an accurate average value within ±0.5 ‰ (2σ) to resolve pH variations of ∼0.1. Based on our findings, we suggest preferring the conventional bulk solution MC-ICPMS over the in situ methods for paleo-pH studies, for example. However, SIMS and LA provide powerful tools for high-resolution paleoreconstructions, or for investigating ontogenetic trends in δ11B.