Modeling δ15N evolution: First palaeoceanographic applications in a coastal upwelling system

The δ15N signal in marine sediments appears to be a good palaeoceanographic tracer. It records biological processes in the water column and is transferred to and preserved in the sediments. Changes in forcing factors in upwelling systems may be recorded by δ15N. These forcing conditions can be of a biogeochemical nature, such as the initial isotopic signal of the nutrients or the trophic structure, or of a physical nature, such as wind stress, insolation, temperature or dynamic recycling. A simple nitrogen-based trophic chain model was used to follow the development of the nitrogen isotopic signal in nutrients, phytoplankton, zooplankton and detritus. Detrital δ15N, influenced by the isotopic signature of the upwelled nutrients and isotopic fractionation along the trophic chain (photosynthesis and zooplankton excretion), was then compared to the sedimentary signal measured off Mauritania. In our model, the biological variables are transported at shallow depths by a simple circulation scheme perpendicular to the coast depicting a continental shelf recirculation cell. Because cell length depends on the extension of the continental shelf, modifications of the cell length mimic sea level changes. Long cell length (high sea level) scenarios produce higher δ15N values whereas short cell length scenarios result in lower values as in the glacial low sea level periods. Despite changes in many climatic parameters throughout this period, our results show that changing the sea level is sufficient to reconstruct the main pattern of the sedimentary δ15N variations offshore of the Mauritanian upwelling, i.e. an increase from about 3‰ to 7‰ during the deglaciation, without invoking any change in nitrogen fixation or denitrification

Climatically-Active Gases in the Eastern Boundary Upwelling and Oxygen Minimum Zone (OMZ) Systems

International audienceThe EBUS (Eastern Boundary Upwelling Systems) and OMZs (Oxygen Minimum Zone) contribute very significantly to the gas exchange between the ocean and the atmosphere, notably with respect to the greenhouse gases (hereafter GHG). From in-situ ocean measurements, the uncertainty of the net global ocean-atmosphere CO2 fluxes is between 20 and 30%, and could be much higher in the EBUS-OMZ. Off Peru, very few in-situ data are available presently, which justifies alternative approaches for assessing these fluxes. GHG air-sea fluxes determination can be inferred from inverse modeling applied to Vertical Column Densities (VCDs) from GOSAT, using state of the art modeling, at low spatial resolution. For accurately linking sources of GHGs to EBUS and OMZs, the resolution of the source regions needs to be increased. This task develops on new non-linear and multiscale processing methods for complex signals to infer a higher spatial resolution mapping of the fluxes and the associated sinks and sources between the atmosphere and the ocean. The use of coupled satellite data (e.g. SST and/or Ocean colour) that carry turbulence information associated to ocean dynamics is taken into account at unprecedented detail level to incorporate turbulence effects in the evaluation of the air-sea fluxes. We will present a framework as described above for determining sources and sinks of GHG from satellite remote sensing with the Peru OMZ as a test bed

The impacts of ocean acidification on marine trace gases and the implications for atmospheric chemistry and climate

Surface ocean biogeochemistry and photochemistry regulate ocean–atmosphere fluxes of trace gases critical for Earth’s atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or pCO2) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N2O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes

Sulphur plume characteristics in the northern Benguela upwelling system

We investigated the seasonal and annual variability of surface sulphur plumes in the northern Benguela upwelling system off Namibia because of their significant impacts on the marine ecosystem, fishing industry, aquaculture farming and tourism due to their toxic properties. We identified the sulphur plumes in ocean colour satellite data of the medium resolution imaging spectrometer (MERIS) for the 2002-2012 time period using the differences in the spectral properties of Namibian Benguela optical water types. The sulphur events have a strong seasonal cycle with pronounced main and off-seasons forced by local and remote-driven processes. The main peak season is in late austral summer and early austral autumn at the beginning of the annual upwelling cycle caused by increasing equatorwards alongshore winds. The sulphur plume activity is high between February and April during the seasonal oxygen minimum associated with the seasonal reduction of cross-shore ventilation of the bottom waters, the seasonal southernmost position of the Angola Benguela Frontal Zone, the seasonal maximum of water mass fractions of South Atlantic and Angola Gyre Central Waters as well as the seasonal arrival of the downwelling coastal trapped waves. The off-season is in austral spring and early austral summer during increased upwelling intensity and enhanced oxygen supply. The annual variability of sulphur events is characterized by very high activities in years 2004, 2005 and 2010 interrupted by periods of lower activity in years 2002 to 2003, 2006 to 2009 and 2011 to 2012. This result can be explained by the relative contributions or adding effects of local and remote-driven forces (from the equatorial area). The probability for the occurrence of sulphur plumes is enhanced in years with a lower annual mean of upwelling intensity, decreased oxygen supply associated with decreased lateral ventilation of bottom waters, more southern position of the Angola Benguela Frontal Zone, increased mass fraction of South Atlantic Central Water and stronger downwelling coastal trapped waves. Understanding of the variability and forcing processes of the toxic sulphur events will help in the future to monitor and forecast them as well as to manage their social and economic consequences in the northern Benguela upwelling system off Namibia

Seasonal and annual variability of coastal sulphur plumes in the northern Benguela upwelling system

International audienceWe investigated the seasonal and annual variability of surface sulphur plumes in the northern Benguela upwelling system off Namibia because of their significant impacts on the marine ecosystem, fishing industry, aquaculture farming and tourism due to their toxic properties. We identified the sulphur plumes in ocean colour satellite data of the medium resolution imaging spectrometer (MERIS) for the 2002–2012 time period using the differences in the spectral properties of Namibian Benguela optical water types. The sulphur events have a strong seasonal cycle with pronounced main and off-seasons forced by local and remote-driven processes. The main peak season is in late austral summer and early austral autumn at the beginning of the annual upwelling cycle caused by increasing equatorwards alongshore winds. The sulphur plume activity is high between February and April during the seasonal oxygen minimum associated with the seasonal reduction of cross-shore ventilation of the bottom waters, the seasonal southernmost position of the Angola Benguela Frontal Zone, the seasonal maximum of water mass fractions of South Atlantic and Angola Gyre Central Waters as well as the seasonal arrival of the downwelling coastal trapped waves. The off-season is in austral spring and early austral summer during increased upwelling intensity and enhanced oxygen supply. The annual variability of sulphur events is characterized by very high activities in years 2004, 2005 and 2010 interrupted by periods of lower activity in years 2002 to 2003, 2006 to 2009 and 2011 to 2012. This result can be explained by the relative contributions or adding effects of local and remote-driven forces (from theequatorial area). The probability for the occurrence of sulphur plumes is enhanced in years with a lower annual mean of upwelling intensity, decreased oxygen supply associated with decreased lateral ventilation of bottom waters, more southern position of the Angola Benguela Frontal Zone, increased mass fraction of South Atlantic Central Water and stronger downwelling coastal trapped waves. Understanding of the variability and forcing processes of the toxic sulphur events will help in the future to monitor and forecast them as well as to manage their social and economic consequences in the northern Benguela upwelling system off Namibia

Seasonal variability of sulphur plumes in relation to remote forcing.

<p>Seasonal climatology of sulphur plume size and intensity, of AGCW fraction, of CARS oxygen concentration in the bottom water layer (A) and of sea level anomaly between 18°S and 19°S within the 1° width coastal band (B). The yellow and red curves correspond to the monthly size and intensity of sulphur plumes, respectively. The turquoise and green curves represent the AGCW fractions for the depths of 63 m and 93 m, respectively. The pink curve stands for the oxygen concentration. Box-Whisker plots with monthly minimum, maximum, median, 25^th and 75^th percentiles can be calculated only for the SLA product. The calculation is not performed for AGCW fraction and oxygen concentration because not the full data sets are available. The blue curves represent the median values.</p

Seasonal variability of sulphur plumes.

<p>(A): Seasonal variability of sulphur plume size derived from all MERIS data of years 2002 to 2012. The light yellow bar in April represents the sulphur plume size including the strong events of year 2005. (B): Seasonal variability of the sulphur plume intensity with monthly minimum, maximum, median, 25^th and 75^th percentiles. (C): Number of MERIS scenes available for each month. (D): Sulphur plume size, mean area-averaged cloud fraction including the standard deviation (Mcfm), number of days in percentage where the cloud fraction is above 2/3 on 3 consecutive days (Ncfm3d in %).</p

Annual variability of sulphur plumes in relation to local forcing.

<p>Annual anomalies of sulphur plume size and intensity, of wind speed at 10 m high of CCMP, TMI, QuikScat and ASCAT (A), of pseudo wind stress v-component at 10 m high of CCMP, QuikScat and ASCAT (B), of corresponding pseudo wind stress u-component (C) and of sea surface temperature of TMI and MODIS (D). The annual anomalies were calculated from the difference of yearly means and the global mean of years 2002 to 2012.</p

Annual variability of sulphur plumes in relation to remote forcing.

<p>Annual anomalies of sulphur plume size and intensity, of TMI and MODIS SST over the Angola-Benguela area (A) and of sea level anomaly between 18°S and 19°S within the 1° width coastal band (B). The SLA positive trend from 2002 to 2012 was removed.</p

Sulphur plumes seen from satellites.

<p>(A) and (B): Each of the mosaics of MODIS and MERIS is composed from two RGB images. The grey-line is the limit between them. The locations of in-situ measurements which were performed in the upper surface water layer during sulphur plumes seen from MODIS are marked with M1 to M3 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192140#pone.0192140.ref016" target="_blank">16</a>]. The measurements of Lavik et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192140#pone.0192140.ref010" target="_blank">10</a>] are marked with the same isolines as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192140#pone.0192140.g001" target="_blank">Fig 1</a>. The triangle St3 corresponds to the measurements of Brüchert et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192140#pone.0192140.ref024" target="_blank">24</a>]. The dashed lines represent zooms in specific areas. (C): The MODIS scene is overlayed by the only available HICO scene. (D): The spatial high resolution MSI scene (10 m) characterizes the fine structures of sulphur plumes.</p