Description: Lake Tahoe is an ultra-oligotrophic subalpine lake that is renowned for its clarity. The region experiences little cloud cover and is one of the most UV transparent lakes in the world. As such, it is an ideal environment to study the role of UV radiation in aquatic ecosystems. Long-term trends in Secchi depths showed that water transparency to visible light has decreased in recent decades, but limited data are available on the UV transparency of the lake. Here we examine how ultraviolet radiation varies relative to longer-wavelength photosynthetically active radiation (PAR, 400-700 nm, visible wavelengths) horizontally along inshore-offshore transects in the lake and vertically within the water column as well as temporally throughout 2007. UV transparency was more variable than PAR transparency horizontally across the lake and throughout the year. Seasonal patterns of Secchi transparency differed from both UV and PAR, indicating that different substances may be responsible for controlling transparency to UV, PAR, and Secchi. In surface waters, UVA (380 nm) often attenuated more slowly than PAR, a pattern visible in only exceptionally transparent waters with very low dissolved organic carbon. On many sampling dates, UV transparency decreased progressively with depth suggesting surface photobleaching, reductions in particulate matter, increasing chlorophyll a, or some combination of these increased during summer months. Combining these patterns of UV transparency with data on visible light provides a more comprehensive understanding of ecosystem structure, function, and effects of environmental change in highly transparent alpine and subalpine lakes such as Tahoe.

Description: During the SAMUM field campaign in southern Morocco in May and June 2006 density currents generated by evaporative cooling after convective precipitation were frequently observed at the Sahara side of the Atlas Mountain chain. The associated strong surface cold-air outflow during such events has been observed to lead to dust mobilization in the foothills. Here a regional model system is used to simulate a density current case on 3 June 2006 and the subsequent dust emission. The model studies are performed with different parameterization schemes for convection, and with different horizontal model grid resolutions to examine to which extent the model system can be used for reproducing dust emissions in this region. The effect of increasing the horizontal model grid resolution from 14 km to 2.8 km on the strength on the density currents and thus on dust emission is smaller than the differences due to different convection parameterization schemes in this case study. While the results in reproducing the observed density current at the Atlas Mountain varied with different convection parameterizations, the most realistic representation of the density current is obtained at 2.8 km grid resolution at which no parameterization of deep convection is needed.

Description: The main physical and biological processes that control the seasonal cycle of the plankton dynamics over the Western Black Sea were explored by means of a three‐dimensional, 7‐compartment, on‐line coupled biophysical model that was developed for this study. Adopting high frequency forcing in terms of air‐sea interaction and Danube river inputs, we performed a simulation of the coupled model during the 2002–2003 period. A series of 8‐day Chl‐a SeaWiFS images provided a validation tool that guided us, along with available in situ measurements, to the improvement of model parameterizations and the calibration of the biological parameters. The simulation of the seasonal phytoplankton variability over the entire Western Black Sea, extending from the highly eutrophic river influenced area to the open sea area, was a major challenge that made necessary the representation of both the spatial and time variability of several processes. Despite the model simplicity, the simulated Chl‐a patterns presented a good agreement as compared to the SeaWiFS and in situ data. During winter, phytoplankton in coastal areas was shown to be limited by light availability, primarily due to the increased particulate matter concentrations, as a result of resuspension from the sediment and the increased river loads. During summer, the primary production was mostly sustained by riverine nutrients and regeneration processes and thus was strongly linked to the evolution of the Danube plume. The limiting nutrients showed deviations from the observed concentrations, indicating the necessity for a more realistic phytoplankton growth model.

Description: A 2-year record of mixed layer measurements of CO2 partial pressure (pCO2), nitrate, and other physical, chemical, and biological parameters at a time series site in the northeast Atlantic Ocean (49N/16.5W) is presented. The data show average undersaturation of surface waters with respect to atmospheric CO2 levels by about 40 ± 15 matm, which gives rise to a perennial CO2 sink of 3.2 ± 1.3 mol m2 a1. The seasonal pCO2 cycle is characterized by a summer minimum (winter maximum), which is due to the dominance of biological forcing over physical forcing. Our data document a rapid transition from deep mixing to shallow summer stratification. At the onset of shallow stratification, up to one third of the mixed layer net community production during the productive season had already been accomplished. The combination of high prestratification productivity and rapid onset of tratification appears to have caused the observed particle flux peak early in the season. Mixed layer deepening during fall and winter reventilated CO2 from subsurface respiration of newly exported organic matter, thereby negating more than one third of the carbon drawdown by net community production in the mixed layer. Chemical signatures of both net community production and respiration are indicative of carbon overconsumption, the effects of which may be restricted, though, to the upper ocean. A comparison of the estimated net community production with satellite-based estimates of net primary production shows fundamental discrepancies in the timing of ocean productivity.

Description: We measured halogen concentrations and I-129/I ratios in five drilling sites of Integrated Ocean Drilling Program Expedition 311 (offshore Vancouver Island, Canada) in order to identify potential sources of fluids and methane in gas hydrate fields. Iodine is dominated by organic decomposition and transports with fluids in reducing environments and the presence of the cosmogenic radioisotope I-129 (T-1/2 = 15.7 Ma) allows the age determination of organic sources for iodine. Here we report halogen concentrations in 135 pore water samples, I concentrations in 48 sediment samples, and I-129/I ratios measured in a subset of 20 pore water samples. Most I-129/I ratios fall into a range around 500 x 10(-15), corresponding to a minimum age of 25 Ma and the lowest ratio of 188 x 10(-15) (T-min = 47 Ma) was observed at 208 m below sea floor (mbsf) in Site 1326. These ages are considerably older than that of the local sediments in the gas hydrate fields and that of the subducting sediments on the Juan de Fuca plate, indicating that old, accreted sediments in the accretionary wedge contribute a significant amount of iodide and, by association, of methane to the gas hydrate occurrences. A geochemical transport-reaction model was applied to simulate the advection of deeply sourced fluids and the release of iodide, bromide, and ammonia in the host sediments due to organic matter degradation. The model was first tested with data from two well studied areas, Ocean Drilling Program Site 1230 (Peru margin) and Site 1245 (Hydrate Ridge). The model results for the Expedition 311 sites indicate that the in situ release of young iodine is relatively minor in comparison to the contribution of migrating fluids, carrying large amounts of old iodine from deep sources. The comparison between the sites demonstrates that the total organic content has a strong effect on the rate of in situ iodine release and that lateral flows along fractures can have a significant influence on pore water chemistry, especially at the Cascadia margin. The iodine results indicate that mobilization and transport of methane from sources in the upper plate of active margins is an important process which can also play a substantial role in the formation of gas hydrate fields.

Description: There is growing concern about the transfer of methane originating from water bodies to the atmosphere. Methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport. This work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere. The bubble model predicts the evolving bubble size, gas composition, and rise distance and is suitable for almost all aquatic environments. The model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep, oxic, saline water with excellent results. Methane bubbles from within the hydrate stability zone (typically below ∼500 m water depth in the ocean) are believed to form an outer hydrate rim. To explain the subsequent slow dissolution, a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone. The calibrated model explains the impressively tall flares (〉1300 m) observed in the hydrate stability zone of the Black Sea. This study suggests that only a small amount of methane reaches the surface at active seep sites in the Black Sea, and this only from very shallow water areas (〈100 m). Clearly, the Black Sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere, although substantial amounts of methane may reach the surface in shallow lakes and reservoirs.

Description: Combined measurements of Mg/Ca and stable oxygen isotopes in tests of the planktonic foraminifer G. bulloides from Ocean Drilling Program (ODP) Site 1172A (East Tasman Plateau) allowed us to reconstruct sea surface temperature (SSTMg/Ca), sea surface salinity (SSS), and hence variations in the Subtropical Convergence (STC) in the southwestern Tasman Sea over the last four major glacial-interglacial changes. During interglacials the commonly enhanced SSTMg/Ca and SSS correspond to a lowered marine productivity and a lowered terrigenous flux, implying that the STC separating cool, high-nutrient Subantarctic Surface Water from warm, saline, oligotrophic Subtropical Surface Water and hence the band of zonal westerlies responsible for the eolian dust flux were located south of East Tasman Plateau. The warm East Australian Current was well established during warm periods and propagated far south. During glacial times, SSTMg/Ca and SSS were lower, while both marine productivity and eolian flux increased. Such conditions prevailed during glacial Marine Isotope Stages MIS 12, MIS 10, and to a lesser degree MIS 6 and implied the extended northward influence of Subantarctic SurfaceWater and a shift of the STC to 〈44�S. The overall climatic signal at Site 1172A appears to be largely attenuated when compared to published climate records from comparable latitudes to the west and to the east. SSTMg/Ca amplitudes were more pronounced in the subantarctic Indian Ocean and at Chatham Rise. They exhibit a consistent pattern suggesting that latitudinal shifts of the STC occurred synchronously in the subantarctic Indian Ocean and at Chatham Rise but were largely damped at East Tasman Plateau due to the influence of the East Australian Current.