Beryllium-7 wet deposition variation with storm height, synoptic classification, and tree canopy state in the mid-Atlantic USA

This is the peer reviewed version of the article which has been published in final form at 10.1002/hyp.10571. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving: http://olabout.wiley.com/WileyCDA/Section/id-820227.html#termsShort-lived fallout isotopes, such as beryllium-7 (7Be), are increasingly used as erosion and sediment tracers in watersheds. Beryllium-7 is produced in the atmosphere and delivered to Earth's surface primarily in precipitation. However, relatively little has been published about the variation in 7Be wet deposition caused by storm type and vegetation cover. Our analysis of precipitation, throughfall, and sediments in two forested, headwater catchments in the mid-Atlantic USA indicates significant variation in isotope deposition with storm type and storm height. Individual summer convective thunderstorms were associated with 7Be activity concentrations up to 5.0 Bq L−1 in precipitation and 4.7 Bq L−1 in throughfall while single-event wet depositional fluxes reached 168 Bq m−2 in precipitation and 103 Bq m−2 in throughfall. Storms originating from the continental USA were associated with lower 7Be activity concentrations and single-event wet depositional fluxes for precipitation (0.7 – 1.2 Bq L−1 and 15.8 – 65.0 Bq m−2) and throughfall (0.1 – 0.3 Bq L−1 and 13.5 – 98.9 Bq m−2). Tropical systems had relatively low activity concentrations, 0.2 – 0.5 Bq L−1 in precipitation and 0.2 – 1.0 Bq L−1 in throughfall, but relatively high single-event depositional fluxes due to large rainfall volumes, 32.8 – 67.6 Bq m−2 in precipitation and 25.7 – 134 Bq m−2 in throughfall. The largest sources of 7Be depositional variation were attributed to storm characteristics including precipitation amount and maximum storm height. 7Be activity associated with fluvial suspended sediments also exhibited the highest concentration and variability in summer (175 – 1450 Bq kg−1). We conclude the dominant source of variation on event-level 7Be deposition is storm type. Our results illustrate the complex relationships between 7Be deposition in precipitation and throughfall and demonstrate event-scale relationships between the 7Be in precipitation and on suspended sediment.National Science Foundatio

Summer CO2 evasion from streams and rivers in the Kolyma River basin, north-east Siberia

Inland water systems are generally supersaturated in carbon dioxide (CO2) and are increasingly recognized as playing an important role in the global carbon cycle. The Arctic may be particularly important in this respect, given the abundance of inland waters and carbon contained in Arctic soils; however, a lack of trace gas measurements from small streams in the Arctic currently limits this understanding.We investigated the spatial variability of CO2 evasion during the summer low-flow period from streams and rivers in the northern portion of the Kolyma River basin in north-eastern Siberia. To this end, partial pressure of carbon dioxide (pCO2) and gas exchange velocities (k) were measured at a diverse set of streams and rivers to calculate CO2 evasion fluxes. We combined these CO2 evasion estimates with satellite remote sensing and geographic information system techniques to calculate total areal CO2 emissions. Our results show that small streams are substantial sources of atmospheric CO2 owing to high pCO2 and k, despite being a small portion of total inland water surface area. In contrast, large rivers were generally near equilibrium with atmospheric CO2. Extrapolating our findings across the Panteleikha-Ambolikha sub-watersheds demonstrated that small streams play a major role in CO2 evasion, accounting for 86% of the total summer CO2 emissions from inland waters within these two sub-watersheds. Further expansion of these regional CO2 emission estimates across time and space will be critical to accurately quantify and understand the role of Arctic streams and rivers in the global carbon budget

Silicon-nitrogen coupling in the equatorial Pacific upwelling zone

We describe the role of diatoms on nitrogen and silicon cycling in the equatorial Pacific upwelling zone (EUZ) using water column nutrient data from 19 equatorial cruises and particle concentration, new production, and sediment trap data from the U.S. Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac), France (JGOFS) fluxes in the Pacific (FLUPAC), and U.S. Zonal Flux cruises. Our results suggest that production and sinking of diatoms dominate particulate nitrogen export at silicate concentrations above 4 micrometers M. Below this level, silicate remains at concentrations of 1-2 micrometers M and is completely exhausted only under nonsteady state conditions. This lower nutrient condition accounts for a majority of particulate nitrogen export in the EUZ with minor loss of particulate silicon. Retention of silicon relative to nitrogen appears due to a combination of new production by nondiatoms, dissolution of silica frustules after grazing, iron limitation, and steady state upwelling. This synthesis supports the argument that diatom production was tightly coupled to new production during the U.S. JGOFS EqPac survey 2 cruise (Dugdale and Wilkerson, 1998). However, this compilation suggests EqPac survey 2 cruise took place during a period of atypically high subsurface nutrients. We conclude that silicon and nitrogen are tightly coupled only at periods of very high nutrient concentration and nonsteady state. In addition, nutrient cycling in the EUZ is consistent at all times with a mechanism of combined iron and grazing control of phytoplankton size classes. (Résumé d'auteur

An initial investigation into the organic matter biogeochemistry of the Congo River

The Congo River, which drains pristine tropical forest and savannah and is the second largest exporter of terrestrial carbon to the ocean, was sampled in early 2008 to investigate organic matter (OM) dynamics in this historically understudied river basin. We examined the elemental (%OC, %N, C:N), isotopic (δ13C, Δ14C, δ15N) and biochemical composition (lignin phenols) of coarse particulate (>63 μm; CPOM) and fine particulate (0.7–63 μm; FPOM) OM and DOC, δ13C, Δ14C and lignin phenol composition with respect to dissolved OM (<0.7 μm; DOM) from five sites in the Congo River Basin. At all sample locations the organic carbon load was dominated by the dissolved phase (∼82–89% of total organic carbon) and the total suspended sediment load was principally fine particulate material (∼81–91% fine suspended sediment). Distinct compositional and isotopic differences were observed between all fractions. Congo CPOM, FPOM and DOM all originated from vegetation and soil inputs as evidenced by elemental, isotopic and lignin phenol data, however FPOM was derived from much older carbon pools (mean Δ14C = −62.2 ± −13.2‰, n = 5) compared to CPOM and DOM (mean Δ14C = 55.7 ± 30.6‰, n = 4 and 73.4 ± 16.1‰, n = 5 respectively). The modern radiocarbon ages for DOM belie a degraded lignin compositional signature (i.e. elevated acid:aldehyde ratios (Ad:Al) relative to CPOM and FPOM), and indicate that the application of OM degradation patterns derived from particulate phase studies to dissolved samples needs to be reassessed: these elevated ratios are likely attributable to fractionation processes during solubilization of plant material. The relatively low DOM carbon-normalized lignin yields (Λ8; 0.67–1.12 (mg(100 mg OC)−1)) could also reflect fractionation processes, however, they have also been interpreted as an indication of significant microbial or algal sources of DOM. CPOM appears to be well preserved higher vascular plant material as evidenced by its modern radiocarbon age, elevated C:N (17.2–27.1) and Λ8 values (4.56–7.59 (mg(100 mg OC)−1)). In relation to CPOM, the aged FPOM fraction (320–580 ybp 14C ages) was comparatively degraded, as demonstrated by its nitrogen enrichment (C:N 11.4–14.3), lower Λ8 (2.80–4.31 (mg(100 mg OC)−1)) and elevated lignin Ad:Al values similar to soil derived OM. In this study we observed little modification of the OM signature from sample sites near the cities of Brazzaville and Kinshasa to the head of the estuary (∼350 km) highlighting the potential for future studies to assess seasonal and long-term OM dynamics from this logistically feasible location and derive relevant information with respect to OM exported to the Atlantic Ocean. The relative lack of OM data for the Congo River Basin highlights the importance of studies such as this for establishing baselines upon which to gauge future change